
[Federal Register Volume 88, Number 214 (Tuesday, November 7, 2023)]
[Notices]
[Pages 76760-76770]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2023-24548]


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DEPARTMENT OF HEALTH AND HUMAN SERVICES

Food and Drug Administration

[Docket No. FDA-2020-N-0955]


Phibro Animal Health Corp.; Carbadox in Medicated Swine Feed; 
Revocation of Approved Method

AGENCY: Food and Drug Administration, HHS.

ACTION: Final order.

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SUMMARY: The Food and Drug Administration (FDA) is issuing a final 
order to revoke the approved method for detecting residues of carbadox, 
a carcinogenic new animal drug used in swine feed. An approved method 
is required by the Federal Food, Drug, and Cosmetic Act (FD&C Act), as 
implemented by regulation, to show that no residue of carcinogenic 
concern from a new animal drug persists in any edible tissue or in any 
food derived from treated animals. The approved method measures 
quinoxaline-2-carboxylic acid (QCA) as a marker residue to detect the 
presence of any residue of carcinogenic concern. QCA is a metabolite of 
carbadox that FDA has judged does not present a carcinogenic risk. FDA 
is revoking the approved method for carbadox based on its determination 
that the method is inadequate to monitor the residue of carcinogenic 
concern in compliance with FDA's operational definition of no residue 
because there is no established relationship between the concentration 
of QCA residues as measured by the approved method and the 
concentration of the residue of carcinogenic concern.

DATES: This order is effective November 7, 2023.

FOR FURTHER INFORMATION CONTACT: Diane Heinz, Center for Veterinary 
Medicine (HFV-6), Food and Drug Administration, 7500 Standish Pl., 
Rockville, MD 20855, 240-402-5692.

SUPPLEMENTARY INFORMATION: 

I. Introduction

    On July 20, 2020, FDA's Center for Veterinary Medicine (CVM), the 
Center within FDA that reviews and approves new animal drug 
applications and supplemental applications, proposed to revoke the 
approved method for carbadox (Ref. 1), which measures QCA as the marker 
residue \1\ to determine whether residues of carcinogenic concern \2\ 
of carbadox are present (85 FR 43853, July 20, 2020). QCA is a 
metabolite of carbadox that FDA has judged does not present a 
carcinogenic risk. The proposal to revoke the approved method was based 
on FDA's determination that the method does not adequately monitor the 
residue of carcinogenic concern in compliance with FDA's operational 
definition of no residue (Sec.  500.82(b) (21 CFR 500.82(b)(defining 
``no residue''; Sec.  500.84(c)(3) (21 CFR 500.84(c)(3))). That is 
because the sponsor has not established the relationship between the 
concentration of the marker residue QCA and the concentration of the 
residue of carcinogenic concern.
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    \1\ See Sec.  500.82(b) (defining ``marker residue'' as the 
residue whose concentration is in a known relationship to the 
concentration of the residue of carcinogenic concern in the last 
tissue to deplete to the Sm and defining 
``Sm'' as the concentration of a residue of carcinogenic 
concern in a specific edible tissue corresponding to no significant 
increase in the risk of cancer to the human consumer).
    \2\ Consistent with FDA regulations, CVM treats unidentified 
residues of a carcinogenic drug as carcinogenic. See Sec.  500.82(b) 
(defining ``residue of carcinogenic concern'' as all compounds in 
the total residue of a demonstrated carcinogen excluding any 
compounds judged by FDA not to present a carcinogenic risk).
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    On March 10, 2022, FDA held a public hearing under 21 CFR part 15, 
entitled, ``Scientific Data and Information Related to the Residue of 
Carcinogenic Concern for the New Animal Drug Carbadox'' to gather 
additional data and information. When FDA announced the hearing (87 FR 
2093, January 13, 2022; https://www.fda.gov/animal-veterinary/workshops-conferences-meetings/part-15-public-hearing-scientific-data-and-information-related-residue-carcinogenic-concern-new), we requested 
public comments and presentations at the public hearing, particularly: 
(1) on data to inform our knowledge of the residue of carcinogenic 
concern not summarized in the FOI Summary for the 1998 supplemental 
approvals, including additional data regarding the fraction of 
noncarcinogenic residues in the total radiolabeled residues of 
carbadox; (2) for any given concentration of a marker residue, the 
corresponding

[[Page 76761]]

concentration of the residue of carcinogenic concern; (3) on additional 
information related to the adequacy of the current approved method to 
measure QCA as a marker residue for the residue of carcinogenic concern 
for the new animal drug carbadox not already contained in Docket No. 
FDA-2020-N-0955, ``Phibro Animal Health Corp.; Carbadox in Medicated 
Swine Feed; Revocation of Approved Method''; (4) on any method, other 
than the current approved method, that demonstrates ``no residue'' for 
the new animal drug carbadox in conformance with 21 CFR part 500, 
subpart E; and (5) on detailed information on the conduct and quality 
of studies providing data to support the points above, including 
information on the extraction process and the stability of residues 
being analyzed.
    In addition to presentations from CVM and from the sponsor of the 
carbadox approved applications, several other stakeholders gave 
presentations. FDA also opened a docket (Docket No. FDA-2021-N-1326) to 
receive additional stakeholder comment on the topics listed above. 
After reviewing the comments to this docket (FDA-2020-N-0955), and 
presentations and the comments received in the docket for the public 
hearing (Docket No. FDA-2021-N-1326), FDA is now finalizing the order 
revoking the approved method for detecting residues of carbadox.
    Elsewhere in this issue of the Federal Register, FDA is publishing 
a notice of opportunity for hearing (NOOH) proposing to withdraw 
approval of all new animal drug applications for use of carbadox based 
on the lack of an approved method for measuring the residue of 
carcinogenic concern. An approved method for measuring the residue of 
carcinogenic concern that complies with part 500, subpart E (21 CFR 
part 500, subpart E) is required by section 512(d)(1)(I) of the FD&C 
Act (21 U.S.C. 360b(d)(1)(I)).

II. Background

A. Regulation of Carcinogenic New Animal Drugs

    The Delaney Clause of the FD&C Act generally prohibits the approval 
of carcinogenic animal drugs unless the ``Diethylstilbestrol (DES) 
Proviso'' applies. See section 512(d)(1)(I) of the FD&C Act. Under the 
DES Proviso exception, a carcinogenic new animal drug may be approved 
if, among other things, no residue of such drug will be found by 
methods of examination prescribed or approved by the Secretary of 
Health and Human Services (HHS) by regulations in any edible portion of 
such animals after slaughter or in any food yielded by or derived from 
the living animals.
    As part of a new animal drug application (NADA), the sponsor must 
include a description of practicable methods for determining the 
quantity, if any, of the new animal drug in or on food and any 
substance formed in or on food because of its use, and the proposed 
tolerance or withdrawal period or other use restrictions to ensure that 
the proposed use of this drug will be safe (Sec.  514.1(b)(7) (21 CFR 
514.1(b)(7))). Carcinogenic drugs, such as carbadox, must also meet the 
requirements in part 500, subpart E (Sec.  514.1(b)(7)(ii)). These 
regulations, known as the sensitivity of the method (SOM) regulations, 
set out the requirements to demonstrate that no residues of the drug 
will be found by an approved method in any edible tissues of or in any 
foods obtained from the animal, as required to comply with the DES 
Proviso.
    Specifically, the SOM regulations require FDA to determine if any 
animal drug or any of its metabolites is a carcinogen (Sec.  
500.84(a)). For the drug and each metabolite that FDA decides should be 
regulated as a carcinogen, FDA calculates, based on submitted assays, 
the concentration of the test compound in the total diet of the test 
animal that corresponds to a maximum lifetime risk of cancer in the 
test animal of 1 in 1 million (Sec.  500.84(c)(1)). FDA designates the 
lowest concentration (i.e., the concentration of the most potent 
carcinogen) thus calculated as the So (Sec.  500.84(c)(1)). 
The So corresponds to a concentration of residue of 
carcinogenic concern in the total human diet that represents no 
significant increase in the risk of cancer to people (Sec.  500.82(b)). 
Because FDA relies on the So from the most potent 
carcinogen, this approach ensures that use of the drug does not present 
a significant increase in the risk of cancer when considering all 
residues in edible tissues.
    Because the total human diet is not derived only from food-
producing animals, the SOM regulations make adjustments for human food 
intake of edible tissues and determine the concentration of residue of 
carcinogenic concern in a specific edible tissue (such as muscle, 
liver, kidney, milk, or eggs) that corresponds to no significant 
increase in the risk of cancer to the human consumer. FDA assumes for 
purposes of these regulations that this value will correspond to the 
concentration of residues in a specific edible tissue that corresponds 
to a maximum lifetime risk of cancer in the test animals of 1 in 1 
million. This value is designated as the Sm (Sec. Sec.  
500.82(b) and 500.84(c)(1)). By limiting the concentration of residue 
of carcinogenic concern to a value at or below the Sm, 
consumers can eat a specific edible tissue every day for an entire 
lifetime with no significant increase in the risk of cancer.
    Based on data submitted by a sponsor, FDA selects a target tissue 
\3\ and a marker residue \4\ and designates the concentration of the 
marker residue that the method must be able to detect in the target 
tissue (Sec.  500.86(a) through (c) (21 CFR 500.86(a) through (c))). 
This value, termed the Rm, is the concentration of a marker 
residue in the target tissue when the residue of carcinogenic concern 
is equal to Sm (500.82(b)). When the marker residue is at or 
below the Rm, the residue of carcinogenic concern in the 
human diet does not exceed So (Sec.  500.86(c)). This 
regulation ensures that when the marker residue is no longer 
detectable, the residue of carcinogenic concern does not exceed 
Sm in any of the edible tissues (Sec. Sec.  500.82(b) and 
500.86(c)). For any given drug, there may be several different 
compounds to consider for use as a marker residue. The Rm 
would be different depending upon the compound selected as the marker 
residue.
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    \3\ See Sec.  500.82(b) (defining target tissue as the edible 
tissue selected to monitor for residues in the target animals, 
including, where appropriate, milk or eggs).
    \4\ See supra note 1 (defining ``marker residue'').
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    A sponsor must submit a method that is able to detect the marker 
residue at or below the Rm (Sec. Sec.  500.88(b) (21 CFR 
500.88(b)) and 500.84(c)(2)). There may be multiple methods available 
to detect a particular marker residue; however, under the SOM 
regulations, a method must be able to confirm the identity of the 
marker residue in the target tissue at a minimum concentration 
corresponding to the Rm. The Limit of Detection (LOD) for 
the method must be less than or equal to the Rm (Sec.  
500.84(c)(2)). FDA will determine the LOD from the submitted analytical 
method validation data (Sec.  500.88(b)).\5\ If

[[Page 76762]]

a method is not developed that can detect the marker residue at or 
below the Rm, the requirements of the SOM regulations are 
not satisfied, and FDA cannot approve the drug (see 21 U.S.C. 
360b(d)(1)(I); Sec.  500.88).
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    \5\ As discussed above, the Delaney Clause prohibits the use of 
carcinogenic animal drugs unless the DES Proviso applies (see 
section 512(d)(1)(I) of the FD&C Act). The DES Proviso requires 
that, among other things, no residue of such drug will be found (by 
methods of examination prescribed or approved by the Secretary of 
HHS by regulations) in any edible portion of such animals after 
slaughter or in any food yielded by or derived from the living 
animals. FDA's SOM regulations establish the process by which a 
carcinogenic new animal drug may satisfy the DES Proviso. The SOM 
regulations were amended in 2002 to revise the operational 
definition of the term ``no residue.'' Previously, FDA determined 
there was ``no residue'' in edible tissues when the concentration of 
the marker residue was at or below Rm. However, in 1995, 
the Department of Justice (DOJ)'s Office of Legal Counsel determined 
that FDA's interpretation was not legally supportable. Specifically, 
it opined that, if a method detected residue (even if the 
concentration of that residue fell below the Rm) the DES 
Proviso requirement for ``no residue'' was not satisfied. 
Accordingly, in 2002, FDA revised the definition of ``no residue'' 
to mean when the concentration of the marker residue is below the 
LOD of the method, meaning nondetectable by the method (67 FR 78174; 
see also DOJ, Mem. Op. for the Assistant Administrator & Gen. 
Counsel EPA & Gen. Counsel DHHS (October 13, 1995), https://www.justice.gov/d9/olc/opinions/1995/10/31/op-olc-v019-p0247_0.pdf).
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B. History of Carbadox Approvals

    Currently, there are three approved NADAs for use of carbadox in 
medicated swine feed, either alone or in combination with other 
approved new animal drugs. Carbadox, a quinoxaline derivative, is a 
synthetic antimicrobial used to manufacture medicated feeds that are 
administered ad libitum (available at all times) to swine. Phibro 
Animal Health Corp. (Phibro), GlenPointe Centre East, 3d Floor, 300 
Frank W. Burr Blvd., Suite 21, Teaneck, NJ 07666, is currently the 
sponsor of all three approved NADAs.
1. NADA 041-061
    NADA 041-061, originally approved in 1972 (37 FR 20683, October 3, 
1972), provides for the use of MECADOX 10 (carbadox) Type A medicated 
article to manufacture single-ingredient Type C medicated swine feeds 
at the rate of 10 to 25 grams per ton (g/ton) of feed for increased 
rate of weight gain and improved feed efficiency; and at 50 g/ton of 
feed for control of swine dysentery (vibrionic dysentery, bloody 
scours, or hemorrhagic dysentery), control of bacterial swine enteritis 
(salmonellosis or necrotic enteritis caused by Salmonella 
choleraesuis), and for increased rate of weight gain and improved feed 
efficiency.
    In January 1998, CVM approved a supplemental application to NADA 
041-061, which included the approved method (Ref. 2). However, this 
method was not published in the Federal Register as required in Sec.  
500.88, and the method that had been published for the 1972 approval 
was removed from the Code of Federal Regulations. Nevertheless, since 
the January 1998 approval of the supplemental NADA, CVM and the sponsor 
have treated the method approved as part of the 1998 supplemental 
application as the method of examination prescribed or approved by the 
Secretary of HHS by regulations for purposes of applying section 
512(d)(1)(I) of the FD&C Act, the Delaney Clause, to carbadox.
    In October 1998, CVM approved an additional supplemental NADA for 
NADA 041-061, changing the withdrawal period for carbadox medicated 
feeds from 70 days to 42 days. This supplemental NADA was approved 
based on the previous approval of a tolerance of 30 parts per billion 
(ppb) for QCA as the marker residue and a residue depletion study using 
the approved method that showed residues of QCA in liver depleted below 
30 ppb by 42 days (Ref. 3).
2. NADA 092-955
    NADA 092-955, originally approved in 1975 (40 FR 45164, October 1, 
1975), provides for the use of MECADOX 10 (carbadox) Type A medicated 
article with BANMINTH (pyrantel tartrate) Type A medicated article to 
manufacture two-way, combination drug Type C medicated swine feeds at 
50 g/ton of feed plus pyrantel tartrate at 96 g/ton of feed for control 
of swine dysentery (vibrionic dysentery, bloody scours, or hemorrhagic 
dysentery), control of bacterial swine enteritis (salmonellosis or 
necrotic enteritis caused by S. choleraesuis), as an aid in the 
prevention of migration and establishment of large roundworm (Ascaris 
suum) infections, and as an aid in the prevention of establishment of 
nodular worm (Oesophagostomum) infections. The withdrawal period for 
the use of this drug combination is 70 days (Sec.  558.115(d)(3)(ii) 
(21 CFR 558.115(d)(3)(ii))).
3. NADA 141-211
    NADA 141-211, originally approved in 2004 (69 FR 51173, August 18, 
2004), provides for the use of MECADOX 10 (carbadox) Type A medicated 
article with TERRAMYCIN 50, TERRAMYCIN 100, or TERRAMYCIN 200 
(oxytetracycline) Type A medicated articles to manufacture two-way, 
combination drug Type C medicated swine feeds at 10 to 25 g/ton of feed 
plus oxytetracycline at levels in feed to deliver 10 mg carbadox per 
pound of body weight for treatment of bacterial enteritis caused by 
Escherichia coli and S. choleraesuis susceptible to oxytetracycline, 
for treatment of bacterial pneumonia caused by Pasteurella multocida 
susceptible to oxytetracycline, and for increased rate of weight gain 
and improved feed efficiency. The withdrawal period for the use of this 
animal drug combination is 42 days (Sec.  558.115(d)(4); Sec.  
558.450(e)(3)(iii) (21 CFR 558.450(e)(3)(iii)).

C. Statutory Authority To Issue Order

    Under 5 U.S.C. 554(e) (section 5(d) of the Administrative Procedure 
Act (APA)), an agency, in its sound discretion, may issue a declaratory 
order to terminate a controversy or remove uncertainty. The APA defines 
``order'' as the whole or a part of a final disposition, whether 
affirmative, negative, injunctive, or declaratory in form, of an agency 
in a matter other than rulemaking but including licensing (5 U.S.C. 
551(6)). The APA defines ``adjudication'' as agency process for the 
formulation of an order (5 U.S.C. 551(7)). FDA's regulations, 
consistent with the APA, define ``order'' to mean the final Agency 
disposition, other than the issuance of a regulation, in a proceeding 
concerning any matter (Sec.  10.3(a) (21 CFR 10.3(a)). Our regulations 
also define ``proceeding and administrative proceeding'' to mean any 
undertaking to issue, amend, or revoke a regulation or order, or to 
take or not to take any other form of administrative action, under the 
laws administered by FDA (Sec.  10.3(a)). Moreover, our regulations 
establish that the Commissioner of Food and Drugs may initiate an 
administrative proceeding to issue, amend, or revoke an order (Sec.  
10.25(b) (21 CFR 10.25(b)).
    On our own initiative, FDA is issuing a 5 U.S.C. 554(e) declaratory 
order to remove uncertainty regarding the approved method for carbadox 
that measures QCA as a marker residue. An order is the most appropriate 
procedure to revoke the approved method because there is no rule to 
amend. The approved method is not currently published in the Federal 
Register, contrary to Sec.  500.88, and the method that had been 
published for the 1972 approval was removed from the Code of Federal 
Regulations in 1998 and is no longer the approved method. The FD&C Act 
does not provide the procedure we must use to determine whether an 
approved method of examination that was never published in the Code of 
Federal Regulations satisfies the regulatory requirements of part 500, 
subpart E. Thus, we are choosing to issue a declaratory order to remove 
uncertainty.

III. Discussion

    When CVM approved the supplemental NADA for carbadox in January 
1998, it did not require the sponsor to provide data establishing a 
known relationship between the concentration of the marker residue 
(QCA) and the concentration of the

[[Page 76763]]

residue of carcinogenic concern (Sec.  500.86). At that time, CVM did 
not believe that such information was necessary because of previous 
conclusions that it had made about the persistence of carcinogenic 
residue in the edible tissues of animals administered carbadox. CVM's 
understanding, at that time, was that carcinogenic residues, including 
desoxycarbadox (DCBX), a known carcinogenic metabolite of carbadox, 
depleted quickly (within 72 hours) while QCA residues depleted more 
slowly. However, results from subsequent studies led CVM to reexamine 
the conclusions it made in 1998 and conclude, based on data from these 
studies, that it is necessary to establish a known relationship between 
the marker residue and the residue of carcinogenic concern, as required 
by regulation.
    FDA is revoking the approved method for carbadox that measures QCA 
as the marker residue because it is inadequate to monitor the residue 
of carcinogenic concern. The approved method cannot adequately monitor 
residue of carcinogenic concern because CVM is not aware of any data to 
establish a relationship between QCA and the residue of carcinogenic 
concern. That means that determining the concentration of QCA in animal 
tissue does not allow CVM to determine whether the residue of 
carcinogenic concern remains in the edible tissue. Thus, the approved 
method does not comply with part 500, subpart E, and therefore does not 
satisfy the statutory requirement of section 512(d)(1)(I) of the FD&C 
Act.

A. CVM's Conclusions in the January 1998 Approval

    In reviewing information for the supplemental NADA for carbadox in 
January 1998, CVM relied on studies conducted by the sponsor \6\ and 
academic researchers (Ref. 2) to establish an So and an 
Sm for the most potent of the carcinogenic compounds. As 
part of the supplemental NADA, the sponsor submitted toxicology 
studies, including carcinogenicity bioassays with carbadox, DCBX, and 
hydrazine (another carcinogenic metabolite of carbadox). These studies 
indicated that DCBX was the most potent of the three identified 
carcinogenic residues of carbadox. Based on the carcinogenicity of 
DCBX, CVM calculated an So of 0.061 ppb for residue of 
carcinogenic concern for carbadox in the total diet. CVM calculated an 
Sm value for the residue of carcinogenic concern in muscle 
at 0.305 ppb, in liver at 0.915 ppb, and in kidney and fat at 1.830 
ppb. Because liver residues persist the longest, CVM assigned it as the 
target tissue. Therefore, 0.915 ppb is the Sm value for the 
residue of carcinogenic concern for carbadox and liver is the target 
tissue (Ref. 2).
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    \6\ Pfizer, Inc. was the sponsor for carbadox until 2001. The 
current sponsor is Phibro.
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    Based on information submitted as part of the supplemental NADA 
approved in January 1998, CVM made conclusions about how long 
carcinogenic residues persist in the edible tissues of swine after 
treatment with carbadox and about the appropriate marker residue to 
select to monitor carbadox use. As stated in the FOI Summary for the 
January 1998 approval of the supplemental NADA, CVM concluded that the 
data:

    [S]how that carbadox, desoxycarbadox and hydrazine do not 
persist in edible tissue as detectable residues beyond 72 hours. The 
agency's evaluation of these data, and the new information provided 
by the sponsor, demonstrate that following administration, parent 
carbadox is rapidly metabolized; that the metabolism of carbadox is 
similar among species; that the in vivo metabolism of the compounds 
of carcinogenic concern is also rapid and irreversible such that the 
resulting metabolic products cannot regenerate compounds of 
carcinogenic concern; that the unextractable residues are related to 
noncarcinogenic compounds, quinoxaline-2-carboxylic acid (QCA) and 
quinoxaline-2-carboxaldehyde; and that QCA is the only residue 
detectable in the edible tissues beyond 72 hours post dosing. Thus, 
the agency concludes that the unextractable bound residue is not of 
carcinogenic concern and that QCA is a reliable marker residue for 
carbadox.

    CVM made the following conclusions during the review of the 
supplemental NADA for carbadox approved in January 1998:
    1. Carcinogenic residues do not persist in animal tissue beyond 72 
hours postdosing.
    2. Extractable QCA is the only residue detectable in edible tissues 
72 hours postdosing.
    3. Unextractable residues are noncarcinogenic residues related to 
QCA.
    4. QCA is a reliable marker residue for carbadox and its 
metabolites.
    5. No residue of carcinogenic concern, even below the 
So, is detectable by any method after 72-hours postdosing.
    Because of the conclusions made at that time, CVM did not require 
the sponsor to submit data to meet the requirements of the part 500, 
subpart E, regulations \7\ despite the fact that carbadox is a 
carcinogen. CVM instead established a tolerance of 30 ppb for QCA and 
granted the supplemental approval for carbadox.
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    \7\ These regulations require the sponsor to submit data that 
allows FDA to designate an Rm (the concentration of the 
marker residue in the target tissue at which the residue of 
carcinogenic concern in the diet of people represents no significant 
increase in the risk of cancer to people) based on a known 
relationship between the marker residue and the residue of 
carcinogenic concern. In addition, the sponsor must provide a method 
that can detect the marker residue at or below the Rm. 
Under Sec.  500.86, the necessary steps to meet the operational 
definition of ``no residue'' for carbadox are: (1) measure the 
depletion of the residue of carcinogenic concern until its 
concentration is at or below the Sm (0.915 ppb) in liver; 
(2) measure the depletion of the marker residue until the 
concentration of the residue of carcinogenic concern is at or below 
the Sm; (3) use the information in (1) and (2) to 
establish an Rm; and, (4) according to the regulations as 
they existed in 1998, develop a method that could detect the marker 
residue of the drug, as long as the marker residue would only be 
detected at or below the Rm under the proposed conditions 
of use. According to the current regulations, step (4) requires the 
development of a method that complies with the operational 
definition of no residue (the method's LOD is less than or equal to 
the Rm and the marker residue depletes to a concentration 
that cannot be detected by the method).
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B. The Approved Method That Measures QCA as the Marker Residue for 
Carbadox Is Inadequate

    Under section 512(d)(1)(I) of the FD&C Act, carcinogenic new animal 
drugs, such as carbadox, must have a method of detection, prescribed or 
approved by regulation, to ensure that no residue of carcinogenic 
concern persists in any edible portion of the treated animals after 
slaughter or in any food derived from treated animals. FDA has 
implemented this statutory requirement through its SOM regulations in 
part 500, subpart E, which require that each carcinogenic new animal 
drug have a marker residue with a known relationship to the residue of 
carcinogenic concern. This relationship is necessary to establish a 
concentration of the marker residue (the Rm) that ensures 
any residue of carcinogenic concern in a specific edible tissue is 
below the level corresponding to maximum lifetime risk of cancer in the 
test animal of 1 in 1 million (the Sm), based on 
calculations that consider the entire human diet (the So). 
The approved method must have a limit of detection less than or equal 
to the Rm.
    Although CVM approved the method for carbadox as part of the 
supplemental NADA in January 1998 and designated the Sm and 
So, it did not require the sponsor to provide data showing 
the relationship between QCA and the residue of carcinogenic concern 
and therefore could not designate an Rm. Nor did CVM require 
the sponsor to identify

[[Page 76764]]

a method with a limit of detection less than or equal to the 
Rm. Without an Rm and an appropriate method for 
detecting the marker residue (i.e., a method sensitive enough to detect 
residues at or below the Rm), it is impossible to determine 
that the residue of carcinogenic concern falls below the Sm. 
Accordingly, based on information currently available to CVM, it is 
impossible to use the approved method or any other method to ensure 
compliance with the operational definition of no residue.
    Furthermore, based on studies conducted since 1998, CVM reevaluated 
the conclusions that originally led it to determine that assignment of 
a tolerance of 30 ppb for QCA in swine liver would ensure that the 
residue of carcinogenic concern would remain at or below its respective 
So in all edible tissues (Refs. 4-6). Based on a review of 
these data, CVM concluded that: (1) carcinogenic residues persist in 
animal tissue more than 72 hours postdosing and (2) QCA is not the only 
residue detectable in animal tissue after 72 hours postdosing.
    For the 2003 Joint Food and Agriculture Organization (FAO)/World 
Health Organization (WHO) Expert Committee on Food Additives (JECFA) 
meeting, the sponsor provided data in which it reported that DCBX is 
measurable quantitatively (specific concentration measured) at 15 days 
postdosing (the last sampling timepoint in the study) (Refs. 4 and 5). 
Based on those studies, which showed the persistence of genotoxic, 
carcinogenic residues, JECFA recommended withdrawal of the previously 
established Codex Alimentarius Commission \8\ (Codex) Maximum Residue 
Limit (MRL). Codex subsequently agreed because the amount of residues 
of carbadox in human food that would have no adverse health effects in 
consumers could not be determined. Following that meeting, the Codex 
Committee on Residues of Veterinary Drugs in Foods withdrew the MRL for 
carbadox (Ref. 7). Carbadox has been removed from the market in many 
foreign jurisdictions, including the European Union (Ref. 8), Canada 
(Ref. 9), and Australia (Ref. 10).
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    \8\ For more information about Codex, see https://www.fao.org/fao-who-codexalimentarius/committees/cac/about/en/.
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    In 2005, the sponsor provided CVM with summary reports for the 
studies evaluated by the 2003 JECFA. CVM responded later that year, 
informing the sponsor that: (1) because the summaries indicated that 
carcinogenic residues persist longer than previously known and there is 
no established relationship between QCA and the residue of carcinogenic 
concern, CVM was concerned that the use of the 30 ppb tolerance for QCA 
and the use of QCA generally as a marker residue may not be appropriate 
and (2) accordingly, the sponsor would need to submit existing or new 
studies to address the relationship of QCA at 30 ppb and the residue of 
carcinogenic concern. CVM also told the sponsor that, if it was 
determined that QCA is not appropriate as the marker residue, the 
sponsor would need to conduct additional metabolism and residue 
depletion studies to identify an appropriate marker residue and 
tolerance in order to maintain the carbadox approvals.
    Between 2005 and 2011, CVM continued to meet with the sponsor and 
to review various submissions from the sponsor, including but not 
limited to a study the sponsor conducted in 2008 to 2009 and submitted 
in 2009 (hereinafter ``the 2008 study''). None of the submissions, 
however, contained reports of studies that were designed to generate 
the needed information. Therefore, in 2011, pursuant to section 
512(l)(1) of the FD&C Act, FDA ordered the sponsor to provide FDA with 
all data, studies, analyses, reviews, reports, or other scientific 
evaluations in its possession related to the persistence of DCBX in 
edible tissues, the appropriateness of QCA as an analyte for residue 
monitoring and for establishing a withdrawal time for the use of 
carbadox in pigs, and whether an analytical method for monitoring 
carbadox-related carcinogenic residues in edible tissues can be 
developed that would comply with part 500, subpart E. The sponsor 
responded with, among other submissions, the complete study reports for 
the studies evaluated by the 2003 JECFA. CVM reviewed the reports and 
determined that the data show qualitatively (specific concentration not 
measured) that carbadox and DCBX are present in liver tissue samples at 
48 hours and at 15 days withdrawal, respectively. For samples exposed 
to enzymes to mimic human digestion, CVM concluded that the mass 
spectrometry chromatograms and the reported DCBX concentration data 
provide qualitative confirmation of the presence of DCBX at 15 days 
withdrawal. These reports show that the known carcinogenic residues 
(DCBX) persist beyond 72 hours and that QCA is not the only residue 
detectable after 72 hours.
    In response to CVM's proposal to withdraw approval of the carbadox 
containing new animal drug applications in 2016,\9\ the sponsor 
submitted reports from six studies (hereinafter ``the 2016 studies''). 
These studies, some of which began in 2012, were initiated without 
agreement from CVM that they would provide the necessary data to 
address CVM's concerns (specifically, data to demonstrate that the 
approved method was adequate to measure the residue of carcinogenic 
concern in compliance with FDA's SOM regulations, or that an 
alternative method to do so was available).
---------------------------------------------------------------------------

    \9\ CVM issued a Notice of Opportunity for Hearing (NOOH) on a 
proposal to withdraw approval of the carbadox containing NADAs on 
April 12, 2016. [81 FR 21559; (Correction published on April 21, 
2016 (81 FR 23499), to correct the telephone number for the 
individual to be contacted for further information. The address for 
Phibro Animal Health Corp. was also corrected.)] Phibro submitted 
data from the 2008 study in its Request for a Hearing in response to 
the NOOH. [https://www.regulations.gov/document/FDA-2016-N-0832-0029] Phibro also submitted to that same docket reports from 
additional studies in July 2016. CVM withdrew the 2016 NOOH on July 
20, 2020 (85 FR 43852).
---------------------------------------------------------------------------

    Finally, the sponsor and others submitted presentations, documents, 
and information in response to the 2020 proposed order, at the March 
10, 2022, public hearing, and/or to the docket for the public hearing. 
CVM reviewed the presentations, documents, and information, and 
determined that they were not sufficient to establish a relationship 
between QCA and the residue of carcinogenic concern, which includes 
carbadox and DCBX. Additionally, there were no data to establish the 
residue level of QCA at which the residue of carcinogenic concern in 
the diet of people represents no significant increase in the risk of 
cancer to people. Without these data, CVM cannot establish the 
Rm and the sponsor cannot demonstrate ``no residue'' of 
carcinogenic concern as required by the SOM regulations in part 500, 
subpart E, which implement the FD&C Act at 21 U.S.C. 360b(d)(1)(I).
    In sum, based on review of data submitted following the 1998 
approval of the method, CVM concludes that: (1) carcinogenic residues 
persist in animal tissue more than 72 hours postdosing and (2) QCA is 
not the only residue detectable in animal tissue after 72 hours 
postdosing. CVM also concludes that data and information submitted 
since 1998, including to this docket and to Docket No. FDA-2021-N-1326 
by the sponsor and others, do not provide information needed to 
establish the relationship between QCA and the residue of carcinogenic 
concern. Without knowing this relationship and without a method for 
measuring a marker residue with a limit of detection

[[Page 76765]]

at or below the Rm, the approved method is inadequate for 
monitoring compliance with FDA's operational definition of no residue 
(see Sec.  500.84(c)(3)). Accordingly, the approved method for carbadox 
does not satisfy the statutory or regulatory requirements and is being 
revoked.

IV. Comments Received on the Proposed Order and Public Hearing

A. Comments Submitted by the Sponsor

    The sponsor of the carbadox NADAs submitted information to the 
docket of the proposed order, presented information at the public 
hearing, and submitted information to the docket for that hearing. 
CVM's scientific review of the sponsor's submitted data, analysis, and 
comments prior to the hearing is discussed below and in ``CVM Response 
to Phibro Animal Health Corporation's September 18, 2020 Comments on 
CVM's July 20, 2020 Proposed Order to Revoke the Regulatory Method for 
Carbadox'' (January 6, 2022), which was posted to the public docket 
before the hearing (Ref. 6). Information submitted during or after the 
hearing is discussed below and in ``CVM's review of documents Phibro 
submitted to Docket No. FDA-2021-N-1326 and presentation at the March 
10, 2022 Part 15 Hearing'' (October 30, 2023) (Ref. 11), and ``CVM 
review of comments on the Zhang Article that Phibro references in the 
document submitted to the Part 15 Hearing docket under cover letter 
dated June 9, 2022, and entitled, `Phibro Animal Health Corporation's 
Reply to the January 6, 2022 ``CVM Response to Phibro Animal Health 
Corporation's September 18, 2020 Comments on CVM's July 20, 2020 
Proposed Order to Revoke the Regulatory Method for Carbadox'' ' '' 
(October 30, 2023) (Ref. 12). CVM's review of the sponsor's procedural 
and policy objections is reflected below and in the denials of the 
sponsor's citizen petition (Docket No. FDA-2020-P-2312) and petition 
for stay of action (Docket No. FDA-2020-P-2313), available at https://www.regulations.gov.
    In the sponsor's comments and oral presentation, it argued that QCA 
is an adequate marker residue and defended the approved method, which 
measures QCA. The comments defended the use of the 30 ppb QCA tolerance 
and 42-day withdrawal period as sufficient to protect human and animal 
safety. The sponsor alternatively suggested use of the U.S. Department 
of Agriculture Food Safety and Inspection Service (FSIS) method to 
measure QCA. The sponsor also proposed that DCBX could be used as a 
marker residue. For measuring DCBX, the sponsor proposed the Canadian 
Food inspection Agency (CFIA) method. The sponsor also suggested that 
other unnamed methods were available. Finally, the sponsor argued that 
a final order was not the appropriate process to revoke an approved 
method and that an NOOH is required instead.
    Comment on use of QCA as a marker residue. The sponsor states that 
an Rm can be calculated for QCA based on the available data 
and submitted an expert opinion about the Rm for QCA. By 
analyzing QCA and DCBX concentrations, the sponsor's expert states that 
the Rm for QCA is either 28.49 ppb (using the 2008 study 
data and the approved method) or 28.61 ppb (using the data submitted 
for the 1998 supplemental approval and the approved method). The 
sponsor also asserted that even if DCBX residues persist longer than 
previously known, no residue of carcinogenic concern persists beyond 
the current 42-day withdrawal period. The sponsor stated that either 
the approved method or FSIS method could be used to measure QCA.
    Response to use of QCA as a marker residue. After reviewing the 
sponsor's studies submitted to the 2003 JECFA, the 2008 study, the 2016 
studies, and other comments and analyses provided by the sponsor, CVM 
concludes that it lacks the data to establish an Rm for QCA 
or any other marker residue. The sponsor's expert opinion estimated the 
concentration of QCA when DCBX is 0.915 ppb (the Sm for the 
residue of carcinogenic concern in liver for carbadox). This analysis 
relied solely on residues of DCBX instead of considering the residue of 
carcinogenic concern. DCBX is only one metabolite of carbadox and 
therefore just one component of the residue of carcinogenic concern, 
which includes all compounds in the total residue of a demonstrated 
carcinogen excluding any compounds judged by FDA not to present a 
carcinogenic risk (Sec.  500.82). Because QCA and another metabolite, 
methyl carbazate, are the only compounds of carbadox that FDA has 
judged to not present a carcinogenic risk, the residue of carcinogenic 
concern for carbadox includes all carbadox residues except for QCA and 
methyl carbazate. The sponsor did not provide an Rm for the 
marker residue QCA that accounted for the residue of carcinogenic 
concern, nor is CVM able to calculate one based on the data available. 
Without an Rm, CVM cannot determine if the approved method, 
FSIS method, or any other method that measures QCA as the marker 
residue is sufficiently sensitive to satisfy the regulatory and 
statutory requirements.
    Contrary to the sponsor's assertion that the residue of 
carcinogenic concern does not persist beyond the 42-day withdrawal 
period, data quantifying the residue of carcinogenic concern for 
carbadox from the 1998 supplemental approval indicates that a marker 
residue would exceed the Rm (the concentration associated 
with no increase in risk to the human consumer) more than 70 days post-
dosing. The data submitted for the 1998 supplemental approval showed 
that the total radiolabeled residues have a concentration of 13.3 ppb 
at 70 days post-dosing, the last timepoint in the study. After removing 
the 9.9 percent QCA residues detected at 70 days,\10\ the remaining 
residue has a concentration of 11.98 ppb. This concentration far 
exceeds the Sm value of 0.915 ppb for carbadox and therefore 
these data cannot be used to calculate an Rm. At most, these 
data indicate that a marker residue would not reach the Rm 
until more than 70 days post-dosing, well past the current 42-day 
withdrawal period.
---------------------------------------------------------------------------

    \10\ According to the 1998 FOI Summary, QCA and methyl carbazate 
are noncarcinogenic metabolites of carbadox (Ref. 2). The sponsor 
provided quantitative measurements for QCA, but not for methyl 
carbazate.
---------------------------------------------------------------------------

    The sponsor's 2008 study and 2016 studies did not provide the 
information to determine the residue of carcinogenic concern. The 
sponsor's 2008 study does not provide information on the residue of 
carcinogenic concern because it measured only QCA and DCBX, not total 
residues of carbadox. In addition, CVM concluded that the data from 
that study cannot be considered quantitative because of poor method 
performance. Likewise, the sponsor's 2016 studies do not provide 
quantitative data on the residue of carcinogenic concern. Additionally, 
although the sponsor attempted to separate the residues and measure the 
presence of each compound individually, it failed to demonstrate that 
the analytical procedures used did not cause carcinogenic compounds to 
degrade to noncarcinogenic compounds. CVM's review of the method 
performance issues and analytical flaws in the sponsor's studies is 
discussed in greater detail in Refs. 6 and 11.
    CVM also reviewed the information provided by the sponsor during 
the public hearing and to the docket following the hearing and 
concluded that such information does not allow CVM to determine an 
Rm for the approved method. The new information concerns the 
procedures, analysis, and documentation for the 2016 studies; however, 
none of the new information

[[Page 76766]]

provides the data necessary to calculate an Rm because the 
studies were not designed to generate the quantitative data necessary 
to make these calculations. CVM's review of the new information is 
discussed in greater detail in Ref. 11.
    Comment on use of DCBX as a marker residue. The sponsor proposed 
the use of DCBX as a marker residue and suggested the CFIA method for 
detecting DCBX. According to an expert opinion submitted by the 
sponsor, DCBX depletes to a concentration of 0.915 ppb at approximately 
23 days post-dosing and depletes to the 0.015 ppb detection limit for 
the CFIA method at 75 days post-dosing.
    Response to use of DCBX as a marker residue. Because DCBX is only 
part of the residue of carcinogenic concern, the sponsor's expert 
opinion and analysis are insufficient to ensure compliance with the SOM 
regulations. The residue of carcinogenic concern for carbadox includes 
all carbadox residues excluding residues judged by FDA not to present a 
carcinogenic risk (Sec.  500.82(b)). For carbadox, only the compounds 
QCA and methyl carbazate have been judged by FDA to be noncarcinogenic. 
All other compounds cannot be excluded from the residue of carcinogenic 
concern. At most, the expert's opinion indicates that the concentration 
of the residue of carcinogenic concern would reach the Sm at 
some point after 23 days (since DCBX is only part of the residue of 
carcinogenic concern) and that detectable residues of a carcinogenic 
new animal drug are present at 75 days post-dosing, which is 33 days 
longer than the current withdrawal period and 72 days longer than was 
known in 1998. This information is insufficient to determine an 
Rm for DCBX as a marker residue. Without an Rm, 
CVM cannot determine if the CFIA method or any other method to measure 
DCBX is sufficiently sensitive to satisfy the regulatory and statutory 
requirements (Sec.  500.88(b)).
    Comment on carbadox metabolism. During the public hearing, the 
sponsor stated that the metabolism for carbadox is well-known and 
asserted that carbadox depletes to DCBX, which in turn depletes to the 
noncarcinogenic QCA. The sponsor addressed an April 2022 study (Ref. 
13) about the metabolism and residue depletion of carbadox and asserted 
that compounds other than DCBX and QCA are intermediates that are 
present ``only fleetingly.'' The sponsor also stated during the public 
hearing that it would be willing to conduct additional studies.
    Response on carbadox metabolism. CVM reviewed the sponsor's 
comments regarding a study published in April 2022 that describes 
metabolism and residue depletion of carbadox (Ref. 12). The study 
identified eight different metabolites of carbadox (DCBX, QCA, and six 
others) and proposed two different metabolic pathways for the 
degradation of carbadox. The study contradicts the sponsor's claim that 
DCBX represents the entirety of the residue of carcinogenic concern. 
Although the sponsor states that the six non-QCA, non-DCBX carbadox 
residues identified in the April 2022 study are present ``only 
fleetingly,'' the method used in that study was not capable of 
detecting carbadox metabolites below 20 ppb, a concentration far 
greater than the Sm. Further, FDA regulations prohibit us 
from excluding compounds from the residue of carcinogenic concern until 
they have been judged to be noncarcinogenic. Only compounds known to be 
noncarcinogenic can be subtracted from the total residues for the 
determination of residue of carcinogenic concern. Although the 2022 
study adds to our knowledge about previously unidentified carbadox 
residues, it does not provide total residue data that could be used to 
calculate the residue of carcinogenic concern or to determine a 
relationship between a marker residue and the residue of carcinogenic 
concern for establishment of an Rm. Finally, although the 
sponsor stated that it would be willing to conduct additional studies, 
it has not submitted additional studies to date.
    Comment on process to revoke the method. The sponsor also argued 
that CVM cannot lawfully revoke an approved method using a final order 
under the FD&C Act and its implementing regulations, agency precedent, 
the Administrative Procedure Act, and the Due Process Clause of the 
U.S. Constitution and must rely instead on an NOOH and an evidentiary 
hearing before an impartial adjudicator to address the adequacy of the 
approved method. Alternatively, the sponsor asserted that revocation of 
the method requires rulemaking under the APA instead of a declaratory 
order. The sponsor also argued that it is arbitrary and capricious to 
revoke an approved method without establishing an alternative method 
and that a public hearing is not a substitute for a formal evidentiary 
hearing.
    Response on process to revoke the method. It is appropriate under 
the FD&C Act and its regulations, agency precedent, the Administrative 
Procedure Act, and the Due Process Clause of the U.S. Constitution to 
address the adequacy of the approved method through a declaratory order 
as a threshold matter before proceeding to an NOOH on withdrawal of the 
drug's approval. Although the FD&C Act requires an opportunity for a 
hearing prior to withdrawing an animal drug approval (which FDA is 
providing by issuing an NOOH and considering any request for hearing it 
receives), the FD&C Act does not require a specific procedure to 
determine whether a particular method of examination satisfies the 
statutory and regulatory requirements, nor does it address the 
situation when an agency did not follow a regulatory requirement to 
publish that method in the Federal Register. A declaratory order is an 
appropriate process under the FD&C Act and APA to determine whether a 
statutory exclusion applies. See Weinberger v. Hynson, Westcott & 
Dunning, Inc., 412 U.S. 609, 626 (1973) (holding that FDA could issue a 
declaratory order to terminate controversy and remove uncertainty 
regarding whether a new drug and ``me-too'' drugs were exempt from 
providing efficacy data).
    In Weinberger, the Supreme Court agreed with FDA's conclusion that 
efficacy data was required for a class of drugs but held that a hearing 
was necessary before withdrawal because the drug sponsor had submitted 
substantial evidence of efficacy in line with FDA's regulatory 
requirements for well-controlled studies. Id. at 622-23. Here, FDA 
concludes that the approved method, which relies on a tolerance of 30 
ppb for QCA, does not comply with the statute and implementing 
regulations because there is no Rm for the marker residue 
QCA and no determination that the approved method is sufficiently 
sensitive to detect the marker residue at or below the Rm. 
Unlike the situation in Weinberger, where the drug sponsor submitted 
efficacy data in line with the regulatory and statutory requirements, 
the drug sponsor does not assert here that the current tolerance of 30 
ppb for QCA has a known relationship with the residue of carcinogenic 
concern and therefore has not submitted evidence that the approved 
method satisfies the statutory and regulatory requirements. Instead, 
the drug sponsor's expert states that the Rm for QCA is 
either 28.49 ppb (using the 2008 data and the approved method) or 28.61 
ppb (using the data submitted for the 1998 supplemental approval and 
the approved method) based on a calculation that estimates 
concentrations of QCA when the estimated concentration of DCBX is 0.915 
ppb. DCBX is not the only

[[Page 76767]]

carcinogenic residue that must be considered when determining an 
Rm, so the sponsor's calculations do not account for the 
entire residue of carcinogenic concern. However, even if we were to 
assume that DCBX is the only carcinogenic residue present, the 
sponsor's assertion essentially admits that its own expert does not 
think the current tolerance satisfies the regulatory requirements 
because the current tolerance of 30 ppb is more than 28.49 ppb or 28.61 
ppb (the Rm identified by the sponsor's expert).
    Currently, edible tissues may enter the food supply if they contain 
a concentration of QCA at or below 30 ppb. According to the expert's 
calculation, when QCA is more than 28.49 ppb or 28.61 ppb, edible 
tissues would still contain carcinogenic DCBX above 0.915 ppb, the 
level that corresponds to no significant increase in the risk of cancer 
to the human consumer. If we accept the expert's calculations as 
true,\11\ edible tissues with a QCA concentration of 29 ppb, for 
example, could contain carcinogenic residues above 0.915 ppb, yet those 
edible tissues could enter the food supply because the QCA tolerance 
would be satisfied. The sponsor argues that the current 42-day 
withdrawal period provides an additional margin of safety sufficient to 
meet the statutory and regulatory requirements because the sponsor's 
expert estimates that DCBX depletes to 0.915 ppb at 23 days, 19 days 
before the end of the withdrawal period. However, edible tissues are 
analyzed for residue concentrations; the length of time since the 
animal was treated is not measurable from tissue analysis. Thus, safety 
is assured by measuring the concentration of a marker residue that 
tracks the residue of carcinogenic concern in edible tissues to 
determine whether the concentration is below or above the 
Rm. Regardless of the length of the withdrawal period, the 
``no residue'' requirement cannot be met if the marker residue is above 
the Rm. Even if we accepted the sponsor's calculations as 
true, a tolerance of 30 ppb for QCA would not be at or below the 
Rm (calculated by the sponsor's expert as 28.49 ppb or 28.61 
ppb) in edible tissues of treated swine. Thus, even the sponsor's own 
expert opinion supports FDA's conclusion that the approved method does 
not satisfy the statutory and regulatory requirements.
---------------------------------------------------------------------------

    \11\ As discussed above, DCBX is not the only residue of 
carcinogenic concern and we have concerns regarding the quality of 
data from the 2008 study.
---------------------------------------------------------------------------

    CVM spent a decade (2005 to 2015) in discussions with the sponsor 
regarding the data necessary to identify an adequate method and did, 
and continues to, invite the sponsor to provide that data. At this 
time, as discussed above, the sponsor has not submitted that data.
    The method revocation and withdrawal of NADA approvals are not so 
intertwined as to require a hearing on revocation under the statute or 
FDA's regulations. While a sponsor may have an opportunity at a hearing 
held on either NADA approvability or NADA withdrawal to show whether 
there is an approvable method to meet the DES Proviso, the FD&C Act 
does not require an opportunity for a hearing on the interlocutory 
revocation of an approved method. 21 U.S.C. 360b(c)(1) and (e)(1)(B). 
Furthermore, CVM's decision to revoke the method separately from (and 
before) taking action on the NADA is consistent with D.C. Circuit 
opinions regarding the DES withdrawal proceedings, which declined to 
apply the Delaney Clause when there were currently approved methods 
that did not result in detectable levels of residue. In Hess & Clark, 
Division of Rhodia, Inc. v. FDA, 495 F.2d 975 (D.C. Cir. 1974), and its 
companion case, Chemetron Corp. v. U.S. Dep't of Health, Educ. & 
Welfare, 495 F.2d 995 (D.C. Cir. 1974), the court overturned FDA's 
withdrawal of approvals of DES because it held that the NOOH preceding 
the withdrawals did not adequately provide notice and a meaningful 
opportunity to respond to test results that FDA claimed supported 
withdrawal. Hess & Clark, 495 F.2d at 983; Chemetron, 495 F.2d at 999. 
Notably, the test results were from a method that the U.S. Department 
of Agriculture (USDA) utilized that was different from the approved 
methods for DES. In discussing the USDA method, the court stated that 
``the Delaney Clause is plainly inapplicable'' where ``the only method 
by which residues have been detected is [an unapproved method].'' Hess 
& Clark, 495 F.2d at 991; see also Chemetron, 495 F.2d at 999 (``The 
`DES' exception to the Delaney Clause . . . continues effective unless 
the agency detects residues in a slaughtered animal while using an 
approved test method. And the residues detected by [USDA] were not 
found by an `approved method.' ''). Under this logic, the Delaney 
Clause will only apply after the approved method has been revoked or 
residue is found by the approved method. Consistent with these cases 
(the only court cases that address the applicability of the Delaney 
Clause when there is still an approved method), CVM is addressing the 
adequacy of the approved method for carbadox before relying on the 
Delaney Clause to take action to withdraw the NADAs.\12\ FDA's decision 
to revoke the approved method relies on the information submitted to 
date by the drug sponsor. This revocation does not prevent the drug 
sponsor from providing new or additional data to establish an 
Rm for a marker residue in accordance with the statute and 
regulations.
---------------------------------------------------------------------------

    \12\ While, subsequent to the 1974 DES decisions, FDA proceeded 
to a hearing on the withdrawal of DES without revoking the method 
first, FDA relied on both the general safety clause and the Delaney 
Clause as the basis for withdrawal and, upon subsequent challenge, 
the D.C. Circuit declined to address FDA's application of, or 
procedure regarding, the Delaney Clause. Rhone-Poulenc, Inc., Hess & 
Clark Division v. FDA, 636 F.2d 750, 751-52 & n.2 (D.C. Cir. 1980).
---------------------------------------------------------------------------

    On the two previous occasions when FDA withdrew approval for 
carcinogenic animal drugs (DES and a class of drugs called 
``nitrofurans''), FDA relied on both the Delaney Clause and the general 
safety clause, so these prior situations differ significantly from a 
withdrawal based solely on the Delaney Clause. Furthermore, both sets 
of withdrawal proceedings began before FDA finalized the SOM 
regulations in 1987 and therefore provide no guidance on the 
appropriate process to determine whether a method complies with the SOM 
regulations. The SOM regulations (which implement the DES Proviso) are 
a rule of general applicability because they set forth the general 
requirements for all regulatory methods for carcinogenic new animal 
drugs; by contrast, this final order revoking the method is appropriate 
as a declaratory order because it determines whether one specific 
method satisfies these general requirements. Notably, FDA does not 
approve regulatory methods through notice-and-comment rulemaking under 
the APA. See 76 FR 72617, November 25, 2011 (publishing regulatory 
method to detect residues of carcinogen without notice-and-comment 
rulemaking). Because notice-and-comment rulemaking is not required to 
publish a regulatory method, it is not required to revoke a regulatory 
method. See Perez v. Mortg. Bankers Ass'n, 575 U.S. 92, 101 (2015).
    CVM provided notice of the proposed order and a meaningful 
opportunity to be heard. The drug sponsor and other interested parties 
had an opportunity to provide comments and other information. The 
public hearing served as an additional opportunity for the sponsor and 
the public to comment on this matter. The sponsor presented orally and 
submitted additional comments to the public hearing docket. In 
addition, the sponsor remains able to market carbadox lawfully, so the

[[Page 76768]]

sponsor has not been deprived of a property right.
    CVM, as the component of FDA charged with applying the Delaney 
Clause and DES Proviso, is appropriately advising on this order and its 
involvement does not infect any subsequent proceedings with any bias. 
Elsewhere in this issue of the Federal Register, FDA is publishing an 
NOOH and, pursuant to FDA regulations, were the sponsor to request a 
hearing, the adjudicator of that request would be affiliated with FDA's 
Office of the Commissioner and would have had no previous role in the 
proceedings to date.
    Comment on policy considerations. The sponsor asserts that revoking 
the approved method for carbadox and the resulting withdrawal of 
carbadox, if it were to occur, would be poor policy because carbadox 
supports animal health and serves the public interest in preventing 
antimicrobial resistance and because the swine industry and U.S. 
economy would face significant costs following revocation of the method 
and/or withdrawal of approval of the NADAs. The sponsor also asserts 
that carbadox is safe in that it has been used for over 50 years and 
has not been linked to a single instance of cancer in pigs or humans.
    Response to comment on policy considerations. These comments are 
not relevant to whether the approved method meets our regulatory 
requirements and is adequate to monitor the residue of carcinogenic 
concern in compliance with FDA's operational definition of no residue 
or provide information needed to establish the relationship of QCA to 
the residue of carcinogenic concern. Without an adequate method, the 
drug cannot meet the DES Proviso in section 512(d)(1)(I) of the FD&C 
Act that permits the approval of carcinogenic animal drugs under 
certain conditions. The carcinogenicity studies of carbadox provided 
clear evidence that carbadox caused cancer in mice and rats under 
laboratory conditions; therefore, the Delaney Clause applies because 
``such drug induces cancer when ingested by man or animal.'' 21 U.S.C. 
360b(d)(1)(I).
    CVM considered the sponsor's other comments and concluded that they 
were not relevant to determining whether the approved method, the CFIA 
method, the FSIS method, or any other method complies with the 
regulatory and statutory requirements. The comments are discussed in 
greater detail in CVM's memoranda regarding carbadox (Refs. 6, 11, and 
12) and denials of the sponsor's citizen petition (Docket No. FDA-2020-
P-2312) and petition for stay of action (Docket No. FDA-2020-P-2313). 
Based on the available evidence, there currently is no analytical 
method for which CVM can conclude that the SOM regulations are met, nor 
has the sponsor provided the data to establish an Rm for any 
marker residue. Without this information, CVM is unable to conclude 
that there is no residue of carcinogenic concern in swine treated with 
carbadox.

B. Comments Submitted by Other Stakeholders

    The non-sponsor comments submitted to this docket and to Docket No. 
FDA-2021-N-1326, and non-sponsor presentations at the part 15 hearing, 
generally concerned the need for carbadox for animal health and 
projected economic losses to the swine industry from a decrease in 
animal health; the increase in the use of medically important 
antimicrobials if carbadox were no longer available; human food safety 
and environmental safety; and requests for FDA to work with the sponsor 
to develop and approve an adequate method. However, none of the non-
sponsor comments contained any data or information demonstrating that 
the approved method meets our regulatory requirements and is adequate 
to monitor the residue of carcinogenic concern in compliance with FDA's 
operational definition of no residue or that a different method meets 
the requirements.
    Comments on animal health and projected economic losses to the 
swine industry. FDA received several comments stating that carbadox is 
the only effective option for stopping swine dysentery and that 
alternatives (including vaccines) either do not exist or do not work as 
well. Several comments indicated that removing carbadox from the market 
would lead to animal suffering and death, and several cited a survey of 
veterinarians conducted in 2016 and again in 2020 that estimates the 
removal of carbadox would result annually in sickness for 53.5 million 
otherwise healthy pigs and cost the nation's hog industry $5.3 billion 
over the next decade. Other comments noted that the approved uses of 
carbadox are limited to growth promotion, the control of swine 
dysentery, and control of salmonellosis caused by Salmonella 
choleraesuis. A comment stated that swine dysentery and S. choleraesuis 
are rare in U.S. swine herds and can be managed without antibiotics, 
pointing to countries that have banned the use of carbadox.
    Comments on antimicrobial resistance. Some comments stated that the 
only alternatives to carbadox that could be used to treat swine 
dysentery are medically important antibiotics for humans, such as 
aminoglycosides, and that removing carbadox is contrary to FDA's 
strategy with respect to antimicrobial resistance. We also received 
comments stating that research has shown that the use of carbadox in 
swine increases gene transfer, creating its own resistance problems.
    Comments on human food safety and environmental safety issues. We 
received several comments defending the human food safety of swine 
administered carbadox. One comment pointed out that Salmonella is 
zoonotic and could result in food safety issues if not controlled and 
that there is an expectation that Salmonella and Brachyspira would make 
their way into slaughterhouses, potentially resulting in lower meat 
quality and increased contamination if carbadox is no longer available. 
We also received comments that asserted that the use of carbadox 
creates dangerous residues in food products and results in residues of 
carbadox and its metabolites in surface waters in states with large 
numbers of pig-producing facilities, and that carbadox poses allergen 
and genotoxicity hazards to the farm and feed mill workers who handle 
products containing the drug.
    Response to comments on animal health, industry economic losses, 
antimicrobial resistance, and human food safety. These comments are not 
relevant to whether the approved method meets our regulatory 
requirements and is adequate to monitor the residue of carcinogenic 
concern in compliance with FDA's operational definition of no residue 
or provide information needed to establish the relationship of QCA or 
any other marker residue to the residue of carcinogenic concern. 
Without an adequate method, the drug cannot meet the provisions of 
section 512(d)(1)(I) of the FD&C Act.
    Comments on process to develop a new method. Several comments 
requested that FDA work with the sponsor to develop and approve a new 
method. Comments also presented the view that FDA did not provide the 
sponsor of carbadox with a clear path forward and that FDA diverged 
from its established process, urging that FDA work with the sponsor or 
publish an NOOH regarding the adequacy of the approved method.
    Response to comments on process to develop a new method. Before 
publishing the proposed order, CVM worked with the sponsor for many 
years (from 2005 to 2015), during which time it described the steps 
needed to be completed to obtain the necessary data

[[Page 76769]]

to establish an Rm. CVM has repeatedly requested data from 
the sponsor to establish the relationship between QCA and the residue 
of carcinogenic concern. During this time, the sponsor chose not to 
submit protocols for our review under CVM's generally available 
protocol review process, except for one study protocol submitted in 
2006. That study would have been conducted under FDA's Good Laboratory 
Practices and would have provided preliminary information about residue 
depletion (although not the data necessary to establish an 
Rm), but the sponsor did not submit a report from this study 
and it does not appear this study was ever conducted.
    These decade-long communications, along with the clear requirements 
of the regulatory text, provided the sponsor with notice of what is 
needed to meet the statutory requirements as well as ample time to 
carry out the necessary studies. To date, CVM has not received data 
demonstrating the approved method is adequate to measure the residue of 
carcinogenic concern in compliance with the requirements of FDA 
regulations or that an alternative analytical method would meet such 
requirements.
    CVM, too, has made the swine industry and general public aware of 
its concerns with the adequacy of the approved method for carbadox. Its 
concern was discussed in the 2016 NOOH, the 2020 Proposed Order, and 
during the subsequent public hearing. Indeed, members of the industry 
and the general public submitted comments to the dockets and made oral 
presentations at the public hearing. While we take seriously the 
concept that the sponsor, veterinarians, swine producers, and consumers 
have relied on the existence of the approved method for carbadox for 
the last 25 years (and the prior approved method for more than two 
decades before that) in the form of monetary and physical resource 
allocation decisions (including inventory decisions on the part of the 
industry), decisions about animal health, and consumer spending and 
costs, they have received notice of, and an opportunity to comment on, 
CVM's concerns and proposed actions. Additionally, were the sponsor to 
request a hearing in response to the NOOH and point to new or 
additional data to support the approved method or another approvable 
method, it may follow that a hearing is granted on that basis and/or 
that the carbadox NADAs are not withdrawn for that or any other 
applicable reason. Those considerations together with the 
considerations discussed throughout this order--including that the 
larger purpose of an approved method is to protect against the presence 
of residue of carcinogenic concern in animal tissues consumed by the 
public--outweigh any such reliance interests.

V. Conclusion and Order

    Although CVM previously determined that carbadox and its 
metabolites, including DCBX, induce cancer in animals, in the January 
1998 approval of the supplemental NADA for carbadox, CVM determined 
that no such residues of the drug would be found in edible tissues 
after the preslaughter withdrawal period by the approved method. The 
failure to establish an Rm (which depends on knowing the 
relationship between a marker residue and the residue of carcinogenic 
concern) during the 1998 process, coupled with analysis of new 
information showing that carcinogenic residues persist longer than 
previously known, means that the approved method does not meet the 
requirements of the FD&C Act and the SOM regulations and is inadequate 
to monitor carbadox residues in compliance with FDA's operational 
definition of no residue. The new information available since the 
approval of the January 1998 supplemental NADA reinforces the 
importance of having an approved method that complies with the SOM 
regulations.
    Nothing submitted to this docket or presented at the public hearing 
or submitted to Docket No. FDA-2021-N-1326 demonstrates that the 
approved method is adequate to monitor the residue of carcinogenic 
concern in compliance with FDA's operational definition of ``no 
residue.'' No new information was submitted or presented that 
establishes the relationship between QCA and the residue of 
carcinogenic concern. Such a relationship must be known in order for 
the method to determine that there is no residue of carcinogenic 
concern. In addition, no information was submitted or presented that 
demonstrates an alternative method is adequate to monitor the residue 
of carcinogenic concern in compliance with FDA's regulations.
    Therefore, FDA is revoking the approved method.

VI. References

    The following references are on display at the Dockets Management 
Staff (see ADDRESSES) and are available for viewing by interested 
persons between 9 a.m. and 4 p.m., Monday through Friday, and are 
available electronically at https://www.regulations.gov. Although FDA 
verified the website addresses in this document, please note that 
websites are subject to change over time.

1. ``Determination of Carbadox as Quinoxaline-2-Carboxylic Residues 
in Swine Liver and Muscle Tissues after Drug Withdrawal.'' Available 
at https://www.fda.gov/media/136267/download.
2. FDA, Freedom of Information (FOI) Summary, NADA 041-061, MECADOX 
10 (carbadox) Type A medicated article, supplemental approval 
January 30, 1998. Available at https://animaldrugsatfda.fda.gov/adafda/app/search/public/document/downloadFoi/308.
3. FDA, FOI Summary, NADA 041-061, MECADOX 10 (carbadox) Type A 
medicated article, supplemental approval October 5, 1998. Available 
at https://animaldrugsatfda.fda.gov/adafda/app/search/public/document/downloadFoi/1673.
4. Su[aacute]rez, A.F. and Arnold, D., Addendum to the carbadox 
monograph prepared by the 36th meeting of the Committee and 
published in the FAO Food and Nutrition Paper 41/3, Rome 1991, 
http://www.fao.org/fileadmin/user_upload/vetdrug/docs/41-15-carbadox.pdf.
5. Evaluations of the Joint FAO/WHO Expert Committee on Food 
Additives (JECFA). Carbadox, https://apps.who.int/food-additives-contaminants-jecfa-database/chemical.aspx?chemID=2176.
6. Memorandum to File entitled, ``CVM Response to Phibro Animal 
Health Corporation's September 18, 2020 Comments on CVM's July 20, 
2020 Proposed Order to Revoke the Regulatory Method for Carbadox'' 
(January 6, 2022).
7. Report from Codex Alimentarius International Food Standards FAO/
WHO ``Maximum Residue Limits (MRLs) and Risk Management 
Recommendations (RMRs) for Residues of Veterinary Drugs in Foods, 
CX/MRL 2-2021,'' Carbadox, page 47, https://www.fao.org/fao-who-codexalimentarius/sh-proxy/en/?lnk=1&url=https://workspace.fao.org/sites/codex/Standards/CXM+2/MRL2e.pdf.
8. WHO FOOD ADDITIVES SERIES: 51 CARBADOX (addendum), 
``Toxicological Evaluation of Certain Veterinary Drug Residues in 
Food,'' prepared by the Sixtieth Meeting of the Joint FAO/WHO Expert 
Committee on Food Additives (JECFA), WHO, Geneva, 2003, https://iris.who.int/bitstream/handle/10665/42800/924166051X.pdf?sequence=1.
9. Health Canada, Drug and health products, Veterinary drugs, ``List 
of Maximum Residue Limits (MRLs) for Veterinary Drugs in Foods,'' 
https://www.canada.ca/en/health-canada/services/drugs-health-products/veterinary-drugs/maximum-residue-limits-mrls/list-maximum-residue-limits-mrls-veterinary-drugs-foods.html (not listing 
carbadox as an approved veterinary drug in food).
10. Australian Pesticides and Veterinary Medicines Authority, 
``Substances Not Permitted for Use on Food-Producing Animals in 
Australia,'' https://apvma.gov.au/node/11626.

[[Page 76770]]

11. Memorandum to File entitled, ``CVM's review of documents Phibro 
submitted to Docket No. FDA-2021-N-1326 and presentation at the 
March 10, 2022 Part 15 Hearing'' (October 30, 2023).
12. Memorandum to File entitled, ``CVM review of comments on the 
Zhang Article that Phibro references in the document submitted to 
the Part 15 Hearing docket under cover letter dated June 9, 2022, 
and entitled, `Phibro Animal Health Corporation's Reply to the 
January 6, 2022 ``CVM Response to Phibro Animal Health Corporation's 
September 18, 2020 Comments on CVM's July 20, 2020 Proposed Order to 
Revoke the Regulatory Method for Carbadox'' ' '' (October 30, 2023).
13. Zhang, J., W. Qu, Z. Wang, and Y. Pan, ``Metabolism and Tissue 
Depletion of Carbadox in Swine, Broilers, and Rats,'' ACS 
Agricultural Science & Technology 2022 2(3), 477-485. Abstract is 
available at https://pubs.acs.org/doi/abs/10.1021/acsagscitech.1c00260.

    Dated: November 1, 2023.
Kimberlee Trzeciak,
Deputy Commissioner for Policy, Legislation, and International Affairs.
[FR Doc. 2023-24548 Filed 11-6-23; 8:45 am]
BILLING CODE 4164-01-P


