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Chapter
III.
Toxicokinetics
Limited
data
on
the
toxicokinetics
of
cyanogen
chloride
are
available,
primarily
from
old
studies
conducted
prior
to
the
development
of
modern
toxicology
methods,
and
from
basic
principles
of
chemistry.

A.
Absorption
No
data
were
located
on
the
rate
or
degree
of
absorption
of
inhaled
cyanogen
chloride.

However,
based
on
the
rapid
onset
of
systemic
symptoms
in
animals
exposed
to
cyanogen
chloride
via
the
inhalation
route
(
Aldridge
and
Evans,
1946),
it
appears
that
absorption
via
this
route
is
rapid.
No
information
on
the
absorption
of
cyanogen
chloride
via
the
oral
or
dermal
routes
was
located.
It
is
likely,
however,
that
oral
absorption
of
cyanogen
chloride
is
rapid,
based
on
analogy
to
cyanide.
Hydrogen
cyanide
is
moderately
lipid
soluble,
and
at
least
50%
of
an
oral
dose
is
absorbed
within
24
hours
of
ingestion
(
ATSDR,
1997).
Cyanogen
chloride
would
be
expected
to
remain
in
the
parent
form
(
rather
than
the
hydrolyzed
form)
in
the
acidic
conditions
of
the
stomach
(
see
Section
III.
C),
and
so
have
absorption
characteristics
similar
to
those
of
hydrogen
cyanide.
Based
on
analogy
to
cyanide,
it
is
also
likely
that
cyanogen
chloride
can
be
absorbed
following
dermal
exposure.

Because
the
generation
of
cyanogen
chloride
metabolites,
such
as
cyanide,
thiocyanate,

cyanate,
cyanamide,
or
chloride,
occurs
from
cyanogen
chloride
that
has
already
been
absorbed,

absorption
of
metabolites
was
not
investigated
in
detail
for
this
project.
However,
absorption
can
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be
relevant
to
the
assessment
for
two
reasons.
First,
the
degree
or
rate
of
absorption
may
be
greater
under
certain
dosing
conditions,
which
may
affect
the
toxicity.
For
example,
food
delays
the
absorption
of
cyanide,
reducing
its
toxicity
(
reviewed
in
NTP,
1993)
compared
to
dosing
in
drinking
water.
Slower
absorption
means
that
the
cyanide
would
be
less
likely
to
overwhelm
the
detoxification
capacity
of
the
enzyme
rhodanese
in
the
liver,
and
thus
less
likely
to
have
toxic
effects.
The
other
way
in
which
absorption
may
be
relevant
to
the
assessment
is
if
cyanogen
chloride
and
a
metabolite
have
markedly
different
degrees
of
absorption.
For
example,
imagine
that
absorption
via
the
oral
route
is
90%
for
cyanogen
chloride,
but
20%
for
the
metabolite
used
as
a
surrogate
to
estimate
cyanogen
chloride
toxicity.
Not
taking
into
account
the
differences
in
absorption
would
result
in
an
underestimate
of
the
toxicity
(
based
on
ingested
amount)
of
cyanogen
chloride.
As
noted
above,
however,
the
close
chemical
similarity
of
hydrogen
cyanide
and
cyanogen
chloride
suggests
that
their
absorption
is
quantitatively
similar.

B.
Distribution
Cyanogen
chloride.
No
information
on
the
distribution
of
cyanogen
chloride
following
oral,
inhalation,
or
dermal
exposure
was
located.
However,
cyanogen
chloride
is
rapidly
metabolized
in
blood,
as
discussed
in
Section
III.
C.
This
indicates
that
cyanogen
chloride
metabolites
may
be
distributed
systemically,
but
it
suggests
the
parent
compound
would
be
unlikely
to
reach
tissues
beyond
the
portal
of
entry
and
immediately­
adjacent
blood
stream.

Cyanide.
The
distribution
of
cyanide
has
been
discussed
in
detail
by
ATSDR
(
1997)
and
is
summarized
here.
Small
levels
of
cyanide
are
normally
present
in
blood
plasma,
and
cyanide
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appears
to
rapidly
distribute
throughout
the
body
following
absorption.
Cyanide
has
been
found
in
the
lung,
heart,
blood,
kidney,
and
brain
of
people
who
died
following
cyanide
inhalation.
In
dogs
exposed
to
cyanide
by
inhalation,
the
highest
concentrations
of
cyanide
were
found
in
the
lungs,
blood,
and
heart.
In
rats
exposed
by
inhalation,
the
highest
tissue
concentrations
were
detected
in
the
lungs,
blood,
liver,
brain,
and
spleen.
In
rabbits
exposed
by
inhalation,
the
highest
tissue
concentrations
were
detected
in
the
heart,
lung,
brain,
spleen,
and
kidney.
Following
oral
exposure
in
humans,
stomach
contents
appear
to
contain
the
highest
concentration
of
cyanide.

Other
tissues
containing
cyanide
include
brain,
blood,
lungs,
kidney,
and
liver.
In
rats
exposed
by
the
oral
route,
the
highest
tissue
concentrations
of
cyanide
were
in
the
liver,
lung,
blood,
spleen,

and
brain.
In
rabbits
exposed
dermally,
the
highest
tissue
concentrations
of
cyanide
were
in
the
lungs,
heart,
brain,
liver,
and
spleen.

Thiocyanate.
Data
on
thiocyanate
distribution
show
that
thiocyanate
distributes
freely
throughout
the
body,
but
does
not
cross
the
blood­
brain
barrier
(
Wood,
1975).
Thiocyanate
occurs
in
the
saliva
of
unexposed
humans,
possibly
from
the
metabolism
of
cyanide
produced
in
the
digestion
of
protein
(
as
reviewed
by
Anderson
and
Chen,
1940).
In
mice,
thiocyanate
was
found
in
greater
amounts
in
the
walls
of
large
blood
vessels,
in
stomach,
thyroid,
and
salivary
glands
(
route
and
compound
of
exposure
not
specified,
Wood,
1975).
Because
thiocyanate
has
an
ionic
size
similar
to
that
of
iodide,
it
competitively
inhibits
iodide
binding
at
the
Na/
I
symporter
(
Wolff
and
Maurey,
1963).
Therefore
it
is
expected
that
thiocyanate
would
be
actively
taken
up
into
tissues
that
take
up
iodide,
including
the
thyroid,
mammary
gland,
skin,
and
gastrointestinal
tract.
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Thiocyanate
can
cross
the
placenta
to
a
limited
degree.
Kreutler
et
al.
(
1978)
administered
sodium
thiocyanate
in
drinking
water
to
pregnant
rats
at
0
­
160
mg
SCN/
L
during
gestation,
and
then
measured
plasma
thiocyanate
levels
in
the
dams
and
pups
at
5
and
10
days
postpartum.

Plasma
thiocyanate
increased
from
0.25
mg/
100
mL
in
the
control
dams
to
1.5
mg/
100
mL
at
the
high
dose
at
postpartum
day
5.
Plasma
thiocyanate
also
increased
in
a
dose­
related
manner
in
the
pup,
but
to
a
much
lower
degree.
On
day
5,
the
control
pups
had
0.15
mg
SCN/
100
mL,
while
the
high­
dose
pups
had
only
0.28
mg
SCN/
100
mL
plasma.
These
general
results
are
supported
by
the
findings
of
Boulos
et
al.
(
1973).
These
authors
administered
sodium
thiocyanate
intravenously
to
goats,
and
found
that
the
plasma
concentration
in
the
fetus
reached
steady
state
approximately
90
minutes
after
maternal
dosing,
and
was
about
33%
of
the
maternal
plasma
concentration.

Cyanate.
Only
one
study
regarding
the
distribution
of
cyanate
was
located.
Following
intraperitoneal
injection
of
160
mg/
kg
[
14C]
potassium
cyanate
in
male
Swiss­
Webster
mice,
peak
blood
levels
were
observed
within
5
minutes
(
Johnson
et
al.,
1985).
At
10
minutes
after
administration,
0.25

mol
[
14C]
cyanate
was
bound
per
mL
of
blood.
This
dropped
to
0.01

mol/
mL
during
the
60­
to
210­
minute
post­
administration
time
period.
The
authors
concluded
that
this
bound
activity
reflected
carbamylated
hemoglobin.

Cyanamide.
Very
limited
information
on
the
distribution
of
cyanamide
is
available.

Following
administration
of
35
mg/
kg
cyanamide
by
oral
gavage,
peak
plasma
concentrations
of
cyanamide
occurred
at
6
minutes
in
fasted
rats
and
at
15
minutes
in
unfasted
rats
(
Obach
et
al.,

1986a).
Following
administration
of
2
mg/
kg
cyanamide
to
rats
or
4
mg/
kg
cyanamide
to
dogs
by
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oral
gavage,
peak
plasma
cyanamide
concentrations
occurred
at
5
minutes
in
rats
and
33
minutes
in
dogs
(
Obach
et
al.,
1989).
In
contrast,
peak
plasma
levels
of
calcium
cyanamide
occurred
1
hour
after
oral
administration
(
Loomis
and
Brien,
1983;
not
available
for
review,
cited
in
Obach
et
al.,
1986a).

HCl.
Under
physiological
conditions,
HCl
rapidly
dissociates
into
hydrogen
and
chloride
ions.
These
physiological
ions
are
freely
soluble
in
water,
and
are
readily
dispersed
throughout
the
body.

C.
Metabolism
Cyanogen
chloride.
Although
the
data
on
cyanogen
chloride
metabolism
are
based
primarily
on
older
studies
using
outdated
methods,
the
key
results
of
these
studies
are
supported
by
a
recent
in
vitro
study
(
Midwest
Research
Institute,
1997).

Aldridge
and
Evans
(
1946)
reported
that
injection
of
1.9
mg
cyanogen
chloride
into
the
portal
vein
of
a
cat
produced
minimal
or
no
effects
(
based
on
measurements
of
respiration
and
arterial
pressure),
perhaps
due
to
the
rapid
detoxification
in
the
liver.
By
contrast,
injection
of
0.95
mg
cyanogen
chloride
into
the
femoral
artery
produced
marked
effects
(
transitory
rise
and
then
a
fall
in
arterial
pressure,
and
acceleration
followed
by
slowing
in
heart
rate).
This
difference
between
injection
sites
was
attributed
to
the
greater
distance
traveled
before
the
compound
reached
the
liver
for
detoxification.
The
authors
remarked
that
direct
evidence
from
perfusion
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studies
showed
detoxification
of
cyanogen
chloride
in
the
liver,
although
it
was
not
rapid.
No
further
quantitation
and
no
supporting
data
were
provided.

Other
experiments
by
these
authors
(
Aldridge
and
Evans,
1946)
provide
information
on
cyanogen
chloride
metabolism
by
comparing
the
relative
potency
of
cyanogen
chloride
and
cyanide
in
affecting
the
arterial
pressure
and
respiration
rate
under
various
exposure
conditions.

The
comparisons
were
semi­
quantitative,
based
on
visual
evaluation
of
data
tracings.
When
equivalent
amounts
of
cyanogen
chloride
and
hydrogen
cyanide
(
HCN)
(
based
on
molar
equivalents
of
cyanide)
were
injected
intravenously,
less
severe
effects
were
seen
with
cyanogen
chloride.
This
result
suggests
that
a
portion
of
the
cyanogen
chloride
was
metabolized
to
a
product(
s)
other
than
cyanide.
Alternatively,
the
difference
in
toxicity
may
have
been
related
to
the
difference
in
injection
volumes
(
4
mL
of
cyanogen
chloride,
compared
to
0.35
mL
of
HCN).

The
study
authors
calculated
that
the
toxic
effect
of
intravenously­
administered
cyanogen
chloride
on
respiration
rate
was
approximately
30%
that
of
HCN.
This
estimate
was
confirmed
by
quantitatively
comparing
the
effect
of
injecting
cats
with
0.95
mg
cyanogen
chloride
or
with
0.122
or
0.144
mg
HCN
(
equivalent
to
30­
35%
of
the
CNCl
dose
on
a
moles
cyanide
basis,
injection
volumes
not
reported);
similar
toxic
effects
were
observed.
In
contrast
with
this
estimate,

Aldridge
and
Evans
(
1946)
used
a
comparison
of
intravenous
LD
50
values
to
estimate
that
approximately
75%
of
high
doses
of
cyanogen
chloride
are
converted
to
cyanide.
The
intravenous
LD
50
for
HCN
in
rabbits
was
0.8
mg/
kg,
while
the
LD
50
for
cyanogen
chloride
was
approximately
2.5
mg/
kg.
The
reason
for
the
apparent
inconsistency
between
the
two
estimates
of
the
degree
of
conversion
of
cyanogen
chloride
to
cyanide
is
unclear.
Although
an
effect
endpoint
was
measured
in
these
studies,
the
quantitative
differences
can
be
attributed
to
the
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degree
of
cyanogen
chloride
conversion
to
cyanide.
Differences
in
the
magnitude
of
the
effect
cannot
be
attributed
to
toxicodynamic
differences,
because
the
measured
endpoint
in
both
cases
is
related
to
cyanide
and
because
exposure
in
these
studies
was
via
injection.

Aldridge
and
Evans
(
1946)
exposed
anesthetized
animals
(
species
not
reported,
but
apparently
cat)
to
3000
mg/
m3
cyanogen
chloride
and
analyzed
the
blood
for
cyanogen
chloride
and
HCN
at
30
seconds
through
6
minutes
(
Table
III­
1).
No
cyanogen
chloride
was
found
at
any
time
point,
but
HCN
was
found,
beginning
at
1
minute
of
exposure.
The
level
of
HCN
in
the
blood
rose
rapidly
and
showed
a
slower
increase
through
the
end
of
sampling
(
and
apparently
the
end
of
exposure)
at
6
minutes.
In
a
further
experiment,
a
cat
inhaled
1000
mg/
m3
hydrogen
cyanide,
a
concentration
that
is
chemically
equivalent
(
on
a
moles
cyanide
basis)
to
75%
of
the
cyanogen
chloride
concentration
in
the
previous
experiment.
Therefore,
if
all
of
the
cyanogen
chloride
had
been
converted
to
HCN,
the
HCN
concentration
in
blood
following
cyanide
exposure
would
have
been
approximately
75%
of
that
following
cyanogen
chloride
exposure.

Instead,
exposure
to
HCN
resulted
in
a
blood
HCN
concentration
(
following
4
minutes
of
exposure)
that
was
approximately
twice
the
blood
HCN
concentration
after
exposure
to
cyanogen
chloride
(
at
the
6
minute
time
point)
(
Table
III­
1).
This
suggests
that
the
concentrations
would
have
been
equivalent
if
the
authors
had
exposed
the
cat
to
half
as
much
HCN
as
was
actually
used.
Based
on
these
considerations,
the
authors
concluded
that
approximately
30%
of
the
cyanogen
chloride
(
0.75
×
0.5)
was
converted
to
HCN.

Follow­
up
in
vitro
experiments
with
defibrinated
rabbit
blood
supported
the
conclusion
that
cyanogen
chloride
is
metabolized
via
routes
other
than
HCN.
The
authors
found
that
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Table
III­
1.
HCN
and
HSCN
Levels
Following
Inhalation
of
CNCl
or
HCN1
Time
Point
(
minutes)
HCN
Content
of
Blood
(

g/
mL)
HSCN
Content
of
Plasma
(

g/
mL)

CNCl
Inhalation
(
at
3000
mg/
m3)

Pre­
exposure
0
NE
0.5
ND
NE
1
3.0
NE
3.5
4.1
NE
6
4.4
NE
HCN
Inhalation
(
at
1000
mg/
m3)

Pre­
exposure
0
3.9
0.58
3.0
4.2
4
9.8
4.4
4.4
­
Exposure
ended,
no
blood
sampling
6.25
6.2
5.0
1Data
from
Aldridge
and
Evans,
1946
ND=
Not
detected
NE
=
Not
evaluated
approximately
30­
39%
of
the
cyanogen
chloride
was
converted
to
HCN
at
cyanogen
chloride
concentrations
of
21­
130

g/
mL.
However,
when
only
9.6

g/
mL
cyanogen
chloride
was
added
to
blood,
59%
was
converted
to
cyanide,
suggesting
that
formation
of
cyanide
is
dose­
related,
and
is
saturated
at
high
doses.
No
HCN
or
thiocyanate
were
produced
when
cyanogen
chloride
was
added
to
serum
alone.
Instead,
the
reaction
of
cyanogen
chloride
with
serum
or
plasma
resulted
in
the
rapid
conversion
of
cyanogen
chloride
into
an
unidentified
metabolite(
s)
that
was
not
cyanide.
The
absence
of
cyanide
conversion
to
thiocyanate
in
the
study
with
serum
is
consistent
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with
the
finding
that
an
enzyme
that
converts
cyanide
to
thiocyanate
is
found
in
red
blood
cells,
as
discussed
below
in
the
context
of
cyanide
metabolism.

The
experiments
of
Aldridge
and
Evans
(
1946)
with
cyanogen
chloride
in
vivo
and
with
rabbit
blood
provided
very
little
information
on
levels
of
thiocyanate.
In
the
study
described
above
in
which
blood
levels
of
cyanogen
chloride
and
HCN
were
monitored
in
the
blood
of
a
cat
exposed
via
inhalation
to
3000
mg/
m3,
the
study
authors
reported
that
blood
thiocyanate
levels
were
approximately
double
the
background
level,
but
they
did
not
provide
additional
quantitative
information
or
any
information
on
the
time
point
evaluated.
The
study
authors
also
noted
that
thiocyanate
could
not
be
accurately
estimated
in
whole
blood.
Thiocyanate
was
also
found
in
the
blood
and
saliva
(
concentrations
not
specified)
of
a
dog
exposed
to
cyanogen
chloride
via
inhalation
(
approximately
50
mg/
m3
for
about
3.75
hours,
followed
by
approximately
100
mg/
m3
for
2.75
hours).

Based
on
the
above
results,
the
authors
concluded
that
conversion
of
cyanogen
chloride
into
cyanide
is
not
quantitative.
They
estimated
the
conversion
at
30%
in
vitro,
but
stated
that
there
may
be
more
conversion
in
isolated
red
blood
cells
or
in
vivo.
However,
interpretation
of
their
data
is
limited
by
the
absence
of
mass
balance
data,
particularly
the
absence
of
information
on
the
amount
of
thiocyanate
formed.
Although
30%
conversion
at
the
doses
tested
is
plausible,

an
alternative
interpretation
of
the
data
is
that,
in
addition
to
the
thiocyanate
formation
from
cyanide,
there
is
a
pathway
by
which
thiocyanate
can
be
formed
directly
from
cyanogen
chloride.

This
hypothesis
is
consistent
with
the
limited
quantitative
data
available
from
the
experiment
described
above
and
presented
in
Table
III­
1.
Thiocyanate
levels
in
blood
doubled
following
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exposure
of
a
cat
to
3000
mg/
m3
cyanogen
chloride.
If
concentrations
in
blood
and
plasma
are
similar,
consideration
of
the
background
level
in
Table
III­
1
suggests
that
thiocyanate
levels
in
blood
were
increased
by
approximately
3.9

g/
mL.
Following
exposure
of
a
cat
to
1000
mg/
m3
HCN
(
equivalent
to
75%
of
the
cyanogen
chloride
exposure,
on
a
moles
cyanide
basis),

thiocyanate
levels
in
plasma
increased
from
a
background
level
of
3.9

g/
mL
to
4.4

g/
mL,
an
increase
of
0.5

g/
mL,
instead
of
the
2.9

g/
mL
(
i.
e.,
0.75
×
3.9)
expected
based
on
comparison
to
cyanogen
chloride.
This
rough
quantitation
is
limited,
however,
by
any
differences
between
thiocyanate
levels
in
blood
and
plasma,
the
inaccuracy
of
thiocyanate
estimates
in
plasma,
and
the
small
sample
size.
In
addition,
although
cyanogen
chloride
can
react
with
nucleophiles
to
form
thiocyanate,
it
is
unclear
whether
this
reaction
would
occur
under
physiological
conditions.
A
third
hypothesis
is
that
the
lower
concentration
of
HCN
following
cyanogen
chloride
exposure
is
due
to
rapid
conversion
of
HCN
to
thiocyanate
(
SCN).
However,
in
light
of
the
nearinstantaneous
disappearance
of
injected
cyanogen
chloride
from
blood
(
as
described
above),
there
is
no
clear
reason
why
this
reaction
would
be
faster
following
inhalation
exposure
to
cyanogen
chloride
than
following
inhalation
exposure
to
HCN.
Regardless
of
the
exact
quantitation,
these
data
indicate
that
the
disposition
of
high
doses
of
cyanogen
chloride
in
vivo
involves
other
reactions
besides
the
reduction
to
cyanide.
As
described
below,
the
unidentified
reaction
products
may
also
result
from
reaction
with
other
cellular
nucleophiles.

In
confirmation
of
the
results
of
Aldridge
and
Evans
(
1946),
Aldridge
(
1951)
found
that
cyanogen
chloride
reacts
rapidly
with
rat
hemoglobin,
with
less
than
1%
of
the
cyanogen
chloride
detectable
within
5
seconds.
This
study
also
found
that:
the
conversion
of
cyanogen
chloride
to
cyanide
was
higher
in
a
red­
blood­
cell
suspension
than
in
whole
blood,
the
percent
conversion
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was
further
increased
by
reduction
of
the
blood
hemoglobin
(
by
treatment
with
nitrogen),
and
the
percent
conversion
was
decreased
by
dialysis
of
the
blood,
which
removed
small
molecules
such
as
glutathione.
(
All
solutions
were
normalized
to
the
same
hemoglobin
content.)
Aldridge
and
Evans
(
1946)
found
that
the
degree
of
conversion
of
cyanogen
chloride
to
cyanide
by
washed
red
cells
decreased
with
increasing
concentration
of
cyanogen
chloride,
and
ranged
from
73%
at
9.7

g
cyanogen
chloride/
mL,
to
25%
at
126

g/
mL,
indicating
saturation
of
the
capacity
for
converting
cyanogen
chloride
to
cyanide.
In
more
detailed
studies
with
high
concentrations
(
50­

100

g/
mL)
of
cyanogen
chloride,
a
rapid
reaction
with
hemoglobin
was
followed
by
a
slower
reaction.
Cyanide
was
not
liberated
during
the
initial
reaction,
but
cyanide
was
released
when
hemolysed
red
cells
were
added
to
the
products
of
the
initial
reaction.
This
release
of
cyanide
was
not
produced
by
the
addition
of
plasma
or
serum.
In
other
words,
cyanide
is
produced
in
a
2­

stage
reaction
in
which
the
first
stage
is
a
rapid
reaction
of
cyanogen
chloride
with
oxy­,

carboxyor
reduced
hemoglobin.
In
the
second
stage,
the
product
produced
in
the
first
stage
reacts
more
slowly
with
a
material
present
in
red
blood
cells,
but
not
in
plasma,
to
produce
cyanide.

Cyanogen
chloride
reacts
with
methemoglobin,
but
produces
cyanomethemoglobin,
rather
than
cyanide.
The
authors
noted
that
cyanide
is
not
produced
if
cyanogen
chloride
is
mixed
first
with
serum,
and
then
hemolysed
red
cells
are
added.
The
chemical
nature
of
the
product
formed
by
reaction
of
cyanogen
chloride
with
hemoglobin
was
not
identified.
However,
based
on
the
observation
that
the
formation
of
cyanomethemoglobin
from
cyanogen
chloride
and
methemoglobin
is
much
slower,
and
occurs
in
much
lower
yields
than
the
reaction
of
methemoglobin
and
cyanide,
the
authors
concluded
that
cyanogen
chloride
does
not
react
with
the
heme
portion
of
hemoglobin.
The
authors
suggested
that
glutathione
was
the
key
component
of
the
second
phase
of
the
reaction.
They
noted,
however,
that
glutathione
also
reacts
directly
with
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cyanogen
chloride,
reducing
the
amount
of
cyanogen
chloride
to
1%
of
the
initial
amount
within
5
seconds.
Cyanide
is
also
slowly
liberated
from
the
reaction
of
cyanogen
chloride
and
glutathione.

The
study
authors
concluded
that
cyanogen
chloride
may
be
converted
to
cyanide
both
by
direct
reaction
with
hemoglobin,
and
by
reaction
with
glutathione.
The
reaction
of
cyanogen
chloride
with
glutathione
was
further
characterized
and
a
reaction
pathway
was
proposed
(
Figure
III­
1).

In
the
initial
fast
reaction,
cyanogen
chloride
reacts
with
one
molecule
of
glutathione.
The
production
of
free
cyanide
requires
the
supply
of
additional
free
SH
groups.
Cysteine
could
also
react
with
cyanogen
chloride,
but
no
cyanide
was
produced,
even
in
the
presence
of
excess
glutathione,
suggesting
that
the
ring
structure
formed
with
cysteine
is
more
stable
than
that
formed
with
glutathione.
Metabolism
of
cyanide
is
discussed
below
and
shown
in
Figure
III­
2.

The
rapid
disappearance
of
cyanogen
chloride
from
blood
was
confirmed
in
a
study
conducted
using
modern
toxicology
methods.
Midwest
Research
Institute
(
1997)
investigated
the
toxicokinetics
of
cyanogen
chloride
in
blood
in
an
unpublished
in
vitro
experiment
conducted
according
to
GLP
guidelines.
In
preliminary
experiments,
cyanogen
chloride
was
added
to
rat
blood
at
10
or
58
µ
g/
mL;
cyanogen
chloride
levels
were
at
the
background
value
by
the
time
of
the
first
measurement
(
1
minute).
This
was
followed
by
time­
course
experiments,
in
which
cyanogen
chloride
was
added
to
rat
blood
at
9.3
or
50
µ
g/
mL,
and
samples
were
removed
at
30,

60,
120,
300,
and
600
seconds.
As
in
the
preliminary
experiment,
cyanogen
chloride
rapidly
reacted
with
blood
components,
and
was
not
detected
at
levels
found
in
blank
samples
of
untreated
blood.
To
analyze
free
cyanide
levels,
the
authors
also
treated
the
timed
samples
with
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CNCl
RSCN
ROCN
R
OH
R
SH
SH
CH
NH
CH
2
NH
CH
NH
S
CH
2
C
SH
CH
NH
CH
2
HCN
RNHCN
RNH
2
HCl
HCl
HCl
R
1
R
2
R1
R
2
HCl
2
GSH
R
1
R
2
See
Figure
2
(
8)
(
7)
(
1)

(
10)
(
9)
(
11)

(
12)
(
2)
(
3)
(
4)
(
5)
(
2)
(
6)

Figure
III­
1.
Reactions
of
cyanogen
chloride
(
1)
with
various
nucleophiles.
Cyanogen
chloride
can
react
with
the
­
SH
group
of
the
cysteine
moiety
in
hemoglobin
or
glutathione
(
2),
producing
HCl
(
3)
and
an
intermediate
(
4).
This
intermediate
then
reacts
with
glutathione
(
5),
regenerating
the
original
­
SH
group
donor
and
producing
hydrogen
cyanide
(
6).
In
other
reactions,
cyanogen
chloride
can
react
with
sulfhydryls
(
7)
to
produce
thiocyanates
(
8)
and
HCl.
Cyanogen
chloride
can
react
with
alcohols
or
phenoxy
groups
(
9)

to
produce
cyanates
(
10)
and
HCl.
(
In
the
simplest
form,
this
reaction
is
hydrolysis
under
basic
conditions,
forming
cyanate.)

Cyanogen
chloride
can
also
react
with
ammonia
and
amines
(
11)
to
produce
cyanamide
and
substituted
cyanamides
(
12).
All
of
these
latter
reactions
also
produce
HCl
as
a
product.
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chloramine­
T,
which
converts
free
cyanide
to
cyanogen
chloride,
followed
by
barbituric
acid,

which
forms
a
colored
product
that
can
be
measured
spectraphotometrically.
In
the
samples
treated
with
10
µ
g/
mL
cyanogen
chloride,
cyanide
was
present
at
all
time
points,
with
the
peak
cyanide
concentration
at
300
seconds
after
dosing,
and
blood­
cyanide
concentrations
decreasing
thereafter.
The
peak
blood
concentration
of
cyanide
was
80%
of
the
theoretical
maximum.
When
blood
was
spiked
with
50
µ
g/
mL,
however,
the
free
cyanide
accounted
for
only
about
25%

percent
of
expected
peak
concentration,
suggesting
that
conversion
of
cyanogen
chloride
to
cyanide
is
dose
dependent.
Free
cyanide
levels
were
generally
constant
at
all
sampling
times
after
mixing
the
blood
with
50
µ
g/
mL
cyanogen
chloride,
although
the
highest
levels
were
measured
at
the
300
second
and
600
second
time
points.
The
study
authors
concluded
that
cyanogen
chloride
rapidly
reacts
in
blood,
and
that
some
of
the
cyanogen
chloride
is
converted
to
free
cyanide.
Both
cyanogen
chloride
and
cyanide
were
presumed
to
react
with
protein,
primarily
hemoglobin.

However,
no
other
cyanide
metabolites
or
reaction
products
were
evaluated,
and
no
mass­
balance
analysis
was
conducted.
Although
an
accompanying
memorandum
(
Boorman,
1998)
indicates
that
the
results
indicate
that
an
evaluation
of
cyanogen
chloride
would
be
"
essentially
an
evaluation
of
cyanide,"
the
data
do
not
address
that
conclusion,
since
no
other
metabolites
or
reaction
products
were
evaluated.

Two
studies,
Aldridge
and
Evans
(
1946)
and
Midwest
Research
Institute
(
1997),
both
examined
the
quantitative
relationship
between
cyanogen
chloride
concentration
and
the
formation
of
cyanide,
and
found
that
the
amount
of
cyanogen
chloride
found
as
cyanide
ranged
from
about
25%
to
80%.
It
is
reasonable
to
question
the
reasons
for
the
wide
variation
in
cyanogen
chloride
metabolism.
Aldridge
and
Evans
(
1946)
added
cyanogen
chloride
to
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defibrinated
rabbit
blood
and
observed
about
60%
was
converted
to
cyanide
at
cyanogen
chloride
concentrations
of
about
10
µ
g/
mL,
compared
to
30­
40%
converted
at
cyanogen
chloride
concentrations
of
21­
130
µ
g/
mL.
When
washed
red
blood
cells
from
rabbits
were
used,
the
degree
of
conversion
was
slightly
increased,
with
73%
of
the
cyanogen
chloride
converted
to
cyanide
at
a
cyanogen
chloride
concentration
of
about
10
µ
g/
mL,
decreasing
to
25%
of
cyanogen
chloride
converted
to
cyanide
at
a
cyanogen
chloride
concentration
of
126
µ
g/
mL.
Midwest
Research
Institute
(
1997)
also
observed
an
inverse
relationship
between
cyanogen
chloride
concentration
added
to
blood
and
the
degree
of
conversion
of
cyanogen
chloride
to
cyanide.

When
cyanogen
chloride
was
added
to
rat
whole
blood
at
concentrations
of
about
10
µ
g/
mL,

approximately
80%
was
converted
to
cyanide.
However,
when
cyanogen
chloride
was
added
at
concentrations
of
about
60
µ
g/
mL,
only
25%
of
cyanogen
chloride
was
converted
to
cyanide.

These
data
suggest
that
three
variables
could
be
contributing
to
the
wide
variation
in
cyanogen
chloride:
species
differences,
the
type
of
blood
components
used,
or
dose.
Although
the
Aldridge
and
Evans
(
1946)
studies
were
in
rabbit
blood
and
the
Midwest
Research
Institute
(
1997)
studies
were
in
rat
blood,
there
is
a
reasonable
agreement
in
the
results,
with
the
suggestion
that
rat
blood
may
convert
cyanogen
chloride
to
a
slightly
higher
degree
at
low
doses.
On
the
other
hand,

Aldridge
and
Evans
clearly
demonstrated
that
the
type
of
blood
component
studied
also
contributes
to
the
amount
of
cyanogen
chloride
converted
to
cyanide.
No
cyanide
was
produced
when
serum
alone
was
used,
cyanide
conversion
increased
when
whole
blood
was
used,
and
conversion
was
the
highest
when
concentrated
red
blood
cells
were
used.
Thus,
although
it
is
possible
that
species
differences
contribute
to
the
differences
between
the
two
studies,
the
differences
could
also
be
plausibly
explained
by
the
fact
that
in
the
rabbit
defibrinated
blood
or
washed
red
blood
cells
were
used,
while
in
the
rat,
whole
blood
was
used.
Nonetheless,
all
of
the
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experiments,
regardless
of
species
or
type
of
blood
component
used,
clearly
described
a
concentration­
dependent
relationship
for
the
conversion
of
cyanogen
chloride
to
cyanide,
in
which
2­
to
3­
fold
more
cyanide
is
produced
at
cyanogen
chloride
concentrations
of
approximately
10
µ
g/
mL
than
when
cyanogen
chloride
concentrations
were
higher.
Thus,
while
the
maximum
capacity
for
conversion
of
cyanogen
chloride
to
cyanide
may
be
related
to
the
species
tested
or
the
blood
component
studied,
there
is
a
consistently
observed
concentration
dependence.

Although
cyanide
and
thiocyanate
are
the
only
metabolites
of
cyanogen
chloride
that
have
been
detected
to
date
in
biological
systems,
studies
of
the
aqueous
chemistry
of
cyanogen
chloride
suggest
that
several
other
compounds
are
potential
metabolites.
Price
et
al.
(
1947)
investigated
the
reactivity
of
dilute
cyanogen
chloride
with
a
variety
of
compounds.
In
both
alkali
solution
(
pH
10)
and
tap
water,
cyanogen
chloride
was
hydrolyzed
by
hydroxyl
ion
to
form
cyanate
(
HOCN)
and
chloride
ion.
This
reaction
can
be
expressed
by
the
equation:

ClCN
+
OH­

HOCN
+
Cl­

The
rate
appeared
to
be
the
same
for
both
solutions,
6
x
102/
mmol/
min.
Hydrolysis
in
tap
water
appeared
to
be
complete
in
about
48
hours.
At
pH
8,
there
was
no
measurable
reaction
between
cyanogen
chloride
and
up
to
4.1E­
4M
ammonia;
at
8.8E­
4
M
ammonia
the
reaction
was
slightly
accelerated.
The
authors
concluded
that
the
reaction
of
cyanogen
chloride
with
ammonia
to
form
cyanamide
(
NH
2
CN)
is
very
slow
because
free
ammonia
is
only
a
small
percentage
of
the
total
ammonia
present.
In
contrast,
the
reaction
of
cyanogen
chloride
with
sulfide
ion
(
as
sodium
sulfide)
to
form
thiocyanate
was
complete
in
less
than
30
minutes.
Finally,
cyanogen
chloride
reacts
with
hypochlorite
to
form
hydrogen
chloride,
carbon
dioxide,
and
nitrogen
gas;
this
reaction
was
complete
within
5
minutes.
Drinking
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Cyanogen
Chloride
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Potential
Metabolites
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Final
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17
Edwards
et
al.
(
1986)
evaluated
the
rate
of
cyanogen
chloride
hydrolysis
at
a
range
of
pHs
in
order
to
obtain
a
complete
pH­
rate
profile.
The
hydrolysis
rate
was
constant
in
the
pH
range
1­

5,
with
an
average
of
5.7
x
10­
7/
second
at
25

C.
The
hydrolysis
rate
increased
with
increasing
pH
up
to
pH
10.
This
suggests
that
the
rate
of
cyanogen
chloride
hydrolysis
would
be
lower
under
the
acidic
conditions
of
the
stomach
than
in
the
respiratory
tract
or
other
regions
of
the
body.
The
authors
also
reported
that
nucleophilic
compounds,
such
as
amines,
attack
the
carbon
in
cyanogen
chloride,
resulting
in
the
release
of
the
chloride
ion.

Cyanogen
chloride
also
reacts
with
a
variety
of
nucleophiles
(
Migridichian,
1946).
The
general
equation
for
these
reactions
is:

Nu
 
H
+
ClCN

Nu
 
CN
+
HCl
Cyanogen
chloride
may
react
in
this
manner
with
ammonia,
alkyl
amines,
and
aromatic
amines,
to
form
cyanamide
and
substituted
cyanamides.
For
example:

NH
3
+
ClCN

NH
2
CN
+
HCl
RNH
2
+
ClCN

RNHCN
+
HCl
The
rate
of
this
reaction
is
pH­
dependent,
since
the
form
reacting
with
cyanogen
chloride
is
the
free
base.
Therefore,
at
pH
7,
weak
bases
(
pKa
<
7)
are
more
reactive
than
strong
bases.

Cyanogen
chloride
also
reacts
with
sulfhydryls,
alcohols,
and
phenoxy
compounds
to
form
thiocyanates
and
cyanates:

R
 
SH
+
ClCN

RSCN
+
HCl
R
 
OH
+
ClCN

ROCN
+
HCl
Drinking
Water
Criteria
Document
for
Cyanogen
Chloride
and
Potential
Metabolites
EPA/
OW/
OST/
HECD
Final
Draft
III­
18
In
summary,
no
information
is
available
on
the
metabolism
of
orally­
administered
cyanogen
chloride,
but
information
on
the
metabolic
pathway
can
be
deduced
from
inhalation
and
injection
data,
as
well
as
in
vitro
studies.
Cyanogen
chloride
is
rapidly
metabolized
in
the
blood
to
cyanide.
This
reaction
occurs
by
reduction
of
cyanogen
chloride
via
interaction
with
the
­
SH
group
in
glutathione
or
hemoglobin;
direct
reaction
with
the
­
SH
group
in
proteins
may
also
occur.
It
appears
that
this
reaction
is
so
rapid
that
all
of
the
metabolism
after
injection
of
cyanogen
chloride
takes
place
in
the
blood
itself,
rather
than
in
the
liver
or
other
organs;
it
is
not
known
whether
first­
pass
metabolism
in
the
liver
following
oral
exposure
is
also
important.

Thiocyanate
was
also
observed
in
the
blood
of
animals
exposed
to
cyanogen
chloride,
but
quantitative
measurements
are
not
available.
It
is
not
known
whether
the
thiocyanate
was
produced
directly
from
the
cyanogen
chloride
or
as
a
metabolite
of
cyanide.
As
discussed
below,

cyanide
is
metabolized
to
thiocyanate
in
the
liver,
kidneys,
blood,
and
other
organs.
Thus,

thiocyanate
might
have
been
produced
in
the
in
vitro
studies
in
which
cyanogen
chloride
was
mixed
with
blood,
but
thiocyanate
levels
were
not
evaluated
in
those
studies.
Measurements
of
blood
cyanide
levels
after
mixing
cyanogen
chloride
with
blood
indicate
that
only
approximately
30­
40%
of
the
cyanogen
chloride
is
observed
as
cyanide
at
higher
doses,
while
the
percent
present
as
cyanide
increases
to
approximately
60­
80%
at
lower
doses.
If
a
constant
percentage
of
the
cyanide
is
converted
to
thiocyanate,
these
data
suggest
that
metabolism
of
cyanogen
chloride
to
cyanide
is
capacity­
limited
at
high
doses.
The
degree
of
conversion
of
cyanogen
chloride
to
cyanide
at
environmentally
relevant
doses
is
not
known.
However,
it
would
be
expected
that
conversion
to
cyanide
at
these
lower
doses
would
be
>
80%.
It
is
also
not
known
whether
the
unidentified
material
represents
cyanogen
chloride
that
was
converted
into
other
metabolites,
or
whether
the
rest
of
the
cyanide
was
converted
to
thiocyanate.
The
increasing
percent
cyanide
Drinking
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Final
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19
yield
at
decreasing
doses
suggests,
however,
that
other
metabolites
are
formed.
No
in
vitro
or
in
vivo
studies
have
been
conducted
to
identify
the
other
possible
metabolites,
but
water
chemistry
data
indicate
that
cyanogen
chloride
can
be
converted
(
non­
reductively)
to
cyanate,
cyanamide,

and
chloride.
Production
of
these
compounds
is
higher
in
basic
solution,
and
so
would
not
be
expected
to
occur
in
the
stomach
in
the
absence
of
enzyme
catalysis.
Some
of
this
conversion
may
occur,
however,
in
tap
water.

Based
on
the
in
vivo
data
from
cyanogen
chloride
inhalation
and
injection,
and
the
in
vitro
data,
the
following
pathway
may
be
hypothesized
for
ingested
cyanogen
chloride.
Since
the
rate
of
cyanogen
chloride
hydrolysis
is
slow
under
acidic
and
neutral
conditions,
little
reaction
would
occur
in
the
stomach
or
intestine.
The
degree
and
rate
of
cyanogen
chloride
reaction
with
nucleophiles
(
e.
g.,
thiols
or
amines
in
proteins)
in
vivo
is
not
known,
but
such
reactions
with
nucleophiles
would
be
slower
in
the
stomach
(
where
the
acidic
conditions
would
protonate
the
nucleophile)
than
in
the
neutral
conditions
of
the
intestine.
This
suggests
that
cyanogen
chloride
is
absorbed
from
the
stomach
and/
or
intestine
as
the
parent
chemical.
The
products
of
cyanogen
chloride
reaction
with
macromolecules
in
the
intestinal
contents
may
also
be
absorbed,
but
these
products
would
not
react
further.
Absorbed
cyanogen
chloride
would
enter
the
blood
stream,

where
it
would
be
rapidly
reduced
to
cyanide
by
glutathione.
Other
reactions
with
blood
constituents
may
also
occur.
All
parent
compound
would
be
expected
to
disappear
within
seconds
of
entering
the
blood.
Some
absorption
of
cyanogen
chloride
would
occur
directly
from
the
stomach
to
the
portal
vein,
where
it
would
enter
the
liver.
It
is
likely
that
any
cyanogen
chloride
that
is
still
present
in
the
blood
entering
the
liver
would
rapidly
react
with
the
high
concentrations
of
glutathione
present
in
zone
1
(
the
portion
of
the
liver
adjacent
to
the
portal
Drinking
Water
Criteria
Document
for
Cyanogen
Chloride
and
Potential
Metabolites
EPA/
OW/
OST/
HECD
Final
Draft
III­
20
vein).
Cyanide
entering
the
liver
or
formed
in
the
liver
could
then
be
converted
to
thiocyanate.

Since
high
levels
of
rhodanese
(
the
enzyme
that
metabolizes
cyanide
­
see
the
next
section)
are
found
in
the
liver
and
the
lung,
metabolism
at
the
portal
of
entry
is
important
for
cyanogen
chloride
exposure
via
both
the
oral
and
inhalation
routes.
In
light
of
this
hypothesized
mechanism,
it
is
of
interest
that
injection
of
cyanogen
chloride
into
the
portal
vein
produced
minimal
acute
toxicity
compared
to
intravenous
injection
(
Aldridge
and
Evans,
1946,
see
beginning
of
this
section).

This
hypothesized
metabolic
pathway
for
cyanogen
chloride
suggests
that
much
of
the
ingested
cyanogen
chloride
is
metabolized
in
the
gastrointestinal
tract
or
the
liver,
primarily
in
the
portal
vein
and
liver,
with
some
metabolism
possibly
occurring
in
the
blood
after
absorption
from
the
intestine.
Similarly,
HCl
formation
as
a
byproduct
of
cyanogen
chloride
metabolism
is
likely
to
be
limited
to
these
tissues.
The
HCl
produced
might
result
in
transient
decreases
in
pH
of
the
blood
or
liver,
but
it
appears
that
systemic
acidosis
would
be
unlikely
at
environmentally­
relevant
doses
(
see
Appendix
E,
Section
III).

Cyanide.
The
major
metabolic
pathway
for
cyanide
is
conversion
to
thiocyanate
by
either
rhodanese
or
3­
mercaptopyruvate
sulfur
transferase.
This
pathway
accounts
for
60­
80%
of
a
cyanide
dose.
Minor
pathways
include
incorporation
into
a
1­
carbon
metabolic
pool,
or
conversion
to
2­
aminothiazoline­
4­
carboxylic
acid
(
ATSDR,
1997).
These
pathways
are
shown
in
Figure
III­
2.
Drinking
Water
Criteria
Document
for
Cyanogen
Chloride
and
Potential
Metabolites
EPA/
OW/
OST/
HECD
Final
draft
III­
21
CO
2
­
HCN
Hydrogen
Cyanide
(
in
expired
air)
Carbon
dioxide
H
+
CN
­

CNCyanide
2­
Aminothiazoline­
4­
carboxylic
acid
&

2­
Iminothiazolidine­
4­
carboxylic
acid
Minor
Path
(
Pool)
Thiocyanate
(
SCN­)

Major
Path
(
80%)
Urinary
Excretion
HCNO
Cyanate
HCOOH
Formic
Acid
Metabolism
of
onecarbon
compounds
Some
excreted
in
urine
Formates
+

Figure
III­
2.
Cyanide
Primary
Metabolic
Pathways.
Adapted
from
ATSDR
(
1997).
Drinking
Water
Criteria
Document
for
Cyanogen
Chloride
and
Potential
Metabolites
EPA/
OW/
OST/
HECD
Final
draft
III­
22
Rhodanese,
a
mitochondrial
enzyme
that
converts
cyanide
to
thiocyanate,
facilitates
transfer
of
a
sulfur
atom
to
cyanide
from
a
sulfane­
sulfur
donor
such
as
thiosulfate.
Rhodanese
is
widely
distributed
throughout
the
body.
Through
immunohistochemical
staining
techniques,

rhodanese
in
rabbits
has
been
located
in
the
liver,
where
it
is
most
abundant
in
the
hepatocytes
near
blood
vessels
(
Sylvester
and
Sander,
1990).
It
was
also
found
in
the
lung,
localized
in
epithelial
cells
that
formed
the
barrier
between
inhaled
air
and
blood
vessels.
In
the
kidney,

rhodanese
was
found
in
tubules
closest
to
the
glomeruli.
Therefore,
these
authors
noted
that
the
most
abundant
sites
of
rhodanese
are
located
to
maximize
conversion
of
cyanide
to
thiocyanate
following
both
oral
and
inhalation
exposure
(
Sylvester
and
Sander,
1990).

Devlin
et
al.
(
1989a)
evaluated
rhodanese
activity
in
rat
liver
and
skeletal
muscle.
Using
histochemical­
staining
techniques,
the
authors
noted
that
only
low
levels
of
rhodanese
activity
were
present
in
the
blood
vessels.
In
contrast,
high
levels
of
rhodanese
were
detected
in
the
liver
and
skeletal
muscle.
Although,
the
concentration
of
rhodanese
in
muscle
was
lower
than
in
liver,

the
authors
concluded
that
total
skeletal­
body
mass
of
muscle
makes
a
significant
contribution
to
the
whole­
body
metabolism
of
cyanide.
In
a
follow­
up
study
in
perfused
liver
and
hindlimb
muscle,
Devlin
et
al.
(
1989b)
observed
that
liver
extracted
80%
of
the
available
cyanide
compared
to
18%
for
hindlimbs.
However,
when
the
hindlimb
data
were
extrapolated
to
total
muscle
mass,

muscle
cleared
cyanide
2.6­
fold
faster
than
liver,
in
the
absence
of
exogenous
thiosulfate.

Lewis
et
al.
(
1991)
observed
the
presence
of
rhodanese
in
the
epithelium
of
human
maxilloturbinates.
Compared
to
rhodanese
in
human
liver,
the
rhodanese
in
nasal
tissue
exhibited
a
higher
affinity
(
lower
Km)
for
cyanide
and
a
lower
maximum
velocity
(
lower
Vmax).
The
EPA/
OW/
OST/
HECD
Final
draft
III­
23
human
enzymes
exhibited
a
lower
affinity
(
higher
Km)
and
lower
maximum
velocity
(
lower
Vmax)
than
rhodanese
in
rats.
In
addition,
rhodanese
activity
in
human
nasal
epithelium
was
higher
in
nonsmokers
than
smokers.

The
tissue
distribution
of
rhodanese
is
highly
variable
among
species.
Himwich
and
Saunders
(
1948)
observed
that
in
dogs,
the
highest
activity
of
rhodanese
was
observed
in
the
adrenal
glands,
followed
by
liver.
Brain,
spinal
cord,
kidney,
and
testes
also
had
large
amounts
of
rhodanese.
Monkeys,
rats,
and
rabbits,
in
general,
had
much
higher
concentrations
of
rhodanese
than
dogs,
with
liver
and
kidney
containing
the
highest
activity
of
rhodanese.
Drawbaugh
and
Marrs
(
1987)
also
studied
the
tissue
distribution
of
rhodanese
in
several
species,
including
marmoset,
rats,
hamster,
rabbit,
guinea
pig,
dog,
and
pigeon.
In
general,
rhodanese
activity
was
higher
in
the
liver
than
in
the
kidney
of
various
species,
except
for
rabbits.
The
highest
rhodanese
activities
were
found
in
rats,
hamsters
and
guinea
pigs;
the
lowest
in
pigeon,
marmoset,
and
dogs.

A
second
enzyme
which
converts
cyanide
to
thiocyanate
is
mercaptopyruvate
sulfurtransferase
(
MPST).
This
enzyme
differs
from
rhodanese
in
that
it
catalyzes
the
transfer
of
sulfur
from
an
organic
thiol
to
cyanide
(
Wing
and
Baskin,
1992).
Therefore,
this
enzyme
breaks
a
carbon­
sulfur
bond
to
facilitate
transfer
of
sulfur
to
cyanide,
compared
to
rhodanese,
which
breaks
a
sulfur­
sulfur
bond.
MPST
is
located
in
the
red
blood
cells
and
kidney,
and
thus
appears
to
have
a
different
tissue
distribution
from
that
of
rhodanese.
MPST
is
located
in
both
the
mitochondria
and
the
cytosol,
making
it
more
accessible
than
rhodanese
for
conversion
of
cyanide
(
Wing
and
Baskin,
1992).
Support
for
the
role
of
MPST
in
cyanide
detoxification
was
provided
by
Huang
et
al.
(
1998).
These
authors
demonstrated
that
addition
of
L­
or
D­
cysteine
to
hepatocytes
in
vitro
EPA/
OW/
OST/
HECD
Final
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24
prevented
cyanide
cytotoxicity
and
enhanced
the
formation
of
thiocyanate.
Mercaptopyruvate
and
thiocystine,
metabolites
of
L­
and
D­
cysteine,
are
substrates
of
MPST.
Huang
et
al.
(
1998)

observed
that
when
formation
of
these
metabolites
was
prevented,
the
formation
of
thiocyanate
was
also
inhibited.
However,
it
is
not
clear
whether
MPST
directly
transfers
sulfur
to
cyanide
or
whether
it
acts
indirectly
by
transferring
sulfur
to
albumin
in
the
liver.
The
modified
albumin
could
then
be
excreted
to
form
a
sulfane­
sulfur
pool
that
is
available
to
react
with
cyanide
via
rhodanese
(
Wing
and
Baskin,
1992).

Thiocyanate.
Thiocyanate
and
cyanide
are
in
equilibrium
in
the
body;
thiocyanate
is
converted
back
to
cyanide
and
sulfate
by
the
action
of
thiocyanate
oxidase
located
in
the
red
blood
cells,
lymphocytes,
mammary
gland,
and
thyroid
(
Wood,
1975).
These
enzymes
catalyze
the
reaction
of
hydrogen
peroxide
and
thiocyanate
to
form
cyanide
and
sulfate.
In
addition,
these
enzymes
produce
an
intermediate
oxidation
product
of
thiocyanate
(
OSCN­),
which
reacts
with
cyanide
to
form
cyanate,
which
is
then
hydrolyzed
to
ammonia
and
carbon
dioxide
(
Wood,
1975).

Cyanate.
Holtham
and
Schutz
(
1948,
not
available
for
review,
cited
in
Johnson
et
al.,

1985)
have
shown
that
cyanate
is
hydrolyzed
to
carbon
dioxide
and
ammonia
by
the
enzyme
cyanase,
which
is
present
in
kidney,
liver,
and
red
blood
cells.
When
mice
were
injected
intraperitoneally
with
160
mg/
kg
[
14C]
potassium
cyanate,
70%
of
the
activity
was
recovered
in
expired
air
as
14CO
2
(
Johnson
et
al.,
1985).
The
authors
concluded
that
the
conversion
of
cyanate
to
CO
2
follows
first­
order
kinetics
and
is
dependent
on
the
blood
levels
of
cyanate.
EPA/
OW/
OST/
HECD
Final
draft
III­
25
Cyanamide.
Cyanamide
appears
to
be
metabolized
by
two
pathways.
The
major
metabolite
is
N­
acetylcyanamide.
Shirota
et
al.
(
1984,
cited
in
Obach
et
al.,
1989)
demonstrated
that
after
giving
[
14C]
cyanamide
to
dogs,
87%
of
the
radioactivity
was
recovered
in
the
urine
as
N­
acetylcyanamide
and
11%
was
recovered
in
the
urine
as
cyanamide.
Mertschenk
et
al.
(
1991)

demonstrated
that
in
Wistar
rats
administered
10
mg/
kg
cyanamide
by
oral
gavage,
43%
of
the
administered
dose
was
excreted
in
the
urine
as
N­
acetlycyanamide
within
22
hours.
In
humans
who
ingested
a
single
dose
of
0.25
mg/
kg,
40%
of
the
administered
dose
was
excreted
in
the
urine
as
N­
acetylcyanamide
(
Mertschenk
et
al.,
1991).

A
second
potential
pathway
for
the
metabolism
of
cyanamide
involves
the
interaction
of
hydrogen
peroxide
and
cyanamide
via
the
enzyme
catalase
to
form
the
unstable
intermediate
Nhydroxycyanamide
which
then
spontaneously
decomposes
to
cyanide
and
nitroxyl
(
HNO).
In
support
of
this
theory,
Shirota
et
al.
(
1987)
demonstrated
that
cyanide
is
produced
in
vitro
when
cyanamide
is
incubated
with
bovine­
liver
catalase
and
a
glucose/
glucose
oxidase
system
to
generate
a
steady
supply
of
hydrogen
peroxide.
The
amount
of
cyanide
formed
was
directly
related
to
the
cyanamide
and
catalase
concentrations.
However,
in
an
investigation
of
human
volunteers
who
ingested
0.25
mg/
kg
cyanamide,
Mertschenk
et
al.
(
1991)
did
not
detect
cyanide
in
the
blood,
or
increased
thiocyanate
in
the
urine.

HCl.
The
hydrogen
and
chloride
ions
are
not
subject
to
enzymatic
metabolism.
However,

the
hydrogen
ion
readily
interacts
with,
and
is
neutralized
by,
physiological
bases
such
as
the
bicarbonate
ion
(
HCO
3
­).
The
acid
and
base
levels
in
blood
and
tissues
are
tightly
controlled
(
reviewed
in
Chan,
1983).
The
body
rapidly
responds
to
metabolic
acidosis
by
increasing
the
EPA/
OW/
OST/
HECD
Final
draft
III­
26
respiration
rate
and
increasing
carbon­
dioxide
expiration.
If
the
acidosis
is
not
corrected,
the
kidney
follows
with
stepwise
increments
of
increased
net­
acid
excretion
over
3
to
4
days,
until
maximal
renal
acidification
occurs.

D.
Excretion
Cyanogen
chloride.
No
information
on
the
excretion
of
cyanogen
chloride
following
oral
or
dermal
exposure
was
located.
Thiocyanate
levels
in
saliva
were
elevated
several
days
following
a
6.5
hour
exposure
of
a
single
dog
to
50­
100
mg/
m3
cyanogen
chloride,
suggesting
that
elimination
occurs
over
at
least
several
days
(
Aldridge
and
Evans,
1946).

Cyanide.
Cyanide
is
primarily
excreted
in
the
urine
as
thiocyanate
following
both
inhalation
and
oral
exposure.
Following
occupational
exposure
to
0.19­
0.75
ppm
hydrogen
cyanide,
urinary­
thiocyanate
levels
in
exposed
workers
were
approximately
twice
the
levels
in
controls
(
5.4
vs
2.2

g/
mL;
Chandra
et
al.,
1980,
as
cited
in
ATSDR,
1997).
Following
a
single
subcutaneous
injection
of
rats
with
[
14C]
potassium
cyanide,
89%
of
the
radioactivity
was
detected
as
thiocyanate
in
urine
within
24
hours;
about
4%
was
expired
in
air
as
carbon
dioxide
(
Okoh,
1983,
cited
in
ATSDR).

Leuschner
et
al.
(
1991)
evaluated
the
elimination
of
potassium
cyanide
following
both
acute
and
subchronic
exposure.
For
the
acute
study,
three
male
Sprague­
Dawley
rats
were
treated
by
gavage
with
1
mg/
kg
potassium
cyanide.
Blood
was
collected
at
regular
intervals
for
up
to
1
hour
following
administration.
A
peak
blood
level
of
6.2
nmol
cyanide/
mL
blood
was
observed
2
EPA/
OW/
OST/
HECD
Final
draft
III­
27
minutes
after
treatment;
by
60
minutes
the
blood
levels
had
dropped
to
the
detection
limit.
The
authors
calculated
an
elimination
half­
life
of
14.1
minutes.

For
the
subchronic
study,
male
Sprague­
Dawley
rats
(
26­
40/
group)
received
potassium
cyanide
in
their
drinking
water
at
doses
of
0,
40,
80,
or
140/
160
mg/
kg­
day
for
13
weeks
(
Leuschner
et
al.,
1991).
Blood
was
collected
every
two
weeks
for
analysis
of
cyanide
and
thiocyanate
levels.
Urine
was
collected
over
a
16­
hour
period
during
weeks
6
and
13
of
the
study
for
analysis
of
cyanide
and
thiocyanate
levels.
For
both
cyanide
and
thiocyanate,
blood
levels
were
dose
related;
although
within
each
dose
group,
the
levels
of
both
cyanide
and
thiocyanate
remained
fairly
constant
over
the
13­
week
exposure
period.
Cyanide
levels
in
the
blood
were
16­

25
nmol
CN/
mL
blood;
thiocyanate
levels
were
341­
877
nmoles
thiocyanate/
mL
plasma.
(
The
study
authors
did
not
report
why
cyanide
levels
were
reported
in
blood,
whereas
thiocyanate
levels
were
reported
in
plasma,
although
this
may
relate
to
better
methods
for
measuring
thiocyanate
in
plasma
than
in
blood.)
Small
amounts
of
thiocyanate
were
also
detected
in
the
control
animals
at
concentrations
of
11­
53
nmol
thiocyanate/
mL
plasma.
The
same
patterns
were
observed
for
excretion
of
cyanide
and
thiocyanate
in
urine.
A
dose­
response
relationship
was
observed
for
the
concentration
of
both
cyanide
and
thiocyanate
in
urine,
and
a
small
amount
of
thiocyanate
was
observed
in
the
urine
of
the
controls.
The
levels
of
cyanide
in
the
urine
were
much
lower
than
the
thiocyanate
levels;
the
ratio
of
cyanide
to
thiocyanate
was
about
1
to
1000.

Approximately
11%
of
the
administered
cyanide
was
eliminated
per
day
as
thiocyanate
in
the
urine
during
the
dosing
period,
while
only
about
0.003%
was
excreted
per
day
unchanged.
The
study
authors
did
not
report
how
they
estimated
the
percent
of
total
dose
eliminated;
radiolabeled
material
was
not
used.
The
authors
also
did
not
address
the
disposition
of
the
~
90%
of
the
EPA/
OW/
OST/
HECD
Final
draft
III­
28
administered
cyanide
that
was
not
accounted
for.
Although
no
elimination
half­
life
was
calculated
under
the
subchronic
conditions,
these
results
suggest
that
the
elimination
half­
life
is
longer
under
subchronic
than
acute
exposure
conditions.
Blood
levels
of
cyanide
and
thiocyanate
were
fairly
consistent
with
time.
Some
elimination
may
have
occurred
as
exhaled
HCN,
but
HCN
is
reported
to
account
for
<
10%
of
an
ingested
cyanide
dose
(
ATSDR,
1997).
The
study
authors
noted,

however,
that
the
percent
administered
cyanide
excreted
via
the
urine
was
unchanged
between
weeks
6
and
13,
indicating
that
detoxification
pathways
were
not
saturated
and
the
mode
of
cyanide
excretion
was
not
affected
over
this
duration.

Blood
cyanide
and
plasma
thiocyanate
levels
were
monitored
in
a
patient
during
hospital
treatment
for
a
lethal
case
of
cyanide
poisoning
(
Singh
et
al.,
1989).
Sampling
was
done
at
0.5,
2,

4,
12,
18,
and
24
hours
after
admission.
The
highest
blood­
cyanide
level
of
819

mol/
L
was
measured
at
2
hours,
and
had
decreased
to
23

mol/
L
by
12
hours,
following
cobalt
edetate
therapy.
Plasma
thiocyanate
levels
peaked
at
345

mol/
L,
at
12
hours
post­
admission.

In
Cynomolgus
monkeys
exposed
for
up
to
30
minutes
to
approximately
100­
170
mg
CN/
m3,
blood­
cyanide
levels
remained
nearly
constant
after
approximately
the
first
10­
15
minutes
of
exposure,
and
for
60
minutes
after
the
termination
of
exposure
(
Purser
et
al.,
1984).
These
data
indicate
a
relatively
long
half­
life
in
monkeys,
based
on
the
slow
decrease
in
blood
levels
after
exposure
was
terminated.

Thiocyanate.
Thiocyanate
is
primarily
excreted
unchanged
in
the
urine.
In
humans
administered
1.2
to
1.5
g
of
sodium
thiocyanate,
96­
99%
was
excreted
in
the
urine
within
5­
14
EPA/
OW/
OST/
HECD
Final
draft
III­
29
days
(
Wood,
1975).
In
rats
injected
with
9­
15
mg
of
[
35S]
potassium
thiocyanate,
81%
of
the
radiolabel
was
recovered
unchanged
in
the
urine.
Sulfate
in
the
urine
accounted
for
4.5%
of
the
label
and
less
than
1%
was
found
in
the
feces
over
a
23­
day
period
(
Wood,
1975).

Considerable
variability,
on
the
order
of
4­
5­
fold,
has
been
reported
in
the
daily
excretion
of
thiocyanate
by
human
subjects
(
Gorman
et
al.,
1949).
This
variability
may
be
due
to
both
interindividual
variability
and
changes
in
the
excretion
rate
with
continued
dosing.
In
addition,
renal
disease
can
be
related
to
hypertension,
and
would
have
affected
the
excretion
rate
in
the
subjects
studied.
A
human
clinical
study
found
that
(
thio)
cyanate
clearance
through
the
kidney
increased
as
subjects
were
exposed
repeatedly
to
thiocyanate
(
Barker,
1936).
This
meant
that
higher
administered
doses
were
required
to
maintain
the
target
blood
concentration
of
thiocyanate.

Similar
results
were
found
in
an
animal
study.
Kreutler
et
al.
(
1978)
administered
radiolabeled
SCN
intraperitoneally
to
control
lactating
rats
and
to
lactating
rats
that
had
been
exposed
to
sodium
thiocyanate
in
drinking
water
during
gestation
and
lactation.
In
contrast
to
the
elevated
levels
of
unlabeled
thiocyanate
in
the
plasma
of
the
exposed
rats,
the
radiolabeled
material
appeared
at
decreased
levels
in
the
plasma
and
milk,
compared
to
the
controls.
The
authors
suggested
that
this
reflected
increased
urinary
clearance
of
thiocyanate
after
repeated
dosing.

Limited
data
are
available
on
the
rate
of
thiocyanate
excretion.
Anderson
and
Chen
(
1940)
administered
100
mg/
kg
of
sodium
or
potassium
thiocyanate
to
rabbits,
and
found
that
thiocyanate
levels
in
blood
peaked
within
6
hours
for
sodium
thiocyanate,
and
within
6­
24
hours
for
potassium
thiocyanate.
Thiocyanate
was
still
detectable
in
the
blood
1
week
after
rabbits
were
dosed
with
300
mg/
kg
sodium
thiocyanate.
Boulos
et
al.
(
1973)
found
that
the
half­
life
of
sodium
EPA/
OW/
OST/
HECD
Final
draft
III­
30
thiocyanate
in
non­
pregnant
goats
was
16
hours.
Although
the
species
used
is
not
a
standard
laboratory
species
and
the
dose
was
administered
intravenously,
this
study
still
provides
a
general
estimate
of
the
half­
life
of
absorbed
thiocyanate
in
other
species.

Cyanate.
Johnson
et
al.
(
1985)
determined
that
the
primary
route
of
excretion
for
cyanate
is
through
the
lungs
as
CO
2.
When
mice
were
injected
intraperitoneally
with
160
mg/
kg
[
14C]
potassium
cyanate,
70%
of
the
activity
was
recovered
in
expired
air
as
14CO
2
.
Peak
recovery
of
CO
2
occurred
within
10
minutes,
and
the
elimination
half­
life
was
estimated
to
be
43
minutes.

Cyanamide.
Cyanamide
is
primarily
excreted
in
the
urine
as
the
metabolite
Nacetylcyanamide
Dietrich
et
al.
(
1976,
not
available
for
review,
cited
in
Mertschenk
et
al.,
1991)

determined
that
when
[
14C]
cyanamide
was
administered
intraperitoneally
to
rats,
93.9%
of
the
applied
dose
was
excreted
in
the
urine
within
6
hours
and
1.39%
of
the
dose
was
excreted
in
the
expired
CO
2.
Obach
et
al.
(
1989)
studied
the
excretion
of
cyanamide
in
rats
and
dogs
following
oral
administration.
Male
Sprague­
Dawley
rats
received
2
mg/
kg
cyanamide
by
either
intravenous
injection
or
oral
gavage.
Male­
beagle
dogs
received
either
1,
2,
or
4
mg/
kg
cyanamide
by
intravenous
injection
or
4
mg/
kg
cyanamide
by
oral
gavage.
Blood
samples
were
collected
at
regular
intervals
following
treatment
and
were
analyzed
for
plasma
cyanamide
concentrations.
In
dogs,
the
elimination
half­
life
increased
with
increasing
dose;
the
measured
half­
life
was
39,
47,

and
61
minutes
following
doses
of
1,
2,
and
4
mg/
kg
respectively.
However,
the
elimination
halflife
was
constant
regardless
of
route
of
exposure.
In
dogs
treated
with
4
mg/
kg,
the
half­
life
was
61
minutes
following
intravenous
injection
and
62
minutes
following
oral
gavage.
In
rats
treated
EPA/
OW/
OST/
HECD
Final
draft
III­
31
with
2
mg/
kg,
the
half­
life
was
33
minutes
following
intravenous
injection
and
27
minutes
following
oral
gavage.
In
contrast,
the
elimination
half­
life
for
calcium
cyanamide
following
oral
administration
is
reported
to
be
92
minutes
(
Loomis
and
Brien,
1983,
not
available
for
review,

cited
in
Obach
et
al.,
1989)

E.
Bioaccumulation,
Retention,
and
Body
Burden
Little
information
is
available
on
the
bioaccumulation
of
cyanogen
chloride
or
its
metabolites.

Cyanogen
chloride.
The
limited
available
data
(
Aldridge
and
Evans,
1946)
indicate
that
elimination
may
be
slower
than
intake
at
moderate
doses,
leading
to
some
accumulation.

Elevated
thiocyanate
levels
were
observed
in
the
saliva
of
a
single
dog
for
several
days
after
a
single
6­
hour
inhalation
exposure
to
cyanogen
chloride
(
Aldridge
and
Evans,
1946);
interpretation
of
this
result
is
limited
by
the
small
sample
size.
There
is
no
information
in
the
available
literature
on
the
body
burden
of
cyanogen
chloride.

Hydrogen
cyanide
and
thiocyanate.
These
two
chemicals
are
discussed
together,
since
they
readily
interconvert.
A
study
on
the
toxicokinetics
of
subchronic
oral
exposure
of
rats
to
cyanide
found
that
only
~
10%
of
the
cyanide
dose
was
eliminated
per
day
(
Leuschner
et
al.,

1991).
The
authors
did
not
conduct
a
mass
balance
analysis,
but
this
result
suggests
that
the
detoxified
cyanide
was
retained
to
some
degree,
and
that
the
cyanide
half­
life
following
subchronic
exposure
was
longer
than
the
half­
life
following
acute
exposure.
Data
from
a
study
of
EPA/
OW/
OST/
HECD
Final
draft
III­
32
monkeys
exposed
to
cyanide
via
inhalation
also
indicate
a
relatively
long
half­
life
(
Purser
et
al.,

1984).
The
conclusion
of
slower
excretion
is
supported
by
the
finding
that
thiocyanate
was
still
detectable
in
the
blood
one
week
following
a
single
exposure
(
Anderson
and
Chen,
1940).

Information
on
the
body
burden
of
cyanide
is
available
from
the
ATSDR
(
1997)

Toxicological
Profile
for
Cyanide.

Concentrations
of
HCN
and
its
metabolite
thiocyanate
in
blood
serum
and
plasma,
urine,

and
saliva
have
been
used
as
indicators
of
HCN
exposure,
particularly
with
populations
with
potentially
high
exposures
(
workers
at
risk
of
occupational
exposure,
smokers,
nonsmokers
exposed
to
sidestream
smoke,
and
those
exposed
to
high
dietary
levels
of
HCN).
In
a
study
conducted
by
Chandra
et
al.
(
1988),
workers
exposed
to
approximately
0.2
to
0.8
ppm
(
0.2
­
0.9
mg/
m3)
HCN
in
air
and
controls
exposed
to
0
to
14

g/
100
mL,
had
blood­
HCN
concentrations
ranging
from
0.54
to
28.4

g/
100
mL
for
both
workers
and
for
controls.
Urinary
thiocyanate
levels
ranged
from
0.05
to
2.8
mg/
mL
for
exposed
workers,
and
0.02
to
0.88
mg/
mL
for
the
controls.

A
number
of
investigators
studied
HCN
levels
in
the
blood
serum
and
thiocyanate
levels
in
the
blood
serum
and
plasma,
urine,
and
saliva
of
smokers
and
nonsmokers.
In
general,
serum
HCN
levels
and
plasma,
serum,
and
saliva
thiocyanate
levels
could
distinguish
between
the
two
populations.
The
authors
concluded
that
plasma­
thiocyanate
levels
below
20

mole/
L
(
1200

g/
L)
indicated
that
passive
smoking
was
unlikely,
and
concentrations
above
80
to
85

mole/
L
(
4600
­
4900

g/
L)
were
a
reliable
indication
of
active
smoking.
However,
Yamanaka
et
al.
EPA/
OW/
OST/
HECD
Final
draft
III­
33
(
1991)
found
that
urine­
thiocyanate
concentrations
of
smokers
and
nonsmokers
were
not
significantly
different.
Chen
et
al.
(
1990)
found
that
serum­
thiocyanate
levels
in
18­
month­
old
infants
exposed
to
heavy
environmental
tobacco
smoke
(>
20
cigarettes
a
day
smoked
in
the
home)
were
significantly
higher
than
those
of
unexposed
infants
(
p
<
0.05).
Mean
concentrations
were
36.2

mole/
L
and
27.7

mole/
L
for
the
exposed
and
unexposed
infants,
respectively.

Bottoms
et
al.
(
1982)
and
Hauth
et
al.
(
1984)
found
positive
correlations
between
fetal
umbilical
serum­
thiocyanate
levels
and
serum­
thiocyanate
levels
of
smoking
mothers.
Hauth
et
al.

(
1984)
found
that
the
mean
serum­
thiocyanate
level
of
smoking
mothers
(
95

mole/
L;
5.5

g/
mL)
was
significantly
higher
(
p
<
0.001)
than
that
of
mothers
exposed
to
passive
smoke
(
35.9

mole/
L;
2.1

g/
mL)
or
that
of
nonsmoking
mothers
(
32.3

mole/
L;
1.9

g/
mL).
Similarly,
the
mean
umbilical
serum­
thiocyanate
concentration
(
72

mole/
L;
4.8

g/
mL)
in
newborn
infants
of
smoking
mothers
was
significantly
higher
than
the
concentrations
in
newborn
infants
of
passive
smokers
(
25

mole/
L;
1.5

g/
mL)
and
nonsmokers
(
23

mole/
L;
1.3

g/
mL).

Data
on
elevated
levels
of
thiocyanate
in
body
fluids
resulting
from
consumption
of
cyanide­
containing
foods
come
primarily
for
populations
in
tropical
regions
that
may
consume
large
quantities
of
improperly
processed
food.
Mlingi
et
al.
(
1992,
1993)
and
Tylleskar
et
al.

(
1992)
reported
urinary­
thiocyanate
levels
in
the
normal
population
of
less
than
100

mole/
L
(
5.8

g/
L)
as
compared
to
concentrations
ranging
from
350
to
1,120

mole/
L
(
20
to
265

g/
L)

among
4
populations
exposed
to
high
levels
of
dietary
HCN
caused
by
incomplete
processing
of
cassava
during
drought.
EPA/
OW/
OST/
HECD
Final
draft
III­
34
Cyanate
and
Cyanamide.
Little
information
is
available
on
the
bioaccumulation
of
cyanate
and
cyanamide.
The
half­
life
for
cyanate
and
cyanamide
following
a
single
oral
dose
was
less
than
two
hours
(
Johnson
et
al.,
1985;
Obach
et
al.,
1989).
These
data
suggest
that
these
metabolites
would
be
unlikely
to
be
retained,
but
information
on
their
half­
lives
following
repeated
dosing
would
also
be
useful.
No
information
was
identified
on
the
body
burden
of
cyanate
and
cyanamide.

F.
Summary
of
Toxicokinetics
No
data
were
located
regarding
the
absorption
of
cyanogen
chloride
by
the
oral
or
dermal
routes
of
exposure,
although
cyanogen
chloride
appears
to
be
rapidly
absorbed
following
inhalation
exposure
(
Aldridge
and
Evans,
1946).
The
absorption
of
known
or
potential
metabolites
of
cyanogen
chloride
is
not
of
concern,
since
these
would
be
formed
after
cyanogen
chloride
has
already
been
absorbed.
No
data
were
located
regarding
the
distribution
of
cyanogen
chloride.
Both
cyanide
and
thiocyanate
appear
to
distribute
freely
through
the
body
following
absorption.
Following
absorption
via
all
routes
of
exposure,
cyanide
has
been
found
in
the
lung,

heart,
blood,
liver,
brain,
spleen,
and
kidney
(
ATSDR,
1997).
Thiocyanate
does
not
cross
the
blood­
brain
barrier
(
Wood,
1975),
but
can
cross
the
placenta
(
Kreutler
et
al.,
1978).
Very
little
data
are
available
on
the
distribution
of
cyanate
and
cyanamide.
Both
compounds
were
detected
in
the
blood
within
minutes
of
administration
in
mice,
rats,
or
dogs
(
Johnson
et
al.,
1985;
Obach
et
al.,
1986a).
However,
the
calcium
form
of
cyanamide
appears
to
distribute
more
slowly,

peaking
in
the
blood
by
1
hour
following
oral
administration
(
Loomis
and
Brien,
1983).
EPA/
OW/
OST/
HECD
Final
draft
III­
35
Early
studies
of
cyanogen
chloride
(
Aldridge
and
Evans,
1946;
Aldridge,
1951)
indicate
that,
in
animals
exposed
to
cyanogen
chloride
by
either
intravenous
injection
or
inhalation,

cyanogen
chloride
was
not
detected
in
the
blood,
but
cyanide
(
CN
­)
was
detected
as
soon
as
1
minute
following
the
start
of
exposure.
In
addition,
thiocyanate
(
SCN
­)
was
detected
at
double
the
background
levels
in
blood
and
in
saliva.
Further
studies
characterizing
the
reaction
products
showed
that
cyanogen
chloride
is
rapidly
reduced
to
cyanide
by
glutathione
in
the
blood
(
Aldridge,
1951).
These
early
studies
concluded
that
at
least
30­
40%
of
the
cyanogen
chloride
had
been
converted
to
cyanide
at
higher
doses,
while
at
least
60­
80%
was
converted
to
cyanide
at
lower
doses
(
Aldridge
and
Evans,
1946;
Midwest
Research
Institute,
1997).
It
is
possible
that
the
test
system
(
species
differences
or
the
specific
blood
component
studied)
affected
the
observed
range
of
results.
However,
a
consistent
dose­
dependence
of
the
amount
of
cyanogen
chloride
converted
to
cyanide
has
been
observed
regardless
of
the
species
from
which
blood
was
drawn
or
the
component
of
blood
studied.
The
degree
of
conversion
to
cyanide
can
not
be
determined
more
precisely,
because
none
of
the
authors
determined
how
much
of
the
cyanogen
chloride
had
been
converted
to
thiocyanate
(
either
directly,
or
via
cyanide).
In
addition,
the
authors
did
not
determine
if
the
total
amount
of
cyanide
and
thiocyanate
accounted
for
the
total
cyanogen
chloride
dose
or
if
additional,
unidentified
metabolites
were
present.
The
degree
of
conversion
of
cyanogen
chloride
to
cyanide
at
environmentally­
relevant
doses
is
not
known,
but
it
would
be
expected
that
conversion
to
cyanide
at
these
lower
doses
would
be
>
80%.
Based
on
limited
information
regarding
the
aqueous
chemistry
of
cyanogen
chloride,
other
potential
metabolites
of
cyanogen
chloride
include
cyanate
(
OCN
­),
cyanamide
(
H
2
NCN),
and
chloride
ion
(
Cl­),
as
well
as
the
products
of
reactions
with
a
variety
of
other
cellular
nucleophiles.
Note,
however,
that
no
EPA/
OW/
OST/
HECD
Final
draft
III­
36
studies
have
been
conducted
to
determine
if
these
compounds
are
detected
in
vivo
following
cyanogen
chloride
administration.

The
major
metabolic
pathway
for
cyanide
is
conversion
to
thiocyanate
by
either
rhodanese
or
3­
mercaptopyruvate
sulfur
transferase.
This
pathway
accounts
for
60­
80%
of
a
cyanide
dose.

Minor
pathways
include
incorporation
into
a
1­
carbon
metabolic
pool,
or
conversion
to
2­

aminothiazoline­
4­
carboxylic
acid
(
ATSDR,
1997).
Rhodanese
is
a
mitochondrial
enzyme
that
is
widely
distributed
throughout
the
body.
It
has
been
found
in
liver,
lung,
nasal
passages,
kidney,

and
muscle
(
Sylvester
and
Sander,
1990;
Devlin
et
al.,
1989a,
b;
Lewis
et
al.,
1991);
although
the
distribution
of
rhodanese
among
tissues
varies
widely
in
different
species
(
Himwich
and
Saunders,

1948;
Drawbaugh
and
Marrs,
1987).
Cyanide
appears
to
be
in
equilibrium
with
thiocyanate
in
the
body.
Although
the
action
of
rhodanese
is
not
reversible,
there
is
an
enzyme
system,
thiocyanate
oxidase,
that
catalyzes
the
reaction
of
thiocyanate
and
hydrogen
peroxide
to
form
cyanide
and
sulfate
(
Wood,
1975).
Cyanate
appears
to
be
hydrolyzed
to
carbon
dioxide
and
ammonia
by
the
enzyme
cyanase,
which
is
located
in
the
kidney,
liver,
and
red
blood
cells
(
Johnson
et
al.,
1985).

The
primary
metabolic
pathway
for
cyanamide
is
acetylation
to
form
N­
acetylcyanamide
(
Mertschenk
et
al.,
1991).
A
second
pathway
has
been
demonstrated
in
vivo,
but
not
in
vitro.
In
this
pathway,
the
enzyme
catalase
facilitates
the
interaction
of
cyanamide
and
hydrogen
peroxide
to
form
an
unstable
intermediate,
which
spontaneously
decomposes
to
form
cyanide
and
nitroxyl
(
Shirota
et
al.,
1987).

No
information
is
available
regarding
the
excretion
of
cyanogen
chloride.
Cyanide
is
primarily
excreted
in
the
urine
as
thiocyanate,
although
a
small
amount
appears
to
be
excreted
in
EPA/
OW/
OST/
HECD
Final
draft
III­
37
expired
air
as
carbon
dioxide
(
ATSDR,
1997).
The
elimination
half­
life
in
rats
following
acute
oral
administration
has
been
estimated
to
be
14
minutes,
with
an
undetermined
longer
half­
life
following
subchronic
exposure
(
Leuschner
et
al.,
1991).
Data
in
monkeys
suggest
a
longer
halflife
based
on
the
slow
decrease
in
blood
levels
following
a
30­
minute
inhalation
exposure
(
Purser
et
al.,
1984).
Thiocyanate
appears
to
be
excreted
in
the
urine
unchanged
(
Wood
et
al.,
1975).

Thiocyanate
has
a
long
half­
life;
it
was
still
detectable
in
the
blood
one
week
following
exposure
(
Anderson
and
Chen,
1940).
The
half­
life
in
nonpregnant
goats
was
16
hours
(
Boulos
et
al.,

1973).
Cyanate
is
primarily
excreted
in
expired
air
as
carbon
dioxide,
with
a
half­
life
of
43
minutes
(
Johnson
et
al,
1985).
Cyanamide
is
primarily
excreted
in
the
urine
as
Nacetylcyanamide
although
a
small
amount
is
excreted
in
expired
air
as
carbon
dioxide
(
Dietrich
et
al.,
1976).
The
elimination
half­
life
is
reported
to
be
62
minutes
in
dogs,
and
27
minutes
in
rats
following
oral
exposure
(
Obach
et
al.,
1989).
No
information
was
available
on
the
half­
life
of
cyanate
or
cyanamide
following
repeated
exposures.
Overall,
these
data
suggest
the
potential
for
some
degree
of
thiocyanate
retention
after
repeated
exposure
to
sufficiently
high
levels
of
cyanogen
chloride.
Information
on
body
burden
was
available
only
for
cyanide
and
its
metabolite
thiocyanate.
Concentrations
of
cyanide
and
its
metabolite
thiocyanate
in
blood
serum
and
plasma,

urine,
and
saliva
have
been
used
as
indicators
of
cyanide
exposure,
and
are
elevated
in
cigarette
smokers
and
populations
consuming
large
quantities
of
food
containing
cyanide
(
e.
g.,
improperly
processed
cassava).

Based
on
consideration
of
cyanogen
chloride
biochemistry
and
that
of
its
known
and
putative
metabolites,
it
is
plausible
that
ingested
cyanogen
chloride
is
absorbed
from
the
stomach
and/
or
intestine
as
the
parent
compound,
and
that
most
of
its
metabolism
occurs
in
the
EPA/
OW/
OST/
HECD
Final
draft
III­
38
gastrointestinal
tract
and
liver.
Metabolism
in
the
blood
after
absorption
from
the
intestine
is
also
possible,
as
is
reaction
with
nucleophiles
in
the
intestine,
or
its
contents.
The
high
concentration
of
rhodanese
in
the
liver
would
enhance
the
rate
of
conversion
to
thiocyanate
from
cyanide
that
is
produced
in
the
portal
vein
or
liver.
HCl
production
might
result
in
transient
pH
decreases,
but
it
appears
that
systemic
acidosis
would
be
unlikely
at
environmentally­
relevant
doses.

Significant
uncertainties
exist
in
the
kinetic
data
for
cyanogen
chloride,
particularly
in
the
studies
which
are
relied
upon
to
provide
key
conclusions
regarding
the
metabolism
of
cyanogen
chloride.
These
studies
(
Aldridge
and
Evans,
1946;
Aldridge,
1951)
were
conducted
long
before
Good
Laboratory
Practices
(
GLP)
were
implemented
and
before
modern
technology
afforded
the
opportunity
for
sophisticated
measurements.
As
a
result,
these
authors'
conclusions
regarding
cyanogen
chloride
metabolism
in
vivo
are
based
on
physiological
responses
rather
than
a
more
accurate
measurement
of
tissue
concentrations
or
metabolites.
These
uncertainties
are
compounded
by
the
more
modern
study
(
Midwest
Research
Institute,
1997),
which
was
only
a
screening
level
in
vitro
assay
and
also
did
not
quantify
all
possible
metabolites.
Nonetheless,
the
body
of
data
do
provide
certain
data
that
allow
risk
assessment
conclusions
to
be
drawn
with
a
reasonable
degree
of
confidence.

Once
cyanogen
chloride
reaches
the
blood
stream,
it
is
rapidly
metabolized
(
within
one
minute)
and
the
parent
compound
is
no
longer
detected.
Both
cyanide
and
thiocyanate
have
been
identified
as
metabolites
following
cyanogen
chloride
exposure.
Although
thiocyanate
production
could
not
be
measured,
cyanide
production
has
been
roughly
quantified.
An
in
vivo
assay
that
measured
physiological
response
to
cyanogen
chloride
exposure
compared
to
cyanide
exposure
EPA/
OW/
OST/
HECD
Final
draft
III­
39
suggested
that
about
30%
of
cyanogen
chloride
is
converted
to
cyanide
(
Aldridge
and
Evans,

1946).
In
vitro
studies
that
measured
cyanide
production
when
cyanogen
chloride
was
added
to
either
whole
blood
or
washed
red
blood
cells
confirmed
that
at
the
doses
tested,
at
least
30%
of
cyanogen
chloride
is
converted
to
cyanide
in
the
blood
(
actually,
in
the
red
blood
cells)
(
Aldridge
and
Evans,
1946;
Midwest
Research
Institute,
1997).
The
in
vitro
studies
also
indicate
that
cyanide
production
is
dependent
on
cyanogen
chloride
concentration.
When
10
µ
g/
L
cyanogen
chloride
was
added
to
blood,
60­
80%
was
converted
to
cyanide;
when

50
µ
g/
L
cyanogen
chloride
30­
40%
was
converted
to
cyanide.
Species
differences
(
rabbit
vs.
rat)
or
different
blood
components
(
whole
blood
vs.
washed
red
blood
cells)
could
be
contributing
to
the
wide
range
of
responses.
Metabolites
other
than
cyanide
and
thiocyanate
have
not
been
identified,
but
may
have
been
formed.

Given
the
available
kinetic
data
for
cyanogen
chloride,
the
following
risk
assessment
assumptions
are
reasonable
for
the
development
of
health
values
for
cyanogen
chloride
(
e.
g.,

Health
Advisories
and
reference
dose).
First,
toxicity
of
cyanogen
chloride
is
likely
to
be
due
to
metabolites
rather
than
the
parent
compound.
Therefore,
it
is
reasonable
to
base
a
risk
assessment
on
metabolites
given
the
lack
of
toxicity
data
on
cyanogen
chloride
itself.
Second,
cyanide
and
thiocyanate
are
known,
observed
metabolites
of
cyanogen
chloride.
Therefore,
it
is
reasonable
to
consider
these
chemicals
as
surrogates
for
cyanogen
chloride
in
a
quantitative
risk
assessment,

even
though
the
exact
quantitative
relationship
between
cyanogen
chloride
and
cyanide
or
thiocyanate
production
has
not
been
adequately
determined.
The
health­
protective
nature
of
this
approach,
and
associated
uncertainties,
are
discussed
in
the
context
of
the
quantitation
in
Chapter
8,
and
the
risk
characterization
in
Chapter
9.
EPA/
OW/
OST/
HECD
Final
draft
III­
40
It
is
plausible
that
additional
metabolites
are
formed
following
cyanogen
chloride
exposure,
in
light
of
the
limited
available
quantitative
data
on
metabolism.
Given
the
available
information
on
water
chemistry
and
metabolic
pathways,
three
additional
chemicals
(
cyanate,

cyanamide,
HCl)
have
been
discussed
here
as
potential
metabolites.
However,
these
chemicals
have
been
dropped
from
the
remainder
of
this
document
as
potential
surrogates
for
cyanogen
chloride,
primarily
because
they
have
not
been
identified
experimentally.
In
addition,
a
preliminary
toxicity
assessment
of
these
chemicals
(
see
results
in
Appendices
C,
D,
and
E)

indicates
that
a
quantitative
assessment
based
on
data
from
cyanide
or
thiocyanate
will
be
more
conservative.
Therefore,
the
remainder
of
this
document
focuses
only
on
cyanogen
chloride,

cyanide,
and
thiocyanate.
