Ammonium mercaptoacetate

Endpoint summary

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

key study

Study period:

Aug 1992

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

test procedure in accordance with generally accepted scientific standards and described in sufficient detail

Objective of study:

absorption

Principles of method if other than guideline:

Percutaneous absorption of the 14C-labelled test substance calculated from the amount of 14C eliminated already from the body (72 h after administration) plus the amount of 14C still being present in the carcass.

GLP compliance:

yes

Radiolabelling:

yes

Remarks:

14C-labelling

Species:

rat

Strain:

Sprague-Dawley

Sex:

male/female

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Forschungsinstitut für Versuchstierzucht, A-2325 Himberg, Austria
- Age at study initiation: males ca. 6 - 7 weeks, females 9 - 10 weeks
- Weight at study initiation: approx. 200 g
- Fasting period before study:
- Housing: single caging. Metabolism cages (UNO B.V., Zevenaar,Holland)
- Individual metabolism cages: yes
- Diet (e.g. ad libitum): Altromin 1321ff, diet for rats, ad libitum
- Water (e.g. ad libitum): tap water in Makrolon bottles with stainless steel canules, ad libitum
- Acclimation period: 1 week


ENVIRONMENTAL CONDITIONS
- Temperature (°C): average of 23
- Humidity (%): average of 75
- Air changes (per hr): 12
- Photoperiod (hrs dark / hrs light): 12/12

Route of administration:

dermal

Vehicle:

other: A solution of 14C-thioglycolic acid was gradually neutralized with a 25 % solution of ammonia resulting in three final solutions of 11 % ammonium 14C-thioglycolate with pH 6, pH 7 and pH 8, respectively (concentration calculated as thioglycolic acid).

Details on exposure:

TEST SITE
- Area of exposure: 3x3 cm on the dorsal, median thoracic to lumbar area
- % coverage: 2.65% of body surface
- Type of wrap if used: After rinsing and neutralisation, the skin was covered with 4 layers of gauze fixed by adhesive tape. Additional covering by fixation of an air permeable, plastic, truncated cone to prevent licking of the treated area.
- Time intervals for shavings or clipplings: one day before application of the test substance


REMOVAL OF TEST SUBSTANCE
- Washing (if done): The solution was rinsed off with water of about 37 °C. Then the skin was dabbed dry with absorbent cellulose tissue.
- Time after start of exposure: 30 min


TEST MATERIAL
- Amount(s) applied (volume or weight with unit):
Mean values
Study A: 0.295 g of the solution applied, corresponding to 32.5 mg of the test substance
Study B: 0.300 g of the solution applied, corresponding to 33.0 mg of the test substance
Study C: 0.304 g of the solution applied, corresponding to 33.4 mg of the test substance.
- concentration (if solution): 11% (w/w)

Duration and frequency of treatment / exposure:

30 min, one application

Dose / conc.:

158 mg/kg bw/day

Remarks:

Study A

Dose / conc.:

164 mg/kg bw/day

Remarks:

Study B

Dose / conc.:

166 mg/kg bw/day

Remarks:

Study C

No. of animals per sex per dose / concentration:

5

Control animals:

no

Details on study design:

- Dose selection rationale: In humans a maximum of 75 mL of the 11% test substance solution is applied to the hair. The scalp which has an estimated average surface of 500 cm² is only reached by a small part of the solution estimated to be at most 1/10, corresponding to 0.015 mL/cm².
In order to have reproducible conditions the fur of the rats was clipped, the test substance was therefore applied mainly on the skin. 0.3 mL were used on an area of 9 cm², larger amounts of test substance solution would flow off during exposure time. The applied volume was with 0.033 mL/cm² two times higher than in humans which is a "worst case".

Details on dosing and sampling:

PHARMACOKINETIC STUDY (Absorption, distribution, excretion)
- Tissues and body fluids sampled (delete / add / specify): urine, faeces, carcass, cage washes
- Time and frequency of sampling: daily

Statistics:

t-test was used for analysis of differences between the sexes and the studies. P = 0.05.

Details on absorption:

Study A: 0.27% of the applied dose
Study B: 0.24% of the applied dose
Study C: 0.26% of the applied dose

Details on excretion:

Urine: 76.5 - 80.3% of the eliminated 14C
Faeces: 19.7 - 23.5% of the eliminated 14C

Metabolites identified:

not measured

No pH dependence of the results was detectable. The majority of the applied 14C was removed from the skin by rinsing after the actual exposure and after the neutralization step (means of 96.1 to 96.8% of the applied dose). The mean 14C-content of the skin at the site of application was 0.82%, 0.57% and 0.60% (study A, B and C, respectively) of the administered 14C-amount. Means of 0.139%, 0.119% and 0.137% of the applied 14C were recovered in urine and faeces within 72 hours. 14C was excreted to a larger extent via urine (means 79%, 76% and 80% of the eliminated amounts). The mean excretion was fast:

92%, 83% and 93% of the totally eliminated amounts were excreted in the first 24 hours after application. The remaining mean amounts of 14C in the carcass 3 days after the application were 0.126%, 0.116% and 0.121% of the administered 14C-amount. This corresponds to 46.7%, 48.3% and 46.5% of the absorbed 14C-amount.

The red foci that appeared in the application site of several animals of all pH groups did not have a distinct influence on absorption.

In the three studies the cutaneous absorption as well as the 14C-concentration found in urine, faeces and carcass were higher in males than in females without gaining statistical significance. Means of 97% to 97.9% of the applied 14C-doses were recovered in the various types of samples in the three studies. From these results the mean absorption was calculated to be 0.27%, 0.24 % and 0.26% of the applied dose.

Conclusions:

The mean absorption was calculated to be 0.24-0.27 % of the applied dose. Only ca. 0.1% of the applied dose was found in the carcass 3 d after exposure, indicating no potential for bioaccumulation.

Reference 2

Endpoint:

dermal absorption in vivo

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

test procedure in accordance with generally accepted scientific standards and described in sufficient detail

Qualifier:

no guideline followed

Principles of method if other than guideline:

The aim of the study was to investigate the influence of the pH (pH 6, 7, 8) on the percutaneous absorption of thioglycolic acid

GLP compliance:

yes

Radiolabelling:

yes

Remarks:

14C-labelled thioglycolate anion

Species:

rat

Strain:

Sprague-Dawley

Sex:

male/female

Details on test animals or test system and environmental conditions:

ANIMALS:
Supplier: Forschnugsinstitut für Versuchstierzucht, A-2325 Himberg, Austria.
Weight: approx. 200 g
Age: male ca. 6-7weeks, females ca. 9-10 weeks
Housing: Single caging in metabolism cages (UNO B.V., Zevenaar, Holland)
Diet: ad libitum Altromin 1321ff
Water: ad libitum tap water
Acclimatization: 1 week

CONDITIONS:
Temperature: average 23°C
Relative humidity: average 75%
Air changes: 12/hour
Light-Dark cycle: 12/12

Type of coverage:

open

Vehicle:

water

Duration of exposure:

30 min

Doses:

165 mg/kg bw

No. of animals per group:

5

Control animals:

no

Details on study design:

Groups of 5 male and 5 female Sprague-Dawley rats were used to investigate the dermal bioavailability and excretion of ammonium thioglycolate.

Approx. 300 mg of the solutions (equivalent to 165 mg/kg bw) were applied to the clipped dorsal skin of the rats for 30 min and then washed off. The treatment was followed by a neutralization step where 0.3 ml of a "Natural styling solution" containing 2.1 % hydrogen peroxide was applied for 10 min and then washed off. The treated area was then covered with 4 layers of gauze fixed by adhesive tape and the animals were subsequently placed into metabolism cages for 72 hours.

The concentration of the test material (11 %), the time of exposure (30 min), the area of exposure (ratio application site to body surface: 0.026) and the neutralization step were chosen to mimic human exposure during the application of hair waiving products.
Solutions from the two rinsings were united. Urine and faeces were collected daily.
The animals were killed after 72 hours.
The application area plus some surrounding skin was excised and dissolved in Soluene-350 for analysis of radioactivity.
The radioactivity of the carcass was determined after removal of the remaining skin to avoid false 14C concentrations in the carcass due to skin, possibly contaminated e.g. by urine.

Signs and symptoms of toxicity:

not specified

Dermal irritation:

yes

Remarks:

small focal redness appeared on the treated skin during application and remained at least until the area was covered

Absorption in different matrices:

Most of the 14C was removed from the rat skin during washing of the test material and neutralization solution (mean 96.1 - 96.8%). The mean 14C recovered in urine and faeces in the pH 6, pH 7, and pH 8 exposure groups was 0.139%, 0.119%, and 0.137%, respectively. The mean 14C-content of the skin at the application site for the pH 6, pH 7, and pH 8 exposure groups was 0.82%, 0.57%, and 0.60%, respectively. The mean cutaneous absorption for 11% Ammonium 14C-thioglycolate at pH 6, pH 7, and pH 8 was 1.09%, 0.81%, and 0.86%, respectively. Cutaneous absorption and 14C concentrations in urine, feces, and carcasses were higher in males than females, but it was determined that this was not statistically significant

Total recovery:

Total mean (sd) recovery was 97.9% (1.1), 97.5% (2.1) and 97.0% (1.5) at pH 6, 7 and 8, respectively.

Key result

Dose:

165 mg/kg bw

Parameter:

percentage

Absorption:

ca. 1 %

Remarks on result:

other: rinsing after 30 min

Recovery of the radioactivity after dermal administration of ammonium¹⁴C-thioglycolate to rats

Analysed sample	^14C-activity in % of applied dose, mean (SD)
	pH 6	pH 7	pH 8
Rinsings	96.8 (1.2)	96.7 (2.1)	96.1 (1.4)
Adsorption (application site)	0.82 (0.43)	0.57 (0.24)	0.60 (0.34)
Urine (0-72 h)	0.11 (0.12)	0.091 (0.073)	0.11 (0.11)
Faeces (0-72 h)	0.029 (0.032)	0.028 (0.025)	0.027 (0.025)
Carcass	0.126 (0.087)	0.116 (0.076)	0.121 (0.089)
Total recovery*	97.9 (1.1)	97.5 (2.1)	97.0 (1.5)
Cutaneous absorption	1.09	0.81	0.86

*Total of urine, faeces, adsorption at the application site and carcass¹⁴C recovery.

Conclusions:

The mean cutaneous absorption for 11% ammonium thioglycolate at pH 6, pH 7, and pH 8 was 1.09%, 0.81%, and 0.86%, respectively.

Executive summary:

Three groups of rats (5/sex; ~200 g) received on the clipped dorsal skin approximately 300 mg of ammonium¹⁴C-thioglycolate (radiochemical purity 97.6%) as an 11% solution (equivalent to 165 mg a.i./kg bw or 133 mg/kg bw as thioglycolic acid) at pH 6, pH 7, and pH 8 for 30 minutes followed by a washing of the site. The test site was then neutralized with 0.3 ml of a “natural styling solution” containing 2.1% hydrogen peroxide for 10 minutes followed by a washing of the site. These applications were to mimic human exposure to hair waving products. After the second wash, the test sites were covered with four layers of gauze and the rats were placed into metabolism cages for 72 hours. Following the observation period, the animals were sacrificed. The test sites and surrounding skin were excised and dissolved in Soluene-350 for radioactivity analysis. The radioactivity of the waste wash water, urine, and feces as well as the carcasses was also measured.

Most of the¹⁴C was removed from the rat skin during washing of the test material and neutralization solution (mean 96.1 - 96.8%). The mean¹⁴C recovered in urine and faeces in the pH 6, pH 7, and pH 8 exposure groups was 0.139%, 0.119%, and 0.137%, respectively. The mean¹⁴C-content of the skin at the application site for the pH 6, pH 7, and pH 8 exposure groups was 0.82%, 0.57%, and 0.60%, respectively. The mean cutaneous absorption for 11% Ammonium¹⁴C-thioglycolate at pH 6, pH 7, and pH 8 was 1.09%, 0.81%, and 0.86%, respectively. Cutaneous absorption and¹⁴C concentrations in urine, feces, and carcasses were higher in males than females, but it was determined that this was not statistically significant.

Reference 3

Endpoint:

dermal absorption in vitro / ex vivo

Type of information:

experimental study

Adequacy of study:

key study

Study period:

29-Jan- 2007 - 29-May-2007

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 428 (Skin Absorption: In Vitro Method)

Qualifier:

according to guideline

Guideline:

other: Official recommendation of the European Scientific Committee on Consumer Products (SCCP)

GLP compliance:

yes

Radiolabelling:

yes

Species:

pig

Strain:

other: Landrace large white cross

Sex:

male/female

Type of coverage:

open

Vehicle:

other: hair waving formulation

Duration of exposure:

30 min

Doses:

ca 20 mg of the hair waving formulation/cm2, the resulting dose of ATG was approx. 2.63 mg/cm² skin.

Details on in vitro test system (if applicable):

The dermal absorption/percutaneous penetration of [14C]-radiolabelled Ammonium Thioglycolate out of a representative permanent hair waiving formulation was studied on the clipped excised skin of four Landrace large white cross pigs (two male, two female). The pig skin, dermatomed to a mean thickness of 0.80 mm, was used because it shares essential penetration characteristics with human skin . Pigs were control/spare animals from separate studies at the test institute.

The skin integrity of frozen (at -20 °C) skin discs was checked by measuring the trans-dermal electrical resistance. The intact, clipped excised pig skin of the cosmetic formulation without occlusion for a representative exposure time (see below).

Before topical application to skin the respective [14C]-radiolabelled ammonium thioglycolate salt was added to the hair waving formulation.
The dermal absorption/percutaneous penetration of the test substance was investigated for the open application of about 20 mg formulation per cm² pig skin. Therefore the resulting dose of ATGA was approx. 2.63 mg/cm² skin. Skin discs of about 3.14 cm² were exposed to the formulation for 30 min, terminated by gently rinsing with a commercial shampoo solution diluted with water and drying the skin surface with tissue paper (tissue swabs). At 24 h post dose, the washing procedure was repeated. The skin was then removed from the static diffusion cells, dried and the stratum corneum was removed with 20 successive tape strips. The remaining skin was divided into exposed and unexposed skin and solubilised with Solvable® tissue solubiliser. The receptor fluid in the receptor chamber was removed and split into fractions for analysis. All samples were analysed by liquid scintillation counting. The formulation was analysed with nine replicates obtained from three different animals per experiment for absorbed and penetrated amount of the test substance. The receptor fluid used was phosphate buffered saline. Samples of the receptor fluid were drawn before the application of the test substance formulation and 0.5, 1, 2, 4, 6, and 24 hours after application. The removed volume was replaced by fresh receptor fluid.

Absorption in different matrices:

For [14C]-ammonium thioglycolate in permanent hair waving formulation (13%, w/w) applied to dermatomed pig skin in vitro, most of the applied dose (98.69%) was removed by washing at 30 min post dose. At 24 h post dose, the total dislodgeable dose was 98.98% of the applied dose. The total unabsorbed dose was 99.04% of the applied dose. The stratum corneum (Dermal Adsorption) retained 0.17% of the applied dose. The percutaneous penetration and dermal bioavailability were 0.22% (5.23 µg equiv./cm2) and 0.77% (19.83 µg equiv./cm2) of the applied dose, respectively. The mass balance was complete with 99.97% of the applied dose recovered. Both the amounts absorbed and penetrated were taken as systemically available.

Total recovery:

99.97%

Dose:

2.63 mg/cm² skin

Parameter:

percentage

Absorption:

0.8 %

Remarks on result:

other: 24h

Remarks:

The amount of test item penetrating and /or remaining in the exposed skin after removing the stratum corneum

ANALYSED SAMPLE 13% ATG (or 11% TGA) in a permanent hair waving formulation

Exposure time : 30 min

[% of dose] [µg/cm²]

Unadsorbed Dose* 99.04 2610.93

Adsorption (stratum corneum) 0.17 4.58

Not Bioavailable 99.21 2615.51

Absorption (epidermis/dermis) 0.55 14.60

Penetration (receptor fluid) 0.22 5.23

Bioavailable 0.77 19.83

Total recovery / mass balance 99.97 2635.34

* Unadsorbed dose: skin washes + tissue swabs + pipette tips + donor chamber wash + unexposed skin

Conclusions:

In this in vitro dermal penetration study the amount of Ammonium Thioglycolate (ATG) systemically available from a representative permanent hair waving formulation was found to be 19.83 µg/cm² (0.77%), corresponding to 16.74 µg/cm² when calculated for Thioglycolic Acid (TGA).

Executive summary:

The three [¹⁴C]-radiolabelled test items (ammonium thioglycolate (ATG), calcium thioglycolate (CaTG) and monoethanolamine thioglycolate (MeaTG)) are used in basic formulations (test preparations). The formulations are typical

for a permanent hair waving formulation, a depilatory formulation and a hair straightening formulation.

As part of a safety evaluation of each test item, this study was required to assess their extent of dermal bioavailability, following topical application of ATG in a permanent hair waving formulation (13%, w/w), CaTG in a depilatory formulation (10%, w/w) and MeaTG in a hair straightening formulation (18%, w/w) to excised pig skin. The skin surface was washed at 30, 15 and 45 min, respectively, for each formulation to reflect the in use conditions. The skin was again washed at 24 h post dose for all groups as a final wash off before test run termination.

Previously frozen dermatomed pig skin was mounted into static diffusion cells containing receptor fluid (phosphate buffered saline, ca 10 mL) in the receptor chamber. The skin surface temperature was maintained at ca 32°C throughout the experiment. An electrical resistance barrier integrity test was performed and any pig skin sample exhibiting a resistance <4 kΩ was excluded from subsequent dermal bioavailability measurements.

The permanent hair waving formulation, depilatory formulation and hair straightening formulation containing [¹⁴C]-ATG, [¹⁴C]-CaTG and [¹⁴C]-MeaTG, respectively, were applied, at an application volume of ca 20 mg/cm², to dermatomed pig skin mounted into static diffusion cells in vitro.

Dermal bioavailability was assessed by collecting receptor fluid aliquots (250 µL) at 0.5, 1, 2, 4, 6 and 24 h post dose. At 30, 15 and 45 min post dose, respectively, for ATG in a permanent hair waving formulation, CaTG in a depilatory formulation and MeaTG in a hair straightening formulation, exposure was terminated by washing the skin surface with a dilute shampoo solution and drying the skin surface with tissue paper (tissue swabs). At 24 h post dose, the washing procedure was repeated. The skin was then removed from the static diffusion cells, dried and the stratum corneum was removed with 20 successive tape strips.

The remaining skin was divided into exposed and unexposed skin and solubilised with Solvable® tissue solubiliser. The receptor fluid in the receptor chamber was removed and split into fractions for analysis. All samples were analysed by liquid scintillation counting.

A summary of the mean results are provided in the tables overleaf:

Test Preparation	ATG in Permanent Hair Waving Formulation	CaTG in Depilatory Formulation	MeaTG in Hair Straightening Formulation
Test Item Concentration (w/w)	13	10	18
Mean Application Rate of Formulation (mg/cm^2)	20.28	20.11	20.12
Mean Application Rate of Test Item (µg equiv./cm^2)	2636.13	2010.74	3621.78
Dislodgeable Dose at 30, 15, 45 min, respectively (% Applied Dose)	98.69	104.70	98.14
Total Dislodgeable Dose* (% Applied Dose)	98.98	105.07	98.36
Unabsorbed Dose** (% Applied Dose)	99.04	105.08	98.41
Dermal Adsorption (% Applied Dose)	0.17	0.07	0.09
Percutaneous Penetration (% Applied Dose)	0.22	0.02	0.17
Dermal Bioavailability (% Applied Dose)	0.77	0.20	0.38
Mass Balance (% Applied Dose)	99.97	105.36	98.87
Dislodgeable Dose at 30, 15, 45 min, respectively (µg equiv./cm^2)	2601.66	2105.30	3554.39
Total Dislodgeable Dose* (µg equiv./cm^2)	2609.26	2112.73	3562.29
Unabsorbed Dose** (µg equiv./cm^2)	2610.93	2112.94	3564.05
Dermal Adsorption (µg equiv./cm^2)	4.58	1.48	3.15
Percutaneous Penetration (µg equiv./cm^2)	5.23	0.41	5.83
Dermal Bioavailability (µg equiv./cm^2)	19.83	4.09	13.64
Mass Balance (µg equiv./cm^2)	2635.34	2118.52	3580.85

* Total dislodgeable dose = skin washes + tissue swabs + pipette tips + donor chamber wash

** Unabsorbed dose = total dislogeable Dose + unexposed skin

For [¹⁴C]-ATG in permanent hair waving formulation (13%, w/w) applied to dermatomed pig skin in vitro, most of the applied dose (98.69%) was removed by washing at 30 min postdose. At 24 h post dose, the total dislodgeable dose was 98.98% of the applied dose. The total unabsorbed dose was 99.04% of the applied dose. The stratum corneum (dermal adsorption) retained 0.17% of the applied dose. The percutaneous penetration and dermalbioavailability were 0.22% (5.23 µg equiv./cm²) and 0.77% (19.83 µg equiv./cm²) of the applied dose, respectively. The mass balance was complete with 99.97% of the applied dose recovered.

For [¹⁴C]-CaTG in depilatory formulation (10%, w/w) applied to dermatomed pig skin in vitro, most of the applied dose (104.70%) was removed by washing at 15 min post dose. At 24 h post dose, the total dislodgeable dose was 105.07% of the applied dose. The total unabsorbed dose was 105.08% of the applied dose. The stratum corneum (dermal adsorption) retained 0.07% of the applied dose. The percutaneous penetration and dermal bioavailability were 0.02% (0.41 µg equiv./cm²) and 0.20% (4.09 µg equiv./cm²) of theapplied dose, respectively. The mass balance was complete with 105.36% of the applied doserecovered.

For [¹⁴C]-MeaTG in a permanent hair waving formulation (18%, w/w) applied to dermatomed pig skin in vitro, most of the applied dose (98.14%) was removed by washing at 45 min post dose. At 24 h post dose, the total dislodgeable dose was 98.36% of the applieddose. The total unabsorbed dose was 98.41% of the applied dose. The stratum corneum (dermal adsorption) retained 0.09% of the applied dose. The percutaneous penetration and dermal bioavailability were 0.17% (5.83 µg equiv./cm²) and 0.38% (13.64 µg equiv./cm²) of the applied dose, respectively. The mass balance was complete with 98.87% of the applied dose recovered.

Description of key information

No kinetic data are available on the absorption of thioglycolic acid and/or its salts by inhalation or oral exposure. However, the physico-chemical properties of thioglycolates, small ionisable water-soluble molecules with a very low log Kow, as well as, the acute oral and inhalation toxicity data suggest that thioglycolic acid and its salts are significantly absorbed by the inhalation and oral routes.
Short description of key information on dermal absorption rate: 
Under testing conditions that take into account realistic use conditions for cosmetic formulations (30-min exposure and rinsing), the dermal absorption of ammonium thioglycolate (pH between 6 and 9) seems very limited, about 1% of a dose of 133 mg/kg bw (as thioglycolic acid) was absorbed within 72h in rats and 0.77% of a dose of 2.1 mg thioglycolic acid/cm² was systemically available in an in vitro dermal absorption/penetration assay with excised skin of pigs.
Overall, for the purpose of risk assessment, a dermal absorption rate of 1%  is assumed for ammonium thioglycolate.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - dermal (%):

1

Additional information

No data are available on the absorption of thioglycolic acid and its salts by inhalation or oral exposure. However, the physical-chemical properties of the thioglycolates, small ionisable water-soluble molecules with a very low logK_ow as well as the acute oral and inhalation toxicity data suggest that thioglycolic acid and/or its salts are significantly absorbed by the inhalative and oral routes.

Regarding dermal absorption, it seems that the dermal penetration of thioglycolate is pH-dependent, the more acidic the pH, the fewer molecules are ionized and the more it is absorbed. A former study, performed under exaggerated exposure conditions (no rinsing) and at an unknown pH, suggests an extensive dermal absorption of the sodium salt in the rabbits, with at least 30-40% of the dose (up to 660 mg/kg bw as thioglycolic acid) excreted in the urine within 5 hours in the form of neutral sulphate (Freeman, 1956). However, under testing conditions that take into account realistic use conditions for cosmetic formulations (30-min exposure and rinsing), the dermal absorption of ammonium thioglycolate (pH between 6 and 9) seems very limited, about 1% of a dose of 133 mg/kg bw (as thioglycolic acid) was absorbed within 72h in rats (Reindl, 1993). In an in vitro dermal absorption/penetration assay with excised skin of pigs performed according to OECD Guideline 428, the amount of ammonium thioglycolate systemically available (epidermis/dermis plus receptor fluid) was found to be 19.83 µg/cm², corresponding to 0.77% of a dose of 2.63 mg ammonium thioglycolate/cm² (Toner, 2007).

After i.v. injection,³⁵S-thioglycolate is mainly distributed in the kidneys, lungs, and spleen of a female monkey (Freeman 1956), in the small intestine and kidneys of a rat (Bakshy and Gershbein, 1972). Residual ³⁵S blood concentrations at 0.5 to 7 h post-injection did not exceed 5.3% in rats (Bakshy and Gershbein, 1972). Significant concentrations of dithiodiglycolate were detected in the urine of rabbits 24 h after thioglycolic acid was injected i. p. negligible concentrations of thioglycolic acid were detected (Bakshy and Gershbein, 1972).

Absorption and excretion

The pulmonary excretion of sodium thioglycolate as hydrogen sulphide was investigated in the rat (weight and strain not stated). The animal was injected i. p. with 150 mg/kg of sodium thioglycolate. Expired air from the animal was analyzed for hydrogen sulphide over a period of 10 h. Hydrogen sulphide was not detected in expired air at any time during the study (Freeman et al. 1956a; reliability 2).

The urinary excretion of sodium thioglycolate was evaluated using rabbits (weights and strain not stated). Four animals were injected i. v. with a 5% solution of sodium35S-thioglycolate (doses of 70, 80, 80, and 123 mg/kg, respectively). Two animals served as controls. Urine was then collected over a period of 24 h. A few drops of liquid petrolatum were placed in each container to prevent air oxidation of possible sulfhydryl compounds. Quantities of organic sulphate, inorganic sulphate, and neutral sulphur in each urine sample were expressed as the percentage of administered radioactivity. Sodium thioglycolate caused a considerable increase in excretion of iodine reducing material; more than enough to account for the compound administered indicating the breakdown of body constituent and was excreted mostly as inorganic sulphate and neutral sulphur. The radioactivity in the urine indicated that 63-83% of the compound was excreted in the first 24 hours after its administration (Freeman et al. 1956a; reliability 2).

The urinary excretion of sodium thioglycolate was also evaluated in rats (weight and strain not stated) injected i.p. with 12.5 to 75.0 mg/kg of a 2.5% solution of sodium ³⁵S-thioglycolate. Urine was collected over a period of 24 h. Quantities of inorganic sulphate excreted, expressed as % of administered radioactivity, ranged from 23 to 72%. The total labeled sulphur excreted during the first 24 hours was 59-96% of the dose. Two of the rats excreted 9% or 11% on the second day and 2% or 6% on the third day respectively (Freeman et al. 1956a; reliability 2).

³⁵S-thioglycolic acid (100 mg/kg adjusted to pH 7.2-7.4 with NaOH) was administered to Holtzman rats (weight = 200-250 g), 12 rats were injected i.v. and to 10 i.p. Also, 2 rats were each given 75 mg/kg via i.p. injection. Animals injected i.v. (12 rats) comprised one group, and those injected intraperitoneally (12 rats) comprised the other. Urine samples were collected 24 h after injection, after which the administered35S was excreted, and excretion percentages were determined. The mean urine sulphate content for i.v. dosed rats was 82.3 ± 1.6% and for i. p. dosed rats was 90.6 ± 1.8%. Most of the radioactivity was excreted in the form of neutral sulphate (Bakshy and Gershbein, 1972; reliability 2).

Two male New Zealand rabbits (weights not stated) were injected i.p. with ³⁵S-thioglycolic acid (100 mg/kg adjusted to pH 7.2-7.4 with NaOH) and one rabbit was injected i.p. with 200 mg/kg. Urine samples were collected 24 h after injection. The mean urine sulphur content of the 3 rabbits was 88% of the administered dose. Most of the radioactivity was excreted in the form of neutral sulphate (Bakshy and Gershbein, 1972; reliability 2).

Distribution

The distribution of radioactivity was determined two hour after i.v. injection of 50 mg/kg ³⁵S-thioglycolic acid (adjusted to pH 7.2-7.4 with NaOH) to one Holtzman rat. The small intestine, kidney, liver and stomach exhibited the greatest activity, respectively 0.07, 0.03, 0.02 and 0.02 % of the dose. It is possibly consistent with the generally rapid elimination of thioglycolate in the urine and bile. The greatest content of35S, 0.66% of the total administered, was detected in the feces. This observation may have been due to contamination of the feces with urine missed during the rinsing of urine residue from the cage after collection (Bakshy and Gershbein, 1972; reliability 2).

The distribution of35S thioglycolic acid (adjusted to pH 7.2-7.4 with NaOH) in whole blood was evaluated in five Holtzman rats injected i.v. with 100 mg/kg of the test substance and bled during periods of up to 7 h. Four of the 5 had less than 3% residual activity at 1 hour, while one had 5.3% residual activity. At 4-7 hours after the injection, only 0.1% activity or less remained (Bakshy and Gershbein, 1972; reliability 2).

The distribution of ³⁵S-thioglycolic acid in the blood was further investigated in the New Zealand rabbit after i.v. injection of ³⁵S-thioglycolic acid (adjusted to pH 7.2-7.4 with NaOH), with emphasis on binding to the following serum protein fractions: α1, α2, β, and γ-globulins and albumin. The test substance (75 mg/kg) was injected i.v. Most of the radioactivity was bound to albumin. The extent of this uptake amounted to 0.14% at 20 min post-injection and had diminished to 0.016% at 3 h. The small amount of radioactivity detected in albumin might have been due to isotopic exchange (Bakshy and Gershbein, 1972; reliability 2).

A female monkey given 300 mg ³⁵S-labelled sodium thioglycolate/kg bw by i.v. injection, excreted labeled sulphur in the urine (for up to 10 hours) entirely as neutral "sulphur". Tissue samples from 10 organs showed the largest amounts of label in the kidney, lungs and spleen (Freeman et al. 1956a; reliability 2).

 

Metabolism

Unlabeled thioglycolic acid (100 or150 mg/kg) was administered to a group of seven rats via i.p. injection. Significant concentrations of dithiodiglycolate (average concentration 28%) were detected in the urine at 24 h post-injection. Only negligible concentrations of thioglycolate were detected (Bakshy and Gershbein, 1972; reliability 2).

Discussion on absorption rate:

Studies performed with ammonium thioglycolate based cosmetic formulations are available

In vitro study

The dermal absorption/percutaneous penetration of [14C]-radiolabelled ammonium thioglycolate out of a representative permanent hair waiving formulation (13% in the formulation, pH 9.5) was studied on the clipped excised skin of four Landrace large white cross pigs. The pig skin, dermatomed to a mean thickness of 0.80 mm, was used because it shares essential penetration characteristics with human skin. The dermal absorption/percutaneous penetration of the test substance were investigated for the open application of about 20 mg formulation per cm² pig skin. Therefore the resulting dose of ammonium thioglycolate was approximately 2.63 mg/cm² skin (equivalent to 2.1 mg thioglycolic acid/cm²). Skin discs of about 3.14 cm² were exposed to the formulations for 30 min., terminated by gently rinsing with a commercial shampoo solution diluted with water. The amount of ammonium thioglycolate systemically available (epidermis/dermis plus receptor fluid) was found to be 19.83 µg/cm² (0.77%), corresponding to 16.74 µg/cm² when calculated for thioglycolic acid (Toner, 2007).

In vivo study

Three groups of rats (5/sex; ~200 g) received on the clipped dorsal skin approximately 300 mg of ammonium ¹⁴C-thioglycolate (radiochemical purity 97.6%) as an 11% solution (equivalent to 165 mg a. i./kg bw or 133 mg/kg bw as thioglycolic acid) at pH 6, pH 7, and pH 8 for 30 minutes followed by a washing of the site. The test site was then neutralized with 0.3 ml of a “natural styling solution” containing 2.1% hydrogen peroxide for 10 minutes followed by a washing of the site. These applications were to mimic human exposure to hair waving products. After the second wash, the test sites were covered with four layers of gauze and the rats were placed into metabolism cages for 72 hours. Following the observation period, the animals were sacrificed. The test sites and surrounding skin were excised and dissolved in Soluene-350 for radioactivity analysis. The radioactivity of the waste wash water, urine, and feces as well as the carcasses was also measured. The results of the radioactivity count are presented in the following table.

Analysed sample	^14C-activity in % of applied dose, mean (SD)
	pH 6	pH 7	pH 8
Rinsings	96.8 (1.2)	96.7 (2.1)	96.1 (1.4)
Adsorption (application site)	0.82 (0.43)	0.57 (0.24)	0.60 (0.34)
Urine (0-72 h)	0.11 (0.12)	0.091 (0.073)	0.11 (0.11)
Feces (0-72 h)	0.029 (0.032)	0.028 (0.025)	0.027 (0.025)
Carcass	0.126 (0.087)	0.116 (0.076)	0.121 (0.089)
Total recovery	97.9 (1.1)	97.5 (2.1)	97.0 (1.5)
Cutaneous absorption^[1]	1.09	0.81	0.86

^[1] total of urine, faeces, adsorption at the application site and carcass¹⁴C recovery.

Most of the ¹⁴C was removed from the rat skin during washing of the test material and neutralization solution (mean 96.1 - 96.8%). The mean ¹⁴C recovered in urine and feces in the pH 6, pH 7, and pH 8 exposure groups was 0.139%, 0.119%, and 0.137%, respectively. The mean14C-content of the skin at the application site for the pH 6, pH 7, and pH 8 exposure groups was 0.82%, 0.57%, and 0.60%, respectively. The mean cutaneous absorption for 11% Ammonium14C-thioglycolate at pH 6, pH 7, and pH 8 was 1.09%, 0.81%, and 0.86%, respectively. Cutaneous absorption and ¹⁴C concentrations in urine, feces, and carcasses were higher in males than females, but it was determined that this was not statistically significant (Reindl, 1993).

The urinary excretion of an ammonium thioglycolate solution (0.6 N, pH 9.3) was evaluated in rabbits (2.3-3.0 kg, strain not stated). A single application (1.0 ml/kg, equivalent to 65.5 mg/kg bw) of the solution containing 0.10 to 0.20 µCi of35S was made via a syringe to a clipped area (15% of body surface) on an animal's right side (apparently without occlusion and rinsing). At 24 hours, 16.22 ± 0.55 % of the labeled sulphur had been excreted in the urine, while in the following 48 hours 6.46% was excreted. When the same volume was applied on 4 successive days, approximately 35-40% of the total ³⁵S had been excreted in the urine within this time (Gershbein, 1979).

The range finding study of an in vivo micronucleus test (Haddouk, 2006)) showed that pH can significantly affect the systemic toxicity of thioglycolic acid after a dermal application to mice. No mortality and no clinical signs were observed at 1491 mg thioglycolate/kg/day at pH 7 for 2 days. In contrast, 2 out of 3 males died after the first treatment with 1500 mg thioglycolate/kg/day at pH 4. Thus, it seems that the dermal penetration of thioglycolic acid is pH-dependent, the more the pH is acidic, the less the molecule is ionized and more it is absorbed.