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Administrative data

dermal absorption
Adequacy of study:
weight of evidence
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Published study by a reputable laboratory

Data source

Reference Type:
Methods for in vitro percutaneous absorption studies I. Comparison with in vivo results
Bronaugh, R.L., Stewart, R.F., Congdon, E.R. & Giles, A.L. Jr
Bibliographic source:
Toxicology and Applied Pharmacology 62(3): 474-480

Materials and methods

Test guideline
no guideline followed
Principles of method if other than guideline:
The authors performed both in vivo and in vitro dermal absorption studies, to facilitate comparison of the methods.
GLP compliance:
published study

Test material

Details on test material:
[14C]urea (53 mCi/mmol), obtained from New England Nuclear, Boston, Massachusetts. All compounds were determined by the manufacturer to have a radiochemical purity of >99%.
14C radio labelled urea

Test animals

Details on test animals and environmental conditions:
The animals were female Osborne-Mendel rats, aged 3 to 6 months old.

Administration / exposure

Type of coverage:
Duration of exposure:
5 days - in vivo.
In vivo: Approximately 160 ng of urea was dissolved per 1 mg of petrolatum. 50 mg of the vehicle plus urea was applied to the skin giving an applied dose of 0.008 mg.
In vitro: As above, but 30 mg was applied giving an applied dose of 0.0048 mg.
No. of animals per group:
Four to eight.
Control animals:
Data were compared between the two methods for several compounds
Details on study design:
Absorption was determined from the amount of compound excreted in the urine in 5 days, and the results expressed as percentages of the applied dose. To correct for the amount that was percutaneously absorbed but not recovered in the urine samples, 1 µCi of the compound was injected into the tail vein of separate animals and the percentage recovery of this dose in the urine was determined.
Skin on the midback was lightly shaved with electric clippers, with care being taken to prevent damage to the skin. A nylon ring with an inside area of 2 cm² was attached securely to the skin with a cyanoacrylate adhesive. The legs of each rat were prevented from scratching the back by wrapping rubber tubing around the body immediately behind the front legs, and immediately in front of the back legs. 50 mg of the [14C]urea in petroalutum mixture was applied with a spatula to completely cover the skin inside the ring. The wire mesh cover was attached, and the rats were placed in separate metabolism cages for collection of 24 hour urine samples for a 5 day period.
Details on in vitro test system (if applicable):
Absorption was measured by standard diffusion cell techniques. Full-thickness skin (lightly shaved, subcutaneous fat carefully removed) was clamped tightly between two glass chambers (four to five determinations). The lower chamber, which had a volume of approximately 2.5 ml, contained normal saline that was continuously mixed with a magnetic stirring bar. Thimerosol (0.01%) was added to the saline to retard bacterial growth. The upper chamber was open at the top so that the surface of the skin was exposed to the ambient temperature. Petrolatum containing the radiolabelled compound was applied in a thin layer to the surface of the skin (area 1.13 cm²) with a spatula. The 30 mg of petrolatum applied resulted in a calculated thickness of the vehicle of 0.02 mm, containing an excess of test compound, resulting in the attainment of a steady-state or linear rate of skin permeation. The temperature of the cells was maintained at 32°C in a water bath. Samples of the receptor solution were withdrawn periodically from a sidearm with a microlitre syringe for determination of radioactivity.

Results and discussion

Signs and symptoms of toxicity:
not examined
Dermal irritation:
not examined
Absorption in different matrices:
Percentage of absorbed urea in the in vivo experiments was low; less than 3% of the applied dose was absorbed per day, with the total absorbed dose being 8.1%. Similar results were obtained in vitro, with the total absorbed dose after 5 days being 7.2%.
Total recovery:
No information available.
Percutaneous absorptionopen allclose all
0.008 mg
8.1 %
Remarks on result:
other: 5 days
in vivo
0.0048 mg
7.2 %
Remarks on result:
other: 5 days
in vitro
Conversion factor human vs. animal skin:
No information available

Any other information on results incl. tables

The permeability constant for the in vitro experiment was 1.6x10-5 cm/hr, and 7.2% of the applied dose was absorbed.

Applicant's summary and conclusion

Urea showed low dermal absorption capabilities, with 7 -8% of the applied dose being absorbed after 5 days.
Executive summary:

The percutaneous absorption of selected radiolabelled compounds through female rat skin was determined after their application in a petrolatum vehicle. Absorption was measured during a 5-day period by in vivo and in vitro techniques. Benzoic acid, acetylsalicylic acid, urea, and caffeine were selected because of their differing skin permeability rates. Absorption was measured in vivo from urinary excretion data and in vitro with excised skin in diffusion cells. When skin absorption was expressed as the percentage of applied dose, the values obtained in vitro with urea was 7.2%. Since similar values were obtained in vivo, permeability measurements of these compounds with excised skin appear reliable. With the in vitro data, the rate of skin absorption was more accurately stated in the form of a permeability constant.