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Background: A new dermatopharmacokinetic (DPK) approach has been proposed for bioequivalence determination of topical drug products by comparing the drug content kinetics in stratum corneum., Objective: We sought to establish any correlation between clinical safety/efficacy and DPK approach in bioequivalence determination of tretinoin gel 0.025%., Methods: Tretinoin and isotretinoin were quantified in human volar forearm stratum corneum as a function of time with 3 tretinoin gel 0.025% products in 49 patients. Stratum corneum layers were harvested using multiple adhesive disks, which were subsequently extracted and quantified for both isomers by high-performance liquid chromatography., Results: Products with similar composition and therapeutic equivalence were found bioequivalent, and products with different composition and clinical profiles were found bioinequivalent by DPK methodology., Conclusions: There is a direct correlation between DPK parameters in healthy patients and clinical safety/efficacy of tretinoin gel products in patients with acne. Data support the use of DPK parameters and methodology in the bioequivalence assessment of topical tretinoin gel products.

The tissue distribution of retinoic acid (RA) throughout development is highly restricted, defined by the expression patterns of enzymes involved in RA synthesis and catabolism. Presented is a summary of recent research that examines the role of some of the enzymes involved in RA distribution, particularly those involved in RA catabolism (P450RAI). These latter enzymes protect against premature exposure to RA, and the implications of these findings are discussed.

Background: A physiologically based pharmacokinetic (PBPK) model for all-trans-retinoic acid (tretinoin) was developed to provide a coherent description of tretinoin absorption, distribution metabolism, and excretion across species and routes of administration., Objective: The goal of developing such a model is to provide a measure of internal dose that would be a biologically relevant surrogate for administered dose in assessing human teratogenic risk from topically applied tretinoin emollient cream., Methods: The developed PBPK model included compartments for plasma, liver, gut, intestinal lumen, fat, skin, richly and slowly perfused tissues, placenta, and embryo. Tretinoin metabolism to 13-cis retinoic acid, oxidation, and glucuronidation were incorporated. Dose surrogates, including the maximum plasma concentration (Cmax) and area under the concentration-versus-time curve were calculated from the model., Results: The ability of the model to predict tretinoin pharmacokinetics and to extrapolate across species and routes of administration was tested and validated. Model-derived estimates of dose surrogates demonstrated that the internal exposure to retinoids after topical treatment with 0.05% tretinoin emollient cream is minimal in comparison to that for teratogenic oral doses. The ratio of areas under the curve for total active retinoids after teratogenic oral doses in monkeys versus therapeutic topical doses in human beings, for example, was greater than 450,000 to 1., Conclusion: For topical application of tretinoin in human beings, detoxification via the glucuronidation pathway predominates, resulting in a much lower internal exposure to active retinoids than was inferred from total radioactivity data. The model predicts that topical application of tretinoin results in an internal exposure that is four to six orders of magnitude lower than a minimally teratogenic dose.

Background: The percutaneous absorption of topically applied tretinoin cream and emollient cream formulations has not been comprehensively studied., Objective: To assess tretinoin absorption and plasma levels of tretinoin and its metabolites after single and repeated topical tretinoin doses., Methods: In study 1, 28 subjects were equally divided into four treatment groups that received a single dose of tritiated tretinoin in a 0.05% formulation of emollient cream (Renova, Retinova) or cream (Retin-A) alone or after 28 days of repeated nonradioactive doses. In study 2, subjects received single topical doses of tritiated tretinoin cream alone (n = 5) or after 1 year of nonradioactive applications (n = 4). Plasma, urine, and fecal samples were analyzed to determine absorption; plasma samples in study 1 were also analyzed for concentrations of tretinoin and its metabolites., Results: Percutaneous absorption of tretinoin was approximately 2% after a single dose and after 28 days of daily application. In patients receiving long-term therapy (i.e., > 1 year), absorption averaged 1.1%. Mean plasma concentrations of tretinoin after 28 days of treatment with either tretinoin emollient cream or tretinoin cream were not significantly changed when compared with the corresponding endogenous concentrations before treatment., Conclusion: Minimal percutaneous absorption of tretinoin was obtained after its topical application in cream formulations. Neither single-dose nor long-term treatment with topical tretinoin formulations appeared to affect the endogenous levels of tretinoin or its metabolites.

Background: Vitamin A ingestion raises plasma levels of several potentially teratogenic retinoic acids (RAs) with all-trans and 13-cis configuration, but definitive data concerning the consequences of topical administration of all-trans-RA are lacking., Objective: The study objective was to investigate the potential for inducing systemic activity after topical administration of 0.025% all-trans-RA by measuring plasma retinoid levels., Methods: Plasma levels of all-trans-RA, 13-cis-RA, and 4-oxo-13-cis-RA were measured in four healthy subjects before, during, and after 14 daily topical applications of all-trans-RA., Results: Topical administration of all-trans-RA did not significantly increase plasma levels of all-trans-RA, 13-cis-RA, and 4-oxo-13-cis-RA. Significant decreases in levels of these RAs observed during the night may reflect diurnal variations of retinoid metabolism or lowered absorption of dietary vitamin A., Conclusion: Diurnal and nutritional factors influence plasma levels of endogenous retinoids to a greater extent than topical administration of all-trans-RA at doses used for acne therapy, which on the basis of these results appears unlikely to induce systemic effects.

175 patients with severe psoriasis of different types were treated with 10, 25, or 50 mg acitretin and compared with patients receiving 50 mg etretinate over a period of 8 weeks in a randomized, double-blind multicenter study in the Federal Republic of Germany. Plasma concentrations of etretinate and its metabolite acitretin were measured during therapy and also 3 weeks after cessation of treatment. After 4 weeks of treatment, a trend toward clinical improvement was shown in all groups with increasing dosage. Those groups receiving the lower acitretin doses (i.e., 10 and 25 mg/day) had more dropouts than the groups taking 50 mg acitretin or 50 mg etretinate. Complete remissions before the end of therapy occurred only among those receiving higher doses. Enlargement of psoriatic lesions, however, could be observed during treatment with both retinoids, despite improvement of other parameters, as measured by psoriasis area and severity index (PASI) and psoriasis severity index (PSI). After 8 weeks, a significant improvement was calculated by the PASI score and by a newly defined, corrected PASI score for all four dose regimens compared with baseline levels. A greater than 50% PSI score improvement was seen in 50% of patients treated with 10 mg acitretin, 40.5% with 25 mg acitretin, 53.8% with 50 mg acitretin, and 61.1% with 50 mg etretinate. No statistical differences were found among these groups at any time during the 8-week period. No new or unexpected side effects occurred during acitretin treatment. Moreover, cholesterol levels did not significantly change. Three weeks after cessation of drug administration, the plasma concentrations of acitretin were below the sensitivity level of the assay, whereas etretinate was still quantifiable. It is interesting that acitretin plasma concentrations during therapy with 50 mg acitretin were markedly lower (means = 18 ng/ml) than were acitretin levels during treatment with 50 mg etretinate (means = 36 ng/ml).