The percutaneous absorption of topically applied tretinoin and its effect on endogenous concentrations of tretinoin and its metabolites after single doses or long-term use

50 Years of Topical Retinoids for Acne: Evolution of Treatment

Since the US Food and Drug Administration (FDA) approved tretinoin in 1971, retinoids alone or combined with other agents have become the mainstay of acne treatment. Retinoids act through binding to retinoic acid receptors, altering expression levels of hundreds of cellular proteins affecting multiple pathways involved in acne pathogenesis. Retinoids have evolved from first-generation agents, such as tretinoin, through chemical modifications resulting in a second generation (etretinate and acitretin for psoriasis), a third generation (adapalene and tazarotene) and, most recently, a fourth (trifarotene). For all topical retinoids, local irritation has been associated with poor tolerability and suboptimal adherence. Efforts to improve tolerability have utilized novel delivery systems and/or novel agents. This qualitative literature review summarizes the evolution of the four topical single-agent retinoids available for the treatment of acne in the US today and their various formulations, presenting the rationale behind their development and data from key studies.

Trifarotene for the Treatment of Facial and Truncal Acne

OBJECTIVE

This article reviews clinical trials to assess the efficacy, safety, and clinical application of trifarotene 0.005% cream (Aklief).

DATA SOURCES

A systematic review of the literature was performed using the terms trifarotene OR Aklief OR CD5789 in MEDLINE (PubMed) and EMBASE databases. Articles prior to May 2020 were considered for inclusion. Bibliographies and ClinicalTrials.gov were also searched to identify further studies.

STUDY SELECTION AND DATA EXTRACTION

Relevant English language and human studies related to pharmacology, clinical trials, and safety were considered.

DATA SYNTHESIS

In the 52-week phase III trial, treatment success rates for facial acne (Investigator Global Assessment [IGA] rating of no or almost no acne) and truncal acne (Physician's Global Assessment [PGA] rating of no or almost no acne) were 65.1% and 66.9%, respectively. Overall success rates (IGA and PGA success in the same patient) were 57.9%; 52.8% of patients had a Dermatology Quality of Life Index score of 0 or 1, compared with 22.6% at baseline. Trifarotene was well tolerated, with pruritus, irritation, and sunburn as the most common adverse effects.

RELEVANCE TO PATIENT CARE AND CLINICAL PRACTICE

Trifarotene is a newly Food and Drug Administration-labeled fourth-generation topical retinoid that shows particular promise in the treatment of facial and truncal acne vulgaris. It is an effective and safe addition to currently available retinoids.

CONCLUSION

Trifarotene is effective and safe for treatment of facial and truncal acne. Future trials should compare its efficacy and tolerability with that of the older, clinically established retinoids. Despite efficacy, cost may be a prohibitive factor.

Identifying pregnancies in insurance claims data: Methods and application to retinoid teratogenic surveillance

PURPOSE

The purpose of the study is to develop an algorithm to identify pregnancies in administrative databases and apply it to assess pregnancy rates and outcomes in women prescribed isotretinoin or tretinoin.

METHODS

Using the 2011 to 2015 Truven Health MarketScan Database, we identified pregnancies, including losses and terminations. In a cohort design, nonpregnant women filling a prescription for isotretinoin or tretinoin were matched to five women without either prescription. Women were followed for 365 days or until conception, medication discontinuation, or enrollment discontinuation ("prescription episode"). Rates of pregnancy, risks of pregnancy losses, and prevalence of infant malformations at birth were assessed by exposure.

RESULTS

We identified 2 179 192 livebirths, 8434 stillbirths, 2521 mixed births, 415 110 spontaneous abortions, 124 556 elective terminations, and 8974 unspecified abortions. There were 86 834 isotretinoin and 973 587 tretinoin episodes, matched to 5 302 105 unexposed women. Pregnancy rates were 3 (isotretinoin), 19 (tretinoin), and 34 (unexposed) per 1000 person-years. Risk of spontaneous pregnancy losses were similar; however, terminations were more common in the isotretinoin-exposed (28% [95% CI: 21%-36%]) than the tretinoin-exposed (10% [95% CI: 9%-11%]) or unexposed pregnancies (6%). Malformations occurred in 4.5% (95% CI: 3.5%-5.6%) of the tretinoin-exposed pregnancies and 4.2% of the unexposed pregnancies (adjusted odds ratio: 1.16 [95% CI: 0.85-1.58]); isotretinoin-exposed births were too few to assess malformations.

CONCLUSIONS

Administrative databases can complement risk evaluation and mitigation strategies (REMS) for known teratogens and contribute to safety surveillance for other medications. Here, isotretinoin-exposed pregnancy rates were low, but existent, and many pregnancies were terminated. Tretinoin exposure was not associated with a meaningfully elevated risk of losses or malformations as compared with unexposed pregnancies.

Cutaneous Aging: A Review

A general overview of the aging process is presented herein including its pathogenesis, the theories that attempt to explain it, and the histological changes that support its clinical picture. The concepts of intrinsic aging and photoaging and their differences are discussed and a review of the therapeutic options is made.

The efficacy of topical 0.1% adapalene gel for use in the treatment of childhood acanthosis nigricans: a pilot study

AIMS

To assess the degree of improvement of neck hyperpigmentation in childhood acanthosis nigricans (AN) after treatment with topical 0.1% adapalene gel and the assessment of localized tissue tolerance to the gel.

SUBJECTS AND METHODS

A split comparison study of the hyperpigmentation on the neck was conducted in patients diagnosed with childhood AN. Patients were treated with topical 0.1% adapalene gel for a period of 4 weeks. The skin color of their neck was evaluated at baseline, 2 weeks and 4 weeks using a skin color chart. Skin color ratio (the skin on their necks compared with the skin on their backs) was calculated for all subjects. The investigator's global evaluation (IGE) scale and the parent's global evaluation (PGE) scale were used to assess the efficacy of the patients' treatment at the end of the 4(th) week.

RESULTS

The mean skin color ratio of therapeutic side was significantly decreased from the baseline scores, at weeks 2 and 4, respectively (30.1%, 18.3%, and 12.9%, P < 0.001), with marked skin improvement at 60.7% ± 28.5%. The percentage of changes of skin color ratio was consistent with the efficacy evaluations as performed by the IGE and PGE scales. Treatment-related cutaneous irritation was minimal, predominantly in the first 2 weeks of treatment and was shown to be well-tolerated at week 4 following a modification of the treatment regimen.

CONCLUSIONS

The study has shown the efficacy of topical 0.1% adapalene gel in the treatment of AN, specifically, in regards to the skin darkening with minimal cutaneous irritation.

Topical retinoids in the management of photodamaged skin: from theory to evidence-based practical approach

Skin, being exposed directly to the environment, represents a unique model for demonstrating the synergistic effects of intrinsic and extrinsic factors on the ageing process. Ultraviolet radiation (UVR) is the major factor among exogenous stressors responsible for premature skin ageing. The problem of skin ageing has captured public attention and has an important social impact. Different therapeutic approaches have been developed to treat cutaneous ageing and to diminish or prevent the negative effects of UVR. Topical retinoids represent an important and powerful class of molecules in the dermatologist's hands for the treatment of photodamaged skin. Since their introduction more than 20 years ago, topical retinoids have shown beneficial efficacy and good safety profiles in the management of photodamaged skin, and as therapeutic anti-ageing agents. This review provides a brief retrospective of the development of topical retinoids in the treatment of photodamaged skin, elucidates their mechanism of action, delineates their use and addresses clinical, pharmaceutical and regulatory issues in connection with their intended use.

Topical retinoids in acne--an evidence-based overview

Topical retinoids are important tools in the management of acne because they act against comedones and microcomedones and have direct anti-inflammatory effects. The substances approved for acne treatment comprise tretinoin (all-trans-retinoic acid), isotretinoin (13-cis retinoic acid) as well as the synthetic third-generation polyaromatic retinoids adapalene and tazarotene, the latter being approved for acne treatment in the US only. Retinaldehyde is used in cosmetic preparations against acne. All topical retinoids are effective as single agents in mild to moderate acne but differ in efficacy and tolerability. Tazarotene 0.1% is more effective than tretinoin 0.025% or 0.1% microsphere gel or adapalene 0.1% gel or cream (EBM-level 2c). Adapalene 0.1% is equally effective to tretinoin 0.025% or tretinoin microsphere 0.1% gel or tretinoin 0.05% cream or isotretinoin 0.05% gel (EBM-level 2c). Adapalene 0.1% gel is significantly better tolerated than tazarotene 0.1% gel, tretinoin 0.025% and tretinoin 0.05% gel, tretinoin 0.05% cream, tretinoin microsphere 0.1% gel or isotretinoin 0.05% gel (EBM-level 2c). The safety profile of topical retinoids differs from their systemic counterparts and is related mainly to local adverse effects, such as erythema, dryness, itching and stinging. The currently available evidence justifies the use of topical retinoids in most types of acne and during maintenance treatment.

Retinoids in the treatment of skin aging: an overview of clinical efficacy and safety

Aging of skin is an intricate biological process consisting of two types. While intrinsic or chronological aging is an inevitable process, photoaging involves the premature aging of skin occurring due to cumulative exposure to ultraviolet radiation. Chronological and photoaging both have clinically differentiable manifestations. Various natural and synthetic retinoids have been explored for the treatment of aging and many of them have shown histological and clinical improvement, but most of the studies have been carried out in patients presenting with photoaged skin. Amongst the retinoids, tretinoin possibly is the most potent and certainly the most widely investigated retinoid for photoaging therapy. Although retinoids show promise in the treatment of skin aging, irritant reactions such as burning, scaling or dermatitis associated with retinoid therapy limit their acceptance by patients. This problem is more prominent with tretinoin and tazarotene whereas other retinoids mainly represented by retinaldehyde and retinol are considerably less irritating. In order to minimize these side effects, various novel drug delivery systems have been developed. In particular, nanoparticles have shown a good potential in improving the stability, tolerability and efficacy of retinoids like tretinoin and retinol. However, more elaborate clinical studies are required to confirm their advantage in the delivery of topical retinoids.

Repeated topical treatment, in contrast to single oral doses, with Vitamin A-containing preparations does not affect plasma concentrations of retinol, retinyl esters or retinoic acids in female subjects of child-bearing age

BACKGROUND

Vitamin A is widely used in cosmetic preparations. Given that oral Vitamin A and its metabolites present a potential reproductive risk, the present study investigated the effect of topical Vitamin A on human endogenous plasma levels of Vitamin A and its metabolites.

METHODS

Two groups of 14 female volunteers of child-bearing age were kept on a Vitamin A-poor diet and treated topically for 21 days with creams containing 0.30% retinol or 0.55% retinyl palmitate on approximately 3000 cm2 of their body surface area, amounting to a total of approximately 30,000 IU Vitamin A/subject/day. After a 12-day wash-out period, the study groups received single oral doses of 10,000 IU or 30,000 IU retinyl palmitate (RP), corresponding to the maximal EU allowance during pregnancy or three-times higher, respectively. Blood samples were collected over 24h on study days -3 (pre-study), 1, 21 (first and last days of topical treatment) and 34 (oral administration) at 0, 1, 2, 4, 6, 8, 12, 14-16 h and 24 h after treatment for determination of plasma concentrations of retinol (REL), retinyl palmitate (RP), oleate (RO) and stearate (RS), 9-cis-, 13-cis-, all-trans- (AT), 13-cis-4-oxo- or AT-4-oxo-retinoic acids (RAs).

RESULTS

With the exception of transient mild (RP-group) to moderate (REL-group) local irritation on the treatment sites, no adverse local or systemic effects were noted. On days 1 or 21 of topical treatment, no changes were measured in individual or group mean plasma Cmax, AUC0-24 h or other pharmacokinetic parameters of REL, retinyl esters or RAs relative to pre-study data. In contrast, single oral doses of RP at 10,000 IU or 30,000 IU produced dose-related and sustained increases in Cmax and AUC0-24 h values of plasma RP, RO, RS, 13-cis- and 13-cis-4-oxo-RAs, as well as a transient increase in AT-RA. In conclusion, our results provide evidence that human topical exposure to retinol- or retinyl ester-containing cosmetic creams at 30,000 IU/day and maximal use concentrations do not affect plasma levels of retinol, retinyl esters or RAs, whereas single oral doses at 10,000 IU or 30,000 IU produce significant increases in plasma retinyl esters and RAs.

Topical vitamin A, or its derivatives, for treating and preventing napkin dermatitis in infants

BACKGROUND

Napkin dermatitis (nappy or diaper rash) is a non-specific term used to describe inflammatory eruptions (rashes) in the napkin area. Most infants develop napkin dermatitis at least once during their infancy. Topical vitamin A has been suggested as a treatment for napkin dermatitis.

OBJECTIVES

To determine if treatment with topical vitamin A is successful in either preventing napkin dermatitis, or producing resolution or decreasing the severity of napkin dermatitis.

SEARCH STRATEGY

We searched the Cochrane Skin Group Specialised Register (May 2005); Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library Issue 3, 2005); Ovid MEDLINE from 1966 to August 2005; EMBASE (2003 to May 2005); Ovid OLDMEDLINE (1950 to 1965); and CINAHL (1982 to August 2005). We also searched reference lists of articles.

SELECTION CRITERIA

Randomised controlled trials, where the topical application of medication containing vitamin A (or its derivatives) was compared with either placebo, no treatment or other topical medication, for the prevention or treatment of napkin dermatitis in infants aged from zero to two years.

DATA COLLECTION AND ANALYSIS

Two authors (AJD and MWD) identified and checked titles and abstracts obtained from the searches, and reviewed the full text where necessary. They decided which trials met the inclusion criteria, and recorded their methodological quality. They assessed studies as either adequate, unclear or inadequate using the following key criteria: (a) randomisation (method of generation and concealment of allocation); (b) blinding; (c) loss to follow-up.

MAIN RESULTS

We did not find any studies for the treatment of napkin dermatitis. We found only one study comparing the use of topical application of medication containing vitamin A, with another topical medication or placebo, to prevent napkin dermatitis. This included study, of 114 newborn infants, reported no significant differences between groups with regard to the severity or duration of napkin dermatitis.

AUTHORS' CONCLUSIONS

For the treatment of napkin dermatitis there is no evidence to support or refute the use of topical vitamin A preparations. For the prevention of napkin dermatitis there is no evidence to suggest that topical vitamin A alters the development of napkin dermatitis. Further RCTs are required to determine whether topical vitamin A is efficacious in treating or preventing napkin dermatitis.

Influence of ion pairing on topical delivery of retinoic acid from microemulsions

The purpose of the present study was to determinate the significance of ion pairing on the topical permeation of retinoic acid (R.A) using microemulsions as delivery vehicles. Phenylalanine methyl ester, phenylalanine ethylester, histidine methyl ester, tryptophan methyl ester and valine methyl ester were used as counter ions. Results of diffusion studies through polydimethylsiloxane membrane (PDMS) indicate that retinoic acid permeation from ethanol-pH 6.4 buffer mixture significantly increased in the presence of counter ions. A linear relationship was found between apparent partition coefficients and permeation coefficients. The highest values were with valine methyl ester and phenylalanine ethyl ester. In order to develop alternative formulations for topical administration of R.A, microemulsions were evaluated as delivery vehicles. Oil-in-water (O/W) and water-in-oil (W/O) microemulsion formulations were prepared using water, isopropyl myristate, lecithin, caprylyl-capryl glucoside and ethanol or 1,2 hexanediol. Experiments with PDMS membranes showed decreasing permeabilities of R.A from microemulsions in the presence of counter ions. This was related to the increased lipophilicity and different vehicle membrane affinity of the ion pairs The ability of the systems to deliver R.A through the skin was evaluated in vitro using pig-skin. R.A permeabilities were much lower with microemulsions than with solution, while a large increase in R.A skin deposition was observed only from O/W microemulsions in the presence of counter ions. The depth of skin accumulation was below 100 microm after 24 h application. The results suggest that O/W microemulsions containing a counter ion can be used to optimise drug targeting without a concomitant increase in systemic absorption.

Developments in topical retinoid therapy for acne

Topical retinoic acid was introduced for acne treatment three decades ago. Since that time, researchers have discovered thousands of retinoids, originally defined as chemical analogs of vitamin A. After the identification of nuclear retinoid receptors in 1987, the definition of this class expanded to include molecules that bind to and activate such receptors. The receptor-selective retinoid agents, adapalene and tazarotene, were developed in the 1990s. Other innovations of the past decade include retinoid formulations and methods aimed at limiting retinoid absorption. Cutaneous irritation may be reduced without losing retinoid efficacy by inhibiting retinoid penetration into the deep epidermis and dermis. Examples include tretinoin in slow-release vehicles and the short-contact method of tazarotene gel therapy. Only trace amounts of adapalene are absorbed after topical application, perhaps explaining its relatively low irritancy. New formulations of existing agents, such as additional concentrations of tretinoin in microsphere gel and cream formulations of tazarotene, are now under investigation for acne. Current research focused on receptor selectivity holds the promise of yielding new retinoid molecules with improved benefits and safety.

Pharmacokinetics and safety of tazarotene

The pharmacokinetic profile of tazarotene helps to ensure that systemic exposure to the drug and its metabolites is minimal. First, percutaneous penetration is limited, with less than 6% of the applied drug being absorbed into the bloodstream. Second, tazarotene is rapidly metabolized into tazarotenic acid and other metabolites that are not lipophilic. Third, tazarotene and its metabolites are rapidly eliminated from the blood in the urine and feces. These three pharmacokinetic features help to ensure that post-treatment plasma levels of tazarotene and its metabolites are comparable to those of endogenous retinoids, which suggests that the risk of teratogenic effects is minimal. Limited systemic exposure to the drug also ensures that any adverse effects are local effects rather than systemic effects. Overall, tazarotene has a good safety profile and is not associated with contact sensitization, phototoxicity, photoallergic reactions, mutagenicity, or carcinogenicity.

Percutaneous absorption of [3H]tretinoin and systemic exposure to mequinol after dermal application of 2% mequinol/0.01% [3H]tretinoin (Solagé) solution in healthy volunteers

Solagé is a combination product composed of 2% mequinol (4-hydroxyanisole) and 0.01% tretinoin (all-trans-retinoic acid) in an ethanolic solution, which is being studied for its safety and efficacy as a topical treatment for disorders of skin hyperpigmentation. The purpose of this study was to evaluate the extent of percutaneous absorption of [3H]tretinoin and to estimate the systemic exposure to mequinol from this combination product when topically applied to the backs of healthy subjects. Eight subjects received bid topical applications of nonradiolabelled 2% mequinol/0.01% tretinoin solution on a 400 cm2 area of the back for 14 days. The subjects then received a single topical application of 2% mequinol/0.01% [3H]tretinoin solution. After 12 h, the radiolabelled dose was removed and bid treatment with nonradiolabelled 2% mequinol/0.01% tretinoin solution was continued for 7 days. Plasma, urine and faecal samples were analysed for total radioactivity and plasma was analysed for both mequinol and tretinoin by GC/MS procedure. Mean percutaneous absorption of [3H]tretinoin based on the cumulative recoveries of radioactivity in the urine and faeces was about 4.5% (median 2.18%). Tretinoin concentrations in plasma did not increase above endogenous levels. This was consistent with the concentrations of radioactivity in plasma, which showed an average Cmax of 91 pg-eq/mL (median 26 ng/mL). Average Cmax and AUC(0-12 h) values for mequinol were 10 ng/mL and 33 ng h/mL, respectively. Based on the results of this study, systemic toxicity from topical application of tretinoin in this formulation is unlikely, because percutaneous absorption of tretinoin is minimal and because endogenous levels of tretinoin are not increased following bid dosing with this combination formulation. The safety of mequinol in this combination formulation is supported by the low systemic exposures of the subjects in this study compared with the systemic exposures at the highest doses in the dermal toxicity studies in mice (16.6-fold) and rats (34.6-fold).

Topical tocoretinate improved hypertrophic scar, skin sclerosis in systemic sclerosis and morphea

Four patients with systemic scleroderma (SSc), 4 patients with morphea, and 4 patients with hypertrophic scar were treated with topical tocoretinate for 6 months to 3 years and studied clinically and histopathologically. Clinically, all of the lesions responded to this therapy. The stiffness of the skin lesions, glossy appearance of the lesions, and telangiectasia improved. Histopathologically, the proliferated collagen fibers decreased in thickness, and the inter-fiber spaces increased. Immunoreactive tenascin-C expressed in the proliferated deep dermal fibers of the SSc and hypertrophic scar lesions was markedly decreased compared with the level before the topical tocoretinate therapy. Topical tocoretinate has been used for the treatment of ulcers; it is also a potent treatment for sclerotic skin diseases.

Pyogenic granuloma-like lesions in a patient using topical tretinoin

A 16-year-old male developed numerous pyogenic granuloma like-lesions across his neck, chest and back after 6 weeks isotretinoin therapy for cystic acne. The isotretinoin was ceased and he was commenced on oral steroids. After 6 weeks, the lesions were almost completely healed. However, due to worsening comedonal acne, the patient was commenced on topical tretinoin cream 0.05% twice daily to his chest. He was reviewed 2 weeks later and, surprisingly, 2 new pyogenic granuloma-like lesions had developed on his chest. These lesions persisted until the topical tretinoin was ceased 3 months later.

Treatment of photodamaged skin with topical tretinoin: an update

The utility of topical tretinoin in combination with sun protection has now been formally established as a useful approach to the treatment of sun-damaged skin. The early observations of our group have been confirmed in numerous well controlled clinical trials. Moreover, a great deal is known about structural and even molecular changes induced by topical tretinoin, which account for the clinical benefits achieved by this agent. A great deal has been learned in a very short period of time, and these findings represent another major use of retinoid therapy in dermatologic disease.

Transdermal absorption of topical anti-acne agents in man; review of clinical pharmacokinetic data

BACKGROUND

Apart from oral drug treatment, drug therapy in acne vulgaris comprises topical treatment with agents with a primarily keratolytic action (e.g. tretinoin and benzoylperoxide), and with antibiotics (clindamycin, erythromycin, and erythromycin-zinc complex). The acne grade in the particular patient usually determines the selection of the preferred route of administration, viz. topical or oral, or a combination of both, and topical treatment is usually preferred in mild to moderate acne. The fact that a topically applied compound may also become systemically available to a quantifiable extent, is not generally considered.

AIM

The present paper reviews the clinical data on transdermal uptake of anti-acne agents in man, also with respect to their relevance for daily clinical practice.

OUTCOME

The majority of published data on transdermal penetration of topical anti-acne agents focuses on the retinoid tretinoin, and on the antimicrobial agent clindamycin. This interest emerges from the fact that these agents have been associated with embryotoxicity/teratogenicity, and pseudomembranous colitis, respectively. For both compounds the extent of systemic availability after topical application is low, viz. 5-7% and 8%, respectively, at its highest. The height and variability in endogenous retinoid levels is very likely to outweigh any contribution of exogenously applied tretinoin, but a full consensus on the safe use of topical tretinoin in pregnancy is still lacking. With respect to clindamycin, the suggested association between its topical use and the occurrence of pseudomembranous colitis appears not to be of clinical relevance. In order to reduce systemic exposure to clindamycin as much as possible, topical application of clindamycin phosphate is to be preferred over clindamycin hydrochloride salt. Regarding other topical anti-acne agents, it has been suggested that topical zinc-erythromycin is to be preferred over erythromycin, both from clinical efficacy and safety viewpoints. With respect to the currently used compounds like benzoylperoxide, azelaic acid, and adapalene, available clinical pharmacokinetic data are scarce, and significant safety concerns did not emerge as yet.

CONCLUSION

The limited transdermal uptake of topical anti-acne agents underpins their safe use in daily clinical practice. With respect to topical retinoids, formal consensus is lacking regarding their use in pregnancy.

A physiologically based pharmacokinetic model for retinoic acid and its metabolites

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.

A risk assessment of topical tretinoin as a potential human developmental toxin based on animal and comparative human data

BACKGROUND

Although topically applied all-trans-retinoic acid (tretinoin) undergoes minimal absorption and adds negligibly to normal endogenous levels, its safety in humans is occasionally questioned because oral ingestion of retinoids at therapeutic levels is known to entail teratogenic risks.

OBJECTIVE

To assess the actual potential for developmental toxicity from treatment with topical tretinoin.

METHODS

Risk assessments were conducted on four known human developmental toxicants (valproic acid, methotrexate, thalidomide, and isotretinoin) and a potential developmental toxicant (acetylsalicylic acid). The margin of safety for each chemical was calculated from the ratio of animal no-observed adverse effect levels to human lowest-observed adverse effect levels or estimated exposure doses.

RESULTS

The derived safety margin of more than 100 for topical tretinoin (with 2% absorption) contrasted sharply with the near unity values for valproic acid, methotrexate, thalidomide, and isotretinoin and was larger than that for acetylsalicylic acid.

CONCLUSION

These data support other epidemiologic and animal data that topical tretinoin is not a potential human developmental toxicant.

Tretinoin: a review of the nonclinical developmental toxicology experience

Tretinoin has been thoroughly evaluated for its potential as an embryofetal developmental toxicant. Oral tretinoin produces developmental anomalies in animal models; the minimal teratogenic dose is consistently 2.5 to 10 mg/kg. In contrast, topical application does not induce developmental malformations in laboratory animals. A structurally related compound, isotretinoin, is a potent toxicant in humans and animals; the lowest systemic dose that induces fetal anomalies varies more than 100-fold depending on the model. Oral isotretinoin is a more potent developmental toxicant than oral tretinoin in monkeys. Between-drug differences in the metabolism and transplacental transfer of the two retinoids account for the differences in toxicant potency. Pharmacokinetic studies reveal that absorption of tretinoin from the skin is poor and yields maternal plasma concentrations below the developmentally toxic threshold established after oral administration. Analysis of outcomes of developmental toxicology and pharmacokinetic studies suggests that the human risk of fetal anomalies is negligible after therapeutic application of topical tretinoin.

Treatment of photodamage with topical tretinoin: an overview

Topical administration of tretinoin has proved to be effective in treating clinical signs of photodamaged skin. In large-scale, double-blind, placebo-controlled, 6-month trials, 0.05% tretinoin emollient cream (Renova, Retinova) reduced fine wrinkles and skin roughness, and it produced histologic changes such as epidermal thickening, increased granular layer thickness, stratum comeum compaction, and decreased melanin content. Smaller changes were also observed at lower tretinoin concentrations. Continued for another 6 months, 0.05% tretinoin emollient cream produced some additional clinical improvement but the histologic changes observed in the epidermis (with the exception of melanin content) regressed toward baseline, raising questions as to what was responsible for the clinical improvement. After 12 months of treatment, there were additional signs of tissue normalization including deposition of new collagen in the papillary dermis and ultrastructural evidence of dermal reconstruction with improvement in the dermoepidermal junction and correction of keratinocyte degeneration, changes that presumably relate directly to tretinoin's mechanism of action. There was no suggestion of cytologic atypia in these studies or in biopsy specimens obtained after up to 4 years of continued use. Mild to moderate dermatitis was the only common adverse reaction to tretinoin use. Percutaneous tretinoin absorption is low, raising plasma levels by amounts that are negligible compared with the normally low endogenous tretinoin levels. No teratogenic effects have been observed in retrospective studies of topical tretinoin application during the first trimester of pregnancy. Thus, topical tretinoin is safe and effective in the treatment of photodamage.

A developmental toxicity study of tretinoin emollient cream (Renova) applied topically to New Zealand white rabbits

BACKGROUND

Embryofetal developmental toxicity associated with oral administration of vitamin A analogs has led to concern about risks from topical application.

OBJECTIVE

This study was conducted to evaluate the potential developmental toxicity of tretinoin emollient cream when applied to the skin of pregnant New Zealand white rabbits during organogenesis (gestational days 7 through 19).

METHODS

Twenty rabbits each were randomly assigned to a control group (group I) or to receive vehicle (group II) or tretinoin emollient cream topically at dosages of 10 (0.05 mg/kg*, group III) or 100 (0.5 mg/kg*, group IV) times that used clinically in humans. Does and fetuses were examined for tretinoin-induced toxic effects, and maternal plasma tretinoin and metabolite levels were measured.

RESULTS

The rate of abortion was increased significantly in does in group IV (p < or = 0.01) compared with the control group. Dosage-dependent increases in incidence and severity of skin reactions occurred in groups administered the vehicle and the two dosages of tretinoin. Does in groups III and IV had clinical and necropsy observations that were considered direct or indirect effects of tretinoin administration, persistent weight loss, and reduced feed consumption. Maternal endogenous plasma tretinoin levels were below the lower limit of quantitation of 5 ng/ml and were not significantly altered with treatment. Group IV had significantly reduced mean fetal body weight (p < or = 0.01) and a greater frequency of resorptions compared with group I. Although external, visceral, or skeletal alterations occurred at significantly greater levels in group III, they were unrelated to tretinoin administration because the fetal incidences were not dosage dependent, and the litter incidence did not significantly differ from the control group values.

CONCLUSION

Maternally toxic dosages of tretinoin were associated with an increased incidence of abortions and resorptions and reduced fetal body weight, two end points of developmental toxicity. Consistent with the absence of detectable tretinoin plasma levels, however, no changes in fetal morphology were attributable to tretinoin administration. *The milligrams per kilogram dosage refers to the amount of active ingredient (tretinoin). The 0.05 mg/kg and 0.5 mg/kg groups were treated with 0.005% and 0.05% wt/wt tretinoin emollient cream formulation. The 0.05% tretinoin emollient cream is the Renova clinical formulation. The 10 and 100 times clinical multiples refer to Renova clinical multiples and are based on a 50 kg adult patient's applying 500 mg of 0.05% tretinoin emollient cream formulation daily to yield a clinical dosage of 0.005 mg/kg.