Paul A. Lehman

Use of excised human skin to assess the bioequivalence of topical products.

Background: Establishing the bioequivalence of topical drug products is a costly and time-consuming process since, with few exceptions, clinical efficacy trials are required. Objective: To develop a surrogate for clinical bioequivalence testing through evaluation of the kinetics of drug absorption in vitro through excised human skin. Methods: The percutaneous absorption of seven approved generic topical drug products was compared with their corresponding reference products during preclinical development using the Franz diffusion cell. Thereafter, following the conduct of bioequivalence trials and regulatory approval of these products in the United States, clinical data became available to which the in vitro data were compared. Results: In six of the seven cases the in vitro test:reference ratio for total absorption was close to one and indicated that the products were equivalent, in agreement with the clinical data. Results from the seventh case, in which the test:reference ratio was only 0.63, indicated that the in vitro model actually had greater sensitivity than the clinical method to detect small differences between products. Conclusion: These data demonstrate the relevance and predictive power of the in vitro human skin model and strongly support its use as a surrogate for in vivo bioequivalence studies.

Percutaneous absorption in man: in vitro-in vivo correlation.

Aims: To examine the existing literature to determine the degree to which percutaneous absorption data obtained using the excised human skin model match those obtained from living man. Methods: The scientific literature was reviewed to collect data on compounds whose percutaneous absorption through human skin had been measured under both in vitro and in vivo conditions. The in vitro-in vivo (IVIV) correlation was evaluated by computing the in vitro/in vivo ratio using total absorption (percent of applied dose) as the metric for comparison. Results: A total of 92 data sets were collected from 30 published studies. The average IVIV ratio across all values was 1.6, though for any single data set there could be a nearly 20-fold difference between the in vitro and in vivo values. In 85% of the cases, however, the difference was less than 3-fold. The correlation was significantly improved when data were excluded from studies in which the protocols for both studies were not fully harmonized. For harmonized data sets the average IVIV ratio was 0.96 and there was a less than 2-fold difference between the in vitro and in vivo results for any one compound, with IVIV ratios ranging from 0.58 to 1.28. The dominant factors leading to exclusion of data were the use of skin from different anatomical sites and vehicles of differing composition. Conclusions: Percutaneous absorption data obtained from the excised human skin model closely approximate those obtained from living man when the two study protocols are appropriately matched.

Pharmacokinetics-Based Approaches for Bioequivalence Evaluation of Topical Dermatological Drug Products

The pharmacokinetic approach has accelerated the development of high-quality generic medicines with extraordinary cost savings, transforming the pharmaceutical industry and healthcare system in the USA. While this is true for systemically absorbed drug products, the availability of generic versions of topical dermatological products remains constrained due to the limited methods accepted for bioequivalence evaluation of these products. The current review explores the possibility of developing appropriate bioequivalence approaches based on pharmacokinetic principles for topical dermatological products. This review focuses on the strengths and limitations of the three most promising pharmacokinetics-based methods to evaluate the performance and bioequivalence of topical dermatological products, which include in vivo skin stripping, in vivo microdialysis, and in vitro permeation testing (IVPT) with excised human skin. It is hoped that recent advances in pharmaceutical and regulatory science will facilitate the development of robust bioequivalence approaches for these dosage forms, enable more efficient methodologies to compare the performance of new drug products in certain pre-approval or post-approval change situations, and promote the availability of high-quality generic versions of topical dermatological products.

Quantitative vasoconstrictor assay for topical corticosteroids: The puzzling case of fluocinolone acetonide

BACKGROUND The validity of the vasoconstrictor test has been questioned because of the apparent nonequivalence of some generic corticosteroid products. OBJECTIVE Our purpose was to determine the vasoconstrictor activity of a generic 0.025% fluocinolone acetonide ointment by two different methods, dilution by volume and dilution by area of application. METHODS Vasoconstrictor activity on a visual 4-point scale was measured in 20 volunteers. Topical fluocinolone acetonide ointment (0.025%) was tested by comparing dilution by volume (1:1, 1:3, and 1:7) in petrolatum and dilution by area of application. Fluocinolone release from the diluted ointments was also measured with an in vitro method. RESULTS Dilution by area gave the expected results, a decreasing average vasoconstrictor score; but dilution by volume did not. Both the 1:1 and 1:3 dilutions inexplicably showed greater activity than the full-strength product; this finding was similarly reflected in the in vitro studies that showed a greater rate of drug release from the 1:1 and 1:3 dilutions. CONCLUSION Dilution by area provides a useful quantitative assay for comparative vasoconstrictor activity.

On Topical Timolol Gel-Forming Solution for Infantile Hemangiomas

To the Editor: We read with interest the recent commentary by McMahon et al (1) entitled “Topical Timolol for Infantile Hemangiomas: Putting a Note of Caution in ‘Cautiously Optimistic.’ We applaud these important efforts to estimate the systemic absorption of topical timolol gel-forming solution (GFS) and to determine equivalent oral propranolol doses, but we think that the article is overly cautious, particularly regarding the estimated amount of timolol that is absorbed and available to have systemic effects, as well as the recommendation to check heart rates of infants using timolol GFS for hemangiomas. In their calculations, McMahon et al (1) estimate the bioavailability of topical timolol GFS as being between 50% and 80%. We believe that the bioavailabilities that McMahon et al (1) used in their analyses were significantly overestimated, which explains the troubling calculations they reported. Their estimates were based on two studies, including one by Korte et al (2) evaluating the systemic effects after ocular, not cutaneous, delivery and one by Vlasses et al (3) involving a 30-hour nonocclusive transdermal patch consisting of polyethylene glycol in a mineral oil vehicle subsequently covered with a nonocclusive plastic dressing. In the latter study, the estimated bioavailability was not based on plasma timolol levels but rather on measuring the amount of timolol remaining in the ointment that was removed after 30 hours. In this transdermal system, the stratum corneum and viable epidermis may be a depot for the drug, and the authors did not account for the amount of drug that could remain in and on the skin after vehicle removal. Timolol GFS uses a water-soluble vehicle with xanthan gum. The vehicle in the topical study by Vlasses et al (3) was 5% polyethylene glycol in mineral oil, which formed an oil-based gel. It is not correct to compare the percutaneous absorption of timolol GFS with that of timolol in mineral oil without first assessing their percutaneous absorption kinetics. Also, the surface area of the product used by Vlasses et al (3) (25 cm) is significantly larger than the surface area that one drop (0.05 mL) of timolol GFS covers. In the study by Vlasses et al (3), 30 to 60 mg/25 cm was applied, which is equivalent to 1.2 to 2.4 mg/cm, whereas with timolol GFS, only 0.125 to 0.25 mg/cm is applied, assuming that one drop covers 1 to 2 cm. The greater surface area and applied dose that Vlasses et al (3) used would probably lead to greater absorption. Finally, another thing to consider is the gel properties when timolol GFS is applied to skin. The GFS solution is more viscous than regular eye drops and is designed for gel formation when the drop meets enzymes in tears. What happens on the skin in the absence of these enzymes is less clear. In general, the percutaneous systemic absorption of most topical drugs is small—only a small percentage (4,5).Using a revised andprobably still overly cautious estimate of the systemic bioavailability of topical timololGFSof 5% to 10%would greatly alter Table 1 in their article, yielding equivalent oral propranolol doses of only 0.2 to 1.0 mg, or 0.05 to 0.25 mg/kg for a 4-kg infant (0.03–0.17 mg/kg for a 6-kg infant, i.e., a 3-month-old in the 50th percentile for weight). These estimates suggest far less need for caution. Even an estimated systemic bioavailability of 25% would yield equivalent oral propranolol doses of only 1 to 2.5 mg, or 0.25 to 0.63 mg/kg for a 4-kg infant (0.17– 0.43 for a 6-kg infant). Moreover, it is important to distinguish between systemic and local bioavailability. Systemic bioavailability does not apply well to topical medicines because topical medicines rarely lead to significant plasma levels, as seen with rapid gastrointestinal absorption, because they are designed for slow cutaneous delivery for local skin effects. In light of the widespread, off-label use of topical timolol GFS for infantile hemangiomas and its still poorly defined topical pharmacokinetics, we agree withMcMahon et al (1) that it is important to proceed with caution and encourage clinical trials. In particular, it is prudent to be cautious when using topical timolol GFS on large, ulcerated, or mucosal or perimucosal hemangiomas, where the skin barrier function is compromised and when occlusion is likely (e.g., diaper hemangiomas); almost all patients with these lesions should be treated with oral propranolol instead. It is reassuring that no side effects other than one questionable case of sleep disturbance (6) have been reported despite widespread use of topical timolol for hemangiomas over the past 4 years. As McMahon et al (1) note, the literature is still sparse, although no serious side effects have been reported in more than 100 cases in multiple series (6–12). At the St. Louis Letters to the Editor are welcomed for publication (subject to editing). Letters must be signed by all authors, and must not exceed two pages of text including references. Letters should not duplicate material submitted or published in other journals. Prepublication proofs will not be provided.

Assessing Topical Bioavailability and Bioequivalence: A Comparison of the In vitro Permeation Test and the Vasoconstrictor Assay

PurposeTo compare the sensitivity of a pharmacokinetic assay, the in vitro permeation test (IVPT), with that of a pharmacodynamic assay, the human skin blanching or vasoconstrictor (VC) assay, in assessing the relative bioavailability of topical clobetasol propionate products.MethodsThe percutaneous absorption of clobetasol propionate from five commercial products was measured in vitro using cryopreserved human skin. The pharmacodynamic potency of the same five products was also assessed in vivo using the VC assay, the surrogate method by which regulatory authorities in the United States establish the bioequivalence of generic topical glucocorticoid products.ResultsIVPT found total clobetasol absorption varying ten-fold from highest to lowest product, whereas the VC assay found this same difference was less than two-fold. The coefficient of variation ranged from 78 to 126% in the VC assay, but only 30–43% for IVPT. Statistically, IVPT could separate the 5 products into three groups: 1) ointment, 2) cream and gel, 3) emollient cream and solution). Due to its greater variability as well as saturation of the pharmacodynamic response at higher flux levels, the VC assay found all products except the solution to be equipotent.ConclusionsIVPT was found to be substantially more sensitive and less variable than the VC assay for assessing clobetasol bioavailability.

A Simplified Approach for Estimating Skin Permeation Parameters from In Vitro Finite Dose Absorption Studies

Historically, percutaneous absorption permeation parameters have been derived from in vitro infinite dose studies, yet there is uncertainty in their accuracy if the applied vehicle saturates or damages the stratum corneum, or when the permeation parameters are inappropriately derived from cumulative absorption data. An approach is provided for determining penetration parameters from in vitro finite dose data. Key variables, and equations for their derivation, are identified from the literature and provide permeation parameters that use only Tmax , AUC, and AUMC from finite dose data. The equations are tested with computer-generated model data and to actual study data. Derived permeation parameters obtained from the computer model data match those used in generating the simulated finite dose data. Parameters obtained from actual study data reasonably and acceptably model the penetration profile kinetics of the study data. From in vitro finite dose absorption data, three parameters can be obtained: the diffusion transit time (td ), which characterizes the diffusion coefficient, the partition volume (Vm P), which characterizes the partition coefficient, and the permeation coefficient (Kp ). These parameters can be obtained from finite dose data without having to know the length of the diffusion pathway through the membrane.

Evaluation of creams and ointments as suitable formulations for peldesine

In-vitro studies were conducted to study the efficacy of mixed and self-emulsifying creams and hydrophobic ointment formulations in delivering peldesine (BCX-34) into and across cryopreserved human cadaver skin (HCS). Oil-in-water cream formulations, containing 1% w/w of radiolabeled C(14) BCX-34 and propylene glycol (PG), glycerin (GLY), isopropyl myristate (IPM), oleic acid (OA) and capric-caprylic esters (CE) were prepared. Petrolatum and lanolin based ointments were also prepared with PG. Sections of the HCS, 250 microm thick, were fitted to vertical Franz diffusion chambers containing a receptor medium of pH 7.4 phosphate buffer solution maintained at 37 degrees C. Using the finite dose technique, 4-6 mg of a formulation sample was applied to the epidermal surface of each section and drug diffusion was permitted for 12 and 24 h periods. The distribution of drug into the HCS epidermis, dermis and into the receptor medium was measured by scintillation spectroscopy. The results show good correlation of the calculated in-vitro values for flux and skin-vehicle partition coefficients against the observed amounts of drug detected in the HCS. The mixed emulsion cream formulation containing PG delivered higher amounts of drug into the skin when compared to the same formulation containing GLY cream. The self-emulsifying cream formulation containing IPM had a higher skin-vehicle partition coefficient and delivered more drug into the dermis when compared to those formulations that contained OA and CE. The petrolatum ointment delivered six times more drug into the epidermis than the lanolin ointment, and had higher skin-vehicle partition values. In conclusion, creams containing PG and petrolatum-base formulations would be suitable for BCX-34 dermal delivery.

Effect of Induced Acute Diabetes and Insulin Therapy on Stratum Corneum Barrier Function in Rat Skin

Background: A wide variety of cutaneous manifestations are associated with diabetes. However, there is a paucity of information on stratum corneum barrier function in diabetics, with and without insulin therapy. Methods: To assess for alteration of the stratum corneum, its barrier function was tested by evaluating the percutaneous absorption of water, ethanol, lidocaine and hydrocortisone, in vitro, on normal control, 4-week diabetic and 8-day insulin-treated diabetic Sprague-Dawley CD rats. Results: Total water penetration was not different between the 3 groups though flux profiles were different. Both total penetration and peak flux of lidocaine and hydrocortisone increased slightly in the diabetic rats over the control group. However, total penetration and peak flux (including ethanol) were significantly increased in the insulin-treated rats. Conclusion: The data indicate that diabetes modestly alters stratum corneum physiology but less so than that seen following insulin therapy.

Percutaneous absorption and metabolism of lonapalene in psoriatic skin

The percutaneous absorption and metabolism of lonapalene (6-chloro-2,3-dimethoxynaphthalene-1,4-diol-diacetate; RS-43179), a topically effective 5-lipoxygenase inhibitor, has been measured in six subjects with stable plaque-type psoriasis of the lower extremities. Lonapalene readily penetrates psoriatic skin, is rapidly and completely metabolized, and is almost entirely excreted in the urine. Unexpectedly we observed a trend for thigh (T) plaque skin to be more permeable than lower leg (LL) plaque skin as measured by total absorption (T, 44.8 ± 13.4%; LL, 24.9 ± 12.6% applied dose excreted), peak plasma levels (T, 209 ± 107; LL, 146 ± 81 ng Eq/ ml), and peak rate of urinary excretion (T, 591.7 ± 112.2; LL, 318.4 ± 143.9 µg Eq/hr). There were also differences in the metabolic profiles between the two sites as measured by the quantity and proportion of dealkylated and conjugated products excreted in the urine.