Bioequivalence for Topical Products—An Update

Investigation of the suitability of confocal Raman spectroscopy for the demonstration of bioequivalence of topical products

Bioequivalence studies of topical formulations have attracted increased interest as the European Medicines Agencies "Guideline on quality and equivalence on locally applied, locally acting cutaneous products" describes them in the context of the approval of generics. Since the guideline only proposes tape stripping as a destructive method for bioequivalence testing in in vitro skin penetration, the aim of this study was to investigate the suitability of confocal Raman spectroscopy (CRS) as a non-destructive alternative. To validate the CRS results, tape stripping and CRS experiments using ketoprofen as a model API were performed consecutively on the same samples of ex vivo porcine skin after frozen storage and compared. All experiments were also carried out on two different animal test subjects to investigate a possible influence of inter-individual variations. Furthermore, the influence of the chosen incubation times inside and outside of the steady state was determined. We were able to show that CRS and tape stripping results were very similar both in overall detected API amounts as well as in the results of bioequivalence testing, proving CRS is not only suitable for quantitative skin penetration experiments but also for bioequivalence testing. Inter-individual variations were found to be relevant when comparing formulations measured on different subjects. Bioequivalence testing however reached the same results on both subjects. Finally, the chosen incubation time was limited by skin disintegration, the reaching of steady state however did not influence the results of bioequivalence testing.

In vitro studies into establishing therapeutic bioequivalence of complex topical products: Weight of evidence

Over the past decade, topically applied drug products have experienced extraordinary price increases, due to the shortage of multisource generic drug products. This occurrence is mainly related to the underlying challenges evolved in topical bioequivalence documentation. Although there has been continuing regulatory efforts to present surrogate in vitro methods to clinical endpoint studies, there is still a continued need for cost- and time-efficient alternatives that account for product specificities. Hence, this work intended to expose bioequivalence assessment issues for complex topical formulations, and more specifically those related with product efficacy guidance. As a model drug and product, a bifonazole 10 mg/g cream formulation was selected and two different batches of the commercially available Reference Product (RP) were used: RP1 that displayed lower viscosity and RP4 which presented high, but not the highest, viscosity. In vitro human skin permeation testing (IVPT) was carried out and the results were evaluated by means of the traditional bioequivalence assessment approach proposed by the EMA, as well as by the Scaled Average Bioequivalence assessment approach proposed by the FDA. Based on previous experience, there was an expectation of a high level of variability in the results, thus alternative methods to evaluate local drug skin availability were developed. More specifically, an infected skin disease model, where ex vivo human skin was infected and ATP levels were used as a biological marker for monitoring antifungal activity after product application. The results showed that permeation equivalence could not be supported between the different RP batches. In contrast, this statistical difference between the formulation batches was not indicated in the disease model. Nevertheless, in pivotal IVPT studies, the lowest permeant formulation (RP4) evidenced a higher antifungal in vitro activity as reported by the lower levels of ATP. A critical appraisal of the results is likewise presented, focusing on an outlook of the real applicability of the regulatory guidances on this subject.

Update on the advances and challenges in bioequivalence testing methods for complex topical generic products

Most of the government regulatory agencies, including the United States Food and Drug Administration and the European Medicine Agency, demand that the generic complex topical products prove pharmaceutical and bioequivalence. The evaluation of bioequivalence for complex topical dermatological formulations is a challenging task that requires careful consideration of several factors. Although comparative clinical studies are still considered the gold standard approach for establishing bioequivalence in most formulations, these studies can be costly and insensitive to detect formulation differences. Therefore, significant efforts have been made to develop and validate alternative approaches that demonstrate bioequivalence and expedite the availability of high-quality generic topical dermatological products. This article reviews the current methods for determining the bioequivalence of topical formulations in humans, with particular emphasis on recent advances in these methodologies. Most of the alternative methods are sensitive and reproducible, with the capability to ease the financial burden of comparative clinical studies within a short delivery time. The limitations associated with each technique are reviewed in detail.

Confocal Raman Spectroscopy for Assessing Bioequivalence of Topical Formulations

The evaluation of bioequivalence (BE) for topical dermatological drug products is challenging, and there has been significant interest from regulatory authorities in developing new BE methodologies in recent years. Currently, BE is demonstrated by comparative clinical endpoint studies; these are costly and time-consuming and often lack sensitivity and reproducibility. Previously, we reported excellent correlations between in vivo Confocal Raman Spectroscopy in human subjects and in vitro skin permeation testing (IVPT) with the human epidermis for skin delivery of ibuprofen and a number of excipients. The aim of the present proof-of-concept study was to evaluate CRS as a method to assess BE of topical products. Two commercially available formulations, Nurofen Max Strength 10% Gel and Ibuleve Speed Relief Max Strength 10% Gel, were selected for evaluation. Delivery of ibuprofen (IBU) to the skin was determined in vitro and in vivo by IVPT and CRS, respectively. The formulations examined were found to deliver comparable amounts of IBU across the skin over 24 h in vitro (p > 0.05). Additionally, the formulations resulted in similar skin uptake values measured with CRS in vivo, either at 1 h or 2 h after application (p > 0.05). This is the first study to report the capability of CRS for the demonstration of BE of dermal products. Future studies will focus on the standardisation of the CRS methodology for a robust and reproducible pharmacokinetic (PK)-based evaluation of topical BE.

Franz Cell Diffusion Testing and Quantitative Confocal Raman Spectroscopy: In Vitro-In Vivo Correlation

Previously, we reported the use of Confocal Raman Spectroscopy (CRS) to investigate the topical delivery of actives and excipients. We have also correlated the results from CRS with findings from in vitro diffusion studies in human skin. However, until now CRS has only been used as a semi-quantitative method of determining the skin uptake of molecules, with results expressed as arbitrary units of signal intensity. Clearly, this posed challenges for using CRS to determine skin delivery and to assess the drug bioavailability and bioequivalence of topical formulations. In the present work, the permeation of niacinamide (NIA) from various formulations in human skin was studied in vitro using conventional Franz cells and in vivo using a quantitative CRS method under finite dose conditions. The selection of NIA was based on its wide use in pharmaceutical and personal care formulations for many years. This is the first fully quantitative study to compare these methods. The vehicles investigated were neat Transcutol^® P (TC); binary combinations of propylene glycol (PG) with propylene glycol monolaurate (PGML); and ternary mixtures of PG, PGML, and isopropyl myristate (IPM). These solvents were selected to encompass a range of physicochemical properties. NIA permeation was evident from all formulations in vitro and in vivo. The vehicles PG:PGML and PG:PGML:IPM delivered comparable amounts across the skin in vitro at 24 h (100.3–106.7 µg/cm², p > 0.05) that were significantly higher compared with those of TC (1.3 µg/cm², p < 0.05). An excellent in vitro in vivo correlation (R² = 0.98) was found following the linear regression of the cumulative amounts of NIA permeated in vitro and the amounts of NIA at 2 μm in the skin measured with CRS. A very good correlation between the cumulative permeation of NIA in vitro and the total amount of NIA that penetrated the stratum corneum (SC) per unit of surface area (μg/cm²) in vivo was also observed, with a Pearson correlation coefficient (R²) of 0.94. The findings support the use of CRS for the quantitative measurement of actives delivered to the skin in vivo. Future studies will focus on exploring the reproducibility and reliability of the method by investigating the delivery of different actives from a wider range of vehicles. Additionally, quantitative CRS will be evaluated further as a method for assessing the bioequivalence of topical formulations.

Stratum Corneum Sampling to Assess Bioequivalence between Topical Acyclovir Products

PURPOSE

To examine the potential of stratum corneum (SC) sampling via tape-stripping in humans to assess bioequivalence of topical acyclovir drug products, and to explore the potential value of alternative metrics of local skin bioavailability calculable from SC sampling experiments.

METHODS

Three acyclovir creams were considered in two separate studies in which drug amounts in the SC after uptake and clearance periods were measured and used to assess bioequivalence. In each study, a "reference" formulation (evaluated twice) was compared to the "test" in 10 subjects. Each application site was replicated to achieve greater statistical power with fewer volunteers.

RESULTS

SC sampling revealed similarities and differences between products consistent with results from other surrogate bioequivalence measures, including dermal open-flow microperfusion experiments. Further analysis of the tape-stripping data permitted acyclovir flux into the viable skin to be deduced and drug concentration in that 'compartment' to be estimated.

CONCLUSIONS

Acyclovir quantities determined in the SC, following a single-time point uptake and clearance protocol, can be judiciously used both to objectively compare product performance in vivo and to assess delivery of the active into skin tissue below the barrier, thereby permitting local concentrations at or near to the site of action to be determined.

Cutaneous Biodistribution: A High-Resolution Methodology to Assess Bioequivalence in Topical Skin Delivery

A draft guideline from the European Medicines Agency (EMA) highlights the need for methods to assess the quality/equivalence of topical drug formulations. The “cutaneous biodistribution method”, which provides insight into a drug’s spatial distribution in the epidermis/dermis, was used to compare cutaneous bioavailability of econazole nitrate (ECZ) from a reference medicinal product (RMP) and two approved bioequivalent generic creams under finite dose conditions. Statistically significant differences between the ECZ biodistributions from the RMP/Generics were determined and used with acceptance criteria based on those from the EMA to evaluate bioequivalence. In porcine skin, ECZ deposition in total skin, epidermis, upper and lower dermis from Generic 1 was within the acceptance interval, contrary to Generic 2, which was marginally below it. For human skin, Generic 1 deposition was marginally above the acceptance interval and not bioequivalent. The results were consistent with those using the EMA’s acceptance intervals using the ratio of the mean ECZ depositions of Generic 1 and the RMP. Differences identified using this data-rich technique may not translate to observable differences in clinical efficacy; however, generics with non-statistically different biodistributions to the RMP should have a comparable clinical effect. The cutaneous biodistribution method could benchmark the development of topical generic products.

aQbD as a platform for IVRT method development - A regulatory oriented approach

The EMA draft guideline on quality and equivalence of topical products and the FDA non-binding product specific guidances release has encouraged the establishment of a regulatory background for in vitro release testing (IVRT). Herein, a novel framework applicable to the development of a discriminatory IVRT method is described, according to analytical quality by design (aQbD) principles. A commercially available diclofenac emulgel formulation was used as model product. Through the definition of IVRT analytical target profile, a risk assessment analysis was carried out, in which the critical analytical attributes (in vitro release rate, cumulative amount released at an initial/final point and dose depletion) and critical method variables (medium, membrane and dosage regimen) were identified. Based on this information, a 3 × 2 × 3 full factorial design was performed. Statistical modeling and system desirability assessment enabled the selection of the most suitable IVRT parameters, which were fully validated according with new EMA requirements. These consisted of PBS:Ethanol (80:20, pH = 7.4), Tuffryn membranes and 300 mg of applied product. aQbD provided a comprehensive framework for developing a reliable and effective IVRT method. A thorough analysis of the new EMA draft guideline requirements revealed that some of the established criteria may be challenging to attain.

Bioequivalence of topical generic products. Part 1: Where are we now?

Regulatory accepted methods for bioequivalence assessment of topical generic products generally involve long and expensive clinical endpoint studies. The only alternative relies on pharmacodynamic trials, solely applicable to corticosteroids. Considerable efforts have been channeled towards the development and validation of other analytical surrogates. The majority of these alternative methods rely on in vitro methodologies that allow a more sensitive and reproducible bioequivalence assessment, avoiding at the same time the financial burden that deeply characterizes clinical trials. The development and validation of these methods represent interesting areas of opportunities for generic drugs, since by enabling faster submission and approval processes, an enlargement of topical drug products with generic version is more easily attainable. This review aims to present a critical discussion of the most promising alternative methods, with particular emphasis on in vitro permeation studies and near infrared spectroscopy studies. Since the last technique is not broadly forecast as a bioequivalence assessment tool, its suitability is assessed by a careful analysis of patents that claim the use of NIR radiation in the skin. In fact, the extensive coverage of the devices that use this technology highlights its applicability towards a better understanding of the mechanism underlying topical drug delivery.

Bioequivalence of topical generic products. Part 2. Paving the way to a tailored regulatory system

Hitherto, for the approval of a topical generic drug product, the majority of the regulatory agencies require clinical endpoint studies to prove its therapeutic equivalence in relation to a reference product. Pharmacodynamic studies are also available to support bioequivalence, however, these are solely applicable for corticosteroids. The first strategy is considered the "gold standard", since it can be applied to all drug products. Nevertheless, the high variability intrinsic to topical drug delivery makes this analysis relatively insensitive, costly, time-consuming, besides requiring a large number of subjects. There are, however, alternative methods capable of providing a more rigorous analysis and requiring a lower cost. Amongst them, in vitro methods have sparked considerable attention, not only in the academic field, but also in the pharmaceutical industry and regulatory agencies. In this context, this review attempts to discuss the main regulatory constraints and the recent advances in the regulatory science of topical generic drugs bioequivalence assessment. Initiatives, such as the Strawman decision tree and the topical drug classification system are specially referred, since these highlight the importance of establishing a broader concept of pharmaceutical equivalence for topical generic drugs, similar to the one already set for orally administered conventional dosage forms, such as tablets and capsules. Finally, the FDA Product-Specific Guidances for Generic Drug Development released for topical products in recent years and particular European Public Assessment Reports are presented and discussed, to illustrate the change of paradigm which is occurring in this regulatory field.

Application of an Optimized Tape Stripping Method for the Bioequivalence Assessment of Topical Acyclovir Creams

This study indicates the application of tape stripping (TS) for bioequivalence (BE) assessment of a topical cream product containing 5% acyclovir. A TS method, previously used successfully to assess BE of topical clobetasol propionate and clotrimazole formulations, was used to assess BE of an acyclovir cream (5%) formulation as well as a diluted acyclovir formulation (1.5%) applied to the skin of healthy humans. An appropriate application time was established by conducting a dose duration study using the innovator product, Zovirax® cream. Transepidermal water loss was measured and used to normalize thicknesses between subjects. The area under the curve (AUC) from a plot of amount of acyclovir/strip vs cumulative fraction of stratum corneum (SC) removed was calculated for each application site. BE was assessed using Fieller's theorem in accordance with FDA's guidance for assessment of BE of topical corticosteroids. Adco-acyclovir cream (5%) was found to be BE to Zovirax® cream, where the mean test/reference (T/R) ratio of the AUC's was 0.96 and the bioequivalence interval using a 90% confidence interval was 0.91-1.01 with a statistical power > 95%, whereas the diluted test product fell outside the BE acceptance criteria with T/R ratio of AUC of 0.23 and a 90% CI of 0.20-0.26. This study indicates that the data resulting from the application of this TS procedure has reinforced the potential for its use to assess BE of topical drug products intended for local action, thereby obviating the necessity to undertake clinical trials in patients.

Skin models for the testing of transdermal drugs

The assessment of percutaneous permeation of molecules is a key step in the evaluation of dermal or transdermal delivery systems. If the drugs are intended for delivery to humans, the most appropriate setting in which to do the assessment is the in vivo human. However, this may not be possible for ethical, practical, or economic reasons, particularly in the early phases of development. It is thus necessary to find alternative methods using accessible and reproducible surrogates for in vivo human skin. A range of models has been developed, including ex vivo human skin, usually obtained from cadavers or plastic surgery patients, ex vivo animal skin, and artificial or reconstructed skin models. Increasingly, largely driven by regulatory authorities and industry, there is a focus on developing standardized techniques and protocols. With this comes the need to demonstrate that the surrogate models produce results that correlate with those from in vivo human studies and that they can be used to show bioequivalence of different topical products. This review discusses the alternative skin models that have been developed as surrogates for normal and diseased skin and examines the concepts of using model systems for in vitro–in vivo correlation and the demonstration of bioequivalence.

[Bioequivalence of dermatological topical medicines:the Brazilian scenario and the challenges for health surveillance]

The comparative evaluation required for the registration of generic topical medicines in Brazil is conducted by means of a pharmaceutical equivalence study, which merely assesses the physical/chemical and microbiological parameters of the formulations. At the international level, clinical or pharmacodynamic studies are now being required to prove the efficacy and safety of semisolid topical generic formulations. This work presents a comparison of the different requirements for the registration of topical formulations, taking into consideration the various regulatory authorities, and presents a survey of topical medicines registered in Brazil prior to 2013. The survey revealed that in comparison with the USA there were many more copies of these formulations registered in Brazil. This fact, together with the large number of studies in the literature showing the lack of bioequivalence of topical medication, is clear proof of the major importance of the need to realign Brazilian legislation with respect to the technical requirements for the registration of generic and similar medication for dermatological topical application in Brazil.

Survey of international regulatory bioequivalence recommendations for approval of generic topical dermatological drug products

The objective of this article is to discuss the similarities and differences in accepted bioequivalence (BE) approaches for generic topical dermatological drug products between international regulatory authorities and organizations. These drug products are locally applied and not intended for systemic absorption. Therefore, the BE approaches which serve as surrogates to establish safety and efficacy for topical dosage forms tend to differ from the traditional solid oral dosage forms. We focused on 15 different international jurisdictions and organizations that currently participate in the International Generic Drug Regulators Pilot Project. These are Australia, Brazil, Canada, China, Chinese Taipei, the European Medicines Association (EMA), Japan, Mexico, New Zealand, Singapore (a member of the Association of Southeast Asian Nations), South Africa, South Korea, Switzerland, the USA and the World Health Organization (WHO). Upon evaluation, we observed that currently only Canada, the EMA, Japan, and the USA have specific guidance documents for topical drug products. Across all jurisdictions and organizations, the three approaches consistently required are (1) BE studies with clinical endpoints for most topical drug products; (2) in vivo pharmacodynamic studies, in particular the vasoconstrictor assay for topical corticosteroids; and (3) waivers from BE study requirements for topical solutions. Japan, South Africa, the USA, and the WHO are also making strides to accept other BE approaches such as in vivo pharmacokinetic studies for BE assessment, in vivo dermatopharmacokinetic studies and/or BE studies with in vitro endpoints.

Development of performance matrix for generic product equivalence of acyclovir topical creams

The effect of process variability on physicochemical characteristics and in vitro performance of qualitatively (Q1) and quantitatively (Q2) equivalent generic acyclovir topical dermatological creams was investigated to develop a matrix of standards for determining their in vitro bioequivalence with reference listed drug (RLD) product (Zovirax®). A fractional factorial design of experiment (DOE) with triplicate center point was used to create 11 acyclovir cream formulations with manufacturing variables such as pH of aqueous phase, emulsification time, homogenization speed, and emulsification temperature. Three more formulations (F-12-F-14) with drug particle size representing RLD were also prepared where the pH of the final product was adjusted. The formulations were subjected to physicochemical characterization (drug particle size, spreadability, viscosity, pH, and drug concentration in aqueous phase) and in vitro drug release studies against RLD. The results demonstrated that DOE formulations were structurally and functionally (e.g., drug release) similar (Q3) to RLD. Moreover, in vitro drug permeation studies showed that extent of drug bioavailability/retention in human epidermis from F-12-F-14 were similar to RLD, although differed in rate of permeation. The results suggested generic acyclovir creams can be manufactured to obtain identical performance as that of RLD with Q1/Q2/Q3.

Skin uptake and clearance of ciclopirox following topical application

The assessment of skin uptake and clearance are important to determine the efficiency and systemic safety of dermatological formulations. The objective of this study was to assess the skin uptake, clearance and possible systemic delivery of ciclopirox following topical application in Wistar rats. In vitro studies (3 h) were carried out in excised pig skin to assess the permeation and retention capacity of ciclopirox in skin layers using gel formulations (1% and 2% w/v). In vivo dermatopharmacokinetics (DPK) parameters were determined by measuring the drug levels in the skin as a function of time post application (0.5, 1, 1.5 and 2 h) and post removal (3, 4, 6 and 8 h) of the formulation in Wistar rats. The plasma drug concentrations were also determined in the same animals. In vitro data indicate the low permeability and high retention of ciclopirox in the stratum corneum. The DPK data observed indicate a higher Cmax value (175.43 ± 25.62 μg/cm2) and AUC (632.14 ± 102.26 μg.h/cm2) with the 2% (w/v) gel formulation. Further, the skin elimination of ciclopirox follows first order kinetics with a short half-life (t1/2 ~2 h). The fraction of drug reaching the systemic circulation was found to be significantly low (~0.15% of the applied dose). A relation between the drug concentration in the skin layers and the plasma was observed with a short lag period. The topical availability of ciclopirox was found to be relatively low and endured rapid clearance with minimal systemic uptake.

Generic development of topical dermatologic products: formulation development, process development, and testing of topical dermatologic products

This review presents considerations which can be employed during the development of a semi-solid topical generic product. This includes a discussion on the implementation of quality by design concepts during development to ensure the generic drug product has similar desired quality attributes to the reference-listed drug (RLD) and ensure batch to batch consistency through commercial production. This encompasses the concept of reverse-engineering to copy the RLD as a strategy during product development to ensure qualitative (Q1) and quantitative (Q2) formulation similarity, as well as similarity in formulation microstructure (Q3). The concept of utilizing in vitro skin permeation studies as a tool to justify formulation differences between the test generic product and the RLD to ensure a successful pharmacodynamic or clinical endpoint bioequivalence study is discussed. The review concludes with a discussion on drug product evaluation and quality tests as well as in vivo bioequivalence studies.

Challenges obtaining a biowaiver for topical veterinary dosage forms

Obtaining a biowaiver for topical drugs used in veterinary species faces many of the same challenges associated with human topicals. However, the skin of domestic animals varies anatomically and biochemically and experimental approaches to assess bioequivalence (BE) in veterinary species have challenges that are not often encountered with human skin. This is especially the situation with locally acting drugs. The focus of this paper is to address several of the challenges associated with (i) determining the BE of these locally acting drugs and (ii) critically examine the current technological advances that can act as a surrogate for clinical trials.

Generic drugs in dermatology: part I

The cost of health care in the United States is increasing. In order to help control these rising costs, all parties involved in the delivery of health care, including dermatologists, need to be part of the solution of ethically reducing the cost of delivery of care. One potential means of meeting this goal is to increase the use of generic medications in daily practice. Generic medications can offer equally efficacious therapy at significantly lower prices, which can translate into large scale savings for the individual patient, the payer, and the overall health care system. Herein we provide an overview of new drug development, review the history of the generic drug industry, describe how generic drugs are approved by the US Food and Drug Administration, and define the concepts of bioequivalence and therapeutic equivalence. In part II, we explore various factors impacting generic drug use, provide cost analyses of dermatologic brand name and generic drugs, and review data addressing potential differences in the effectiveness of brand name versus generic drugs in dermatology. The cost of brand name and generic medications is highly variable by pharmacy, state, and payer. We used one source (www.drugstore.com) as an example and for consistency across all medications discussed herein. Prices included here may not reflect actual retail prices across the United States.

Assessment of topical bioequivalence using dermal microdialysis and tape stripping methods

PURPOSE

To assess the bioequivalence of two commercial topical formulations of oxytetracycline HCl by tape stripping and microdialysis in healthy volunteers.

METHODS

Tape stripping study was conducted on 12 healthy volunteers. After a 30-minute application of the formulations, adhesive tapes were used to sample stratum corneum at 0.25, 0.5, 1, 1.5, 2, 3, 4 hr. Ten of these volunteers were included in the microdialysis study with a period of 4 weeks between the experiments. Microdialysis probes were inserted into the dermis of the forearm. Following the application of the test and reference simultaneously, dialysates were collected in 30-minute sampling intervals up to 4 hr.

RESULTS

Pharmacokinetic evaluation by microdialysis yielded that the test could not be said to be bioequivalent to the reference at 90% CI. The intersubject variability of oxytetracycline content in stratum corneum was moderate when it was compared to the dermal levels. The test was found to be bioequivalent to reference according to the dermatopharmacokinetic evaluation by tape stripping.

CONCLUSIONS

No significant correlations were found between microdialysis and tape stripping methods as regarding the topical bioequivalence of oxytetracycline HCl formulations.