Correlation of in vivo topical efficacies with in vitro predictions using acyclovir formulations in the treatment of cutaneous HSV-1 infections in hairless mice: an evaluation of the predictive value of the C* concept

Assessment of dermal bioavailability: predicting the input function for topical glucocorticoids using stratum corneum sampling

Predicting the dermal bioavailability of topically delivered drugs is challenging. In this work, minimally invasive stratum corneum (SC) sampling was used to quantify the delivery of betamethasone valerate (BMV) into the viable skin. Betnovate® cream (0.1% w/w BMV) was applied at three doses (2, 5, and 10 mg cm⁻²) to the ventral forearms of 12 healthy volunteers. The mass of drug in the SC was measured using a validated tape-stripping method (a) after a 4-h “uptake” period, and (b) following a 6-h “clearance” period subsequent to cream removal. Concomitantly, the skin blanching responses to the same doses were assessed with a chromameter over 22 h post-application. BMV uptake into the SC was significantly higher for the 5 mg cm⁻² dose compared to those of 2 and 10 mg cm⁻². In all cases, ~30% of the drug in the SC at the end of the uptake period was cleared in the subsequent 6 h. From the SC sampling data, the average drug flux into the viable epidermis and its first-order elimination rate constant from the SC were estimated as 4 ng cm⁻² h⁻¹ and 0.07 h⁻¹, respectively. In contrast, skin blanching results were highly variable and insensitive to the dose of cream applied. The SC sampling method was able to detect a 50% difference between two applied doses with 80% power; detection of a 20% difference would require a larger sample size. SC sampling enabled quantitative metrics describing corticosteroid delivery to the viable epidermis to be determined.

Topical drug delivery: History, percutaneous absorption, and product development

Topical products, widely used to manage skin conditions, have evolved from simple potions to sophisticated delivery systems. Their development has been facilitated by advances in percutaneous absorption and product design based on an increasingly mechanistic understanding of drug-product-skin interactions, associated experiments, and a quality-by-design framework. Topical drug delivery involves drug transport from a product on the skin to a local target site and then clearance by diffusion, metabolism, and the dermal circulation to the rest of the body and deeper tissues. Insights have been provided by Quantitative Structure Permeability Relationships (QSPR), molecular dynamics simulations, and dermal Physiologically Based PharmacoKinetics (PBPK). Currently, generic product equivalents of reference-listed products dominate the topical delivery market. There is an increasing regulatory interest in understanding topical product delivery behavior under 'in use' conditions and predicting in vivo response for population variations in skin barrier function and response using in silico and in vitro findings.

Assessment of Drug Delivery Kinetics to Epidermal Targets In Vivo

It has proven challenging to quantify ‘drug input’ from a formulation to the viable skin because the epidermal and dermal targets of topically applied drugs are difficult, if not impossible, to access in vivo. Defining the drug input function to the viable skin with a straightforward and practical experimental approach would enable a key component of dermal pharmacokinetics to be characterised. It has been hypothesised that measuring drug uptake into and clearance from the stratum corneum (SC) by tape-stripping allows estimation of a topical drug’s input function into the viable tissue. This study aimed to test this idea by determining the input of nicotine and lidocaine into the viable skin, following the application of commercialised transdermal patches to healthy human volunteers. The known input rates of these delivery systems were used to validate and assess the results from the tape-stripping protocol. The drug input rates from in vivo tape-stripping agreed well with the claimed delivery rates of the patches. The experimental approach was then used to determine the input of lidocaine from a marketed cream, a typical topical product for which the amount of drug absorbed has not been well-characterised. A significantly higher delivery of lidocaine from the cream than from the patch was found. The different input rates between drugs and formulations in vivo were confirmed qualitatively and quantitatively in vitro in conventional diffusion cells using dermatomed abdominal pig skin.

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.

Efficacy, Safety and Targets in Topical and Transdermal Active and Excipient Delivery

A key requirement for topical and transdermal active delivery is the effective delivery of an active to a desired target site, to achieve both safe and efficacious outcomes. This chapter seeks to explore the importance of the pharmacological, toxicological and therapeutic properties of actives and excipients, as well as the site of action as complementary components in percutaneous absorption. This is crucial for optimized topical and transdermal product design.

Passive and iontophoretic transport through the skin polar pathway

The purpose of the present article is to briefly recount the contributions of Prof. William I. Higuchi to the area of skin transport. These contributions include developing fundamental knowledge of the barrier properties of the stratum corneum, mechanisms of skin transport, concentration gradient across skin in topical drug applications that target the viable epidermal layer, and permeation enhancement by chemical and electrical means. The complex and changeable nature of the skin barrier makes it difficult to assess and characterize the critical parameters that influence skin permeation. The systematic and mechanistic approaches taken by Dr. Higuchi in studying these parameters provided fundamental knowledge in this area and had a measured and lasting influence upon this field of study. This article specifically reviews the validation and characterization of the polar permeation pathway, the mechanistic model of skin transport, the influence of the dermis on the target skin concentration concept, and iontophoretic transport across the polar pathway of skin including the effects of electroosmosis and electropermeabilization.

Effect of Azone upon the in vivo antiviral efficacy of cidofovir or acyclovir topical formulations in treatment/prevention of cutaneous HSV-1 infections and its correlation with skin target site free drug concentration in hairless mice

The purpose of this study is to examine the influence of Azone upon the skin target site free drug concentration (C(*)) and its correlation with the in vivo antiviral efficacies of cidofovir (HPMPC) and acyclovir (ACV) against HSV-1 infections. Formulations of HPMPC and ACV with or without Azone were used. The in vitro skin flux experiments were performed and the C(*) values were calculated. For the in vivo efficacy studies, hairless mice cutaneously infected with HSV-1 were used and three different treatment protocols were carried out. The protocols were chosen based upon when therapy is initiated and terminated in such a way to assess the efficacy of the test drug to cure and/or prevent HSV-1 infections. A finite dose of the formulation was topically applied twice a day for the predetermined time course for each protocol and the lesions were scored on the fifth day. For ACV formulation with Azone, the C(*) values and hence the in vivo efficacy were much higher than those for that without Azone. In protocol #1, however, early treatment did not increase the in vivo efficacy of ACV when compared with the standard treatment protocol #3. In protocol #2 where the treatment was terminated on the day of virus inoculation, the efficacies for both ACV formulations were completely absent. Although the estimated C(*) values for HPMPC formulations with and without Azone were comparable, formulation with Azone was much more effective than that without Azone in all treatment protocols. HPMPC formulations with Azone at similar flux values were much more effective in "treating and preventing" HSV-1 infections than those without Azone. For ACV formulations, in contrast, addition of Azone has failed to show any effect on the preventive in vivo antiviral efficacy and the enhancement of ACV in vivo antiviral efficacy was merely the skin permeation enhancement effect of Azone.

Influence of the treatment protocol upon the in vivo efficacy of cidofovir (HPMPC) and of acyclovir (ACV) formulations in topical treatment of cutaneous HSV-1 infection in hairless mice

In recent studies we found that the topical effectiveness of acyclovir (ACV) formulations was a single-valued function of C-the target site free drug concentration. The topical efficacy was the same when the therapy was initiated 0, 1, or 2 days after intracutaneous herpes simplex virus type-1 (HSV-1) inoculation in hairless mice. The purpose of the present study was to examine the hypothesis that the topical effectiveness of cidofovir (HPMPC) would not be a single valued function of C and that it would be dependent upon when the therapy was initiated relative to the time of viral infection. Formulations of HPMPC and ACV in 95% DMSO as a vehicle were used. Hairless mice intracutaneously infected with HSV-1 were used, and 20 microL of the test formulation was topically applied twice a day. In protocol A, the treatment was continued until the fourth day after virus inoculation, whereas in protocol B the treatment was terminated on the day of virus inoculation. Treatment was initiated on various days ranging from day -6 to day 4, and the lesions were scored on day 5. Treatment of ACV according to protocol A proved efficacious whether started as early as 6 days before virus inoculation or later, whereas the efficacy of ACV was annihilated if applied following protocol B. For HPMPC, on the other hand, the in vivo efficacies were found to be strongly dependent on how early the therapy was initiated, and significant efficacy was observed even when the treatment was terminated on the day of virus inoculation. This difference was attributed to the virus-independent intracellular phosphorylation of HPMPC and slow clearance of its metabolites from the cell. It was also noted that, similar to ACV, for HPMPC the topical efficacy is likely to be a function of C for a fixed protocol. However, unlike for ACV, for HPMPC the efficacy was not a single-valued function of C.

Assessment of correlation between skin target site free drug concentration and the in vivo topical antiviral efficacy in hairless mice for (E)-5-(2-bromovinyl)-2'-deoxyuridine and acyclovir formulations

Recently, we reported that the in vivo efficacy of acyclovir (ACV) formulations was a single valued function of skin target site free drug concentration (C) irrespective of the formulation compositions. A long-term objective of this research has been to generalize the C concept using model drugs which are similar to as well as different from ACV in their mechanism of actions. (Bromovinyl)deoxyuridine (BVDU) was selected as a model drug based on the reported similarity in its mechanism of action with ACV. The relationship between the C predictions and the in vivo efficacies for some topical formulations containing different concentrations (0.05-10%) of either ACV or BVDU in 95% DMSO as a vehicle with or without 5% Azone as skin permeation enhancer was examined. Hairless mice infected cutaneously with HSV-1 were used to quantitatively estimate the in vivo topical antiviral efficacy. A finite dose of the test antiviral formulation was applied twice a day for 4 days, starting the day after virus inoculation. On the fifth day, the lesions were scored and the efficacy values were calculated. For each formulation, in vitro flux experiments were performed in an in vivo-in vitro experimental design that closely approximated the in vivo study protocol. As was previously shown, with all ACV formulations, a good correlation was found between the C predictions and the in vivo topical efficacy. With the BVDU formulations, on the other hand, this was found not to be the case. BVDU formulations with 5% Azone were generally much more effective than those without Azone at comparable C values. This finding is believed to be the first of its kind showing that skin "permeation enhancers" may enhance efficacy by more than simply increasing skin permeation rates.

Relationship of skin target site free drug concentration (C*) to the in vivo efficacy: an extensive evaluation of the predictive value of the C* concept using acyclovir as a model drug

For the past few years, our laboratory has been involved in the development of a novel approach for predicting topical in vivo efficacy based on the estimation of skin target site free drug concentration (C*) from in vitro flux data. We have used acyclovir (ACV) as a model drug in the treatment of cutaneous herpes simplex virus type 1 infections in hairless mice. The goal of this study was to rigorously evaluate the applicability of this approach over the entire range of topical efficacy (i.e., from 0 to 100%). We employed a variety of ACV formulations differing in solvent compositions, enhancers, and excipients (and therefore in their efficacies) to achieve this goal. The C* values were estimated from the in vitro flux data obtained in an in vivo-in vitro experimental design that closely approximated the in vivo treatment protocol. For the in vivo antiviral efficacy studies, a finite dose of ACV formulation was applied twice a day, beginning the day after virus inoculation, for 4 days. The lesions were scored on the fifth day, and the efficacies were calculated as described earlier. Our results indicate that, for a variety of formulations over a wide range of efficacies, the predictions based on C* are in good agreement with the observed in vivo efficacies. These findings strongly demonstrate the predictive value of C* over the entire range of topical efficacy, thereby further strengthening its potential for future studies. The findings also indicate that although the excipients in a formulation may alter the rate and extent of available drug at the target site, in these cases, they do not seem to have any effect on the in vivo potency of the drug.