Current challenges in bioequivalence, quality, and novel assessment technologies for topical products.

This paper summarises the proceedings of a recent workshop which brought together pharmaceutical scientists and dermatologists from academia, industry and regulatory agencies to discuss current regulatory issues and industry practices for establishing therapeutic bioequivalence (BE) of dermatologic topical products. The methods currently available for assessment of BE were reviewed as well as alternatives and the advantages and disadvantages of each method were considered. Guidance on quality and performance of topical products was reviewed and a framework to categorise existing and alternative methods for evaluation of BE was discussed. The outcome of the workshop emphasized both a need for greater attention to quality, possibly, via a Quality-By-Design (QBD) approach and a need to develop a "whole toolkit" approach towards the problem of determination of rate and extent in the assessment of topical bioavailability. The discussion on the BE and clinical equivalence of topical products revealed considerable concerns about the variability present in the current methodologies utilized by the industry and regulatory agencies. It was proposed that academicians, researchers, the pharmaceutical industry and regulators work together to evaluate and validate alternative methods that are based on both the underlying science and are adapted to the drug product itself instead of single "universal" method.

In vitro and in vivo evaluation of three metronidazole topical products.

Two 1% and one 0.75% metronidazole cream products were approved as bioequivalent products. These products were evaluated for their in vivo cutaneous penetration characteristics by dermatopharmacokinetic (DPK) and dermal microdialysis (DMD) sampling methodologies. The same three products were also evaluated for their rheological and in vitro drug release (IVR) properties. Structural differences were observed in the resulting flow curves. However, similar IVR profiles were obtained for the two topical semisolid dosage forms containing 1% metronidazole. For the lower strength product, a higher IVR rate was associated with the lower DPK profile. All three products exhibited similar values of area under the curve when investigated by DMD. This in vitro evaluation corroborated the divergent penetration characteristics found using in vivo methodologies.

A prospective study of patient adherence to topical treatments: 95% of patients underdose.

BACKGROUND: Adherence is essential to a positive treatment outcome. Whether adequate doses of topically applied drugs are used among patients with dermatologic conditions has not been investigated adequately. OBJECTIVES: The objective of this prospective study was to determine to what extent first-time outpatients with dermatologic conditions apply the appropriate dosage of initial treatment with topically administered medication. METHODS: Consultations with first-time patients in the dermatologic outpatient clinic were observed. Patients receiving a prescription for a previously untried topically administered drug were eligible. The expected quantity of topical treatment to be used by each patient was calculated from the affected skin area to be treated. The affected area was estimated in numbers of palms of the hand, and an amount of 0.25 g of ointment or cream was chosen as sufficient treatment to cover the area of one palm. Two weeks after the consultation, a questionnaire with items regarding the size of the affected area, intentions to follow the treatment, and the applied dose the previous day was mailed to each patient. The questionnaires were personally collected from the patients' homes and at the same visit the patients' topical drug containers were weighed on a balance. Both patients and staff were blinded to the particular study purpose concerning adherence and dosing. RESULTS: In all, 17 patients were eligible. The majority received a prescription for topical corticosteroids, and the median area to be treated was 3 palms of the hand (interquartile range: 1.5-8). Two patients did not redeem their prescriptions. Only one patient used the expected dosage; in general median 35% (interquartile range: 22%-50%) of the expected individual dosages were used. LIMITATIONS: Only first-time patients who received a new and previously untried topical treatment were included, resulting in a small study size. CONCLUSIONS: Most first-time patients with dermatologic conditions underdose new topical treatments. Consequently, clinicians should always consider nonadherence when topical therapies fail.

One in 3 prescriptions are never redeemed: primary nonadherence in an outpatient clinic.

BACKGROUND: Despite being essential to medication adherence, redemption of initial prescriptions (ie, primary adherence) has been investigated only sparsely. OBJECTIVES: The objectives were to determine the frequency and risk factors for primary nonadherence among outpatients with dermatologic conditions. METHODS: Every 15th day during 2006, all patients receiving a prescription for an initial treatment with a previously untried medication were studied. Redemptions were traced in an electronic register after 4 weeks. Exclusions were a result of identical treatments within the last 6 months or hospitalizations within 4 weeks. RESULTS: In all, 30.7% of the 322 eligible patients did not collect their medication. Patients with psoriasis were least adherent with nearly 50% of the prescriptions being unredeemed. LIMITATIONS: Only initial prescriptions for previously untried medications issued to hospital outpatients were studied. CONCLUSIONS: For the clinician, primary nonadherence is an essential differential diagnosis when a given therapy fails.

The effect of irritant dermatitis on cutaneous bioavailability of a metronidazole formulation, investigated by microdialysis and dermatopharmacokinetic method.

BACKGROUND: Determination of drug penetration in diseased skin represents a challenge. OBJECTIVE: To compare dermal microdialysis and tape-strip sampling of drug penetration in normal skin and skin with irritant dermatitis. METHODS: The two methodologies were employed simultaneously in 16 healthy volunteers. Samples were collected in a study of the penetration of a metronidazole cream formulation (Flagyl 1%) applied to forearm skin in both areas with irritant dermatitis and normal skin. Barrier perturbation and the depth of microdialysis probes were quantified by non-invasive bioengineering methods. RESULTS: Microdialysis showed a significant threefold increase in metronidazole penetration in skin with irritant dermatitis compared with unmodified skin. Conversely, the concentration of metronidazole in tape-strip samples was significantly decreased in irritant dermatitis. CONCLUSION: The selection of sampling methodology should be based on the skin layer of interest as well as the integrity of the skin barrier. Whenever the dermal tissue is the target for topical treatment, microdialysis sampling should be the method of choice.

Penetration through the Skin Barrier.

The skin is a strong and flexible organ with barrier properties essential for maintaining homeostasis and thereby human life. Characterizing this barrier is the ability to prevent some chemicals from crossing the barrier while allowing others, including medicinal products, to pass at varying rates. During recent decades, the latter has received increased attention as a route for intentionally delivering drugs to patients. This has stimulated research in methods for sampling, measuring and predicting percutaneous penetration. Previous chapters have described how different endogenous, genetic and exogenous factors may affect barrier characteristics. The present chapter introduces the theory for barrier penetration (Fick's law), and describes and discusses different methods for measuring the kinetics of percutaneous penetration of chemicals, including in vitro methods (static and flow-through diffusion cells) as well as in vivo methods (microdialysis and microperfusion). Then follows a discussion with examples of how different characteristics of the skin (age, site and integrity) and of the penetrants (size, solubility, ionization, logPow and vehicles) affect the kinetics of percutaneous penetration. Finally, a short discussion of the advantages and challenges of each method is provided, which will hopefully allow the reader to improve decision making and treatment planning, as well as the evaluation of experimental studies of percutaneous penetration of chemicals.

Artefactual skin lesions in children and adolescents: review of the literature and two cases of factitious purpura.

BACKGROUND: Self harm is a great diagnostic and treatment challenge. In addition, psychocutaneous conditions are rare in the pediatric population and may therefore be misdiagnosed. Dermatitis artefacta is a psychocutaneous syndrome, which is a subgroup of the general spectrum of self-inflicted skin lesions. Dermatitis artefacta encompasses an array of different clinical manifestations, including purpura. Factitious purpura has rarely been reported in children. METHODS: Case report and review of the literature. RESULTS: We describe two Caucasian patients (9-year-old boy and 10-year-old girl) with striking purpuric lesions diagnosed as factitious purpura. The clinical lesions were similar, but the underlying psychological problems differed significantly (depression and stress). The current state of knowledge of dermatitis artefacta in children and adolescents was reviewed. CONCLUSION: The presence of purpura in children and adolescents typically causes extensive intervention programs due to the possible serious pathological consequences. The two cases demonstrate a need for a high degree of attention to psychological disturbances, lesional evolution, and distribution once the suspicion is established.

Differentiated in vivo skin penetration of salicylic compounds in hairless rats measured by cutaneous microdialysis.

The purpose was to investigate the in vivo skin penetration of four 14C-salicylic compounds using microdialysis and to relate dermal concentrations to structural features. Furthermore, to compare two in vivo retrodialysis recovery methods for estimation of true unbound extracellular concentrations. Microdialysis probes were inserted in the dermis of hairless rats. Equimolal 14C-salicylic formulations were applied topically and dialysate sampled consecutively for 4h. True extracellular concentrations were estimated by retrodialysis by drug method (the 14C-salicylic compounds themselves) and by retrodialysis by calibrator method (3H-salicylic acid as internal standard). Probe depth was measured by ultrasound scanning. High dermal concentrations were found after application of 14C-salicylamide (low protein-binding) and the lipophilic ester 14C-butyl salicylate, which was completely hydrolysed to 14C-salicylic acid during skin diffusion. Protein binding and dissociation may explain the lower dermal concentrations of 14C-salicylic acid and 14C-diethylamine salicylate, respectively. Probe depth did not significantly influence dialysate concentrations. The two in vivo recovery correction methods did not reduce the variation in concentration-time curves. In conclusion, differentiated penetration kinetics was found ranking: 14C-salicylamide >/= 14C-butyl salicylate > 14C-salicylic acid > 14C-diethylamine salicylate. Dermal concentrations were related to structural features of the model compounds. The two correction methods performed alike; however, the calibrator method has the advantage of serving as a quality control during experiments.

Percutaneous penetration--methodological considerations.

Studies on percutaneous penetration are needed to assess the hazards after unintended occupational skin exposures to industrial products as well as the efficacy after intended consumer exposure to topically applied medicinal or cosmetic products. During recent decades, a number of methods have been developed to replace methods involving experimental animals. The results obtained from these methods are decided not only by the chemical or product tested, but to a significant degree also by the experimental set-up and decisions made by the investigator during the planning phase. The present MiniReview discusses some of the existing and well-known experimental in vitro and in vivo methods for studies of percutaneous penetration together with some more recent and promising methods. After this, some considerations and recommendations about advantages and limitations of the different methods and their relevance for the prediction of percutaneous penetration are given. Which method to prefer will depend on the product to be tested and the question asked. Regulatory guidelines exist for studies on percutaneous penetration, but researchers as well as regulatory bodies need to pay specific attention to the vehicles and solvents used in donor and sampling fluids so that it reflects in-use conditions as closely as possible. Based on available experimental data, mathematical models have been developed to aid predictions of skin penetration. The authors question the general use of the present mathematical models in hazard assessment, as they seem to ignore outliers among chemicals as well as the heterogeneity of skin barrier properties and skin conditions within the exposed populations.

Dermal PK/PD of a lipophilic topical drug in psoriatic patients by continuous intradermal membrane-free sampling.

BACKGROUND: Methodologies for continuous sampling of lipophilic drugs and high-molecular solutes in the dermis are currently lacking. We investigated the feasibility of sampling a lipophilic topical drug and the locally released biomarker in the dermis of non-lesional and lesional skin of psoriatic patients over 25h by means of membrane-free dermal open-flow microperfusion probes (dOFM) and novel wearable multi-channel pumps. METHODS: Nine psoriatic patients received a topical p-38 inhibitor (BCT194, 0.5% cream) on a lesional and a non-lesional application site once daily for 8 days. Multiple dOFM sampling was performed for 25 h from each site on day 1 and day 8. Patients were mobile as dOFM probes were operated by a novel light-weight push-pull pump. Ultrasound was used to verify intradermal probe placement, cap-LC-MS/MS for BCT194 and ELISA for TNFα analysis. RESULTS: dOFM was well tolerated and demonstrated significant drug concentrations in lesional as well as non-lesional skin after 8 days, but did not show significant differences between tissues. On day 8, TNFα release following probe insertion was significantly reduced compared to day 1. CONCLUSIONS: Novel membrane-free probes and wearable multi-channel pumps allowed prolonged intradermal PK/PD profiling of a lipophilic topical drug in psoriatic patients. This initial study shows that dOFM overcomes limitations of microdialysis sampling methodology, and it demonstrates the potential for PK/PD studies of topical products and formulations in a clinical setting.

Basic drug information given by physicians is deficient, and patients' knowledge low.

OBJECTIVES: Our aim was to assess basic drug information given by physicians when prescribing new topical medication and to compare this with outpatients' subsequent knowledge. METHODS: Basic drug information was noted during consultations. Subsequently, patients answered a questionnaire regarding diagnosis, treatment, and satisfaction with the consultation. Neither physicians nor patients were informed about the specific study objective. RESULTS: A total of 17 patients were included. Information provided during the consultations versus patients' knowledge after 2 weeks was: diagnosis (65% vs 41%), dosage by number (100% vs 71%), dosage by quantity (18% vs 12%), and treatment duration (65% vs 35%). None was informed about adverse effects and 47% were fully satisfied with the consultation. CONCLUSIONS: Basic drug information provided by physicians was deficient and patients' knowledge was even lower.

Application of dermal microdialysis for the evaluation of bioequivalence of a ketoprofen topical gel.

The purpose was to investigate dermal microdialysis (DMD) for the assessment of the bioavailability of a ketoprofen topical gel formulation and to evaluate this technique as a tool for the determination of bioequivalence. Four microdialysis probes were inserted into the dermis on the volar aspect of the forearms of 18 human subjects and the probes were perfused with normal saline for 60 min. A ketoprofen (2.5%, m/m) gel formulation (50mg) was applied to the skin directly overlying the probes and samples were collected at 30 min intervals for 5h. With the probes still in place in the dermis each site was scanned by ultrasound to determine the implantation depth of these probes. Ketoprofen concentration in dialysates was determined by LC-MS/MS. The area under the curve obtained from the concentration-time profiles from pairs of application sites in each subject was evaluated in order to assess bioequivalence. Ninety percent confidence intervals were calculated using the two one-sided test procedure and limits of 80-125% based on log-transformed data were used as acceptance criteria to declare bioequivalence. The intra-subject variability was 10% between probes whereas inter-subject variability was 68% (n=18). Bioequivalence was confirmed with a power greater than 90%.

Impact of adult atopic dermatitis on topical drug penetration: assessment by cutaneous microdialysis and tape stripping.

Appropriate methodologies for the determination of drug penetration in diseased skin have not yet been established. The aim of this study was to determine the cutaneous penetration of a metronidazole cream formulation in atopic dermatitis, employing dermal microdialysis and tape strip sampling techniques. Non-invasive measuring methods were used for the quantification of the severity of the dermatitis. Skin thickness and the depth of the microdialysis probes in the skin were measured by 20 MHz ultrasound scanning. Metronidazole concentration, sampled by microdialysis, was 2.4-fold higher in the atopic dermatitis compared with uninvolved skin (p<0.001). Tape stripping methodology did not disclose this difference in penetration. Thus, the skin layer of interest and the integrity of the skin barrier should be considered when selecting sampling methodology. Microdialysis sampling is the method of choice whenever the dermis is the target tissue for topical treatment and a skin disease affecting the barrier function is present.

Bioequivalence of topical formulations in humans: evaluation by dermal microdialysis sampling and the dermatopharmacokinetic method.

The aim of this study was to evaluate the relationship between dermal microdialysis (DMD) sampling and the dermatopharmacokinetic method when employed simultaneously for bioequivalence (BE) investigations of topical formulations. Topical lidocaine cream and ointment (both 5%) was investigated in eight healthy human volunteers (four male, four female). On one forearm, four microdialysis probes in two penetration areas sampled for 5 hours, and on the other arm, tape stripping was performed 30 and 120 minutes after product application. Lidocaine content in samples was analyzed by HPLC-mass spectrometry. The two methods were in agreement showing 3- to 5-fold higher lidocaine penetration from cream formulation than from ointment. A rank-order correlation between the two methods was demonstrated for lidocaine contents in microdialysates versus tape strip at 120 minutes, significant for the ointment formulation and for both formulations analyzed together. Analysis of variance demonstrated reproducible lidocaine concentrations in microdialysates with an intrasubject variability of 19% between probes and 20% between the two penetration areas. Thus, intersubject variability accounted for 61% of the variance. DMD sampling proved effective and variability analyses demonstrated the feasibility of BE studies in as little as 18 subjects.

AAPS-FDA workshop white paper: microdialysis principles, application and regulatory perspectives.

Many decisions in drug development and medical practice are based on measuring blood concentrations of endogenous and exogenous molecules. Yet most biochemical and pharmacological events take place in the tissues. Also, most drugs with few notable exceptions exert their effects not within the bloodstream, but in defined target tissues into which drugs have to distribute from the central compartment. Assessing tissue drug chemistry has, thus, for long been viewed as a more rational way to provide clinically meaningful data rather than gaining information from blood samples. More specifically, it is often the extracellular (interstitial) tissue space that is most closely related to the site of action (biophase) of the drug. Currently microdialysis (microD) is the only tool available that explicitly provides data on the extracellular space. Although microD as a preclinical and clinical tool has been available for two decades, there is still uncertainty about the use of microD in drug research and development, both from a methodological and a regulatory point of view. In an attempt to reduce this uncertainty and to provide an overview of the principles and applications of microD in preclinical and clinical settings, an AAPS-FDA workshop took place in November 2005 in Nashville, TN, USA. Stakeholders from academia, industry and regulatory agencies presented their views on microD as a tool in drug research and development.

Use of an intraluminal guide wire in linear microdialysis probes: effect on recovery?

BACKGROUND/PURPOSE: For microdialysis studies in the skin, laboratory-made linear probes are often used. Probes can be assembled with or without a stainless-steel guide wire in the lumen of the dialysis fibre. From a theoretical viewpoint, this will alter the performance of the probe. The aim of this study was to compare the in vitro microdialysis recovery using probes with and without an internal guide wire, employing two different model drugs and a range of perfusate flow rates. METHODS: In vitro microdialysis recovery experiments were performed with and without internal guide wires at perfusion flow rates from 1 to 15 microL/min, using either salicylic acid or metronidazole as test compounds at 5 mg/L. Phosphate buffer (pH 7.4) was used as perfusate. Dialysates were analysed by a validated HPLC assay with spectrophotometric detection. RESULTS: For both test compounds, the mean microdialysis recovery did not differ significantly between microdialysis probes with and without an internal guide wire at lower flow rates. At very high flow rates (10-15 microL/min), a significant difference in recovery could be seen for probes with internal guide wires when compared with probes without. CONCLUSIONS: The use of a stabilizing internal guide wire in linear microdialysis probes did not influence the in vitro recovery of microdialysis at volumetric flow rates below 10 microL/min. These are the flow rates most frequently used for in vivo microdialysis studies in the skin.