In vivo percutaneous penetration/absorption, Washington, D.C., May 1989

In vitro release test (IVRT): Principles and applications

In vitro drug release test has become one of the most important tools for drug development and approval process of semisolid dosage forms. In vitro release test (IVRT) has the ability to reflect the combined effects of several physicochemical characteristics, particle or droplet size, viscosity, microstructure arrangement of the matter and state of aggregation of dosage form. Genesis of IVRT, its principles and rank order relationship with pharmacodynamic response such as vasoconstriction or dermatopharmacokinetic (skin stripping) results and the evolution of test requirements for regulatory approval is discussed. IVRT reflects various parameters and is an essential part of the stepwise approach to compare topical formulation and its ability to release active in similar quantity at similar rate. Therefore, it is an essential tool, in addition to similar qualitative and quantitative composition (Q1 Q2), to assess the similarity of microstructural arrangement (Q3) as proposed in the Topical drug Classification System (TCS) approach of classes 1 and 3. The TCS system along with evolving concept for topical dermatological drug products from Q1, Q2, Q3 sameness to Q1, Q2, Q3 similar allowing greater permissiveness in formulation changes is discussed.

Skin models for dermal exposure assessment of phthalates

Phthalates are a class of compounds that have found widespread use in industrial applications, in particular in the polymer, cosmetics and pharmaceutical industries. While ingestion, and to a lesser degree inhalation, have been considered as the major exposure routes, especially for higher molecular weight phthalates, dermal exposure is an important route for lower weight phthalates such as diethyl phthalate (DEP). Assessing the dermal permeability of such compounds is of great importance for evaluating the impact and toxicity of such compounds in humans. While human skin is still the best model for studying dermal permeation, availability, cost and ethical concerns may preclude or restrict its use. A range of alternative models has been developed over time to substitute for human skin, especially in the early phases of research. These include ex vivo animal skin, human reconstructed skin and artificial skin models. While the results obtained using such alternative models correlate to a lesser or greater degree with those from in vivo human studies, the use of such models is nevertheless vital in dermal permeation research. This review discusses the alternative skin models that are available, their use in phthalate permeation studies and possible new avenues of phthalate research using skin models that have not been used so far.

In Vitro Skin Permeation Methodology for Over-The-Counter Topical Dermatologic Products

For topically applied over-the-counter (OTC) products, the association of unwanted systemic exposure and adverse events may be difficult to ascertain without a recognition or determination of in vivo absorption. Evaluation of skin permeability using a validated in vitro permeation methodology can provide important information for both initial formulation selection and reformulation during the product life cycle. Additionally, a comparison of permeation rates between formulations using a validated methodology could reduce the number of nonclinical studies needed as part of reformulation. However, many in vitro permeation tests (IVPTs) have produced results with high variability and low reproducibility between study sites. It is unclear if this is due to a lack of a standardized protocol, or lack of control of multiple key experimental factors including skin source, preparation, receptor fluid, and study design. This review presents the authors perspective on the potential regulatory utility of IVPT and proposes steps to improve the accuracy and reproducibility of IVPT. The focus of this review is on topical dermatologic drugs with an initial emphasis on the OTC marketplace where reformulations are more common.

A history of biopharmaceutics in the Food and Drug Administration 1968-1993

The history of biopharmaceutics is reviewed, beginning with its origin out of the Division of Clinical Research in The Bureau of Medicine. The reason for the creation of the Division of Biopharmaceutics, the certification of Food and Drug Administration authority over the functions it was to have, and the implementation of that authority are described. The determination of bioequivalence, the bioavailability decision rules, pharmacokinetics, and drug metabolism are explained. The reason for the development of the Scale-Up and Post Approval Regulations and how they were developed are also explained.

In vivo methods for the assessment of topical drug bioavailability

This paper reviews some current methods for the in vivo assessment of local cutaneous bioavailability in humans after topical drug application. After an introduction discussing the importance of local drug bioavailability assessment and the limitations of model-based predictions, the focus turns to the relevance of experimental studies. The available techniques are then reviewed in detail, with particular emphasis on the tape stripping and microdialysis methodologies. Other less developed techniques, including the skin biopsy, suction blister, follicle removal and confocal Raman spectroscopy techniques are also described.

Pig ear skin ex vivo as a model for in vivo dermatopharmacokinetic studies in man

OBJECTIVE

The objective was to investigate pig ear skin as a surrogate for human skin in the assessment of topical drug bioavailability by sequential tape-stripping of the stratum corneum (SC). The potential benefits of ex vivo investigations are manifold: ethical approval is not required, multiple replicate experiments are more easily performed, and toxic compounds can be evaluated.

MATERIALS AND METHODS

Ex vivo experiments on isolated pig ears were compared with in vivo studies in human volunteers. Four formulations, comprising the model drug, ibuprofen, in different propylene glycol (PG)-water mixtures (25:75, 50:50, 75:25 and 100:0), were compared.

RESULTS

Derived dermatopharmacokinetic parameters characterizing the diffusion and partitioning of the drug in the SC ex vivo were consistent with those in vivo following a 30-minute application period. Further, the non-steady-state ex vivo results could be used to predict the in vivo concentration profile of the drug across the SC when a formulation was administered for 3 h (i.e., close to steady-state).

CONCLUSIONS

Taken together, the results obtained suggest that pig ear skin ex vivo has promise as a tool for topical formulation evaluation and optimization.

In vivo skin penetration of salicylic compounds in hairless rats

The in vivo skin penetration of four salicylic compounds was investigated using a hairless rat model, which allowed for non-occluded, finite dose application, and free mobility of the rats throughout the test period. The model compounds were applied in equimolal concentrations of 0.4 mmol/g dimethyl isosorbide. At certain times (0.5-24 h) the rats were killed, and the amount of test compound on the skin surface, in the stratum corneum, and in the deeper viable skin layers was determined. Significant different skin concentrations were found with the following ranking: [(14)C]diethylamine salicylate>[(14)C]salicylic acid>[(14)C]salicylamide>[(14)C]butyl salicylate. In addition, the in vivo percutaneous rate of absorption was in the following order: [(14)C]butyl salicylate>[(14)C]salicylic acid> or =[(14)C]salicylamide>[(14)C]diethylamine salicylate. [(14)C]Butyl salicylate was rapidly absorbed and completely depleted from the surface 3 h post application. In comparison with [(14)C]salicylic acid, the ionic [(14)C]diethylamine salicylate had larger surface depots and penetrated the skin at a lower rate. The relatively hydrophilic [(14)C]salicylamide also had larger surface depots but much lower skin levels. For comparison, the in vitro permeation of the formulations was studied through freshly excised hairless rat skin using Franz diffusions cells, and an agreement between the techniques was found.

Noninvasive in vivo percutaneous absorption measurements using X-ray fluorescence

X-ray fluorescence (XRF) has been used to determine in vivo the percutaneous absorption of 5-iodouracil (5IU) in dimethyl sulfoxide (DMSO) on female Sprague-Dawley rats. An average absorption rate constant of 122 +/- 34 micrograms/cm2-hr was obtained from the XRF measurements on four rats. A comparative study was performed with radiolabeled (125I) 5IU in which the absorption rate constant was determined to be 126 +/- 20 micrograms/cm2-hr. The XRF system described provides a simple, noninvasive means of measuring the percutaneous absorption rate of select compounds by the surface disappearance method.

Percutaneous absorption of amorolfine following a single topical application of an amorolfine cream formulation

In an open-label, parallel-group, randomized study percutaneous absorption of 14C-labelled amorolfine incorporated into a cream formulation was assessed in healthy male volunteers (n = 12). A single dose of 0.5 g of the 0.25% cream formulation was applied to 100 cm2 of intact (n = 6) and stripped (n = 6) skin for 24 h using occlusive dressing. The remaining drug was removed and the treated skin area of both groups was stripped with adhesive tape. Total urine and faeces were collected in portions up to 3 weeks after the experiment and blood samples were taken at intervals for 3 weeks. Radioactivity was measured in the skin strippings and in the urine, faeces and plasma samples. The intact drug was assessed in the plasma samples. Using mass balance techniques it could be shown that a mean of 92% (range: 84-101%) of the applied radioactivity could be recovered. Small differences in the absorption and elimination of the radioactivity were observed between the two groups but they were not statistically significant (alpha = 0.05). Therefore data from the two groups were pooled. Elimination of drug and drug-related material from the body was very slow. During the 3-week collection period, a mean of 7% (range: 3.8-10.2%) of the dose was excreted in urine and faeces. Another 0.9-3.3% of the dose was retained in the upper layers of the skin as shown by the skin strippings after treatment. Levels of radioactivity and of intact drug in plasma were below the detection limit of 0.5 ng-equiv./ml, respectively. Present data suggest that mean percutaneous absorption of amorolfine following topical application of the 0.25% cream formulation should not exceed 8-10% of the dose applied.