Dermatopharmacokinetic prediction of topical drug bioavailability in vivo

Preparation and in vitro evaluation of topical formulations based on polystyrene-poly-2-hydroxyl methacrylate nanoparticles

The skin disposition of topically applied nanoparticles with varying degrees of hydrophobicity, composed of different proportions of polystyrene (PS) and poly-(2-hydroxyethyl methacrylate) (HEMA), and of an associated, model "active" (Nile Red), was investigated. PS-HEMA copolymer nanoparticles were fluorescently labeled, via the covalent incorporation of a small quantity of fluorescein methacrylate, and characterized by dynamic light scattering, transmission electron microscopy and NMR. The fluorophore, Nile Red, was dispersed into the nanoparticles and its loading was determined by ultracentrifugation. Skin uptake was assessed in vitro following a 6 h application of the nanoparticle formulation, via stratum corneum (SC) tape-stripping and confocal microscopy. Nanoparticle diameters were below 100 nm. Progressive introduction of HEMA decreased particle hydrophobicity and reduced Nile Red loading. Uptake of Nile Red into the skin, as assessed both by the amounts extracted from the SC and by confocal microscopy, decreased as the percentage HEMA increased. Confocal microscopy confirmed that nanoparticles could not move beyond the superficial SC, but did show affinity for hair follicle openings. In conclusion, the loading of a lipophilic "active" into nanoparticles, and its subsequent release when these formulations are applied topically, are sensitive to the composition and relative hydrophobicity of the carrier.

Pharmacodynamics and dermatopharmacokinetics of betamethasone 17-valerate: assessment of topical bioavailability

BACKGROUND

The bioavailability of most topically delivered drugs is difficult to quantify, but is generally believed to be very low. With the exception of the vasoconstrictor assay for corticosteroids, no methodology to quantify the rate and extent of drug delivery to the skin has been validated. Recent research has examined the dermatopharmacokinetic (DPK) technique, which is based on stratum corneum (SC) tape-stripping.

OBJECTIVE

To compare the in vivo bioavailability of different topical formulations of betamethasone 17-valerate (BMV) using the vasoconstrictor assay and the DPK method.

METHODS

BMV was formulated in different vehicles and the drug concentration was adjusted to either (i) equal thermodynamic activity, or (ii) a range of values up to that corresponding to 80% of maximum thermodynamic activity. Vasoconstriction, an accepted and widely used method to determine bioavailability and bioequivalence of topical steroids, was quantified with a chromameter over 24 h post-removal of the formulation. Drug uptake into the SC was assessed by tape-stripping.

RESULTS

BMV at the same thermodynamic activity in different vehicles provoked similar skin blanching responses, while DPK profiles distinguished between the formulations. Further, skin blanching responses and drug uptake into the SC clearly depended upon the absolute BMV concentration applied. However, while the saturable nature of the pharmacodynamic response was clear, the tape-stripping method distinguished unequivocally between the different formulations and different concentrations.

CONCLUSIONS

The DPK approach offers a reliable metric with which to quantify transfer of drug from the vehicle to the SC, and may be useful for topical bioavailability and bioequivalence determinations.

The determination of stratum corneum thickness: an alternative approach

The individual thickness of the stratum corneum is required to normalise drug permeation profiles in dermato-pharmacokinetic studies. The thickness is often estimated using tape-stripping combined with transepidermal water loss measurements. A linear transformation of Fick's first law is used to relate the progressively thinner barrier with the corresponding increase in transepidermal water loss and to estimate the thickness by linear regression. However, the data from an important subset of subjects are poorly fitted to this linear model. This is typically due to the removal of loose outer layers of stratum corneum, which do not contribute significantly to barrier function. This work proposes two alternative non-linear models. All three models were used to fit data from 31 in vivo tape-stripping experiments and their outcomes and goodness-of-fit compared. The results suggest that the linear model may overestimate the stratum corneum thickness and is open to subjectivity regarding the selection of data points to be fitted. The non-linear models satisfactorily fitted all the data, including all data points. No significant differences were found between the thicknesses derived from the two non-linear models. However, the analysis of the goodness-of-fit of the models to the data suggests a preference for a baseline-corrected approach.

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.