Topical drug delivery from thin applications: theoretical predictions and experimental results

Is the Skin Absorption of Hydrocortisone Modified by the Variability in Dosing Topical Products?

Fingertip units have been proposed as a tool to standardize topical therapy with semisolid formulations. However, no studies to date have characterized the variability in dosing by patients using this concept and whether this variability ultimately affects the topical absorption of drugs. This work aimed to answer these two questions. A first study determined the dose measured, the area of spread and the area-normalized dose for a 1% hydrocortisone cream and ointment applied by members of the public using this dosing approach before and after brief counselling. Then, in vivo tape-stripping and in vitro permeation studies investigated whether the variability in the area-normalized dose altered the skin absorption of hydrocortisone. Participants applied greater doses and spread them over larger areas after a short counselling intervention leading to smaller area-normalized doses. In vivo hydrocortisone uptake by the stratum corneum was significantly greater for the higher normalized dose and the differences were further supported by the in vitro permeation studies. However, these differences were relatively small and not proportional to the increase in normalized dose. This work shows that, following brief advice, patients and carers can apply consistent and sufficient doses of corticosteroids whilst minimizing risks and variability in hydrocortisone absorption.

How accurate is the determination of the relative bioavailability of transdermal drug formulations from pharmacodynamic response data?

The relative bioavailability of transdermal drug formulations may be quantified by measurement of the pharmacodynamic response. Parameters such as the latency time of onset and the duration of the response may be used to obtain dose-response curves. The horizontal distance between the parallel sections of a standard and a test curve is a measure of the relative bioavailability. High penetration rate constants often lead to an insufficient parallelism of the dose-response curves because of drug depletion in the vehicle. Drug depletion may be simulated with Fick's First Law and with the Bateman equation which allows the calculation of dose-response curves for both response parameters. The simulated curves for the parameter duration are not parallel to each other and the calculated bioavailability factors do not reach the theoretical values. In contrast, the curves obtained with the response parameter 1/latency time show a sufficient parallelism at high response levels and provide accurate bioavailability data in this curve area.

Accurate determination of skin flux from flow-through diffusion cell data

PURPOSE

The goal of this investigation was to demonstrate whether the intrinsic flux of a drug diffusing across a membrane mounted in a flow-through diffusion cell may be accurately and easily determined by accounting for the accumulation in the receiver chamber.

METHODS

Mathematical modeling, applied to transdermal diffusion, was used to calculate receiver concentration data for single layer and bilayer membranes. The data were interpreted using two apparent flux values, Japp1 and Japp2. Japp1 has been used extensively in the literature, but did not account for accumulation in the receiver. Japp2 did take the accumulation into consideration.

RESULTS

The results confirm that, generally, Japp1 values were not accurate estimates of the intrinsic flux. Japp2 values were significantly more accurate, especially prior to the maximum in receiver concentration.

CONCLUSIONS

Japp2 was an accurate measurement of intrinsic flux over the entire experimental time period, except at time zero. It was more accurate because it accounted for solute accumulation in the receiver compartment. The accuracy of the Japp2 approximation was practically independent of receiver volume, flow rate and donor volume. For very slowly permeating drugs, or a very small receiver volume combined with a high flow rate, the Japp1 estimate accurately reflected the intrinsic flux. Early time data were required to properly account for accumulation in the receiver cell. If such data were not available, the inverse Laplace method of determining intrinsic flux was preferable to the Japp2 calculation.

Enhancing effects of lipophilic vehicles on skin penetration of methyl nicotinate in vivo

Vehicle effects may be caused by thermodynamic effects and by specific (penetration enhancing) effects. To investigate the effects of various lipophilic vehicles on drug penetration, an in vivo permeability study was conducted with methyl nicotinate as the model drug. The drug was dissolved in the respective vehicles at concentrations that provide equal drug escaping tendencies. Drug solutions were applied to the upper arms of volunteers with a glass chamber system. To avoid drug depletion effects, drug disappearance rates were measured under steady-state conditions by the difference method. Enhancement factors were calculated from the steady-state flux values (i.e., drug disappearance rates per area unit) and compared with results from non-steady-state experiments. Significant enhancing effects (p < 0.01) were observed with dibutyl adipate, caprylic/capric acid triglycerides containing 5% phospholipids, isopropyl myristate, and mineral oil. Caprylic/capric acid triglycerides, cetearyl isooctanoate, and the standard vehicle dimethicone 100 were without effect on drug penetration. The explanation for the observed enhancing effects may be an interaction of the lipophilic liquids with the lipid bilayers of the stratum corneum that leads to a decrease of the barrier resistance.

Flow-through system effects on in vitro analysis of transdermal systems

The development of transdermal therapeutic systems (TTS) often involves in vitro evaluation of formulations and prototypes using flow-through diffusion cells. The apparent flux obtained from such methodologies does not accurately represent the actual (intrinsic) permeation of the compound through the skin. Flow-through system parameters, i.e., fraction collector tube volume, receiver cell volume, flow rate, and sampling interval, modify the flux yielding an apparent flux. Both finite-dose flux profiles and infinite-dose diffusional lag times are modified by these parameters. In this study, a transfer function is derived which describes the effect of these parameters. The intrinsic flux is calculated from apparent flux data using the transfer function and experimental flow-through system parameters. This allows the calculation of permeant flux profiles devoid of modification by the experimental methodology.

The determination of diffusion coefficients in semisolids by Fourier transform infrared (FT-IR) spectroscopy

A Fourier transform infrared (FT-IR) spectrometer with a horizontal attenuated total reflectance (ATR) cell was used to determine the diffusion coefficients of several liquids in two semisolid materials. The experimental setup was that of a system with one open and one closed boundary wherein the open boundary was maintained at constant concentration. While the liquid of interest was diffusing through the film of ointment, the concentration of liquid at the film surface in contact with the ATR crystal was determined at various times by means of IR absorption measurements. The depth of penetration of the IR radiation into the sample was approximately 0.6-0.9 microns at the wavelengths of analysis. Since the ointment thickness was 157 microns, it was reasonable to assume that only the penetrant reaching the lower boundary was being measured. The values of the diffusion coefficients were then calculated using an equation that appropriately modeled the aforementioned conditions. The liquids tested exhibited diffusion coefficients in anhydrous lanolin and in polyethylene glycol ointment that ranged from 0.56 to 7.2 x 10(-7) and 0.68 to 5.7 x 10(-7) cm2/sec, respectively. The expected molecular weight dependency was observed.