The prediction of plasma levels of drugs following transdermal application

Mathematical modeling of transdermal delivery of topical drug formulations in a dynamic microfluidic diffusion chamber in health and disease

Mathematical models of epidermal and dermal transport are essential for optimization and development of products for percutaneous delivery both for local and systemic indication and for evaluation of dermal exposure to chemicals for assessing their toxicity. These models often help directly by providing information on the rate of drug penetration through the skin and thus on the dermal or systemic concentration of drugs which is the base of their pharmacological effect. The simulations are also helpful in analyzing experimental data, reducing the number of experiments and translating the in vitro investigations to an in-vivo setting. In this study skin penetration of topically administered caffeine cream was investigated in a skin-on-a-chip microfluidic diffusion chamber at room temperature and at 32°C. Also the transdermal penetration of caffeine in healthy and diseased conditions was compared in mouse skins from intact, psoriatic and allergic animals. In the last experimental setup dexamethasone, indomethacin, piroxicam and diclofenac were examined as a cream formulation for absorption across the dermal barrier. All the measured data were used for making mathematical simulation in a three-compartmental model. The calculated and measured results showed a good match, which findings indicate that our mathematical model might be applied for prediction of drug delivery through the skin under different circumstances and for various drugs in the novel, miniaturized diffusion chamber.

Evaluation of the predicted time-concentration profile of serum tulobuterol in human after transdermal application

We proposed an in vitro/in vivo/in silico method for evaluating the clinical performance of matrix type transdermal therapeutic systems (TTSs). This method is based on the following four approaches: (1) drug release experiment, (2) in vitro penetration experiment using excised hairless mouse skin, (3) clinical pharmacokinetic study, and (4) mathematical model for evaluating the pharmacokinetic profile. The tulobuterol TTS was used as an example of a matrix type TTS in this study. The drug diffusion coefficient in the matrix device was calculated from the result of the release experiment. The drug diffusion coefficient and the partition coefficient in the skin were calculated from the results of in vitro skin penetration experiments where hairless mice and rats were used. Those parameters were used as substitutes of human. Further, these parameters were used for solving the governing partial differential equation on skin penetration. The time profiles of the serum concentration in human after applying the tulobuterol TTS were predicted and compared with the clinical data. The predicted profiles obtained from the data of hairless mice reproduced the influence of drug depletion adequately and well agreed with the clinical data, while those from the data of rats differed clearly in the initial rise. This method is useful for prediction of pharmacokinetic profiles of TTSs.

The prediction of plasma and brain levels of 2,3,5,6-tetramethylpyrazine following transdermal application

The purpose of this study was to construct a pharmacokinetic (PK) model and to determine PK parameters of 2,3,5,6-tetramethylpyrazine (TMP) after application of TMP transdermal delivery system. Data were obtained in Sprague-Dawley (SD) rats following a single dose of TMP transdermal delivery system. Blood samples were obtained at 0, 0.25, 0.5, 1, 2, 4, 6, 16, and 24 hours after the transdermal application. In the brain level study, 18 SD rats were divided into 6 groups. Three SD rats before and after transdermal application were culled and sacrificed at each of the following time intervals: 2, 4, 6, 16, and 24 hours after the TMP-TTS application. TMP concentrations in plasma and brain tissues were determined using high performance liquid chromatography and data were fitted using a zero-order absorption and a first-order-elimination 3-compartment PK model. Fitted parameters included 2 volumes of distribution (V1, V2) and 2 elimination rate constants (k10, k20). The elimination half-life for TMP in plasma and brain was 26.5 and 31.2 minutes, respectively. The proposed PK model fit observed concentrations of TMP very well. This model is useful for predicting drug concentrations in plasma and brain and for assisting in the development of transdermal systems.

Terbutaline transdermal delivery: preformulation studies and limitations of in-vitro predictive parameters

A transdermal dosage form of terbutaline may be useful to prevent nocturnal wheezing by providing prolonged duration of action. It will also improve patient compliance and bioavailability. Controlled input of the drug would be an additional advantage as this will reduce the intersubject variability. Preformulation studies were conducted to determine the feasibility of a transdermal dosage form of terbutaline. The drug solubility in propylene glycol was 6.3 mg mL-1. The apparent partition coefficient (n-octanol/ deionized-water, pH 6.5) of terbutaline was 0.03. A pH-partition coefficient (octanol/buffer) profile indicated that the partition coefficient values were 0.02, 0.05 and 0.4 in buffers of pH 3, 7.4 and 9, respectively. The required drug flux through the human skin to attain therapeutic concentrations in the blood was calculated to be 3.3 micrograms cm-2 h-1 for a 10-cm2 transdermal delivery system. Rabbit, guinea-pig and human skin was tested as the penetration barrier using modified Franz diffusion cells. Terbutaline flux values through the rabbit and guinea-pig skin were 8.3 and 7.7 micrograms cm-2 h-1, respectively. The flux through human full-thickness skin and human epidermis were 0.6 and 3.6 micrograms cm-2 h-1. Azone (3% w/v), a skin penetration enhancer, significantly increased the drug flux through all the membranes tested. Based on these studies, transdermal delivery of terbutaline appears to be promising.

Comparative in vitro diffusion studies for atenolol transdermal delivery system

The purpose of this study was to screen atenolol as a candidate for transdermal drug delivery, and to study the release of atenolol from various gels. The percent atenolol release over time profiles indicates that for all the gelling agents used the amount of atenolol released is decreased by increasing the polymer concentration in the gel. The amount of atenolol released was found to be higher from Klucel gels, compared to Methocel and Carbopol gels, with Carbopol gels giving the least release. In studying the effect of atenolol concentration in the gel, it was observed that the amount of atenolol released was increased by increasing the drug concentration in the donor up to a limit corresponding to an atenolol concentration of approximately 2%. This seems to resemble the saturation solubility of atenolol into the gels. Hairless mouse skin and TESTSKIN LSE were used as in vitro skin models.

Bioavailability of topically administered steroids: a "mass balance" technique

The percutaneous absorption of four steroids (hydrocortisone, estradiol, testosterone, and progesterone) has been measured in vivo in man under occluded and "protected" (i.e., covered, but non-occlusive) conditions. The experimental approach, involving simple modifications of standard radiochemical methodology, has enabled excellent "mass balance" and dose accountability to be achieved. Consequently, the utility of the procedure for the measurement of in vivo topical bioavailability can be inferred. In addition, because of the precision and accountability of the results, the technique offers a potential means to establish quantitative structure-penetration relationships for skin absorption in man. It was found that steroid absorption increased with increasing lipophilicity up to a point, but that penetration of progesterone (the most hydophobic analog studied) did not continue the trend and was at least partly rate-limited by slow interfacial transport at the stratum corneum-viable epidermis boundary. Comparison of data obtained from the occluded and "protected" experiments permitted the effect of occlusion (defined as the complete impairment of passive transepidermal water loss at the application site) to be assessed. Occlusion significantly increased percutaneous absorption of estradiol, testosterone, and progesterone but did not effect the penetration of hydrocortisone. A mechanism is proposed to explain why the absorption of the more lipophilic steroids is enhanced by occlusion but that of the most water-soluble (i.e., hydrocortisone) is not. It is suggested that the rate-determining role of the sequential steps involved in percutaneous absorption can be revealed by experiments of the type described using related series of homologous or analogous chemicals.

Transdermal drug delivery and cutaneous metabolism

The delivery of drugs via the skin to achieve systemic therapeutic effect is currently under intense investigation. The skin offers unique advantages and limitations for drug input into the body. For example, while hepatic first pass may be circumvented, the excellent barrier function of the stratum corneum (the thin outermost layer of skin) precludes, at present, all but the most potent drugs from this route of administration. Examples of approved transdermally delivered drugs are scopolamine, nitroglycerin, clonidine and estradiol. The delivery systems which have been formulated for these agents have been designed to provide essentially zero-order input kinetics for between 1 and 7 days. The impact of cutaneous metabolism on transdermal drug delivery has not yet been evaluated rigorously. Limited in vivo data for nitroglycerin suggest a cutaneous first pass effect of between 10 and 20%. More work has been directed towards the use of topical prodrugs and the design of molecules better able to transport across the stratum corneum and then undergo local enzymatic activation. Further research in this area will require a more specific quantitative understanding of the metabolic capabilities of human skin in vivo.

Absorption of materials by or through the living skin

Synopsis The skin comes into contact with a large range of materials either deliberately or inadvertently. It should be possible to predict the exact transport rates of these materials through the skin as a function of the physicochemical properties of the different compounds. With this sort of knowledge it is possible to predict the exact disposition of compounds and use this in the formulation of new products both in the pharmaceutical and cosmetic field. The information will also be useful from the standpoint of skin toxicology and environmental health. In order to be able to predict this complex process it is necessary to split the overall transport into different component parts. This article will identify these components and provide illustrations. The major areas discussed will be barrier function of the skin, the release properties of different topical formulations and how these may be monitored. Novel means of enhancing the penetration of drugs will be discussed and how some additives that are incorporated into formulations will perhaps alter the barrier function of skin. A mathematical model describing skin penetration has been developed and its use in predicting blood levels will be described. This model has been tested both in animal experiments and in limited human studies and its relevance to these situations will be highlighted. Absorption cutanée et transcutanée in vivo.