Estimation of intradermal disposition kinetics of drugs: I. Analysis by compartment model with contralateral tissues

Dermatopharmacokinetics of salicylate following topical injection in rats: Effect of osmotic pressure and injection volume on salicylate disposition

Using advanced topical formulations containing potential chemical enhancer(s) or physical penetration-enhancing tools capable of delivering entrapped drug(s) directly into skin tissues with little influence of the stratum corneum barrier, local and systemic drug disposition may be markedly similar to direct injection into the skin and muscle. The objective of this study is to investigate the dermatopharmacokinetics and systemic drug disposition after topical application and topical injection. Salicylate (SA) disposition in the skin and muscle as administration sites, and in the systemic circulation were evaluated following intracutaneous (i.c.) injection of an isotonic solution of SA-Na (dose; 3.08 micromol). Subcutaneous (s.c.) and intramuscular (i.m.) injection were also evaluated for comparison. Dermatopharmacokinetics and systemic disposition of SA after i.c. and s.c. injections were analyzed using a 4-compartment model consisting of skin, muscle, and central and peripheral compartments, whereas SA disposition after i.m. injection was analyzed using a 3-compartment model consisting of muscle, and central and peripheral compartments. Moreover, the absorption rate constant of SA after i.c. injection (0.073 min(-1)) was slightly lower than that after s.c. injection (0.083 min(-1)), and much lower than that after i.m. injection (0.327 min(-1)). In addition, higher osmolarity and a larger volume of SA-Na injectant increased the retention of SA in the skin and decreased the absorption rate to the systemic circulation after i.c. injection. The effect of injection volume on SA disposition after i.c. injection was not so marked compared with that of osmotic pressure. These results are useful to design an injection-type topical delivery system.

Enhancement of topical delivery of drugs via direct penetration by reducing blood flow rate in skin

The purpose of this work was to investigate the effect of blood flow in the skin on the direct penetration of topically applied drugs into the muscular layer, and to show that the skin blood flow could also be one of the important factors determining the direct penetration of drugs to the muscular layer. In vivo percutaneous absorption study was performed for antipyrine, salicylic acid or diclofenac by using rats with tape-stripped skin. Phenylephrine, which is well known to reduce the local blood flow by vasoconstrictor action, was topically applied to decrease the local blood flow in the skin. The concentrations of drugs in viable skin and muscle, and the local blood flow in the skin under the applied and the contralateral sites were determined to evaluate the effect of the local blood flow on the delivery of topically applied drugs into the muscular layer. Dose dependency for the effect of phenylephrine was, first of all, investigated for antipyrine in the range from 0.4 to 10 micromol. The distribution of antipyrine into the viable skin and muscular layer 2 h after topical application significantly increased, but the effect of phenylephrine was saturated around 2 micromol and the dose-dependent profiles for both tissues were almost superimposed. On the other hand, the fraction dose absorbed, plasma concentration and concentrations in viable skin and muscular layer under the contralateral site showed the decreasing tendency and the saturation of the effect around 2 micromol. To confirm the effect of phenylephrine on the local blood flow in the skin, the skin blood flow was measured 2 h after topical application of 2 micromol phenylephrine, and the significant decrease in the blood flow was recognized. In vivo percutaneous absorption studies were performed for salicylic acid and diclofenac, too. Extensive enhancement of penetration into the viable skin and muscular layer was observed for both drugs, although total absorption from the donor cell showed the decreasing tendency. In conclusion, direct penetration of drugs applied topically is enhanced by reducing the local blood flow in the skin, which would be a possible approach to improve the local delivery of drugs applied topically.

Estimation of intradermal disposition kinetics of drugs: II. Factors determining penetration of drugs from viable skin to muscular layer

To develop a more efficient transdermal delivery system, it is very important to regulate the intradermal disposition of drugs after topical application. We tried to elucidate the factors determining the intradermal disposition kinetics, especially drug penetration from the viable skin to the muscular layer, mainly based on the six-compartment model, including the contralateral skin and muscle for ten model drugs with different physicochemical characteristics. In vivo transdermal absorption study was performed for six model drugs using the stripped-skin rats. The fitting analyses by the six-compartment model gave the theoretical curves describing the observed data very well and the reasonable pharmacokinetic parameters, showing the pharmacokinetic model should be useful for the estimation of the intradermal disposition kinetics of drugs applied topically again. The simulation study using the pharmacokinetic parameters obtained above could show the relative contribution of the direct penetration and the distribution from the systemic circulation to the muscular distribution of drugs. The largest contribution of direct penetration was observed for antipyrine (90.8%) and the smallest was for felbinac (43.3%). Among the pharmacokinetic parameters obtained above, the clearance from the viable skin to the muscle (CL(vs-m)) was found to be significantly correlated with the unbound fraction of drugs in the viable skin (fu(vs)). Although the clearance from the viable skin to the plasma (CL(vs-p)) also tended to increase as fu(vs) increased, the ratio of CL(vs-m) to CL(vs-p) was significantly correlated with fu(vs), meaning that the larger amount of unbound drug in the viable skin significantly contributes to the direct penetration into the muscle more than to the systemic absorption. On the other hand, k(direct) values obtained in in vitro penetration study-the penetration rate constant of drugs from the viable skin to the muscular layer-were found to be correlated with CL(vs-m) values for seven model drugs. Therefore, adding the in vitro experiments for the other three model drugs, the multiple linear regression analysis of k(direct) was performed for ten model drugs in terms of fu(vs), logarithm of the partition coefficient (Log P) and molecular weight. The results clearly showed the largest and significant contribution of fu(vs) to the direct penetration of drugs from the viable skin to the muscular layer, indicating that a drug with the higher value of fu(vs) in the viable skin can penetrate more into the muscular layer.

Prediction of optimal injection protocol for tumor detection in contrast-enhanced dynamic hepatic CT using simulation of lesion-to-liver contrast difference

The detection of hepatic lesions in computed tomography (CT) is improved when a contrasting material is administered intravenously, but it depends considerably on how this agent is injected. In this study, we constructed a compartment model for a liver containing a hepato-cellular carcinoma (HCC), and simulated the difference in lesion-to-liver contrast enhancement. Using this model, we simulated the effects of various injection factors on the detectability of a lesion and searched for the optimal injection protocol for detecting tumors in contrast-enhanced dynamic hepatic CTs. We found that the mass injection rate, or k-value (in mg/s), (which is the volume injection rate, in ml/s, multiplied by the contrast agent concentration, in mg/ml) plays a critical role in the injection protocol optimized for tumor detection, and has a unique value for a given body weight independent of the total injection dose.