No abstract

The effect of drug lipophilicity on in vivo iontophoretic transdermal absorption was evaluated. Non-steroidal anti-inflammatory drugs (NSAIDs) were selected as model drugs with a wide range of lipophilicity: salicylic acid (SA), ketoprofen (KP), naproxen (NP) and indomethacin (IM). Cathodal iontophoresis of NSAIDs was conducted in rats (0.625 mA/cm2; 90 min), and drug concentrations in skin, cutaneous vein and systemic vein were determined. Skin concentrations of NSAID were higher in the case of lipophilic drugs (SA=KP=NPKP=NP>IM). Additionally, the dependence of drug lipophilicity on systemic plasma concentration was similar to cutaneous plasma concentration. The transfer rate from skin to cutaneous vein (R(SC)) was calculated from the arterio-venous plasma concentration difference of drug in the skin. Normalized R(SC) by skin concentration (R(SC)/X(S)) yielded a negative correlation with the logarithm of n-octanol/buffer partition coefficient (Log P at pH 7.4), suggesting that transfer of NSAIDs from skin to cutaneous vein decreased with increasing lipophilicity (SA>KP=NP>IM). This correlation means that drug partitioning between stratum corneum and viable epidermis might be a dominant step.

Transdermal drug absorption includes several sequential processes, as follows: (1) drug absorption into the skin; (2) transfer from skin to cutaneous vein; (3) transport to systemic circulation by blood flow. In systemic therapy, it is necessary to transfer drug molecules from skin (application site) to cutaneous blood flow and to transport to the target organ through systemic circulation. [2][3][4][5][6][7] On the other hand, local therapy needs to accumulate drug molecules at the local site (skin, muscle and joint, etc.), but not to eliminate into the blood flow.2-7) Therefore, the evaluation of drug transfer properties into local blood flow would be expected to predict the systemic and local therapeutic effects.Previously, we have developed a method to investigate transdermal absorption processes in vivo, whereby the drug concentrations in skin, cutaneous vein and systemic vein are measured after iontophoretic delivery of drugs to rats. 8) Further, kinetic analysis of drug transfer from skin to cutaneous vein has been established by modifying a physiological pharmacokinetic model. 9) In the previous study, we investigated the effect of drug lipophilicity on the iontophoretic transdermal absorption of non-steroidal anti-inflammatory drugs (NSAIDs) such as salicylic acid, ketoprofen, naproxen and indomethacin.10) The transfer properties of these NSAIDs from skin to cutaneous vein decreased with an increase in their lipophilicity.The NSAIDs examined previously are acidic drugs with a carboxyl group. In the present study, b-blockers were selected as basic drugs having an amino group and included atenolol (AT), pindolol (PD), metoprolol (MP), acebutolol (AB), oxprenolol (OX) and propranolol (PP). The chemical structures and physicochemical properties are listed in Table 1. 11-13) These b-blockers are adequate for evaluating the effect of lipophilicity on transdermal absorption properties, because the molecular weights are of narrow range. As an indication of drug lipophilicity, we used the logarithm of n-octanol/buffer partition coefficient (Log P), which varied from Ϫ0.11 to 1.49 (pH 7.4, 37°C).12) The difference in the lipophilicity is a consequence of the difference in the aromatic substituents of the b-blockers.Beta-blockers are used in the treatment of hypertension, angina pectoris and cardiac arrhythmia.14) Because of their short elimination half-life (from several to less than 6 h), 15) sustained release formulations for oral administration have been developed to prolong the therapeutic effects.16-21) However, the oral absorption of b-blockers is influenced by hepatic first-pass metabolism. 15,22) Thus, the development of transdermal delivery systems may be expected to achieve good maintenance of optimal blood levels and the avoidance of first-pass effects. Anodal Iontophoresis Anodal iontophoresis was performed as described previously. 10) Briefly, male SpragueDawley rats (8-10 weeks old, Charles River Japan) were anesthetized with ether throughout the experiment, and their body temperature was maintained...