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The pharmacokinetics of aniracetam (AP), a new cognitive performance enhancer, and its main metabolites was investigated after intravenous (iv) and oral administrations to rat. The plasma levels of AP, 4-p-anisamidobutyric acid (ABA), and p-anisic acid (AA) were determined simultaneously by the HPLC method. The plasma concentrations of the parent drug and ABA quickly declined in a biexponential manner, with rapid terminal decay and a small mean residence time. However, AA yielded nonlinearly high levels at the initial times and the plasma concentrations of 2-pyrrolidinone (PD) were sustained over a relatively long time. When AA was administered intravenously, nonlinearity of the plasma concentrations was also found at higher doses. To describe the time course of the plasma levels of AP and its metabolites after iv administration, a pharmacokinetic model with seven compartments was applied, which included 10 first-order rate constants and one Michaelis-Menten constant. An approximate fit was obtained between the observed and calculated curves based on the model, except for the plasma concentrations of ABA. The plasma concentration-time profiles of AP and its metabolites following oral administration of AP (50 and 100 mg/kg) were similar to those after iv dosing, with the exception of PD, which showed much lower plasma levels than those after iv administration. Elimination of AP and ABA was rapid after oral dosing, and the bioavailability of AP was extremely small (11.4 and 8.6%). As a result, AP was largely metabolized to ABA, AA, and PD in rat.

To identify the mechanism involved in the enhancement effect of enhancers on the intercellular penetration of large polar molecules, the skin penetration of fluorescein isothiocyanate (FITC)-dextrans (average molecular weight; 4400, 9400, and 69000 Da) and the lipid removal from the intercellular spaces by enhancers were studied using hairless rat skin. Pretreatment of hairless rat skin with enhancers such as n-octanol (20%), laurocapram (2%), isopropylmyristate (IPM, 20%), oleic acid (5%) and cineol (2%), which are water-immiscible, significantly enhanced the flux of FITC-dextrans, while pretreatment with water-miscible enhancers, i.e. dimethyl sulfoxide (DMSO, 5%) and N-methyl-2-pyrrolidone (NMP) did not increase the flux compared with the control. The penetration of FITC-dextrans was approximately size dependent. n-Octanol, laurocapram, IPM and oleic acid dramatically removed ceramides which are the intercellular lipids, whereas NMP and DMSO partly extracted the sphingolipids. A linear relationship was observed between the flux and removal of ceramides (p < 0.01), indicating that the removal of intercellular lipids would cause dramatic dilations between adherent cornified cells and enhance the penetration through the intercellular pathways. When the penetration of FITC-dextrans through Wistar rat skin was compared with that via hairless rat skin, the steady state flux of FITC-dextrans through Wistar rat skin pretreated with water-immiscible enhancers was 1.2- to 4.9-fold higher, suggesting that the penetration of large polar molecules through follicles may play at least some role in the percutaneous absorption.

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