Synthesis and transdermal penetration of selected lamivudine derivatives

Abstract

The skin is the most extensive and readily accessible organ in the body. The outermost layer of the skin, the stratum corneum, functions as a barrier, limiting the transport of molecules into and across the skin. Transdermal drug delivery offers several advantages over oral and parenteral dosing that include a non-invasive treatment, improving bioavailability and patient compliance, bypassing of hepatic first pass metabolism, decreasing the administered dose and gastrointestinal adverse effects as well as the quick discontinuation of treatment. A hydrophilic compound will have trouble partitioning into the stratum corneum from its vehicle and a lipophilic compound may have difficulty leaving the stratum corneum. Optimal transport through the skin requires a drug to possess lipophilic as well as hydrophilic properties. Research suggests that a drug should have an aqueous solubility of more than 1 mg/ml and an octanol-water partition coefficient (log P) between 1 and 2 to optimally penetrate the skin. Approximately 40.3 million people were living with HIV/AIDS at the end of 2005, which is generally treated with Nucleoside Reverse Transcriptase Inhibitors (NRTls), like zalcitabine and lamivudine. NRTls have a bitter taste and the most common adverse effects occurring with these two compounds are abdominal pain, nausea, vomiting, diarrhoea and mouth ulcers. The aim of this study was primarily to determine the transdermal permeation of zalcitabine, lamivudine and the synthesised amide esters of lamivudine, with and without the use of pheroidTM as delivery system and to establish a correlation, if any, with selected physicochemical properties. The six amide ester derivatives of lamivudine were prepared by acylation esterification of lamivudine with six different acid chlorides. The structures of the products were confirmed by mass spectrometry (MS), nuclear magnetic resonance spectroscopy (NMR) and infrared spectroscopy (I R). The aqueous solubility of all compounds was higher at pH 5 than at pH 7. The aqueous solubility of lamivudine at both pH 5 and 7 (114.36 mg/ml and 91.57 mg/ml, respectively) was lower than that of zalcitabine (144.78 mg/ml and 110.16 mg/ml, respectively), but was distinctly higher than that of the synthesised amide esters of lamivudine (ranging from 1.00 x 10-4 to 8.34 mg/ml and 1.00 x 10-4 to 6.16 mg/ml, respectively). The octanol-PBS partition coefficient (log D) of lamivudine and its amide esters was lower at pH 5 than at pH 7. Of all compounds zalcitabine had the lowest log D at both pH 5 and 7 (-1.50 and -1.78, respectively). The log D of lamivudine at both pH 5 and 7 (-1.19 and -1.15, respectively) was lower than that of the amide esters (ranging from 0.12 to 4.55 and 0.25 to 4.88, respectively). Hence, there was a direct correlation between the aqueous solubility and the log D at both pH 5 and 7 for all compounds. A comparison between average and median flux of the amide esters of lamivudine show that there is a good correlation between the flux values in PBS and in pheroidTM, (except for N-butyryllamivudine-5'-buterate). Compounds with higher flux, like zalcitabine and lamivudine, seem to be prone to larger differences between average and median flux. In the occurrence of large variation and skewed distributions of experimental values, the median flux is a more robust measurement. Therefore median flux was used as a more accurate method for determining flux. In vitro penetration was measured through excised female human abdominal skin in Franz diffusion cells. The median flux of lamivudine (4.289 mol.cm-2.h-2) in PBS was higher than that of zalcitabine (0.442 mol.cm-2.h-1), but in pheroidTM, zalcitabine had a slightly higher median flux (0.015 mol.cm-2.h-1) than lamivudine (0.011 mol.cm-2.h-1). In both PBS and pheroidTM, the median flux of lamivudine was higher than that of the amide esters (2.0 x10-4 to 0.046 mol.cm-2.h-1 in PBS and 2.0 x 10-4 to 9.3 x 10-3 mol.cm-2.h-1 in pheroidTM). Of all the amide esters of lamivudine, N-acetyllamivudine-5'-acetate (in PBS) and N-propionyllamivudine- 5'-propionate (in pheroidTM) presented the highest flux. When comparing flux in PBS with that in pheroidTM it is observed that all the compounds have lower flux in pheroidTM except N-hexanoyllamivudine-5'-hexanoate. Hence, pheroidTM does not improve transdermal flux of this series of compounds. In this study a direct correlation between the aqueous solubility and transdermal flux was found. A strong statistically significant correlation was observed between flux in both PBS and in pheroidTM and each of molecular weight, aqueous solubility (at pH 5 and 7), and log D (at pH 5 and 7); as was determined with a 5 % level of confidence using the Spearman correlation. Yellow spots were observed in the confocal laser scanning microscopy (CLSM) micrographs which confirmed that the compounds were entrapped in pheroidTM. The more hydrophilic compounds had a decrease in microsponge size as they became more lipophilic from zalcitabine to N-butyryllamivudine-5'-buterate and thereafter an increase was noticed from N-butyryllamivudine-5'-buterate to N-decanoyllamivudine-5'-decanoate. Hence, it seems that hydrophilic drugs permeate easier when entrapped in pheroidTM than lipophilic compounds.