|dc.description.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
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
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.||