Percutaneous absorption of cyclizine and its alkyl analogues
Monene, Lesibana Mishack
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Percutaneous delivery of drugs promises many advantages over oral or intravenous administration, such as a better control of blood levels, a reduced incidence of systemic toxicity, an absence of hepatic first-pass metabolism, better patient compliance, etc. However, the dermal drug transport is limited by the unsuitable physicochemical properties of most drugs and the efficient barrier function of the skin. Thus, numerous attempts have been reported to improve topical absorption of drugs, concentrating mainly on the barrier function of the stratum corneum by use of penetration enhancers and/or skin warming. An alternative and interesting possibility for improved dermal permeability is the synthesis of derivatives or analogues with the aim of changing the physicochemical properties in favour of skin permeation, efficacy and therapeutic value. Cyclizine (I) is an anti-emetic drug primarily indicated for the prophylaxis and treatment of nausea and vomiting associated with motion sickness, post operation and Meniere's disease. It acts both on the emetic trigger zone and by damping the labyrinthine sensitivity. Pharmacologically it has anti-histaminic, antiserotonergic, local anaesthetic and vagolytic actions. It is widely used and also suitable for children from six year of age. Percutaneous absorption of (I) can, among others, avoid the "first-pass" effect and the discomfort of injection. The main objective of this study was to explore the feasibility of percutaneous absorption of (I) and its alkyl analogues via physicochemical characterization and assessment of their permeation parameters. The intent was also to establish a correlation between the physicochemical properties of these compounds and their percutaneous rate of absorption. To achieve these objectives, the study was undertaken by synthesizing the alkyl analogues and determining the physicochemical parameters relevant to skin transport. Identification and level of purity for the prepared analogues were confirmed by mass spectrometry (MS), nuclear magnetic resonance (NMR) spectrometry and infrared (IR) spectrometry. Experimental aqueous solubility (25 °c & 32 °C) and partition coefficient for each compound were determined. In vitro permeation studies were performed at pH 7.4, using Franz diffusion cells with human epidermal membranes. Diffusion experiments were conducted over a period of 24 hours maintaining a constant temperature (37 DC) by means of water bath. All samples were analysed by high pressure liquid chromatography (HPLC). Cyclizine (I) has a methyl group at N-4. Increasing the alkyl chain length on N-4 of the piperazine ring resulted' in compounds with lower melting points and higher water solubility than (I). (II) exhibited 3-fold increase in water solubility, followed by (IV) with about 2.5 fold increase. The water solubility of (III) was almost the same as that of (I). Log partition coefficients increased linearly with increasing alkyl chain length. The analogues therefore, possessed more favourable physicochemical properties to be delivered percutaneously. Indeed, the in vitro skin permeation data proved that these analogues could be delivered more easily than (I) itself. The flux of (I) was 0.132 ug/cm2/h in a saturated aqueous solution. Compound (II) resulted in a 53-fold (6.952 ug/cm2/h) increase in permeation compared to (I). (III) and (IV) resulted in a 2- and 5fold enhancement of permeation respectively. Based on the results of the study, it seems that increased aqueous solubility and low level of crystallinity play a vital role in optimizing percutaneous absorption of (I) and its alkyl analogues. But the importance of the effect of increased lipophilicity cannot be ignored. The low percutaneous• absorption of (I) might be attributed to its low aqueous solubility and increased crystallinity, as is evident from the higher melting point than the analogues. From all the permeability data using aqueous solutions, it is clear that compound (II) is the best permeant of this series and in addition it is known that this compound antagonizes the effects of histamine.
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