Percutaneous absorption of cyclizine and its alkyl analogues

Cyclizine (I) alkyl analogues (II-IV) were synthesized and their skin permeation parameters evaluated in vitro. It was hoped that these compounds would possess physicochemical properties more favourable for percutaneous delivery than (I). The identification and levels of purity for the compounds were confirmed by mass spectrometry (MS), nuclear magnetic resonance (NMR) spectrometry, and infrared spectrometry (IR) while melting points were determined by an electrothermal digital Bϋchi melting point apparatus. Aqueous solubilities (25 °C) and partition coefficients were determined and in vitro permeation studies were performed in buffer (37 °C) at pH 7.4 over a period of 24 h, using Franz diffusion cells fitted with human epidermal membranes. Generally, the analogues were more lipophilic, but nevertheless possessed higher aqueous solubilities as compared to (I). (II) and (IV) exhibited two- to three-fold increase in aqueous solubility and their melting temperatures dropped by more than 55 °C. Compound (III) had similar aqueous solubility to (I), but its melting point dropped by about 35 °C. Measured steady-state fluxes indicated that (II) is a far better penetrant (J = 6.95 μg/cm2/h) of human epidermis than (I). Although fluxes of (III) and (IV) drop off markedly from that of (II), they remained above the flux of (I), which is (0.132 μg/cm2/h). In conclusion, (II) was the best skin permeant and also exhibited the highest aqueous solubility and lowest level of crystallinity as compared to (I) and other analogues. (III) and (IV) were more lipophilic. The overall permeation data of this series indicated that the more water-soluble and the lowest melting point compound was the best skin permeant.