No abstract

The rheology of a new topical vehicle FAPG base was investigated over the temperature ranges of 25-37'. Continuous shear rheograms obtained with a Ferranti-Shirley cone and plate viscometer were hysteresis loops with spur points; loop areas, yield values and apparent viscosities decreased with increase in temperature and an activation energy for viscous flow of 10.4 kcal mol-l (43.5 kJ mol-l) was derived. In creep (concentric cylinder geometry) the base was viscoelastic with a low limit of linearity with respect to strain. Creep compliance curves showed an initial elastic response, a region of retarded elasticity and a viscous region ; compliances increased and viscosities decreased with temperature rise. Patient acceptance of skin spreadability of the base was assessed using a master curve concept; the spreading properties were close to the preferred values for maximum patient acceptance. It is concluded that FAPG base deserves further study as a vehicle for dermatological use.For skin therapy, the vehicle used should have optimum physicochemical properties for assuring drug stability and release besides inhibiting microbial growth and being aesthetically satisfactory. Recently, a new topical vehicle 'FAPG base was reported to have possible significant advantages over traditional ointments and creams (British Patent No. 1,259,858 and other Patents pending; Garnier, 1971a,b;Rhodes, 1971). We have examined the rheological properties of the material in continuous shear and in creep and have attempted to assess patient acceptance of its spreadability. METHODS A N D RESULTS Material.A production sample of FAPG base was provided by Syntex Pharmaceuticals Ltd., St. Ives House, Maidenhead, Berks. It is a mixture of propylene glycol, stearyl alcohol, polyethylene glycol and glycerol combined to form a gel-like structure with a crystalline network. The product is a smooth, white, soft, non-aqueous, hydrophilic semisolid with a slight pearly sheen (Garnier, 1971b).Microscopy. The base was examined with a Kofler Micro Hot Stage attached to a polarizing microscope. At room temperature the material was anisotropic (Fig. 1A) and slight melting occurred between 32-34'. Between 47-49' the preparation melted to form an isotropic solution which recrystallized in a different form on cooling. The flocculated suspension appearance was replaced by a mosaic of thin, flat, anisotropic, polyhedral crystals (Fig. IB). This implies that FAPG base may change its crystalline form on storage to yield a product with different mechanical properties.A Ferranti-Shirley cone and plate viscometer with automatic flow curve recorder unit was used with a large cone (diameter 7 cm, Continuous shear rheology.