Effect of latex and plasticizer concentration on glucocorticoid release from ointments

The in vitro release of two glucocorticoids, betamethasone disodium, phosphate (BMNaP) and betamethasone acetate (BMA), is analyzed as a function of time in formulations containing Aquacoat (ethylcellulose latex) and cetylic excipient. An effect of the latex concentration is only observable in BMNaP formulations; the larger the latex concentration the faster the release rate. It is suggested that the aqueous solubility of BMNaP can account for this result; the availability of the corticoid is higher when the aqueous phase fraction in the system is larger. Unlike BMNaP, BMA availability is low whatever the latex concentration in the formulation; since BMA is insoluble in both the aqueous phase and the lipophobic excipient, the relationship between solubility and release behaviour is confirmed. In order to achieve a better control on the release characteristics of the formulation, the effect of the addition of the plasticizers, dibutyl phtalate and propylene glycol, to the ointments is also investigated.