On the mechanism of drug release from oil suspensions in vitro using local anesthetics as model drug compounds.

The objective of this study was to gain insight into factors influencing the drug release kinetics from oil suspensions. The in vitro drug release from suspensions was investigated at pH 7.4 using the local anesthetics, bupivacaine and ropivacaine, as model drug compounds. Two dialysis membrane-based in vitro release models differing with respect to stirring of the donor compartment were employed to study the release characteristics of oil suspensions comprising the free base or the corresponding drug hydrochloride salt. In the rotating dialysis cell model identical release profiles from aqueous and oil suspensions of the base form were obtained for both ropivacaine and bupivacaine. From the steady state fluxes, drug concentrations in the aqueous donor compartment were found to be in agreement with drug solubilities at pH 7.4. Also relatively fast transformation of a sesame oil suspension of the oil insoluble ropivacaine hydrochloride salt into an aqueous suspension of ropivacaine base was observed. Collectively, these observations indicate a lability of the oil film surrounding the solid particles eventually caused by rotation of the donor cell. In the Float A Lyzer model, which operates at much less intensive stirring, significantly slower release rates from aqueous and oil suspensions of ropivacaine base were obtained. In the latter model, ropivacaine was released faster from oil suspensions containing the hydrochloride salt than from the corresponding oil suspensions of the free base form. These findings suggest that the oil film surrounding the particles also is instable in the absence of significant shear forces.