RHEOLOGICAL CHARACTERIZATION OF RBST OIL SUSPENSIONS

The low shear theology of numerous rbSt sustained release oil suspensi ons was studied in order to assess the impact of formulation variables on product characteristics. A Haake CV 100 was used to obtain rheogra ms of rbSt suspensions in a Miglyol 812/Tween 80 vehicle as a function of drug concentration, drug particle shape and Tween 80 concentration . Rheograms of active formulations revealed a drug-Tween-Miglyol compl ex which increased in structure as a function of Tween 80 concentratio n. However, corresponding placebo formulations failed to show any sign ificant change in Miglyol theology regardless of the Tween 80 concentr ation. The active formulations at 12.5% (w/v) rbSt showed increased de viation from Newtonian behavior as the Tween 80 concentration increase d from 0 to 1.0% (w/v), but thixotropy was not apparent. Increasing th e rbSt concentration to 24% (w/v) while keeping Tween 80 at 0.1% (w/v) caused slight thixotropy and a substantial deviation from Newtonian f low. The above tests were performed using a lot of spray-dried drug co ntaining a high percentage of shattered particles. Further studies wit h a lot of spray-dried drug containing predominantly spherical particl es showed similar theology. Oil suspensions made from lyophilized drug exhibited Newtonian behavior and minimal viscosity. Photomicrographs of the suspensions made with spray-dried drug revealed a dense populat ion of small particles consistent with a high degree of structural the ology while those of the lyophilized drug suspensions showed an open n etwork of overlapping platelet structures consistent with the minimal viscosity observed.