Dissolution/permeation with PermeaLoop™: Experience and IVIVC exemplified by dipyridamole enabling formulations

It is our hypothesis that the presence of an absorptive sink for in-vitro dissolution experiments is decisive to predict extent and duration of super-saturation of low soluble drugs in formulations expected to increase oral absorption, often called enabling formulations. Combined dissolution-/permeation-testing may provide such absorptive sink. Commonly used in-vitro dissolution-/permeation tools have a limited interfacial area-to-donor-volume-ratio (A/V), far below the physiological one which is estimated for humans. In consequence, super-saturation is expected to be more pronounced and thus precipitation to occur more readily in these models as compared to the in-vivo situation. In the current study, a PermeaLoop™ prototype a of a novel in-vitro dissolution-/permeation-tool with a substantially larger A/V was employed to investigate the dissolution and permeation behaviour of model formulations of dipyridamole containing fumaric acid as modifier of the micro-environmental pH. After identifying the most suitable experimental conditions in terms of donor- and acceptor pH and composition, dose, flow-rate and sampling intervals, both the dissolution and the permeation were simultaneously assessed over time and the extent and duration of super-saturation monitored. The importance of biomimetic media in the donor was revealed not only in terms of increasing the dissolution but also the permeation. The formulations were ranked in terms of their performance (cumulative amount permeated). As a result the data generated by PermeaLoop experiments showed for the same formulations a superior correlation with in rat bioavailability data than obtained from a traditional side-by-side Dissolution-/Permeation-system with a Caco-2-cell membrane (D/P-system). The insights into the effects of solubilisers and pH conditions gained in the present study contribute to an improved mechanistic understanding of dynamic dissolution/permeation behaviour of weakly basic drugs and their enabling formulations. Challenges with the current PermeaLoop prototype are still to be solved, as dispersed drug still tends to get stuck inside the system, but gained experiences are helpful for the improvement of the design.