How agarose gels surrounding PLGA implants limit swelling and slow down drug release

The aim of this study was to better understand to which extent and in which way the presence of an agarose gel (mimicking living tissue) around a PLGA [poly(lactic-co-glycolic acid)] implant affects the resulting drug release kinetics. Ibuprofen-loaded implants were prepared by hot melt extrusion. Drug release was measured upon exposure to phosphate buffer pH 7.4 in Eppendorf tubes, as well as upon inclusion into an agarose gel which was exposed to phosphate buffer pH 7.4 in an Eppendorf tube or in a transwell plate. Dynamic changes in the implants' dry & wet mass and dimensions were monitored gravimetrically and by optical macroscopy. Implant erosion and polymer degradation were observed by SEM and GPC. Different pH indicators were used to measure pH changes in the bulk fluids, gels and within the implants during drug release. Ibuprofen release was bi-phasic in all cases: A zero order release phase (~20% of the dose) was followed by a more rapid, final drug release phase. Interestingly, the presence of the hydrogel delayed the onset of the 2nd release phase. This could be attributed to the sterical hindrance of implant swelling: After a certain lag time, the degrading PLGA matrix becomes sufficiently hydrophilic and mechanically instable to allow for the penetration of substantial amounts of water into the system. This fundamentally changes the conditions for drug release: The latter becomes much more mobile and is more rapidly released. A gel surrounding the implant mechanically hinders system swelling and, thus, slows down drug release. These observations also strengthen the hypothesis of the "orchestrating" role of PLGA swelling for the control of drug release and can help developing more realistic in vitro release set-ups.