Purpose: In situ forming implants (ISFIs) are attractive formulations for long-acting drug release due to their: i) ability to control drug release; ii) simple manufacturing process; and iii) minimally invasive administration. These implants form solid depots in the subcutaneous tissue through solvent-induced phase inversion of biodegradable polymers (such as poly(lactic-co-glycolic acid) (PLGA) and poly(lactic acid) (PLA)). However, the mechanism of implant formation and the changes in their microstructure that determine drug release behavior have not been fully investigated. In the present study, implant formation was recreated in vitro using five different methods to determine the effect of implant formation on the drug release kinetics of ISFIs as well as the microstructures of the implants.
Methods: Five implant formation methods were investigated to determine the effects of implant formation on drug release kinetics of ISFIs. ISFI formulations were prepared via syringe-to-syringe mixing of poly (lactic-co-glycolic acid) (PLGA) in an aprotic solvent (N-methyl-2-pyrrolidone, NMP) with lyophilized leuprolide acetate. Brief descriptions of the methods are as follows: i) medium injection method (direct injection into release medium); ii) flash freezing method (frozen formulation); iii) gelatin capsule method (formulation within gelatin capsules); iv) dialysis tube method (formulation within dialysis tubes); and v) PVA thin film method (formulation within water-dissolvable PVA film sacs). After formation, the implants were cryosectioned using a cryostat (CM3050S, Leica). The implant sections were evaluated using a light microscope. The amount of leuprolide and NMP released from the implants during the first 24 h was determined in order to understand the effect of implant formation on the initial burst release of drug.
Results: Figure 1 shows the effect of implant formation on the initial burst release of leuprolide and NMP. Significant differences were observed between groups. A high burst release was detected in the implants from following three groups (medium injection, flash freezing, and gelatin capsule methods), whereas the other two groups (dialysis and PVA thin film methods) showed a low burst release during the first 24 hr. The media injection, flash freezing, and the gelatin capsule methods generated implants with unexpected expansion or breakage leading to an increase in the surface area, whereas the implants prepared using dialysis tube and PVA thin film methods maintained consistent shape and size. The cryosection images of the implants prepared by the media injection, flash freezing, and gelatin capsule methods showed two phases (solid and gel phases) with irregular depth of the solid phase, whereas the PVA thin film method presented three phases (solid, semi-solid, and gel) as shown in Figure 2.
Conclusion: In this work, the impact of the ISFI implant formation methods on the in vitro drug release is discussed for the first time. In addition, this work reports the first morphological and microstructural demonstration of implant formation of ISFIs. The results revealed that the implant formation method significantly impacts the drug release kinetics of ISFI formulations. Accordingly, choosing an appropriate method to form ISFIs in vitro is crucial in ensuring the effectiveness and bio-relevance of dissolution quality testing.
Acknowledgements: Financial support was provided by the U.S. Food and Drug Administration, 1 U01 FD005169-01. This article reflects the views of the authors and should not be construed to represent FDA's views or policies.
Michail Kastellorizios– Assistant Professor, University of North Texas Health Science Center
Yuan Zou– Staff Fellow, United States Food and Drug Administration
Yan Wang– Staff Fellow, United States Food and Drug Administration
Stephanie Choi– Chemist, US Food and Drug Administration, Maryland
Diane Burgess– Distinguished Professor of Pharmaceutics, University of Connecticut, Storrs, Connecticut