Biodegradable microspheres for the sustained release of ppB-HSA to target PDGFβ-receptors in fibrotic tissues

Introduction pPB-HSA is a protein construct, which consists of human serum albumin (HSA), coupled to a cyclic peptide (pPB). This cyclic peptide binds specifically to the PDGF receptor without eliciting an intracellular response. Hence, this construct can be used as a carrier vehicle to target drugs to fibrotic tissues in which PDGF receptors are abundantly present [1,2]. Its efficacy has been shown in several in vivo studies in different animal models [2]. When formulated as a solution for parenteral administration, treatment of fibrosis with this proteinaceous drug construct would involve long-term or even lifelong daily injections, which may jeopardize patient compliance. Therefore, the aim of this study was to develop a solid formulation for the sustained release of pPB-HSA and assess the delivery of the intact protein construct in vivo. pPB-HSA was encapsulated in biodegradable polymeric microspheres using a W/O/W emulsification process. Various mixtures of two semi-crystalline block copolymers consisting of poly -caprolactone, poly ethylene glycol and poly L-lactic acid of different block ratios were used to tailor the release characteristics. The delivery and targeting of the protein construct was evaluated in vivo using a mouse unilateral ureteral obstruction (UUO) kidney fibrosis model. Results and Discussion By varying the ratio of the two polymers, the release of HSA, the model protein, from the microspheres could be tailored from 7 days to 2 months without burst release. To fit the design of the in vivo study, the microsphere formulation with a 50:50 ratio of the polymers, showing a sustained first order release of 14 days, was selected for further optimization. The optimized production process resulted in an encapsulation efficiency of 83% and an in vitro release of >90% after 14 days for the final pPB-HSA formulation. The median particle size of these microspheres was 25 m, which makes them injectable with a 20G needle. All UUO mice were sacrificed 7 days after microsphere administration and ureteral obstruction. The obstructed kidneys had developed fibrosis. The plasma concentration of pPB-HSA in the pPB-HSA treated mice (n=3) was 15 ± 4 ng/mL. Immunohistochemical staining showed the presence of pPB-HSA in the fibrotic kidney of these mice, while the sham kidney was negative for pPB-HSA. Conclusion The pPB-HSA construct was successfully formulated in polymeric microspheres produced by a W/O/W method, which showed a first order release profile in vitro. Furthermore, pPB-HSA was released from these microspheres in vivo. In addition, pPB-HSA was specifically localized in fibrotic tissue, where the target receptor, the PDGF receptor, is abundantly present. The delivery and site specific targeting of pPB-HSA for polymeric microspheres is thus confirmed in this study. Future studies include a pharmacokinetic in vivo study of the pPB-HSA microspheres in a mouse liver fibrosis model. References [1] Poosti F, Bansal R, Yazdani S et al. FASEB J. 2015;29(3):1029–42. [2] Beljaars L, Weert B, Geerts A, Meijer DK, Poelstra K. Biochem Pharmacol. 2003;66(7):1307-17.