Rapamycin-Loaded Boronic Acid-Based Hydrogel as Artificial Perivascular Tissue for Prevention of Vascular Graft Failure

Introduction: Excessive proliferation of subendothelial smooth muscle cells (SMC) leads to anastomotic stenosis, resulting in an increase in graft failure rate, a big issue of vascular graft transplantation. Perivascular adipose tissue (PVAT), an adipose tissue adherent on the vessel’s external surface, has been shown to inhibit the excessive proliferation of SMC via releasing biomolecules¹. However, the PVAT is mostly absent in vascular grafts involving native and artificial ones. Phenylboronic acid (PBA)-based polymer has been reported to form a hydrogel with synthetic polyvinyl alcohol (PVA) and bind to cell membranes and tissue surfaces via PBA-sialic acid integration². Rapamycin (RPM) has been clinically approved as an anti-stenosis drug because of its cell anti-proliferative ability³. Recently, we synthesized poly (3-acrylamidophenylboronic acid-co-acrylamide) (BAAm) polymer. In this study, we proposed RPM-loaded BAAm/PVA (BAVA) hydrogel serving as an artificial PVAT⁴, thus its tissue adhesive and drug release characterization and anti-SMC proliferative function were investigated. Furthermore, we tested its effectiveness on graft patency in a porcine model.<br/>Method: Two hydrogels with 25 and 50 mg/mL of BAAm (named BAVA25 and BAVA50, respectively) were prepared. A decellularized vascular graft with 2.5mm in inner diameter was selected as the graft model⁵. To examine the tissue adhesive ability, the lap-shear test was done after loading the hydrogel on the graft’s external surface. To assess drug release ability, the RPM-loaded BAVA hydrogel was immersed in PBS solution, and the released RPM was measured every day of 6d after incubation. To test the anti-cell proliferative function, the BAVA hydrogel with or without RPM was co-cultured with porcine SMC cells, then the cell viability was examined at 1d and 3d. Finally, a porcine carotid artery interposition grafting was performed. The external surface of the transplanted graft was then coated with the RPM-loaded BAVA hydrogel, and its graft patency was followed for six months by computer tomography.<br/>Result: The BAVA25 and BAVA50 hydrogels could adhere to the graft surface, and their adhesion was stable for 15d after immersion in PBS solution. In the lap shear adhesion test, initial loading slope (1.5) of the BAVA25 hydrogels was similar to that (1.3) of the BAVA50 hydrogels, and the displacement at fraction point of the BAVA25 hydrogels was approximately two-fold higher than that of the BAVA50 hydrogels. As incubated in PBS solution, the BAVA25 and BAVA50 hydrogels released 83 and 73% of the initial loaded RPM within the first day. As co-cultured with SMC cells, the RPM-loaded BAVA25 and RPM-loaded BAVA50 hydrogels significantly reduced cell proliferation after one day and three days of incubation, respectively. In porcine experiments, the graft without any treatment or with RPM-loaded BAVA50 hydrogel was occluded within three days after transplantation. However, in three cases treated with RPM-loaded BAVA25 hydrogel, two grafts were patent for 10 and 17d and one graft was patent for at least 180d.<br/>Conclusion: The BAVA25 hydrogel had a higher tissue adhesive and more rapid drug release potential than the BAVA50 hydrogel. By loading with RPM, the BAVA25 and the BAVA50 hydrogel showed the in-vitro anti-SMC proliferative function. In the porcine model, the RPM-loaded BAVA25 hydrogel effectively maintained the patency of small-diameter decellularized vascular grafts. The RPM-loaded BAVA25 hydrogel, as an artificial PVAT, might be a therapeutic approach for preventing graft occlusion by suppressing SMC proliferation.<br/>Reference: 1) M. Takaoka, et al. Circ. Res. 105 (2009), 906–911. 2) J. Li, et al., Cell Metab. 19 (2014), 373. 3) M. Li, et al. Carbohydr. Polym. 296 (2022), 119953. 4) H.T. Le, et al. Biomater. Adv. 147 (2023), 213324. 5) A. Mahara et al. Biomaterials, 58 (2015), 54-62.