Tumor-Selective Vascular Modulation by Acid-Cleavable Retro-Inverso Bradykinin Conjugates Enhances the Antitumor Efficacy of Nanomedicine.

Enhancing the tumor accumulation of nanomedicines through vascular modulation represents a promising strategy to improve anticancer drug delivery. Bradykinin (BK), a potent vasoactive peptide, can enhance intratumoral blood flow and vascular permeability; however, its clinical application is hampered by rapid proteolytic degradation and systemic side effects, including hypotension. To address these limitations, we developed P-RIBKs, N-(2-hydroxypropyl)-methacrylamide (HPMA) copolymer conjugates of retro-inverso bradykinin, in which a proteolysis-resistant retro-inverso analog of BK (RIBK) was conjugated via an acid-cleavable hydrazone linkage. RIBK retained vascular permeability-enhancing activity comparable to native BK while exhibiting resistance to proteolytic degradation in plasma. RIBK activated G protein-dependent calcium signaling while failing to induce B2 receptor internalization. Conjugation of RIBK to HPMA copolymer via hydrazone linkage attenuated its vasoactive activity under physiological conditions while enabling tumor-selective release within the acidic microenvironment. The release kinetics of RIBK from P-RIBKs were modulable by varying the substituent adjacent to the hydrazone linkage, and sustained release, as achieved with the phenyl-substituted conjugate (P-PhRIBK), was found to enhance tumor accumulation of a model nanomedicine. Furthermore, pretreatment with P-PhRIBK augmented the antitumor efficacy of liposomal doxorubicin (DOXIL) in a murine colon tumor model. These results demonstrate that P-PhRIBK provides a tumor vascular modulator for improving the delivery and therapeutic efficacy of anticancer nanomedicines.