Development of Self-Associating SN-38-Conjugated Poly(ethylene oxide)-Poly(ester) Micelles for Colorectal Cancer Therapy

The clinical use of 7-ethyl-10-hydroxy-camptothecin (SN-38), which is the active metabolite of irinotecan, has been hampered because of its practical water-insolubility. In this study, we successfully synthesized two self-associating SN-38-polymer drug conjugates to improve the water-solubility of SN-38, while retaining its anticancer activity. The polymeric micellar SN-38 conjugates were composed of either methoxy-poly(ethylene oxide)-block-poly(α-benzyl carboxylate-ε-caprolactone) conjugated to SN-38 at the PBCL end (mPEO-b-PBCL/SN-38) or mPEO-block-poly(α-carboxyl-ε-caprolactone) attached to SN-38 from the pendent-free carboxyl site (mPEO-b-PCCL/SN-38). The chemical structure of block copolymers was confirmed by (1)H NMR. The physicochemical characterizations of their self-assembled structures including size, surface charge, polydispersity, critical micellar concentration, conjugation content and efficiency, morphology, kinetic stability, and in vitro release of SN-38 were compared between the two formulations. In vitro anticancer activities were evaluated by measuring cellular cytotoxicity and caspase activation by MTS and Caspase-Glo 3/7 assays, respectively. The hemolytic activity of both micellar structures against rat red blood cells was also measured. The results showed the formation of SN-38-polymeric micellar conjugates at diameters < 50 nm with a narrow size distribution and sustained release of SN-38 for both structures. The loading content of SN-38 in mPEO-b-PBCL and mPEO-b-PCCL were 11.47 ± 0.10 and 12.03 ± 0.17 (% w/w), respectively. The mPEO-b-PBCL/SN-38, end-capped micelles were kinetically more stable than mPEO-b-PCCL/SN-38. The self-assembled mPEO-b-PBCL/SN-38 and mPEO-b-PCCL/SN-38 micelles resulted in significantly higher cytotoxic effects than irinotecan against human colorectal cancer cell lines HCT116, HT-29, and SW20. The CRC cells were found to be 70-fold to 330-fold more sensitive to micellar SN-38 than irinotecan, on average. Both SN-38-incorporated micelles showed two-fold higher caspase-3/7 activation levels than irinotecan. The mPEO-b-PBCL/SN-38 micelles were not hemolytic, but mPEO-b-PCCL/SN-38 showed some hemolysis. The overall results from this study uphold mPEO-b-PBCL/SN-38 over mPEO-b-PCCL/SN-38 micellar formulation as an effective delivery system of SN-38 that warrants further preclinical investigation.