Characterization of an anti-Trop-2-SN-38 antibody-drug conjugate (IMMU-132) with potent activity against solid cancers.

3107 Background: IMMU-132 is an antibody-drug conjugate (ADC) made from a humanized anti-Trop-2 mAb (hRS7) coupled through a linker to SN-38, the active metabolite of CPT-11. Trop-2 is found in a wide range of tumor, including gastric, pancreatic, triple-negative breast (TNBC), colonic, prostate, and lung. Current Phase I/II clinical trials confirm the anticancer activity of IMMU-132 in cancers expressing Trop-2. The current studies further characterize IMMU-132 in terms of conjugation, mechanism of action (MoA), and efficacy. Methods: SN-38 conjugation to hRS7 was analyzed by HIC, LC-MS, and HPLC. MoA was assessed by comparisons in binding, ADCC, and pro-apoptotic signaling pathways. Efficacy studies were performed in mice bearing human tumor xenografts. Results: IMMU-132 has a drug to antibody ratio of 7.6, <1% aggregation, and no loss in binding to cells or to a Trop-2 chip (cells: KD = 0.63 ± 0.26 nM v 0.54 ± 0.17 nM; BIACore: 0.26 ± 0.14 v 0.51 ± 0.04 nM, IMMU-132 and hRS7, respectively). Linkage chemistry should preserve the active lactone ring form; initial in vivo data suggest that conjugated SN-38 may not be glucuronidated until released. IMMU-132 retains binding to the neonatal receptor, but lost 65% of ADCC activity. Free SN-38 and IMMU-132 mediated the same signaling pathways in cells, with p21WAF1/Cip1 up-regulation, cleavage of caspase 3 and 9, and poly-ADP-ribose polymerase cleavage. IMMU-132 treatment of gastric cancer xenografts (17.5 mg/kg; 2xwk x 4wks) result in significant anti-tumor effects compared to non-specific control ADC (P<0.0001). In tumors not typically treated with irinotecan as a single agent (pancreatic and TNBC), IMMU-132 provided significant tumor growth inhibition. Mice with a TNBC xenograft treated with 12.5 mg/kg IMMU-132 (q4dx4) resulted in 7/8 mice alive at study conclusion (day 105) v only 1/9 in saline-control group (P=0.001). Tumor regressions occur in mice bearing aggressive pancreatic tumor xenografts (untreated median survival = 11 d) when clinically relevant doses of IMMU-132 is given (HED=8 mg/kg; P=0.0003). Conclusions: Conjugation of SN-38 to hRS7 results in a potent ADC that retains characteristics of both the antibody and the drug.