Purpose: This study was designed to evaluate the in vitro cytotoxicity and in vivo efficacy of TRA-8, a mouse monoclonal antibody that binds to the DR5 death receptor for tumor necrosis factor-related apoptosis-inducing ligand (also called Apo2L), alone and in combination with CPT-11, against human colon cancer cells and xenografts.
Experimental design: DR5 expression was assessed on human colon cancer cell lines using flow cytometry, and cellular cytotoxicity after TRA-8 treatment, alone and in combination with SN-38, was determined by measuring cellular ATP levels. Tumor growth inhibition and regression rates of well-established subcutaneous COLO 205, SW948, HCT116, and HT-29 colon cancer xenografts in athymic nude mice treated with TRA-8 or CPT-11 alone and in combination were determined. (99m)Tc-TRA-8 was used to examine tumor localization of TRA-8 in animals bearing each of the four xenografts. In addition, whole-body biodistribution and imaging was carried out in COLO 205-bearing animals using in vivo single-photon emission computed tomography imaging and tissue counting.
Results: DR5 expression was highest on HCT116, intermediate on SW948 and COLO 205 cells, and lowest on HT-29 cells. COLO 205 cells were the most sensitive to TRA-8-induced cytotoxicity in vitro, SW948 and HCT116 cell lines were moderately sensitive, and HT-29 cells were resistant. Combination treatment with TRA-8 and SN-38 produced additive to synergistic cytotoxicity against all cell lines compared with either single agent. The levels of apoptosis in all cell lines, including HT-29, were increased by combination treatment with SN-38. In vivo, combination therapy with TRA-8 and CPT-11 was superior to either single-agent regimen for three of the xenografts: COLO 205, SW948, and HCT116. COLO 205 tumors were most responsive to therapy with 73% complete regressions after combination therapy. HT-29 cells derived no antitumor efficacy from TRA-8 therapy. Tumor xenografts established from the four colon cancer cell lines had comparable specific localization of (99m)Tc-TRA-8.
Conclusions: In vitro and in vivo effects of TRA-8 anti-DR5 monoclonal antibody on four different colon cancer cell lines and xenografts were quite variable. The HT-29 cell line had low surface DR5 expression and was resistant to TRA-8 both in vitro and in vivo. Three cell lines (COLO 205, SW948, and HCT116) exhibited moderate to high sensitivity to TRA-8-mediated cytotoxicity which was further enhanced by the addition of SN-38, the active metabolite of CPT-11. In vivo, the combination of TRA-8 and CPT-11 treatment produced the highest antitumor efficacy against xenografts established from the three TRA-8-sensitive tumor cell lines. All four colon cancer xenografts had comparable localization of (99m)Tc-TRA-8. These studies support the strategy of TRA-8/CPT-11 combined treatment in human colon cancer clinical trials.