Purpose: To improve the efficacy and reduce the gastrointestinal toxicity of the cancer prodrug, CPT-11, we have developed immunoconjugates of its active form, SN-38, and an anti-CEACAM5 antibody for targeted chemotherapy.
Experimental design: SN-38 conjugates of the anti-CEACAM5 monoclonal antibody, labetuzumab (hMN-14), varying in the nature of the cross-linker attachment at the drug's 20-hydroxyl position, were evaluated in vitro, in metastatic and/or s.c. human colonic and pancreatic cancer xenografts in nude mice using appropriate controls, and in a CEACAM5-negative tumor model.
Results: A pilot study in a s.c. LS174T model of human colonic carcinoma established the relative effectiveness of different conjugates. In the lung metastatic model of GW-39 human colonic carcinoma in nude mice, therapy with two specific labetuzumab-SN-38 conjugates, using 0.25 mg SN-38 equivalent/kg, q4d x 8, significantly extended median survival time versus controls (P < 0.002). In an expanded evaluation in the s.c. LS174T xenograft model, specific SN-38 conjugates produced significant tumor growth control and increases in median survival time versus other controls, including CPT-11 at a 33-fold greater cumulative dose (P < 0.01). An improvement was also observed in the therapy of a s.c. human pancreatic tumor xenograft. In a CEACAM5-negative systemic lymphoma xenograft, one labetuzumab-SN-38 conjugate examined was ineffective, whereas the conjugate specific for the tumor model produced 100% survival.
Conclusions: The promising labetuzumab-SN-38 conjugates developed showed selective therapeutic efficacy in human tumor models at nontoxic doses that were a fraction of the CPT-11 doses used.