Abstract 2462: In Vivo efficacy of a novel anti-CD33 targeted thorium conjugate (TTC) in mouse models of acute myeloid leukemia (AML)

Abstract

Abstract Introduction: The sialoadhesin receptor CD33 is a well-established target in the treatment of acute myeloid leukemia (AML). CD33 is expressed on leukemic blasts in AML and possibly also on leukemic stem cells in some patients (1). Thorium-227, an alpha-particle emitting radionuclide with a half-life of 18.7 days, is being studied for use as a possible cytotoxic agent. We have developed a novel Targeted Thorium Conjugate (TTC), comprising an anti-CD33 monoclonal antibody conjugated to a chelator for the efficient delivery of high-energy alpha-particles. We present data from in vitro cell cytotoxicity assays as well as biodistribution and efficacy data from preclinical animal models of AML. Experimental procedures: Antibody conjugates were prepared and labeled with 227Th at room temperature giving high radiochemical yields and purity. In vitro cell cytotoxicity experiments were carried out on HL-60 cells and the multi-drug resistant cell line KG-1 by measuring cell viability and monitoring apoptosis. In vivo efficacy and biodistribution were performed in a sub-cutaneous model of AML in athymic nude (nu/nu) mice using HL-60 cells. The 227Th-anti-CD33 conjugate was further tested in a disseminated model of AML using HL-60 cells, injected intravenously 5 days prior to treatment in CB.17 SCID mice with hind-leg paralysis as endpoint. Results: The 227Th-anti-CD33 conjugate was a potent and specific inducer of in vitro cell cytotoxicity independent of MDR-status. Mechanistic studies demonstrated cell arrest in G2 phase with cytotoxicity associated with apoptotic pathways. In vivo biodistribution in a sub-cutaneous model demonstrated a high tumor to blood ratio (TBR ∼ 6) at day 7 post injection based on measured 227Th-activity. Significant efficacy was achieved in two models of AML using human HL-60 cells. In athymic nude mice, a single injection of 227Th-anti-CD33 conjugate at 700 kBq/kg resulted in complete regression of established sub-cutaneous tumors. Importantly, no significant loss in body weight was observed during the course of the study, demonstrating that the dose was well tolerated. In parallel, statistically significant dose dependency of 227Th-anti-CD33 conjugate was demonstrated in a disseminated model of AML with median survival times (MST) of 90 days and 116 days for animals receiving 50 and 150 kBq/kg. Around 80% of animals dosed with either 2 × 150 or 1 × 300 kBq/kg where still alive at the end of the study with no gross pathological findings observed. Conclusions: The data presented are supportive of the further investigation of the 227Th-anti-CD33 conjugate for the treatment of AML.