Lintuzumab-Ac225, a CD33-Directed Antibody Radiotherapy, Targets AML in a Mutation Agnostic Manner
Authors: Chin Amanda , Jiao Rubin , Allen Kevin J.H. , Li Jason , Chen Mary , Vusirikala Madhuri , Xu Le-Cun , Brodin Patrik , Roy Monideepa , Malo Mackenzie , Van der Touw William , Dadachova Ekaterina , Beckford-Vera Denis , Kotanides Helen 
Published: December 2023

Introduction
Acute myeloid leukemia (AML) is a highly heterogeneous hematologic malignancy with a poor survival prognosis. Recurrent gene mutations are the major obstacle that impact treatment efficacy of AML patients. Actimab-A, an anti-CD33 antibody conjugated with Actinium-225 (Ac-225) alpha particle-emitting radionuclide (lintuzumab-Ac225), has demonstrated promising therapeutic responses in AML patients, including in combination trials with chemotherapy regimen CLAG-M and targeted therapy venetoclax. Because lintuzumab-Ac225 broadly targets myeloid cells via CD33, we hypothesized that anti-leukemic activity is agnostic of genetic abnormalities commonly found in relapsed/refractory AML, including FMS-like tyrosine kinase-3 ( FLT3) mutations and mixed lineage leukemia ( MLL) rearrangements. While small molecule FLT3 and menin inhibitors that target these patient subpopulations show anti-leukemic benefit, most patients eventually relapse. This study uses preclinical models to evaluate the anti-leukemic activity of lintuzumab-Ac225 in FLT3 and MLL mutation harboring AML cells as either a single agent or in combination with FLT3 or menin inhibitors.
Methods
Lintuzumab-Ac225 was generated by conjugating lintuzumab with p-SCN-Bn-DOTA and subsequently radiolabeled with Ac-225. A viability assay was performed using flow cytometry to assess the response of the FLT3 mutant and MLL rearranged AML cell line MV-4-11 to lintuzumab-Ac225, FLT3 inhibitor (Gilteritinib) or MLL inhibitor (Revumenib) single agents and the combinations. Furthermore, the effects of lintuzumab-Ac225 on AML growth in vivo were investigated using the subcutaneously injected MV-4-11 leukemia xenograft model in nude mice.
Results
Treatment of MV-4-11 AML cells with lintuzumab-Ac225 revealed a dose-dependent reduction of leukemia cell viability which was significantly more potent relative to cold lintuzumab. Single agent cytotoxicity of Gilteritinib or Revumenib was also observed, but the combination of lintuzumab-Ac225 with FLT3 or MLL inhibitors was additive and synergistic at all dose levels studied. In a preclinical AML model in vivo, each inhibitor delayed tumor growth as a monotherapy. However, the combination of each of these inhibitors with lintuzumab-Ac225 demonstrated statistically significant enhanced tumor control in the MV-4-11 AML model.
Conclusions
Lintuzumab-Ac225 has potent anti-leukemic activity in AML cells harboring FLT3 or MLL genetic abnormalities. Combination of CD33-targeted radiation with either FLT3 or menin inhibitors significantly improves AML control, suggesting how FLT3 and/or menin efficacy may be augmented by radiation-induced DNA damage. The combination of lintuzumab-Ac225 to existing mutation-targeted standard-of-care may enhance AML targeting and treatment durability versus single-agent approaches.