ONC001: A first-in-class ADC targeting CD64 for the treatment of monocytic leukemia developed through patient-guided target and payload selection

Abstract Background: Chronic myelomonocytic and acute monocytic leukemias (CMML and M4/M5 AML) display poor response to SoC therapies and low CR rates. The basis of this unmet medical need is likely related to the intrinsic refractory phenotype of malignant monocytes to the MOAs of current SoC drugs. Identifying novel druggable targets with broad expression across the monocytic maturation lineage, yet with low/null expression in HSCs and other hematopoietic lineages remains a challenge, in particular for the development of antibody drug conjugates (ADCs) for which the therapeutic index is critical. Our ex vivo profiling of 100 myeloid leukemia patients in multiple clinical studies (Bach et al, Blood 2024 ; Andino et al Blood 2023) not only robustly confirmed the refractory nature of malignant monocytes to SoC but also unveiled that the Fc gamma receptor I (CD64) is highly expressed in monocytic precursors and mature monocytes found in CMML and AML patients (up to 3-fold higher than in monocytes from healthy donors). In addition, recent findings indicate that CD64 is a distinctive biomarker of leukemic stem cells from the monocytic lineage (mLSCs), which do not express classical LSC markers such as CD34 or monocytic markers such as CD14 (Pei et al, Cancer Discovery 2023). Taken together, this evidence drove our decision to pursue the development of an ADC targeting CD64. Results: We used OncoPrecision's Patient Micro-Avatar (PMA) ex vivo platform (Garcia et al, Blood 2022) to screen CD64-ADCs conjugated to different payloads in PBMC/BMMC samples from monocytic leukemia patients with differential expression patterns of CD64, including CMML, M4-AML and M5-AML, as well as primitive leukemia patients with myeloid cells negative for CD64 expression (M0-AML). These experiments confirmed the efficient druggability of CD64 by an ADC approach (target engagement + internalization) and enabled us to identify PNU-159682 (PNU) as the payload with the optimal MOA to target monocytic blasts. Interestingly, payloads with proven clinical success in other other malignancies, such as Exatecan and MMAE, displayed poor/null activity against malignant monocytes, thus highlighting a non-obvious vulnerability of these cells to PNU. ONC001, our lead ADC conjugated to PNU through a non-cleavable linker, displays picomolar activity against a battery of monocytic cell lines in vitro and against malignant monocytes derived from 27 patients ex vivo (CD64-pos). Such activity is entirely lost on myeloid cells without CD64 expression, such as M0-AML blasts, as well as in patients' cells derived from the lymphoid compartment (T-cells, B-cells, NK). Notably, strong single agentex vivo activity is observed in CMML and M4/M5 AML patients, substantially outperforming SoC drugs such as Azacitidine (Aza) and Venetoclax (Ven). In addition, we observed a strong combinatory potential with Aza+Ven in an expanded cohort of 44 patients, particularly within M1/M2 AML patients. ONC001 also displayed remarkable single agent efficacy at 1 mg/kg in vivo when evaluated in two complementary mouse CDX models grafted with different human monocytic cell lines (THP-1 and MV-4-11). A kinetic subcutaneous model showed complete remission in all tested animals after a single injection of the ADC, with durability of response for up to 7 weeks after single or double dosing. An orthotopic disseminated leukemia model at a single 1-week endpoint also showed complete remissions after a single injection of the ADC in all tested animals. Importantly, no signs of off-target toxicity were observed in both xenograft models, with no changes in body weight nor any significant alterations in complete blood count and clinical chemistry to monitor liver and kidney function. Conclusions: By leveraging high-throughput patient-derived biology we exposed a bona fide vulnerability of malignant monocytic cells that can be therapeutically exploited by our first-in-class ADC targeting CD64. Given the unique expression profile of CD64 across the monocytic differentiation lineage, with low/null expression in the lymphoid compartment and other non-hematologic tissues, ONC001 emerges as a promising new treatment with monotherapy potential in areas of high unmet need such as CMML and M4/M5 AML.