Abstract 2709: Evaluation of CD37 expression and binding of AGS67E, an antibody-drug conjugate (ADC) against CD37, on white blood cells (WBCs) collected from phase I non-Hodgkin lymphoma (NHL) patients

AGS67E is an antibody drug conjugate (ADC) against CD37 conjugated to monomethyl auristatin E (MMAE). CD37 is expressed on normal WBCs, but is also highly expressed in NHL, CLL and AML (Pereira et al., 2015). A phase I study is currently evaluating the safety, PK and anti-cancer activity of AGS67E with or without growth factor (GF) in subjects with relapsed/refractory NHL. To assess CD37 expression on WBCs, binding of AGS67E, and potential pharmacodynamic effects, samples from subjects were collected at pre-dose, D2, D8, and D15 and analyzed by flow cytometry. CD37 expression on subject tumor samples was also evaluated by immunohistochemistry (IHC). Our results demonstrated that CD37 was highly expressed in tumor samples and that AGS67E binds to WBCs causing down-regulation of CD37, achieving saturation of binding at 24 hours post-treatment (earliest time measured) at or above 0.9 mg/kg. A dose-dependent decrease in the number of all cell types examined was observed with a nadir occurring at D8, with partial or full recovery at D15, except for neutrophils. NK and T cell counts appeared to be least impacted while neutrophils were most affected. B cell counts were extremely low pre-dose for some patients, presumably from prior therapies. In patients treated at 0.9 mg/kg and higher without GF, recovery of neutrophils was delayed beyond D15. At doses of 1.2 mg/kg and higher, use of GF resulted in a significant recovery of neutrophils by D15. The extent of cell count decreases did not correlate to the proportion of cells expressing CD37. For example, decreases in NK cells, monocytes, and, in some cases, T cells, were much greater than the proportion of cells expressing CD37. Furthermore, mature WBCs are unlikely to be affected by AGS67E. This raises the possibility that the main effect of AGS67E may be on rapidly growing precursor cells and that cells with low, or no, CD37 expression may be impacted by the membrane permeable MMAE through a by-stander effect. The effect of AGS67E on neutrophils was investigated in an in vitro assay where hematopoietic stem cells were differentiated into neutrophils. Using this method, we showed that when AGS67C antibody was conjugated to a non-cleavable, membrane impermeable auristatin (mcMMAF) less cytotoxicity to differentiating neutrophils was observed compared to AGS67E. Previously, we have shown that neutrophils secrete proteases that can liberate MMAE from ADCs (Zhao et al, 2016). These results suggest that AGS67E contributes to neutropenia through a by-stander effect, in addition to the CD37-mediated internalization of the ADC. In conclusion, the results showed that AGS67E bound to its target CD37, modulated its expression, achieved saturation of binding at doses at or above 0.9 mg/kg, and reversibly depleted WBCs, with the exception of neutrophils for which GF administration appeared to significantly improve recovery rate. Citation Format: Sher Karki, Hector Avina, Jacqueline Lackey, Ahmed Sawas, Kerry J. Savage, Raymond Perez, Ranjana Advani, Jasmine Zain, Owen A. O9Connor, Sara Gulesserian, Hui Zhao, Peng Yang, Karen Morrison, Leonard Reyno, Fernando Donate. Evaluation of CD37 expression and binding of AGS67E, an antibody-drug conjugate (ADC) against CD37, on white blood cells (WBCs) collected from phase I non-Hodgkin lymphoma (NHL) patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2709. doi:10.1158/1538-7445.AM2017-2709