1179 Understanding the impact of magrolimab on red blood cell dynamics in humanized CD47-SIRPα mouse model

<h3>Background</h3> Magrolimab is a monoclonal IgG4 antibody that binds to CD47, blocking the CD47-SIRPα checkpoint interaction. Previous clinical trials demonstrated on-target anemia mitigated by administrating a low ‘priming’ dose of antibody.^1–3 While CD47 is critical for RBC homeostasis and clinical findings suggest priming affords RBC protection, the mechanism of this effect is unknown. Here we use a human CD47/SIRPα double knock-in (DKI) preclinical model to elucidate mechanisms of magrolimab-induced anemia and recovery. <h3>Methods</h3> Patient’s blood samples were collected from the ELEVATE CRC study (NCT05330429). The central lab test results, magrolimab occupancy, and surface CD47 expression were analyzed. Blood samples from DKI mice were analyzed post dosing of magrolimab bearing a murine IgG1. Macrophages were ablated in mice by clodronate-liposome administration. <h3>Results</h3> In patients, an acute transient decrease in hematocrit occurred post priming without clinical signs of anemia, followed by a prolonged and reversible reduction of HGB.³ Loss of surface CD47 on RBC was observed at C1D8. These findings were recapitulated in DKI mice with magrolimab treatment. The acute (2hr) RBC drop (to 22% of control) in mice was followed by a rapid recovery by 24hr (to 40% of control), while CD47 expression on RBC was partially decreased by 2hr and almost completely lost by 24hr. Viable blood cell aggregates were detected by flow cytometry at 2hr post priming and lost by 24hr, suggesting potential release of CD47^LO RBC from the aggregates due to their diminished binding to magrolimab by 24hr. This phenomenon could explain the rapid RBC recovery in the absence of sufficient reticulocyte production. Chronic RBC reduction with more pronounced reticulocyte production was observed at 72hr. Clodronate liposome treatment did not impact the RBC drop at 2hr, excluding the possibility that acute anemia was driven by macrophages. However, by 72hr, macrophage-mediated clearance of CD47^LO RBC was observed. Long-term continuous dosing of magrolimab led to 50% lower RBC counts, with increased erythropoiesis, suggesting adaptation to macrophage RBC clearance mechanisms. <h3>Conclusions</h3> Overall, our findings are consistent with clinical findings of an acute but transient reduction in RBC followed by a secondary, more gradual RBC decrease that recovers over time, even with continuous dosing. With a translatable preclinical model, we elucidate two different mechanisms of magrolimab-triggered anemia: an early acute anemia due to viable RBC aggregation with rapid recovery and concomitant with loss of CD47, followed by a more gradual macrophage-dependent RBC loss which ultimately resolves by reaching a new homeostasis of clearance and erythropoiesis. <h3>References</h3> Vyas P, <i>et al</i>. Initial phase 1 results of the first-in-class anti-CD47 antibody Hu5F9-G4 in relapsed/refractory acute myeloid leukemia patients.<i> EHA poster</i> 2018. Chen JY, <i>et al</i>. Impact of magrolimab treatment in combination with azacytidine on red blood cells in higher risk myelodysplastic syndrome (HR-MDS) patients. <i>ASCO poster</i>. 2022. Sikic BI, <i>et al</i>. First-in-human, first-in-class phase 1 trial of the anti-CD47 antibody Hu5F9-G4 in patients with advanced cancers.<i> J Clin Oncol</i> 2019;<b>37</b>:946–953.