Impact of Daratumumab on Stem Cell Collection, Graft Composition and Engraftment Among Multiple Myeloma Patients Undergoing Autologous Stem Cell Transplant

Background High dose chemotherapy followed by autologous stem cell transplantation (ASCT) plays an important role in the treatment of transplant-eligible multiple myeloma (MM) patients and yields deep responses, prolongs progression-free survival (PFS) and overall survival (OS). Daratumumab (Dara), a humanized monoclonal antibody that binds to CD38+ on the surface of myeloma cells and is increasingly used upfront to treat MM. Up to 75% of mobilized CD34+ hematopoietic progenitors also express CD38 and, hence exposure to Dara may potentially impact stem cell mobilization, and, given the extended half-life of Dara, also impair engraftment. The Cassiopeia trial (Moreau, P. 2019) showed the utility of Dara in combination with bortezomib and thalidomide as upfront treatment for MM without a negative impact on stem cell collection yield. Similarly, the Griffin trial (Voorhees, PM, 2020) demonstrated the safety of upfront Dara when combined with bortezomib and lenalidomide also with no impact on stem cell collection or engraftment. However, others(Al Saleh, A. 2019) reported delayed neutrophil engraftment in patients treated with Dara. Given the conflicting findings of these studies and scarcity of data on the direct impact of Dara on erythroid and myeloid progenitors, we investigated the effect of Dara on stem cell collection yield, graft composition and time to engraftment among patients who underwent ASCT. Methods We evaluated all patients with MM who underwent ASCT at our institute between 2017-2020 and excluded patients undergoing 2nd transplant or tandem ASCT. Disease risk stratification was assessed as per International Staging System (ISS) for MM. Treatment response prior to transplant was assessed as per response criteria definitions by International Myeloma Working Group (IMWG). Stem cells were collected according to institutional protocol and prior to cryopreservation, 1x105 peripheral blood cells are plated in duplicate in semi-solid media- MethoCult™ H4434 (Stem Cell Technologies, Vancouver, BC) and cultured in 37°C, 5% CO2 following which CFU-GM (colony forming unit- granulocyte macrophage) and BFU-E (burst forming units- erythroid) colonies are scored on day 14 based on morphological recognition, and reported as 104 colonies per kg of recipient weight. CFU-C (colony forming unit combined) is calculated by combining BFU-E and CFU-GM. Results Patients (N=108) that underwent ASCT between 2017-2020 were identified and demographic data are summarized (Table 1). Sixteen patients received Dara as part of upfront treatment prior to stem cell collection. Median age was 62 years old for the entire group with no significant age difference between patients who received Dara vs. those who did not receive. Similarly, there was no difference in race, ISS stage, pre-transplant response, plerixafor or lenalidomide use between the groups. Pts who received Dara were more likely to have received more than one chemotherapy regimen prior to transplant (62.5% vs. 30%, p-value=0.014). Although, total and day 1 collection of CD34+ stem cells was lower in the Dara treated group (7.18 x 106/kg) compared to Dara untreated (8.78 x 106/kg), the difference was not statistically significant. (p-value=0.151). Stem cell product composition based on measurement of BFU-E, CFU-GM and CFU-C colonies were similar independent of Dara use. Prior exposure to Dara was also not predictive of day 1 stem cell collection failure (defined as < 5x106 CD34+ cells/kg) as determined by multivariate analysis. Median time to absolute neutrophil count (ANC) recovery (defined as ANC >1500) was 12 days in both groups (p-value=0.09). Median time to platelet recovery (defined as platelet count >20k) was 12 days in the Darauntreated group vs. 13 days Dara treated group (p-value=0.06). Discussion In this single institute experience, there was a trend towards lower CD34+ collection as well as lower progenitor scoring with Dara use but was not statistically significant and also there was no difference in time to ANC or platelet recovery. The majority of patients in this study received plerixafor for mobilization, which might abrogate the effect of Dara and lenalidomide on the graft and stem cell collection. Further studies to investigate the impact of Dara on CD38+ mobilized hematopoietic cells is warranted. Disclosures Manjappa: Pfizer: Research Funding. Caimi:Amgen: Other: Advisory Board; Bayer: Other: Advisory Board; Verastem: Other: Advisory Board; Celgene: Speakers Bureau; Kite pharmaceuticals: Other: Advisory Board; ADC therapeutics: Other: Advisory Board, Research Funding. de Lima:BMS: Other: Personal Fees, advisory board; Incyte: Other: Personal Fees, advisory board; Kadmon: Other: Personal Fees, Advisory board; Celgene: Research Funding; Pfizer: Other: Personal fees, advisory board, Research Funding. Malek:Clegene: Other: Advisory board , Speakers Bureau; Janssen: Other: Advisory board, Speakers Bureau; Sanofi: Other: Advisory board; Amgen: Honoraria; Medpacto: Research Funding; Cumberland: Research Funding; Bluespark: Research Funding; Takeda: Other: Advisory board , Speakers Bureau.