The Effect of Skipped Collections on Stem Cell Product Yields in Difficult to Mobilize Multiple Myeloma Patients

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Multiple Myeloma (MM) is incurable disease and autologous stem cell transplant (ASCT) remains an important modality of treatment. Current mobilization strategies result in mobilization failure rates as high as 5-10% among patients with MM1. According to our institutes policies, patients are prepared for stem cell collection with a Cytoxan based mobilization regimen plus GCSF/Mozobil (chemo-mobilization), or GCSF/Mozobil (growth factor) alone. Most often, patients undergoing chemo-mobilization and growth factor-based mobilization achieve adequate CD34 count in peripheral blood by day 14 and 5, respectively. Apheresis is performed after adequate CD34 is measured in the peripheral blood. We identified 18 MM patients, during 2016-2019, who did not achieve adequate CD34 count and were classified as difficult mobilizers. In this cohort, the median age was 57 years, 84% had diffuse marrow involvement and/or multiple lytic lesions, 39% had received an alkylating agent and 28% had received at least two previous lines of chemotherapy. 13 patients who were chemo-mobilized, did not achieve adequate CD34 yield, after 5 days of continuous apheresis. Similarly, 5 patients in GCSF/Mozobil arm did not achieve adequate CD34 yield after 5 days of continuous apheresis. Hence, we gave them planned 48 hours break from further GCSF or Mozobil exposure. After the break, we resumed GCSF/Mozobil for 1 day, and subsequently we noticed significant rise in peripheral blood CD34 count in some of the patients. We achieved a median collection of 3.98 CD34+ cell/kg in these difficult mobilizers. We compared collection to the easy mobilizers during the same time period and the median collection for easy mobilizers was 4.48 CD34+ cell/kg. The skipped day strategy led to adequate yields for ASCT in 100% of the difficult to mobilize patients. One possible explanation for the higher number of cells collected after the skipped day is that continuous exposure to GCSF leads to tachyphylaxis of GCSF receptors, and the break relieves this phenomenon, allowing better collection after the break. Further studies exploring this planned break strategy needs to be conducted in so called difficult mobilizer patient group.