In Vivo Potency of Hematopoietic Stem Cells after Long-Term Storage at-80C in DMSO+HES Cryprotectant Mixture: Comparing Engraftment Kinetics and Efficacy Data in First and Second Autologous Stem Cell Transplantation in Myeloma

Background Two methods of hematopoietic stem cell (HSC) cryopreservation exist: 10% DMSO alone using rate-controlled freezing with storage in liquid nitrogen (LN2) and 5% DMSO/6%HES with ‘dump’ into and storage in mechanical freezers at -80C. Long-term storage at LN2 is clinically efficacious, but clinical data for long-term storage in DMSO/HES is lacking. To assess clinical benefit of HSC stored long term in DMSO+HES at -80C, we report on first and second autologous stem cell transplants (ASCT1 and 2) in patients undergoing sequential transplants for multiple myeloma (MM) using HSC stored >2 years. Method Patients undergoing ASCT2 for MM from 1990-2023 using only HSC collected pre-ASCT1 were eligible. The primary outcome was to determine if differences in neutrophil and platelet engraftment existed between transplants. Secondary outcomes were progression free survival (PFS) and overall survival (OS). We reviewed patient characteristics, and disease state and engraftment kinetics for both. Outcomes were analyzed by Kaplan Meier (KM) and longrank test, and median PFS (mPFS) and OS (mOS) were determined by reverse KM. Engraftment times were compared using the Kruskal-Wallis test. Results 35 patients were transplanted using their HSC stored at -80C for > 2 years. Median age was 60. Additional characteristics and results in Table 1. Median storage time was 4.6 yrs. Engraftment times were identical between transplants. Comparing HSC stored between 2-5 years and >5 years, no statistical difference in ANC (p=0.85) or platelet engraftment (p=0.29) was seen. No graft failures were seen. The mOS was 4.8 years from AHSCT2; mPFS varied dramatically by disease state; those in CR pre-ASCT2 had a mPFS of 7.8 years, those in VGPR 4.3 years, and those in PR 1.3 years. Discussion Our results indicate that HSC cryopreserved in DMSO+HES and stored at -80°C does not adversely affect quality even up to 15.8 years. Compared to LN2 storage, this method is simpler, cost-effective, and poses no risk to the working environment. This is particularly important given the efficacy of second transplants for those in a deep second remission, more possible now with effective salvage therapies. This simplified method allows for the continued use of AHSCT2 as part of second line therapy for many with MM, being not only efficacious but also more cost effective than the prolonged use of new novel salvage therapies.