Sustained Hematopoietic Engraftment Potential after Prolonged Storage of Cryopreserved Hematopoietic Stem Cells (HSC) Used in Salvage Autologous Stem Cell Transplantation: Prolonged Storage of HSCs Sustains Hematological Reconstitution Potential

BACKGROUND

Salvage autologous hematopoietic stem cell transplantation (HSCT) is an effective treatment for patients with relapsed multiple myeloma (MM). Peripheral blood stem cells (PBSCs), a source of hematopoietic stem cells (HSCs), are collected prior to first transplant and adequate quantities of PBSCs can be collected and stored potentially for years to support at least two transplants for eligible patients. To ensure the safety of salvage HSCT used in the treatment of patients in subsequent relapse, PBSCs must retain the potential to engraft even after several years of cryopreservation. Although PBSC viability has been extensively studied using in vitro techniques, few publications describe the most rigorous functional potency measure: of patients receiving a myeloablative conditioning regimen. This study describes a large single-institution experience evaluating the engraftment kinetics of PBSCs used in salvage transplantation after multiple years of storage in comparison to first transplantation of the same patients in the treatment of MM.

STUDY DESIGN AND METHODS

A retrospective chart review of patients with MM undergoing HSCT from 2000 to 2021, identified 89 patients who received salvage autologous PBSC stored > 1 year after first HSCT. PBSC were cryopreserved and stored in vapor-phase liquid nitrogen refrigerators at a temperature of ≤ -150°C. All patients received a PBSC product for both transplants from the same collection cycle. Differences in CD34⁺ cell doses and days to engraftment between the first and salvage transplant were tested using a paired 2-tailed t-test and Wilcoxon signed-rank test. Univariate and multivariable linear regressions were used to determine association between storage time and days to engraftment, adjusting for CD34+ cell dose and conditioning regimen in the multivariable model.

RESULTS

The median storage time between day of initial collection and salvage transplant was 5.4 years (range, 1.0 - 19.7). Engraftment kinetics demonstrated a sustained neutrophil engraftment (absolute neutrophil count (ANC) > 0.5 × 10⁹ cells/L) at a median of 11 days after both the first and salvage transplant (ranges, 8 - 15 and 8 - 19 respectively, p<0.05). The median time to sustained platelet engraftment (> 20 × 10⁹ cells/L without transfusion support) was 13.5 days after first HSCT and 14 days after salvage HSCT (ranges, 9 - 27 and 10 - 56 respectively, p = 0.616). After adjusting for CD34+ cell doses and conditioning regimens, there was no association between the duration of cryopreservation and days to neutrophil (r = 0.178, p = 0.130) or platelet (r = 0.244, p = 0.100) engraftments.

CONCLUSION

Engraftment kinetics of the salvage HSCT are comparable to the first HSCT even when products are stored in vapor-phase nitrogen refrigerators for a median time of 5.4 years. There is no association between storage duration and time to engraftment when controlling for CD34+ dose and conditioning regimens. Prolonged storage of cryopreserved HSC products is a safe practice for MM patients undergoing salvage autologous HSCT.