Sustained Hematopoietic Recovery after Salvage Autologous Stem Cell Transplants after Long Term Storage of Cryopreserved Hematopoietic Progenitor Stem Cells (HPSC)

Background: Salvage autologous hematopoietic stem cell transplantation (HSCT) is an effective treatment for patients with relapsed multiple myeloma (MM). Adequate HPSC after multiple cycles of dose-intense melphalan (mel) can be harvested, cryopreserved, and stored to allow for such future transplants. Many investigators reported sustained CD34+ cell viability using in vitro assays such as colony forming units (CFU-GM) potency of HPSCs after long term storage. Data on engraftment outcomes using these products demonstrating in vivo viability are limited. This study describes a large single center experience evaluating the engraftment potential of HPSC used in salvage transplantation after long-term storage in patients with MM, in comparison to initial treatment.

Study Design and Methods: We conducted a retrospective chart review of patients with MM undergoing salvage HSCT, whose initial cell collection occurred between January 2002 and May 2016. This review identified 59 patients, all conditioned for initial transplants with dose-intense Mel 200 mg/m², and who received autologous HPSC stored > 1 year after initial HSCT. HPSC were cryopreserved and stored in vapor phase liquid nitrogen at a temperature of ≤-150°C. Conditioning regimens for salvage transplants were mel (n=11), mel/bortezomib (bor) (n=32), mel/bor/thalidomide (n=6), BEAM (n=1), and Super BEAM (n=9). Patients who received a planned tandem transplant only (less than a year apart) were excluded. For patients receiving tandem HSCT followed by salvage HSCT, the first of the tandem HSCTs was considered for this analysis (n=5). Differences in CD34+ cell doses and days to engraftment between first and salvage transplant were tested using a paired 2-tailed t-test. Univariate and multivariable linear regressions were used to determine association between storage time and days to engraftment.

Results: From 2002 to 2017, transplant data from 59 MM patients were analyzed (Table 1). Forty-nine (83%) patients had a Salmon Durie stage IIIA or IIIB at first diagnosis. The median age at first diagnosis was 57.5 (range, 36-73) years. A median collection dose of 16.0 × 10⁶ CD34+ cells per kilogram (range, 7.9-62.5) was reached during HPSC collection with a median of 3 collections (range, 1-7) per patient. All 59 patients collected upon first mobilization attempt. The median age of patients at time of first and salvage transplants was 59 and 62 years, respectively. As predicted, the patient's age at salvage transplant was significantly greater than the age at first transplant (p<0.001). The median storage time between day of initial collection and salvage transplant was 4.0 years (range 1-14.6), with 37% (n=22) and 8% (n=5) stored for over 5 and 8 years, respectively. The median time between first and salvage transplants was 4.0 years (range 1-14.5). Patients achieved sustained neutrophil engraftment (ANC>500 /uL) at a median of 11 days after both the first and salvage transplant (ranges, 9-18 and 8-15 respectively, p=0.041) (Figure 1). The median time to sustained platelet engraftment (>20 x 10⁹/L) was 13 days (range, 9-36) after first HSCT and 14 days (range 8-45) after salvage HSCT (p=0.842) (Figure 1). The CD34 dose for the salvage transplant was significantly higher than the first transplant, with patients undergoing the first HSCT receiving a median CD34+ cell dose of 5.3 × 10⁶/kg (range 3.0-14.1), compared to a median of 6.1 × 10⁶/kg (range 3.4-13.8) for the salvage HSCT (p=0.04). There was no association between the CD34 dose infused and days to ANC (p=0.755) or platelet (p=0.669) engraftment. After adjusting for age at transplant and CD34 dose, there was no association between duration of cryopreservation and days to ANC (p=0.658) and platelet (p=0.725) engraftment (Figure 2). There were no graft failures reported in either the first or salvage HSCT.

Conclusion: Long-term cryopreservation did not affect engraftment outcomes in patients with MM receiving salvage autologous HSCT, despite the addition of salvage chemotherapy. There was no association between engraftment kinetics and storage duration in patients receiving a salvage transplant when controlling for CD34 dose and recipient age. Although it is possible that these cell products may have lost HPSC viability but still contained more than adequate viable HSC for HSCT, there was no evidence of delayed engraftment, particularly of platelets, suggestive of low numbers of viable HSCT.

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