Most umbilical cord blood (UCB) transplant studies indicate that the minimum nucleated cell dose needed for adequate engraftment is > 2 x 107 cells/kg, which eliminates most adults. Using a novel, post-thaw cell manipulation without washing, we explored the notion that the minimum ‘safe’ transplant cell dose/kg could be lowered to 1 x 107/kg, which would permit most adults to be transplanted. Washing UCB cells post-thaw before transplantation is a common practice, but is associated with a modest, non-specific cell loss. Our method utilizes rapid thawing at 37°C followed by a slow, progressive dilution to a final DMSO concentration of 1% over 10 minutes with a 10% Dextran, 8% human albumin solution, without washing and with subsequent infusion over 15 minutes. To date 27 adults who did not have a matched sibling (5/6, 6/6) or unrelated (8/8) matched donor have been transplanted using cells processed by this method. Their median age was 42 (22–58) and their median weight was 84 (49–109) kg. Of the group, 18 (67%) had an active refractory hematologic disorder, and only 1 was in CR1 (AML with monosomy 7), and 11 (41%) had failed a prior ablative transplant. HLA matching (serologic for class I, and allele for class II) was 4/6 in 14, 5/6 in 11, and 6/6 in 2. Preparative regimens were ATG plus Cy/TBI in 14, Busulfan/cyclophosphamide in 3 or high dose BCNU-based in 10. GVHD prophylaxis consisted of tacrolimus (Day −2 to Day + 100) and steroids (D + 7 to day +80). Median cell doses infused were: nucleated cells: 1.47 x 107/kg and CD34: 0.48 x 105/kg; 22 (82%) received a nucleated cell dose < 2 x 107/kg. Median post-thaw cell recovery and viability were 97% and 97.5% respectively. Survivals at 100 days and 1 year were 76% (95% CI = 59 – 93)and 44 % (95% CI = 22 – 67) respectively. Median nucleated cell dose infused for the 1+ year survivors was 1.32 x 107/kg with all but one transplanted with < 2 x 107 cells/kg. There were 4 early deaths, early on in the trial at day +5, +13, +17 and +22, all in patients with refractory disease, due to sepsis. The median time to neutrophil engraftment (> 500/μl) was 19 days (range = 11–38 days) and time to platelet engraftment (> 20,000/μl) was 45 days (18–114+); however 2 patients died at day + 33 (VOD) and +114 (fungal pneumonia complicating refractory acute GVHD) without platelet recovery. Acute GVHD was seen in 56% (30% grade III/IV); chronic GVHD was seen in 57%. Within the limited cell dose range used, increasing nucleated cell and CD34 doses, as well as HLA match and disease stage were not predictive of survival or ANC engraftment in univariate analyses. In summary, our survivals are similar to those seen when blood, marrow or UCB stem cells with higher minimum cell dose thresholds are used as the transplant source in similar patients. Using a non-wash post-thaw procedure for our 27 transplants, few viable cells are lost allowing successful UCB transplantation at nucleated cell doses < 2 x 107/kg in adults who do not have a living HLA-matched related or unrelated donor and who otherwise have no option for potentially curative therapy.

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