Engraftment and Immune Recovery (IR) in Good Risk Patients Undergoing Hematopoietic Stem Cell Transplantation (HSCT): Comparison of Two Different Approaches Using Cyclophosphamide (CY) for T-Cell Tolerization

We developed a 2 step haploidentical HSCT where 76 patients received myeloablative or reduced intensity regimens followed by a DLI containing a fixed dose of 2.0 x 10⁸/kg T cells (HSCT step 1).  After 2 days, 60mg/kg/d x 2 of CY was infused, followed a day later by a CD34 selected PBSC product [HSCT step 2 median dose 5 x 10⁶/kg (range 1.64-10)].  We concurrently used a 1 step HSCT approach in a group of 16 patients, 8 with unrelated donors (URD) and 8 patients (6 with haploidentical, 2 with matched related donors) with comorbidities precluding eligibility for the 2 step protocol.  The 1 step patients were given the same conditioning, but afterwards received unmanipulated PBSC’s [median CD 34 dose 7.18x10⁶/kg, (range 4.41-10); median T cell dose 2.9 x 10⁸/kg, (range 1.4-3.90)].  CY 60mg/kg/d x 2 was infused 48 hours later.  Because avoidance of stem cell exposure to CY occurs only in the 2 step approach, we compared engraftment rates and IR between the 2 groups.  All 92 patients had good risk disease.  In the 1 step vs the 2 step study, median time to ANC > 500/ul was 19(range 15-28 days) vs 11[range 9-16 days (p=0.000-Mann-Whitney)], and for platelets > 20,000/ul, 29(range 18-52 days) vs 17(range 12-173 days) respectively (ns).  The significant difference in time to ANC recovery in the 1 step group was possibly from the exposure of the donor PBSC’s to CY. When we accounted for the later occurrence of day 0 in the 2 step group, the median time to ANC recovery was still 3 days longer in the 1 step group even though this group received a higher median CD 34 dose.  Possibly due to the earlier count recovery in the 2-step group, the median CD3/4 count at day 28 was 20(range 5-50/ul) in the 1 step group vs 54(range 11-299/ul) in the 2 step group.  By day 90, differences between the groups resolved, with a median CD3/4 count of 157(range 29-397/ul) vs 147(range 10-814/ul) in the 1 and 2 step groups respectively.  Median CD3/8 count at day 28 was 40 (range 3-157/ul) vs 57 (range 4-2682/ul) and at day 90 was 239(range 12-1439/ul) vs 204(range 2-2379/ul) in the 1 and 2 step groups respectively.  The percentages of patients on steroids for GVHD at day 28 (13% in the 1 step group vs 31%) and 90 (18% vs 33%) was not significantly different between the two groups (p=0.215 and 0.413 respectively, Pearson Chi Square).  The median length of stay (LOS) was 41(range 15-99 days) vs 32(range 15-156 days) in the 1 and 2 step groups respectively.  The 2 step approach to HSCT allows for the administration of a fixed dose of T cells from which to optimize outcomes and circumvents exposure of donor cells to the effects of CY.  Our experience with a 1 step approach suggests later ANC recovery and possibly initial T cell recovery versus the 2 step approach.  There may be a slightly longer LOS in the 1 step group possibly from longer time to count recovery. Formal analyses of the differences between the two approaches will be performed when more patients are treated at our institution with the 1 step approach.