Effect of T Cell Recovery on Overall Survival (OS) Following Pentostatin Conditioning for Nonmyeloablative Allogeneic Stem Cell Transplantation (NST).

NST provides graft-versus-leukemia (GVL) activity with reduced regimen related toxicity. We used a conditioning regimen consisting of Pentostatin 4 mg/m² daily × 3 days and 200 cGy TBI 24 hrs prior to stem cell infusion in patients (pts) with high-risk/relapsed/refractory hematologic malignancies. The importance of the timing of stem cell infusion relative to Pentostatin administration was examined in a comparison of two sequential protocols. In cohort 1 (n=39) Pentostatin was given on days -21, -20, and -19. In cohort 2 (n=39) Pentostatin was given on days -10, -9, and -8. The median age of the day -21 cohort was 52 years (range 22–70) and the day -10 cohort was 59.5 years (range 36–70). The median number of prior therapies in the day -21 cohort was 4 (range 0–8), including prior autologous SCT in 22 pts, while in the day -10 cohort, the median number of prior therapies was 6 (range 0–18), including prior autologous SCT in 9 pts. Post-grafting immunosuppression was cyclosporine (CsA)/mycophenolate mofetil (MMF). In both protocols, CsA 2.0 mg/kg IV q12 hrs was administered on days -1, 0, and +1, and then converted to oral 5 mg/kg PO BID with a taper beginning on day +70. In the second protocol, pts with unrelated donor transplants tapered CsA starting at day +100. MMF at 15 mg/kg PO BID was administered on days 0–27 for related donor transplants in both protocols and in the second protocol this was extended until day +40 for the unrelated donor transplants. In the day -21 protocol, SCTs were obtained from matched related (n=14) or unrelated (n=25) donors. The cumulative incidence of all grades of acute GVHD at day 100 was 40% and was more common in unrelated donor (60%) vs. related donors (15%) transplants. In the day -10 protocol, SCT was performed with products from matched related (n=15) or unrelated (n=21) donors. The cumulative incidence of grade II–IV acute GVHD was approximately 35% (30% in related versus 40% in unrelated donors). In both protocols, the median chimerism values for CD3⁺ cells and white blood cells at day 28 were >80% and 90% donor cells, respectively. T, B and natural killer (NK) lymphocyte frequency and numbers were identical at protocol entry. One day following TBI conditioning, there was a 57% depression in CD3⁺ cells in the day -21 protocol that was retained on day 28 post transplant (55%). In the day -10 protocol, there was a significant depression (47%) in the frequency of CD3⁺ cells one day following TBI, which was depressed further (68%) on day 28 post transplant. The dendritic cells (DCs), both CD123⁺ and CD11c⁺, were not suppressed by the conditioning regimen. Separation of pts on day 28 into cohorts with an absolute number of CD3⁺ cells >0.25×10⁶/ml revealed that those pts with a higher CD3⁺ cell count had a median OS time of 36.7 weeks, which was significantly longer than pts whose absolute number of CD3⁺ cells <0.25×10⁶ (median OS of 21 weeks). This differential effect on OS was not observed when pts were separated on the basis of either CD4⁺ or CD8⁺ cells. In conclusion, there is a lower incidence of acute GVHD in the day -10 protocol, which can be associated with a significant reduction in the frequency and absolute number of circulating T cells. The significant reduction in T cells, observed one day following TBI and 28 days post transplant, is associated with the timing of transplant relative to Pentostatin conditioning and a significant prolongation of OS.