Phase II Study of CD4+-Guided Pentostatin Lymphodepletion and Pharmacokinetically Targeted Busulfan as Conditioning for Hematopoietic Cell Allografting

One limitation of reduced-intensity preparative regimens is potential for graft failure. We have developed a regimen that targets CD4+ lymphodepletion to ensure early and durable engraftment. The primary endpoint was achievement of ≥50% CD3+ donor chimerism by day +28. Forty-two patients (median age, 53 years; range, 29 to 73 years) received pentostatin 4 mg/m2 i.v. on days −28, −21, and −14 when the CD4+ cell count was >100 cells/μL and on days −4 and −3 regardless of CD4+ level. Rituximab 375 mg/m2 was administered to patients with CD20+ malignancies on days −21, −14, −7, +1, and +8. Busulfan 200 mg/m2 i.v. was administered on days −4 and −2 at a dose to target a cumulative AUC dose of 16,000 (±10%) μmol·min/L. Graft-versus-host disease (GVHD) prophylaxis consisted of tacrolimus plus methotrexate in 86% of patients. Donors were matched-related (47%), matched unrelated (43%), or mismatched unrelated (10%). Chronic lymphocytic leukemia (45%) and follicular non-Hodgkin lymphoma (14%) were the most common diagnoses. Disease status at initiation of the preparative regimen was complete remission in 22%, partial response in 55%, and stable/progression in 24%. The median percent CD4+ cell count decrease from baseline (day −28) was 52% to day −21, 66% to day −14, 62% to day −7, and 91% to day 0. At day +28, all 42 patients (100%) had ≥50% CD3+ donor chimerism. No patient experienced graft failure. Overall response rate was 82% (complete remisson, 67%). The day +100 cumulative incidence of grade II-IV acute GVHD was 59% (grade III-IV acute GVHD, 19%), and the 2 year cumulative incidence of chronic GVHD was 69% (moderate/severe, 58%). Nonrelapse mortality was 2% at day +100 and 17% at 2 years. Two-year PFS was 55%, and OS was 68%. This regimen ensures durable engraftment, is effective against persistent disease, and results in relatively low mortality from causes other than relapse.