Increased stem cell dose, as obtained using currently available technology, may not be sufficient for engraftment of haploidentical stem cell transplants.

The best strategies for haploidentical stem cell transplants are not known. We used a standard myeloablative pretransplant conditioning regimen (30 mg/kg VP-16, 120 mg/kg cyclophosphamide, and 12 Gy of TBI in six fractions), an increased peripheral stem cell dose of > 10 x 10(6) CD34+ cells/kg, T cell depletion (with CD34+ cell selection and CD4/CD8 depletion steps) to < 1 x 10(5) CD3+ cells/kg and cyclosporine post transplant. Ten patients (7M/3F, median age 11 (3-33) years) with high-risk leukemia (AML in 4, MDS in 2, CML in 1 and T-ALL in 3) received a hemopoietic stem cell transplant (HSCT) from a haploidentical father or sibling. The median number of CD34+ cells was 12.9 (9.5-45.7) x 10(6) cells/kg; median number of CD3+ cells was 0.41 (0.09-1.89) x 10(5) CD3+ cells/kg. All patients initially achieved 0.5 x 10(9)/l neutrophils at a median 12 (10-21) days. Graft failure in two consecutive patients out of four on the original protocol led to a modification adding ATG pretransplant and OKT3 post transplant. Graft failure was observed in one out of six subsequent patients. Acute GVHD > or = grade II was observed in three patients. Three of 10 patients are alive in CR at > 24 and >3 (2) months after transplant. Seven patients died: four of transplant related complications and three of relapse. Increased stem cell dose (> or = 10 x 10(6) CD34+ cells/kg) as obtained using currently available technology may not be sufficient to ensure stable engraftment in patients with high-risk leukemia using standard myeloablative conditioning regimens.