Phase I/II study of gemtuzumab ozogamicin added to fludarabine, melphalan and allogeneic hematopoietic stem cell transplantation for high-risk CD33 positive myeloid leukemias and myelodysplastic syndrome

We investigated the hypothesis that gemtuzumab ozogamicin (GO), an anti-CD33 immunotoxin would improve the efficacy of fludarabine/ melphalan as a preparative regimen for allogeneic hematopoietic stem cell transplantation (HSCT) in a phase I/II trial. Toxicity was defined as grades III-IV organ damage, engraftment failure or death within 30 days. 'Response' was engraftment and remission (CR) on day +30. We sought to determine the GO dose (2, 4 or 6 mg m^-2) giving the best trade-off between toxicity and response. All patients were not candidates for myeloablative regimens. Treatment plan: GO (day -12), fludarabine 30 mg m^-2 (days -5 to -2), melphalan 140 mg m^-2 (day -2) and HSCT (day 0). GVHD prophylaxis was tacrolimus and mini-methotrexate. Diagnoses were AML (n=47), MDS (n=4) or CML (n=1). Median age was 53 years (range, 13-72). All but three patients were not in CR. Donors were related (n=33) or unrelated (n=19). Toxicity and response rates at 4 mg m^-2 were 50% (n=4) and 50% (n=4). GO dose was de-escalated to 2 mg m^-2: 18% had toxicity (n=8) and 82% responded (n=36). 100-day TRM was 15%; one patient had reversible hepatic VOD. Median follow-up was 37 months. Median event-free and overall survival was 6 and 11 months. GO 2 mg m^-2 can be safely added to fludarabine/melphalan, and this regimen merits further evaluation.