The level of blast CD33 expression positively impacts the effect of gemtuzumab ozogamicin in patients with acute myeloid leukemia

Gemtuzumab ozogamicin (GO) is an immunoconjugate, combining an anti-CD33 monoclonal antibody to calicheamicin, a highly cytotoxic antibiotic. First developed as single agent in adults with relapsed acute myeloid leukemia (AML), it was then evaluated in combination with chemotherapy in newly diagnosed patients. In the randomized Acute Leukemia French Association (ALFA)–0701 study, we reported that sequential administration of a lower dose of GO allowed the safe delivery of a high cumulative dose associated with a substantial improvement in patient outcome. A recent meta-analysis has confirmed that adding GO to chemotherapy may provide a survival advantage in patients without adverse cytogenetic characteristics. However, these results were obtained regardless of the level of blast CD33 expression. In vitro, a clear relationship between CD33 expression and GO efficacy has nevertheless been shown. In vivo, contradictory results have been reported so far. No impact was found when CD33 expression was evaluated as a continuous covariable or using a 20% cutoff. Higher response rates were nevertheless reported for patients with CD33+ expression ≥98% in 1 phase 2 study or when showing CD33 expression as mean fluorescence intensity (MFI) using an isotype antibody as control. To further evaluate the impact of CD33 expression on GO treatment effect, we retrospectively analyzed the results of the ALFA-0701 study.