Persistence of CD33 expression at relapse in CD33+ acute myeloid leukaemia patients after receiving Gemtuzumab in the course of the disease

Gemtuzumab is a humanized anti-CD33 antibody conjugated with a cytotoxic antitumour antibiotic, calicheamicin-g1, currently approved in the USA for CD33 acute myeloid leukaemia (AML) patients in first relapse. In this setting, Gemtuzumab as a single agent has shown a 26% response rate, with a median disease-free-survival (DFS) of 6Æ4 months in those patients achieving complete remission (Tsimberidou et al, 2006). Recently, the UK Medical Research Council (MRC) AML15 trial demonstrated an improved DFS without extra toxicity when adding Gemtuzumab at a dose of 3 mg/m on day 1 of induction chemotherapy (Burnett et al, 2006). As a consequence, the combination of Gemtuzumab and chemotherapy might rapidly become a common induction regimen for CD33 AML patients. Moreover, the combination of Gemtuzumab and chemotherapy at relapse may also be superior to salvage chemotherapy alone (Chevallier et al, 2005). However, despite these enthusiastic results, it is still unknown if, when patients have already received Gemtuzumab in the course of their therapy, the expression level of CD33 is affected at the time of relapse. Also, Gemtuzumab could not be re-administered at relapse if the use of this drug in the induction regimen produced a CD33 negative leukaemic blast population in AML patients. So far, no report has evaluated CD33 expression in CD33 AML patients who have relapsed after Gemtuzumab administration. This single centre retrospective study included 21 patients who have received at least one course of Gemtuzumab in combination with chemotherapy in the course of their disease (Table I). Patient selection was restricted to those whose CD33 expression on AML blasts was quantified prior to Gemtuzumab administration and at time of disease relapse. Median age was 58 years (range: 2–70 years). Initial administration of Gemtuzumab + chemotherapy was given as first line therapy in four patients: three of them received a combination of intermediate dose Cytarabine and Gemtuzumab without anthracycline because of a cardiac contra-indication, and one have been included in a protocol testing the association of Idarubicin + Cytarabine + Gemtuzumab in elderly AML patients with poor-risk cytogenetics (Harousseau et al, 2007). Interestingly, one of the three patients with cardiac contraindication was considered CD33 at time of diagnosis but at time of this retrospective study he was found to be CD33 negative. Four patients received the MIDAM rotocol (Gemtuzumab, AraC, Mitoxantrone) (Chevallier et al, 2005) for refractory disease. Finally, 13 patients received Gemtuzumab + chemotherapy at relapse: 10 received the MIDAM protocol and three received the MIDAM without anthacycline (MIDA) because of cardiac contra-indications. Overall, all patients received 9 mg/m of Gemtuzumab as a single dose in combination with the chemotherapy, except for one patient who received 6 mg/m in first line therapy and one patient in relapse who received two courses of Gemtuzumab 9 mg/m (one in induction and one in consolidation) for a total dose of 18 mg/m before secondary phenotype analysis. CD33 expression was determined by fluorescence-activated cell sorting (FACS) analysis using the anti-CD33-phycoerythrin (PE) antibody from Becton Dickinson (San Jose, CA, USA). CD33 positivity was defined by dividing the mean fluorescence intensity (MFI) of CD33 expression on the leukaemic blasts by the MFI of its isotypic control. The threshold for positivity was defined by a relative fluorescence intensity (RFI) ‡2. Samples were collected from peripheral blood or bone marrow as detailed in Table I. The median time between the initial course of Gemtuzumab and phenotypic analysis at relapse was 4 (range: 1– 48) months, with a majority of patients assessed less than 6 months from Gemtuzumab. Median CD33 expression before Gemtuzumab was 99% (range: 80–99%) with a median intensity ratio of 24 (range: 4Æ2–370). Median CD33 expression after Gemtuzumab was 99% (range: 66– 99%) with a median intensity ratio of 30Æ4 (range: 3Æ9–215, P = NS). None of the CD33 AML patients have become CD33-negative at time of relapse. The only CD33-negative AML patient remained CD33-negative at time of relapse. Only two patients showed some differences of CD33 expression between the two phenotypic analyses. Both patients were assessed early after Gemtuzumab administration as they failed to respond to their treatment and both samples for comparison were from bone marrow before and after Gemtuzumab: one patient showed a decrease of CD33 expression of 33% at relapse (66% vs. 99% before Gemtuzumab). Conversely the second one showed an increase of CD33 expression of 15% (98% against 83% before Gemtuzumab). To our knowledge, this is the first report assessing CD33 expression at relapse in AML patients after receiving Gemtuzumab in the course of their therapy. Overall, our results suggest that Gemtuzumab administration for CD33 AML or correspondence