Targeted immunotherapy with gemtuzumab ozogamicin (GO) following reduced intensity allogeneic stem cell transplantation (RI AlloSCT) to reduce minimal residual disease (MRD) in children with acute myeloid leukemia (AML): MRD monitoring by Wilms tumor gene

Abstract

9532 Background: RI vs myeloablative (MA) AlloSCT in adults with AML is associated with significantly less transplant related mortality but increased rate of relapse, presumably secondary to MRD (Alyea et al; Blood, 2005). Quantification of the WT1 has been used successfully to detect MRD in children with acute myeloid leukemia (AML) (Kletzel et al; Pediatr Dev Pathol, 2002). GO has induced ORR in >30% of patients with overt relapsed CD33+ AML and may have a better effect in the setting of MRD (Sievers et al; J Clin Oncol, 2001). We have shown that RI AlloSCT followed by 2 doses of GO in average risk AML has resulted in >94% donor chimerism (Roman/Cairo, Clin Can Res, 2005). We explored changes in MRD prior to and after consolidation with 2 doses of GO targeted immunotherapy in children with average risk AML post RI AlloSCT. Methods: 11 pts, 11 years (0.5–21) with CD33+ (8 CR1, 3 CR2) received fludarabine 30 mg/m2x6d and busulfan 3.2–4 mg/kgx2d. Donors: 7 6/6 HLA related peripheral blood stem cells (RPBSC), 1 5/6 RPBSC, 1 6/6 related cord blood (CB) and 2 4/6 unrelated CB. GOx2 was administered =60d post AlloSCTx2 (8wks apart), at 4.5–7.5 mg/m2 (dose escalation). Samples were collected for WT1 analysis prior to and after each dose of GO (n=5). WT1 levels were determined by quantitative RT-PCR and compared with WT1 expression in a K562 leukemia line. Positive MRD was a ratio of K562 WT/patient's WT1>0.5. Results: The MTD for GO post RI AlloSCT has not been reached. 8/11 are NED (3 deaths: 1 relapse, 1 graft failure, 1 cGVHD) Pre GO WT1–0.079±0.07 (n=5), pre GO-dose #2–0.012±0.016 (n=3). One of these patients had detectable MRD (WT1–1.56) prior to the GO-dose #2, no MRD at 1year and is alive and in remission at 2 years post GOx2. The fifth patient entered transplant with +MRD (WT-0.866), relapsed after the first dose of GO and died of relapse prior to receiving the second dose. Conclusions: Targeted therapy with GOx2 may be useful in eradicating MRD in patients post RI AlloSCT. A larger cohort is necessary to assess the impact of GO (at the MTD) on MRD post RI AlloSCT in children with AML. No significant financial relationships to disclose.