Lintuzumab and Low-Dose Cytarabine Compared to Placebo and Low-Dose Cytarabine in Patients with Untreated Acute Myeloid Leukemia (AML) 60 Years and Older: Results of a Randomized, Double-Blinded Phase 2b Study,

Older adults with AML who decline intensive induction chemotherapy lack effective therapeutic options, as evidenced by a median overall survival (OS) of ∼5 months with low-dose cytarabine treatment (Burnett 2007). Leukemic blast cells express CD33 in most patients with AML. While antitumor activity has been previously demonstrated by CD33-directed agents, none of these are currently available. Lintuzumab (SGN-33) is a humanized antibody directed to CD33 that was studied in patients with myeloid malignancies and demonstrated tolerability with modest monotherapy activity. To determine if the addition of lintuzumab to low-dose cytarabine could prolong survival in older patients with untreated AML, a randomized, placebo-controlled, double-blinded phase 2b study was conducted (ClinicalTrials.gov #NCT00528333).

The study was conducted at 72 international sites between November 2007 and August 2010. Patients were >60 years of age with centrally confirmed, previously untreated AML; the minimum blast percentage required was 20% with ≥50% CD33 expression. Maximum allowed WBC at baseline was 30,000/μL. All patients declined treatment with intensive induction chemotherapy. Cytarabine 20 mg was administered SC twice daily on Days 1–10 of each 28-day cycle (12 cycles maximum). Patients were randomized to additionally receive either lintuzumab 600 mg or placebo IV: in Cycle 1 on Days 1, 8, 15, and 22, and on Days 1 and 15 of subsequent cycles. Patients were stratified according to age, ECOG performance status (PS), and history of antecedent hematologic disorder (AHD). Cytogenetic risk was determined at baseline according to Fröling 2006. The primary endpoint of the study was OS, determined using an unstratified log rank test. Secondary endpoints included peripheral blood count recovery, transfusion requirements, infection rates, and quality of life. As patients choosing less-intensive therapy often incorporate quality of life into their treatment decision, follow-up bone marrow biopsies were not required in the study.

211 patients were randomized in the study (107 lintuzumab arm, 104 placebo arm). The study arms appeared balanced; overall, the median patient age was 70 years (range 60–90), 53% were female, and baseline ECOG PS was 0/1 (55%) or 2 (45%). Baseline blast percentages were ≥30% in 74% of patients and 23% had a history of AHD. A median of 95% CD33 expression was documented at baseline in either bone marrow or peripheral blood. Median number of treatment cycles was 4 in the lintuzumab arm and 3 in the placebo arm. At the time of the analysis, 24 patients were alive and the maximum follow-up was 32 months. The estimated median OS for the lintuzumab + low-dose cytarabine arm was 4.7 months compared with 5.1 months for the placebo + low-dose cytarabine arm with a hazard ratio (lintuzumab to placebo) of 0.96, indicating that lintuzumab was not associated with a treatment effect (P value 0.76, 95% CI [0.72, 1.28]). Rates of infections or fevers of unknown origin requiring hospitalization or IV antibiotics did not differ between the study arms. Overall, 28% of patients were observed to have no peripheral blasts, ANC >1.0 x10⁹/L, platelets >100 × 10⁹/L, and no transfusions for at least 1 week. The median OS among all patients was 5.0 months, which was significantly impacted by cytogenetic risk (8.7 months for standard-risk compared with 4.5 months for high-risk; P value 0.0024). The incidence of adverse events (AEs) was comparable between the treatment groups, with the exception of infusion-related reactions which occurred more frequently in the lintuzumab arm. The most common ≥ Grade 3 non-hematologic AEs in the study were pneumonia (12%) and sepsis (8%).

In this randomized, placebo-controlled, double-blinded phase 2b trial, the combination of lintuzumab and low-dose cytarabine did not improve OS compared with placebo and low-dose cytarabine. While lintuzumab was not effective, CD33 remains a compelling target for AML because it is widely expressed by malignant cells and appears to be absent from pluripotent hematopoietic stem cells. This was the largest study to date of patients treated with low-dose cytarabine; the median OS across both treatment arms was 5 months. A similar outcome was observed in the subgroup of patients with high-risk cytogenetics, confirming that low-dose cytarabine remains a valid comparator in trials of older patients with AML.

Lancet:Seattle Genetics, Inc.: Research Funding. Off Label Use: Lintuzumab is an investigational monoclonal antibody directed against CD33. Sekeres:Celgene Corp.: Membership on an entity's Board of Directors or advisory committees; Celgene Corp.: Honoraria; Seattle Genetics, Inc.: Research Funding; Amgen: Membership on an entity's Board of Directors or advisory committees. Wood:Seattle Genetics, Inc.: Research Funding. Grove:Seattle Genetics, Inc.: Equity Ownership; Seattle Genetics, Inc.: Employment. Sandalic:Seattle Genetics, Inc.: Equity Ownership; Seattle Genetics, Inc.: Employment. Sievers:Seattle Genetics: Employment, Equity Ownership. Jurcic:Seattle Genetics, Inc.: Consultancy; Seattle Genetics, Inc.: Membership on an entity's Board of Directors or advisory committees; Seattle Genetics, Inc.: Research Funding.