A Phase 1b Study of Vadastuximab Talirine in Combination with 7+3 Induction Therapy for Patients with Newly Diagnosed Acute Myeloid Leukemia (AML)

Background

For patients who are less than 65 years with newly diagnosed AML, standard induction treatment is continuous infusion cytarabine for 7 days and an anthracycline for 3 days (7+3). Although a high percentage of patients achieve an initial CR by morphologic criteria, some requiring a 2nd induction, a significant number of patients are either primarily resistant to treatment or achieve a morphologic CR but with flow cytometric or molecular evidence of minimal residual disease (MRD). CD33, a cell surface antigen, is expressed in approximately 90% of AML, representing a promising target of therapy regardless of genetic or mutational heterogeneity. Vadastuximab talirine (33A) is a CD33-directed antibody conjugated to 2 molecules of a pyrrolobenzodiazepine (PBD) dimer. Upon binding, 33A is internalized and transported to the lysosomes where PBD dimer is released via proteolytic cleavage of the linker, crosslinking DNA, and leading to cell death. Addition of 33A to 7+3 chemotherapy could result in enhanced and deeper (MRD negative) remissions, resulting in reduced relapse rates and improved overall survival.

Methods

This phase 1b study (NCT02326584) evaluated the safety and antileukemic activity of escalating doses of 33A in combination with 7+3 induction therapy (cytarabine 100 mg/m² and daunorubicin 60 mg/m²). AML patients (ECOG status 0-1) must be eligible to receive induction therapy. 33A is given on Days 1 and 4 concomitantly with the combination treatment. Response assessments occur on Days 15 and 28; investigator assessment of response is per IWG criteria (Cheson 2003). A 2nd induction regimen and post-remission therapies were per investigator choice and did not include additional administration of 33A. MRD was assessed centrally by bone marrow examination using a multi-parametric flow cytometric assay at Days 15 and Day 28.

Results

To date, 42 patients (36% male) with a median age of 45.5 years (range, 18-65) have been treated with 10+10 (n=4) and 20+10 (n=38) mcg/kg of 33A. Most patients had intermediate (40%) or adverse (43%) cytogenetic risk by MRC criteria and 17% of patients had secondary AML. As expected, all patients experienced Grade 4 myelosuppression. In patients who achieved CR or CRi, the estimated median time to count recovery from Day 1 of therapy was 33 days for neutrophils (≥1K) and 35 days for platelets (≥100K). Three DLTs (lack of recovery of platelets [25K] and/or ANC [500] by Day 42) occurred at the 20+10 mcg/kg dose level, which was determined to be the maximum tolerated dose (MTD) of 33A in combination with 7+3. No non-hematologic treatment emergent adverse events (AEs) ≥Grade 3 were reported in >15% of patients; Grade 1 or 2 non-hematologic AEs occurring in >15% of patients were nausea (55%), diarrhea (33%), constipation (31%), decreased appetite (19%), fatigue (19%), and vomiting (17%); no infusion-related reactions occurred. No veno-occlusive disease (VOD) or significant hepatotoxicity was observed. The 30- and 60-day mortality rates were 0% and 7%, respectively. Of the 40 efficacy evaluable patients, best responses include 24 CR (60%), 7 CRi (18%), and 4 morphologic leukemia-free state (10%) with a CR+CRi (CRc) rate of 78%; 94% of CR or CRi responses occurred with 1 cycle of induction therapy. Twenty-three of 31 (74%) patients attaining CR or CRi achieved MRD negative status. Median OS is not yet reached; 36 patients were alive at the time of this data cut with 6 patients (14%) still on treatment. Pharmacokinetic data demonstrate rapid elimination of 33A.

Conclusions

33A can be safely combined with 7+3 with acceptable count recovery and the recommended phase 2 dose is 20+10 mcg/kg on Days 1 and 4. An alternate schedule of single-day dosing on Day 1 is under investigation and enrollment continues. Extramedullary AEs, including hepatic toxicity, and induction mortality rates were similar to reported rates for 7+3 alone in this AML population. A high remission rate within the 1st induction cycle was observed, the majority of which were MRD negative.