SGN-CD33A Plus Hypomethylating Agents: A Novel, Well-Tolerated Regimen with High Remission Rate in Frontline Unfit AML

Background

Older patients with AML who are not candidates for intensive therapy are typically treated with hypomethylating agents (HMAs) or other low intensity therapy. HMAs have been shown to upregulate CD33 and to increase sensitivity to cytotoxic chemotherapy by decreasing apoptotic threshold in tumor cells. SGN-CD33A (or 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. In preclinical studies combining 33A with an HMA (azacitidine and decitabine), synergy has been demonstrated in multidrug resistant AML models (Sutherland ASH 2014).

Methods

A combination cohort in a phase 1 study (NCT01902329) was designed to evaluate the safety, tolerability, pharmacokinetics (PK), and anti-leukemic activity of 33A in combination with an HMA. Eligible patients (ECOG 0-1) must have previously untreated CD33-positive AML, and have declined intensive therapy. A single dose level of 33A, 10 mcg/kg, was administered outpatient IV every 4 weeks on the last day of HMA (azacitidine or decitabine [5 day regimen], standard dosing). Patients with clinical benefit may continue treatment until relapse or unacceptable toxicity. Investigator assessment of response is per IWG criteria; CRi requires either platelet count of ≥100,000/µL or neutrophils of ≥1,000/µL (Cheson 2003).

Results

To date, 24 patients (63% male) with a median age of 77 years (range, 66-83) have been treated with the combination therapy. 42% of patients had adverse cytogenetics (MRC), 23 patients were treatment naïve and 1 patient had received prior low intensity therapy for MDS. At baseline, patients had a median of 60% BM blasts (range, 2%-90%) and a median of WBC of 2.2 (range, 1-132). At the time of this interim analysis, patients were on treatment for a median of 13.5+ weeks with 17 patients continuing treatment; no DLTs have been reported. Grade 3 or higher adverse events (AE) reported in >20% of patients were fatigue (54%), febrile neutropenia (46%), anemia (25%), neutropenia (25%), and thrombocytopenia (21%). Other treatment-emergent AEs regardless of relationship to study treatment reported in ˃20% of patients were nausea (29%), decreased appetite (25%), and constipation (21%). Thirty- and 60-day mortality rates are 0% and 4% respectively with no treatment-related deaths reported. Fifteen of the 23 efficacy evaluable patients (65%) achieved CR (5) or CRi (10).  Remissions were generally obtained after 2 cycles of treatment and were observed in many patients with adverse risk including underlying myelodysplasia (6/8, 75%) and adverse cytogenetics (8/9, 89%). Median OS has not been reached with 20 patients alive at the time of this data cut.

Conclusions

The combination of 33A with HMA appears to be well-tolerated, active, and has no identified off-target toxicities. Activity with the combination compares favorably with historical experience with HMAs alone in this patient population. The CR+CRi rate of 65% in AML patients with poor risk factors with the observed low 60-day mortality (4%) are particularly encouraging. These promising data warrant further evaluation in future trials.