Phase 1 study of the safety, pharmacokinetics, and antitumour activity of the BCL2 inhibitor navitoclax in combination with rituximab in patients with relapsed or refractory CD20⁺ lymphoid malignancies

The oral BCL2 inhibitor navitoclax has moderate single-agent efficacy in chronic lymphocytic leukaemia (CLL) and minor activity in lymphoma in Phase 1 trials. Navitoclax synergizes with rituximab in preclinical models of B-cell lymphoid cancers. We report the safety, pharmacokinetics and clinical activity of this combination. Patients received navitoclax (200-325 mg) daily and four standard weekly doses of rituximab. Twenty-nine patients were enrolled across three dose-escalation cohorts and a safety expansion cohort (250 mg/d navitoclax). The combination was well tolerated. Common toxicities were mild diarrhoea (79%) and nausea (72%). Grade 4 thrombocytopenia occurred in 17% of patients (dose limiting at 325 mg/d). CD19⁺ counts were severely reduced, while CD3⁺ cells (∼ 20%) and serum immunoglobulin M levels (∼ 33%) were also reduced during the first year. The maximum tolerated dose for navitoclax in combination was 250 mg/d. Pharmacokinetic analyses revealed no apparent interactions between the drugs. The response rate in patients with follicular lymphoma was 9/12, including five complete responses. All five patients with CLL/small lymphocytic leukaemia achieved partial responses. One of nine patients with aggressive lymphoma responded. The addition of rituximab to navitoclax 250 mg/d is safe; the combination demonstrates higher response rates for low-grade lymphoid cancers than observed for either agent alone in previous Phase 1 trials.