Safety and Efficacy of a Combination of Venetoclax (GDC-0199/ABT-199) and Obinutuzumab in Patients with Relapsed/Refractory or Previously Untreated Chronic Lymphocytic Leukemia - Results from a Phase 1b Study (GP28331)

Introduction

Treatment of patients (pts) with relapsed/refractory (R/R) chronic lymphocytic leukemia (CLL) with the combination of venetoclax (VEN), an oral, selective Bcl-2 inhibitor, and rituximab yielded an ORR of 84% (Roberts et al. Haematologica 2015). Treatment of such pts with VEN in combination with obinutuzumab (Gazyva®, Gazyvaro™, G), a Type II, glycoengineered anti-CD20 antibody, may yield even better treatment outcomes.  We present preliminary efficacy and updated safety data from an ongoing phase 1b study (NCT01685892) evaluating this combination in R/R or treatment-naïve (TN) pts with CLL in alternate treatment schedules.   

Methods

Pts with CLL with an ECOG PS ≤1 and adequate organ function are enrolled in a study with a 3+3 design and cohorts ranging from 100 to 600 mg/day of VEN. Pts are assigned to one of two dosing schedules, starting treatment with either VEN (Schedule A) or G (Schedule B). Both schedules include tumor lysis syndrome (TLS) risk mitigation based on disease burden at screening, which includes a gradual VEN ramp-up to the assigned cohort dose. Six cycles of combination therapy will be given and then pts with R/R disease continue single-agent VEN until disease progression; TN pts will receive single-agent VEN for an additional 6 months. Dose-limiting toxicities (DLTs) are identified during the first 21 days of combination therapy in Schedule A or the first 35 days of combination therapy in Schedule B, and focus on TLS, infusion related reactions, and cytopenias. Based on a safety review of data from this trial, the 600 mg cohort will not be explored. Response is first assessed before Cycle 4 according to 2008 International Workshop on CLL guidelines.

Results

As of April 20, 2015, 32 pts (26 R/R and 6 TN) have been enrolled. Four R/R pts were unenrolled after a sponsor-initiated clinical hold secondary to TLS events in other VEN studies. Patient characteristics include a median age 62.5 (range, 45-80) years, and 62.5% male pts. TLS risk was assessed in 28 pts following protocol modifications adopted after a Sponsor-initiated clinical hold; 96.4% were at medium or high risk for TLS. The highest VEN dose administered in this study was 400 mg/day (administered to 11 R/R and 6 TN pts). Median time on study was 5.5 (range, 0.1-19.6) mo. for all pts and 2.8 (range, 0.9-2.8) mo. for TN pts. Among pts exposed to VEN, dose interruptions were observed in 17/27 (63%) pts.  A summary of AEs is presented in Figure 1.  

Laboratory TLS was observed in 4/32 (12.5%) pts and all were able to continue study treatment after resolution of electrolyte changes; no cases of clinical TLS occurred. One pt with R/R disease in cohort 1 discontinued study participation following disease progression (the pt completed 6 cycles of combination treatment). A second pt with R/R disease in cohort 1 died secondary to acute respiratory failure; Richter’s transformation also was suspected in this pt but not confirmed. Twenty pts with R/R disease and 6 TN pts remain on the study. At least 1 response evaluation has been performed in 17 pts with R/R disease. The overall response rate (ORR) by investigator assessment was 100%; 4/17 (23.5%) pts achieved complete response/complete response with incomplete bone marrow recovery (CR/CRi). Among the 13 (76.5%) pts with PRs after 3 cycles of therapy, 3 have improved to CR/CRi at assessments 28 days after completing C6D1. Full MRD data will be available in the near future but early analyses suggest some patients may achieve MRD negative status by Cycle 4.

Conclusion  

These preliminary data suggest that VEN + G can be safely administered in pts with CLL with no difference in tolerability between R/R and TN subgroups.  AEs appear to be manageable and no pt has discontinued study participation secondary to cytopenia, the most frequently observed AE. Data suggests that the TLS prophylaxis measures are effective even in patients with a higher disease burden. An expansion phase is planned using a 400 mg per day dose of VEN in R/R and TN pts following a review of safety data assessing potential differences between dosing schedules. The preliminary efficacy data suggest this regimen may be an important option in patients with CLL; a phase 3 study evaluating VEN+G is ongoing.