Final Results of a Phase 1/2 Study of SYK Inhibitor Entospletinib in Combination with Obinutuzumab in Patients with Relapsed/Refractory (R/R) Chronic Lymphocytic Leukemia (CLL)

Therapeutic resistance and intolerance of Bruton tyrosine kinase (BTK) inhibitors is an emerging need in CLL. SYK is integral to the activation of BTK and the B-cell receptor (BCR) signaling cascade and is overexpressed in CLL. We have shown that BAFF-mediated SYK activation triggered BCR signaling and rendered protection of CLL cells from spontaneous apoptosis in vitro. Single agent small molecule SYK inhibitor entospletinib was efficacious in treatment of patients with R/R CLL. Here we report final results of a single arm, open label, investigator-initiated phase 1/2 clinical trial which evaluated safety and efficacy of entospletinib in combination with obinutuzumab, a glycoengineered monoclonal anti-CD20 antibody, in patients with R/R CLL (NCT03010358).

Patients enrolled in the Phase 1 dose-escalation portion of the trial included adults with CLL or non-Hodgkin lymphoma (Phase 1 part only) after ≥1 prior therapy. Patients were enrolled at 2 dose levels to receive entospletinib 200 or 400 mg twice-daily orally according to 3+3 design. The primary endpoint for the phase 1 portion of the study was to determine the RP2D of the combination. All patients received single agent entospletinib as part of a 7-day run-in. Thereafter, patients received entospletinib on days 1-28 of each 28-day cycle continuously, and obinutuzumab intravenously in standard doses for 6 cycles. Once the RP2D was determined, a phase 2 study enrolled patients with R/R CLL only, where complete response (CR) was the primary endpoint.

A total of 24 patients (22 CLL, 2 follicular lymphoma) were enrolled. Twelve patients were enrolled in the phase 1 part of the study. The phase 2 part of the study included 17 patients with CLL. Of 6 patients who received entospletinib 200 mg on the Phase 1 part of the study, one patient experienced a DLT (grade 3 asymptomatic AST/ALT abnormalities) attributed to entospletinib. No DLTs were observed among the six patients who received entospletinib 400 mg. Thus, entospletinib 400 mg twice-daily was determined to be the RP2D in combination with obinutuzumab.

Efficacy of entospletinib+obinutuzumab was analyzed in the 21 patients with CLL, of which 17 received entospletinib at RP2D (400 mg twice daily). Patients with CLL had a median age of 66 years. Thirteen patients (62%) had TP53 aberration (n=9), complex karyotype (n=6), or NOTCH1 or SF3B1 mutation. The median number of prior therapies was two (range, 1-6). Seven patients had received prior ibrutinib (4 patients discontinued due to intolerance and 2 due to progression). Median follow-up was 31 months.

Among the 21 efficacy-evaluable participants with CLL, the ORR was 67% (95%CI, 43-85%). Three patients (14%, 95%CI 3-36%) achieved a CR, and 11 patients (53%) had a partial response (PR). patients with confirmed CR had undetectable MRD in the bone marrow. Median event-free survival was 27.5 months (95%CI: 16 months-NR), treatment duration - 31 months (95%CI: 27-40; Figure). Thirteen patients with high-risk CLL had an ORR of 54% (5 PRs and 2 CRs). Among the eight patients who had previously received kinase inhibitors, ORR was 62.5% (all PRs).

Treatment-related adverse events were reported in 96% of patients (Table). Grade 3 or higher AEs occurred in 65%. Neutropenia (43.5%; including 4 patients [17%] who had transient grade 4 neutropenia attributed to obinutuzumab) was the most common grade ≥3 hematologic toxicity. The median onset of neutropenia was 7 days after the first obinutuzumab infusion, median duration was 28 days. Growth factor support was not required and grade ≥3 infection occurred in only 1 patient. Only one patient on study discontinued therapy due to adverse events (recurrent AST/ALT abnormalities which resolved upon cessation of entospletinib).

Pharmacodynamic analysis demonstrated that treatment with entospletinib led to rapid downmodulation of pSTAT3 and the anti-apoptotic protein MCL1 in CLL cells. Furthermore, six months of combination therapy was accompanied by a reduction in IFNγ secretion in CD4 ⁺ T-cells and a reversal of exhausted phenotype, as evidenced by downregulation of PD-1.

Thus, the combination of entospletinib and obinutuzumab shows an acceptable safety profile. Efficacy of this combination (EFS 27.5 months in predominantly high-risk population ) compares favorably with chlorambucil/obinutuzumab in R/R CLL (13 months), thus warranting continued exploration of the regimen.

Figure 1

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Figure 1

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