SAN FRANCISCO – Combined therapy using an experimental programmed cell death protein 1 (PD-1) inhibitor and experimental poly ADP-ribose polymerase (PARP) 1/2 inhibitor was generally well tolerated and showed efficacy in a phase 1 study of patients with advanced solid tumors.
Tislelizumab, the anti–PD-1 agent in development for solid and hematologic malignancies, is a humanized IgG4 monoclonal antibody engineered to have minimal Fc-gamma receptor binding. Pamiparib, the PARP 1/2 inhibitor, is hypothesized to promote neoantigen release that may boost the efficacy of tislelizumab. At the Jan. 4 data cutoff, 2 of 49 patients treated with one of five planned dose levels experienced a complete response, 8 had a confirmed partial response, and 4 had an unconfirmed partial response,, reported at the ASCO-SITC Clinical Immuno-Oncology Symposium.
Thus, the objective response rate was 20%, she said, noting that the clinical benefit rate, which encompasses all those with a response as well as those with durable stable disease after at least 24 weeks, was 39%. The median duration of response was 168.5 days
As of the data cutoff, 11 patients, including all those with a complete or partial response, remained on treatment, and 10 patients remained on treatment beyond 200 days, said Dr. Mileshkin of Peter MacCallum Cancer Centre, Melbourne.
Study participants were 42 women and 7 men, with a mean age of 63 years and measurable disease treated with at least 1 prior line of therapy (median of 4), but with no prior exposure to a PARP inhibitor or PD-1 therapy. Primary tumor sites included ovarian/fallopian tube/peritoneal (34 patients); pancreas, prostate, and breast cancer (3 patients each); and bile duct, bladder, cervix, lung, peripheral nerve sheath, and uterus (1 patient each). For the current dose-finding phase of the study (phase 1a), cohorts of 6-13 patients were treated with either tislelizumab at an intravenous dose of 2 mg/kg every 3 weeks plus either an oral dose of 20, 40, or 60 mg of pamiparib (dose levels 1, 2, and 3, respectively), or with tislelizumab at an intravenous dose of 200 mg every 3 weeks plus pamiparib at an oral dose of 40 or 60 mg twice daily (dose levels 4 and 5, respectively), Dr. Mileshkin said.
Phase 1b will be a disease-specific expansion to evaluate preliminary antitumor activity.
The rationale for combining the two agents was “to prove that we could get some up-regulation of tumor-associated antigens by using a PARP inhibitor that may then allow us to improve the antitumor activity of the checkpoint inhibitor,” she said.
Further, the malignancies in the patients include those likely to harbor DNA damage repair deficiencies, she added.
All patients experienced at least one treatment-emergent adverse event; 21 experienced a serious event, and 23 had immune-related adverse events.
“Despite the number of events, most patients were able to continue therapy,” Dr. Mileshkin said, noting that more discontinuation was associated with the PD-1 therapy than with the PARP inhibitor.
Non–immune-related adverse events related to the PARP inhibitor included mostly grade 1 and 2 nausea, fatigue, and diarrhea. Anemia also occurred and was grade 3 in 12% of patients.
“In terms of the PD-1 therapy, we mostly saw grade 1 or 2 nonimmune adverse events and very few grade 3 and 4 events,” she said.
One or more grade 3 immune-related adverse events occurred in 12 patients, and based on reports from participating centers, there “seemed to be a signal here that we were seeing more hepatic toxicity than we might have expected,” she said, explaining that these included increases in transaminases and hepatitis.
“We ended up with 13 patients who had a hepatic adverse event thought to be related to treatment. The median time to onset was 55 days ... and there were 9 patients in whom these events were grade 3 or 4,” she said.
Ten had grade 2 or higher transaminitis, which was most likely autoimmune in nature.
“All of these patients were treated with corticosteroids, and they all recovered promptly from the event. Subsequently, the protocol was amended to try increase real-time hepatic safety monitoring,” she said, noting that the rate of hepatic events has fallen since these changes were made.
“We need to closely monitor that moving forward to try to understand it better,” she said of the hepatic events.
Dose level 4 (tislelizumab at 200 mg every 3 weeks plus pamiparib at 40 mg twice daily) was determined to be the maximum tolerated dose and is the recommended phase 2 dose, she noted.
“We were pleased to see 10 patients respond, with these responses appearing to be durable with this currently short follow-up period, and we look forward to seeing the results of part B,” she concluded, noting that enrollment of patients into disease-specific cohorts for that next phase is ongoing.
This study is sponsored by BeiGene. Dr. Mileshkin reported receiving payment for travel, accommodations, and/or expenses from BeiGene and Roche.