Phase I Safety, Pharmacokinetic, and Pharmacodynamic Study of the Poly(ADP-ribose) Polymerase (PARP) Inhibitor Veliparib (ABT-888) in Combination with Irinotecan in Patients with Advanced Solid Tumors

PURPOSE

PARP is essential for recognition and repair of DNA damage. In preclinical models, PARP inhibitors modulate topoisomerase I inhibitor-mediated DNA damage. This phase I study determined the MTD, dose-limiting toxicities (DLT), pharmacokinetics (PK), and pharmacodynamics (PD) of veliparib, an orally bioavailable PARP1/2 inhibitor, in combination with irinotecan.

EXPERIMENTAL DESIGN

Patients with advanced solid tumors were treated with 100 mg/m(2) irinotecan on days 1 and 8 of a 21-day cycle. Twice-daily oral dosing of veliparib (10-50 mg) occurred on days 3 to 14 (cycle 1) and days -1 to 14 (subsequent cycles) followed by a 6-day rest. PK studies were conducted with both agents alone and in combination. Paired tumor biopsies were obtained after irinotecan alone and veliparib/irinotecan to evaluate PARP1/2 inhibition and explore DNA damage signals (nuclear γ-H2AX and pNBS1).

RESULTS

Thirty-five patients were treated. DLTs included fatigue, diarrhea, febrile neutropenia, and neutropenia. The MTD was 100 mg/m(2) irinotecan (days 1 and 8) combined with veliparib 40 mg twice daily (days -1-14) on a 21-day cycle. Of 31 response-evaluable patients, there were six (19%) partial responses. Veliparib exhibited linear PK, and there were no apparent PK interactions between veliparib and irinotecan. At all dose levels, veliparib reduced tumor poly(ADP-ribose) (PAR) content in the presence of irinotecan. Several samples showed increases in γ-H2AX and pNBS1 after veliparib/irinotecan compared with irinotecan alone.

CONCLUSIONS

Veliparib can be safely combined with irinotecan at doses that inhibit PARP catalytic activity. Preliminary antitumor activity justifies further evaluation of the combination. Clin Cancer Res; 22(13); 3227-37. ©2016 AACR.

Targeted radiosensitization with PARP1 inhibition: optimization of therapy and identification of biomarkers of response in breast cancer

Sustained locoregional control of breast cancer is a significant issue for certain patients. Inhibition of PARP1 is a promising strategy for radiosensitization (RS). We sought to optimize therapy with PARP1 inhibition and radiation (RT) by establishing the most effective treatment schedule, degree of PARP1-mediated RS, and identify early biomarkers predictive of efficacy in breast cancer models. Using clonogenic survival assays, we assessed intrinsic radiosensitivity and RS induced by PARP1 inhibition in breast cancer cell lines. Potential biomarkers of response were evaluated using western blotting, flow cytometry, and immunofluorescence with validation in vivo using tumor xenograft experiments. Across a panel of BC and normal breast epithelial cell lines, the PARP1 inhibitor ABT-888 preferentially radiosensitizes breast cancer (vs. normal) cells with enhancement ratios (EnhR) up to 2.3 independent of intrinsic BC subtype or BRCA mutational status. Concurrent and adjuvant therapy resulted in the highest EnhR of all schedules tested. The degree of RS did not correlate with pretreatment markers of PARP1 activity, DNA damage/repair, or cell cycle distribution. Increases in PARP1 activity 24 h after RT were associated with sensitivity after combination treatment. Findings were confirmed in breast cancer xenograft models. Our study demonstrates that PARP1 inhibition improves the therapeutic index of RT independent of BC subtype or BRCA1 mutational status and that PARP1 activity may serve as a clinically relevant biomarker of response. These studies have led to a clinical trial (TBCRC024) incorporating intratreatment biomarker analyses of PARP1 inhibitors and RT in breast cancer patients.