PARP Inhibitors in Combination with Radiotherapy: To Do or Not to Do?

BACKGROUND: Despite the large use of inhibitors of Poly-ADP ribose polymerase (PARP-I), the feasibility and safety of their combination with radiotherapy (RT) is unclear. AIM: We conducted a literature analysis with the aim to evaluate the efficacy and safety profile of a combination with RT and PARP-I. METHOD: The key issues for the current review were expressed in two questions according to the Population, Intervention, Control, Outcome (PICO) criteria: 1. What is the outcome and 2. What is the toxicity in patients treated with a combination of PARP-I and RT for a newly diagnosed or recurrent tumors? RESULTS: A total of 12 clinical studies met the inclusion criteria including seven single-arm dose-escalation phase I studies, two phase II (two- and three-arms controlled trials) trials, one parallel-arm phase I study, and two phase I/II studies published between 2015 and 2021. RT was performed with photon beams and several schedules according to the clinical situation. The acute toxicity ≥ grade 3 ranged between 25% and >96%, which was divided into hematological or non-hematological adverse events. CONCLUSIONS: despite the heterogeneity of the evaluated patient populations and tumor types, and the limited number of the studies, this review suggests that a combination approach is feasible even though the efficacy profile remains unclear.

Polymerase θ Coordinates Multiple Intrinsic Enzymatic Activities during DNA Repair.

The POLQ gene encodes DNA polymerase θ, a 2590 amino acid protein product harboring DNA-dependent ATPase, template-dependent DNA polymerase, dNTP-dependent endonuclease, and 5'-dRP lyase functions. Polymerase θ participates at an essential step of a DNA double-strand break repair pathway able to join 5'-resected substrates by locating and pairing microhomologies present in 3'-overhanging single-stranded tails, cleaving the extraneous 3'-DNA by dNTP-dependent end-processing, before extending the nascent 3' end from the microhomology annealing site. Metazoans require polymerase θ for full resistance to DNA double-strand break inducing agents but can survive knockout of the POLQ gene. Cancer cells with compromised homologous recombination, or other DNA repair defects, over-utilize end-joining by polymerase θ and often over-express the POLQ gene. This dependency points to polymerase θ as an ideal drug target candidate and multiple drug-development programs are now preparing to enter clinical trials with small-molecule inhibitors. Specific inhibitors of polymerase θ would not only be predicted to treat BRCA-mutant cancers, but could thwart accumulated resistance to current standard-of-care cancer therapies and overcome PARP-inhibitor resistance in patients. This article will discuss synthetic lethal strategies targeting polymerase θ in DNA damage-response-deficient cancers and summarize data, describing molecular structures and enzymatic functions.

PARP-inhibitor potpourri: A comparative review of class safety, efficacy, and cost.

Purpose: To summarize similarities and differences in efficacy, safety, and cost of available PARP-inhibitors and offers pearls to distinguish subtle nuances between each agent to help guide therapy. Summary: Currently, four PARP-inhibitors (olaparib, rucaparib, niraparib, and talazoparib) are FDA-approved, with olaparib, rucaparib, and niraparib approved for treatment and/or maintenance or ovarian cancer and olaparib and talazoparib approved for the treatment of recurrent metastatic BRCA-mutant, HER2-negative breast cancer. While the PARP-inhibitor class is generally are well-tolerated, each agent does possess a unique side-effect profile. Niraparib and talazoparib have more prominent hematologic adverse event profiles, while niraparib has an increased risk of cardiac events. In patients using other medications with known drug interactions, niraparib may be the preferred option for patients with ovarian cancer, and talazoparib may be the preferred option for patients with breast cancer because neither of these agents undergo hepatic metabolism. These agents also can incur large financial toxicities for patients, and olaparib currently has the broadest range of options for financial assistance. Conclusion: Although these agents have similar approved indications, efficacy, and toxicity profiles, there are notable differences that may help direct choice of therapy and optimize treatment for patients. It is important to incorporate patient-specific factors to optimize PARP-inhibitor therapy for patients.