PARP Inhibitors as Monotherapy in Daily Practice for Advanced Prostate Cancers

Despite recent improvements in survival, metastatic castration-resistant prostate cancers (mCRPCs) remain lethal. Alterations in genes involved in the homologous recombination repair (HRR) pathway are associated with poor prognosis. Poly-ADP-ribose polymerase (PARP) inhibitors (PARPis) have demonstrated anti-tumoral effects by synthetic lethality in patients with mCRPCs harboring HRR gene alterations, in particular BRCA2. While both olaparib and rucaparib have obtained government approvals for use, the selection of eligible patients as well as the prescription of these treatments within the clinical urology community are challenging. This review proposes a brief review of the rationale and outcomes of PARPi treatment, then a pragmatic vision of PARPi use in terms of prescription and the selection of patients based on molecular screening, which can involve potential genetic counseling in the case of associated germinal alterations.

Talazoparib plus enzalutamide in metastatic castration-resistant prostate cancer: TALAPRO-2 Phase III study design

PARP inhibitors in combination with androgen receptor-targeted therapy have demonstrated potential in the treatment of metastatic castration-resistant prostate cancer (mCRPC). Here, we describe the design and rationale of the multinational, Phase III, two-part TALAPRO-2 study comparing talazoparib plus enzalutamide versus placebo plus enzalutamide as a first-line treatment for patients with mCRPC with or without DNA damage response (DDR) alterations. This study has two co-primary end points: radiographic progression-free survival (rPFS) by blinded independent clinical review in all-comers (Cohort 1) and in patients with DDR alterations (Cohort 2). TALAPRO-2 will demonstrate whether talazoparib plus enzalutamide can significantly improve the efficacy of enzalutamide in terms of rPFS in both molecularly unselected and DDR-deficient patients with mCRPC (NCT03395197). Clinical Trial Registration: NCT03395197 (ClinicalTrials.gov).

Genetics of prostate cancer and its utility in treatment and screening

Prostate cancer heritability is attributed to a combination of rare, moderate to highly penetrant genetic variants as well as commonly occurring variants conferring modest risks [single nucleotide polymorphisms (SNPs)]. Some of the former type of variants (e.g., BRCA2 mutations) predispose particularly to aggressive prostate cancer and confer poorer prognoses compared to men who do not carry mutations. Molecularly targeted treatments such as PARP inhibitors have improved outcomes in men carrying somatic and/or germline DNA repair gene mutations. Ongoing clinical trials are exploring other molecular targeted approaches based on prostate cancer somatic alterations. Genome wide association studies have identified >250 loci that associate with prostate cancer risk. Multi-ancestry analyses have identified shared as well as population specific risk SNPs. Prostate cancer risk SNPs can be used to estimate a polygenic risk score (PRS) to determine an individual's genetic risk of prostate cancer. The odds ratio of prostate cancer development in men whose PRS lies in the top 1% of the risk profile ranges from 9 to 11. Ongoing studies are investigating the utility of a prostate cancer PRS to target population screening to those at highest risk. With the advent of personalized medicine and development of DNA sequencing technologies, access to clinical genetic testing is increasing, and oncology guidelines from bodies such as NCCN and ESMO have been updated to provide criteria for germline testing of "at risk" healthy men as well as those with prostate cancer. Both germline and somatic prostate cancer research have significantly evolved in the past decade and will lead to further development of precision medicine approaches to prostate cancer treatment as well as potentially developing precision population screening models.

Clinical Utility of Olaparib in the Treatment of Metastatic Castration-Resistant Prostate Cancer: A Review of Current Evidence and Patient Selection

Metastatic castration-resistant prostate cancer (mCRPC) is an aggressive and fatal disease with a median survival of 36 months. With the advent of genetic sequencing to identify individual genomic profiles and acquired tumor-specific pathways, targeted therapies have revolutionized cancer treatment, including the treatment strategy in mCRPC. Poly(adenosine 5'-diphosphate) ribose polymerase inhibitors (PARPi) are oral drugs that target mutations in the homologous recombination repair (HRR) pathway, which are found in approximately 27% of prostate cancer patients. In May 2020, the first PARP inhibitor, olaparib, was approved by the US Food and Drug Administration for men with mCRPC with HHR gene mutations based on the findings of the Phase III PROfound trial that showed improved overall survival in men with mCRPC who received olaparib and whose disease had progressed on a novel hormonal agent. This review summarizes the current evidence and clinical utility of olaparib as treatment in men with mCRPC. We describe the mechanism of action of PARPi, key clinical trials of olaparib in men with mCRPC, and ongoing Phase II and III clinical trials investigating olaparib in combination therapy and as front-line therapy in mCRPC.

Changing the History of Prostate Cancer with New Targeted Therapies

The therapeutic landscape of metastatic castration-resistant prostate cancer (mCRPC) is changing due to the emergence of new targeted therapies for the treatment of different molecular subtypes. Some biomarkers are described as potential molecular targets different from classic androgen receptors (AR). Approximately 20–25% of mCRPCs have somatic or germline alterations in DNA repair genes involved in homologous recombination. These subtypes are usually associated with more aggressive disease. Inhibitors of the enzyme poly ADP ribose polymerase (PARPi) have demonstrated an important benefit in the treatment of these subtypes of tumors. However, tumors that resistant to PARPi and wildtype BRCA tumors do not benefit from these therapies. Recent studies are exploring drug combinations with phosphatidylinositol-3-kinase (PI3K) or protein kinase B (AKT) inhibitors, as mechanisms to overcome resistance or to induce BRCAness and synthetic lethality. This article reviews various different novel strategies to improve outcomes in patients with prostate cancer.

Prostate cancer and PARP inhibitors: progress and challenges

Despite survival improvements achieved over the last two decades, prostate cancer remains lethal at the metastatic castration-resistant stage (mCRPC) and new therapeutic approaches are needed. Germinal and/or somatic alterations of DNA-damage response pathway genes are found in a substantial number of patients with advanced prostate cancers, mainly of poor prognosis. Such alterations induce a dependency for single strand break reparation through the poly(adenosine diphosphate-ribose) polymerase (PARP) system, providing the rationale to develop PARP inhibitors. In solid tumors, the first demonstration of an improvement in overall survival was provided by olaparib in patients with mCRPC harboring homologous recombination repair deficiencies. Although this represents a major milestone, a number of issues relating to PARP inhibitors remain. This timely review synthesizes and discusses the rationale and development of PARP inhibitors, biomarker-based approaches associated and the future challenges related to their prescription as well as patient pathways.

Niraparib with androgen receptor-axis-targeted therapy in patients with metastatic castration-resistant prostate cancer: safety and pharmacokinetic results from a phase 1b study (BEDIVERE)

Purpose

To assess the safety and pharmacokinetics and determine the recommended phase 2 dose (RP2D) of niraparib with apalutamide or abiraterone acetate plus prednisone (AAP) in patients with metastatic castration-resistant prostate cancer (mCRPC).

Methods

BEDIVERE was a multicenter, open-label, phase 1b study of niraparib 200 or 300 mg/day with apalutamide 240 mg or AAP (abiraterone acetate 1000 mg; prednisone 10 mg). Patients with mCRPC were previously treated with ≥ 2 lines of systemic therapy, including ≥ 1 androgen receptor-axis-targeted therapy for prostate cancer.

Results

Thirty-three patients were enrolled (niraparib-apalutamide, 6; niraparib-AAP, 27). No dose-limiting toxicities (DLTs) were reported when combinations included niraparib 200 mg; five patients receiving niraparib 300 mg experienced DLTs [niraparib-apalutamide, 2/3 patients (66.7%); niraparib-AAP, 3/8 patients (37.5%)]. Although data are limited, niraparib exposures were lower when given with apalutamide compared with historical niraparib monotherapy exposures in patients with solid tumors. Because of the higher incidence of DLTs, the niraparib–apalutamide combination and niraparib 300 mg combination with AAP were not further evaluated. Niraparib 200 mg was selected as the RP2D with AAP. Of 19 patients receiving niraparib 200 mg with AAP, 12 (63.2%) had grade 3/4 treatment-emergent adverse events, the most common being thrombocytopenia (26.3%) and hypertension (21.1%). Five patients (26.3%) had adverse events leading to treatment discontinuation.

Conclusions

These results support the choice of niraparib 200 mg as the RP2D with AAP. The niraparib–AAP combination was tolerable in patients with mCRPC, with no new safety signals. An ongoing phase 3 study is further assessing this combination in patients with mCRPC.

Trial registration no.

NCT02924766 (ClinicalTrials.gov).

Supplementary Information

The online version contains supplementary material available at 10.1007/s00280-021-04249-7.

PARP Inhibitors and Prostate Cancer: To Infinity and Beyond BRCA

The U.S. Food and Drug Administration recently approved two poly-adenosine diphosphate-ribose polymerase (PARP) inhibitors, olaparib and rucaparib, for treatment of biomarker-positive metastatic castrate resistant prostate cancer. The benefits of PARP inhibition have been well characterized in patients who have BRCA1 and BRCA2 mutations in several forms of cancer. BRCA1 and BRCA2 occupy key roles in DNA damage repair, which is comprised of several different pathways with numerous participants. Patients with mutations in other key genes within the DNA damage repair pathway may also respond to treatment with PARP inhibitors, and identification of these alterations could significantly increase the percentage of patients that may benefit from PARP inhibition. This review focuses on the potential for synthetically lethal interactions between PARP inhibitors and non-BRCA DNA damage repair genes. IMPLICATIONS FOR PRACTICE: The treatment potential of PARP inhibition has been well characterized in patients with BRCA1 and BRCA2 mutations, but there is compelling evidence for expanding the use of PARP inhibitors to mutations of other non-BRCA DNA damage repair (DDR) genes. This could increase the percentage of patients that may benefit from treatment with PARP inhibitors alone or in combination with other therapies. Understanding the significance of PARP inhibitor-sensitizing alterations in other common non-BRCA DDR genes will help guide clinical decisions to provide targeted treatment options to a wider population of patients.

PARP Inhibitors in Metastatic Prostate Cancer: Evidence to Date

Poly (ADP-ribose) polymerase inhibitors (PARPi) are a unique class of antineoplastic agents that function by inducing synthetic lethality. Synthetic lethality occurs when PARPi and either another agent or an underlying genetic alteration together lead to overwhelming DNA damage and ultimately cell death. PARPi first showed promise as a cancer therapy in patients with BRCA1/2 mutations and have become part of standard treatment for breast and ovarian cancer. In prostate cancer, two PARPi, rucaparib and olaparib, have been FDA approved for the treatment of metastatic castration-resistant prostate cancer (mCRPC). While both agents are approved for tumors with BRCA1/2 alterations, for olaparib the indication is also expanded to patients with 12 other homologous recombination deficiency (HRD) gene alterations including ATM and PALB2. PARPi differ in their pharmacokinetics and pharmacodynamics, and additional studies are being conducted with niraparib, veliparib, and talazoparib in prostate cancer. While PARPi are fairly well tolerated, common toxicities include hematologic (anemia/thrombocytopenia) and gastrointestinal effects (nausea/vomiting). Ongoing studies are being conducted combining PARPi with other agents in patients with and without HRD alterations. Early data are promising for the combination of PARPi with second-generation antiandrogens and with immunotherapy. As additional trials are developed and reported, the hope is that the patient population who may benefit from PARPi will continue to expand.

When and How to Use PARP Inhibitors in Prostate Cancer: A Systematic Review of the Literature with an Update on On-Going Trials

CONTEXT

The goal of precision oncology is to use the underlying genomic characteristics of the patient and the cancer to select the optimal treatment at a given time. The recent Food and Drug Administration (FDA) approval of the poly(ADP-ribose) polymerase (PARP) inhibitors olaparib and rucaparib for the treatment of advanced prostate cancer heralds the onset of precision medicine for this disease.

OBJECTIVE

To discuss the emerging role that PARP inhibitors may play as a personalised future treatment option in patients with prostate cancer, with a focus on patients with metastatic castration-resistant prostate cancer (mCRPC) whose tumour cells harbour mutations resulting from deficient homologous recombination repair (HRR).

EVIDENCE ACQUISITION

To identify publications relevant to this review, a systematic literature search of PubMed was conducted for articles and proceedings of relevant major congresses, published between January 2010 and March 2020, reporting the use of PARP inhibitors in the treatment of cancers.

EVIDENCE SYNTHESIS

A total of 168 publications were identified, and 18 of these met the criteria for subsequent review. In addition, 15 phase 2 or on-going phase 3 (mCRPC) studies evaluating PARP inhibitors as monotherapy or in combination, which had not yet reported data, were identified through ClinicalTrials.gov. Emerging data suggest that the greatest efficacy with single-agent PARP inhibitors is seen in mCRPC patients with germline or somatic BRCA1/2 alterations (especially BRCA2 or biallelic mutations), with potential efficacy also observed in men with PALB2 and FANCA mutations.

CONCLUSIONS

PARP inhibitors have demonstrated efficacy in mCRPC, and similar to ovarian and breast cancers, the greatest effect is observed in patients with HRR deficiency. The PARP inhibitors olaparib and rucaparib are now FDA approved for mCRPC patients with HRR mutations and BRCA1/2 mutations, respectively. Furthermore, when PARP inhibition is combined with novel hormonal therapies, a treatment benefit may be observed regardless of the HRR deficiency status. Gaps in the knowledge and understanding around PARP inhibitor use in prostate cancer, including the most appropriate diagnostic testing method for identifying an HRR mutation, remain to be resolved.

PATIENT SUMMARY

The poly(ADP-ribose) polymerase (PARP) inhibitors olaparib and rucaparib are now approved by the Food and Drug Administration for the treatment of advanced prostate cancer. Here, we reviewed the literature and proceedings from meeting presentations and published papers relevant to the use of PARP inhibitors in the treatment of prostate cancer. Testing methods for detecting homologous recombination repair gene mutations, as diagnostic tools to help identify patients most likely to benefit from PARP inhibitor treatment, are also discussed.

DNA Repair and Prostate Cancer: A Field Ripe for Harvest

Recent data have revealed antitumor activity for four PARP inhibitors, two of which (olaparib and rucaparib) are approved by the US Food and Drug Administration for metastatic castrate-resistant prostate cancer with selected DNA repair defects. Additional clinical trials are in progress for talazoparib, veliparib, and niraparib. More progress can be anticipated in the near future.