Phase I, Dose-Escalation, Two-Part Trial of the PARP Inhibitor Talazoparib in Patients with Advanced Germline BRCA1/2 Mutations and Selected Sporadic Cancers

PARP inhibitors in platinum-sensitive high-grade serous ovarian cancer

Purpose

Poly(ADP-ribose) polymerase inhibitors (PARPi) have changed the management of high-grade serous ovarian cancer (HGSOC). The rationale for the development of PARPi was based on the concept of synthetic lethality, in which a cell can survive a deficiency of one gene/gene product, but may die if there is a deficiency in a combination of genes/gene products. In women with BRCA1/2 deficiency within their ovarian cancer tissue, inhibition of PARP imposes an intolerable burden of DNA damage repair deficiency and may induce cell death.

Methods

Clinical trials have evaluated PARPi as single-agent therapeutics and as maintenance treatment following platinum-based chemotherapy for HGSOC. Clinical data suggest the most impressive anti-tumour activity occurs in women with platinum-sensitive ovarian cancer and germline or somatic BRCA1/2 mutations (g/sBRCAmt).

Results

In the maintenance setting, randomised trials have shown that PARPi compared to placebo reduce the hazard ratio for the development of progressive disease to 0.2–0.27 for patients with a g/sBRCAmt; to 0.34–0.38 for patients with putative evidence of DNA damage repair deficiency; and to 0.35–0.45 in an unselected population with HGSOC. Furthermore, phase 1/2 trials have reported single-agent anti-tumour response rates in gBRCAmt of approximately 50% in platinum-sensitive and 25% in platinum-resistant disease.

Conclusion

Here, we discuss the evidence for the use of PARPi as single-agent therapeutics and maintenance treatment in HGSOC and evaluate the genetic assays used in clinical trials so far. We discuss the emerging role of platinum sensitivity as a broad eligibility criteria for the use of PARPi.

Targeting DNA damage repair in small cell lung cancer and the biomarker landscape

Small cell lung cancer (SCLC) is an aggressive malignancy that accounts for 14% of all lung cancer diagnoses. Despite decades of active research, treatment options for SCLC are limited and resistance to the few Food and Drug Administration (FDA) approved therapies develops rapidly. With no approved targeted agents to date, new therapeutic strategies are desperately needed. SCLC is characterized by high mutation burden, ubiquitous loss of TP53 and RB1, mutually exclusive amplification of MYC family members, thereby, high genomic instability. Studies in the past few years have demonstrated the potential of targeting the DNA damage response (DDR) pathway as a promising therapeutic strategy for SCLC. Inhibitors targeting DDR proteins have shown promise in preclinical models, and are under clinical investigation as single agents and in combination with cytotoxic therapies. Recent efforts to expand the therapeutic arsenal toward SCLC have focused in part on immune checkpoint inhibitors, such as agents targeting the receptor-ligand pair programmed cell death protein 1 (PD-1) and programmed death-ligand 1 (PD-L1). Clinical trials have confirmed activity of these agents in extensive stage (ES)-SCLC. However, while several patients had dramatic responses, overall response rates to immune checkpoint blockade (ICB) remain poor. As a result, there is an urgent need to develop rational combination therapies to enhance response rates to immunotherapy in SCLC. Identification of predictive biomarkers for patient stratification, identifying effective combinations to overcome adaptive resistance to DDR-targeted therapies and identifying strategies to enhance response to immunotherapy are areas of active investigation in SCLC.

Inference of Germline Mutational Status and Evaluation of Loss of Heterozygosity in High-Depth, Tumor-Only Sequencing Data

PURPOSE

Inherited germline defects are implicated in up to 10% of human tumors, with particularly well-known roles in breast and ovarian cancers that harbor BRCA1/2-mutated genes. There is also increasing evidence for the role of germline alterations in other malignancies such as colon and pancreatic cancers. Mutations in familial cancer genes can be detected by high throughput sequencing (HTS), when applied to formalin-fixed paraffin-embedded (FFPE) tumor specimens. However, due to often lack of patient-matched control normal DNA and/or low tumor purity, there is limited ability to determine the genomic status of these alterations (germline versus somatic) and to assess the presence of loss of heterozygosity (LOH). These analyses, especially when applied to genes such as BRCA1/2, can have significant clinical implications for patient care.

METHODS

LOHGIC (LOH-Germline Inference Calculator) is a statistical model selection method to determine somatic-versus-germline status and predict LOH for mutations identified via clinical grade, high-depth, hybrid-capture tumor-only sequencing. LOHGIC incorporates statistical uncertainties inherent to HTS as well as specimen biases in tumor purity estimates, which we use to assess BRCA1/2 mutations in 1,636 specimens sequenced at Rutgers Cancer Institute of New Jersey.

RESULTS

Evaluation of LOHGIC with available germline sequencing from BRCA1/2 testing, demonstrates 93% accuracy, 100% precision, and 96% recall. This analysis highlights a differential tumor spectrum associated with BRCA1/2 mutations.

CONCLUSION

LOHGIC can assess LOH status for both germline and somatic mutations. It also can be applied to any gene with candidate, inherited mutations. This approach demonstrates the clinical utility of targeted sequencing in both identifying patients with potential germline alterations in tumor suppressor genes as well as estimating LOH occurrence in cancer cells, which may confer therapeutic relevance.

Targeting DNA damage in SCLC

SCLC accounts for 15% of lung cancer worldwide. Characterised by early dissemination and rapid development of chemo-resistant disease, less than 5% of patients survive 5 years. Despite 3 decades of clinical trials there has been no change to the standard platinum and etoposide regimen for first line treatment developed in the 1970's. The exceptionally high number of genomic aberrations observed in SCLC combined with the characteristic rapid cellular proliferation results in accumulation of DNA damage and genomic instability. To flourish in this precarious genomic context, SCLC cells are reliant on functional DNA damage repair pathways and cell cycle checkpoints. Current cytotoxic drugs and radiotherapy treatments for SCLC have long been known to act by induction of DNA damage and the response of cancer cells to such damage determines treatment efficacy. Recent years have witnessed improved understanding of strategies to exploit DNA damage and repair mechanisms in order to increase treatment efficacy. This review will summarise the rationale to target DNA damage response in SCLC, the progress made in evaluating novel DDR inhibitors and highlight various ongoing challenges for their clinical development in this disease.

[DNA repair as a therapeutic target]

The transmission of an intact and stable genetic code at each cell division relies on different DNA repair systems. Germline mutations of some of these genes cause cancer predisposition, whereas somatic mutations are frequently found in various cancer types, generating genomic instability. As a consequence, cancer cell becomes more susceptible to additional DNA damage. Pharmacological inhibition of DNA repair pathways exploits this frailty: it triggers more damages than cancer cell can tolerate, finally leading to apoptosis. The success of PARP (poly-ADP-ribose polymerase) inhibitors in BRCA1/2-mutated ovarian cancer shows the clinical relevance of this strategy. Herein, we explain the functioning of different DNA-repair pathways, describe the implicated proteins, and their close relation with cell-cycle checkpoints. We focus on novel therapeutic agents targeting DNA repair, their clinical results, and discuss challenges of combination therapies.

Current status of poly(ADP-ribose) polymerase inhibitors and future directions

Inhibitors of poly(ADP-ribose) polymerases (PARPs), which play a key role in DNA damage/repair pathways, have been developed as antitumor agents based on the concept of synthetic lethality. Synthetic lethality is the idea that cell death would be efficiently induced by simultaneous loss of function of plural key molecules, for example, by exposing tumor cells with inactivating gene mutation of BRCA-mediated DNA repair to chemically induced inhibition of PARPs. Indeed, three PARP inhibitors, olaparib, rucaparib and niraparib have already been approved in the US or Europe, mainly for the treatment of BRCA-mutant ovarian cancer. Clinical trials of various combinations of PARP inhibitors with cytotoxic or molecular-targeted agents are also underway. In particular, expanded applications of PARP inhibitors are anticipated following recent reports that defects in homologous recombination repair (HRR) are associated with mutations in repair genes other than BRCA1/BRCA2, such as ATM, ATR, PALB2, RAD51, CHEK1 and CHEK2, as well as with epigenetic loss of BRCA1 function through promoter methylation or overexpression of the BRCA2-interacting transcriptional repressor EMSY. Current topics of interest include selection of the best agent in each clinical context, identification of new treatment targets for HRR-proficient cases, and development of PARP inhibitor-based regimens that are less toxic and that prolong overall survival as well as progression-free survival. In addition, potential long-term side effects and suitable biomarkers for predicting efficacy and mechanisms of clinical resistance are in discussion. This review summarizes representative preclinical and clinical data for PARP inhibitors and discusses their potential for future applications to treat various malignancies.

PARP inhibitors: Clinical utility and possibilities of overcoming resistance

PARP inhibitors represent a major breakthrough in ovarian cancer care. Almost half of all ovarian cancers have deficiencies in the homologous recombination (HR) DNA repair pathway, namely BRCA1/2 mutations. Given the limited therapeutic options for recurrent ovarian cancer patients there has been a significant effort to develop novel therapies to exploit DNA repair deficiencies. In 2005 and 2006, inhibiting PARP enzymes was first observed to be highly effective against cancers with HR deficiencies. PARP inhibitors are being utilized in the clinic to manage recurrent ovarian cancers that display defects in the HR repair pathway. However, PARP inhibitors also show significant clinical benefit in patients without HR deficiencies. There are currently three FDA-approved PARP inhibitors for recurrent ovarian cancer and an additional two PARP inhibitors being evaluated in late stage clinical trials. Given the expanding clinical use of PARP inhibitors and the high likelihood of acquired resistance, there is a significant need for clinical strategies to manage PARP inhibitor resistant disease. This review will examine PARP inhibitors in the context of: indications and toxicities, novel biomarkers to predict response, targeted-therapy resistance, and potential approaches to manage resistant disease.

Germline genetic variants with implications for disease risk and therapeutic outcomes

Genetic testing has multiple clinical applications including disease risk assessment, diagnosis, and pharmacogenomics. Pharmacogenomics can be utilized to predict whether a pharmacologic therapy will be effective or to identify patients at risk for treatment-related toxicity. Although genetic tests are typically ordered for a distinct clinical purpose, the genetic variants that are found may have additional implications for either disease or pharmacology. This review will address multiple examples of germline genetic variants that are informative for both disease and pharmacogenomics. The discussed relationships are diverse. Some of the agents are targeted for the disease-causing genetic variant, while others, although not targeted therapies, have implications for the disease they are used to treat. It is also possible that the disease implications of a genetic variant are unrelated to the pharmacogenomic implications. Some of these examples are considered clinically actionable pharmacogenes, with evidence-based, pharmacologic treatment recommendations, while others are still investigative as areas for additional research. It is important that clinicians are aware of both the disease and pharmacogenomic associations of these germline genetic variants to ensure patients are receiving comprehensive personalized care.

Circulating Cell-Free DNA to Guide Prostate Cancer Treatment with PARP Inhibition

Biomarkers for more precise patient care are needed in metastatic prostate cancer. We have reported a phase II trial (TOPARP-A) of the PARP inhibitor olaparib in metastatic prostate cancer, demonstrating antitumor activity associating with homologous recombination DNA repair defects. We now report targeted and whole-exome sequencing of serial circulating cell-free DNA (cfDNA) samples collected during this trial. Decreases in cfDNA concentration independently associated with outcome in multivariable analyses (HR for overall survival at week 8: 0.19; 95% CI, 0.06-0.56; P = 0.003). All tumor tissue somatic DNA repair mutations were detectable in cfDNA; allele frequency of somatic mutations decreased selectively in responding patients (χ2P < 0.001). At disease progression, following response to olaparib, multiple subclonal aberrations reverting germline and somatic DNA repair mutations (BRCA2, PALB2) back in frame emerged as mechanisms of resistance. These data support the role of liquid biopsies as a predictive, prognostic, response, and resistance biomarker in metastatic prostate cancer.Significance: We report prospectively planned, serial, cfDNA analyses from patients with metastatic prostate cancer treated on an investigator-initiated phase II trial of olaparib. These analyses provide predictive, prognostic, response, and resistance data with "second hit" mutations first detectable at disease progression, suggesting clonal evolution from treatment-selective pressure and platinum resistance. Cancer Discov; 7(9); 1006-17. ©2017 AACR.See related commentary by Domchek, p. 937See related article by Kondrashova et al., p. 984See related article by Quigley et al., p. 999This article is highlighted in the In This Issue feature, p. 920.

Unravelling the biology of SCLC: implications for therapy

Small-cell lung cancer (SCLC) is an aggressive malignancy associated with a poor prognosis. First-line treatment has remained unchanged for decades, and a paucity of effective treatment options exists for recurrent disease. Nonetheless, advances in our understanding of SCLC biology have led to the development of novel experimental therapies. Poly [ADP-ribose] polymerase (PARP) inhibitors have shown promise in preclinical models, and are under clinical investigation in combination with cytotoxic therapies and inhibitors of cell-cycle checkpoints.Preclinical data indicate that targeting of histone-lysine N-methyltransferase EZH2, a regulator of chromatin remodelling implicated in acquired therapeutic resistance, might augment and prolong chemotherapy responses. High expression of the inhibitory Notch ligand Delta-like protein 3 (DLL3) in most SCLCs has been linked to expression of Achaete-scute homologue 1 (ASCL1; also known as ASH-1), a key transcription factor driving SCLC oncogenesis; encouraging preclinical and clinical activity has been demonstrated for an anti-DLL3-antibody-drug conjugate. The immune microenvironment of SCLC seems to be distinct from that of other solid tumours, with few tumour-infiltrating lymphocytes and low levels of the immune-checkpoint protein programmed cell death 1 ligand 1 (PD-L1). Nonetheless, immunotherapy with immune-checkpoint inhibitors holds promise for patients with this disease, independent of PD-L1 status. Herein, we review the progress made in uncovering aspects of the biology of SCLC and its microenvironment that are defining new therapeutic strategies and offering renewed hope for patients.

Understanding Resistance Mechanisms and Expanding the Therapeutic Utility of PARP Inhibitors

Poly-(ADP-ribose) polymerase (PARP) inhibitors act through synthetic lethality in cells with defects in homologous recombination (HR) DNA repair caused by molecular aberrations such as BRCA mutations, and is approved for treatment in ovarian cancer, with promising clinical activity against other HR defective tumors including breast and prostate cancers. Three PARP inhibitors have been FDA approved, while another two have shown promising activity and are in late stage development. Nonetheless, both primary and secondary resistance to PARP inhibition have led to treatment failure, and the development of predictive biomarkers and the ability to identify and overcome mechanisms of resistance is vital for optimization of its clinical utility. Additionally, there has been evidence that PARP inhibition may have a therapeutic role beyond HR deficient tumors which warrants further investigation, both as single agent and in combination with other therapeutic modalities like cytotoxic chemotherapy, radiation, targeted therapy and immunotherapy. With new strategies to overcome resistance and expand its therapeutic utility, PARP inhibitors are likely to become a staple in our armamentarium of drugs in cancer therapeutics.

Olaparib for Metastatic Breast Cancer in Patients with a Germline BRCA Mutation

BACKGROUND

Olaparib is an oral poly(adenosine diphosphate-ribose) polymerase inhibitor that has promising antitumor activity in patients with metastatic breast cancer and a germline BRCA mutation.

METHODS

We conducted a randomized, open-label, phase 3 trial in which olaparib monotherapy was compared with standard therapy in patients with a germline BRCA mutation and human epidermal growth factor receptor type 2 (HER2)-negative metastatic breast cancer who had received no more than two previous chemotherapy regimens for metastatic disease. Patients were randomly assigned, in a 2:1 ratio, to receive olaparib tablets (300 mg twice daily) or standard therapy with single-agent chemotherapy of the physician's choice (capecitabine, eribulin, or vinorelbine in 21-day cycles). The primary end point was progression-free survival, which was assessed by blinded independent central review and was analyzed on an intention-to-treat basis.

RESULTS

Of the 302 patients who underwent randomization, 205 were assigned to receive olaparib and 97 were assigned to receive standard therapy. Median progression-free survival was significantly longer in the olaparib group than in the standard-therapy group (7.0 months vs. 4.2 months; hazard ratio for disease progression or death, 0.58; 95% confidence interval, 0.43 to 0.80; P<0.001). The response rate was 59.9% in the olaparib group and 28.8% in the standard-therapy group. The rate of grade 3 or higher adverse events was 36.6% in the olaparib group and 50.5% in the standard-therapy group, and the rate of treatment discontinuation due to toxic effects was 4.9% and 7.7%, respectively.

CONCLUSIONS

Among patients with HER2-negative metastatic breast cancer and a germline BRCA mutation, olaparib monotherapy provided a significant benefit over standard therapy; median progression-free survival was 2.8 months longer and the risk of disease progression or death was 42% lower with olaparib monotherapy than with standard therapy. (Funded by AstraZeneca; OlympiAD ClinicalTrials.gov number, NCT02000622 .).

Differential Toxicity in Patients with and without DNA Repair Mutations: Phase I Study of Carboplatin and Talazoparib in Advanced Solid Tumors

Purpose: The PARP inhibitor (PARPi) talazoparib may potentiate activity of chemotherapy and toxicity in cells vulnerable to DNA damage.Experimental Design: This phase I study evaluated the safety, tolerability, pharmacokinetics, and efficacy of talazoparib and carboplatin. Pharmacokinetic modeling explored associations between DNA vulnerability and hematologic toxicity.Results: Twenty-four patients (eight males; 16 females) with solid tumors were enrolled in four cohorts at 0.75 and 1 mg daily talazoparib and weekly carboplatin (AUC 1 and 1.5, every 2 weeks or every 3 weeks), including 14 patients (58%) with prior platinum treatment. Dose-limiting toxicities included grade 3 fatigue and grade 4 thrombocytopenia; the MTD was not reached. Grade 3/4 toxicities included fatigue (13%), neutropenia (63%), thrombocytopenia (29%), and anemia (38%). After cycle 2's dose, delays/reductions were required in all patients. One complete and two partial responses occurred in germline BRCA1/2 (gBRCA1/2) patients. Four patients showed stable disease beyond 4 months, three of which had known mutations in DNA repair pathways. Pharmacokinetic toxicity modeling suggests that after three cycles of carboplatin AUC 1.5 every 3 weeks and talazoparib 1 mg daily, neutrophil counts decreased 78% [confidence interval (CI), 87-68] from baseline in gBRCA carriers and 63% (CI, 72-55) in noncarriers (P < 0.001). Pharmacokinetic toxicity modeling suggests an intermittent, pulse dosing schedule of PARP inhibition, differentiated by gBRCA mutation status, may improve the benefit/risk ratio of combination therapy.Conclusions: Carboplatin and talazoparib showed efficacy in DNA damage mutation carriers, but hematologic toxicity was more pronounced in gBRCA carriers. Carboplatin is best combined with intermittent talazoparib dosing differentiated by germline and somatic DNA damage mutation carriers. Clin Cancer Res; 23(21); 6400-10. ©2017 AACR.