Beyond monotherapy: An era ushering in combinations of PARP inhibitors with immune checkpoint inhibitors for solid tumors

The introduction of PARP inhibitors (PARPis) and immune checkpoint inhibitors (ICIs) has marked a significant shift in the treatment landscape for solid tumors. Emerging preclinical evidence and initial clinical trials have indicated that the synergistic application of PARPis and ICIs may enhance treatment efficacy and potentially improve long-term patient outcomes. Nonetheless, how to identify specific tumor types and molecular subgroups most likely to benefit from this combination remains an area of ongoing research. This review thoroughly examines current studies on the co-administration of PARPis and ICIs across various solid tumors. It explores the underlying mechanisms of action, evaluates clinical efficacy, identifies potential responder populations, and delineates common adverse events alongside strategic management approaches. The aim is to offer a detailed understanding of this combination therapy, potentially guiding future therapeutic strategies for solid tumors.

Development of Tumor Markers for Breast Cancer Immunotherapy

Although breast cancer treatment has been developed remarkably in recent years, it remains the primary cause of death among women. Immune checkpoint blockade therapy has significantly altered the way breast cancer is treated, although not all patients benefit from the changes. At present, the most effective mechanism of immune checkpoint blockade application in malignant tumors is not clear and efficacy may be influenced by many factors, including host, tumor, and tumor microenvironment dynamics. Therefore, there is a pressing need for tumor immunomarkers that can be used to screen patients and help determine which of them would benefit from breast cancer immunotherapy. At present, no single tumor marker can predict treatment efficacy with sufficient accuracy. Multiple markers may be combined to more accurately pinpoint patients who will respond favorably to immune checkpoint blockade medication. In this review, we have examined the breast cancer treatments, developments in research on the role of tumor markers in maximizing the clinical efficacy of immune checkpoint inhibitors, prospects for the identification of novel therapeutic targets, and the creation of individualized treatment plans. We also discuss how tumor markers can provide guidance for clinical practice.

An overview of the research progress of BRCA gene mutations in breast cancer

The breast cancer susceptibility gene (BRCA) is an important tumor suppressor gene, including BRCA1 and BRCA2, a biomarker that assesses the risk of breast cancer and influences a patient's individualized treatment options. BRCA1/2 mutation (BRCAm) increases the risk of breast cancer. However, breast-conserving surgery is still an option for BRCAm, and prophylactic mastectomy and nipple-sparing mastectomy may also reduce the risk of breast cancer. BRCAm is sensitive to Poly (ADP-ribose) polymerase inhibitor (PARPi) therapy due to specific types of DNA repair defects, and its combination with other DNA damage pathway inhibitors and endocrine therapy and immunotherapy are also used for the treatment of BRCAm breast cancer. The current treatment and research progress of BRCA1/2 mutant breast cancer in this review provides a basis for the individualized treatment of patients with this type of breast cancer.

Emerging strategies: PARP inhibitors in combination with immune checkpoint blockade in BRCA1 and BRCA2 mutation-associated and triple-negative breast cancer

Poly (adenosine diphosphate-ribose) polymerase (PARP) inhibitor monotherapy in germline BRCA1 and BRCA2 mutation-associated metastatic breast cancer is a well-tolerated and an effective therapeutic strategy, however, the durability of response can be limited. Checkpoint inhibitors targeting the PD-1/PD-L1 axis as monotherapy in metastatic triple-negative breast cancer (mTNBC) have a limited role due to low response rates, but are capable of long, durable responses. Combination PARP inhibition with checkpoint blockade is an emerging area of investigation with potential synergy to produce robust responses with durability. Mechanistically, PARP inhibition activates the stimulator of interferon gene (STING) pathway to promote dendritic cell and T lymphocyte recruitment, increases tumor neoantigens, and upregulates PD-L1 expression to increase the immunogenicity of the tumor and thereby potentially enhance responses to immunotherapy. Several clinical trials have reported early results on PARP inhibitor and PD-1/PD-L1 checkpoint inhibitor combinations. All studies have shown safety and tolerability of this combination regimen. In advanced breast cancer associated with a germline BRCA1 or BRCA2 mutation, response rates have been high and similar to what is observed with PARP inhibitor monotherapy. Additional follow-up is needed to see if combination with a checkpoint inhibitor can lead to a clinically meaningful extension of durability of response in patients with germline mutations in BRCA1 and BRCA2. In unselected mTNBC in the 1st-3rd line setting, response rates of combined PARP inhibitor and PD-1/PD-L1 inhibitors have ranged from 18-21%, with higher rates of response among those with alterations in homologous recombination DNA repair pathway genes. Multiple ongoing studies will report additional data on combinations of PARP inhibitors and checkpoint blockade in the future and this combination strategy remains an active area of investigation.

Immune-Checkpoint Inhibitors: A New Line of Attack in Triple-Negative Breast Cancer

Poor prognosis is a distinctive feature of triple-negative breast cancer (TNBC). Chemotherapy has long represented the main and unique treatment for patients with TNBC. Recently, immune checkpoint inhibitors (ICIs) were investigated in several clinical trials and were approved for clinical use in TNBC patients that express programmed cell death protein-1 (PD-1) in combination with chemotherapy in the first-line setting. ICIs are also being investigated in the neoadjuvant and adjuvant settings for TNBC. This chapter aims to discuss different ICIs used to treat all TNBC stages to date.

Therapeutic Targeting of DNA Damage Repair in the Era of Precision Oncology and Immune Checkpoint Inhibitors

Cancer manifestation is a multistep process involving accumulation of various genetic and epigenetic changes that results in oncogenic "hallmarks of cancer" processes including genomic instability. Exploitation of aberrant DNA-damage response (DDR) mechanisms in cancer is in part a goal of many therapeutic strategies, and recent evidence supports the role of targeting DDR in modulating the tumor immune microenvironment to enhance immunotherapeutic response. Improved cancer profiling, including next-generation and whole-genome mutational sequencing of tumor tissue, as well as circulating nucleic acids, has enhanced our understanding of the genetic and epigenetic molecular mechanisms in tumorigenesis and will become fundamental to precisely target tumors and achieve cancer control. With the successes of poly(ADP-ribose) polymerase inhibitors (PARPi) and immunotherapies, the intersection of DDR molecular machinery and corresponding antitumor immune response has gained much interest with a focus on achieving therapeutic synergy using DNA damage-targeting agents and immunotherapy. In this review, we provide a bench-to-bedside overview of the fundamentals of DDR signaling and repair as they relate to cancer therapeutic strategies including novel DDR-targeting agents. We also discuss the underlying mechanisms that link DDR signaling to antitumor immunity and immunotherapy efficacy, and how this knowledge can be used to improve precision medicine approaches in the treatment of cancer.

Role of PARP Inhibitors in Cancer Immunotherapy: Potential Friends to Immune Activating Molecules and Foes to Immune Checkpoints

Poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi) induce cytotoxic effects as single agents in tumors characterized by defective repair of DNA double-strand breaks deriving from BRCA1/2 mutations or other abnormalities in genes associated with homologous recombination. Preclinical studies have shown that PARPi-induced DNA damage may affect the tumor immune microenvironment and immune-mediated anti-tumor response through several mechanisms. In particular, increased DNA damage has been shown to induce the activation of type I interferon pathway and up-regulation of PD-L1 expression in cancer cells, which can both enhance sensitivity to Immune Checkpoint Inhibitors (ICIs). Despite the recent approval of ICIs for a number of advanced cancer types based on their ability to reinvigorate T-cell-mediated antitumor immune responses, a consistent percentage of treated patients fail to respond, strongly encouraging the identification of combination therapies to overcome resistance. In the present review, we analyzed both established and unexplored mechanisms that may be elicited by PARPi, supporting immune reactivation and their potential synergism with currently used ICIs. This analysis may indicate novel and possibly patient-specific immune features that might represent new pharmacological targets of PARPi, potentially leading to the identification of predictive biomarkers of response to their combination with ICIs.

The Role of PARP Inhibitors in Patients with Primary Malignant Central Nervous System Tumors

OPINION STATEMENT

Primary malignant central nervous (CNS) tumors are a devastating group of diseases with urgent need for improved treatment options. Surgery, radiation, and cytotoxic chemotherapy remain the primary standard treatment modalities, with molecularly targeted therapies having proven efficacy in only small subsets of cases. Poly(ADP-ribose) polymerase (PARP) inhibitors, which have had immense success in the treatment of extracranial cancers with homologous recombination deficiency (HRD), are emerging as a potential targeted treatment for various CNS tumors. Although few primary CNS tumors display canonical BRCA gene defects, preclinical evidence suggests that PARP inhibitors may benefit certain CNS tumors with functional HRD or elevated replication stress. In addition, other preclinical studies indicate that PARP inhibitors may synergize with standard therapies used for CNS tumors including radiation and alkylating agents and may prevent or overcome drug resistance. Thus far, initial clinical trials with early-generation PARP inhibitors, typically as monotherapy or in the absence of selective biomarkers, have shown limited efficacy. However, the scientific rationale remains promising, and many clinical trials are ongoing, including investigations of more CNS penetrant or more potent inhibitors and of combination therapy with immune checkpoint inhibitors. Early phase trials are also critically focusing on determining active drug CNS penetration and identifying biomarkers of therapy response. In this review, we will discuss the preclinical evidence supporting use of PARP inhibitors in primary CNS tumors and clinical trial results to date, highlighting ongoing trials and future directions in the field that may yield important findings and potentially impact the treatment of these devastating malignancies in the coming years.

Overall survival in the OlympiA phase III trial of adjuvant olaparib in patients with germline pathogenic variants in BRCA1/2 and high risk, early breast cancer

BACKGROUND

The randomized, double-blind OlympiA trial compared 1 year of the oral poly(adenosine diphosphate-ribose) polymerase inhibitor, olaparib, to matching placebo as adjuvant therapy for patients with pathogenic or likely pathogenic variants in germline BRCA1 or BRCA2 (gBRCA1/2pv) and high-risk, human epidermal growth factor receptor 2-negative, early breast cancer (EBC). The first pre-specified interim analysis (IA) previously demonstrated statistically significant improvement in invasive disease-free survival (IDFS) and distant disease-free survival (DDFS). The olaparib group had fewer deaths than the placebo group, but the difference did not reach statistical significance for overall survival (OS). We now report the pre-specified second IA of OS with updates of IDFS, DDFS, and safety.

PATIENTS AND METHODS

One thousand eight hundred and thirty-six patients were randomly assigned to olaparib or placebo following (neo)adjuvant chemotherapy, surgery, and radiation therapy if indicated. Endocrine therapy was given concurrently with study medication for hormone receptor-positive cancers. Statistical significance for OS at this IA required P < 0.015.

RESULTS

With a median follow-up of 3.5 years, the second IA of OS demonstrated significant improvement in the olaparib group relative to the placebo group [hazard ratio 0.68; 98.5% confidence interval (CI) 0.47-0.97; P = 0.009]. Four-year OS was 89.8% in the olaparib group and 86.4% in the placebo group (Δ 3.4%, 95% CI -0.1% to 6.8%). Four-year IDFS for the olaparib group versus placebo group was 82.7% versus 75.4% (Δ 7.3%, 95% CI 3.0% to 11.5%) and 4-year DDFS was 86.5% versus 79.1% (Δ 7.4%, 95% CI 3.6% to 11.3%), respectively. Subset analyses for OS, IDFS, and DDFS demonstrated benefit across major subgroups. No new safety signals were identified including no new cases of acute myeloid leukemia or myelodysplastic syndrome.

CONCLUSION

With 3.5 years of median follow-up, OlympiA demonstrates statistically significant improvement in OS with adjuvant olaparib compared with placebo for gBRCA1/2pv-associated EBC and maintained improvements in the previously reported, statistically significant endpoints of IDFS and DDFS with no new safety signals.

PARP inhibition is a modulator of anti-tumor immune response in BRCA-deficient tumors

PARP inhibitors are synthetically lethal with BRCA1/2 mutations, and in this setting, accumulation of DNA damage leads to cell death. Because increased DNA damage and subsequent immune activation can prime an anti-tumor immune response, we studied the impact of olaparib ± immune checkpoint blockade (ICB) on anti-tumor activity and the immune microenvironment. Concurrent combination of olaparib, at clinically relevant exposures, with ICB gave durable and deeper anti-tumor activity in the Brca1m BR5 model vs. monotherapies. Olaparib and combination treatment modulated the immune microenvironment, including increases in CD8+ T cells and NK cells, and upregulation of immune pathways, including type I IFN and STING signaling. Olaparib also induced a dose-dependent upregulation of immune pathways, including JAK/STAT, STING and type I IFN, in the tumor cell compartment of a BRCA1m (HBCx-10) but not a BRCA WT (HBCx-9) breast PDX model. In vitro, olaparib induced BRCAm tumor cell–specific dendritic cell transactivation. Relevance to human disease was assessed using patient samples from the MEDIOLA (NCT02734004) trial, which showed increased type I IFN, STING, and JAK/STAT pathway expression following olaparib treatment, in line with preclinical findings. These data together provide evidence for a mechanism and schedule underpinning potential benefit of ICB combination with olaparib.

Targeting PARP1 to Enhance Anticancer Checkpoint Immunotherapy Response: Rationale and Clinical Implications

Reinvigorating the antitumor immune response using immune checkpoint inhibitors (ICIs) has revolutionized the treatment of several malignancies. However, extended use of ICIs has resulted in a cancer-specific response. In tumors considered to be less immunogenic, the response rates were low or null. To overcome resistance and improve the beneficial effects of ICIs, novel strategies focused on ICI-combined therapies have been tested. In particular, poly ADP-ribose polymerase inhibitors (PARPi) are a class of agents with potential for ICI combined therapy. PARPi impairs single-strand break DNA repair; this mechanism involves synthetic lethality in tumor cells with deficient homologous recombination. More recently, novel evidence indicated that PAPRi has the potential to modulate the antitumor immune response by activating antigen-presenting cells, infiltrating effector lymphocytes, and upregulating programmed death ligand-1 in tumors. This review covers the current advances in the immune effects of PARPi, explores the potential rationale for combined therapy with ICIs, and discusses ongoing clinical trials.

Immunotherapy in Breast Cancer and the Potential Role of Liquid Biopsy

Liquid biopsy biomarkers, such as circulating tumor cells (CTCs) and circulating tumor DNA (ctDNA), are noninvasive diagnostics that could complement predictive and prognostic tools currently used in the clinic. Recent trials of immunotherapy have shown promise in improving outcomes in a subset of breast cancer patients. Biomarkers could improve the efficacy of immune checkpoint inhibitors by identifying patients whose cancers are more likely to respond to immunotherapy. In this review, we discuss the current applications of liquid biopsy and emerging technologies for evaluation of immunotherapy response and outcomes in breast cancer. We also provide an overview of the status of immunotherapy in breast cancer.

Expanding the Use of PARP Inhibitors as Monotherapy and in Combination in Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) is the most aggressive subtype of breast cancer, and is known to be associated with a poor prognosis and limited therapeutic options. Poly (ADP-ribose) polymerase inhibitors (PARPi) are targeted therapeutics that have demonstrated efficacy as monotherapy in metastatic BRCA-mutant (BRCAMUT) TNBC patients. Improved efficacy of PARPi has been demonstrated in BRCAMUT breast cancer patients who have either received fewer lines of chemotherapy or in chemotherapy-naïve patients in the metastatic, adjuvant, and neoadjuvant settings. Moreover, recent trials in smaller cohorts have identified anti-tumor activity of PARPi in TNBC patients, regardless of BRCA-mutation status. While there have been concerns regarding the efficacy and toxicity of the use of PARPi in combination with chemotherapy, these challenges can be mitigated with careful attention to PARPi dosing strategies. To better identify a patient subpopulation that will best respond to PARPi, several genomic biomarkers of homologous recombination deficiency have been tested. However, gene expression signatures associated with PARPi response can integrate different pathways in addition to homologous recombination deficiency and can be implemented in the clinic more readily. Taken together, PARPi have great potential for use in TNBC patients beyond BRCAMUT status, both as a single-agent and in combination.

Emerging Role of PARP Inhibitors in Metastatic Triple Negative Breast Cancer. Current Scenario and Future Perspectives

Triple negative tumors represent 15% of breast cancer and are characterized by the lack of estrogen receptors, progesterone receptor, and HER2 amplification or overexpression. Approximately 25% of patients diagnosed with triple negative breast cancer carry a germline BRCA1 or BRCA2 mutation. They have an aggressive biology, and chemotherapy has been the mainstay of treatment for a long time. Despite intensive therapies, prognosis is still poor, and many patients will eventually relapse or die due to cancer. Therefore, novel targeted agents that can increase the treatment options for this disease are urgently needed. Recently, a new class of molecules has emerged as a standard of care for patients with triple negative breast cancer and germline BRCA1 or BRCA2 mutation: poly (ADP-ribose) (PARP) inhibitors. In the first part of the review, we summarize and discuss evidence supporting the use of PARP inhibitors. Currently, two PARP inhibitors have been approved for triple negative metastatic breast cancer-olaparib and talazoparib-based on two phase III trials, which showed a progression-free survival benefit when compared to chemotherapy. Safety profile was manageable with supportive therapies and dose reductions/interruptions. In addition, other PARP inhibitors are currently under investigation, such as talazoparib, rucaparib, and veliparib. Subsequently, we will discuss the potential role of PARP inhibitors in the future. Clinical research areas are investigating PARP inhibitors in combination with other agents and are including patients without germline BRCA mutations: ongoing phase II/III studies are combining PARP inhibitors with immunotherapy, while phases I and II trials are combining PARP inhibitors with other targeted agents such as ATM and PIK3CA inhibitors. Moreover, several clinical trials are enrolling patients with somatic BRCA mutation or patients carrying mutations in genes, other than BRCA1/2, involved in the homologous recombination repair pathway (e.g., CHECK2, PALB2, RAD51, etc.).

Targeting the DNA damage response: PARP inhibitors and new perspectives in the landscape of cancer treatment

Cancer derives from alterations of pathways responsible for cell survival, differentiation and proliferation. Dysfunctions of mechanisms protecting genome integrity can promote oncogenesis but can also be exploited as therapeutic target. Poly-ADP-Ribose-Polymerase (PARP)-inhibitors, the first approved targeted agents able to tackle DNA damage response (DDR), have demonstrated antitumor activity, particularly when homologous recombination impairment is present. Despite the relevant results achieved, a large proportion of patients fail to obtain durable responses. The development of innovative treatments, able to overcome resistance and ensure long-lasting benefit for a wider population is still an unmet need. Moreover, improvement in biomarker assays is necessary to properly identify patients who can benefit from DDR targeting agents. Here we summarize the main DDR pathways, explain the current role of PARP inhibitors in cancer therapy and illustrate new therapeutic strategies targeting the DDR, focusing on the combinations of PARP inhibitors with other agents and on cell-cycle checkpoint inhibitors.

Multiomics analysis of serial PARP inhibitor treated metastatic TNBC inform on rational combination therapies

In a pilot study, we evaluated the feasibility of real-time deep analysis of serial tumor samples from triple negative breast cancer patients to identify mechanisms of resistance and treatment opportunities as they emerge under therapeutic stress engendered by poly-ADP-ribose polymerase (PARP) inhibitors (PARPi). In a BRCA-mutant basal breast cancer exceptional long-term survivor, a striking tumor destruction was accompanied by a marked infiltration of immune cells containing CD8 effector cells, consistent with pre-clinical evidence for association between STING mediated immune activation and benefit from PARPi and immunotherapy. Tumor cells in the exceptional responder underwent extensive protein network rewiring in response to PARP inhibition. In contrast, there were minimal changes in the ecosystem of a luminal androgen receptor rapid progressor, likely due to indifference to the effects of PARP inhibition. Together, identification of PARPi-induced emergent changes could be used to select patient specific combination therapies, based on tumor and immune state changes.

A review of immune checkpoint blockade in breast cancer

In the recent years characterized by the cancer immunotherapy revolution, attention has turned to how to potentially boost and/or generate an efficient anti-tumor immune response in breast cancer (BC). Clinical activity of immune checkpoint blockade (ICB) targeting PD-1 or PD-L1 in BC has been more evident in the triple negative subtype and in earlier lines of the treatment. Remarkably, some responders to single agent ICB have achieved durable responses with metastatic disease, possibly as a result of treatment-induced immunological memory. However, most BC are immunologically quiescent and current research efforts developing ICB combinations are attempting to convert "cold" into "hot" tumors by manipulating the tumor microenvironment, expanding anti-tumor T cells improving efficient antigen presentation, and suppressing pro-tumor inhibitory cells. The aim of this review is to summarize existing data on the efficacy of immune checkpoint blockers as single agents and combination strategies in all BC subtypes, highlighting the BC subgroups that benefit most from ICB.

Harnessing DNA Repair Defects to Augment Immune-Based Therapies in Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) has poor prognosis with limited treatment options, with little therapeutic progress made during the past several decades. DNA damage response (DDR) associated therapies, including radiation and inhibitors of DDR, demonstrate potential efficacy against TNBC, especially under the guidance of genomic subtype-directed treatment. The tumor immune microenvironment also contributes greatly to TNBC malignancy and response to conventional and targeted therapies. Immunotherapy represents a developing trend in targeted therapies directed against TNBC and strategies combining immunotherapy and modulators of the DDR pathways are being pursued. There is increasing understanding of the potential interplay between DDR pathways and immune-associated signaling. As such, the question of how we treat TNBC regarding novel immuno-molecular strategies is continually evolving. In this review, we explore the current and upcoming treatment options of TNBC in the context of DNA repair mechanisms and immune-based therapies, with a focus on implications of recent genomic analyses and clinical trial findings.

Society for Immunotherapy of Cancer (SITC) clinical practice guideline on immunotherapy for the treatment of breast cancer

Breast cancer has historically been a disease for which immunotherapy was largely unavailable. Recently, the use of immune checkpoint inhibitors (ICIs) in combination with chemotherapy for the treatment of advanced/metastatic triple-negative breast cancer (TNBC) has demonstrated efficacy, including longer progression-free survival and increased overall survival in subsets of patients. Based on clinical benefit in randomized trials, ICIs in combination with chemotherapy for the treatment of some patients with advanced/metastatic TNBC have been approved by the United States (US) Food and Drug Administration (FDA), expanding options for patients. Ongoing questions remain, however, about the optimal chemotherapy backbone for immunotherapy, appropriate biomarker-based selection of patients for treatment, the optimal strategy for immunotherapy treatment in earlier stage disease, and potential use in histological subtypes other than TNBC. To provide guidance to the oncology community on these and other important concerns, the Society for Immunotherapy of Cancer (SITC) convened a multidisciplinary panel of experts to develop a clinical practice guideline (CPG). The expert panel drew upon the published literature as well as their clinical experience to develop recommendations for healthcare professionals on these important aspects of immunotherapeutic treatment for breast cancer, including diagnostic testing, treatment planning, immune-related adverse events (irAEs), and patient quality of life (QOL) considerations. The evidence-based and consensus-based recommendations in this CPG are intended to give guidance to cancer care providers treating patients with breast cancer.

Checkpoint inhibitor therapy for metastatic triple-negative breast cancer

Immunotherapy has become a mainstay of cancer treatment in many malignancies, though its application in breast cancer remains limited. Of the breast cancer subtypes, triple-negative breast cancers (TNBCs) are characterized by immune activation and infiltration and more commonly express biomarkers associated with response to immunotherapy. Checkpoint inhibitor therapy has shown promising activity in metastatic TNBC. In 2019, the US FDA granted accelerated approval of atezolizumab, a programmed death-ligand 1 (PD-L1) inhibitor, in combination with nab-paclitaxel for unresectable locally advanced or metastatic PD-L1-positive TNBC, based on the results of the phase III IMpassion130 trial. In 2020, the FDA also granted accelerated approval of pembrolizumab, a PD-1 inhibitor, in combination with chemotherapy for locally recurrent unresectable and metastatic PD-L1-positive TNBC, based on results of the phase III KEYNOTE-355 trial. Additional combination strategies are being explored in the treatment of metastatic TNBC, with the goal of augmenting antitumor activity. In this review, the clinical development of checkpoint inhibitors in the treatment of metastatic TNBC will be discussed, including clinical outcomes with monotherapy and combination therapy regimens, biomarkers that may predict for benefit, and future directions in the field.

An Overview of PARP Inhibitors for the Treatment of Breast Cancer

Loss-of-function mutations in BRCA1 and BRCA2 are detected in at least 5% of unselected patients with breast cancer (BC). These BC susceptibility genes encode proteins critical for DNA homologous recombination repair (HRR). This review provides an update on oral poly(ADP-ribose) polymerase (PARP) inhibitors for the treatment of BC. Olaparib and talazoparib are PARP inhibitors approved as monotherapies for deleterious/suspected deleterious germline BRCA-mutated, HER2-negative BC. Olaparib is approved in the USA for metastatic BC and in Europe for locally advanced/metastatic BC. Talazoparib is approved for locally advanced/metastatic BC in the USA and Europe. In phase 3 trials, olaparib and talazoparib monotherapies demonstrated significant progression-free survival benefits compared with chemotherapy. Common toxicities were effectively managed by supportive treatment and dose interruptions/reductions. Veliparib combined with platinum-based chemotherapy has also shown promise for locally advanced/metastatic BC in a phase 3 trial. Differences in efficacy and safety across PARP inhibitors (olaparib, talazoparib, veliparib, niraparib, rucaparib) may relate to differences in potency of PARP trapping on DNA and cytotoxic specificity. PARP inhibitors are being investigated in early BC, in novel combinations, and in patients without germline BRCA mutations, including those with somatic BRCA mutations and other HRR gene mutations. Ongoing phase 2/3 studies include PARP inhibitors combined with immune checkpoint inhibitors for the treatment of triple-negative BC. Wider access to testing for BRCA and other mutations, and to genetic counseling, are required to identify patients who could benefit from PARP inhibitor therapy. The advent of PARP inhibitors has potential benefits for BC treatment beyond the locally advanced/metastatic setting.

Emerging Therapeutic Drugs in Metastatic Triple-Negative Breast Cancer

Metastatic triple-negative breast cancer (TNBC) is a heterogeneous disease with a poor prognosis and currently with few treatment options. Treatment of these patients is highly based on systemic chemotherapy. Some targeted drugs were recently approved for these patients: two poly(ADP-ribose) polymerase inhibitors in patients with germline BRCA1/2 mutations (olaparib and talazoparib), immune checkpoint inhibitors in association with chemotherapy if programmed death-ligand 1 positive (atezolizumab plus nabpaclitaxel and pembrolizumab plus chemotherapy [nabpaclitaxel, paclitaxel, and carboplatin plus gemcitabine]), and an antibody-drug conjugate sacituzumab-govitecan in heavily pretreated patients (at least 2 previous lines for the metastatic setting). Combinations using these and other targeted treatment options are under investigation in early and late clinical trials, and we will probably have some practice-changing results in the new future. Other targeted drugs explored in phase II and phase III clinical trials are PI3K/AKT pathway inhibitors and androgen receptor antagonists in patients with alterations in these signaling pathways. The definition of molecular subtypes has been essential for the development of these treatment strategies. Soon, the treatment of metastatic TNBC could be based on personalized medicine using molecular testing for targeted drugs instead of only systemic chemotherapy. The authors present a review of emerging treatment options in metastatic TNBC, focusing on targeted drugs, including the recent data published in 2020.

Combining poly(ADP-ribose) polymerase inhibitors and immune checkpoint inhibitors in breast cancer: rationale and preliminary clinical results

PURPOSE OF REVIEW

Recently, both immune checkpoint inhibitors and poly(ADP-ribose) polymerase inhibitors have demonstrated clinical benefit in some subsets of HER2-negative breast cancer patients. A biological rationale exists supporting a potential synergism between these compounds, which may further increase their antitumor activity in the clinic.

RECENT FINDINGS

PARP inhibitors were shown to activate type I interferon pathway, thus eliciting local and general immune response, while inducing programmed cell death-ligand 1 (PD-L1) up-regulation. In addition, the DNA damages created by PARP inhibition may increase tumor mutational burden and neo-antigens, thereby favoring efficacy of immune checkpoint inhibitors. Accordingly, clinical trials combining PARP inhibitors and agents targeting the PD-1/PD-L1 axis have been initiated in breast cancer in both advanced and early stages, enrolling patients with germline BRCA1/2 mutation, homologous recombination deficiency and/or with triple negative phenotype. Preliminary safety and efficacy results are encouraging, but it is still unclear whether the combination adds benefit compared with each therapeutic administered as single agent.

SUMMARY

Although a strong rationale exists to support the combination of PARP inhibitors with immune checkpoint inhibitors, future clinical trials will have to demonstrate whether it improves outcome and to identify which patients are the most likely to benefit from.

An overview of immune checkpoint inhibitors in breast cancer

Although breast cancer is not traditionally considered an immunogenic type of tumor, the combination of immunotherapy and chemotherapy has recently emerged as a novel treatment option in triple-negative subtype in the advanced setting and other similar combinations of immune checkpoint inhibitors with chemotherapy are expected to become part of the neoadjuvant management in the near future. In addition, encouraging results have been observed with the combination of immune checkpoint blockade with diverse biological agents, including anti-HER2 agents, CDK 4/6 inhibitors, PARP-inhibitors. The present review summarized the available evidence coming from clinical trials on the role of immune checkpoint inhibitors in the management of breast cancer, both in advanced and early setting.

Combined PARP Inhibition and Immune Checkpoint Therapy in Solid Tumors

Genomic instability is a hallmark of cancer related to DNA damage response (DDR) deficiencies, offering vulnerabilities for targeted treatment. Poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi) interfere with the efficient repair of DNA damage, particularly in tumors with existing defects in DNA repair, and induce synthetic lethality. PARPi are active across a range of tumor types harboring BRCA mutations and also BRCA-negative cancers, such as ovarian, breast or prostate cancers with homologous recombination deficiencies (HRD). Depending on immune contexture, immune checkpoint inhibitors (ICIs), such as anti-PD1/PD-L1 and anti-CTLA-4, elicit potent antitumor effects and have been approved in various cancers types. Although major breakthroughs have been performed with either PARPi or ICIs alone in multiple cancers, primary or acquired resistance often leads to tumor escape. PARPi-mediated unrepaired DNA damages modulate the tumor immune microenvironment by a range of molecular and cellular mechanisms, such as increasing genomic instability, immune pathway activation, and PD-L1 expression on cancer cells, which might promote responsiveness to ICIs. In this context, PARPi and ICIs represent a rational combination. In this review, we summarize the basic and translational biology supporting the combined strategy. We also detail preclinical results and early data of ongoing clinical trials indicating the synergistic effect of PARPi and ICIs. Moreover, we discuss the limitations and the future direction of the combination.

PARP Inhibitors in the Treatment of Early Breast Cancer: The Step Beyond?

Exquisitely exploiting defects in homologous recombination process, poly(ADP-ribose) polymerase (PARP) inhibitors have recently emerged as a promising class of therapeutics in human epidermal growth factor receptor 2 (HER2)-negative advanced breast cancer with germline breast cancer 1 (BRCA1) or breast cancer 2 (BRCA2) mutations (gBRCA1/2m). In this setting, PARP inhibitors, either as single agent or in combination with platinum-based chemotherapy, significantly increased progression-free survival, as compared to conventional chemotherapy. Accordingly, further therapeutic advances are expected at an earlier stage of the disease. In the neoadjuvant setting, veliparib failed to increase the pathological complete response rate when added to a carboplatin-based regimen, in unselected triple-negative breast cancer patients. Similarly, when administered before anthracycline-cyclophosphamide, the neoadjuvant olaparib-paclitaxel combination was not superior to carboplatin–paclitaxel, in patients with HER2-negative breast cancer and BRCA1/2 mutation, or homologous recombination defect. Yet, neoadjuvant talazoparib, administered as a single-agent in patients with HER2-negative breast cancer and germline BRCA1/2 mutation, achieved an impressive pathological complete response rate of nearly 50%. In the adjuvant setting, the results from the OlympiA phase III study, evaluating adjuvant olaparib in HER2-negative early breast cancer and germline BRCA1/2 mutations, are eagerly awaited. Ongoing trials should clarify whether PARP inhibitors might improve outcome when administered in the adjuvant or neoadjuvant setting in early breast cancer patients with BRCA1/2 mutation or homologous recombination defect.

Biologic therapy for advanced breast cancer: recent advances and future directions

INTRODUCTION

Advanced breast cancer (ABC) is a leading cause of mortality, morbidity, and disability in women worldwide. For decades, treatment of ABC has relied on chemotherapy and endocrine treatments (ET), until HER2 was recognized as a 'druggable' target in the 1990s. Thereafter, various anti-HER2 drugs have been approved for the HER2-positive subtype, but only in the last few years, biologic agents targeting different pathways have entered the therapeutic arsenal of luminal and triple-negative cancers.

AREAS COVERED

The purpose of the present review is to recapitulate the most promising novel biologic agents being developed for the treatment of ABC. New drugs for all breast cancer subtypes are discussed, as well as some potential future directions in ABC treatment.

EXPERT OPINION

Several biologic drugs have been recently approved, revolutionizing ABC treatment algorithms: key examples are CDK4/6-inhibitors and the PI3K-inhibitor alpelisib for endocrine-positive ABC; atezolizumab for triple-negative cancers; two PARP-inhibitors for HER2-negative germinal BRCA-mutated cancers. Additionally, multiple drugs are demonstrating activity in late-phase clinical trials for all subtypes. While some of these represent pharmacological evolutions of previously approved drugs, some others might pave the way for new paradigms in ABC, challenging both its classification and current treatment algorithms.

Olaparib for advanced breast cancer

Olaparib, an oral PARP-inhibitor, has shown clinical benefit for HER2-negative advanced breast cancer patients carrying a germinal BRCA1/2 mutation. In a randomized Phase III trial, olaparib significantly prolonged progression-free survival as compared with chemotherapy of physician choice. Moreover, in the same trial, a prespecified subgroup analysis reported an overall survival benefit for patients not previously pretreated with chemotherapy for metastatic disease. This review focuses on available preclinical, pharmacokinetic and pharmacodynamic data regarding olaparib and clinical evidence of its antitumor efficacy (both as monotherapy and in combination) and tolerability in breast cancer patients. Open questions, such as use of appropriate biomarkers for patient selection and combination/sequencing with other anticancer drugs, are also addressed.

Role of Immunotherapy in Triple-Negative Breast Cancer

Immune checkpoint inhibitors (ICIs) have led to durable clinical remissions in many metastatic cancers. However, the single-agent efficacy of ICIs in breast cancer is low, including in triple-negative breast cancer (TNBC), which has several key characteristics that enhance ICI responses. Strategies to improve anticancer immune responses in TNBC are urgently needed to extend survival for patients with metastatic disease. This review presents ICI monotherapy response rates and discusses combination strategies with chemotherapy, targeted therapies, and novel immunotherapies. It concludes with a summary of immunotherapy biomarkers in TNBC and a call to action for future directions of research critical to advancing the efficacy of immunotherapy for patients with TNBC.

Combination Strategies of Checkpoint Immunotherapy in Metastatic Breast Cancer

Checkpoint immunotherapy is emerging as a new therapeutic approach for metastatic breast cancer. Monotherapy of immunoagents against PD1/PD-L1 or CTLA-4 has shown little efficacy in these patients. Recently, to determine the optimal use of immunotherapy, there has been a rapid expansion in the number of clinical trials developing immunotherapy combinations. These combination therapeutic approaches can enhance various aspects of cancer immunity, such as tumor antigenicity or intratumor T cell infiltration, which provides a theoretical basis for combining them with checkpoint immunotherapy to achieve synergistic effects. Here, we review the available data and ongoing efforts to establish the safety and efficacy of immunoagents in combination with chemotherapy, radiotherapy, HER2-targeted therapy, CDK4/6 inhibitors, PARP inhibitors, and another checkpoint immunoagents.

Immune Checkpoint Inhibitors in Epithelial Ovarian Cancer: An Overview on Efficacy and Future Perspectives

Epithelial ovarian cancer (EOC) is the leading cause of death among gynecological cancers. Despite improvements in medical treatments, the prognosis for EOC remains poor, and there is an urgent need for new therapeutic strategies. Immune checkpoint inhibitors (CPIs) have dramatically improved survival of several cancers and are under evaluation in OC. Unfortunately, CPIs have shown globally unsatisfactory results. The aim of this manuscript is to critically review the results from early-phase trials with CPIs in terms of safety and activity, discuss the possible reasons for disappointing results and the new therapeutic approaches to improve patient outcomes.