Activation of STING-Dependent Innate Immune Signaling By S-Phase-Specific DNA Damage in Breast Cancer

Biochemical Aspects of PD-L1 Regulation in Cancer Immunotherapy

PD-L1, frequently expressed in human cancers, engages with PD-1 on immune cells and contributes to cancer immune evasion. As such, antibodies blocking the PD-1/PD-L1 interaction reactivate cytotoxic T cells to eradicate cancer cells. However, a majority of cancer patients fail to respond to PD-1/PD-L1 blockade with unclear underlying mechanism(s). Recent studies revealed that PD-L1 expression levels on tumor cells might affect the clinical response to anti-PD-1/PD-L1 therapies. Hence, understanding molecular mechanisms for controlling PD-L1 expression will be important to improve the clinical response rate and efficacy of PD-1/PD-L1 blockade. In this review, we primarily focus on summarizing PD-L1 regulation and its potential roles in regulating antitumor immune response, with purpose to optimize anti-PD-1/PD-L1 therapies, benefiting a wider cancer patient population.

DNA repair defects and implications for immunotherapy

A complex DNA repair machinery has evolved to protect genomic integrity in the face of a myriad of DNA damage sources. When DNA repair fails, this damage can lead to carcinogenesis and tumor genomic instability. Indeed, many heritable cancer predisposition syndromes are attributable to germline defects in DNA repair pathways. On the other hand, these defects may also portend particular vulnerabilities of the cancer and may be exploited therapeutically. Most recently this has been demonstrated in the case of mismatch repair-deficient cancers, in which the immune checkpoint inhibitors have been demonstrated to be highly active. This observation has paved the way for further research investigating other sources of genomic instability that may serve as biomarkers to select patients for immunotherapy.

Functions of the CXC ligand family in the pancreatic tumor microenvironment

Therapeutic resistance is the major contributor to the poor prognosis of and low survival from pancreatic cancer (PC). Cancer progression is a complex process reliant on interactions between the tumor and the tumor microenvironment (TME). Members of the CXCL family of chemokines are present in the pancreatic TME and seem to play a vital role in regulating PC progression. As pancreatic tumors interact with the TME and with PC stem cells (CSCs), determining the roles of specific members of the CXCL family is vital to the development of improved therapies. This review highlights the roles of selected CXCLs in the interactions between pancreatic tumor and its stroma, and in CSC phenotypes, which can be used to identify potential treatment targets.

Recent advances in small molecule based cancer immunotherapy

Immunotherapy has been increasingly utilized for the treatment of cancer. Currently available cancer immunotherapies mainly involve the use of antibodies, which have advantages in terms of pharmacodynamics such as efficacy and specificity, however, they exhibit disadvantages in regard to the pharmacokinetics including but not limited to poor tissue and tumor penetration, very long half-life, and the lack of oral bioavailability. Also they are immunogenic and may cause undesired side effects. In addition, they are difficult and expensive to produce. In contrast to therapeutic antibodies, small molecule immuno-oncology agents generally have favorable pharmacokinetics, for example, better oral bioavailability, higher tissue and tumor penetration, reasonable half-lives etc. Furthermore, some small molecules are highly selective and efficacious with benign toxicity profiles. Therefore, small molecule immuno-oncology agents have the potential to overcome the drawbacks of therapeutic antibodies, and they can complement existing therapeutic antibodies and may also be used in combination with antibodies to achieve synergistic effects. In this article, we summarize the current advances in the field of small molecule approaches in tumor immunology which include the small molecules in clinical trials and preclinical studies, and the reported crystal structures of small molecules and their target proteins as well as the binding interactions between small molecules and the targets. The tumorigenesis mechanism of different targets (the programmed cell death 1/programmed cell death ligand 1(PD1/PD-L1), retinoic acid-related orphan receptor-gamma t (RORγt), Chemokine receptor, Stimulator of Interferon Genes (Sting), Indoleamine 2,3-dioxygenase (IDO), toll-like receptors (TLR) etc.) are also elucidated.

Personalising Treatment for High-Grade Serous Ovarian Carcinoma

Ovarian cancer is a heterogeneous group of cancers that differ by cell of origin and genomic features. High-grade serous ovarian cancer (HGSOC) is the commonest histiotype and is characterized by extreme genomic complexity and dysregulation of DNA damage repair pathways, particularly homologous recombination deficiency. New insights from molecular profiling into homologous recombination deficiency now offers the credible possibility of personalizing treatment choices for women with HGSOC using poly(ADP-ribose) polymerase inhibitor (PARP) therapy. Although the presence of tumour infiltrating lymphocytes (TILs) in the microenvironment is associated with improved survival in HGSOC, the role of anti-angiogenic and immune checkpoint inhibitor therapy remains unclear. PARP inhibition combined with immunotherapy is an exciting combination strategy for future therapeutic development for women with advanced HGSOC.

Is inflammatory micronucleation the key to a successful anti-mitotic cancer drug?

Paclitaxel is a successful anti-cancer drug that kills cancer cells in two-dimensional culture through perturbation of mitosis, but whether it causes tumour regression by anti-mitotic actions is controversial. Drug candidates that specifically target mitosis, including inhibitors of kinesin-5, AurkA, AurkB and Plk1, disappointed in the clinic. Current explanations for this discrepancy include pharmacokinetic differences and hypothetical interphase actions of paclitaxel. Here, we discuss post-mitotic micronucleation as a special activity of taxanes that might explain their higher activity in solid tumours. We review data showing that cells which exit mitosis in paclitaxel are highly micronucleated and suffer post-mitotic DNA damage, and that these effects are much stronger for paclitaxel than kinesin-5 inhibitors. We propose that post-mitotic micronucleation promotes inflammatory signalling via cGAS–STING and other pathways. In tumours, this signalling may recruit cytotoxic leucocytes, damage blood vessels and prime T-cell responses, leading to whole-tumour regression. We discuss experiments that are needed to test the micronucleation hypothesis, and its implications for novel anti-mitotic targets and enhancement of taxane-based therapies.

Alterations in DNA Damage Response and Repair Genes as Potential Marker of Clinical Benefit From PD-1/PD-L1 Blockade in Advanced Urothelial Cancers

Purpose Alterations in DNA damage response and repair (DDR) genes are associated with increased mutation load and improved clinical outcomes in platinum-treated metastatic urothelial carcinoma. We examined the relationship between DDR alterations and response to PD-1/PD-L1 blockade. Methods Detailed demographic, treatment response, and long-term outcome data were collected on patients with metastatic urothelial carcinoma treated with atezolizumab or nivolumab who had targeted exon sequencing performed on pre-immunotherapy tumor specimens. Presence of DDR alterations was correlated with best objective response per Response Evaluation Criteria in Solid Tumors (RECIST) and progression-free and overall survival. Results Sixty patients with urothelial cancer enrolled in prospective trials of anti-PD-1/PD-L1 antibodies met inclusion criteria. Any DDR and known or likely deleterious DDR mutations were identified in 28 (47%) and 15 (25%) patients, respectively. The presence of any DDR alteration was associated with a higher response rate (67.9% v 18.8%; P < .001). A higher response rate was observed in patients whose tumors harbored known or likely deleterious DDR alterations (80%) compared with DDR alterations of unknown significance (54%) and in those whose tumors were wild-type for DDR genes (19%; P < .001). The correlation remained significant in multivariable analysis that included presence of visceral metastases. DDR alterations also were associated with longer progression-free and overall survival. Conclusion DDR alterations are independently associated with response to PD-1/PD-L1 blockade in patients with metastatic urothelial carcinoma. These observations warrant additional study, including prospective validation and exploration of the interaction between tumor DDR alteration and other tumor/host biomarkers of immunotherapy response.

STING-Dependent Interferon-λ1 Induction in HT29 Cells, a Human Colorectal Cancer Cell Line, After Gamma-Radiation

PURPOSE

To investigate the induction of type III interferons (IFNs) in human cancer cells by gamma-rays.

METHODS AND MATERIALS

Type III IFN expression in human cancer cell lines after gamma-ray irradiation in vitro was assessed by reverse transcription-quantitative polymerase chain reaction and enzyme-linked immunosorbent assay. Signaling pathways mediating type III IFN induction were examined by a variety of means, including immunoblotting, flow cytometry, confocal imaging, and reverse transcription-quantitative polymerase chain reaction. Key mediators in these pathways were further explored and validated using gene CRISPR knockout or short hairpin RNA knockdown.

RESULTS

Exposure to gamma-rays directly induced type III IFNs (mainly IFNL1) in human cancer cell lines in dose- and time-dependent fashions. The induction of IFNL1 was primarily mediated by the cytosolic DNA sensors-STING-TBK1-IRF1 signaling axis, with a lesser contribution from the nuclear factor kappa b signaling in HT29 cells. In addition, type III IFN signaling through its receptors serves as a positive feedback loop, further enhancing IFN expression via up-regulation of the kinases in the STING-TBK1 signaling axis.

CONCLUSIONS

Our results suggest that IFNL1 can be up-regulated in human cancer cell lines after gamma-ray treatment. In HT29 cells this induction occurs via the STING pathway, adding another layer of complexity to the understanding of radiation-induced antitumor immunity, and may provide novel insights into IFN-based cancer treatment.

PARP inhibitors in breast cancer: Bringing synthetic lethality to the bedside

Individuals with breast and ovarian cancer susceptibility gene 1 (BRCA1) or BRCA2 germline mutations have a significantly increased lifetime risk for breast and ovarian cancers. BRCA-mutant cancer cells have abnormal homologous recombination (HR) repair of DNA. In these tumors, the base excision repair (BER) pathway is important for cell survival. The poly(adenosine diphosphate-ribose) polymerase (PARP) enzymes play a key role in BER, and PARP inhibitors are effective in causing cell death in BRCA-mutant cells while sparing normal cells-a concept called synthetic lethality. PARP inhibitors are the first cancer therapeutics designed to exploit synthetic lethality. Recent clinical trials in BRCA-mutant, metastatic breast cancer demonstrated improved outcomes with single-agent PARP inhibitors (olaparib and talazoparib) over chemotherapy. However, resistance to PARP inhibitors remains a challenge. Primarily due to myelosuppression, the combination of PARP inhibitors with chemotherapy has been difficult. Novel combinations with chemotherapy, immunotherapy, and other targeted therapies are being pursued. In this review, the authors discuss current knowledge of PARP inhibitors in BRCA-mutant breast cancer and potential future directions for these agents. Cancer 2018;124:2498-506. © 2018 American Cancer Society.

From checkpoint to checkpoint: DNA damage ATR/Chk1 checkpoint signalling elicits PD-L1 immune checkpoint activation

Multiple clinical studies have revealed a link between genomic instability and response to anti-PD-1/PD-L1 therapy in cancer management. A recent study has revealed an important role for the ATR/Chk1 DNA damage checkpoint in regulating PD-L1 expression, raising important clinical and translational questions for therapy selection and study design.

The Impact of Radiation on the Tumor Microenvironment: Effect of Dose and Fractionation Schedules

In addition to inducing lethal DNA damage in tumor and stromal cells, radiation can alter the interactions of tumor cells with their microenvironment. Recent technological advances in planning and delivery of external beam radiotherapy have allowed delivery of larger doses per fraction (hypofractionation) while minimizing dose to normal tissues with higher precision. The effects of radiation on the tumor microenvironment vary with dose and fractionation schedule. In this review, we summarize the effects of conventional and hypofractionated radiation regimens on the immune system and tumor stroma. We discuss how these interactions may provide therapeutic benefit in combination with targeted therapies. Understanding the differential effects of radiation dose and fractionation can have implications for choice of combination therapies.

Expression of STING and PD-L1 in colorectal cancer and their correlation with clinical prognosis

OBJECTIVE

To investigate the expression of PD-L1 and STING in colorectal cancer and analyze their correlation with disease prognosis.

METHODS

The colorectal tissue chip was taken as the research object. Immunohistochemical methods were used to detect expression of PD-L1 and STING of 87 cases of colorectal cancer tissue and corresponding adjacent tissue in the tissue chip. Chi-square test or Fisher's test were used to analyze the relationship of expression of PD-L1 and STING and with the clinical pathological features. Kaplan-Meier curves were used to analyze the relationship between expression of PD-L1 and STING and the prognosis of colorectal cancer.

RESULTS

The positive expression rate of PD-L1 in 87 cases of colorectal cancer tissue and corresponding adjacent tissue was 79.3% and 42.5% respectively, and the difference was significant (P < 0.05). The positive expression rate of PD-L1 in colorectal carcinoma tissues was not related with age, gender, tumor size, differentiation, TNM clinical stage, or depth of invasion (P > 0.05). The positive expression rate of STING in 87 cases colorectal cancer tissues and corresponding adjacent tissue was 9.2% and 40.2% respectively and the difference was significant (P < 0.05). The positive expression rate of STING in colorectal carcinoma tissue was correlated with gender, age, and differentiation (P < 0.05), but not associated with tumor size, depth of invasion, and TNM clinical stage (P > 0.05). The expression rate of PD-L1 was related to the prognosis of colorectal cancer (P=0.018). The expression rate of STING was not correlated with prognosis of colorectal cancer (P=0.784). In four independent groups (PD-L1⁺STING⁺ group, PD-L1^-STING^- group, PD-L1^-STING⁺ group, PD-L1⁺STING^- group), there were significant differences in 5-year survival (P=0.047 < 0.05), the 5-year survival rate of the PD-L1^-STING⁺ group was significantly higher than that of the other three groups. In the STING positive group, the PD-L1⁺STING⁺ group had a worse prognosis than the PD-L1^-STING⁺ group. In the STING negative group, the PD-L1^-STING^-group had a higher survival rate than the PD-L1⁺STING^- group.

CONCLUSION

PD-L1 might promote the occurrence of disease and STING might play an important role in anti-tumor immunity. PD-L1 was related with the prognosis of patients with colorectal cancer but the expression of STING was not obviously associated with the prognosis. Survival rates of patients with colorectal cancer were higher in patients with PD-L1 negative expression or STING positive expression. When PD-L1 expression was negative and STING expression was positive, the 5-year survival rate of patients with colorectal cancer was highest.

Combining DNA damaging therapeutics with immunotherapy: more haste, less speed

The idea that chemotherapy can be used in combination with immunotherapy may seem somewhat counterproductive, as it can theoretically eliminate the immune cells needed for antitumour immunity. However, much preclinical work has now demonstrated that in addition to direct cytotoxic effects on cancer cells, a proportion of DNA damaging agents may actually promote immunogenic cell death, alter the inflammatory milieu of the tumour microenvironment and/or stimulate neoantigen production, thereby activating an antitumour immune response. Some notable combinations have now moved forward into the clinic, showing promise in phase I-III trials, whereas others have proven toxic, and challenging to deliver. In this review, we discuss the emerging data of how DNA damaging agents can enhance the immunogenic properties of malignant cells, focussing especially on immunogenic cell death, and the expansion of neoantigen repertoires. We discuss how best to strategically combine DNA damaging therapeutics with immunotherapy, and the challenges of successfully delivering these combination regimens to patients. With an overwhelming number of chemotherapy/immunotherapy combination trials in process, clear hypothesis-driven trials are needed to refine the choice of combinations, and determine the timing and sequencing of agents in order to stimulate antitumour immunological memory and improve maintained durable response rates, with minimal toxicity.

Towards predictive biomarkers for immunotherapy response in breast cancer patients

Immunotherapy using anti-PD(L)1 has revolutionized treatment for various tumor types. Early data have shown durable responses in a small subgroup of breast cancer patients. So far, the response rates appear higher for breast tumors that are triple negative, PDL1-positive and/or harbor high levels of immune cells. Both comprehensive analyses of the breast tumor microenvironment and exploiting research on biomarkers in other cancer types, such as melanoma and lung cancer, may contribute to the discovery of accurate biomarkers to select breast cancer patients for immunotherapy. Here we summarize key features of the breast tumor microenvironment as well as putative predictive biomarkers established in other tumor types. Insights from both fields can guide future studies to enable personalized breast cancer immunotherapy.

Activation of MAPK signalling results in resistance to saracatinib (AZD0530) in ovarian cancer

SRC tyrosine kinase is frequently overexpressed and activated in late-stage, poor prognosis ovarian tumours, and preclinical studies have supported the use of targeted SRC inhibitors in the treatment of this disease. The SAPPROC trial investigated the addition of the SRC inhibitor saracatinib (AZD0530) to weekly paclitaxel for the treatment of platinum resistant ovarian cancer; however, this drug combination did not provide any benefit to progression free survival (PFS) of women with platinum resistant disease. In this study we aimed to identify mechanisms of resistance to SRC inhibitors in ovarian cancer cells. Using two complementary strategies; a targeted tumour suppressor gene siRNA screen, and a phospho-receptor tyrosine kinase array, we demonstrate that activation of MAPK signalling, via a reduction in NF1 (neurofibromin) expression or overexpression of HER2 and the insulin receptor, can drive resistance to AZD0530. Knockdown of NF1 in two ovarian cancer cell lines resulted in resistance to AZD0530, and was accompanied with activated MEK and ERK signalling. We also show that silencing of HER2 and the insulin receptor can partially resensitize AZD0530 resistant cells, which was associated with decreased phosphorylation of MEK and ERK. Furthermore, we demonstrate a synergistic effect of combining SRC and MEK inhibitors in both AZD0530 sensitive and resistant cells, and that MEK inhibition is sufficient to completely resensitize AZD0530 resistant cells. This work provides a preclinical rationale for the combination of SRC and MEK inhibitors in the treatment of ovarian cancer, and also highlights the need for biomarker driven patient selection for clinical trials.

TPF induction chemotherapy increases PD-L1 expression in tumour cells and immune cells in head and neck squamous cell carcinoma

Background

Antiprogrammed cell death-1/programmed cell death-ligand 1 (PD-1/PD-L1) therapies have demonstrated promising activity in advanced head and neck squamous cell carcinoma (HNSCC), with overall response rates of approximately 20% in unselected populations and survival benefit. Whether induction docetaxel, platinum and fluorouracil (TPF) modifies PD-L1 expression or tumour immune infiltrates is unknown.

Patients and methods

Patients with locally advanced HNSCC treated at Gustave Roussy (Villejuif, France) between 2006 and 2013 by induction TPF followed by surgery were retrospectively considered. Patients with paired samples (pre-TPF and post-TPF) were kept for further analysis. PD-L1 expression was quantified by immunohistochemistry according to a validated protocol. The objective of the study was to compare PD-L1 expression on tumour cells (TC) and immune cells (IC) (positivity threshold of ≥5%) before and after TPF. CD8+ and Foxp3+ lymphocytes densities before and after TPF were also quantified.

Results

Out of 313 patients receiving induction TPF, 86 underwent surgery; paired samples were available for 21 of them. Baseline PD-L1 expression was ≥5% in two and five samples for TC and IC, respectively. A significant increase of PD-L1 expression was observed after TPF, with 15 samples (71%) presenting a positive staining in IC after induction chemotherapy (P=0.003; Wilcoxon rank-sum test) and eight samples (38%) in TC (P=0.005; Wilcoxon rank-sum test). Tumour-infiltrating CD8+ mean densities also significantly increased post-TPF (P=0.01). There was no significant difference in Foxp3+ expression, CD8/Foxp3 ratio or correlation with outcome.

Conclusion

TPF induction chemotherapy in advanced HNSCC increases PD-L1 positivity on tumour-infiltrating ICs, as well as CD8+ lymphocytes density. These results warrant independent validation on larger datasets and might help therapeutic strategy in advanced HNSCC.

STING, DCs and the link between innate and adaptive tumor immunity

Cancer and the immune system are intimately related. Much of the bulk of tumors is comprised of stromal leukocytes with immune functions, which serve to both promote and inhibit tumor growth, invasion and metastasis. The T lymphocytes of the adaptive immune system are essential for tumor immunity, and these T cells are generated by cross-priming against tumor associated antigens. Dendritic cells (DCs) are essential in this process, serving as the cellular link between innate and adaptive immunity. As a prerequisite for priming of adaptive immune responses, DCs must take up tumor antigens, process them and present them in the context of the major histocompatibility complex (MHC). DCs also serve as sensors of innate activation signals from cancer that are necessary for their activation and effective priming of cancer specific T cells. Here we discuss the role of DCs in the sensing of cancer and in priming the adaptive response against tumors. Furthermore, we present the essential role of the Stimulator of Interferon Genes (STING) signaling pathway in producing type I interferons (IFNs) that are essential in this process.

Directing the use of DDR kinase inhibitors in cancer treatment

INTRODUCTION

Defects in the DNA damage response (DDR) drive the development of cancer by fostering DNA mutation but also provide cancer-specific vulnerabilities that can be exploited therapeutically. The recent approval of three different PARP inhibitors for the treatment of ovarian cancer provides the impetus for further developing targeted inhibitors of many of the kinases involved in the DDR, including inhibitors of ATR, ATM, CHEK1, CHEK2, DNAPK and WEE1. Areas covered: We summarise the current stage of development of these novel DDR kinase inhibitors, and describe which predictive biomarkers might be exploited to direct their clinical use. Expert opinion: Novel DDR inhibitors present promising candidates in cancer treatment and have the potential to elicit synthetic lethal effects. In order to fully exploit their potential and maximize their utility, identifying highly penetrant predictive biomarkers of single agent and combinatorial DDR inhibitor sensitivity are critical. Identifying the optimal drug combination regimens that could used with DDR inhibitors is also a key objective.

Surviving Ovarian Cancer: An Affair between Defective DNA Repair and RB1

Detailed clinical and molecular evaluation of large cohorts of exceptional survivors provides an unprecedented opportunity to identify mechanisms underlying long-term survival that can drive future therapeutic approaches and biomarker development. Exceptional survivors of high-grade serous ovarian cancer demonstrate concurrent disruption of homologous recombination DNA repair and retinoblastoma protein. Clin Cancer Res; 24(3); 508-10. ©2017 AACRSee related article by Garsed et al., p. 569.

STING Signaling in Cancer Cells: Important or Not?

Stimulator of interferon genes (STING) is an adaptor protein that plays an important role in the activation of type I interferons in response to cytosolic nucleic acid ligands. Recent evidence indicates involvement of the STING pathway in the induction of antitumor immune response. Therefore, STING agonists are now being extensively developed as a new class of cancer therapeutics. However, little is known about the consequences of activated STING-mediated signaling in cancer cells on the efficacy of the antitumor treatment. It has been shown that activation of the STING-dependent pathway in cancer cells can result in tumor infiltration with immune cells and modulation of the anticancer immune response. Understanding the function of STING pathway in cancer cells might provide important insights into the development of effective therapeutic strategies. This review focuses on the role of STING pathway in cancer cells, the largely unknown topic that has recently emerged to be important in the context of STING-mediated antitumor responses.

DNA Damage and Repair Biomarkers of Immunotherapy Response

DNA-damaging agents are widely used in clinical oncology and exploit deficiencies in tumor DNA repair. Given the expanding role of immune checkpoint blockade as a therapeutic strategy, the interaction of tumor DNA damage with the immune system has recently come into focus, and it is now clear that the tumor DNA repair landscape has an important role in driving response to immune checkpoint blockade. Here, we summarize the mechanisms by which DNA damage and genomic instability have been found to shape the antitumor immune response and describe clinical efforts to use DNA repair biomarkers to guide use of immune-directed therapies.Significance: Only a subset of patients respond to immune checkpoint blockade, and reliable predictive biomarkers of response are needed to guide therapy decisions. DNA repair deficiency is common among tumors, and emerging experimental and clinical evidence suggests that features of genomic instability are associated with response to immune-directed therapies. Cancer Discov; 7(7); 675-93. ©2017 AACR.

Targeting ATR in cancer medicine

DNA damage occurs continually through various intrinsic and extrinsic mechanisms such as ultraviolet radiation, smoking, reactive oxygen species, and errors during replication. The cellular DNA damage response (DDR) comprises signaling networks that regulate a spectrum of processes, including cell cycle progression, which enable DNA repair to occur. Ataxia telangiectasia mutated (ATM) and ataxia telangiectasia mutated and rad3-related (ATR) kinase are 2 key regulators of the DDR cell cycle checkpoints. ATR plays an essential role in the repair of replication-associated DNA damage, while ATM is activated by DNA double-strand breaks. The investigation of cell cycle checkpoint signaling through ATR and ATM, as well as the relevant pathways involved in oncogenesis and cancer progression, has led to the discovery and development of potent and selective ATR inhibitors (ATRi). Preclinical data have demonstrated that ATR inhibition leads to tumor synthetic lethality in specific molecular contexts, and it exhibits synergy in combination with different antitumor therapies, including chemotherapy, radiotherapy, and poly(ADP-ribose) polymerase inhibitors. ATRi are now being assessed in early-phase clinical trials as single agents and in combinatorial regimens, including platinum and other chemotherapies, radiotherapy, poly(ADP-ribose) polymerase inhibitors, and immune checkpoint inhibitors. This article details the preclinical biology leading to the discovery and development of novel ATRi and discusses the rationale for monotherapy and combination antitumor strategies. We focus on the clinical development of ATRi and discuss the progress made in identifying putative predictive biomarkers of response for patient selection, such as p53, ATM, ARID1A, and other DDR aberrations.

PARP inhibitors: Synthetic lethality in the clinic

PARP inhibitors (PARPi), a cancer therapy targeting poly(ADP-ribose) polymerase, are the first clinically approved drugs designed to exploit synthetic lethality, a genetic concept proposed nearly a century ago. Tumors arising in patients who carry germline mutations in either BRCA1 or BRCA2 are sensitive to PARPi because they have a specific type of DNA repair defect. PARPi also show promising activity in more common cancers that share this repair defect. However, as with other targeted therapies, resistance to PARPi arises in advanced disease. In addition, determining the optimal use of PARPi within drug combination approaches has been challenging. Nevertheless, the preclinical discovery of PARPi synthetic lethality and the route to clinical approval provide interesting lessons for the development of other therapies. Here, we discuss current knowledge of PARP inhibitors and potential ways to maximize their clinical effectiveness.

cGAS-STING Activation in the Tumor Microenvironment and Its Role in Cancer Immunity

Stimulator of interferon (IFN) genes (STING) is a key mediator in the immune response to cytoplasmic DNA sensed by cyclic GMP-AMP (cGAMP) synthase (cGAS). After synthesis by cGAS, cGAMP acts as a second messenger activating STING in the cell harboring cytoplasmic DNA but also in adjacent cells through gap junction transfer. While the role of the cGAS-STING pathway in pathogen detection is now well established, its importance in cancer immunity has only recently started to emerge. Nonetheless, STING appears to be an essential component in the recruitment of immune cells to the tumor microenvironment, which is paramount to immune clearance of the tumor. This review presents an overview of the growing literature around the role of the cGAS-STING pathway in the tumor microenvironment, with a specific focus on the role that cancer cells may play in the direct activation of this pathway, and its amplification through cell-cell transfer of cGAMP.