Talazoparib Is a Potent Radiosensitizer in Small Cell Lung Cancer Cell Lines and Xenografts

CRISPR Screen of Druggable Targets in Small Cell Lung Cancer Identified ATM Inhibitor (AZD1390) as a Radiosensitizer

PURPOSE

Small cell lung cancer (SCLC) is an aggressive and lethal form of lung cancer and the overall 5-year survival (OS) for patients is a dismal 7%. Radiation therapy (RT) provides some benefit for selected patients with SCLC but could be improved with radiosensitizing agents. In this study, we identified novel radiosensitizers for SCLC by a CRISPR-Cas9 screen and evaluated the efficacy of ATM inhibitor AZD1390 as a radiosensitizer of SCLC.

METHODS AND MATERIALS

We transduced the SCLC cell line SBC5 with a custom CRISPR sgRNA library focused on druggable gene targets and treated cells with RT. Cells collected at multiple timepoints were subjected to next-generation sequencing. We determined radiosensitization both in vitro with cell lines assessed by short-term viability and clonogenic assays, and in vivo mouse models by tumor growth delay. Pharmacodynamic effects of AZD1390 were quantified by ATM-Ser1981 phosphorylation, and RT-induced DNA damage by comet assay.

RESULTS

Using a CRISPR dropout screen, we identified multiple radiosensitizing genes for SCLC at various timepoints with ATM as a top determinant gene for radiosensitivity. Validation by ATM knockout (KO) demonstrated increased radiosensitivity by short-term viability assay (dose modification factor [DMF]50 = 3.25-3.73 in SBC5 ATM-KO) and clonogenic assays (DMF37 1.25-1.65 in SBC5 ATM-KO). ATM inhibition by AZD1390 effectively abrogated ATM Ser1981 phosphorylation in SCLC cell lines and increased RT-induced DNA damage. AZD1390 synergistically increased the radiosensitivity of SCLC cell lines (cell viability assay: SBC5 DMF37 = 2.19, SHP77 DMF37 = 1.56, H446 DMF37 = 3.27, KP1 DMF37 = 1.65 at 100nM; clonogenic assay: SBC5 DMF37 = 4.23, H1048 DMF37 = 1.91), and in vivo murine syngeneic, KP1, and patient-derived xenograft (PDX) models, JHU-LX108 and JHU-LX33.

CONCLUSIONS

In this study, we demonstrated that genetically and pharmacologically (AZD1390) inhibiting ATM markedly enhanced RT against SCLC, providing a novel pharmacologically tractable radiosensitizing strategy for patients with SCLC.

Actionable Driver Events in Small Cell Lung Cancer

Small cell lung cancer (SCLC) stands out as the most aggressive form of lung cancer, characterized by an extremely high proliferation rate and a very poor prognosis, with a 5-year survival rate that falls below 7%. Approximately two-thirds of patients receive their diagnosis when the disease has already reached a metastatic or extensive stage, leaving chemotherapy as the remaining first-line treatment option. Other than the recent advances in immunotherapy, which have shown moderate results, SCLC patients cannot yet benefit from any approved targeted therapy, meaning that this cancer remains treated as a uniform entity, disregarding intra- or inter-tumoral heterogeneity. Continuous efforts and technological improvements have enabled the identification of new potential targets that could be used to implement novel therapeutic strategies. In this review, we provide an overview of the most recent approaches for SCLC treatment, providing an extensive compilation of the targeted therapies that are currently under clinical evaluation and inhibitor molecules with promising results in vitro and in vivo.

Applications of CRISPR screening to lung cancer treatment

Lung cancer is an extremely aggressive and highly prevalent disease worldwide, and it is one of the leading causes of cancer death. Deciphering intrinsic genetic mechanism, finding new targets, and overcoming drug resistance are the key to lung cancer treatment. High-throughput CRISPR screening has been extensively used to obtain the genes related to cancers including lung cancer. This review describes CRISPR/Cas9 or CRISPR/dCas9-based technologies for high-throughput screening. We summarize the applications of CRISPR screening technology in exploring the mechanism of lung cancer development in vivo or in vitro, overcoming drug resistance, improving the effect of immunotherapy, and discovering new therapeutic targets. This review highlights the potential of CRISPR screening in combination with tumor barcoding and high-throughput sequencing (Tuba-seq) to precisely quantify the impact of alterations in many tumor suppressor genes on lung cancer.

Polyadenosine diphosphate-ribose polymerase inhibitors: advances, implications, and challenges in tumor radiotherapy sensitization

Polyadenosine diphosphate-ribose polymerase (PARP) is a key modifying enzyme in cells, which participates in single-strand break repair and indirectly affects double-strand break repair. PARP inhibitors have shown great potential in oncotherapy by exploiting DNA damage repair pathways, and several small molecule PARP inhibitors have been approved by the U.S. Food and Drug Administration for treating various tumor types. PARP inhibitors not only have significant antitumor effects but also have some synergistic effects when combined with radiotherapy; therefore they have potential as radiation sensitizers. Here, we reviewed the advances and implications of PARP inhibitors in tumor radiotherapy sensitization. First, we summarized the multiple functions of PARP and the mechanisms by which its inhibitors exert antitumor effects. Next, we discuss the immunomodulatory effects of PARP and its inhibitors in tumors. Then, we described the theoretical basis of using PARP inhibitors in combination with radiotherapy and outlined their importance in oncological radiotherapy. Finally, we reviewed the current challenges in this field and elaborated on the future applications of PARP inhibitors as radiation sensitizers. A comprehensive understanding of the mechanism, optimal dosing, long-term safety, and identification of responsive biomarkers remain key challenges to integrating PARP inhibition into the radiotherapy management of cancer patients. Therefore, extensive research in these areas would facilitate the development of precision radiotherapy using PARP inhibitors to improve patient outcomes.

Delta-like ligand 3 in small cell lung cancer: Potential mechanism and treatment progress

Small cell lung cancer (SCLC) is one of a pathological type of lung cancer, and it is characterized by invasiveness, high malignancy and refractoriness. The mortality rate of SCLC is significantly higher than other types of lung cancer, and the treatment options for SCLC patients are limited. Delta-like ligand 3 (DLL3) is a Notch signaling ligand that plays a role in regulating the proliferation, development and metastasis of SCLC cells. Mnay studies have shown that DLL3 is overexpressed on the surface of SCLC cells, suggesting that DLL3 is a potential target for SCLC patients. A series of drug trials targeting DLL3 are underway. The Phase III clinical trials of Rova-T, a drug targeting DLL3, have not yielded the expected results. However, other drugs that target DLL3, such as AMG119, AMG757 and DLL3-targeted NIR-PIT, bring new ideas for SCLC treatment. Overall, DLL3 remains a valuable target for SCLC.

PARP inhibitors combined with radiotherapy: are we ready?

PARP was an enzyme found in the nucleus of eukaryotic cells that played a crucial role in repairing damaged DNA. Recently, PARP inhibitors have demonstrated great potential in cancer treatment. Thus, the FDA has approved several small-molecule PARP inhibitors for cancer maintenance therapy. The combination of PARP inhibitors and radiotherapy relies on synthetic lethality, taking advantage of the flaws in DNA repair pathways to target cancer cells specifically. Studies conducted prior to clinical trials have suggested that the combination of PARP inhibitors and radiotherapy can enhance the sensitivity of cancer cells to radiation, intensify DNA damage, and trigger cell death. Combining radiotherapy with PARP inhibitors in clinical trials has enhanced the response rate and progression-free survival of diverse cancer patients. The theoretical foundation of PARP inhibitors combined with radiotherapy is explained in detail in this article, and the latest advances in preclinical and clinical research on these inhibitors for tumor radiotherapy are summarized. The problems in the current field are recognized in our research and potential therapeutic applications for tumors are suggested. Nevertheless, certain obstacles need to be tackled when implementing PARP inhibitors and radiotherapies in clinical settings. Factors to consider when using the combination therapy are the most suitable schedule and amount of medication, identifying advantageous candidates, and the probable adverse effects linked with the combination. The combination of radiotherapy and PARP inhibitors can greatly enhance the effectiveness of cancer treatment.

Potential agnostic role of BRCA alterations in patients with several solid tumors: One for all, all for one?

Germline BRCA1/2 alterations in the Homologous Recombination (HR) pathway are considered as main susceptibility biomarkers to Hereditary Breast and Ovarian Cancers (HBOC). The modern molecular biology technologies allowed to characterize germline and somatic BRCA1/2 alterations in several malignancies, broadening the landscape of BRCA1/2-alterated tumors. In the last years, BRCA genetic testing, beyond the preventive value, also assumed a predictive and prognostic significance for patient management. The approval of molecules with agnostic indication is leading to a new clinical model, defined "mutational". Among these drugs, the Poly (ADP)-Ribose Polymerase inhibitors (PARPi) for BRCA1/2-deficient tumors were widely studied leading to increasing therapeutic implications. In this Review we provided an overview of the main clinical studies describing the association between BRCA-mutated tumors and PARPi response, focusing on the controversial evidence about the potential agnostic indication based on BRCA1/2 alterations in several solid tumors.

Current Evidence and Future Perspectives about the Role of PARP Inhibitors in the Treatment of Thoracic Cancers

In recent years, poly (ADP-ribose) polymerase (PARP) inhibition has become a promising therapeutic option for several tumors, especially for those harboring a BRCA 1-2 mutation or a deficit in the homologous recombination repair (HRR) pathway. Nevertheless, to date, PARP inhibitors are still not largely used for thoracic malignancies neither as a single agent nor in combination with other treatments. Recently, a deeper understanding of HRR mechanisms, alongside the development of new targeted and immunotherapy agents, particularly against HRR-deficient tumors, traced the path to new treatment strategies for many tumor types including lung cancer and malignant pleural mesothelioma. The aim of this review is to sum up the current knowledge about cancer-DNA damage response pathways inhibition and to update the status of recent clinical trials investigating the use of PARP inhibitors, either as monotherapy or in combination with other agents for the treatment of thoracic malignancies. We will also briefly discuss available evidence on Poly(ADP-Ribose) Glycohydrolase (PARG) inhibitors, a novel promising therapeutic option in oncology.

Holistic immunomodulation for small cell lung cancer

Small cell lung cancer (SCLC) is characterized by a high mortality rate, rapid growth, and early metastasis, which lead to a poor prognosis. Moreover, limited clinical treatment options further lower the survival rate of patients. Therefore, novel technology and agents are urgently required to enhance clinical efficacy. In this review, from a holistic perspective, we summarized the therapeutic targets, agents and strategies with the most potential for treating SCLC, including chimeric antigen receptor (CAR) T therapy, immunomodulating antibodies, traditional Chinese medicines (TCMs), and the microbiota, which have been found recently to improve the clinical outcomes and prognosis of SCLC. Multiomics technologies can be integrated to develop effective diagnostic methods and identify new targets for new drug discovery in SCLC. We discussed in depth the feasibility, potential, and challenges of these new strategies, as well as their combinational treatments, which may provide promising alternatives for enhancing the clinical efficacy of SCLC in the future.

BMN673 Is a PARP Inhibitor with Unique Radiosensitizing Properties: Mechanisms and Potential in Radiation Therapy

BMN673 is a relatively new PARP inhibitor (PARPi) that exhibits superior efficacy in vitro compared to olaparib and other clinically relevant PARPi. BMN673, similar to most clinical PARPi, inhibits the catalytic activities of PARP-1 and PARP-2 and shows impressive anticancer potential as monotherapy in several pre-clinical and clinical studies. Tumor resistance to PARPi poses a significant challenge in the clinic. Thus, combining PARPi with other treatment modalities, such as radiotherapy (RT), is being actively pursued to overcome such resistance. However, the modest to intermediate radiosensitization exerted by olaparib, rucaparib, and veliparib, limits the rationale and the scope of such combinations. The recently reported strong radiosensitizing potential of BMN673 forecasts a paradigm shift on this front. Evidence accumulates that BMN673 may radiosensitize via unique mechanisms causing profound shifts in the balance among DNA double-strand break (DSB) repair pathways. According to one of the emerging models, BMN673 strongly inhibits classical non-homologous end-joining (c-NHEJ) and increases reciprocally and profoundly DSB end-resection, enhancing error-prone DSB processing that robustly potentiates cell killing. In this review, we outline and summarize the work that helped to formulate this model of BMN673 action on DSB repair, analyze the causes of radiosensitization and discuss its potential as a radiosensitizer in the clinic. Finally, we highlight strategies for combining BMN673 with other inhibitors of DNA damage response for further improvements.

The Prognostic and Therapeutic Potential of DNA Damage Repair Pathway Alterations and Homologous Recombination Deficiency in Lung Cancer

Lung cancer remains the second most commonly diagnosed cancer worldwide and the leading cause of cancer-related mortality. The mapping of genomic alterations and their role in lung-cancer progression has been followed by the development of new therapeutic options. Several novel drugs, such as targeted therapy and immunotherapy, have significantly improved outcomes. However, many patients with lung cancer do not benefit from existing therapies or develop progressive disease, leading to increased morbidity and mortality despite initial responses to treatment. Alterations in DNA-damage repair (DDR) genes represent a cancer hallmark that impairs a cell’s ability to prevent deleterious mutation accumulation and repair. These alterations have recently emerged as a therapeutic target in breast, ovarian, prostate, and pancreatic cancers. The role of DDR alterations remains largely unknown in lung cancer. Nevertheless, recent research efforts have highlighted a potential role of some DDR alterations as predictive biomarkers of response to treatment. Despite the failure of PARP inhibitors (main class of DDR targeting agents) to improve outcomes in lung cancer patients, there is some evidence suggesting a role of PARP inhibitors and other DDR targeting agents in benefiting a distinct subset of lung cancer patients. In this review, we will discuss the existing literature on DDR alterations and homologous recombination deficiency (HRD) state as predictive biomarkers and therapeutic targets in both non-small cell lung and small cell lung cancer.

Dissecting Immunotherapy Strategies for Small Cell Lung Cancer: Antibodies, Ionizing Radiation and CAR-T

Small cell lung cancer (SCLC) is a highly aggressive malignancy that accounts for about 14% of all lung cancers. Platinum-based chemotherapy has been the only available treatment for a long time, until the introduction of immune checkpoint inhibitors (ICIs) recently changed first-line standard of care and shed light on the pivotal role of the immune system. Despite improved survival in a subset of patients, a lot of them still do not benefit from first-line chemo-immunotherapy, and several studies are investigating whether different combination strategies (with both systemic and local treatments, such as radiotherapy) may improve patient outcomes. Moreover, research of biomarkers that may be used to predict patients' outcomes is ongoing. In addition to ICIs, immunotherapy offers other different strategies, including naked monoclonal antibodies targeting tumor associated antigens, conjugated antibody, bispecific antibodies and cellular therapies. In this review, we summarize the main evidence available about the use of immunotherapy in SCLC, the rationale behind combination strategies and the studies that are currently ongoing in this setting, in order to give the reader a clear and complete view of this rapidly expanding topic.

PARP inhibitor plus radiotherapy reshapes an inflamed tumor microenvironment that sensitizes small cell lung cancer to the anti-PD-1 immunotherapy

Small cell lung cancer (SCLC) is a highly malignant tumor with extremely poor prognosis. The treatment strategy is very limited, and patient outcomes remain dismal with the 5-year survival rate being mere 3-6%. Thus, novel therapeutic strategies for SCLC patients are urgently needed. In this study, we found that the triple-therapy of poly (ADP-ribose) polymerase (PARP) inhibitor, radiotherapy (RT) and anti-PD-1 treatment significantly inhibited tumor growth and prolonged survival in the syngeneic SCLC models in immunocompetent C57BL/6 mice. Mechanistically, we demonstrated that the combination of PARP inhibitor niraparib and RT reshaped an inflamed tumor microenvironment, including activation of the cGAS/STING immune response pathway, induction of immunogenic cell death, and upregulation of PD-L1 on tumor cells. Furthermore, this triple-therapy substantially augmented CD8⁺ T cell infiltration and activation, and enhanced anti-tumor effects as revealed by increased median survival time and reduced tumor volume without additional myelosuppression or hepatic injury. Together, our studies demonstrated that PARP inhibitor combined with RT potentiated anti-tumor immunity and enhanced the efficacy of anti-PD-1 immunotherapy in preclinical study, which provided a promising therapeutic strategy for SCLC patients in clinic.

Poly(ADP-Ribose) Polymerase Inhibition as a Promising Approach for Hepatocellular Carcinoma Therapy

Primary liver cancer is the sixth most common cancer in men and seventh in women, with hepatocellular carcinoma (HCC) being the most common form (75-85% of primary liver cancer cases) and the most frequent etiology being viral infections (HBV and HCV). In 2020, mortality represented 92% of the incidence-830,180 deaths for 905,677 new cases. Few treatment options exist for advanced or terminal-stage HCC, which will receive systemic therapy or palliative care. Although radiotherapy is used in the treatment of many cancers, it is currently not the treatment of choice for HCC, except in the palliative setting. However, as radiosensitizing drugs, such as inhibitors of DNA repair enzymes, could potentiate the effects of RT in HCC by exploiting the modulation of DNA repair processes found in this tumour type, RT and such drugs could provide a treatment option for HCC. In this review, we provide an overview of PARP1 involvement in DNA damage repair pathway and discuss its potential implication in HCC. In addition, the use of PARP inhibitors and PARP decoys is described for the treatment of HCC and, in particular, in HBV-related HCC.

Novel therapeutic combinations with PARP inhibitors for small cell lung cancer: A bench-to-bedside review

Small cell lung cancer (SCLC) is treated as a monolithic disease despite the evident intra- and intertumoral heterogeneity. Non-specific DNA-damaging agents have remained the first-line treatment for decades. Recently, emerging transcriptomic and genomic profiling of SCLC tumors identified distinct SCLC subtypes and vulnerabilities towards targeted therapeutics, including inhibitors of the nuclear enzyme poly (ADP-ribose) polymerase (PARPi). SCLC cell lines and tumors exhibited an elevated level of PARP1 protein and mRNA compared to healthy lung tissues and other subtypes of lung tumors. Notable responses to PARPi were also observed in preclinical SCLC models. Clinically, PARPi monotherapy exerted variable benefits for SCLC patients. To date, research is being vigorously conducted to examine predictive biomarkers of PARPi response and various PARPi combination strategies to maximize the clinical utility of PARPi. This narrative review summarizes existing preclinical evidence supporting PARPi monotherapy, combination therapy, and respective translation to the clinic. Specifically, we covered the combination of PARPi with DNA-damaging chemotherapy (cisplatin, etoposide, temozolomide), thoracic radiotherapy, immunotherapy (immune checkpoint inhibitors), and many other novel therapeutic agents that target DNA damage response, tumor microenvironment, epigenetic modulation, angiogenesis, the ubiquitin-proteasome system, or autophagy. Putative biomarkers, such as SLFN11 expression, MGMT methylation, E2F1 expression, and platinum sensitivity, which may be predictive of response to distinct therapeutic combinations, were also discussed. The future of SCLC treatment is undergoing rapid change with a focus on tailored and personalized treatment strategies. Further development of cancer therapy with PARPi will immensely benefit at least a subset of biomarker-defined SCLC patients.

PARP Inhibition, a New Therapeutic Avenue in Patients with Prostate Cancer

Up to 25% of patients with metastatic prostate cancer present with germline or somatic DNA damage repair alterations, some of which are associated with aggressive disease and poor outcomes. New data have brought poly(ADP-ribose) polymerase (PARP) inhibitors into sharp focus in the treatment of metastatic castrate-resistant prostate cancer (mCRPC). Olaparib improved survival after at least one new hormonal therapy (NHT) in a cohort of patients harboring BRCA1, BRCA2 or ATM mutations in the PROfound trial, while rucaparib, talazoparib and niraparib demonstrated compelling activity in phase II trials. While patients with prostate cancer and BRCA1 or BRCA2 mutations may derive greatest benefit of PARP inhibition, the magnitude of benefit seems much lower in the context of most other homologous recombination gene mutations. Several PARP inhibitors are currently developed in combination with conventional therapy, including chemotherapy, NHT, and alpha-particle emitters, at different disease stages. Herein, we review the rationale for PARP inhibition in patients with prostate cancer, discuss the impact of PARP inhibitors on outcomes, and explore underlying challenges for future developments.

Radioimmunotherapy Targeting Delta-like Ligand 3 in Small Cell Lung Cancer Exhibits Antitumor Efficacy with Low Toxicity

PURPOSE

Small cell lung cancer (SCLC) is an exceptionally lethal form of lung cancer with limited treatment options. Delta-like ligand 3 (DLL3) is an attractive therapeutic target as surface expression is almost exclusive to tumor cells.

EXPERIMENTAL DESIGN

We radiolabeled the anti-DLL3 mAb SC16 with the therapeutic radioisotope, Lutetium-177. [177Lu]Lu-DTPA-CHX-A"-SC16 binds to DLL3 on SCLC cells and delivers targeted radiotherapy while minimizing radiation to healthy tissue.

RESULTS

[177Lu]Lu-DTPA-CHX-A"-SC16 demonstrated high tumor uptake with DLL3-target specificity in tumor xenografts. Dosimetry analyses of biodistribution studies suggested that the blood and liver were most at risk for toxicity from treatment with high doses of [177Lu]Lu-DTPA-CHX-A"-SC16. In the radioresistant NCI-H82 model, survival studies showed that 500 μCi and 750 μCi doses of [177Lu]Lu-DTPA-CHX-A"-SC16 led to prolonged survival over controls, and 3 of the 8 mice that received high doses of [177Lu]Lu-DTPA-CHX-A"-SC16 had pathologically confirmed complete responses (CR). In the patient-derived xenograft model Lu149, all doses of [177Lu]Lu-DTPA-CHX-A"-SC16 markedly prolonged survival. At the 250 μCi and 500 μCi doses, 5 of 10 and 7 of 9 mice demonstrated pathologically confirmed CRs, respectively. Four of 10 mice that received 750 μCi of [177Lu]Lu-DTPA-CHX-A"-SC16 demonstrated petechiae severe enough to warrant euthanasia, but the remaining 6 mice demonstrated pathologically confirmed CRs. IHC on residual tissues from partial responses confirmed retained DLL3 expression. Hematologic toxicity was dose-dependent and transient, with full recovery within 4 weeks. Hepatotoxicity was not observed.

CONCLUSIONS

Together, the compelling antitumor efficacy, pathologic CRs, and mild and transient toxicity profile demonstrate strong potential for clinical translation of [177Lu]Lu-DTPA-CHX-A"-SC16.

The non-canonical target PARP16 contributes to polypharmacology of the PARP inhibitor talazoparib and its synergy with WEE1 inhibitors

PARP inhibitors (PARPis) display single-agent anticancer activity in small cell lung cancer (SCLC) and other neuroendocrine tumors independent of BRCA1/2 mutations. Here, we determine the differential efficacy of multiple clinical PARPis in SCLC cells. Compared with the other PARPis rucaparib, olaparib, and niraparib, talazoparib displays the highest potency across SCLC, including SLFN11-negative cells. Chemical proteomics identifies PARP16 as a unique talazoparib target in addition to PARP1. Silencing PARP16 significantly reduces cell survival, particularly in combination with PARP1 inhibition. Drug combination screening reveals talazoparib synergy with the WEE1/PLK1 inhibitor adavosertib. Global phosphoproteomics identifies disparate effects on cell-cycle and DNA damage signaling thereby illustrating underlying mechanisms of synergy, which is more pronounced for talazoparib than olaparib. Notably, silencing PARP16 further reduces cell survival in combination with olaparib and adavosertib. Together, these data suggest that PARP16 contributes to talazoparib's overall mechanism of action and constitutes an actionable target in SCLC.

Radiotherapy-Poly(ADP-ribose) Polymerase Inhibitor Combinations: Progress to Date

Radiation resistance remains a huge clinical problem for cancer patients and oncologists in the 21st century. In recent years, the mammalian DNA damage response (DDR) has been extensively characterized and shown to play a key role in determining cellular survival following ionizing radiation exposure. Genomic instability due to altered DDR is a hallmark of cancer, with many tumors exhibiting abnormal DNA repair or lack of redundancy in DDR. Targeting the abnormal DDR phenotype of tumor cells could lead to substantial gains in radiotherapy efficacy, improving local control and survival for patients with cancers that are refractory to current therapies. Poly(ADP-ribose) polymerase inhibitors (PARPi) are the most clinically advanced DDR inhibitors under investigation as radiosensitisers. Preclinical evidence suggests that PARPi may provide tumor specific radiosensitisation in certain contexts. In addition to inhibition of DNA single strand break repair, PARPi may offer other benefits in combination treatment including radiosensitisation of hypoxic cells and targeting of alternative repair pathways such as microhomology mediated end joining which are increasingly recognized to be upregulated in cancer. Several early phase clinical trials of PARPi with radiation have completed or are in progress. Early reports have highlighted tumor specific challenges, with tolerability dependent upon anatomical location and use of concomitant systemic therapies; these challenges were largely predicted by preclinical data. This review discusses the role of PARP in the cellular response to ionizing radiation, summarizes preclinical studies of PARPi in combination with radiotherapy and explores current early phase clinical trials that are evaluating these combinations.

Recent Research Progress of Chiral Small Molecular Antitumor-Targeted Drugs Approved by the FDA From 2011 to 2019

Chiral drugs usually contain chiral centers, which are present as single enantiomers or racemates. Compared with achiral drugs, they have significant advantages in safety and efficacy with high stereoselectivity. Of these drugs, chirality not only exerts influence on the solubility and pharmacokinetic characteristics but also has specific mechanistic characteristics on their targets. We noted that small molecules with unique chiral properties have emerged as novel components of antitumor drugs approved by the FDA in decade. Since approved, these drugs have been continuously explored for new indications, new mechanisms, and novel combinations. In this mini review, recent research progress of twenty-two FDA-approved chiral small molecular-targeted antitumor drugs from 2011 to 2019 is summarized with highlighting the potential and advantages of their applications. We believe that these updated achievements may provide theoretical foundation and stimulate research interests for optimizing drug efficacy, expanding clinical application, overcoming drug resistance, and advancing safety in future clinical administrations of these chiral targeted drugs.

Recent developments in the treatment of small cell lung cancer

Small cell lung cancer (SCLC) comprises about 15% of all lung cancers. It is an aggressive disease, with early metastasis and a poor prognosis. Until recently, SCLC treatment remained relatively unchanged, with chemotherapy remaining the cornerstone of treatment. In this overview we will highlight the recent advances in the field of staging, surgery, radiotherapy and systemic treatment. Nevertheless, the prognosis remains dismal and there is a pressing need for new treatment options. We describe the progress that has been made in systemic treatment by repurposing existing drugs and the addition of targeted treatment. In recent years, immunotherapy entered the clinic with high expectations of its role in the treatment of SCLC. Unravelling of the genomic sequence revealed new possible targets that may act as biomarkers in future treatment of patients with SCLC. Hopefully, in the near future, we will be able to identify patients who may benefit from targeted therapy or immunotherapy to improve prognoses.

Molecular mechanisms of chemo- and radiotherapy resistance and the potential implications for cancer treatment

Cancer is a leading cause of death worldwide. Surgery is the primary treatment approach for cancer, but the survival rate is very low due to the rapid progression of the disease and presence of local and distant metastasis at diagnosis. Adjuvant chemotherapy and radiotherapy are important components of the multidisciplinary approaches for cancer treatment. However, resistance to radiotherapy and chemotherapy may result in treatment failure or even cancer recurrence. Radioresistance in cancer is often caused by the repair response to radiation-induced DNA damage, cell cycle dysregulation, cancer stem cells (CSCs) resilience, and epithelial-mesenchymal transition (EMT). Understanding the molecular alterations that lead to radioresistance may provide new diagnostic markers and therapeutic targets to improve radiotherapy efficacy. Patients who develop resistance to chemotherapy drugs cannot benefit from the cytotoxicity induced by the prescribed drug and will likely have a poor outcome with these treatments. Chemotherapy often shows a low response rate due to various drug resistance mechanisms. This review focuses on the molecular mechanisms of radioresistance and chemoresistance in cancer and discusses recent developments in therapeutic strategies targeting chemoradiotherapy resistance to improve treatment outcomes.

High-throughput compound screening identifies navitoclax combined with irradiation as a candidate therapy for HPV-negative head and neck squamous cell carcinoma

Conventional chemotherapeutic agents are nonselective, often resulting in severe side effects and the development of resistance. Therefore, new molecular-targeted therapies are urgently needed to be integrated into existing treatment regimens. Here, we performed a high-throughput compound screen to identify a synergistic interaction between ionizing radiation and 396 anticancer compounds. The assay was run using five human papillomavirus (HPV)-negative head and neck squamous cell carcinoma (HNSCC) cell lines cultured on the human tumor-derived matrix Myogel. Our screen identified several compounds with strong synergistic and antagonistic effects, which we further investigated using multiple irradiation doses. Navitoclax, which emerged as the most promising radiosensitizer, exhibited synergy with irradiation regardless of the p53 mutation status in all 13 HNSCC cell lines. We performed a live cell apoptosis assay for two representative HNSCC cell lines to examine the effects of navitoclax and irradiation. As a single agent, navitoclax reduced proliferation and induced apoptosis in a dose-dependent manner, whereas the navitoclax-irradiation combination arrested cell cycle progression and resulted in substantially elevated apoptosis. Overall, we demonstrated that combining navitoclax with irradiation resulted in synergistic in vitro antitumor effects in HNSCC cell lines, possibly indicating the therapeutic potential for HNSCC patients.

Repositioning PARP inhibitors in the treatment of thoracic malignancies

The evaluation of the homologous recombination repair (HRR) status is emerging as a predictive tumor agnostic biomarker for poly (ADP-ribose) polymerase (PARP) inhibition across different tumor types and testing for HRR-signature is currently a developing area with promising therapeutic implications. Treatment with PARP inhibitors (PARPi) either as single agent or in combination with chemotherapy have shown so far limited activity in patients with thoracic malignancies. A deeper understanding of the biological background underlying HRR-deficient tumors, along with the recent advent of new effective targeted and immunotherapeutic agents, prompted the design of a new generation of clinical trials investigating novel PARPi-combinations in patients with lung cancer as well as malignant pleural mesothelioma. In this review we briefly summarize the biological basis of the DNA damage response pathway inhibition and provide an updated and detailed overview of clinical trials testing different PARPi-combinations strategies in patients with thoracic malignancies.

CRISPR/Cas9 library screening uncovered methylated PKP2 as a critical driver of lung cancer radioresistance by stabilizing β-catenin

Radiation resistance is a major cause of lung cancer treatment failure. Armadillo (ARM) superfamily proteins participate in various fundamental cellular processes; however, whether ARM proteins regulate radiation resistance is not fully understood. Here, we used an unbiased CRISPR/Cas9 library screen and identified plakophilin 2 (PKP2), a member of the ARM superfamily of proteins, as a critical driver of radiation resistance in lung cancer. The PKP2 level was significantly higher after radiotherapy than before radiotherapy, and high PKP2 expression after radiotherapy predicted poor overall survival (OS) and postprogression survival (PPS). Mechanistically, mass spectrometry analysis identified that PKP2 was methylated at the arginine site and interacted with protein arginine methyltransferase 1 (PRMT1). Methylation of PKP2 by PRMT1 stabilized β-catenin by recruiting USP7, further inducing LIG4, a key DNA ligase in nonhomologous end-joining (NHEJ) repair. Concomitantly, PKP2-induced radioresistance depended on facilitating LIG4-mediated NHEJ repair in lung cancer. More strikingly, after exposure to irradiation, treatment with the PRMT1 inhibitor C-7280948 abolished PKP2-induced radioresistance, and C-7280948 is a potential radiosensitizer in lung cancer. In summary, our results demonstrate that targeting the PRMT1/PKP2/β-catenin/LIG4 pathway is an effective approach to overcome radiation resistance in lung cancer.

Advances in Small-Cell Lung Cancer (SCLC) Translational Research

Over the past several years, we have witnessed a resurgence of interest in the biology and therapeutic vulnerabilities of small-cell lung cancer (SCLC). This has been driven in part through the development of a more extensive array of representative models of disease, including a diverse variety of genetically engineered mouse models and human tumor xenografts. Herein, we review recent progress in SCLC model development, and consider some of the particularly active avenues of translational research in SCLC, including interrogation of intratumoral heterogeneity, insights into the cell of origin and oncogenic drivers, mechanisms of chemoresistance, and new therapeutic opportunities including biomarker-directed targeted therapies and immunotherapies. Whereas SCLC remains a highly lethal disease, these new avenues of translational research, bringing together mechanism-based preclinical and clinical research, offer new hope for patients with SCLC.

Olaparib increases the therapeutic index of hemithoracic irradiation compared with hemithoracic irradiation alone in a mouse lung cancer model

Background

The radiosensitising effect of the poly(ADP-ribose) polymerase inhibitor olaparib on tumours has been reported. However, its effect on normal tissues in combination with radiation has not been well studied. Herein, we investigated the therapeutic index of olaparib combined with hemithoracic radiation in a urethane-induced mouse lung cancer model.

Methods

To assess tolerability, A/J mice were treated with olaparib plus whole thorax radiation (13 Gy), body weight changes were monitored and normal tissue effects were assessed by histology. In anti-tumour (intervention) studies, A/J mice were injected with urethane to induce lung tumours, and were then treated with olaparib alone, left thorax radiation alone or the combination of olaparib plus left thorax radiation at 8 weeks (early intervention) or 18 weeks (late intervention) after urethane injection. Anti-tumour efficacy and normal tissue effects were assessed by visual inspection, magnetic resonance imaging and histology.

Results

Enhanced body weight loss and oesophageal toxicity were observed when olaparib was combined with whole thorax but not hemithorax radiation. In both the early and late intervention studies, olaparib increased the anti-tumour effects of hemithoracic irradiation without increasing lung toxicity.

Conclusions

The addition of olaparib increased the therapeutic index of hemithoracic radiation in a mouse model of lung cancer.

PARP Inhibitors in Cancer Diagnosis and Therapy

Targeting of PARP enzymes has emerged as an effective therapeutic strategy to selectively target cancer cells with deficiencies in homologous recombination signaling. Currently used to treat BRCA-mutated cancers, PARP inhibitors (PARPi) have demonstrated improved outcome in various cancer types as single agents. Ongoing efforts have seen the exploitation of PARPi combination therapies, boosting patient responses as a result of drug synergisms. Despite great successes using PARPi therapy, selecting those patients who will benefit from single agent or combination therapy remains one of the major challenges. Numerous reports have demonstrated that the presence of a BRCA mutation does not always result in synthetic lethality with PARPi therapy in treatment-naïve tumors. Cancer cells can also develop resistance to PARPi therapy. Hence, combination therapy may significantly affect the treatment outcomes. In this review, we discuss the development and utilization of PARPi in different cancer types from preclinical models to clinical trials, provide a current overview of the potential uses of PARP imaging agents in cancer therapy, and discuss the use of radiolabeled PARPi as radionuclide therapies.

Application of Animal Models in Cancer Research: Recent Progress and Future Prospects

Animal models refers to the animal experimental objects and related materials that can simulate human body established in medical research. As the second-largest disease in terms of morbidity and mortality after cardiovascular disease, cancer has always been the focus of human attention all over the world, which makes it a research hotspot in the medical field. At the same time, more and more animal models have been constructed and used in cancer research. With the deepening of research, the construction methods of cancer animal models are becoming more and more diverse, including chemical induction, xenotransplantation, gene programming, and so on. In recent years, patient-derived xenotransplantation (PDX) model has become a research hotspot because it can retain the microenvironment of the primary tumor and the basic characteristics of cells. Animal models can be used not only to study the biochemical and physiological processes of the occurrence and development of cancer in objects but also for the screening of cancer drugs and the exploration of gene therapy. In this paper, several main tumor animal models and the application progress of animal models in tumor research are systematically reviewed. Finally, combined with the latest progress and development trend in this field, the future research of tumor animal model was prospected.

PARP Inhibitors in Small-Cell Lung Cancer: Rational Combinations to Improve Responses

Simple Summary

Small-cell lung cancer carries a dismal prognosis with few long-term treatment options. The enzyme poly-(ADP)-ribose polymerase (PARP), which functions to repair DNA breaks, has emerged as a promising therapeutic target, with modest response rates in early clinical trials prompting investigation of predictive biomarkers and therapeutic combinations. This review summarizes the development and testing of PARP inhibitors in small-cell lung cancer with an emphasis on developing treatment combinations. These combinations can be divided into three categories: (1) contributing to DNA damage; (2) inhibiting the DNA damage response; and (3) activating the immune system. An evolving classification of small-cell lung cancer subtypes and gene expression patterns will guide PARP inhibitor biomarker identification to improve treatments for this challenging cancer.

Abstract

Despite recent advances in first-line treatment for small-cell lung cancer (SCLC), durable responses remain rare. The DNA repair enzyme poly-(ADP)-ribose polymerase (PARP) was identified as a therapeutic target in SCLC using unbiased preclinical screens and confirmed in human and mouse models. Early trials of PARP inhibitors, either alone or in combination with chemotherapy, showed promising but limited responses, suggesting that selecting patient subsets and treatment combinations will prove critical to further clinical development. Expression of SLFN11 and other components of the DNA damage response (DDR) pathway appears to select for improved responses. Combining PARP inhibitors with agents that damage DNA and inhibit DDR appears particularly effective in preclinical and early trial data, as well as strategies that enhance antitumor immunity downstream of DNA damage. A robust understanding of the mechanisms of DDR in SCLC, which exhibits intrinsic replication stress, will improve selection of agents and predictive biomarkers. The most effective combinations will target multiple nodes in the DNA damage/DDR/immune activation cascade to minimize toxicity from synthetic lethality.

Combinatorial Efficacy of Olaparib with Radiation and ATR Inhibitor Requires PARP1 Protein in Homologous Recombination-Proficient Pancreatic Cancer

PARP inhibitor monotherapy (olaparib) was recently FDA approved for the treatment of BRCA1/2-mutant, homologous recombination (HR) repair-deficient pancreatic cancer. Most pancreatic cancers, however, are HR proficient and thus resistant to PARP inhibitor monotherapy. We tested the hypothesis that combined therapy with radiation and ataxia telangiectasia and Rad3-related (ATR) inhibitor (AZD6738) would extend the therapeutic indication of olaparib to HR-proficient pancreatic cancers. We show that olaparib combined with AZD6738 significantly reduced radiation survival relative to either agent alone, regardless of HR status. Whereas catalytic inhibition of PARP with low concentrations of olaparib radiosensitized HR-deficient models, maximal sensitization in HR-proficient models required concentrations of olaparib that induce formation of PARP1-DNA complexes. Furthermore, CRISPR-Cas9-mediated PARP1 deletion failed to recapitulate the effects of olaparib on radiosensitivity and negated the combinatorial efficacy of olaparib and AZD6738 on radiosensitization, suggesting that PARP1-DNA complexes, rather than PARP catalytic inhibition, were responsible for radiosensitization. Mechanistically, therapeutic concentrations of olaparib in combination with radiation and AZD6738 increased DNA double-strand breaks. DNA fiber combing revealed that high concentrations of olaparib did not stall replication forks but instead accelerated replication fork progression in association with an ATR-mediated replication stress response that was antagonized by AZD6738. Finally, in HR-proficient tumor xenografts, the combination of olaparib, radiation, and AZD6738 significantly delayed tumor growth compared with all other treatments. These findings suggest that PARP1-DNA complexes are required for the therapeutic activity of olaparib combined with radiation and ATR inhibitor in HR-proficient pancreatic cancer and support the clinical development of this combination for tumors intrinsically resistant to PARP inhibitors.

Targeting DNA methyltransferases in non-small-cell lung cancer

Despite the advances in treatment using chemotherapy or targeted therapies, due to static survival rates, non-small cell lung cancer (NSCLC) is the major cause of cancer-related deaths worldwide. Epigenetic-based therapies have been developed for NSCLC by targeting DNA methyltransferases (DNMTs) and histone-modifying enzymes. However, treatment using single epigenetic agents on solid tumours has been inadequate; whereas, treatment with a combination of DNMTs inhibitors with chemotherapy and immunotherapy has shown great promise. Dietary sources of phytochemicals could also inhibit DNMTs and cancer stem cells, representing a novel and promising way to prevent and treat cancer. Herein, we will discuss the different DNMTs, DNA methylation profiling in NSCLC as well as current demethylating agents in ongoing clinical trials. Therefore, providing a concise overview of future developments in the field of epigenetic therapy in NSCLC.

Development of new poly(ADP-ribose) polymerase (PARP) inhibitors in ovarian cancer: Quo Vadis?

Epithelial ovarian cancer (EOC) is the fifth leading cause of cancer mortality among women, potentially due to ineffectiveness of screening tests for early detection. Patients typically present with advanced disease at diagnosis, whereas, up to 80% relapse and the estimated median progression-free survival (PFS) is approximately 12–18 months. Increased knowledge on the molecular biology of EOC resulted in the development of several targeted therapies, including poly(ADP-ribose) polymerase (PARP) inhibitors. These agents have changed the therapeutic approach of the EOC and exploit homologous recombination (HR) deficiency through synthetic lethality, especially in breast cancer genes 1 and 2 (BRCA1/2) mutation carriers. Furthermore, BRCA wild-type patients with other defects in the HR repair pathway, or those with platinum-resistant tumors may obtain benefit from this treatment. While PARP inhibitors as a class display many similarities, several differences in structure can translate into differences in tolerability and antitumor activity. Currently, olaparib, rucaparib, and niraparib have been approved by Food and Drug Administration (FDA) and/or European Medicines Agency (EMA) for the treatment of EOC, while veliparib is in the late stage of clinical development. Finally, since October 2018 talazoparib is FDA and EMA approved for BRCA carriers with metastatic breast cancers. In this article, we explore the mechanisms of DNA repair, synthetic lethality, efficiency of PARP inhibition, and provide an overview of early and ongoing clinical investigations of the novel PARP inhibitors veliparib and talazoparib.

Diosmetin enhances the sensitivity of radiotherapy by suppressing homologous recombination in endometrial cancer

Radiotherapy is an essential treatment for endometrial cancer (EC), especially in advanced, metastatic, and recurrent cases. Combining radiotherapy, which mainly causes DNA double-strand breaks (DSBs), with small molecules targeting aberrantly activated homologous recombination (HR) repair pathways holds great potential for treating ECs in advanced stages. Here, we demonstrate that diosmetin (DIO), a natural flavonoid, suppresses HR, therefore inhibiting cell proliferation and enhancing the sensitivity of EC to radiotherapy. Clonogenic experiments revealed that combining DIO and X-ray significantly inhibited the viability of EC cells compared to cells treated with diosmetin or X-ray alone. The survival fraction of EC cells decreased to 40% when combining 0.4 Gy X-ray and 4 μM DIO; however, each treatment alone only caused death in approximately 15% and 22% of cancer cells, respectively. Further mechanistic studies showed that diosmetin inhibited the recruitment of RPA2 and RAD51, two critical factors involved in the HR repair pathway, upon the occurrence of DSBs. Thus, we propose that a combination of diosmetin and irradiation is a promising therapeutic strategy for treating endometrial cancer.

PARP inhibitors for small cell lung cancer and their potential for integration into current treatment approaches

Small cell lung cancer (SCLC) is a very aggressive, highly lethal, neuroendocrine tumor that constitutes 15% of all lung cancer cases. It is characterized by its rapid disease progression and high relapse rate leading to poor survival for diagnosed patients. Recently, poly (ADP-ribose) polymerase inhibitors (PARPi) have emerged as a novel therapeutic strategy for SCLC. Preclinical studies have demonstrated that PARPi possesses cytotoxic activity as a single-agent and in combination with other anti-cancer agents. Predictive biomarkers of response to PARPi, such as SLFN11, have also been described in SCLC. This review aims to summarize the recent preclinical investigations and the relevant clinical trials that evaluate PARPi in SCLC. Here, we highlight the potential role of PARPi in a biomarker-selected manner and in combination with chemotherapy, targeted agents, radiotherapy and immunotherapy.

In Search of the Long-Desired 'Copernican Therapeutic Revolution' in Small-Cell Lung Cancer

Small-cell lung cancer has defied our scientific community for decades. Chemotherapy has been the mainstay treatment for small-cell lung cancer (SCLC) and unlike its counterpart, non-small cell lung cancer, no significant therapeutic breakthroughs have been made since the 1970s. Among the reasons for this slow-paced therapeutic development, one that stands out is the distinctive and almost universal loss of function of the tumour suppressor genes TP53 and RB1 in this disease, for which pharmacological activation has yet to be achieved, despite having been highly sought after. Although no molecularly targeted approach has been approved for clinical practice thus far, several strategies are currently exploring the potential to drug the tumour's "Achilles heel" that stems from essential pathways regulating DNA-damage response. Most recently, we have witnessed newfound reasons to hope, as the combination of immunotherapy and systemic chemotherapy has improved survival outcomes, representing the first landmark achievement in decades and a new standard of care for patients with extensive disease SCLC. However, continuous efforts are still needed towards a better understanding of the molecular pathways that singularise this tumour to eventually identify the predictive biomarkers that might result in the development of a more rational therapeutic approach, including the use of immunotherapy combinations. In this review we aim to uncover critical aspects of the immune microenvironment and biology of SCLC and provide an overview of the current and future landscape of promising therapeutic opportunities. The challenge still stands, but regardless, we are living in exciting times to finally check SCLC off the "bucket list" of our scientific community.

PARP Inhibitors: Clinical Relevance, Mechanisms of Action and Tumor Resistance

The Poly (ADP-ribose) polymerase (PARP) family has many essential functions in cellular processes, including the regulation of transcription, apoptosis and the DNA damage response. PARP1 possesses Poly (ADP-ribose) activity and when activated by DNA damage, adds branched PAR chains to facilitate the recruitment of other repair proteins to promote the repair of DNA single-strand breaks. PARP inhibitors (PARPi) were the first approved cancer drugs that specifically targeted the DNA damage response in BRCA1/2 mutated breast and ovarian cancers. Since then, there has been significant advances in our understanding of the mechanisms behind sensitization of tumors to PARP inhibitors and expansion of the use of PARPi to treat several other cancer types. Here, we review the recent advances in the proposed mechanisms of action of PARPi, biomarkers of the tumor response to PARPi, clinical advances in PARPi therapy, including the potential of combination therapies and mechanisms of tumor resistance.

PARP inhibitors combined with ionizing radiation induce different effects in melanoma cells and healthy fibroblasts

Background

PARP inhibitors niraparib and talazoparib are FDA approved for special cases of breast cancer. PARP is an interesting repair protein which is frequently affected in cancer cells. We studied the combined action of talazoparib or niraparib with ionizing radiation in melanoma cells and healthy fibroblasts.

Methods

Homologous recombination (HR) status in six different melanoma cell lines and healthy fibroblasts was assessed. Cell cultures were treated with PARP inhibitors talazoparib or niraparib and ionizing radiation (IR). Apoptosis, necrosis and cell cycle distribution was analyzed via flow cytometry. Cell migration was studied by scratch assays.

Results

Studied melanoma cell cultures are HR deficient. Studied healthy fibroblasts are HR proficient. Talazoparib and niraparib have congruent effects within the same cell cultures. In all cell cultures, combined treatment increases cell death and G2/M arrest compared to IR. Combined treatment in melanoma cells distinctly increases G2/M arrest. Healthy fibroblasts are less affected by G2/M arrest. Treatment predominantly decelerates or does not modify migration. In two cell cultures migration is enhanced under the inhibitors.

Conclusions

Although the two PARP inhibitors talazoparib and niraparib appear to be suitable for a combination treatment with ionizing radiation in our in vitro studies, a combination treatment cannot generally be recommended. There are clear interindividual differences in the effect of the inhibitors on different melanoma cells. Therefore, the effect on the cancer cells should be studied prior to a combination therapy. Since melanoma cells increase more strongly than fibroblasts in G2/M arrest, the fractional application of combined treatment should be further investigated.

Small Cell Lung Cancer from Traditional to Innovative Therapeutics: Building a Comprehensive Network to Optimize Clinical and Translational Research

Small cell lung cancer (SCLC) is an aggressive, complex disease with a distinct biology that contributes to its poor prognosis. Management of SCLC is still widely limited to chemotherapy and radiation therapy, and research recruitment still poses a considerable challenge. Here, we review the current standard of care for SCLC and advances made in utilizing immunotherapy. We also highlight research in the development of targeted therapies and emphasize the importance of a team-based approach to make clinical advances. Building an integrative network between an academic site and community practice sites optimizes biomarker and drug target discovery for managing and treating a difficult disease like SCLC.

Differential therapeutic effects of PARP and ATR inhibition combined with radiotherapy in the treatment of subcutaneous versus orthotopic lung tumour models

Background

Subcutaneous mouse tumour models are widely used for the screening of novel antitumour treatments, although these models are poor surrogate models of human cancers.

Methods

We compared the antitumour efficacy of the combination of ionising radiation (IR) with two DNA damage response inhibitors, the PARP inhibitor olaparib and the ATR inhibitor AZD6738 (ceralasertib), in subcutaneous versus orthotopic cancer models.

Results

Olaparib delayed the growth of irradiated Lewis lung carcinoma (LL2) subcutaneous tumours, in agreement with previous reports in human cell lines. However, the olaparib plus IR combination showed a very narrow therapeutic window against LL2 lung orthotopic tumours, with nearly no additional antitumour effect compared with that of IR alone, and tolerability issues emerged at high doses. The addition of AZD6738 greatly enhanced the efficacy of the olaparib plus IR combination treatment against subcutaneous but not orthotopic LL2 tumours. Moreover, olaparib plus AZD6738 administration concomitant with IR even worsened the response to radiation of head and neck orthotopic tumours and induced mucositis.

Conclusions

These major differences in the responses to treatments between subcutaneous and orthotopic models highlight the importance of using more pathologically relevant models, such as syngeneic orthotopic models, to determine the most appropriate therapeutic approaches for translation to the clinic.

Targeted Therapies and Biomarkers in Small Cell Lung Cancer

Small cell lung cancer (SCLC) is an aggressive malignancy characterized by rapid growth, early metastasis, and acquired therapeutic resistance. A majority of patients with SCLC have extensive-stage (ES) disease, defined as the presence of metastatic disease outside the hemithorax at first diagnosis. SCLC has been considered “a graveyard for drug development,” with chemotherapy remaining the standard treatment for first- and second-line management until quite recently. In contrast to NSCLC, identifying therapeutic targets in SCLC has been challenging, partly because driver mutations are primarily loss of function, involving the tumor suppressor genes RB1 and TP53 or currently untargetable (e.g., amplification of MYC family members). Recent gene expression profiling of SCLC cells lines, patient samples and representative murine models, have led to a proposed delineation of four major subtypes for SCLC distinguished by differential expression of four key transcriptional regulators (ASCL1, NEUROD1, POU2F3, and YAP1). Our understanding of the biology of SCLC has indeed significantly improved recently due to the continued efforts of the dedicated investigators in this field, but the therapeutic options remain dismal. While recent results from immunotherapy trials are encouraging, most patients demonstrate either primary or rapid acquired resistance to current regimens, highlighting the clear need to improve the effectiveness and expand the scope of current therapeutic strategies. In this opinion article, we will discuss recent developments in the treatment of SCLC, focused on current understanding of the signaling pathways, the role of immunotherapy and targeted therapy, and emerging biomarkers of response to therapy in SCLC.

A novel lncRNA PLK4 up-regulated by talazoparib represses hepatocellular carcinoma progression by promoting YAP-mediated cell senescence

A growing number of studies recognize that long non‐coding RNAs (lncRNAs) are essential to mediate multiple tumorigenic processes, including hepatic tumorigenesis. However, the pathological mechanism of lncRNA‐regulated liver cancer cell growth remains poorly understood. In this study, we identified a novel function lncRNA, named polo‐like kinase 4 associated lncRNA (lncRNA PLK4, GenBank Accession No. RP11‐50D9.3), whose expression was dramatically down‐regulated in hepatocellular carcinoma (HCC) tissues and cells. Interestingly, talazoparib, a novel and highly potent poly‐ADP‐ribose polymerase 1/2 (PARP1/2) inhibitor, could increase lncRNA PLK4 expression in HepG2 cells. Importantly, we showed that talazoparib‐induced lncRNA PLK4 could function as a tumour suppressor gene by Yes‐associated protein (YAP) inactivation and induction of cellular senescence to inhibit liver cancer cell viability and growth. In summary, our findings reveal the molecular mechanism of talazoparib‐induced anti‐tumor effect, and suggest a potential clinical use of talazoparib‐targeted lncRNA PLK4/YAP‐dependent cellular senescence for the treatment of HCC.

New Approaches to SCLC Therapy: From the Laboratory to the Clinic

The outcomes of patients with SCLC have not yet been substantially impacted by the revolution in precision oncology, primarily owing to a paucity of genetic alterations in actionable driver oncogenes. Nevertheless, systemic therapies that include immunotherapy are beginning to show promise in the clinic. Although, these results are encouraging, many patients do not respond to, or rapidly recur after, current regimens, necessitating alternative or complementary therapeutic strategies. In this review, we discuss ongoing investigations into the pathobiology of this recalcitrant cancer and the therapeutic vulnerabilities that are exposed by the disease state. Included within this discussion, is a snapshot of the current biomarker and clinical trial landscapes for SCLC. Finally, we identify key knowledge gaps that should be addressed to advance the field in pursuit of reduced SCLC mortality. This review largely summarizes work presented at the Third Biennial International Association for the Study of Lung Cancer SCLC Meeting.

Pre-clinical models of small cell lung cancer and the validation of therapeutic targets

Introduction: Small-cell lung cancer (SCLC) is an aggressive form of lung cancer that has a dismal prognosis. One of the factors hindering therapeutic developments for SCLC is that most SCLC is not surgically resected resulting in a paucity of material for analysis. To address this, significant efforts have been made by investigators to develop pre-clinical models of SCLC allowing for downstream target identification in this difficult to treat cancer.Areas covered: In this review, we describe the current pre-clinical models that have been developed to interrogate SCLC, and outline the benefits and limitations associated with each. Using examples we show how each has been used to (i) improve our knowledge of this intractable cancer, and (ii) identify and validate potential therapeutic targets that (iii) are currently under development and testing within the clinic.Expert opinion: The large numbers of preclinical models that have been developed have dramatically improved the ways in which we can examine SCLC and test therapeutic targets/interventions. The newer models are rapidly providing novel avenues for the design and testing of new therapeutics. Despite this many of these models have inherent flaws that limit the possibility of their use for individualized therapy decision-making for SCLC.

PARP and PARG inhibitors in cancer treatment

In this review, Slade provides an overview of the molecular mechanisms and cellular consequences of PARP and PARG inhibition. The author also highlights the clinical performance of four PARP inhibitors used in cancer therapy (olaparib, rucaparib, niraparib, and talazoparib) and discusses the predictive biomarkers of inhibitor sensitivity and mechanisms of resistance as well as the means of overcoming them through combination therapy.

Oxidative and replication stress underlie genomic instability of cancer cells. Amplifying genomic instability through radiotherapy and chemotherapy has been a powerful but nonselective means of killing cancer cells. Precision medicine has revolutionized cancer therapy by putting forth the concept of selective targeting of cancer cells. Poly(ADP-ribose) polymerase (PARP) inhibitors represent a successful example of precision medicine as the first drugs targeting DNA damage response to have entered the clinic. PARP inhibitors act through synthetic lethality with mutations in DNA repair genes and were approved for the treatment of BRCA mutated ovarian and breast cancer. PARP inhibitors destabilize replication forks through PARP DNA entrapment and induce cell death through replication stress-induced mitotic catastrophe. Inhibitors of poly(ADP-ribose) glycohydrolase (PARG) exploit and exacerbate replication deficiencies of cancer cells and may complement PARP inhibitors in targeting a broad range of cancer types with different sources of genomic instability. Here I provide an overview of the molecular mechanisms and cellular consequences of PARP and PARG inhibition. I highlight clinical performance of four PARP inhibitors used in cancer therapy (olaparib, rucaparib, niraparib, and talazoparib) and discuss the predictive biomarkers of inhibitor sensitivity, mechanisms of resistance as well as the means of overcoming them through combination therapy.