A first-in-man phase 1 study of the DNA-dependent protein kinase inhibitor peposertib (formerly M3814) in patients with advanced solid tumours

Shaping DNA damage responses: Therapeutic potential of targeting telomeric proteins and DNA repair factors in cancer

Shelterin proteins regulate genomic stability by preventing inappropriate DNA damage responses (DDRs) at telomeres. Unprotected telomeres lead to persistent DDR causing cell cycle inhibition, growth arrest, and apoptosis. Cancer cells rely on DDR to protect themselves from DNA lesions and exogenous DNA-damaging agents such as chemotherapy and radiotherapy. Therefore, targeting DDR machinery is a promising strategy to increase the sensitivity of cancer cells to existing cancer therapies. However, the success of these DDR inhibitors depends on other mutations, and over time, patients develop resistance to these therapies. This suggests the need for alternative approaches. One promising strategy is co-inhibiting shelterin proteins with DDR molecules, which would offset cellular fitness in DNA repair in a mutation-independent manner. This review highlights the associations and dependencies of the shelterin complex with the DDR proteins and discusses potential co-inhibition strategies that might improve the therapeutic potential of current inhibitors.

Differential Distribution of the DNA-PKcs Inhibitor Peposertib Selectively Radiosensitizes Patient-derived Melanoma Brain Metastasis Xenografts

Radioresistance of melanoma brain metastases limits the clinical utility of conventionally fractionated brain radiation in this disease, and strategies to improve radiation response could have significant clinical impact. The catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) is critical for repair of radiation-induced DNA damage, and inhibitors of this kinase can have potent effects on radiation sensitivity. In this study, the radiosensitizing effects of the DNA-PKcs inhibitor peposertib were evaluated in patient-derived xenografts of melanoma brain metastases (M12, M15, M27). In clonogenic survival assays, peposertib augmented radiation-induced killing of M12 cells at concentrations ≥100 nmol/L, and a minimum of 16 hours exposure allowed maximal sensitization. This information was integrated with pharmacokinetic modeling to define an optimal dosing regimen for peposertib of 125 mpk dosed just prior to and 7 hours after irradiation. Using this drug dosing regimen in combination with 2.5 Gy × 5 fractions of radiation, significant prolongation in median survival was observed in M12-eGFP (104%; P = 0.0015) and M15 (50%; P = 0.03), while more limited effects were seen in M27 (16%, P = 0.04). These data support the concept of developing peposertib as a radiosensitizer for brain metastases and provide a paradigm for integrating in vitro and pharmacokinetic data to define an optimal radiosensitizing regimen for potent DNA repair inhibitors.

A Phase 1 Study of the DNA-PK Inhibitor Peposertib in Combination With Radiation Therapy With or Without Cisplatin in Patients With Advanced Head and Neck Tumors

PURPOSE

DNA-dependent protein kinase (DNA-PK) plays a key role in the repair of DNA double strand breaks via nonhomologous end joining. Inhibition of DNA-PK can enhance the effect of DNA double strand break inducing anticancer therapies. Peposertib (formerly "M3814") is an orally administered, potent, and selective small molecule DNA-PK inhibitor that has demonstrated radiosensitizing and antitumor activity in xenograft models and was well-tolerated in monotherapy. This phase 1 trial (National Clinical Trial 02516813) investigated the maximum tolerated dose, recommended phase 2 dose (RP2D), safety, and tolerability of peposertib in combination with palliative radiation therapy (RT) in patients with thoracic or head and neck tumors (arm A) and of peposertib in combination with cisplatin and curative-intent RT in patients with squamous cell carcinoma of the head and neck (arm B).

METHODS AND MATERIALS

Patients received peposertib once daily in ascending dose cohorts as a tablet or capsule in combination with palliative RT (arm A) or in combination with intensity modulated curative-intent RT and cisplatin (arm B).

RESULTS

The most frequently observed treatment-emergent adverse events were radiation skin injury, fatigue, and nausea in arm A (n = 34) and stomatitis, nausea, radiation skin injury, and dysgeusia in arm B (n = 11). Based on evaluations of dose-limiting toxicities, tolerability, and pharmacokinetic data, RP2D for arm A was declared as 200 mg peposertib tablet once daily in combination with RT. In arm B (n = 11), 50 mg peposertib was declared tolerable in combination with curative-intent RT and cisplatin. However, enrollment was discontinued because of insufficient exposure at that dose, and the RP2D was not formally declared.

CONCLUSIONS

Peposertib in combination with palliative RT was well-tolerated up to doses of 200 mg once daily as tablet with each RT fraction. When combined with RT and cisplatin, a tolerable peposertib dose yielded insufficient exposure.

Concomitant inhibition of the thioredoxin system and nonhomologous DNA repair potently sensitizes Philadelphia-positive lymphoid leukemia to tyrosine kinase inhibitors

Breakpoint cluster region-Abelson (BCR::ABL1) gene fusion is an essential oncogene in both chronic myeloid leukemia (CML) and Philadelphia-positive (Ph+) B-cell acute lymphoblastic leukemia (B-ALL). While tyrosine kinase inhibitors (TKIs) are effective in up to 95% of CML patients, 50% of Ph+ B-ALL cases do not respond to treatment or relapse. This calls for new therapeutic approaches for Ph+ B-ALL. Previous studies have shown that inhibitors of the thioredoxin (TXN) system exert antileukemic activity against B-ALL cells, particularly in combination with other drugs. Here, we present that peroxiredoxin-1 (PRDX1), one of the enzymes of the TXN system, is upregulated in Ph+ lymphoid as compared to Ph+ myeloid cells. PRDX1 knockout negatively affects the viability of Ph+ B-ALL cells and sensitizes them to TKIs. Analysis of global gene expression changes in imatinib-treated, PRDX1-deficient cells revealed that the nonhomologous end-joining (NHEJ) DNA repair is a novel vulnerability of Ph+ B-ALL cells. Accordingly, PRDX1-deficient Ph+ B-ALL cells were susceptible to NHEJ inhibitors. Finally, we demonstrated the potent efficacy of a novel combination of TKIs, TXN inhibitors, and NHEJ inhibitors against Ph+ B-ALL cell lines and primary cells, which can be further investigated as a potential therapeutic approach for the treatment of Ph+ B-ALL.

A Phase Ib Study of the DNA-PK Inhibitor Peposertib Combined with Neoadjuvant Chemoradiation in Patients with Locally Advanced Rectal Cancer

PURPOSE

Peposertib-an orally administered DNA-dependent protein kinase inhibitor-has shown potent radiosensitization in preclinical models. This dose-escalation study (NCT03770689) aimed to define the maximum tolerated dose (MTD) and recommended phase II dose (RP2D) of peposertib plus capecitabine-based chemoradiotherapy (CRT) and assessed its safety and efficacy in locally advanced rectal cancer.

PATIENTS AND METHODS

Patients were treated for 5 to 5.5 weeks with 50- to 250-mg peposertib once daily, capecitabine 825 mg/m2 twice daily, and radiotherapy (RT), 5 days per week. Following clinical restaging (8 weeks after CRT completion), patients with clinical complete response (cCR) could opt for surveillance. Total mesorectal excision was recommended upon incomplete response (IR).

RESULTS

Nineteen patients were treated with peposertib at doses of 50 mg (n = 1), 100 mg, 150 mg, and 250 mg (n = 6 each). Dose-limiting toxicities occurred in one out of five (100 mg), one out of six (150 mg), and three out of six (250 mg) evaluable patients. Peposertib ≤150 mg once daily was tolerable in combination with CRT. After 8 weeks of treatment with peposertib and CRT, the cCR was 15.8% (n = 3). Among the three patients with cCR, two underwent surgery and had residual tumors. Among the 16 patients with IR, seven underwent surgery and had residual tumors; five of the remaining nine patients opted for consolidative chemotherapy. The combined cCR/pathologic complete response (pCR) rate was 5.3% (n = 1, 100 mg cohort).

CONCLUSIONS

Peposertib did not improve complete response rates at tolerable dose levels. The study was closed without declaring the MTD/RP2D.

Precise CRISPR-Cas9 gene repair in autologous memory T cells to treat familial hemophagocytic lymphohistiocytosis

Familial hemophagocytic lymphohistiocytosis (FHL) is an inherited, often fatal immune deficiency characterized by severe systemic hyperinflammation. Although allogeneic bone marrow transplantation can be curative, more effective therapies are urgently needed. FHL is caused by inactivating mutations in proteins that regulate cellular immunity. Here, we used an adeno-associated virus-based CRISPR-Cas9 system with an inhibitor of nonhomologous end joining to repair such mutations in potentially long-lived T cells ex vivo. Repaired CD8 memory T cells efficiently cured lethal hyperinflammation in a mouse model of Epstein-Barr virus-triggered FHL2, a subtype caused by perforin-1 (Prf1) deficiency. Furthermore, repair of PRF1 and Munc13-4 (UNC13D)-whose deficiency causes the FHL subtype FHL3-in mutant memory T cells from two critically ill patients with FHL restored T cell cytotoxicity. These results provide a starting point for the treatment of genetic T cell immune dysregulation syndromes with repaired autologous T cells.

Phase I study of peposertib and avelumab with or without palliative radiotherapy in patients with advanced solid tumors

INTRODUCTION

We report results from a phase I, three-part, dose-escalation study of peposertib, a DNA-dependent protein kinase inhibitor, in combination with avelumab, an immune checkpoint inhibitor, with or without radiotherapy in patients with advanced solid tumors.

MATERIALS AND METHODS

Peposertib 100-400 mg twice daily (b.i.d.) or 100-250 mg once daily (q.d.) was administered in combination with avelumab 800 mg every 2 weeks in Part A or avelumab plus radiotherapy (3 Gy/fraction × 10 days) in Part B. Part FE assessed the effect of food on the pharmacokinetics of peposertib plus avelumab. The primary endpoint in Parts A and B was dose-limiting toxicity (DLT). Secondary endpoints were safety, best overall response per RECIST version 1.1, and pharmacokinetics. The recommended phase II dose (RP2D) and maximum tolerated dose (MTD) were determined in Parts A and B.

RESULTS

In Part A, peposertib doses administered were 100 mg (n = 4), 200 mg (n = 11), 250 mg (n = 4), 300 mg (n = 6), and 400 mg (n = 4) b.i.d. Of DLT-evaluable patients, one each had DLT at the 250-mg and 300-mg dose levels and three had DLT at the 400-mg b.i.d. dose level. In Part B, peposertib doses administered were 100 mg (n = 3), 150 mg (n = 3), 200 mg (n = 4), and 250 mg (n = 9) q.d.; no DLT was reported in evaluable patients. Peposertib 200 mg b.i.d. plus avelumab and peposertib 250 mg q.d. plus avelumab and radiotherapy were declared as the RP2D/MTD. No objective responses were observed in Part A or B; one patient had a partial response in Part FE. Peposertib exposure was generally dose proportional.

CONCLUSIONS

Peposertib doses up to 200 mg b.i.d. in combination with avelumab and up to 250 mg q.d. in combination with avelumab and radiotherapy were tolerable in patients with advanced solid tumors; however, antitumor activity was limited.

CLINICALTRIALS

GOV IDENTIFIER

NCT03724890.

Targeting the DNA damage response in hematological malignancies

Deregulation of the DNA damage response (DDR) plays a critical role in the pathogenesis and progression of many cancers. The dependency of certain cancers on DDR pathways has enabled exploitation of such through synthetically lethal relationships e.g., Poly ADP-Ribose Polymerase (PARP) inhibitors for BRCA deficient ovarian cancers. Though lagging behind that of solid cancers, DDR inhibitors (DDRi) are being clinically developed for haematological cancers. Furthermore, a high proliferative index characterize many such cancers, suggesting a rationale for combinatorial strategies targeting DDR and replicative stress. In this review, we summarize pre-clinical and clinical data on DDR inhibition in haematological malignancies and highlight distinct haematological cancer subtypes with activity of DDR agents as single agents or in combination with chemotherapeutics and targeted agents. We aim to provide a framework to guide the design of future clinical trials involving haematological cancers for this important class of drugs.

Discovery, Optimization, and Evaluation of Potent and Selective DNA-PK Inhibitors in Combination with Chemotherapy or Radiotherapy for the Treatment of Malignancies

Given the multifaceted biological functions of DNA-PK encompassing DNA repair pathways and beyond, coupled with the susceptibility of DNA-PK-deficient cells to DNA-damaging agents, significant strides have been made in the pursuit of clinical potential for DNA-PK inhibitors as synergistic adjuncts to chemo- or radiotherapy. Nevertheless, although substantial progress has been made with the discovery of potent inhibitors of DNA-PK, the clinical trial landscape requires even more potent and selective molecules. This necessitates further endeavors to expand the repertoire of clinically accessible DNA-PK inhibitors for the ultimate benefit of patients. Described herein are the obstacles that were encountered and the solutions that were found, which eventually led to the identification of compound 31t. This compound exhibited a remarkable combination of robust potency and exceptional selectivity along with favorable in vivo profiles as substantiated by pharmacokinetic studies in rats and pharmacodynamic assessments in H460, BT474, and A549 xenograft models.

Hyper-Dependence on NHEJ Enables Synergy between DNA-PK Inhibitors and Low-Dose Doxorubicin in Leiomyosarcoma

PURPOSE

Leiomyosarcoma (LMS) is an aggressive sarcoma for which standard chemotherapies achieve response rates under 30%. There are no effective targeted therapies against LMS. Most LMS are characterized by chromosomal instability (CIN), resulting in part from TP53 and RB1 co-inactivation and DNA damage repair defects. We sought to identify therapeutic targets that could exacerbate intrinsic CIN and DNA damage in LMS, inducing lethal genotoxicity.

EXPERIMENTAL DESIGN

We performed clinical targeted sequencing in 287 LMS and genome-wide loss-of-function screens in 3 patient-derived LMS cell lines, to identify LMS-specific dependencies. We validated candidate targets by biochemical and cell-response assays in vitro and in seven mouse models.

RESULTS

Clinical targeted sequencing revealed a high burden of somatic copy-number alterations (median fraction of the genome altered =0.62) and demonstrated homologous recombination deficiency signatures in 35% of LMS. Genome-wide short hairpin RNA screens demonstrated PRKDC (DNA-PKcs) and RPA2 essentiality, consistent with compensatory nonhomologous end joining (NHEJ) hyper-dependence. DNA-PK inhibitor combinations with unconventionally low-dose doxorubicin had synergistic activity in LMS in vitro models. Combination therapy with peposertib and low-dose doxorubicin (standard or liposomal formulations) inhibited growth of 5 of 7 LMS mouse models without toxicity.

CONCLUSIONS

Combinations of DNA-PK inhibitors with unconventionally low, sensitizing, doxorubicin dosing showed synergistic effects in LMS in vitro and in vivo models, without discernable toxicity. These findings underscore the relevance of DNA damage repair alterations in LMS pathogenesis and identify dependence on NHEJ as a clinically actionable vulnerability in LMS.

Mapping combinatorial drug effects to DNA damage response kinase inhibitors

One fundamental principle that underlies various cancer treatments, such as traditional chemotherapy and radiotherapy, involves the induction of catastrophic DNA damage, leading to the apoptosis of cancer cells. In our study, we conduct a comprehensive dose-response combination screening focused on inhibitors that target key kinases involved in the DNA damage response (DDR): ATR, ATM, and DNA-PK. This screening involves 87 anti-cancer agents, including six DDR inhibitors, and encompasses 62 different cell lines spanning 12 types of tumors, resulting in a total of 17,912 combination treatment experiments. Within these combinations, we analyze the most effective and synergistic drug pairs across all tested cell lines, considering the variations among cancers originating from different tissues. Our analysis reveals inhibitors of five DDR-related pathways (DNA topoisomerase, PLK1 kinase, p53-inducible ribonucleotide reductase, PARP, and cell cycle checkpoint proteins) that exhibit strong combinatorial efficacy and synergy when used alongside ATM/ATR/DNA-PK inhibitors.

Phase I crossover study of DNA-protein kinase inhibitor peposertib in healthy volunteers: Effect of food and pharmacokinetics of an oral suspension

Peposertib is an orally administered inhibitor of DNA-dependent protein kinase. We evaluated the effect of food on its pharmacokinetics, and examined the pharmacokinetics of an oral suspension (OS) of disintegrated tablets, in a phase I, open-label, crossover three-period study (NCT04702698). Twelve healthy volunteers were randomized to one of six treatment sequences. They received a single dose of peposertib 100 mg as film-coated tablets under fasted or fed conditions ("tablet fasted" or "tablet fed") or as an OS under fasted conditions ("OS fasted"), with washout between treatments. Using healthy volunteers was possible because, despite its mechanism of action being suppression of DNA repair, peposertib has shown no genotoxic effect in animals. A mild food effect was observed with peposertib tablets. Fed-to-fasted ratios were: area under the curve from time 0 to time t (AUC0-t ), 123.81% (90% confidence interval [CI]: 108.04, 141.87%); AUC from zero to infinity (AUC0-∞ ), 110.28% (90% CI 100.71, 120.77%); and maximum concentration (Cmax ) 104.47% (90% CI: 79.15, 137.90%). Cmax was delayed under fed conditions (median time to maximum concentration [Tmax ] was 3.5 h [tablet fed] vs. 1 h [tablet fasted]). OS-to-tablet (fasted) ratios were: AUC0-t , 124.83% (90% CI: 111.50%, 139.76%); AUC0-∞ , 119.05% (90% CI: 104.47, 135.67%); and Cmax 173.29% (90% CI: 135.78, 221.16%). Median Tmax was 0.5 h (OS fasted) versus 1 h (tablet). All treatments were well-tolerated in healthy volunteers. Peposertib tablets can be taken with or without food; if combined with chemotherapy or radiotherapy, the delay in Cmax must be considered to optimize the chemo- or radiosensitizing effect. The peposertib OS form represents an alternative route of administration in patients with specific cancers causing dysphagia. However, the OS form should be part of future dose optimization strategies in relevant settings.

Advances in molecular targeted therapies to increase efficacy of (chemo)radiation therapy

Recent advances in understanding the tumor's biology in line with a constantly growing number of innovative technologies have prompted characterization of patients' individual malignancies and may display a prerequisite to treat cancer at its patient individual tumor vulnerability. In recent decades, radiation- induced signaling and tumor promoting local events for radiation sensitization were explored in detail, resulting the development of novel molecular targets. A multitude of pharmacological, genetic, and immunological principles, including small molecule- and antibody-based targeted strategies, have been developed that are suitable for combined concepts with radiation (RT) or chemoradiation therapy (CRT). Despite a plethora of promising experimental and preclinical findings, however, so far, only a very limited number of clinical trials have demonstrated a better outcome and/or patient benefit when RT or CRT are combined with targeted agents. The current review aims to summarize recent progress in molecular therapies targeting oncogenic drivers, DNA damage and cell cycle response, apoptosis signaling pathways, cell adhesion molecules, hypoxia, and the tumor microenvironment to impact therapy refractoriness and to boost radiation response. In addition, we will discuss recent advances in nanotechnology, e.g., RNA technologies and protein-degrading proteolysis-targeting chimeras (PROTACs) that may open new and innovative ways to benefit from molecular-targeted therapy approaches with improved efficacy.

Inhibitors of phosphoinositide 3-kinase (PI3K) and phosphoinositide 3-kinase-related protein kinase family (PIKK)

Phosphoinositide 3-kinases (PI3K) and phosphoinositide 3-kinase-related protein kinases (PIKK) are two structurally related families of kinases that play vital roles in cell growth and DNA damage repair. Dysfunction of PIKK members and aberrant stimulation of the PI3K/AKT/mTOR signalling pathway are linked to a plethora of diseases including cancer. In recent decades, numerous inhibitors related to the PI3K/AKT/mTOR signalling have made great strides in cancer treatment, like copanlisib and sirolimus. Notably, most of the PIKK inhibitors (such as VX-970 and M3814) related to DNA damage response have also shown good efficacy in clinical trials. However, these drugs still require a suitable combination therapy to overcome drug resistance or improve antitumor activity. Based on the aforementioned facts, we summarised the efficacy of PIKK, PI3K, and AKT inhibitors in the therapy of human malignancies and the resistance mechanisms of targeted therapy, in order to provide deeper insights into cancer treatment.

Molecular classifications of prostate cancer: basis for individualized risk stratification and precision therapy

Tumour classifications play a pivotal role in prostate cancer (PCa) management. It can predict the clinical outcomes of PCa as early as the disease is diagnosed and then guide therapeutic schemes, such as active monitoring, standalone surgical intervention, or surgery supplemented with postoperative adjunctive therapy, thereby circumventing disease exacerbation and excessive treatment. Classifications based on clinicopathological features, such as prostate cancer-specific antigen, Gleason score, and TNM stage, are still the main risk stratification strategies and have played an essential role in standardized clinical decision-making. However, mounting evidence indicates that clinicopathological parameters in isolation fail to adequately capture the heterogeneity exhibited among distinct PCa patients, such as those sharing identical Gleason scores yet experiencing divergent prognoses. As a remedy, molecular classifications have been introduced. Currently, molecular studies have revealed the characteristic genomic alterations, epigenetic modulations, and tumour microenvironment associated with different types of PCa, which provide a chance for urologists to refine the PCa classification. In this context, numerous invaluable molecular classifications have been devised, employing disparate statistical methodologies and algorithmic approaches, encompassing self-organizing map clustering, unsupervised cluster analysis, and multifarious algorithms. Interestingly, the classifier PAM50 was used in a phase-2 multicentre open-label trial, NRG-GU-006, for further validation, which hints at the promise of molecular classification for clinical use. Consequently, this review examines the extant molecular classifications, delineates the prevailing panorama of clinically pertinent molecular signatures, and delves into eight emblematic molecular classifications, dissecting their methodological underpinnings and clinical utility.

A new wave of innovations within the DNA damage response

Genome instability has been identified as one of the enabling hallmarks in cancer. DNA damage response (DDR) network is responsible for maintenance of genome integrity in cells. As cancer cells frequently carry DDR gene deficiencies or suffer from replicative stress, targeting DDR processes could induce excessive DNA damages (or unrepaired DNA) that eventually lead to cell death. Poly (ADP-ribose) polymerase (PARP) inhibitors have brought impressive benefit to patients with breast cancer gene (BRCA) mutation or homologous recombination deficiency (HRD), which proves the concept of synthetic lethality in cancer treatment. Moreover, the other two scenarios of DDR inhibitor application, replication stress and combination with chemo- or radio- therapy, are under active clinical exploration. In this review, we revisited the progress of DDR targeting therapy beyond the launched first-generation PARP inhibitors. Next generation PARP1 selective inhibitors, which could maintain the efficacy while mitigating side effects, may diversify the application scenarios of PARP inhibitor in clinic. Albeit with unavoidable on-mechanism toxicities, several small molecules targeting DNA damage checkpoints (gatekeepers) have shown great promise in preliminary clinical results, which may warrant further evaluations. In addition, inhibitors for other DNA repair pathways (caretakers) are also under active preclinical or clinical development. With these progresses and efforts, we envision that a new wave of innovations within DDR has come of age.

Inhibition of non-homologous end joining mitigates paclitaxel resistance resulting from mitotic slippage in non-small cell lung cancer

Mitotic slippage, which enables cancer cells to bypass cell death by transitioning from mitosis to the G1 phase without undergoing normal cytokinesis, is one likely mechanism of paclitaxel (PTX) resistance. DNA double-strand breaks (DSBs) in the G1 phase are mainly repaired through non-homologous end joining (NHEJ). Therefore, inhibiting NHEJ could augment the PTX-induced cytotoxicity by impeding the repair of PTX-induced DSBs during the G1 phase following mitotic slippage. We aimed to evaluate the effects of NHEJ inhibition on mitotic slippage after PTX treatment in non-small cell lung cancer (NSCLC). H1299, A549, H1975, and H520 NSCLC cell lines were employed. In addition, A-196 and JQ1 were used as NHEJ inhibitors. H1299 cells were PTX-resistant and exhibited an increased frequency of mitotic slippage upon PTX treatment. NHEJ inhibitors significantly augmented the PTX-induced cytotoxicity, DSBs, and apoptosis in H1299 cells. The newly generated PTX-resistant cells were even more prone to mitotic slippage following PTX treatment and susceptible to the combined therapy. Docetaxel further demonstrated synergistic effects with the NHEJ inhibitor in PTX-resistant cells. NHEJ inhibition may overcome intrinsic or acquired PTX resistance resulting from mitotic slippage by synergistically increasing the cytotoxic effects of antimitotic drugs in NSCLC.

A genome-wide CRISPR/Cas9 screen identifies DNA-PK as a sensitiser to 177Lutetium-DOTA-octreotate radionuclide therapy

Peptide receptor radionuclide therapy (PRRT) using 177Lutetium-DOTA-octreotate (LuTate) for neuroendocrine tumours (NET) is now an approved treatment available in many countries, though primary or secondary resistance continue to limit its effectiveness or durability. We hypothesised that a genome-wide CRISPR/Cas9 screen would identify key mediators of response to LuTate and gene targets that might offer opportunities for novel combination therapies for NET patients. Methods: We utilised a genome-wide CRISPR-Cas9 screen in LuTate-treated cells to identify genes that impact on the sensitivity or resistance of cells to LuTate. Hits were validated through single-gene knockout. LuTate-resistant cells were assessed to confirm LuTate uptake and retention, and persistence of somatostatin receptor 2 (SSTR2) expression. Gene knockouts conferring LuTate sensitivity were further characterised by pharmacological sensitisation using specific inhibitors and in vivo analysis of the efficacy of these inhibitors in combination with LuTate. Results: The CRISPR-Cas9 screen identified several potential targets for both resistance and sensitivity to PRRT. Two gene knockouts which conferred LuTate resistance in vitro, ARRB2 and MVP, have potential mechanisms related to LuTate binding and retention, and modulation of DNA-damage repair (DDR) pathways, respectively. The screen showed that sensitivity to LuTate treatment in vitro can be conferred by the loss of a variety of genes involved in DDR pathways, with loss of genes involved in Non-Homologous End-Joining (NHEJ) being the most lethal. Loss of the key NHEJ gene, PRKDC (DNA-PK), either by gene loss or inhibition by two different inhibitors, resulted in significantly reduced cell survival upon exposure of cells to LuTate. In SSTR2-positive xenograft-bearing mice, the combination of nedisertib (a DNA-PK specific inhibitor) and LuTate produced a more robust control of tumour growth and increased survival compared to LuTate alone. Conclusions: DDR pathways are critical for sensing and repairing radiation-induced DNA damage, and our study shows that regulation of DDR pathways may be involved in both resistance and sensitivity to PRRT. Additionally, the use of a DNA-PK inhibitor in combination with LuTate PRRT significantly improves the efficacy of the treatment in pre-clinical models, providing further evidence for the clinical efficacy of this combination.

Chemical inhibition of DNA-PKcs impairs the activation and cytotoxicity of CD4+ helper and CD8+ effector T cells

Modulation of T cell activity is an effective strategy for the treatment of autoimmune diseases, immune-related disorders and cancer. This highlights a critical need for the identification of proteins that regulate T cell function. The kinase DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is emerging as a potent regulator of the immune system, spurring interest in its use as a therapeutic target. In murine models of immune-related diseases including asthma and rheumatoid arthritis, treatment with small-molecule DNA-PKcs inhibitors decreased the disease severity. Additionally, DNA-PKcs inhibitors reduced T cell-mediated graft rejection in a murine allogenic skin graft model. These in vivo studies suggest the use of DNA-PKcs inhibitors as immunotherapy for autoimmune and T cell-mediated disorders. In this study, we sought to characterize further the effects of DNA-PKcs inhibitors on T cells to better understand their clinical potential. We determined that inhibition of DNA-PKcs using inhibitor NU7441 and the inhibitors currently in clinical trials for cancer therapy, M3184 and AZD7648, abrogated the activation of murine and human CD4+ and CD8+ T cells as evidenced by the reduced expression of the activation markers CD69 and CD25. Furthermore, inhibition of DNA-PKcs impeded metabolic pathways and the proliferation of activated T cells. This reduced the ability of OTI-CD8+ T cells to kill cancer cells and the expression of IFNγ and cytotoxic genes. These results highlight a critical role for DNA-PKcs in T cells and validate future studies using DNA-PKcs inhibitors as immune modulation therapy for the treatment of immune-related diseases.

Ribosome biosynthesis and Hedgehog activity are cooperative actionable signaling mechanisms in breast cancer following radiotherapy

Hyperactivated ribosome biosynthesis is attributed to a need for elevated protein synthesis that accommodates cell growth and division, and is characterized by nucleomorphometric alterations and increased nucleolar counts. Ribosome biogenesis is challenged when DNA-damaging treatments such as radiotherapy are utilized. Tumor cells that survive radiotherapy form the basis of recurrence, tumor progression, and metastasis. In order to survive and become metabolically revitalized, tumor cells need to reactivate RNA Polymerase I (RNA Pol I) to synthesize ribosomal RNA, an integral component of ribosomes. In this study, we showed that following radiation therapy, tumor cells from breast cancer patients demonstrate activation of a ribosome biosynthesis signature concurrent with enrichment of a signature of Hedgehog (Hh) activity. We hypothesized that GLI1 activates RNA Pol I in response to irradiation and licenses the emergence of a radioresistant tumor population. Our work establishes a novel role for GLI1 in orchestrating RNA Pol I activity in irradiated breast cancer cells. Furthermore, we present evidence that in these irradiated tumor cells, Treacle ribosome biogenesis factor 1 (TCOF1), a nucleolar protein that is important in ribosome biogenesis, facilitates nucleolar translocation of GLI1. Inhibiting Hh activity and RNA Pol I activity disabled the outgrowth of breast cancer cells in the lungs. As such, ribosome biosynthesis and Hh activity present as actionable signaling mechanisms to enhance the effectiveness of radiotherapy.

Paediatric Strategy Forum for medicinal product development of DNA damage response pathway inhibitors in children and adolescents with cancer: ACCELERATE in collaboration with the European Medicines Agency with participation of the Food and Drug Administration

DNA damage response inhibitors have a potentially important therapeutic role in paediatric cancers; however, their optimal use, including patient selection and combination strategy, remains unknown. Moreover, there is an imbalance between the number of drugs with diverse mechanisms of action and the limited number of paediatric patients available to be enrolled in early-phase trials, so prioritisation and a strategy are essential. While PARP inhibitors targeting homologous recombination-deficient tumours have been used primarily in the treatment of adult cancers with BRCA1/2 mutations, BRCA1/2 mutations occur infrequently in childhood tumours, and therefore, a specific response hypothesis is required. Combinations with targeted radiotherapy, ATR inhibitors, or antibody drug conjugates with DNA topoisomerase I inhibitor-related warheads warrant evaluation. Additional monotherapy trials of PARP inhibitors with the same mechanism of action are not recommended. PARP1-specific inhibitors and PARP inhibitors with very good central nervous system penetration also deserve evaluation. ATR, ATM, DNA-PK, CHK1, WEE1, DNA polymerase theta and PKMYT1 inhibitors are early in paediatric development. There should be an overall coordinated strategy for their development. Therefore, an academia/industry consensus of the relevant biomarkers will be established and a focused meeting on ATR inhibitors (as proof of principle) held. CHK1 inhibitors have demonstrated activity in desmoplastic small round cell tumours and have a potential role in the treatment of other paediatric malignancies, such as neuroblastoma and Ewing sarcoma. Access to CHK1 inhibitors for paediatric clinical trials is a high priority. The three key elements in evaluating these inhibitors in children are (1) innovative trial design (design driven by a clear hypothesis with the intent to further investigate responders and non-responders with detailed retrospective molecular analyses to generate a revised or new hypothesis); (2) biomarker selection and (3) rational combination therapy, which is limited by overlapping toxicity. To maximally benefit children with cancer, investigators should work collaboratively to learn the lessons from the past and apply them to future studies. Plans should be based on the relevant biology, with a focus on simultaneous and parallel research in preclinical and clinical settings, and an overall integrated and collaborative strategy.

Maintaining Genome Integrity: Protein Kinases and Phosphatases Orchestrate the Balancing Act of DNA Double-Strand Breaks Repair in Cancer

DNA double-strand breaks (DSBs) are the most lethal DNA damages which lead to severe genome instability. Phosphorylation is one of the most important protein post-translation modifications involved in DSBs repair regulation. Kinases and phosphatases play coordinating roles in DSB repair by phosphorylating and dephosphorylating various proteins. Recent research has shed light on the importance of maintaining a balance between kinase and phosphatase activities in DSB repair. The interplay between kinases and phosphatases plays an important role in regulating DNA-repair processes, and alterations in their activity can lead to genomic instability and disease. Therefore, study on the function of kinases and phosphatases in DSBs repair is essential for understanding their roles in cancer development and therapeutics. In this review, we summarize the current knowledge of kinases and phosphatases in DSBs repair regulation and highlight the advancements in the development of cancer therapies targeting kinases or phosphatases in DSBs repair pathways. In conclusion, understanding the balance of kinase and phosphatase activities in DSBs repair provides opportunities for the development of novel cancer therapeutics.

DNA Ligase 4 Contributes to Cell Proliferation against DNA-PK Inhibition in MYCN-Amplified Neuroblastoma IMR32 Cells

Identifying the vulnerability of altered DNA repair machinery that displays synthetic lethality with MYCN amplification is a therapeutic rationale in unfavourable neuroblastoma. However, none of the inhibitors for DNA repair proteins are established as standard therapy in neuroblastoma. Here, we investigated whether DNA-PK inhibitor (DNA-PKi) could inhibit the proliferation of spheroids derived from neuroblastomas of MYCN transgenic mice and MYCN-amplified neuroblastoma cell lines. DNA-PKi exhibited an inhibitory effect on the proliferation of MYCN-driven neuroblastoma spheroids, whereas variable sensitivity was observed in those cell lines. Among them, the accelerated proliferation of IMR32 cells was dependent on DNA ligase 4 (LIG4), which comprises the canonical non-homologous end-joining pathway of DNA repair. Notably, LIG4 was identified as one of the worst prognostic factors in patients with MYCN-amplified neuroblastomas. It may play complementary roles in DNA-PK deficiency, suggesting the therapeutic potential of LIG4 inhibition in combination with DNA-PKi for MYCN-amplified neuroblastomas to overcome resistance to multimodal therapy.

Targeting the DNA damage response for cancer therapy

The DNA damage response (DDR) is an elegant system, coordinating DNA repair with cell cycle checkpoints, that evolved to protect living organisms from the otherwise fatal levels of DNA damage inflicted by endogenous and environmental sources. Since many agents used to treat cancer; radiotherapy and cytotoxic chemotherapy, work by damaging DNA the DDR represents a mechanism of resistance. The original rational for the development of drugs to inhibit the DDR was to overcome this mechanism of resistance but clinical studies using this approach have not led to improvements in the therapeutic index. A more exciting approach is to exploit cancer-specific defects in the DDR, that represent vulnerabilities in the tumour and an opportunity to selectively target the tumour. PARP inhibitors (PARPi) selectively kill homologous recombination repair defective (HRD, e.g. through BRCA mutation) cells. This approach has proven successful clinically and there are now six PARPi approved for cancer therapy. Drugs targeting other aspects of the DDR are under pre-clinical and clinical evaluation as monotherapy agents and in combination studies. For this promising approach to cancer therapy to be fully realised reliable biomarkers are needed to identify tumours with the exploitable defect for monotherapy applications. The possibility that some combinations may result in toxicity to normal tissues also needs to be considered. A brief overview of the DDR, the development of inhibitors targeting the DDR and the current clinical status of such drugs is described here.

Targeting Polymerase Theta (POLθ) for Cancer Therapy

Polymerase theta (POLθ) is the critical multi-domain enzyme in microhomology-mediated end-joining DNA double-stranded break repair. POLθ is expressed at low levels in normal tissue but is often overexpressed in cancers, especially in DNA repair deficient cancers, such as homologous-recombination cancers, rendering them exquisitely sensitive to POLθ inhibition secondary to synthetic lethality. Development of POLθ inhibitors is an active area of investigation with inhibitors of the N-terminal helicase domain or the C-terminal polymerase domain currently in clinical trial. Here, we review POLθ-mediated microhomology-mediated end-joining, the development of POLθ inhibitors, and the potential clinical uses of POLθ inhibitors.

Targeting DNA-PK

This chapter explores the multifaceted roles of DNA-PK with particular focus on its functions in non-homologous end-joining (NHEJ) DNA repair. DNA-PK is the primary orchestrator of NHEJ but also regulates other biologic processes. The growing understanding of varied DNA-PK biologic roles highlights new avenues for cancer treatment. However, these multiple roles also imply challenges, particularly in combination therapies, with perhaps a higher risk of clinical toxicities than was previously envisioned. These considerations underscore the need for compelling and innovative strategies to accomplish effective clinical translation.

Exploiting DNA Replication Stress as a Therapeutic Strategy for Breast Cancer

Proliferating cells rely on DNA replication to ensure accurate genome duplication. Cancer cells, including breast cancer cells, exhibit elevated replication stress (RS) due to the uncontrolled oncogenic activation, loss of key tumor suppressors, and defects in the DNA repair machinery. This intrinsic vulnerability provides a great opportunity for therapeutic exploitation. An increasing number of drug candidates targeting RS in breast cancer are demonstrating promising efficacy in preclinical and early clinical trials. However, unresolved challenges lie in balancing the toxicity of these drugs while maintaining clinical efficacy. Furthermore, biomarkers of RS are urgently required to guide patient selection. In this review, we introduce the concept of targeting RS, detail the current therapies that target RS, and highlight the integration of RS with immunotherapies for breast cancer treatment. Additionally, we discuss the potential biomarkers to optimizing the efficacy of these therapies. Together, the continuous advances in our knowledge of targeting RS would benefit more patients with breast cancer.

Triple kill: DDR inhibitors, radiotherapy and immunotherapy leave cancer cells with no escape

Radiotherapy (RT) has been widely used in the clinical treatment of cancers, but radiotherapy resistance (RR) leads to RT failure, tumor recurrence and metastasis. Many studies have been performed on the potential mechanisms behind RR, and a strong link has been found between RR and DNA damage. RT-induced DNA damage triggers a protective mechanism called the DNA damage response (DDR). DDR consists of several aspects, including the detection of DNA damage and induction of cell cycle checkpoint, DNA repair, and eventual induction of cell death. A large number of studies have shown that DDR inhibition leads to significantly enhanced sensitivity of cancer cells to RT. DDR may be an effective target for radio- and chemo-sensitization during cancer treatment. Therefore, many inhibitors of important enzymes involved in the DDR have been developed, such as PARP inhibitors, DNA-PK inhibitors, and ATM/ATR inhibitors. In addition, DNA damage also triggers the cGAS-STING signaling pathway and the ATM/ATR (CHK)/STAT pathway to induce immune infiltration and T-cell activation. This review discusses the effects of DDR pathway dysregulation on the tumor response to RT and the strategies for targeting these pathways to increase tumor susceptibility to RT. Finally, the potential for the combination treatment of radiation, DDR inhibition, and immunotherapy is described.

Looking Under the Lamppost: The Search for New Cancer Targets in the Human Kinome

The number of cancer drugs is increasing as new chemical entities are developed to target molecules, often protein kinases, driving cancer progression. In 2009, Fedorov et al. identified that of the protein kinases in the human kinome, most of the focus has been on a small subset. They highlighted that many poorly investigated protein kinases were cancer drivers, but there was no relationship between publications and involvement in cancer development or progression. Since 2009, there has been a doubling in the number of publications, patents, and drugs targeting the kinome. To determine whether this was an expansion in knowledge of well-studied targets-searching in the light under the lamppost-or an explosion of investigations into previously poorly investigated targets, we searched the literature for publications on each kinase, updating Federov et al.'s assessment of the druggable kinome. The proportion of papers focusing on the 50 most-studied kinases had not changed, and the makeup of those 50 had barely changed. The majority of new drugs (80%) were against the same group of 50 kinases identified as targets 10 years ago, and the proportion of studies investigating previously poorly investigated kinases (<1%) was unchanged. With three exceptions [p38 mitogenactivated protein kinase (p38a), AMP-activated protein kinase catalytic α-subunit 1,2, and B-Raf proto-oncogene (BRAF) serine/threonine kinase], >95% of publications addressing kinases still focused on a relatively small proportion (<50%) of the human kinome independently of their involvement as cancer drivers. There is, therefore, still extensive scope for discovery of therapeutics targeting different protein kinases in cancer and still a bias toward well-characterized targets over the innovative searchlight into the unknown. SIGNIFICANCE STATEMENT: This study presents evidence that drug discovery efforts in cancer are still to some extent focused on a narrow group of well-studied kinases 10 years after the identification of multiple novel cancer targets in the human kinome. This suggests that there is still room for researchers in academia, industry, and the not-for-profit sector to develop new and diverse therapies targeting kinases for cancer.

Targeting the DNA Damage Response Pathways and Replication Stress in Colorectal Cancer

PURPOSE

Genomic instability is a hallmark of cancer and targeting DNA damage response (DDR) is emerging as a promising therapeutic strategy in different solid tumors. The effectiveness of targeting DDR in colorectal cancer has not been extensively explored.

EXPERIMENTAL DESIGN

We challenged 112 cell models recapitulating the genomic landscape of metastatic colorectal cancer with ATM, ATR, CHK1, WEE1, and DNA-PK inhibitors, in parallel with chemotherapeutic agents. We focused then on ATR inhibitors (ATRi) and, to identify putative biomarkers of response and resistance, we analyzed at multiple levels colorectal cancer models highly sensitive or resistant to these drugs.

RESULTS

We found that around 30% of colorectal cancers, including those carrying KRAS and BRAF mutations and unresponsive to targeted agents, are sensitive to at least one DDR inhibitor. By investigating potential biomarkers of response to ATRi, we found that ATRi-sensitive cells displayed reduced phospho-RPA32 foci at basal level, while ATRi-resistant cells showed increased RAD51 foci formation in response to replication stress. Lack of ATM and RAD51C expression was associated with ATRi sensitivity. Analysis of mutational signatures and HRDetect score identified a subgroup of ATRi-sensitive models. Organoids derived from patients with metastatic colorectal cancer recapitulated findings obtained in cell lines.

CONCLUSIONS

In conclusion, a subset of colorectal cancers refractory to current therapies could benefit from inhibitors of DDR pathways and replication stress. A composite biomarker involving phospho-RPA32 and RAD51 foci, lack of ATM and RAD51C expression, as well as analysis of mutational signatures could be used to identify colorectal cancers likely to respond to ATRi.

The Nitrogen Mustards

The nitrogen mustards are powerful cytotoxic and lymphoablative agents and have been used for more than 60 years. They are employed in the treatment of cancers, sarcomas, and hematologic malignancies. Cyclophosphamide, the most versatile of the nitrogen mustards, also has a place in stem cell transplantation and the therapy of autoimmune diseases. Adverse effects caused by the nitrogen mustards on the central nervous system, kidney, heart, bladder, and gonads remain important issues. Advances in analytical techniques have facilitated the investigation of the pharmacokinetics of the nitrogen mustards, especially the oxazaphosphorines, which are prodrugs requiring metabolic activation. Enzymes involved in the metabolism of cyclophosphamide and ifosfamide are very polymorphic, but a greater understanding of the pharmacogenomic influences on their activity has not yet translated into a personalized medicine approach. In addition to damaging DNA, the nitrogen mustards can act through other mechanisms, such as antiangiogenesis and immunomodulation. The immunomodulatory properties of cyclophosphamide are an area of current exploration. In particular, cyclophosphamide decreases the number and activity of regulatory T cells, and the interaction between cyclophosphamide and the intestinal microbiome is now recognized as an important factor. New derivatives of the nitrogen mustards continue to be assessed. Oxazaphosphorine analogs have been synthesized in attempts to both improve efficacy and reduce toxicity, with varying degrees of success. Combinations of the nitrogen mustards with monoclonal antibodies and small-molecule targeted agents are being evaluated. SIGNIFICANCE STATEMENT: The nitrogen mustards are important, well-established therapeutic agents that are used to treat a variety of diseases. Their role is continuing to evolve.

New Perspectives on the Role of Integrin-Linked Kinase (ILK) Signaling in Cancer Metastasis

Simple Summary

Today, the vast majority of deaths from cancer are due to cancer metastasis. Metastasis requires that cancer cells escape from the initial tumor, travel through blood vessels, and form new tumors in distant host tissues. Integrin-linked kinase (ILK) is overexpressed by many types of cancer cells and provides both structural and signaling functions that are important for successful metastasis. Here, we discuss recent findings that show how ILK is involved in promoting physical changes important for cell motility and invasion, and how ILK relays signals to other machinery components during metastasis, including interactions with components of the immune system and communication between cancer cells and normal cells, to affect the process of metastasis. We also discuss the contribution of ILK to therapeutic resistance and examine efforts to target ILK for the treatment of metastatic disease.

Abstract

Cancer metastasis is a major barrier to the long-term survival of cancer patients. In cancer cells, integrin engagement downstream of cell-extracellular matrix (ECM) interactions results in the recruitment of cytoskeletal and signaling molecules to form multi-protein complexes to promote processes critical for metastasis. One of the major functional components of these complexes is Integrin Linked Kinase (ILK). Here, we discuss recent advances in our understanding of the importance of ILK as a signaling effector in processes linked to tumor progression and metastasis. New mechanistic insights as to the role of ILK in cellular plasticity, epithelial mesenchymal transition (EMT), migration, and invasion, including the impact of ILK on the formation of invadopodia, filopodia-like protrusions (FLPs), and Neutrophil Extracellular Trap (NET)-induced motility are highlighted. Recent findings detailing the contribution of ILK to therapeutic resistance and the importance of ILK as a potentially therapeutically tractable vulnerability in both solid tumors and hematologic malignancies are discussed. Indeed, pharmacologic inhibition of ILK activity using specific small molecule inhibitors is effective in curtailing the contribution of ILK to these processes, potentially offering a novel therapeutic avenue for inhibiting critical steps in the metastatic cascade leading to reduced drug resistance and increased therapeutic efficacy.

Central Nervous System Delivery of the Catalytic Subunit of DNA-Dependent Protein Kinase Inhibitor Peposertib as Radiosensitizer for Brain Metastases

Cytotoxic effects of chemotherapy and radiation therapy (RT) used for the treatment of brain metastases results from DNA damage within cancer cells. Cells rely on highly evolved DNA damage response (DDR) pathways to repair the damage caused by these treatments. Inhibiting these repair pathways can further sensitize cancer cells to chemotherapy and RT. The catalytic subunit of DNA-dependent protein kinase, in a complex with Ku80 and Ku70, is a pivotal regulator of the DDR, and peposertib is a potent inhibitor of this catalytic subunit. The characterization of central nervous system (CNS) distributional kinetics of peposertib is critical in establishing a therapeutic index in the setting of brain metastases. Our studies demonstrate that the delivery of peposertib is severely restricted into the CNS as opposed to peripheral organs, by active efflux at the blood-brain barrier (BBB). Peposertib has a low free fraction in the brain and spinal cord, further reducing the active concentration, and distributes to the same degree within different anatomic regions of the brain. However, peposertib is heterogeneously distributed within the metastatic tumor, where its concentration is highest within the tumor core (with disrupted BBB) and substantially lower within the invasive tumor rim (with a relatively intact BBB) and surrounding normal brain. These findings are critical in guiding the potential clinical deployment of peposertib as a radiosensitizing agent for the safe and effective treatment of brain metastases. SIGNIFICANCE STATEMENT: Effective radiosensitization of brain metastases while avoiding toxicity to the surrounding brain is critical in the development of novel radiosensitizers. The central nervous system distribution of peposertib, a potent catalytic subunit of DNA-dependent protein kinase inhibitor, is restricted by active efflux in the normal blood-brain barrier (BBB) but can reach significant concentrations in the tumor core. This finding suggests that peposertib may be an effective radiosensitizer for intracranial tumors with an open BBB, while limited distribution into normal brain will decrease the risk of enhanced radiation injury.

DNA Damage Response Inhibitors in Cholangiocarcinoma: Current Progress and Perspectives

Cholangiocarcinoma (CCA) is a poorly treatable type of cancer and its incidence is dramatically increasing. The lack of understanding of the biology of this tumor has slowed down the identification of novel targets and the development of effective treatments. Based on next generation sequencing profiling, alterations in DNA damage response (DDR)-related genes are paving the way for DDR-targeting strategies in CCA. Based on the notion of synthetic lethality, several DDR-inhibitors (DDRi) have been developed with the aim of accumulating enough DNA damage to induce cell death in tumor cells. Observing that DDRi alone could be insufficient for clinical use in CCA patients, the combination of DNA-damaging regimens with targeted approaches has started to be considered, as evidenced by many emerging clinical trials. Hence, novel therapeutic strategies combining DDRi with patient-specific targeted drugs could be the next level for treating cholangiocarcinoma.

Development and Evolution of DNA-Dependent Protein Kinase Inhibitors toward Cancer Therapy

DNA double-strand break (DSB) is considered the most deleterious type of DNA damage, which is generated by ionizing radiation (IR) and a subset of anticancer drugs. DNA-dependent protein kinase (DNA-PK), which is composed of a DNA-PK catalytic subunit (DNA-PKcs) and Ku80-Ku70 heterodimer, acts as the molecular sensor for DSB and plays a pivotal role in DSB repair through non-homologous end joining (NHEJ). Cells deficient for DNA-PKcs show hypersensitivity to IR and several DNA-damaging agents. Cellular sensitivity to IR and DNA-damaging agents can be augmented by the inhibition of DNA-PK. A number of small molecules that inhibit DNA-PK have been developed. Here, the development and evolution of inhibitors targeting DNA-PK for cancer therapy is reviewed. Significant parts of the inhibitors were developed based on the structural similarity of DNA-PK to phosphatidylinositol 3-kinases (PI3Ks) and PI3K-related kinases (PIKKs), including Ataxia-telangiectasia mutated (ATM). Some of DNA-PK inhibitors, e.g., NU7026 and NU7441, have been used extensively in the studies for cellular function of DNA-PK. Recently developed inhibitors, e.g., M3814 and AZD7648, are in clinical trials and on the way to be utilized in cancer therapy in combination with radiotherapy and chemotherapy.

Perspective on the Use of DNA Repair Inhibitors as a Tool for Imaging and Radionuclide Therapy of Glioblastoma

Simple Summary

The current routine treatment for glioblastoma (GB), the most lethal high-grade brain tumor in adults, aims to induce DNA damage in the tumor. However, the tumor cells might be able to repair that damage, which leads to therapy resistance. Fortunately, DNA repair defects are common in GB cells, and their survival is often based on a sole backup repair pathway. Hence, targeted drugs inhibiting essential proteins of the DNA damage response have gained momentum and are being introduced in the clinic. This review gives a perspective on the use of radiopharmaceuticals targeting DDR kinases for imaging in order to determine the DNA repair phenotype of GB, as well as for effective radionuclide therapy. Finally, four new promising radiopharmaceuticals are suggested with the potential to lead to a more personalized GB therapy.

Abstract

Despite numerous innovative treatment strategies, the treatment of glioblastoma (GB) remains challenging. With the current state-of-the-art therapy, most GB patients succumb after about a year. In the evolution of personalized medicine, targeted radionuclide therapy (TRT) is gaining momentum, for example, to stratify patients based on specific biomarkers. One of these biomarkers is deficiencies in DNA damage repair (DDR), which give rise to genomic instability and cancer initiation. However, these deficiencies also provide targets to specifically kill cancer cells following the synthetic lethality principle. This led to the increased interest in targeted drugs that inhibit essential DDR kinases (DDRi), of which multiple are undergoing clinical validation. In this review, the current status of DDRi for the treatment of GB is given for selected targets: ATM/ATR, CHK1/2, DNA-PK, and PARP. Furthermore, this review provides a perspective on the use of radiopharmaceuticals targeting these DDR kinases to (1) evaluate the DNA repair phenotype of GB before treatment decisions are made and (2) induce DNA damage via TRT. Finally, by applying in-house selection criteria and analyzing the structural characteristics of the DDRi, four drugs with the potential to become new therapeutic GB radiopharmaceuticals are suggested.

DNA-PK Inhibitor Peposertib Amplifies Radiation-Induced Inflammatory Micronucleation and Enhances TGFβ/PD-L1 Targeted Cancer Immunotherapy

Radiotherapy is the most widely used cancer treatment and improvements in its efficacy and safety are highly sought-after. Peposertib (also known as M3814), a potent and selective DNA-dependent protein kinase (DNA-PK) inhibitor, effectively suppresses the repair of radiation-induced DNA double-strand breaks (DSB) and regresses human xenograft tumors in preclinical models. Irradiated cancer cells devoid of p53 activity are especially sensitive to the DNA-PK inhibitor, as they lose a key cell-cycle checkpoint circuit and enter mitosis with unrepaired DSBs, leading to catastrophic consequences. Here, we show that inhibiting the repair of DSBs induced by ionizing radiation with peposertib offers a powerful new way for improving radiotherapy by simultaneously enhancing cancer cell killing and response to a bifunctional TGFβ "trap"/anti-PD-L1 cancer immunotherapy. By promoting chromosome misalignment and missegregation in p53-deficient cancer cells with unrepaired DSBs, DNA-PK inhibitor accelerated micronuclei formation, a key generator of cytosolic DNA and activator of cGAS/STING-dependent inflammatory signaling as it elevated PD-L1 expression in irradiated cancer cells. Triple combination of radiation, peposertib, and bintrafusp alfa, a fusion protein simultaneously inhibiting the profibrotic TGFβ and immunosuppressive PD-L1 pathways was superior to dual combinations and suggested a novel approach to more efficacious radioimmunotherapy of cancer.

IMPLICATIONS

Selective inhibition of DNA-PK in irradiated cancer cells enhances inflammatory signaling and activity of dual TGFβ/PD-L1 targeted therapy and may offer a more efficacious combination option for the treatment of locally advanced solid tumors.

Inhibition of DNA-PK may improve response to neoadjuvant chemoradiotherapy in rectal cancer

Treatment of locally advanced rectal cancer includes chemoradiation and surgery, but patient response to treatment is variable. Patients who have a complete response have improved outcomes; therefore, there is a critical need to identify mechanisms of resistance to circumvent them. DNA-PK is involved in the repair of DNA double-strand breaks caused by radiation, which we found to be increased in rectal cancer after treatment. We hypothesized that inhibiting this complex with a DNA-PK inhibitor, Peposertib (M3814), would improve treatment response.

We assessed pDNA-PK in a rectal cancer cell line and mouse model utilizing western blotting, viability assays, γH2AX staining, and treatment response. The three treatment groups were: standard of care (SOC) (5-fluorouracil (5FU) with radiation), M3814 with radiation, and M3814 with SOC.

SOC treatment of rectal cancer cells increased pDNA-PK protein and increased γH2AX foci, but this was abrogated by the addition of M3814. Mice with CT26 tumors treated with M3814 with SOC did not differ in average tumor size but individual tumor response varied. The clinical complete response rate improved significantly with the addition of M3814 but pathological complete response did not. We investigated alterations in DNA repair and found that Kap1 and pATM are increased after M3814 addition suggesting this may mediate resistance.

When the DNA-PK inhibitor, M3814, is combined with SOC treatment, response improved in some rectal cancer models but an increase in other repair mechanisms likely diminishes the effect. A clinical trial is ongoing to further explore the role of DNA-PK inhibition in rectal cancer treatment.

DNA repair defects in cancer and therapeutic opportunities

DNA repair and DNA damage signaling pathways are critical for the maintenance of genomic stability. Defects of DNA repair and damage signaling contribute to tumorigenesis, but also render cancer cells vulnerable to DNA damage and reliant on remaining repair and signaling activities. Here, we review the major classes of DNA repair and damage signaling defects in cancer, the genomic instability that they give rise to, and therapeutic strategies to exploit the resulting vulnerabilities. Furthermore, we discuss the impacts of DNA repair defects on both targeted therapy and immunotherapy, and highlight emerging principles for targeting DNA repair defects in cancer therapy.

Phase I trial of ATM inhibitor M3541 in combination with palliative radiotherapy in patients with solid tumors

BACKGROUND

Ataxia telangiectasia mutated (ATM) kinase orchestrates DNA double strand break (DSB) repair; ATM inhibitors may therefore enhance the therapeutic effect of DSB-inducing treatments such as radiotherapy (RT). M3541 is an orally administered selective inhibitor of ATM.

METHODS

This phase I dose-escalation study evaluated the maximum-tolerated dose (MTD), recommended phase II dose(s) (RP2D), safety, pharmacokinetics (PK) and antitumor activity of M3541 in combination with fractionated palliative RT in patients with solid tumors. Fifteen patients received palliative RT (30 Gy in 10 fractions) and escalating doses of M3541 (50-300 mg administered on RT fraction days) guided by a Bayesian 2-parameter logistic regression model with overdose control.

RESULTS

Doses of M3541 up to 300 mg/fraction day were well tolerated. One patient (200 mg group) experienced two dose-limiting toxicities (urinary tract infection, febrile neutropenia) that resolved with antibiotics. All patients reported ≥ 1 treatment-emergent adverse event (TEAE) but none led to treatment discontinuation. No grade ≥ 4 TEAEs were reported and there was no indication of a dose effect for any TEAE. Three patients (20.0%; 95% confidence interval 4.3-48.1) had confirmed complete or partial response. M3541 total plasma levels did not increase with dose following single or repeated dosing. No relationship was observed between dose and changes in the ratio of phosphorylated to total ATM or in immune cell counts.

CONCLUSIONS

The MTD and RP2D could not be established as the study closed early due to the absence of a dose-response relationship and non-optimal PK profile. No further clinical development of M3541 was pursued. (Trial registration number ClinicalTrials.gov NCT03225105. Registration date July 21, 2017).

Therapeutic Targeting of DNA Damage Response in Cancer

DNA damage response (DDR) is critical to ensure genome stability, and defects in this signaling pathway are highly associated with carcinogenesis and tumor progression. Nevertheless, this also provides therapeutic opportunities, as cells with defective DDR signaling are directed to rely on compensatory survival pathways, and these vulnerabilities have been exploited for anticancer treatments. Following the impressive success of PARP inhibitors in the treatment of BRCA-mutated breast and ovarian cancers, extensive research has been conducted toward the development of pharmacologic inhibitors of the key components of the DDR signaling pathway. In this review, we discuss the key elements of the DDR pathway and how these molecular components may serve as anticancer treatment targets. We also summarize the recent promising developments in the field of DDR pathway inhibitors, focusing on novel agents beyond PARP inhibitors. Furthermore, we discuss biomarker studies to identify target patients expected to derive maximal clinical benefits as well as combination strategies with other classes of anticancer agents to synergize and optimize the clinical benefits.

Radiotherapy as a tool to elicit clinically actionable signalling pathways in cancer

A variety of targeted anticancer agents have been successfully introduced into clinical practice, largely reflecting their ability to inhibit specific molecular alterations that are required for disease progression. However, not all malignant cells rely on such alterations to survive, proliferate, disseminate and/or evade anticancer immunity, implying that many tumours are intrinsically resistant to targeted therapies. Radiotherapy is well known for its ability to activate cytotoxic signalling pathways that ultimately promote the death of cancer cells, as well as numerous cytoprotective mechanisms that are elicited by cellular damage. Importantly, many cytoprotective mechanisms elicited by radiotherapy can be abrogated by targeted anticancer agents, suggesting that radiotherapy could be harnessed to enhance the clinical efficacy of these drugs. In this Review, we discuss preclinical and clinical data that introduce radiotherapy as a tool to elicit or amplify clinically actionable signalling pathways in patients with cancer.

Structural insights into inhibitor regulation of the DNA repair protein DNA-PKcs

The DNA-dependent protein kinase catalytic subunit (DNA-PKcs) has a central role in non-homologous end joining, one of the two main pathways that detect and repair DNA double-strand breaks (DSBs) in humans^1,2. DNA-PKcs is of great importance in repairing pathological DSBs, making DNA-PKcs inhibitors attractive therapeutic agents for cancer in combination with DSB-inducing radiotherapy and chemotherapy³. Many of the selective inhibitors of DNA-PKcs that have been developed exhibit potential as treatment for various cancers⁴. Here we report cryo-electron microscopy (cryo-EM) structures of human DNA-PKcs natively purified from HeLa cell nuclear extracts, in complex with adenosine-5′-(γ-thio)-triphosphate (ATPγS) and four inhibitors (wortmannin, NU7441, AZD7648 and M3814), including drug candidates undergoing clinical trials. The structures reveal molecular details of ATP binding at the active site before catalysis and provide insights into the modes of action and specificities of the competitive inhibitors. Of note, binding of the ligands causes movement of the PIKK regulatory domain (PRD), revealing a connection between the p-loop and PRD conformations. Electrophoretic mobility shift assay and cryo-EM studies on the DNA-dependent protein kinase holoenzyme further show that ligand binding does not have a negative allosteric or inhibitory effect on assembly of the holoenzyme complex and that inhibitors function through direct competition with ATP. Overall, the structures described in this study should greatly assist future efforts in rational drug design targeting DNA-PKcs, demonstrating the potential of cryo-EM in structure-guided drug development for large and challenging targets.

Cryo-electron microscopy structures of DNA-dependent protein kinase catalytic subunit bound to ATPγS and four inhibitors (wortmannin, NU7441, AZD7648 and M3814) provide molecular details and insights useful for drug design.

Targeting Non-homologous and Alternative End Joining Repair to Enhance Cancer Radiosensitivity

Many cancer therapies, including radiotherapy, induce DSBs as the major driving mechanism for inducing cancer cell death. Thus, modulating DSB repair has immense potential for radiosensitization, although such interventions must be carefully designed to be tumor selective to ensure that normal tissue toxicities are not also increased. Here, we review mechanisms of error-prone DSB repair through a highly efficient process called end joining. There are two major pathways of end-joining repair: non-homologous end joining (NHEJ) and alternative end joining (a-EJ), both of which can be selectively upregulated in cancer and thus represent attractive therapeutic targets for radiosensitization. These EJ pathways each have therapeutically targetable pioneer factors - DNA-dependent protein kinase catalytic subunit (DNA-PKcs) for NHEJ and DNA Polymerase Theta (Pol θ) for a-EJ. We summarize the current status of therapeutic targeting of NHEJ and a-EJ to enhance the effects of radiotherapy - focusing on challenges that must be overcome and opportunities that require further exploration. By leveraging preclinical insights into mechanisms of altered DSB repair programs in cancer, selective radiosensitization through NHEJ and/or a-EJ targeting remains a highly attractive avenue for ongoing and future clinical investigation.

Targeting DNA repair pathway in cancer: Mechanisms and clinical application

Over the last decades, the growing understanding on DNA damage response (DDR) pathways has broadened the therapeutic landscape in oncology. It is becoming increasingly clear that the genomic instability of cells resulted from deficient DNA damage response contributes to the occurrence of cancer. One the other hand, these defects could also be exploited as a therapeutic opportunity, which is preferentially more deleterious in tumor cells than in normal cells. An expanding repertoire of DDR-targeting agents has rapidly expanded to inhibitors of multiple members involved in DDR pathways, including PARP, ATM, ATR, CHK1, WEE1, and DNA-PK. In this review, we sought to summarize the complex network of DNA repair machinery in cancer cells and discuss the underlying mechanism for the application of DDR inhibitors in cancer. With the past preclinical evidence and ongoing clinical trials, we also provide an overview of the history and current landscape of DDR inhibitors in cancer treatment, with special focus on the combination of DDR-targeted therapies with other cancer treatment strategies.

Radiosensitisation of SCCVII tumours and normal tissues in mice by the DNA-dependent protein kinase inhibitor AZD7648

BACKGROUND AND PURPOSE

Inhibitors of DNA-dependent protein kinase (DNA-PK) are effective radiation sensitisers in preclinical tumours, but little is known about risks of normal tissue radiosensitisation. Here, we evaluate radiosensitisation of head and neck squamous cell carcinoma (HNSCC) cells by DNA-PK inhibitor AZD7648 under oxia and anoxia in vitro, and tumour (SCCVII), oral mucosa and small intestine in mice.

MATERIALS AND METHODS

Radiosensitisation of human (UT-SCC-54C) and murine (SCCVII) HNSCC cells by AZD7648 under oxia and anoxia was evaluated by clonogenic assay. Radiosensitisation of SCCVII tumours in C3H mice by oral AZD7648 (75 mg/kg) was determined by ex vivo clonogenic assay 3.5 days post-irradiation, with evaluation of normal tissue surrogate endpoints using 5-ethynyl-2'-deoxyuridine to facilitate detection of regenerating crypts in the ileum and repopulating S-phase cells in the ileum and oral mucosa of the same animals.

RESULTS

AZD7648 potently radiosensitised both cell lines, with similar sensitiser enhancement ratios for 10% survival (SER₁₀) under oxia and anoxia. AZD7648 diffused rapidly through multicellular layers, suggesting rapid equilibration between plasma and hypoxic zones in tumours. SCCVII tumours were radiosensitised by AZD7648 (SER₁₀ 2.5). AZD7648 also enhanced radiation-induced body weight loss and suppressed regenerating intestinal crypts and repopulating S-phase cells in the ileum and tongue epithelium with SER values similar to SCCVII tumours.

CONCLUSION

AZD7648 is a potent radiation sensitiser of both oxic and anoxic tumour cells, but also markedly radiosensitises stem cells in the small intestine and oral mucosa.