Magnus T. Dillon

Selective Targeting of the G2/M Cell Cycle Checkpoint to Improve the Therapeutic Index of Radiotherapy

Despite tremendous advances in radiotherapy techniques, allowing dose escalation to tumour tissues and sparing of organs at risk, cure rates from radiotherapy or chemoradiotherapy remain suboptimal for most cancers. In tandem with our growing understanding of tumour biology, we are beginning to appreciate that targeting the molecular response to radiation-induced DNA damage holds great promise for selective tumour radiosensitisation. In particular, approaches that inhibit cell cycle checkpoint controls offer a means of exploiting molecular differences between tumour and normal cells, thereby inducing so-called cancer-specific synthetic lethality. In this overview, we discuss cellular responses to radiation-induced damage and discuss the potential of using G2/M cell cycle checkpoint inhibitors as a means of enhancing tumour control rates.

Human Papillomavirus–Negative Pharyngeal Cancer

Human papillomavirus-negative head and neck squamous cell carcinoma (HNSCC) carries a poor prognosis, and despite optimal treatment with chemoradiotherapy to the limit of tolerance, many patients will relapse. A number of methods for intensifying treatment of HNSCC have been investigated, leading to the current standards of care. Novel agents targeting tumor cell and stromal signaling, DNA damage response, and immune system are now reaching clinical trials in combination with chemoradiotherapy. In this review, we discuss the evidence for the current treatment of locally advanced human papillomavirus-negative HNSCC, as well as investigational therapies, such as hypoxia modification, molecular targeting of epidermal growth factor receptor family, vascular endothelial growth factor receptor or DNA damage response proteins in combination with radiation therapy.

Radiosensitization by the ATR Inhibitor AZD6738 through Generation of Acentric Micronuclei.

AZD6738 is an orally active ATR inhibitor (ATRi) currently in phase I clinical trials. We found in vitro growth inhibitory activity of this ATRi in a panel of human cancer cell lines. We demonstrated radiosensitization by AZD6738 to single radiation fractions in multiple cancer cell lines independent of both p53 and BRCA2 status by the clonogenic assay. Radiosensitization by AZD6738 to clinically relevant doses of fractionated radiation was demonstrated in vitro using a 3D tumor spheroid model and, in vivo, AZD6738 radiosensitized by abrogating the radiation-induced G2 cell-cycle checkpoint and inhibiting homologous recombination. Mitosis with damaged DNA resulted in mitotic catastrophe as measured by micronucleus formation by live-cell fluorescent-ubiquitination cell-cycle imaging of cell-cycle progression and nuclear morphology. Induction of micronuclei was significantly more prominent for AZD6738 compared with inhibition of the downstream kinase CHK1 alone at isoeffective doses. Micronuclei were characterized as acentric chromosomal fragments, which displayed characteristics of increased DNA damage and cell-cycle dyssynchrony when compared with the primary nucleus. Mol Cancer Ther; 16(1); 25–34. ©2016 AACR.

An orally bioavailable Chk1 inhibitor, CCT244747, sensitizes bladder and head and neck cancer cell lines to radiation.

PURPOSE Chk1 inhibition increases cell sensitivity to both chemotherapy and radiotherapy in several tumour types and is, therefore, a promising anti-cancer approach. Although several Chk1 inhibitors have been developed, their clinical progress has been hampered by low bioavailability and off-target toxicities. MATERIALS AND METHODS We characterized the radiosensitizing activity of CCT244747, the first orally bioavailable Chk1 inhibitor. We used a panel of bladder and head and neck cancer cell lines and monitored the effect of combining CCT244747 with radiation both in in vitro and in vivo models. RESULTS CCT244747 sensitized cancer cell lines to radiation in vitro and resulted in a growth delay in cancer xenograft models associated with a survival benefit. Radiosensitization was elicited by abrogation of the radiation-induced G2 arrest and premature entry into mitosis. CONCLUSIONS CCT244747 is a potent and specific Chk1 inhibitor that can be administered orally. It radiosensitizes tumour cell lines and represents a new therapy for clinical application in combination with radiotherapy.

PATRIOT: A phase I study to assess the tolerability, safety and biological effects of a specific ataxia telangiectasia and Rad3-related (ATR) inhibitor (AZD6738) as a single agent and in combination with palliative radiation therapy in patients with solid tumours

Highlights • Summary of a protocol for a phase I study combining the ATR inhibitor and palliative radiotherapy.• Study design is discussed.• Study aims and objectives, brief eligibility criteria, DLT criteria and dose escalation is discussed.• Brief summary of translational research to be carried out.

Abstract CT084: A Phase I dose-escalation study of ATR inhibitor monotherapy with AZD6738 in advanced solid tumors (PATRIOT Part A)

Many cancers have high levels of replication stress and a poorly functional G1/S DNA damage checkpoint. This may render them more susceptible than normal tissues to inhibition of ATR, an apical kinase in the DNA damage response and critical part of the response to DNA replication stress. We report the early results of the monotherapy dose-escalation phase of the PATRIOT study of AZD6738, an orally active ATR inhibitor in patients (pts) with advanced solid tumors (NCT02223923), whose endpoints were MTD, safety, tolerability, pharmacokinetics (PK) and preliminary efficacy. 26 pts were enrolled between July 2014 and July 2016 in a 3+3 design. Pts received continuous BD dosing. PK analyses were performed. Dose limiting toxicities (table 1) were thrombocytopenia (G3 with epistaxis, 1 participant; G4, 2 participants), pancytopenia (G4, 1 participant), increased amylase (G3, 1 participant). Other treatment-related AEs (probably or definitely caused by AZD6738) affecting ≥2 participants were fatigue (9; 35% G1-4, 0% G3-4), anemia (7; 23% G1-4, 12% G3-4), nausea (4; 15% G1-4, 0% G3-4), thrombocytopenia (5; 19% G1-4, 15% G3-4), anorexia (3; 12% G1-4, 0% G3-4), dysgeusia (3; 12% G1-4, 0% G3-4), vomiting (2; 8% G1-4, 0% G3-4). The MTD was 160 mg BD, given continuously. Two RECIST partial responses were observed in pts with SCCHN and nasopharyngeal carcinoma, one confirmed. Median duration taking AZD6738 was 97 days, range 30-279 days (evaluable patients only). Two pts remain on treatment, three pts discontinued due to treatment-related toxicity. Expansion cohorts have been initiated at 160mg BD, exploring a number of alternative treatment schedules designed to offset cumulative toxicity and test efficacy of AZD6738 monotherapy and the presence of high replication stress, DNA damage response deficiencies or ATM loss. Schedules include: AZD6738 given at 160mg BD for 21 of a 28 day cycle and 5 days on, 2 days off. A parallel study is investigating AZD6738 in combination with palliative radiotherapy.[Funded by CRUK CRUKD/14/007] Citation Format: Magnus T. Dillon, Aude Espinasse, Sally Ellis, Kabir Mohammed, Lorna G. Grove, Lyndall McLellan, Simon A. Smith, Graham Ross, Sola Adeleke, Kin Woo, Eleni Josephides, James F. Spicer, Martin D. Forster, Kevin J. Harrington. A Phase I dose-escalation study of ATR inhibitor monotherapy with AZD6738 in advanced solid tumors (PATRIOT Part A) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr CT084. doi:10.1158/1538-7445.AM2017-CT084

Targeting ATR for Cancer Therapy: ATR-Targeted Drug Candidates

ATR inhibitors are a new class of anti-cancer compounds reaching early phase clinical trials. They are predicted to have anti-cancer activity as monotherapy, and in combination with DNA damaging chemotherapies and ionizing radiation. We outline the clinical trials in progress using the current clinical candidates VX-970 (M6620) and AZD6738, discuss potential biomarkers for this class of drug and consider future avenues for development of ATR inhibitors.

Combined ATR and DNA-PK Inhibition Radiosensitizes Tumor Cells Independently of Their p53 Status

Head and neck squamous cell carcinoma (HNSCC) is a significant cause of cancer deaths. Cisplatin-based chemoradiotherapy is a standard of care for locally advanced disease. ATR and DNA-PK inhibition (DNA-PKi) are actively being investigated in clinical trials with preclinical data supporting clinical translation as radiosensitizers. Here, we hypothesized that targeting both ATR and DNA-PK with small molecule inhibitors would increase radiosensitization of HNSCC cell lines. Radiosensitization was assessed by Bliss independence analysis of colony survival data. Strong cell cycle perturbing effects were observed with ATR inhibition reversing the G2/M arrest observed for radiation-DNA-PKi. Increased apoptosis in combination groups was measured by Sub-G1 DNA populations. DNA-PKi increased radiation-induced RAD51 and gamma-H2Ax foci, with the addition of ATR inhibition reducing levels of both. A sharp increase in nuclear fragmentation after aberrant mitotic transit appears to be the main driver of decreased survival due to irradiation and dual ATR/DNA-PKi. Dual inhibition of DNA-PK and ATR represents a novel approach in combination with radiation, with efficacy appearing to be independent of p53 status. Due to toxicity concerns, careful assessment is necessary in any future translation of single or dual radiosensitization approaches. Ongoing clinical trials into the ATR inhibitor AZD6738 plus radiation, and the phenotypically similar combination of AZD6738 and the PARP inhibitor olaparib, are likely to be key in ascertaining the toxicity profile of such combinations.