PRAD50: a novel and clinically applicable pharmacodynamic biomarker of both ATM and ATR inhibition identified using mass spectrometry and immunohistochemistry

Gemma N Jones,Lucy A Young,Elizabeth A Harrington,Alan Lau,Jeffrey R Whiteaker,Richard G Ivey,Lei Zhao,William J Howat,Nicola Griffin,Antonio Ramos-Montoya,J Carl Barrett,Elaine Cadogan ,Martine P Roudier ,Gareth Hughes ,Rajesh Odedra,Amanda G Paulovich,Antonio Garcia-Trinidad,Claire Rooney,Zena Wilson,Andrew J Pierce

doi:10.1038/s41416-018-0286-4

Abstract

BackgroundAZD0156 and AZD6738 are potent and selective inhibitors of ataxia-telangiectasia-kinase (ATM) and ataxia-telangiectasia-mutated and Rad3-related (ATR), respectively, important sensors/signallers of DNA damage.MethodsWe used multiplexed targeted-mass-spectrometry to select pRAD50(Ser635) as a pharmacodynamic biomarker for AZD0156-mediated ATM inhibition from a panel of 45 peptides, then developed and tested a clinically applicable immunohistochemistry assay for pRAD50(Ser635) detection in FFPE tissue.ResultsWe found moderate pRAD50 baseline levels across cancer indications. pRAD50 was detectable in 100% gastric cancers (n = 23), 99% colorectal cancers (n = 102), 95% triple-negative-breast cancers (TNBC) (n = 40) and 87.5% glioblastoma-multiformes (n = 16). We demonstrated AZD0156 target inhibition in TNBC patient-derived xenograft models; where AZD0156 monotherapy or post olaparib treatment, resulted in a 34–72% reduction in pRAD50. Similar inhibition of pRAD50 (68%) was observed following ATM inhibitor treatment post irinotecan in a colorectal cancer xenograft model. ATR inhibition, using AZD6738, increased pRAD50 in the ATM-proficient models whilst in ATM-deficient models the opposite was observed, suggesting pRAD50 pharmacodynamics post ATR inhibition may be ATM-dependent and could be useful to determine ATM functionality in patients treated with ATR inhibitors.ConclusionTogether these data support clinical utilisation of pRAD50 as a biomarker of AZD0156 and AZD6738 pharmacology to elucidate clinical pharmacokinetic/pharmacodynamic relationships, thereby informing recommended Phase 2 dose/schedule.

Highlights

Ataxia-telangiectasia-mutated (ATM) is an attractive therapeutic target as constitutional loss of ATM in ataxia-telangiectasia (A-T)patients causes profound sensitivity to DNA damaging agents that induce double-strand breaks.[1,2] ATM is the major signalling kinase involved in the initiation of double-strand break (DSB) repair by homologous recombination (HR) and activated by exposed DNA double-stranded ends.[3]
To determine the responsiveness of phosphorylated RAD50 (pRAD50) as a PD biomarker in an in vivo setting, we examined an SW620 colorectal cancer (CRC) xenograft model treated with irinotecan and a probe compound to AZD0156, AZ31.35 AZ31 and AZD0156 are both potent and specific inhibitors of ATM, AZD0156 was selected as the clinical lead compound subsequent to these experiments when physiologically based pharmacokinetic (PBPK) modelling predicted a longer human plasma half-life for AZD0156 than for AZ31
We used immuno-MRM-MS to guide the selection of pRAD50 as an alternative biomarker for interrogating the ATM pathway signalling and subsequently developed a pRAD50 IHC assay for clinical use

Summary

Introduction

Ataxia-telangiectasia-mutated (ATM) is an attractive therapeutic target as constitutional loss of ATM in ataxia-telangiectasia (A-T)patients causes profound sensitivity to DNA damaging agents that induce double-strand breaks (e.g. ionizing radiation).[1,2] ATM is the major signalling kinase involved in the initiation of double-strand break (DSB) repair by homologous recombination (HR) and activated by exposed DNA double-stranded ends.[3]. ATM dimer to exposed double-strand DNA ends, the interaction results in a conformation change of ATM to an open active ATM dimer and autophosphorylation of ATM at Ser1981 (Supplementary Figure 1).[6] Activated ATM phosphorylates its downstream substrates, including all three components of the MRN complex, which are known to have distinct roles in signal transduction to the DNA repair machinery, cell cycle checkpoint controls and cellular processes.[3,7] Diminished levels of MRE11 functionality, for example from patients with hypomorphic mutations, result in patients with an A-T-like disorder (ATLD), presenting with radiation sensitivity and chromosomal instability.[8]. We demonstrated AZD0156 target inhibition in TNBC patient-derived xenograft models; where AZD0156 monotherapy or post olaparib treatment, resulted in a 34–72% reduction in pRAD50. Similar inhibition of pRAD50 (68%) was observed following ATM inhibitor treatment post irinotecan in a colorectal cancer xenograft model. CONCLUSION: Together these data support clinical utilisation of pRAD50 as a biomarker of AZD0156 and AZD6738 pharmacology to elucidate clinical pharmacokinetic/pharmacodynamic relationships, thereby informing recommended Phase 2 dose/schedule

Methods

Results

Conclusion