To determine the responsiveness of pRAD50 as a PD biomarker in an in vivo setting, we examined an SW620 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, however, 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. Here, ATM is linked to colorectal carcinoma.