Background: AZD6738 is a potent, selective inhibitor of the ataxia telangiectasia and Rad3-related (ATR) serine/threonine-specific protein kinase with good selectivity against other phosphatidylinositol 3-kinase-related kinase (PIKK) family members. To support dose/schedule guidance through mathematical modelling of clinical pharmacokinetic/pharmacodynamic relationships, we sought to determine quantitative pharmacodynamic effects of AZD6738 in peripheral blood with respect to drug exposure. Methods: We assessed peripheral blood cell subpopulations in clinical studies of AZD6738 used either as a monotherapy, a combination with the PARP-1 inhibitor olaparib and in combination with the PD-L1 inhibitor durvalumab through both blood analyser and flow cytometry characterization. Peripheral pharmacokinetics of AZD6738 were also assessed. Results: Levels of peripheral monocytes were suppressed in a dose-dependent manner for the duration of AZD6738 dosing resulting in a decrease of up to 80% within the first 7 consecutive days of AZD6738 treatment; these results were consistent across studies. Upon cessation of an AZD6738 dosing interval monocyte levels return to baseline values within 14 days; in combination with durvalumab there is a trend for the rebound in monocyte counts to surpass baseline levels by up to 20%. Monocyte suppression is specific to AZD6738 and not found with either single agent olaparib or single agent durvalumab, possibly attributable to defective base excision repair in monocytes and lack of PARP-1. Monocytes were found to be the most sensitive cell type to AZD6738 treatment compared with neutrophils, platelets, haemoglobin or lymphocytes. Conclusions: This drug effect of AZD6738 is consistent with natural clearance of peripheral monocytes under suppression of monocyte precursor proliferation in the bone marrow, suggestive of a change in the equilibrium between production and elimination of peripheral monocytes. Used as a surrogate marker of target inhibition, a mathematical model of the AZD6738/monocyte dose-exposure-response relationship is described and has been used to inform AZD6738 Phase 2 dose and schedule.