Patients on treatment with oral fixed dose imatinib are frequently under- or overexposed to the drug. We investigated the association between the gene activity score (GAS) of imatinib-metabolizing cytochromes (CYP3A4, CYP3A5, CYP2D6, CYP2C9, CYP2C19, CYP2C8) and imatinib and nor-imatinib exposure. We also investigated the impact of concurrent drug-drug-interactions (DDIs) on the association between GAS and imatinib exposure.Serial plasma samples were collected from 33 GIST patients treated with imatinib 400 mg daily within a prospective clinical trial. Imatinib and nor-imatinib C were quantified by liquid chromatography with tandem mass spectrometry (LC-MS/MS). Genetic polymorphisms with a functional impact on imatinib-metabolizing cytochromes were identified and a GAS was calculated for each gene. A DDI-adjusted GAS was also generated.Imatinib and nor-imatinib C were measured in 161 plasma samples. CYP2D6 GAS and metabolizer status based on genotype were associated with imatinib and (imatinib + nor-imatinib) C . CYP2D6 poor and intermediate metabolizers were predicted to have a lower nor-imatinib/imatinib metabolic ratio than normal metabolizers (0.197 and 0.193 vs. 0.247, P = .0205), whereas CYP2C8*3 carriers had a higher ratio than CYP2C8*1/*1 patients (0.263 vs. 0.201, P = .0220). CYP2C9 metabolizer status was inversely related to the metabolic ratio with an effect probably driven by the linkage disequilibrium between CYP2C9*2 and CYP2C8*3. The CYP2D6 DDI-adjusted GAS was still predictive of imatinib exposure.These findings highlight that CYP2D6 plays a major role in imatinib pharmacokinetics, but other players (i.e., CYP2C8) may influence imatinib exposure. These findings could drive the selection of patients more susceptible to imatinib under- or overexposure who could be candidates for personalized treatment and intensified monitoring strategies.