Relationship of body mass index and plasma imatinib levels in CML patients and mitigation of clinical impact using PK-guided dosing strategy.

e18522 Background: Imatinib is a mainstay of treatment for patients with chronic myeloid lymphoma (CML). Exposure-response relationships have been demonstrated and trough levels between 800 and 1000 ng/ml are usually associated with Major Molecular Response (MMR), prolonged survival and acceptable safety. The extent to which body mass index (BMI) might influence drug pharmacokinetics (PK) and clinical outcome remains to be investigated. Methods: Therapeutic drug monitoring was performed in 60 adult patients (56 years, M33/27F) starting imatinib (400 mg QD) for CML, and baseline deviation from target Cmin was used to subsequently adjust dosing using Monolix software. Baseline BMI was calculated and collected along with other patient characteristics and comedications likely to alter imatinib PK. Response (MMR at 3 months) and drug-related toxicity (CTCAE grading) were recorded. Patients were divided into three subgroups: BMI24.5. The association between BMI and trough imatinib levels at baseline, further differences in toxic events and response rate between cohorts, as well as differences in dosing after PK-guided dose adjustment, were all tested using appropriate statistical analysis. Results: Mean imatinib trough levels at baseline were 1172 (754-4737), 870 (276-1767) and 820 (334-2217) ng/ml for patients with BMI24.5, respectively. Patients with BMI24.5, but this was not statistically significant. Multivariate analysis confirmed that low BMI was an independent factor for overexposure with imatinib. After PK-guided dosing, a statistically significant difference in dosing after correction was observed, as overexposed patients had their dose reduced to an average of 330 mg QD, whereas underexposed patients had their dose increased to an average of 500 mg QD (p24.5). This could potentially lead to either increased toxicity or lack of efficacy is no adjustment in dosing is performed. Here, TDM with subsequent adaptive dosing helps to correct the dose according to the initial deviation from target, thus smoothing the clinical impact of the differences in exposure observed at the start of treatment.