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Palbociclib is an oral CDK4/6 inhibitor indicated in HR+/HER2- advanced or metastatic breast cancer in combination with hormonotherapy. Its main toxicity is neutropenia. The aim of our study was to describe the kinetics of circulating neutrophils from real-life palbociclib-treated patients. A population pharmacokinetic (popPK) model was first constructed to describe palbociclib pharmacokinetic (PK). Individual PK parameters obtained were then used in the pharmacokinetic/pharmacodynamic (PK/PD) model to depict the relation between palbociclib concentrations and absolute neutrophil counts (ANC). The models were built with a population of 143 patients. Palbociclib samples were routinely collected during therapeutic drug monitoring, whereas ANC were retrospectively retrieved from the patient files. The optimal popPK model was a mono-compartmental model with a first-order absorption constant of 0.187 h−1 and an apparent clearance Cl/F of 57.09 L (32.8% of inter individuality variability (IIV)). The apparent volume of distribution (1580 L) and the lag-time (Tlag: 0.658 h) were fixed to values from the literature. An increase in creatinine clearance and a decrease in alkaline phosphatase led to an increase in palbociclib Cl/F. To describe ANC kinetics during treatment, Friberg’s PK/PD model, with linear drug effect, was used. Parameters estimated were Base (2.92 G/L; 29.6% IIV), Slope (0.0011 L/µg; 28.8% IIV), Mean Transit Time (MTT; 5.29 days; 17.9% IIV) and γ (0.102). The only significant covariate was age on the initial ANC (Base), with lower ANC in younger patients. PK/PD model-based simulations show that the higher the estimated CressSS (trough concentration at steady state), the higher the risk of developing neutropenia. In order to present a risk lower than 20% to developing a grade 4 neutropenia, the patient should show an estimated CressSS lower than 100 µg/L.

Palbociclib is an oral cyclin-dependent kinase inhibitor that is used in combination with aromatase inhibitors in the treatment of postmenopausal women with metastatic breast cancer. Its metabolism profile is associated with an important interpatient variability. We performed a population pharmacokinetics study of palbociclib in women routinely followed in a cancer center. One hundred and fifty-one samples were analyzed. The sampling times after administration ranged from 0.9 to 75 h and the samples were taken between 1 and 21 days after the beginning of the palbociclib cycle. Palbociclib was determined using a validated mass spectrometry method. The best model that described the concentrations was a one-compartment model with first-order absorption and an absorption lag time. Interindividual variability could only be estimated on the clearance and the first-order absorption. Creatinine clearance was found to be a significant covariate for the apparent clearance. No significant covariates could be observed with the first-order absorption. First-order absorption and absorption lag times were difficult to assess because of the constraints linked to the real-world setting due to the small number of samples used during the absorption process. However, palbociclib apparent clearance was satisfactorily estimated. Population pharmacokinetics (POP PK) with palbociclib could help to optimize dosing.