Starting dose selection of palbociclib in Chinese patients with breast cancer based on population kinetic-pharmacodynamic model of neutropenia.

Neutropenia is the most common adverse event (AE) of palbociclib, an oral CDK4/6 inhibitor for breast cancer. Neutropenia increases the risk of infection and is even life threatening. Asian patients generally suffer more severe neutropenia from palbociclib treatment, but the label does not recommend a reduction in the starting dose for Asian patients. Therefore, the study aimed to explore the exposure-response (E-R) relationship in Chinese patients and preliminarily generate a scale for starting dose selection of palbociclib in Chinese patients. After comparing the kinetic-pharmacodynamic (K-PD) and the pharmacokinetic/pharmacodynamic (PK/PD) model, a semi-mechanistic K-PD model was selected and developed on the basis of real-world data from 28 patients with breast cancer to describe the time course of longitudinal absolute neutrophil counts (ANC). The longitudinal ANC data were well described by the population K-PD model with reasonable parameters: mean transit time (MTT) of 198 h, feedback parameter (γ) of 0.317, baseline ANC level (Circ0) of 3.36 × 109 L-1, drug effect coefficient (kd) of 0.0349, and drug effect power (β) of 0.383. No covariate was included in the final model. The model showed that palbociclib dose-dependently reduced ANC levels in a Chinese population, and lower baseline ANC level was associated with more severe neutropenia. The dose selection scale suggested that palbociclib 125 mg daily was appropriate for Chinese patients with Circ0 higher than 3.75 × 109 L-1. In summary, the K-PD model of palbociclib well described the longitudinal ANC in Chinese patients. Besides, the starting dose selection scale may provide reference for clinicians during individualized therapy.