Population Pharmacokinetic and Pharmacodynamic Study of Palbociclib in Children and Young Adults with Recurrent, Progressive, or Refractory Brain Tumors

<b>Background/Objectives:</b> Palbociclib, an oral CDK 4/6 inhibitor, was evaluated in a Pediatric Brain Tumor Consortium (PBTC) phase 1 (NCT02255461; PBTC-042) study to treat children and young adults with recurrent, progressive, or refractory brain tumors. The objectives of this study were to characterize the palbociclib population pharmacokinetics in children enrolled on PBTC-042, to conduct a population pharmacodynamic analysis in this patient population, and to perform a simulation study to assess the role of palbociclib exposure on neutropenia and thrombocytopenia. <b>Methods:</b> The palbociclib population pharmacokinetics and pharmacodynamics were characterized in this patient population (n = 34 patients; 4.9-21.6 years old). Population pharmacokinetics were modeled using a one-compartment model with first-order absorption and elimination. Covariate analysis was performed, evaluating demographics, laboratory values, and concomitant medications. A pharmacodynamic model was used to describe the relation between palbociclib plasma exposure and changes in the ANC and platelet counts. <b>Results:</b> The population estimates for the apparent oral volume, apparent oral clearance, and absorption rate constant were 664.5 L/m², 36.8 L/h/m², and 0.48 h^-1, respectively. The palbociclib apparent oral clearance was decreased in patients with higher AST values (<i>p</i> = 0.0066). The ANC and platelet pharmacodynamic models estimated that the median (5th-95th percentile) time individuals had grade 3 or greater neutropenia was 4 (0, 21) days. Simulations showed that given 75 mg/m² palbociclib, 49% of the individuals were expected to have grade 3 or greater neutropenia. <b>Conclusions:</b> Palbociclib pharmacokinetics and pharmacodynamics were adequately characterized in this patient population, no unexpected adverse reactions were noted, and the drug was well tolerated.