Cerebrospinal fluid penetration of the colony-stimulating factor-1 receptor (CSF-1R) inhibitor, pexidartinib

Purpose: Pexidartinib (PLX3397) is a colony-stimulating factor-1 receptor (CSF-1R) inhibitor under clinical evaluation for potential CNS tumor treatment. This study aims to evaluate plasma pharmacokinetic parameters and estimate CNS penetrance of pexidartinib in a non-human primate (NHP) cerebrospinal fluid (CSF) reservoir model. Methods: Five male rhesus macaques, each with a previously implanted subcutaneous CSF ventricular reservoir and central venous lines, were used. NHPs received a single dose of 40 mg/kg pexidartinib (human equivalent dose of 800 mg/m²), administered orally as 200 mg tablets. Serial paired samples of blood and CSF were collected at 0–8, 24, 48, and 72 h. Pexidartinib concentrations were assayed by Integrated Analytical Solutions, Inc. (Berkeley, CA, USA) using HPLC/MS/MS. Pharmacokinetic (PK) analysis was performed using noncompartmental methods. Results: Samples from four NHPs were evaluable. Average (± SD) plasma PK parameters were as follows: C_max = 16.50 (± 6.67) μg/mL; T_max = 5.00 (± 2.58) h; AUC_last = 250.25 (± 103.76) h*μg/mL; CL = 0.18 (± 0.10) L/h/kg. In CSF, pexidartinib was either quantifiable (n = 2), with C_max values of 16.1 and 10.1 ng/mL achieved 2–4 h after plasma T_max, or undetected at all time points (n = 2, LLOQ_CSF = 5 ng/mL). Conclusion: Pexidartinib was well-tolerated in NHPs, with no Grade 3 or Grade 4 toxicities. The CSF penetration of pexidartinib after single-dose oral administration to NHPs was limited.