Background: This Phase 0 trial evaluates tumor pharmacokinetic (PK) and pharmacodynamic (PD) responses of fibroblast growth factor receptor (FGFR) inhibitor, infigratinib, in participants with recurrent high-grade glioma carrying FGFR1 K656E or FGFR3 K650E mutation or FGFR3-TACC3 translocation. Patients with high unbound drug concentrations in the gadolinium-nonenhancing tumor region were graduated to a therapeutic regimen of infigratinib. Methods: Recurrent high-grade glioma patients received 7 days of infigratinib (125 mg QD) prior to planned resection at 7-9 hours following the last dose. Tumor tissue (enhancing and nonenhancing regions), cerebrospinal fluid (CSF), and plasma were collected. Total and unbound infigratinib concentrations were measured using validated LC-MS/MS methods. A PK ‘trigger’ determined eligibility for the therapeutic expansion phase and was defined as unbound [infigratinib] > 5-fold biochemical IC50 (i.e., 5 nM) in non-enhancing tumor. PD response was assessed by quantification of percentage of positive pERK, MIB-1, and Cleaved Caspase 3 cells in the surgical tissue compared to baseline archival/biopsy tissue. Patients with tumors exceeding the PK threshold for unbound drug concentration were eligible for expansion phase therapeutic dosing of infigratinib. Results: Seven patients were enrolled into the Phase 0 study with 3 patients who met the PK threshold continued to the expansion phase. In non-enhancing tumor region, the mean unbound concentration of infigratinib was 3.5 nM (n=7). The mean non-enhancing tumor-to-plasma coefficient for unbound drug (Kp,uu) was calculated to be 1.4. The suppression of pERK levels and MIB-1 levels was observed in 17% (1/6) and 67% (4/6) of the patients, respectively. At a median follow-up of 16.3 months [range: 2.8-18.7 months], the median progression-free survival (PFS) was 2.8 months (n=3). One patient remains on treatment. No grade 3+ toxicities were observed, and no adverse events resulted in discontinuation of therapy. Conclusions: Despite relatively high unbound tumor-to-plasma (Kp,uu) value, infigratinib achieves sub-optimal unbound drug concentration in the non-enhancing brain tumor tissue. Pharmacodynamic response with archival tissue as comparator did not reveal inhibition of pERK. Clinical trial information: NCT04424966.