Abstract BACKGROUND Checkpoint inhibitors have shown efficacy in other solid tumors and are being studied in glioblastoma. For patients treated with anti-PD-1 monoclonal antibodies, such as pembrolizumab, CSF concentrations and blockade of PD-1 on T cells present in the CSF have not been reported. Such information would provide valuable context for applying checkpoint inhibitors for the treatment of CNS malignancies. METHODS CSF and blood samples were obtained from 6 glioblastoma CAR T cell study patients who received intraventricularly administered CAR T cells. These patients were also treated with pembrolizumab 200 mg intravenously every 3 weeks. Pembrolizumab levels in CSF and blood were measured using an ELISA assay. FACS analysis for PD-1 blockade was performed on endogenous and CAR T cells detected in CSF samples, and the results were compared to cells in CSF without pembrolizumab treatment. RESULTS Data analyzed from 3 patients samples so far show that average pembrolizumab levels in the CSF and serum were 330 ± 114 ng/mL and 64 ± 8 µg/mL, respectively. The average CSF/serum ratio was 0.5 ± 0.1%. In 1 patient from whom multiple CSF/blood samples were obtained over 3 months, steady-state CSF levels ranged from 155–394 ng/mL throughout each 21 day cycle of pembrolizumab. PD-1 was completely blocked on both endogenous T cells as well as CAR T cells that were delivered directly into the CSF. CONCLUSIONS To our knowledge, this is the first report of pembrolizumab concentrations in the CSF after intravenous administration. CSF levels were 0.5% of serum concentrations and remained stable throughout a 21 day cycle of pembrolizumab. PD-1 was found to be blocked on endogenous T cells in the CSF. Furthermore, even though they were relatively low, pembrolizumab levels in the CSF were sufficient to block PD-1 on intraventricularly administered CAR T cells.