Transporters such as ABCB1 and ABCG2 limit the exposure of several anti-cancer drugs to the brain, leading to suboptimal treatment in the central nervous system. The purpose of this study was to investigate the effects of the ABCB1/ABCG2 inhibitor elacridar on erlotinib brain uptake using 11C-erlotinib PET. Methods: Elacridar and cold erlotinib were administered orally to wild type (WT) and Abcb1a/b;Abcg2 knockout mice. In addition, brain uptake was measured using 11C-erlotinib imaging and ex vivo scintillation counting in knockout and WT mice. Six patients with advanced solid tumors underwent 11C-erlotinib PET scans before and after a 1000 mg dose of elacridar. 11C-erlotinib brain uptake was quantified by pharmacokinetic modeling using volume of distribution (VT) as outcome parameter. In addition, 15O-H2O scans were acquired prior to each 11C-erlotinib scan, to measure cerebral blood flow . Results: Brain uptake of 11C-erlotinib was 2.6-fold higher in Abcb1a/b;Abcg2 knockout mice than in WT mice, measured as % of injected dose/gram tissue (P = 0.01). In WT mice, addition of elacridar (at systemic plasma concentrations of ≥200 ng/mL) resulted in increased brain concentration of erlotinib, without affecting erlotinib plasma concentration. In patients, VT of 11C-erlotinib did not increase after intake of elacridar (0.213±0.12 versus 0.205±0.07, P = 0.91). 15O-H2O PET showed no significant changes in cerebral blood flow. Elacridar exposure in patients was 401±154 ng/mL. No increase in VT with increased elacridar plasma exposure was found over the 271-619 ng/mL range. Conclusion: When Abcb1 and Abcg2 were disrupted in mice, brain uptake increased both at a tracer and pharmacological erlotinib dose. In patients, brain uptake of 11C-erlotinib was not higher after administration of elacridar. The more pronounced role that ABCG2 appears to play at the human blood-brain barrier and the lower potency of elacridar to inhibit ABCG2 may be an explanation of these inter-species differences.