Molecular Imaging of ABCB1 and ABCG2 Inhibition at the Human Blood–Brain Barrier Using Elacridar and 11C-Erlotinib PET

Transporters such as ABCB1 and ABCG2 limit the exposure of several anticancer 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 and ABCG2 inhibitor elacridar on 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 1,000-mg dose of elacridar. ¹¹C-erlotinib brain uptake was quantified by pharmacokinetic modeling using volume of distribution (V_T) as the outcome parameter. In addition, ¹⁵O-H₂O scans to measure cerebral blood flow were acquired before each ¹¹C-erlotinib scan. Results: Brain uptake of ¹¹C-erlotinib was 2.6-fold higher in Abcb1a/b;Abcg2 knockout mice than in WT mice, measured as percentage injected dose per gram of tissue (P = 0.01). In WT mice, the addition of elacridar (at systemic plasma concentrations of ≥200 ng/mL) resulted in an increased brain concentration of erlotinib, without affecting erlotinib plasma concentration. In patients, the V_T of ¹¹C-erlotinib did not increase after intake of elacridar (0.213 ± 0.12 vs. 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 V_T 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 of ¹¹C-erlotinib increased both at a tracer dose and at a pharmacologic 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 interspecies differences.