In vitro bidirectional permeability studies identify pharmacokinetic limitations of NKCC1 inhibitor bumetanide

Recently, it has been suggested that bumetanide, an inhibitor of the Na-K-2Cl co-transporter (NKCC1), may be useful in the treatment of central nervous system (CNS) disorders. However, from a physicochemical perspective, bumetanide may not cross the blood-brain barrier to the extent that is necessary for it to be an effective brain NKCC1 inhibitor in vivo. High plasma-protein binding, potentially high brain-tissue binding and putative efflux transporters including organic anion transporter 3 (OAT3) contribute to the poor pharmacokinetic profile of bumetanide. Bidirectional permeability assays are an in vitro method to determine the impact of plasma-protein/brain tissue binding, as well as efflux transport, on the permeability of a compound. We established and validated a cell line stably overexpressing human OAT3 using lentiviral cloning techniques for use in in vitro bidirectional permeability assays. Using efflux transport studies, we show that bumetanide is a transported substrate of human OAT3, exhibiting a transport ratio of >1.5, which is attenuated by OAT3 inhibitors. Bidirectional permeability assays were carried out in the presence and absence of either albumin or brain homogenate to elucidate the effect of plasma-protein/brain tissue binding. These tests confirmed the pharmacokinetic limitations for brain delivery of bumetanide. In this experiment, bumetanide is 53% bound to albumin, 77% bound to brain tissue and accumulates in brain cells. Moreover, we conclusively established that bumetanide is a transported substrate of OAT3. Taken together, these bidirectional permeability studies highlight the potential of efflux transporter inhibition as an augmentation strategy for enhanced delivery of bumetanide to the CNS.