Translational Aspects of Blood-Brain Barrier Transport and Brain Distribution of Drugs in Health and Disease

A high unmet medical need in the area of CNS diseases coincides with high failure rates in CNS drug development. Efficient treatment of CNS disease is constrained by limited entrance of drugs into the brain owing to the blood-brain barrier (BBB), which separates brain from blood. Insufficient inter-species translation and lack of methods to evaluate therapeutic, unbound, drug concentrations in human brain also contribute to development failure. Further disease related changes in BBB properties and tissue composition raise a concern of altered drug neuropharmacokinetics (neuroPK) during disease. This calls for the evaluation of translational aspects of neuroPK parameters in health and disease, and exploration of strategies for neuroPK translations between rodents and humans. Positron emission tomography (PET) enables corresponding PK analysis in various species, although being restricted to measuring total, i.e. both unbound and nonspecifically bound, drug concentrations. However, the current work shows that PET can be used for the estimation of unbound, active, brain concentrations and for assessment of drug BBB transport, if compensation is made for intra-brain drug distribution and binding. Adapted PET designs could be applied in humans where rat estimates of drug intra-brain distribution may be used with reasonable accuracy for concentration conversions in healthy humans, but preferably not in Alzheimer’s disease (AD) patients. As shown in this thesis, a high variability in nonspecific drug tissue binding was observed in AD compared to rats and human controls that might lead to unacceptable bias of outcome values if used in PET. Furthermore, heterogeneity in drug tissue binding among brain regions in both rodents and humans was detected and must be considered in regional investigations of neuroPK. By the use of transgenic animal models of amyloid beta and alpha-synuclein pathology, the work further suggests that the BBB is able to uphold sufficient capacity for the transport of small molecular drugs and integrity towards large molecules despite the presence of hallmarks representative of neurodegenerative diseases. This thesis work provides insight into neurodegenerative disease impact on neuroPK and contributes with translational strategies for neuroPK evaluation from preclinical investigations to the clinic, aimed to aid drug development and optimal disease management