Functional Characterization of 5-HT Receptor Drugs in Nonhuman Primates Using Simultaneous PET-MR.

In the present study, we used a simultaneous PET-MR experimental design to investigate the effects of functionally different compounds (agonist, partial agonist, and antagonist) on 5-HT receptor (5-HTR) occupancy and the associated hemodynamic responses. In anesthetized male nonhuman primates ( = 3), we used positron emission tomography (PET) imaging with the radioligand [C]AZ10419369 administered as a bolus followed by constant infusion to measure changes in 5-HTR occupancy. Simultaneously, we measured changes in cerebral blood volume (CBV) as a proxy of drug effects on neuronal activity. The 5-HTR partial agonist AZ10419369 elicited a dose-dependent biphasic hemodynamic response that was related to the 5-HTR occupancy. The magnitude of the response was spatially overlapping with high cerebral 5-HTR densities. High doses of AZ10419369 exerted an extracranial tissue vasoconstriction that was comparable to the less blood-brain barrier-permeable 5-HTR agonist sumatriptan. By contrast, injection of the antagonist GR127935 did not elicit significant hemodynamic responses, even at a 5-HTR cerebral occupancy similar to the one obtained with a high dose of AZ10419369. Given the knowledge we have of the 5-HTR and its function and distribution in the brain, the hemodynamic response informs us about the functionality of the given drug: changes in CBV are only produced when the receptor is stimulated by the partial agonist AZ10419369 and not by the antagonist GR127935, consistent with low basal occupancy by endogenous serotonin. We here show that combined simultaneous positron emission tomography and magnetic resonance imaging uniquely enables the assessment of CNS active compounds. We conducted a series of pharmacological interventions to interrogate 5-HT receptor binding and function and determined blood-brain barrier passage of drugs and demonstrate target involvement. Importantly, we show how the spatial and temporal effects on brain hemodynamics provide information about pharmacologically driven downstream CNS drug effects; the brain hemodynamic response shows characteristic dose-related effects that differ depending on agonistic or antagonistic drug characteristics and on local 5-HT receptor density. The technique lends itself to a comprehensive investigation and understanding of drugs' effects in the brain.