Pharmacodynamic, pharmacokinetic and rat brain receptor occupancy profile of NLX-112, a highly selective 5-HT receptor biased agonist.

NLX-112 (i.e., F13640, befiradol) exhibits nanomolar affinity, exceptional selectivity and full agonist efficacy at serotonin 5-HT receptors. NLX-112 shows efficacy in rat, marmoset and macaque models of L-DOPA induced dyskinesia (LID) in Parkinson's disease and has shown clinical efficacy in a Phase 2a proof-of-concept study for this indication. Here we investigated, in rats, its pharmacodynamic, pharmacokinetic (PK) and brain 5-HT receptor occupancy profiles, and its PK properties in the absence and presence of L-DOPA. Total and free NLX-112 exposure in plasma, CSF and striatal ECF was dose-proportional over the range tested (0.04, 0.16 and 0.63 mg/kg i.p.). NLX-112 exposure increased rapidly (T 0.25-0.5h) and exhibited approximately threefold longer half-life in brain than in plasma (1.1 and 3.6h, respectively). At a pharmacologically relevant dose of 0.16 mg/kg i.p., previously shown to elicit anti-LID activity in parkinsonian rats, brain concentration of NLX-112 was 51-63 ng/g from 0.15 to 1h. In microPET imaging experiments, NLX-112 showed dose-dependent reduction of F-F13640 (i.e., F-NLX-112) brain 5-HT receptor labeling in cingulate cortex and striatum, regions associated with motor control and mood, with almost complete inhibition of labeling at the dose of 0.63 mg/kg i.p.. Co-administration of L-DOPA (6 mg/kg s.c., a dose used to elicit LID in parkinsonian rats) together with NLX-112 (0.16 mg/kg i.p.) did not modify PK parameters in rat plasma and brain of either NLX-112 or L-DOPA. Here, we demonstrate that NLX-112's profile is compatible with 'druggable' parameters for CNS indications, and the results provide measures of brain concentrations and 5-HT receptor binding parameters relevant to the anti-dyskinetic activity of the compound.