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Blockade of serotonin 2A (5-HT2A) receptors is regarded as an anti-dyskinetic and anti-psychotic strategy in Parkinson's disease (PD). However, the 5-HT2A antagonists tested so far exhibited affinity for other receptors, which might have played a role in their action. EMD-281,014 is the most selective 5-HT2A antagonist available, with approximately 2,000-fold selectivity over serotonin 2C (5-HT2C) receptors. EMD-281,014 was previously tested in the clinic and has high translational potential. In the present study, we assessed the effect of EMD-281,014 on dyskinesia and psychosis in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned common marmoset. We first determined the pharmacokinetic profile of EMD-281,014 in the marmoset, after which doses leading to clinically-relevant plasma levels (0.01, 0.03 and 0.1mg/kg) or vehicle were administered to MPTP-lesioned marmosets, in combination with L-3,4-dihydroxyphenylalanine (l-DOPA). The effects of EMD-281,014 on dyskinesia, psychosis-like behaviours (PLBs) and parkinsonism were then evaluated. When added to l-DOPA, EMD-281,014 (0.03 and 0.1mg/kg) reduced peak dose dyskinesia, by 41.8% and 54.5% (P<0.05 and P<0.001), when compared to l-DOPA/vehicle. EMD-281,014 (0.03 and 0.1mg/kg) also significantly reduced the severity of peak dose PLBs, by 42.5% and 45.9% (P<0.05 and P<0.001), when compared to vehicle. The anti-dyskinetic and anti-psychotic effects of EMD-281,014 were achieved without interfering with l-DOPA anti-parkinsonian action. Our results suggest that highly-selective 5-HT2A receptor blockade with EMD-281,014 is an effective way to alleviate both dyskinesia and psychosis in PD, without adversely affecting parkinsonian disability.EMD-281,014 is a potent and highly selective 5-HT2A antagonist.We have determined the pharmacokinetic profile of EMD-281,014 in the marmoset.EMD-281,014 reduces l-DOPA-induced dyskinesia in the MPTP-lesioned marmoset.EMD-281,014 reduces dopaminergic psychosis in the MPTP-lesioned marmoset.EMD-281,014 does not hinder the anti-parkinsonian effect of l-DOPA.