Dopamine D2 and serotonin 5-HT2 receptors (antagonist)

Class

Risperidone — an established, long-marketed second-generation (atypical) antipsychotic. 'QUAR' (Quarterly Risperidone) is not a new molecule; it is a long-acting intramuscular reformulation of risperidone using Rovi Pharmaceuticals' in-situ microparticle (ISM) depot technology, intended to extend dosing to roughly once every three months (the QUARTZ pharmacokinetics study).

Pathway

Combined antagonism at dopamine D2 and serotonin 5-HT2 receptors, the accepted (though not definitively proven) basis for risperidone's antipsychotic activity per its own FDA label.

This mechanism describes a drug class that blocks two neurotransmitter receptors in the brain: the dopamine D2 receptor and a subtype of serotonin receptor called 5-HT2A. Dopamine D2 receptors are strongly implicated in psychosis, and blocking them reduces hallucinations and delusions, the basis of first-generation antipsychotics. However, D2 blockade alone, especially in brain regions controlling movement, commonly causes stiffness, tremor, and other motor side effects. Second-generation, or atypical, antipsychotics add antagonism at 5-HT2A receptors, which modulates dopamine release in motor pathways and appears to reduce the risk of these movement-related side effects while preserving antipsychotic benefit. This combined receptor profile is thought to give a broader therapeutic margin than D2 blockade alone, though the precise contribution of 5-HT2A antagonism to clinical efficacy is not fully established. This mechanism is central to the treatment of schizophrenia, bipolar disorder with psychotic or manic features, and related conditions where dopaminergic overactivity in certain brain circuits is believed to drive psychotic symptoms. Formulation strategies, including long-acting injectable versions of such molecules, aim to improve consistency of drug exposure over time, which is clinically relevant in conditions where treatment adherence is often difficult to sustain.