Propiomazine is an antagonist at types 1, 2, and 4 dopamine receptors, serotonin (5-HT) receptor types 2A and 2C, muscarinic receptors 1 through 5, alpha(1)-receptors, and histamine H1-receptors. Propiomazine's antipsychotic effect is due to antagonism at dopamine and serotonin type 2 receptors, with greater activity at serotonin 5-HT2 receptors than at dopamine type-2 receptors. This may explain the lack of extrapyramidal effects. Propiomazine does not appear to block dopamine within the tubero-infundibular tract, explaining the lower incidence of hyperprolactinemia than with typical antipsychotic agents or risperidone.
Rare, serious side effects include convulsions (seizures); difficult or unusually fast breathing; fast or irregular heartbeat or pulse; fever (high); high or low blood pressure; loss of bladder control; muscle stiffness (severe); unusual increase in sweating; unusually pale skin; and unusual tiredness or weakness.
Conversion of the parent phenothiazine to the propionamide (via the magnesium salt) serves as a protecting group for the amide as a means of allowing the para-directing effect of the sulfide to prevail. Acylation with propionyl chloride and aluminium chloride thus affords, after saponification of the amide, the ketone (4). This is then treated with phosgene to give 5, and this last esterified with 2-dimethylamino-1-propanol (N,N-dimethylalaninol) to yield the corresponding urethane (6). In an interesting reaction, pyrolysis of the urethane leads to extrusion of carbon dioxide and formation of 7, propiomazine.
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