Antipsychotic Drugs: Typical and Atypical Agents

Typical antipsychotics like chlorpromazine and haloperidol primarily block D2 receptors, while atypical agents such as risperidone and lurasidone block both D2 and 5HT2A receptors. Atypical agents also act as partial 5HT1A receptor agonists, producing anxiolytic effects. This dual receptor activity distinguishes second-generation from first-generation antipsychotics.

Clinical efficiency typically corresponds to D2 receptor occupancy ranging from 60 to 75%. However, occupancy exceeding 78% can lead to extrapyramidal side effects. This narrow therapeutic window means that antipsychotic effectiveness depends on achieving optimal receptor blockade without excessive occupancy that triggers movement disorders.

Atypical antipsychotics act as partial 5HT1A receptor agonists, which produces anxiolytic effects. Additionally, their potent 5HT2A receptor antagonism contributes to their complex pharmacology. This combination of serotonergic activity distinguishes atypical agents from typical antipsychotics that primarily target dopamine receptors alone.

Drugs like clozapine and risperidone show varying degrees of α2 adrenoceptor antagonism. Clozapine and quetiapine mainly exhibit α1 adrenoceptor antagonism, while risperidone, olanzapine, and aripiprazole demonstrate α2 adrenoceptor antagonism. These additional receptor interactions contribute to the overall pharmacological profile of atypical antipsychotics.

Antipsychotics are generally lipophilic, resulting in their distribution and accumulation in well-vascularized tissues, including the brain. They can also cross into fetal circulation and breast milk, posing potential risks during pregnancy and lactation. Their lipophilic nature enables significant first-pass metabolism and prolonged tissue retention.

Clozapine's unique efficacy in refractory schizophrenia may be attributed to its activity at glutamatergic sites, especially the NMDA receptor. This mechanism distinguishes clozapine from other atypical antipsychotics that primarily work through dopamine and serotonin receptor antagonism, offering an alternative therapeutic approach for treatment-resistant cases.

Long-acting formulations can block D2 receptors for weeks, and metabolites can be excreted weeks after the last dose. This extended duration contrasts with standard formulations requiring more frequent dosing. Abrupt discontinuation of drugs like clozapine can lead to rapid and severe symptom relapse due to prolonged receptor occupancy.