Second-generation antipsychotics

Mechanism of Action - Atypical Balancing Act

• Primary Action: Combined Dopamine $D_2$ receptor and Serotonin $5-HT_{2A}$ receptor antagonism.
  • Mesolimbic Pathway: $D_2$ blockade → ↓ positive symptoms (hallucinations, delusions).
  • Mesocortical Pathway: $5-HT_{2A}$ blockade > $D_2$ blockade → ↑ dopamine release → alleviates negative & cognitive symptoms.
  • Nigrostriatal Pathway: Serotonin blockade counters dopamine blockade → protective against extrapyramidal symptoms (EPS).
  • Tuberoinfundibular Pathway: $D_2$ blockade → ↑ prolactin levels (hyperprolactinemia).

⭐ Exam Favorite: The lower risk of EPS with atypicals is attributed to potent $5-HT_{2A}$ antagonism, which stimulates downstream dopamine release in the nigrostriatal pathway, overriding the $D_2$ blockade.

Key Agents - The Atypical Lineup

• Risperidone (Risperdal): Highest risk of hyperprolactinemia among SGAs due to potent D2 blockade. Available as a long-acting injectable (LAI).
• Olanzapine (Zyprexa): High risk for metabolic syndrome (↑ weight, ↑ lipids, ↑ glucose). 📌 "Olanzapine makes you Obese."
• Quetiapine (Seroquel): Highly sedating (H1 blockade), making it useful for patients with insomnia. Low EPS risk.
• Aripiprazole (Abilify): Unique partial D2 agonist mechanism. Can be activating; common side effect is akathisia.
• Ziprasidone (Geodon): Associated with QTc prolongation. ⚠️ Must be taken with a >500 kcal meal.
• Clozapine (Clozaril): Most effective agent, reserved for treatment-resistant schizophrenia.

⭐ Clozapine is uniquely effective but carries a 1% risk of agranulocytosis, requiring mandatory weekly ANC monitoring for the first 6 months.

Adverse Effects - Metabolic Mayhem & More

• Metabolic Syndrome: Major concern, highest risk with clozapine and olanzapine.

  • Weight Gain: Significant and common.
  • Dyslipidemia: ↑ triglycerides and cholesterol.
  • Hyperglycemia: Risk of new-onset Type 2 Diabetes.

• Monitoring Protocol:

• Other Key Adverse Effects:
  • Hyperprolactinemia: Esp. risperidone, paliperidone → gynecomastia, amenorrhea.
  • QTc Prolongation: Highest risk with ziprasidone.
  • Sedation/Orthostasis: Common with clozapine, quetiapine.
  • ⚠️ Agranulocytosis: Clozapine only. Requires strict ANC monitoring.

⭐ Clozapine is reserved for treatment-resistant schizophrenia due to its superior efficacy but carries a ~1% risk of agranulocytosis, requiring absolute neutrophil count (ANC) to be >1500/μL before starting treatment.

Clinical Choice - The Right Drug, Right Time

• Guiding Principle: Individualize choice based on side-effect profile, patient comorbidities, and prior treatment responses.
• Metabolic Syndrome: For patients with or at high risk for obesity/diabetes, prefer agents with lower metabolic risk like Aripiprazole, Lurasidone, or Ziprasidone.
• Non-Adherence: Consider long-acting injectable (LAI) formulations (e.g., Paliperidone, Aripiprazole) to improve compliance.

⭐ Clozapine is uniquely effective for treatment-resistant schizophrenia (failure of 2 other antipsychotics) but requires regular ANC monitoring due to the risk of agranulocytosis.

High‑Yield Points - ⚡ Biggest Takeaways

• Second-generation antipsychotics (SGAs) block both D2 and 5-HT2A receptors.
• They are effective against both positive and negative symptoms of schizophrenia.
• SGAs have a lower risk of EPS and tardive dyskinesia but a higher risk of metabolic syndrome.
• Clozapine is used for treatment-resistant cases and carries a risk of agranulocytosis.
• Risperidone is most associated with hyperprolactinemia.
• Olanzapine and clozapine carry the highest risk for weight gain and metabolic issues.