Residency · Residency · Psychiatry

Antipsychotic Pharmacology: First- and Second-Generation Agents

Dopamine Hypothesis of Psychosis

Classical Dopamine Hypothesis

Hyperactivity of dopaminergic transmission in the mesolimbic pathway drives positive symptoms (hallucinations, delusions, thought disorder) Evidence: dopamine agonists (amphetamine, L-DOPA) can induce psychosis; all effective antipsychotics block D2 receptors. D2 receptor occupancy of 60-80% correlates with antipsychotic efficacy; >80% associated with EPS.

Revised Dopamine Hypothesis

Mesolimbic DA excess: positive symptoms. Mesocortical DA deficit: negative symptoms and cognitive impairment. Nigrostriatal DA blockade: extrapyramidal side effects (EPS) Tuberoinfundibular DA blockade: hyperprolactinemia. Explains why D2 blockade treats positive symptoms but may worsen negative/cognitive symptoms.

Beyond Dopamine

Glutamate hypothesis: NMDA receptor hypofunction (based on PCP/ketamine models producing positive AND negative symptoms) Serotonin modulation: 5-HT2A antagonism contributes to atypical antipsychotic mechanism. Integrated model: dopamine-serotonin-glutamate interactions.

First-Generation Antipsychotics (FGAs / Typical Antipsychotics)

Mechanism

Primarily D2 receptor antagonists. High-potency agents (haloperidol, fluphenazine): tight D2 binding, more EPS, less sedation/anticholinergic effects. Low-potency agents (chlorpromazine, thioridazine): loose D2 binding, more sedation, anticholinergic, and anti-alpha-1 effects, less EPS. Mid-potency: perphenazine, loxapine.

Key Agents

Haloperidol: prototypical high-potency FGA; widely used for acute agitation and psychosis; available oral, IM, IV; strong EPS liability. Fluphenazine: available as long-acting decanoate injection (every 2-4 weeks) Chlorpromazine: first antipsychotic (1952); highly sedating; orthostatic hypotension; photosensitivity; corneal/lens deposits. Perphenazine: moderate potency; CATIE trial comparator; reasonable side-effect profile. Pimozide: used for Tourette syndrome and delusional disorder; QTc prolongation concern.

FGA Side Effects

EPS: acute dystonia (hours-days), akathisia (days-weeks), parkinsonism (weeks), tardive dyskinesia (months-years) Neuroleptic Malignant Syndrome (NMS): rigidity, hyperthermia, autonomic instability, elevated CK; medical emergency. Hyperprolactinemia: galactorrhea, amenorrhea, sexual dysfunction, osteoporosis risk. QTc prolongation: especially thioridazine, pimozide, IV haloperidol. Anticholinergic: dry mouth, constipation, urinary retention, cognitive impairment (low-potency agents) Sedation and orthostatic hypotension: particularly low-potency agents.

Second-Generation Antipsychotics (SGAs / Atypical Antipsychotics)

Defining "Atypicality"

Combined D2 antagonism with 5-HT2A antagonism (Meltzer hypothesis) Lower EPS risk relative to FGAs (though not zero) Broader receptor binding profiles. Some agents have partial D2 agonist properties.

Key Agents and Unique Profiles

Clozapine

Gold standard for treatment-resistant schizophrenia (TRS) Only antipsychotic with demonstrated superiority in TRS (response rate ~30-60% in TRS) Uniquely reduces suicidality in schizophrenia (InterSePT trial) Low D2 affinity, high 5-HT2A/D4/M1/H1/alpha-1 affinity. Loose D2 binding ("fast-off" theory) -- dissociates quickly from D2 receptor, minimizing EPS. See dedicated seminar (Topic 18) for detailed clozapine management.

Risperidone

Potent D2 and 5-HT2A antagonist. Dose-dependent EPS (particularly >6 mg/day) Significant prolactin elevation (highest among SGAs) Available as long-acting injectable (Risperdal Consta, every 2 weeks) Active metabolite: paliperidone (9-hydroxyrisperidone)

Paliperidone

Active metabolite of risperidone; primarily renal excretion. Available as monthly (Invega Sustenna) and 3-monthly (Invega Trinza) LAI, and 6-monthly (Invega Hafyera) Similar receptor profile to risperidone; significant prolactin elevation.

Olanzapine

Structurally related to clozapine; broad receptor binding (D2, 5-HT2A, H1, M1, alpha-1) Effective for positive and negative symptoms; useful in acute mania. MAJOR metabolic liability: highest weight gain and metabolic syndrome risk among SGAs. Available as short-acting IM (acute agitation) and LAI (Zyprexa Relhyprev -- post-injection delirium/sedation syndrome risk, requires 3-hour monitoring)

Quetiapine

Low D2 affinity; significant H1, alpha-1, and 5-HT2A binding. Active metabolite norquetiapine has NET inhibition (antidepressant properties) Dose-dependent indications: low dose (25-100 mg) for insomnia; moderate (300 mg) for bipolar depression; high (400-800 mg) for schizophrenia/mania. Metabolic effects: weight gain, dyslipidemia (less than olanzapine, more than aripiprazole) Very sedating; minimal EPS.

Aripiprazole

D2 and 5-HT1A partial agonist, 5-HT2A antagonist ("dopamine system stabilizer") Lower EPS risk; weight-neutral to mild gain; minimal prolactin elevation. May cause akathisia (most common limiting side effect) and insomnia/activation. Available as oral, monthly LAI (Abilify Maintena), and 2-monthly LAI (Aristada) FDA-approved for schizophrenia, bipolar mania, MDD augmentation, irritability in autism, Tourette syndrome.

Brexpiprazole

Similar to aripiprazole; D2 partial agonist with lower intrinsic activity. Less akathisia risk than aripiprazole. FDA-approved for schizophrenia and MDD augmentation.

Cariprazine

D2/D3 partial agonist with preferential D3 binding. Extremely long half-life (2-4 weeks for active metabolite DDCAR) Evidence for negative symptoms and bipolar depression. Akathisia and EPS possible.

Ziprasidone

D2/5-HT2A antagonist with SERT/NET reuptake inhibition. Weight-neutral; requires twice-daily dosing with food for absorption (500-calorie meal) QTc prolongation concern (though clinical significance debated) Available as IM for acute agitation.

Lurasidone

D2/5-HT2A/5-HT7 antagonist. FDA-approved for schizophrenia and bipolar depression. Must be taken with 350+ calories for absorption. Favorable metabolic profile; minimal weight gain. Akathisia and EPS possible.

Lumateperone

Low D2 occupancy at therapeutic dose; 5-HT2A antagonist; SERT inhibitor; D1 modulator; glutamate modulator. FDA-approved for schizophrenia and bipolar depression (adjunctive or monotherapy) Favorable metabolic and EPS profile. Somnolence most common side effect.

Metabolic Syndrome and Monitoring

Metabolic Risk Hierarchy (Highest to Lowest)

Clozapine, olanzapine (highest risk) Quetiapine, risperidone/paliperidone (moderate risk) Aripiprazole, ziprasidone, lurasidone, cariprazine (lower risk)

ADA/APA Monitoring Guidelines

Baseline: weight/BMI, waist circumference, fasting glucose, fasting lipid panel, blood pressure, personal/family history of metabolic disease. 4 weeks: weight. 8 weeks: weight. 12 weeks: weight, fasting glucose, fasting lipids, blood pressure. Quarterly: weight. Annually: fasting glucose, fasting lipids, blood pressure. Every 5 years: fasting lipids (if normal)

Management of Metabolic Effects

Lifestyle interventions (diet, exercise) -- limited but recommended. Consider switching to metabolically favorable agent. Metformin augmentation: evidence supports 750-2000 mg/day for antipsychotic-induced weight gain. GLP-1 receptor agonists (liraglutide, semaglutide) emerging as effective options.

Extrapyramidal Side Effects

Acute Dystonia

Onset: hours to days. Sustained involuntary muscle contractions (torticollis, oculogyric crisis, laryngospasm) Risk factors: young males, high-potency FGAs, antipsychotic-naive. Treatment: anticholinergics (benztropine 1-2 mg IM/IV, diphenhydramine 50 mg IM/IV)

Akathisia

Subjective restlessness with inability to sit still. Onset: days to weeks. Often misdiagnosed as anxiety or psychotic agitation. Treatment: dose reduction, beta-blocker (propranolol 20-80 mg/day), benzodiazepine, mirtazapine; switch agent if persistent.

Drug-Induced Parkinsonism

Tremor, rigidity, bradykinesia. Onset: weeks to months. Treatment: dose reduction, anticholinergics (benztropine, trihexyphenidyl), amantadine; switch to lower-EPS-risk agent.

Tardive Dyskinesia (TD)

Involuntary choreiform movements, typically orofacial (lip smacking, tongue protrusion) Onset: months to years of exposure. Risk increases with cumulative antipsychotic exposure, age, female sex, FGAs. May be irreversible; early detection critical. Screening: AIMS (Abnormal Involuntary Movement Scale) at baseline and every 6 months. FDA-approved treatments: VMAT2 inhibitors -- valbenazine (Ingrezza) and deutetrabenazine (Austedo) Prevention: use lowest effective dose, prefer lower-risk agents.

QTc Prolongation

All antipsychotics carry some risk. Highest risk: thioridazine, IV haloperidol, ziprasidone, pimozide. Obtain baseline ECG when using high-risk agents or in patients with cardiac risk factors. QTc >500 ms or increase >60 ms from baseline warrants discontinuation or switch. Monitor electrolytes (K+, Mg2+); correct hypokalemia/hypomagnesemia.

<image> A diagram of the four major dopaminergic pathways in the brain shown in a sagittal view. Label and color-code each pathway: (1) mesolimbic pathway (VTA to nucleus accumbens) -- associated with positive symptoms when hyperactive, (2) mesocortical pathway (VTA to prefrontal cortex) -- associated with negative and cognitive symptoms when hypoactive, (3) nigrostriatal pathway (substantia nigra to striatum) -- D2 blockade causes EPS, (4) tuberoinfundibular pathway (hypothalamus to pituitary) -- D2 blockade causes hyperprolactinemia. Include clinical annotations for each pathway. Neuroanatomy atlas style. </image>

<image> A receptor binding affinity comparison chart for major antipsychotics. Create a heat map or bubble chart with antipsychotics on the y-axis (haloperidol, chlorpromazine, clozapine, risperidone, olanzapine, quetiapine, aripiprazole, ziprasidone, lurasidone) and receptors on the x-axis (D1, D2, D3, D4, 5-HT2A, 5-HT2C, 5-HT1A, H1, M1, alpha-1, alpha-2). Color intensity or bubble size represents binding affinity (Ki values). Include a legend. Clean pharmacology reference style. </image>

<image> A metabolic monitoring flowchart for patients on antipsychotic medications following ADA/APA guidelines. Show a timeline from baseline through 12 months with checkpoints at baseline, 4 weeks, 8 weeks, 12 weeks, quarterly, and annually. At each time point, display icons for the required assessments: scale for weight/BMI, tape measure for waist circumference, blood drop for glucose and lipids, blood pressure cuff for BP. Include a decision branch for abnormal results leading to intervention options: lifestyle modification, metformin, or switch antipsychotic. Clean clinical guideline infographic style. </image>

<image> A timeline diagram showing the temporal onset of extrapyramidal side effects from antipsychotic medications. Use a horizontal timeline from hours to years. Plot each EPS type at its typical onset: acute dystonia (hours-days), akathisia (days-weeks), parkinsonism (weeks-months), tardive dyskinesia (months-years). For each, include a small illustration of the characteristic movement and list the first-line treatment. Include NMS as a separate emergency banner across the early timeline. Medical education poster style. </image>

Clinical Pearls

The CATIE trial (2005) showed that perphenazine (an FGA) was as effective as SGAs with similar discontinuation rates, challenging the assumption that SGAs are categorically superior. D2 receptor occupancy of 60-80% is the therapeutic sweet spot; below 60% is subtherapeutic, above 80% increases EPS without additional efficacy (PET imaging studies) Clozapine is the ONLY antipsychotic with Level 1 evidence for treatment-resistant schizophrenia -- it is dramatically underutilized. Olanzapine and clozapine carry the highest metabolic risk; aripiprazole, ziprasidone, and lurasidone are the most metabolically favorable. Always screen for tardive dyskinesia with the AIMS exam at baseline and every 6 months; VMAT2 inhibitors are now available as FDA-approved treatments. Akathisia is frequently misdiagnosed as psychotic agitation, leading to dose increases that worsen the problem; propranolol is first-line treatment. Long-acting injectable antipsychotics should be considered early in treatment, not reserved as a last resort for non-adherent patients. Quetiapine at low doses (25-100 mg) is widely used off-label for insomnia, but this practice exposes patients to metabolic and neurological risks without evidence of benefit beyond short-term use.

References

• Lieberman JA, et al. Effectiveness of antipsychotic drugs in patients with chronic schizophrenia (CATIE). N Engl J Med. 2005;353(12):1209-1223.
• Leucht S, et al. Comparative efficacy and tolerability of 15 antipsychotic drugs in schizophrenia: a multiple-treatments meta-analysis. Lancet. 2013;382(9896):951-962.
• American Diabetes Association, et al. Consensus development conference on antipsychotic drugs and obesity and diabetes. Diabetes Care. 2004;27(2):596-601.
• Stahl SM. Stahl's Essential Psychopharmacology. 5th ed. 2021.
• Howes OD, Kapur S. The dopamine hypothesis of schizophrenia: version III. Schizophr Bull. 2009;35(3):549-562.

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