First- & Second-Generation Antipsychotics: Mechanisms, Adverse Effects, and Clinical Considerations

Mechanism of Action (Both Generations)

• PRIMARY: Antagonism of central dopamine receptors—especially D_2.

  • Rationale: Schizophrenia = relative dopamine excess → blocking brings signaling toward normal.

• OFF-TARGET (non-dopaminergic) blockade produces many effects/adverse effects:

  • M_1 muscarinic (antimuscarinic)

  • H_1 histamine (antihistamine)

  • 5-HT serotonergic

  • \alpha_1 adrenergic

• Result: very broad pharmacodynamic profile → wide spectrum of clinical use AND toxicity.

Pharmacodynamic Spectrum (Receptor Targets)

• D_2: antipsychotic efficacy, extrapyramidal symptoms (EPS), hyperprolactinaemia.

• 5-HT{2A/2C}: mood, weight, metabolic effects; less EPS when blocked concomitantly with D2.

• H_1: sedation, weight gain.

• M_1: dry mouth, blurred vision, constipation, urinary retention, cognitive blunting.

• \alpha_1: orthostatic hypotension, reflex tachycardia.

First-Generation (Typical) Antipsychotics

• Examples (older): chlorpromazine, haloperidol, fluphenazine, pericyazine, thioridazine, etc.

• Dose–response almost linear for D_2 blockade.

• Key characteristics

  • Strong D_2 antagonism → HIGH EPS risk.

  • Also block M1, H1, 5-HT, \alpha_1 → antimuscarinic, sedative, hypotensive effects.

• Major adverse effects

  • Extrapyramidal motor disturbances

  • Acute dystonia (hours–days)

  • Akathisia (days–weeks)

  • Parkinsonism (weeks)

  • Tardive dyskinesia (months–years; often irreversible—e.g., lip-smacking, tongue protrusion, choreo-athetoid movements)

  • Endocrine/hypothalamic

  • Hyperprolactinaemia → galactorrhoea, gynecomastia, menstrual irregularity, sexual dysfunction.

  • Sedation (via H_1)

  • Antimuscarinic: dry mouth, blurred vision, constipation, urinary retention, cognitive slowing.

  • Orthostatic hypotension (\alpha_1 blockade)

  • Neuroleptic Malignant Syndrome (NMS)

  • Life-threatening hyperthermia, rigidity, autonomic instability, ↑CK.

• Clinical use

  • Acute & chronic psychosis (schizophrenia, schizoaffective)

  • Rapid tranquillisation for acute behavioural disturbance

  • Off-label last-line anti-emetic (e.g., refractory hyperemesis gravidarum)

Second-Generation (Atypical) Antipsychotics

• Examples: clozapine, olanzapine, quetiapine, risperidone, paliperidone, ziprasidone, aripiprazole, lurasidone.

• Mechanism tweaks

  • Moderate D2 blockade + potent 5-HT{2A} antagonism → ↓EPS.

  • Still interact with H1, M1, \alpha_1 receptors → own side-effect signature.

• Advantages vs typicals

  • Markedly lower incidence of EPS & tardive dyskinesia.

• New adverse-effect cluster (“METABOLIC & CARDIAC”)

  • Weight gain (↑appetite via H1/5-HT2C)

  • Increased risk of type 2 diabetes mellitus (insulin resistance)

  • Dyslipidaemia

  • Prolonged QT (QT_{c} \ge 450 \text{ ms}) → torsades de pointes risk

  • Orthostatic hypotension

  • Sedation

  • Seizure threshold ↓ (esp. clozapine)

  • Blood dyscrasias (agranulocytosis with clozapine; hence mandatory FBC monitoring)

  • Persisting hyperprolactinaemia (risperidone/paliperidone)

• Clinical considerations

  • Onset: ∼2 weeks for partial response; up to 3 months for full effect → allow adequate trial before switching.

  • Clozapine

  • Reserved for treatment-resistant schizophrenia (after failure of ≥2 antipsychotics)

  • Superiority in ↓suicidality & symptom control

  • Requires prescriber to be psychiatrist + strict monitoring (weekly → monthly FBC, weight, lipids, glucose).

Adverse-Effects Matrix (simplified from lecture table)

Clinical Prescribing Principles

• Patient-specific factors

  • Age, sex (child-bearing potential), comorbidities (DM, CVD, epilepsy), prior response, side-effect tolerance.

• Start low, titrate slowly; aim for minimum effective dose.

• Trial period ≥12 weeks before deeming ineffective (unless severe intolerance).

• Monitor

  • Baseline & periodic: weight/BMI, waist circumference, fasting lipids & glucose, BP, ECG (QT), EPSE scales.

  • Clozapine: FBC (neutrophils), CRP, troponin (myocarditis risk).

• Avoid polypharmacy unless specialist recommendation.

• Use in dementia: short-term, low-dose, closely monitored (↑ stroke/mortality risk).

Review / Key Takeaways

• Antipsychotics work chiefly via D_2 antagonism, but receptor promiscuity explains wide adverse-effect spectrum.

• First-gen = potent D_2 block → EPS, prolactin ↑, NMS.

• Second-gen ↓EPS but ↑metabolic, cardiac & haematologic toxicity.

• Many ADEs are irreversible (e.g., tardive dyskinesia) → early detection critical.

• Drug choice balances efficacy vs side-effect risk; atypicals now preferred first-line.

• Adequate duration, systematic monitoring & patient-centred selection = keys to success.

How They Work (Both Types)

• MAINLY: Block dopamine receptors in the brain, especially D_2.- Why: Schizophrenia involves too much dopamine activity, so blocking it helps bring levels back to normal.

• ALSO: Block other receptors, causing many side effects:

  • M_1 (muscarinic)

  • H_1 (histamine)

  • 5-HT (serotonin)

  • \alpha_1 (adrenergic)

• Result: They affect many body systems, leading to wide uses and many side effects.

What Receptors Do

• D_2: Treats psychosis, but causes movement problems (EPS) and high prolactin.

• 5-HT{2A/2C}: Affects mood, weight, and blood sugar; helps reduce EPS when D2 is also blocked.

• H_1: Causes sleepiness and weight gain.

• M_1: Leads to dry mouth, blurry vision, constipation, trouble urinating, and slowed thinking.

• \alpha_1: Can cause low blood pressure when standing up (orthostatic hypotension) and a fast heart rate.

First-Generation (Older) Antipsychotics

• Examples: chlorpromazine, haloperidol, fluphenazine.

• How they work: Strongly block D_2 receptors.

• Key features:

  • High risk of movement problems (EPS) due to strong D_2 blocking.

  • Also block M1, H1, 5-HT, \alpha_1 receptors, leading to side effects like dry mouth, sedation, and low blood pressure.

• Main side effects:

  • Movement problems (EPS):

    • Acute dystonia: Muscle spasms (hours-days).

    • Akathisia: Restlessness (days-weeks).

    • Parkinsonism: Tremors, stiffness (weeks).

    • Tardive dyskinesia: Involuntary movements, often irreversible (months-years, e.g., lip-smacking).

  • Hormone issues:

    • High prolactin: causes breast milk production, breast growth in men, irregular periods, sexual problems.

  • Sedation (sleepiness from H_1 block).

  • Antimuscarinic effects: dry mouth, blurry vision, constipation, trouble urinating, slowed thinking.

  • Orthostatic hypotension: Low blood pressure when standing (\alpha_1 block).

  • Neuroleptic Malignant Syndrome (NMS):

    • A dangerous condition with high fever, muscle stiffness, unstable vital signs, and muscle breakdown.

• Uses:

  • Treating severe mental illnesses (schizophrenia).

  • Quickly calming agitated patients.

  • Sometimes used for severe nausea (last resort).

Second-Generation (Newer) Antipsychotics

• Examples: clozapine, olanzapine, quetiapine, risperidone.

• How they work: Affect D2 less strongly and also block 5-HT{2A} receptors, which helps reduce EPS.

  • Still affect H1, M1, \alpha_1 receptors, causing their own set of side effects.

• Advantages over older ones:

  • Much lower risk of movement problems (EPS) and tardive dyskinesia.

• New common side effects (Metabolic & Heart-related):

  • Weight gain (from increased appetite, via H1/5-HT{2C}).

  • Higher risk of type 2 diabetes.

  • High cholesterol.

  • Prolonged QT interval on ECG (QT_c \ge 450 \text{ ms}), increasing risk of heart rhythm problems.

  • Orthostatic hypotension.

  • Sedation.

  • Lower seizure threshold (especially clozapine).

  • Blood disorders (e.g., severe drop in white blood cells (agranulocytosis) with clozapine, requiring regular blood tests).

  • Ongoing high prolactin (risperidone/paliperidone).

• Important points:

  • Takes about 2 weeks to see some effect, up to 3 months for full effect. Give them time to work.

  • Clozapine:

    • Used for schizophrenia that hasn't responded to other treatments.

    • Better at reducing suicide risk and symptoms.

    • Needs very strict monitoring (weekly/monthly blood tests, weight, lipids, sugar).

Side-Effects Comparison (Simplified)

Principles for Prescribing

• Consider the patient:

  • Age, gender, other health problems (diabetes, heart disease, epilepsy), past drug responses, ability to tolerate side effects.

• Start with a low dose and increase slowly; aim for the lowest dose that works.

• Try a drug for at least 12 weeks before deciding it's not effective.

• Monitor:

  • Regularly check: weight/BMI, waist size, fasting blood sugar and lipids, blood pressure, ECG (for QT interval), and movement disorder scales.

  • For Clozapine: check blood counts (neutrophils), CRP, and troponin (for heart inflammation risk).

• Avoid using many drugs together unless a specialist recommends it.

• In dementia, use only short-term, low-dose, and with close monitoring (increases stroke/death risk).

Summary

• Antipsychotics work mainly by blocking D_2 dopamine receptors, but affecting other receptors causes many side effects.

• Older (first-gen) drugs strongly block D_2, causing movement problems (EPS), high prolactin, and NMS.

• Newer (second-gen) drugs cause fewer movement problems but more metabolic, heart, and blood-related side effects.

• Many side effects, like tardive dyskinesia, can be permanent, so early detection is key.

• Choosing a drug means balancing how well it works with its side effect risks; newer drugs are often preferred first.

• Giving enough time for the drug to work, monitoring patients carefully, and choosing the right drug for each person are crucial for success.