Medical Pharmacology Chapter 16:  Pharmacology of Antipsychotics Drugs

• Antipsychotic Medications

  • Learning Objectives

    Upon completing this module, the learner will be able to: 1. Describe the historical discovery of chlorpromazine and its significance to psychiatry. 2. Explain the dopamine hypothesis of antipsychotic action and the role of D2 receptor blockade. 3. Classify first-generation antipsychotics (FGA) by potency and identify representative agents in each class. 4. Summarize evidence supporting first-generation antipsychotics (FGA) efficacy in acute psychosis and relapse prevention. 5. Identify, monitor, and manage the major adverse effects of first-generation antipsychotics (FGAs,) including Extrapyramidal Symptoms (EPS), tardive dyskinesia, and neuroleptic malignant syndrome. 6. Compare first-generation antipsychotics (FGAs) with second-generation antipsychotics in terms of efficacy and tolerability. 7. Recognize the role of depot first-generation antipsychotics (FGAs) formulations in improving medication adherence.

   

  • Historical Context

    • The discovery of chlorpromazine in the early 1950s represents one of the most consequential events in the history of psychiatry.

      • Prior to this era, patients with schizophrenia and other psychotic disorders were largely managed with sedation, physical restraints, or invasive procedures such as insulin coma therapy and frontal lobotomy.

      • The introduction of the first antipsychotic drug fundamentally altered that landscape.

      • In 1952, Jean Delay and Pierre Deniker, a psychiatrist and neurologist working at the Hôpital Sainte-Anne in Paris, systematically evaluated the antihistamine promethazine derivative RPh-4560 (later named chlorpromazine) in psychotic patients.

      • Their seminal report, presented at the Congrès des Médecins Aliénistes et Neurologistes de France in 1952,¹ described dramatic reductions in agitation and psychotic symptoms without rendering patients unconscious, a property they distinguished as a novel "neuroleptic" effect, from the Greek meaning "to clasp the neuron."

    • Haloperidol, synthesized by Paul Janssen in 1958 and introduced clinically in the early 1960s, offered greater potency and became the prototypic high-potency FGA.

      •  Together, chlorpromazine and haloperidol defined the clinical and pharmacological profile of the firstgeneration (or "typical" or "conventional") antipsychotics, a class that would dominate schizophrenia pharmacotherapy for the next three decades.

  • Mechanism of Action

    • The Dopamine Hypothesis

      • The mechanistic basis of FGA efficacy was elucidated in the 1960s and 1970s through a series of landmark neurochemical studies.

        •   In 1963, Carlsson and Lindqvist demonstrated that administration of chlorpromazine and haloperidol to mice increased the turnover of dopamine metabolites (3-methoxytyramine and normetanephrine), suggesting that these drugs were blocking postsynaptic dopamine receptors and triggering a compensatory increase in presynaptic dopamine synthesis.²

          • This observation laid the groundwork for the dopamine hypothesis of antipsychotic action.

      • In 1976, two landmark papers confirmed the central role of D₂ dopamine receptors.

        • Creese, Burt, and Snyder demonstrated that the clinical potency of antipsychotic drugs correlated highly with their affinity for dopamine receptors as measured by radioligand binding assays.³ 

        • In the same year, Seeman and colleagues independently confirmed this finding using [³H]-haloperidol binding, establishing that neuroleptic dosing is directly proportional to D₂ receptor affinity.⁴

          • These findings constituted the evidentiary foundation of the dopamine hypothesis: that the antipsychotic efficacy of FGAs derives primarily from blockade of D₂ dopamine receptors, particularly in the mesolimbic pathway.

  •  Dopamine Pathway Pharmacology

    • The brain contains four primary dopamine pathways, each mediating distinct physiological functions.

      • FGA blockade of D₂ receptors in each of these pathways accounts for both the therapeutic effects and the adverse effect profile of this drug class:

    • Other Receptor Effects

      • In addition to D₂ antagonism, FGAs bind with varying affinity to histamine H₁, muscarinic M₁, and α-₁ adrenergic receptors.

        • The clinical consequences of this off-target binding are significant and differ substantially between high- and low-potency agents:

          •  Histamine H₁ blockade

            • Sedation and weight gain.

              • More pronounced with low-potency agents such as chlorpromazine and thioridazine.

          • Muscarinic M₁ blockade

            •  Anticholinergic effects including dry mouth, urinary retention, constipation, blurred vision, and cognitive impairment. Most prominent with low-potency FGAs.

          • α-1 adrenergic blockade

            • Orthostatic hypotension, dizziness, and reflex tachycardia.

              • Also more common with low-potency agents.

    • Classification of First-Generation Antipsychotics

      • FGAs are classified by potency, defined by the milligram dose required to achieve an effect equivalent to 100 mg of chlorpromazine (the chlorpromazine equivalent, or CPZ-eq).

        • Potency correlates inversely with likelihood of sedation and anticholinergic effects, but directly with risk of extrapyramidal side effects.

        • • *Thioridazine is associated with QTc prolongation and retinal pigmentary changes; use is restricted to refractory cases.
            • CPZ-eq = chlorpromazine equivalent.

    • A clinically useful principle: high-potency FGAs trade sedation and anticholinergic burden for a higher EPS risk, while low-potency agents carry the opposite profile.

      • Mid-potency agents such as perphenazine offer an intermediate balance and were specifically studied in the landmark CATIE trial as the FGA comparator.⁷

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