Clozapine

Mechanism

Chemical and Commercial Profile

Common trade names: Leponex, Nemea, Clozaril.
Group: Atypical antipsychotic, derived from dibenzodiazepines.

Mechanism of Action

It shows a multireceptor profile characterized by a weak and transient antagonism of dopamine type 2 receptors (D₂, K_i ≈ 150 nM), rapidly dissociating from the receptor as soon as the plasma peak ceases, which almost completely avoids extrapyramidal effects and hyperprolactinemia.

Exercises its superior antipsychotic efficacy through:

• Powerful blockade of the serotonergic 5-HT_2A receptor (K_i ≈ 5 nM), restoring the release of dopamine in the prefrontal cortex and controlling negative symptoms.
• Selective high-affinity D₄ receptor blockade.
• Secondary antagonism at α₁, α₂, H₁ and muscarinic receptors (especially M₁).

Pharmacokinetics

Pharmacokinetics

• Absorption: Excellent oral absorption, bioavailability of 50-60% due to moderate first-pass metabolism. Not influenced by fat intake.
• Metabolism: Hepatic catalyzed mainly by the isoenzymes CYP1A2 and CYP3A4. It generates the active metabolite desmethylclozapine (norclozapine).
• Excretion: Renal (50%) and biliary/fecal (40%).
• Half-life (t_1/2): 12 to 16 hours.

Indicators and dose

Indications

• Treatment-resistant schizophrenia (defined by documented therapeutic failure after adequate use of at least two different antipsychotics in effective doses for 6-8 weeks, one of them second generation).
• Reduction in the risk of recurrent suicidal behavior in patients diagnosed with schizophrenia or schizoaffective disorder with a high risk of self-lysis.
• Psychosis associated with idiopathic Parkinson's disease refractory to dopaminergic reduction.

Dosage and Settings

• Mandatory initiation protocol: Day 1: 12.5 mg orally once or twice a day. Day 2: 25 mg/day. If there is good tolerance, increase from 25 mg to 25 mg daily until reaching the effective range of 200 mg to 450 mg/day divided into doses (higher dose at night). Absolute maximum: 900 mg/day.
• Treatment interruption: If the patient suspends clozapine for more than 48 hours in a row, autonomic tolerance is lost and the titration must be restarted from 12.5 mg.
• Kidney / Liver Failure: Not recommended in terminal stages of liver failure. In moderate oral stages, limit to a maximum dose of 200 mg/day and increase at intervals of 12.5 mg weekly.

Security

Contraindications

Safety alert · Agranulocytosis

Clozapine induces a ~0.8%-1% risk of developing severe agranulocytosis and potentially lethal neutropenia through direct toxicity to the bone marrow stroma. Its dispensation is legally conditioned to the performance of mandatory serial blood counts: weekly during the first 18 weeks, and then monthly throughout the course of treatment.

• Personal history of agranulocytosis or previous toxic granulocytopenia with clozapine or any other drug.
• Impaired bone marrow function (myelodysplasia, active chemotherapy).
• Uncontrolled refractory epilepsy (clozapine strongly decreases the epileptogenic threshold at doses greater than 450 mg/day).
• Active myocarditis or known secondary cardiomyopathy.

Adverse Effects (ADR)

Clinical Interactions

• Myelotoxic drugs: Co-administration with dipyrone (metamizole), carbamazepine, chemotherapy or synthetic antithyroid drugs is prohibited.
• CYP1A2 inducers and inhibitors: Tobacco is a potent inducer of the CYP1A2 pathway. If a patient enters a smoke-free facility or abruptly stops smoking, the plasma concentration of clozapine may rise by 50-70%, triggering seizures or acute cardiac toxicity.

Pregnancy and Breastfeeding

FDA Category: B. More favorable safety data than other typical antipsychotics in terms of absence of proven direct teratogenicity, although it is associated with the risk of fetal macrosomia due to the induction of maternal gestational diabetes. Breastfeeding: Absolutely contraindicated. It concentrates in breast milk at rates that can depress hematopoiesis or induce critical sedation in the infant.