(sold as Clozaril
in Canada; Clozaril
in the UK; Klozapol
in Poland) is an antipsychotic
medication used in the treatment of schizophrenia
. The first of the atypical antipsychotics
to be developed, it was first introduced in Europe in 1971, but was voluntarily withdrawn by the manufacturer in 1975 after it was shown to cause agranulocytosis
that led to death in some clozapine-treated patients. In 1989, after studies demonstrated that it was more effective than any other antipsychotic for treating schizophrenia, the FDA approved clozapine's use but only for treatment-resistant schizophrenia. The FDA requires weekly blood testing for patients taking clozapine. The FDA also requires clozapine to carry five black box
warnings for agranulocytosis
, seizures, myocarditis, for "other adverse cardiovascular and respiratory effects", and for "increased mortality in elderly patients with dementia-related psychosis. In 2002 the FDA approved clozapine for reducing the risk of suicidal behavior
but only for patients with schizophrenia.
Clozapine has been shown to be the most effective drug in treating schizophrenia but due to its potential to cause many severe side effects, it is relegated to third-line use. Clozapine is only used in patients after other anti-psychotics have failed. Safer use of clozapine requires weekly blood monitoring for around five months followed by four weekly testing thereafter.
Clozapine was developed by Sandoz
, and introduced in Europe
ten years later. In 1975
, after reports of agranulocytosis
leading to death in some clozapine-treated patients, clozapine was voluntarily withdrawn by the manufacturer. Clozapine fell out of favor for more than a decade. However, when studies demonstrated that clozapine was more effective against treatment-resistant schizophrenia
than other antipsychotics
, the FDA
and health authorities in most other countries approved its use only for treatment-resistant schizophrenia, and required regular hematological
monitoring to detect granulocytopenia
, before agranulocytosis
develops. In December 2002, clozapine was also approved for reducing the risk of suicide in schizophrenic or schizoaffective
patients judged to be at chronic risk for suicidal behavior.
Clozapine is used principally in treating treatment-resistant schizophrenia, a term generally used for the failure of symptoms to respond satisfactorily to at least two different antipsychotics; It clearly has been shown to be more effective in reducing symptoms of schizophrenia than the older typical antipsychotics
, with maximal effects in those who have responded poorly to other medication; though the relapse rate is lower and patient acceptability better, this has not translated to significant observed benefits in global functioning.
It is also used for reducing the risk of suicide in patients judged to belong to a high risk group with chronic risk for suicidal behavior. Clozapine was shown to prolong the time to suicidal attempt significantly greater than olanzapine.
Clozapine works well against positive (e.g. delusions, hallucinations) and negative (e.g. emotional and social withdrawal) symptoms of schizophrenia. It has no dyscognitive effect often seen with other psychoactive drugs and is even able to increase the capabilities of the patient to react to this environment and thereby fosters social rehabilitation.
Off-label and investigational drug use
- Treatment of psychosis in L-Dopa treated patients (25 to 50 mg at bedtime is often sufficient); this indication is currently approved in Switzerland
- Treatment of psychotic symptoms occurring in patients with dementia of the Lewy-body-type
- Treatment of otherwise resistant acute episodes of mania
- Treatment of intractable chronic insomnia, if all other measures have failed
- Treatment of schizoid personality disorder
Though much research has been done evaluating the benefit of clozapine in treating the aforementioned conditions, results have been inconclusive regarding its actual efficacy in treating those conditions.
Clozapine is contraindicated
in individuals with uncontrolled epilepsy
, myeloproliferative disease
, or agranulocytosis
with prior clozapine treatment.
Many other (relative) contraindications (e.g. preexisting cardiovascular or liver damage, epilepsy) also exist.
The use of clozapine is associated with a fair number of side effects, many minor though some serious and potentially fatal: the more common include constipation
, and weight gain
. The risks of extrapyramidal symptoms
such as tardive dyskinesia
are much less with clozapine when compared to the typical antipsychotics
; this may be due to clozapine's anticholinergic effects. Extrapyramidal symptoms may subside somewhat after a person switches from another antipsychotic to clozapine.
Clozapine also carries black box warnings for seizures, myocarditis, and "other adverse cardiovascular and respiratory effects." Lowering of the seizure threshold may be dose related and slow initial titration of dose may decrease the risk for precipitating seizures. Slow titration of dosing may also decrease the risk for orthostatic hypotension and other adverse cardiovascular side effects.
Clozapine may have a synergistic effect with the sedating action of other drugs such as benzodiazepines, and thus respiratory depression may result with concomitant use. Care should be taken, especially if the latter drugs are given parenterally.
Many male patients have experienced ceasure of ejaculation during orgasm as a side effect of Clozapine though this is not documented in official drug guides.
Clozapine carries a black box warning
for drug-induced agranulocytosis
. Without monitoring, agranulocytosis occurs in about 1% of patients who take clozapine during the first few months of treatment; the risk of developing it is highest about three months into treatment, and decreases substantially thereafter, to less than 0.01% after one year. Patients who have experienced agranulocytosis with previous treatment of clozapine should not receive it again.
In 2007, a pharmacogenetic test was introduced to measure the probability of developing agranulocytosis. The test has two gradations - Higher and Lower risk, with a relative agranulocytosis risk of 2.5 and 0.5 compared to general level. The company states that the test is based on two SNPs of the HLA-DQB1 gene.
In the USA, patients who take clozapine are required to have a blood cell count every week, for the first six months of therapy. After this, they are required to have a blood cell count every other week for the second six months after therapy. After twelve months, blood cell counts need be performed every four weeks. Patients are advised to inform their doctor if they develop a sore throat, or fever. If the number of white blood-cells drops notably then referral to a hematologist is undertaken. The manufacturers of both the brand and generic clozapine are required by the FDA to track white blood cells counts for patients receiving clozapine, and pharmacies are required to obtain a copy of the CBC prior to dispensing the medication to the patient. The purpose of the monitoring system is to prevent rechallenge with clozapine in patients with a history of clozapine-induced agranulocytosis and to detect leukopenic events among patients taking clozapine. In other countries (e.g. in Europe), restrictions have been eased.
A more recently identified and sometimes fatal side effect is that of myocarditis
which usually develops within the first month of commencement and presents with signs of cardiac failure
and cardiac arrhythmias. Cardiomyopathy
is another potentially fatal cardiac condition which may arise less acutely. More recently, a regular six-monthly echocardiogram
is also recommended to detect myocarditis
Another underrecognized and potentially life-threatening side effect spectrum is gastrointestinal hypomotility, which may manifest as severe constipation, fecal impaction, paralytic ileus, bowel obstruction, ischemia or necrosis. Monitoring of bowel function is recommended, as untreated cases are occasionally fatal.
Central nervous system
Psychotic symptoms can worsen while under influence and following the discontinuation especially after long-term use.
Weight gain and diabetes
The FDA requires the manufacturers of all atypical antipsychotics to include a warning about the risk of hyperglycemia and diabetes
with these medications. Indeed, there are case reports of clozapine-induced hyperglycemia and diabetes; additionally, there are case reports of clozapine-induced diabetic ketoacidosis
. There is data showing that clozapine can decrease insulin sensitivity. Clozapine should be used with caution in patients who are diagnosed with diabetes or in patients at risk for developing diabetes. All patients receiving clozapine should have their fasting blood glucose monitored.
In addition to hyperglycemia, significant weight gain is frequently experienced by patients treated with clozapine. Impaired glucose metabolism and obesity have been shown to be constituents of the metabolic syndrome and may increase the risk of cardiovascular disease. The data suggests that clozapine may be more likely to cause adverse metabolic effects than some of the other atypical antipsychotics. Research has indicated that clozapine may cause a deficiency of selenium.
It is insoluble in water, soluble in acetone
, and very soluble in chloroform
Its solubility in water is 11.8 mg/L (25 C)
The manufacturer Novartis claim a solubility of <0.01% in water
Mechanism of action
Clozapine is classified as an atypical antipsychotic
drug because its profile of binding to serotonergic
as well as dopamine
receptors; its effects on various dopamine mediated behaviors also differ from those exhibited by more typical antipsychotics. In particular, clozapine interferes to a lower extent with the binding of dopamine at D1
receptors, and has a high affinity for the D4
receptor, but it does not induce catalepsy
nor inhibit apomorphine
in animal models as is seen with 'conventional' neuroleptics
. This evidence suggests clozapine is preferentially more active at limbic than at striatal dopamine receptors and may explain the relative freedom of clozapine from extrapyramidal
side effects together with strong anticholinergic
Clozapine is also partial agonists at the 5-HT1A receptor, putatively improving depression, anxiety, and negative/cognitive symptoms.
Clozapine also is a strong antagonist at different subtypes of adrenergic, cholinergic and histaminergic receptors, the last two being predominantly responsible for its side effect profile.
It has approximately the same potency as chlorpromazine.
The absorption of clozapine is almost complete, but the oral bioavailability
is only 60 to 70% due to first-pass metabolism
. The time to peak concentration after oral dosing is about 2.5 hours, and food does not appear to affect the bioavailability of clozapine.
The elimination half-life
of clozapine is about 14 hours at steady state
conditions (varying with daily dose).
Clozapine is extensively metabolized in the liver, via the cytochrome P450 system, to polar metabolites suitable for elimination in the urine and faeces. The major metabolite, norclozapine (desmethyl-clozapine), is pharmacologically active. The cytochrome P450 isoenzyme 1A2 is primarily responsible for clozapine metabolism, but 2C, 2D6, 2E1 and 3A3/4 appear to play roles as well. Agents which induce (e.g. cigarette smoke) or inhibit (e.g. theophylline, ciprofloxacin, fluvoxamine) CYP1A2 may increase or decrease, respectively, the metabolism of clozapine. Smoking increases the metabolism of Clozapine to the extent that approximately double the dose is required in heavy smokers compared to non-smokers.
Clozapine and norclozapine plasma levels may also be monitored, though they show a significant degree of variation and are higher in women and increase with age.
Due to risk of serious side effects, clozapine treatment is commenced at a very low dose and increased slowly until a therapeutic dose is reached. In severely ill and/or younger patients higher doses may be needed, while in the elderly much lower doses may be sufficient. Once the patient is stabilized and the maintenance dose has been determined, the greater part or all of the daily dose may be given at bedtime. This will ameliorate daytime sedation and orthostatic problems; most people benefit from the sedation to get to sleep anyway. Furthermore, compliance on medication taken more frequently than once daily drops off dramatically.
Norclozapine - this is the metabolite of Clozapine and is about 70% of the clozapine in a plasma analysis.
Plasma Value of Clozapine for therapeutic monitoring is 0.3 to 0.6
- Benkert, Hippius: Kompendium der Psychiatrischen Pharmakotherapie (German), 4th. ed., Springer Verlag
- B. Bandelow, S. Bleich, and S. Kropp: Handbuch Psychopharmaka (German), 2nd. ed. Hogrefe
- Crilly JF (2007) The history of clozapine and its emergence in the US Market: A review and Analysis. History of Psychiatry, 18(1): 39-60.