Haloperidol is metabolized by several routes. The major pathways are glucuronidation and ketone reduction. The cytochrome P450 enzyme system is also involved, particularly CYP3A4 and, to a lesser extent, CYP2D6. Inhibition of these routes of metabolism by another drug or a decrease in CYP2D6 enzyme may result in increased haloperidol concentrations. The effect of CYP3A4 inhibition and of decreased CYP2D6 enzyme activity may be additive.
The haloperidol plasma concentrations increased when a CYP3A4 and/or CYP2D6 inhibitor was coadministered with haloperidol. Examples include:
- CYP3A4 inhibitors – alprazolam; itraconazole, ketoconazole, nefazodone, ritonavir.
- CYP2D6 inhibitors – chlorpromazine; promethazine; quinidine; paroxetine, sertraline, venlafaxine.
- Combined CYP3A4 and CYP2D6 inhibitors – fluoxetine, fluvoxamine; ritonavir.
Increased haloperidol plasma concentrations may result in increased risk of adverse events, including QTc-interval prolongation (see WARNINGS – Cardiovascular Effects). Increases in QTc have been observed when haloperidol was given with a combination of the metabolic inhibitors ketoconazole (400 mg/day) and paroxetine (20 mg/day).
It is recommended that patients who take haloperidol concomitantly with such medicinal products be monitored for signs or symptoms of increased or prolonged pharmacologic effects of haloperidol, and the HALDOL dose be decreased as deemed necessary.
Sodium valproate, a drug known to inhibit glucuronidation, does not affect haloperidol plasma concentrations.
Coadministration of haloperidol with potent enzyme inducers of CYP3A4 may gradually decrease the plasma concentrations of haloperidol to such an extent that efficacy may be reduced. Examples include (but are not limited to): carbamazepine, phenobarbital, phenytoin, rifampin, St John’s Wort (Hypericum, perforatum).
In a study of 12 patients with schizophrenia coadministered oral haloperidol and rifampin, plasma haloperidol levels were decreased by a mean of 70% and mean scores on the Brief Psychiatric Rating Scale were increased from baseline. In 5 other patients with schizophrenia treated with oral haloperidol and rifampin, discontinuation of rifampin produced a mean 3.3-fold increase in haloperidol concentrations.
In a study in 11 patients with schizophrenia coadministered haloperidol and increasing doses of carbamazepine, haloperidol plasma concentrations decreased linearly with increasing carbamazepine concentrations.
During combination treatment with inducers of CYP3A4, it is recommended that patients be monitored and the HALDOL dose increased as deemed necessary. After withdrawal of the CYP3A4 inducer, the concentration of haloperidol may gradually increase and therefore it may be necessary to reduce the dose of HALDOL.
Haloperidol is an inhibitor of CYP2D6. Plasma concentrations of CYP2D6 substrates (e.g., tricyclic antidepressants such as desipramine or imipramine) may increase when they are co-administered with haloperidol.
HALDOL may impair the mental and/or physical abilities required for the performance of hazardous tasks such as operating machinery or driving a motor vehicle. The ambulatory patient should be warned accordingly.
The use of alcohol with this drug should be avoided due to possible additive effects and hypotension.
Carcinogenesis, Mutagenesis, and Impairment of Fertility
No mutagenic potential of haloperidol was found in the Ames Salmonella assay. Negative or inconsistent positive findings have been obtained in in vitro and in vivo studies of effects of haloperidol on chromosome structure and number. The available cytogenetic evidence is considered too inconsistent to be conclusive at this time.
Carcinogenicity studies using oral haloperidol were conducted in Wistar rats (dosed at up to 5 mg/kg daily for 24 months) and in Albino Swiss mice (dosed at up to 5 mg/kg daily for 18 months). In the rat study survival was reduced in all dose groups, decreasing the number of rats at risk for developing tumors. However, a relatively greater number of rats survived to the end of the study in high-dose male and female groups, these animals did not have a greater incidence of tumors than control animals. Therefore, although not optimal, this study does suggest the absence of a haloperidol related increase in the incidence of neoplasia in rats at doses up to approximately 2.5 times the maximum recommended human dose (MRHD) of 20 mg/day based on mg/m2 body surface area.
In female mice, there was a statistically significant increase in mammary gland neoplasia and total tumor incidence at doses approximately 0.3 to 1.2 times the MRHD based on mg/m2 body surface area and there was a statistically significant increase in pituitary gland neoplasia at approximately 1.2 times the MRHD. In male mice, no statistically significant differences in incidences of total tumors or specific tumor types were noted.
Antipsychotic drugs elevate prolactin levels; the elevation persists during chronic administration. Tissue culture experiments indicate that approximately one-third of human breast cancers are prolactin dependent in vitro , a factor of potential importance if the prescription of these drugs is contemplated in a patient with a previously detected breast cancer. Although disturbances such as galactorrhea, amenorrhea, gynecomastia, and impotence have been reported, the clinical significance of elevated serum prolactin levels is unknown for most patients. An increase in mammary neoplasms has been found in rodents after chronic administration of antipsychotic drugs. Neither clinical studies nor epidemiologic studies conducted to date, however, have shown an association between chronic administration of these drugs and mammary tumorigenesis; the available evidence is considered too limited to be conclusive at this time.
There are no well controlled studies with HALDOL (haloperidol) in pregnant women. There are reports, however, of cases of limb malformations observed following maternal use of HALDOL along with other drugs which have suspected teratogenic potential during the first trimester of pregnancy. Causal relationships were not established in these cases. Since such experience does not exclude the possibility of fetal damage due to HALDOL, this drug should be used during pregnancy or in women likely to become pregnant only if the benefit clearly justifies a potential risk to the fetus.
Since haloperidol is excreted in human breast milk, infants should not be nursed during drug treatment with haloperidol.
Safety and effectiveness in pediatric patients have not been established.
Clinical studies of haloperidol did not include sufficient numbers of subjects aged 65 and over to determine whether they respond differently from younger subjects. Other reported clinical experience has not consistently identified differences in responses between the elderly and younger patients. However, the prevalence of tardive dyskinesia appears to be highest among the elderly, especially elderly women (see WARNINGS, Tardive Dyskinesia). Also, the pharmacokinetics of haloperidol in geriatric patients generally warrants the use of lower doses (see DOSAGE AND ADMINISTRATION).
Studies in patients with hepatic impairment have not been conducted. Haloperidol concentrations may increase in hepatically impaired patients, because it is primarily metabolized by the liver and protein binding may decrease.
The following adverse reactions are discussed in more detail in other sections of the labeling:
- WARNINGS, Increased mortality in Elderly Patients with Dementia-Related Psychosis
- WARNINGS, Cardiovascular Effects
- WARNINGS, Tardive Dyskinesia
- WARNINGS, Neuroleptic Malignant Syndrome
- WARNINGS, Hypersensitivity Reactions
- WARNINGS, Falls
- WARNINGS, Usage in Pregnancy
- WARNINGS, Combined Use of HALDOL and Lithium
- WARNINGS, General
- PRECAUTIONS, Leukopenia, Neutropenia, and Agranulocytosis
- PRECAUTIONS, Withdrawal Emergent Dyskinesia
- PRECAUTIONS, Other
Clinical Trials Experience
Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared to rates in the clinical trials of another drug, and may not reflect the rates observed in practice.
The data described below reflect exposure to haloperidol in the following:
- 284 patients who participated in 3 double-blind, placebo-controlled clinical trials with haloperidol (oral formulation, 2 to 20 mg/day); two trials were in the treatment of schizophrenia and one in the treatment of bipolar disorder.
- 1295 patients who participated in 16 double-blind, active comparator-controlled clinical trials with haloperidol (injection or oral formulation, 1 to 45 mg/day) in the treatment of schizophrenia.
Based on the pooled safety data, the most common adverse reactions in haloperidol-treated patients from these double-blind placebo-controlled clinical trials (≥5%) were: extrapyramidal disorder, hyperkinesia, tremor, hypertonia, dystonia, and somnolence.
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