For definition of Groups, see Preamble Evaluation.
Vol.: 79 (2001) (p. 161)
Chem. Abstr. Name: 5-Ethyl-5-phenyl-2,4,6(1H,3H,5H)-pyrimidinetrione
Chem. Abstr. Name: 5-Ethyl-5-phenyl-2,4,6(1H,3H,5H)-pyrimidinetrione, monosodium salt
5.1 Exposure data
Phenobarbital and its sodium salt have been very widely used as a mild sedative or hypnotic in the treatment of neuroses and in pre- or post-operative sedation, and as an anticonvulsant in the treatment of epilepsy. Phenobarbital was introduced in 1912. Its use has decreased since the 1960s, but it is still produced worldwide and used extensively.
5.2 Human carcinogenicity data
Three large follow-up studies of cancer, two of incidence and one of mortality, from Denmark, England and the USA of patients treated primarily with phenobarbital for epilepsy showed an occurrence of brain cancer higher than expected. However, in the two incidence studies, the excess numbers of cases of brain cancer occurred within 10 years of hospitalization and decreased significantly over time. This inverse relationship between excess risk and time since hospitalization for epilepsy suggests that the brain tumours of some of the patients were the cause of their seizure disorder and that the association between use of phenobarbital and brain cancer is not causal. The finding in a small case–control study from the USA of an increased risk for brain tumours after prenatal exposure to phenobarbital was not confirmed in a larger case–control study, also from the USA, or in a cohort study from Denmark of transplacental exposure to phenobarbital and other anti-convulsants.
Of the three cohort studies of epilepsy patients, two showed a significant increase in the relative risk for lung cancer, with no clear pattern of risk with length of follow-up. One showed a non-significant increase. Dose–response analyses in a nested case–_control study of lung cancer in the largest of the cohort studies (in Denmark) revealed no consistent relationship between lung cancer and cumulative exposure to phenobarbital. A survey among the controls indicated a higher-than-average prevalence of smoking.
After exclusion from the largest of the cohorts of epilepsy patients known to have received radioactive Thorotrast during cerebral angiography, a slight, non-significant increase in risk for primary liver cancer was seen. However, a nested case–control study of liver cancer with adjustment for other anti-convulsant therapy revealed no association with phenobarbital treatment. No cases of liver cancer were seen in the other two cohort studies, from England and the USA.
In the cohort study in Denmark, the observed number of cases of thyroid cancer was close to that expected in the general Danish population. In the same study, a statistically significant deficit of urinary bladder cancer was noted, which was shown in an analysis of the dose–response relationship to be inversely related to use of phenobarbital.
Use of phenobarbital, mostly as a sedative, was associated with moderately increased risks for cancers of the lung, ovary and gall-bladder in a cohort study based on a prepaid medical care programme in the USA.
5.3 Animal carcinogenicity data
The carcinogenicity of phenobarbital was investigated by oral administration in multiple studies in mice and several studies in rats. Phenobarbital consistently produced hepatocellular adenomas and carcinomas in mice. Hepatocellular adenomas were produced in rats after lifetime exposure in one study. Oral administration of phenobarbital in combination with known carcinogens resulted in the enhancement or inhibition of effects, depending on the carcinogen and the time of administration. In several experiments in mice and rats, sequential exposure to phenobarbital with known carcinogens enhanced the incidences of hepatocellular preneoplastic foci, adenomas and carcinomas. In two studies each, phenobarbital was found to promote liver carcinogenesis in patas monkeys but not in hamsters. Phenobarbital promoted thyroid follicular-cell tumours in one study in mice and in several studies in rats.
5.4 Other relevant data
Most of an administered dose of phenobarbital in humans was excreted in urine. The major urinary excretion products include unmodified phenobarbital, para-hydroxyphenobarbital, phenobarbital-N-glucoside and phenobarbital para-glucuronide. para-Hydroxyphenobarbital can be formed by direct hydroxylation of phenobarbital.
CYP2B1 and CYP2B2 are the primary members of the cytochrome P450 (CYP) superfamily of enzymes that are induced by phenobarbital in vivo. Although phenobarbital causes large increases in the activity of these enzymes in liver, the metabolism of phenobarbital itself is not increased. Phenobarbital has also been found to induce the activities of other CYP enzymes, including benzo[a]pyrene hydroxylase, UDP-glucuronosyl transferase and several glutathione-S-transferases. ‘Phenobarbital-like induction’ describes the effect on liver hepatocyte CYP enzymes of various compounds, including sedatives, pesticides and other compounds that induce a similar spectrum of isozymes.
Cell proliferation is initially stimulated by phenobarbital in normal hepatocytes and lasts a few days. It may even be inhibited by down-regulation of epidermal growth factor receptors. Phenobarbital exerts a selective and sustained mitogenic effect in cells of altered foci that progress to adenomas that are no longer dependent on the mitogenic effects of phenobarbital.
The biochemical mechanisms underlying enhancement of cell proliferation and tumour promotion by phenobarbital may involve alterations in gene regulation. The dose–response relationship for microsomal enzyme induction is similar to that for tumour promotion. Consequently, changes in gene regulation that presumably lead to mitogenesis and up-regulation of growth factors parallel the induction of CYPs.
Phenobarbital has also been shown to inhibit intercellular communication in hepatocytes, which could impede the transmission of growth control signals between normal and altered hepatocytes.
Owing to its effects on the induction of microsomal enzymes, phenobarbital enhances the hepatic disposition of thyroid hormone. The promotion of thyroid gland tumours in rats by phenobarbital has been shown to be mediated by increased secretion of pituitary thyroid-stimulating hormone as a compensatory response to increased thyroid hormone glucuronidation and biliary excretion.
Phenobarbital is a teratogen and developmental neurotoxicant in humans and experimental animals. Exposure of rats in utero induces long-term effects on hepatic drug-metabolizing enzymes. Neuroendocrine effects on reproductive function have been noted in exposed adult male rats and female hamsters.
Phenobarbital did not induce sister chromatid exchange in patients with epilepsy receiving only this drug.
In studies in which rodents were exposed to phenobarbital in vivo, no covalent binding to mouse liver DNA was observed, but the frequency of alkali-labile damage in mouse liver cells was increased. Gene mutation was not induced in a transgenic mouse strain, and sister chromatid exchange, micronuclei and chromosomal aberrations were not induced in mouse bone-marrow cells. Phenobarbital did not increase the frequency of sperm-head abnormalities in mice, but spermatogonial germ-cell chromosomal aberrations were reported in male mice in one laboratory. Further increases in the frequency of chromosomal aberrations were found in liver foci cells of mice treated with phenobarbital after previous treatment with a genotoxic agent.
Chromosomal aberrations but not gene mutations were induced in cultured human lymphocytes.
Tests for the genetic effects of phenobarbital in vitro are numerous and include assays for DNA damage, DNA repair induction, gene mutation and chromosomal aberrations in mammalian cells, tests for gene mutation and mitotic recombination in insects and fungi and tests for gene mutation in bacteria. Although the majority of the test results were negative, the numerous positive results cannot be ignored, even though they do not present a consistent pattern of genetic toxicity. The inconsistency of the results, the absence of any direct evidence of an interaction with DNA and the generally negative in-vivo data lead to the conclusion that phenobarbital is not genotoxic.
Phenobarbital transformed hamster embryo cells. It inhibited gap-junctional intercellular communication in hepatocytes of rats treated in vivo and in primary cultures of hepatocytes from rats and mice but not (in a single study) in primary cultures of human or rhesus monkey hepatocytes.
There is inadequate evidence in humans for the carcinogenicity of phenobarbital.
There is sufficient evidence in experimental animals for the carcinogenicity of phenobarbital.
Phenobarbital is possibly carcinogenic to humans (Group 2B).For definition of the italicized terms, see Preamble Evaluation.
Previous evaluations: Vol. 13 (1977); Suppl. 7 (1987)
See Also: Toxicological Abbreviations Phenobarbital (IARC Summary & Evaluation, Supplement7, 1987) Phenobarbital (IARC Summary & Evaluation, Volume 13, 1977)