Cancer risk in people with epilepsy: the role of antiepileptic drugs.

There has been considerable debate about the relationship between epilepsy and cancer, in particular whether the incidence of cancer is increased in people with epilepsy and whether antiepileptic drugs promote or protect against cancer. We review available evidence from animal experiments, genotoxicity studies and clinico-epidemiological observations, and discuss proposed mechanisms underlying the association between epilepsy and cancer. A carcinoma-promoting effect has been seen unequivocally in rodent models for phenobarbital and phenytoin; phenobarbital promoted liver tumours and phenytoin caused lymphoid cell and liver tumours in rats. Early human epidemiological studies found an association between phenobarbital and hepatocellular carcinoma, and several subsequent studies suggested an association with lung cancer. An association with brain tumours has also been demonstrated. Phenytoin has been causally implicated in three human cancers: lymphoma, myeloma and neuroblastoma, the latter specifically in the setting of foetal hydantoin syndrome. However, despite considerable long-term pharmaco-epidemiological data being available for both antiepileptic drugs, evidence for human carcinogenicity is not consistent and both are considered only possibly carcinogenic to humans. Valproate, however, has been found to exert an antiproliferative effect on certain cancer cell lines both in vitro and in vivo. A corresponding cancer-suppressive effect has not been studied in human epidemiological studies, though there are now preliminary reports of the use of valproate in human haematological and solid tumours. The anticancer activity of valproate appears to be driven by histone deacetylase inhibition and to be independent of hormone or multidrug protein resistance dependent mechanisms. The newer antiepileptic drugs appear to be safe, as no carcinogenicity has been demonstrated either during regulatory testing or in post-marketing surveillance. Nevertheless, the subject of cancers and epilepsy constitutes a promising agenda for clinical and experimental research in the future.