Pharmaco-epidemiology as a Tool in Pharmacovigilance: Studying cancer as adverse drug reaction

Pharmacovigilance is defined by the World Health Organization (WHO) as the science and activities relating to the detection, assessment, understanding and prevention of adverse effects or any other possible drug-related problem. There has been concern about the safety of medicines since the discovery of congenital abnormalities in babies delivered by women who had taken thalidomide during pregnancy in 1961. An adverse drug reaction is defined as a response to a medicinal product which is noxious and unintended and which occurs at doses normally used in man for the prophylaxis, diagnosis or therapy of disease or for the restoration or modification of physiological function. Two types of adverse drug reactions are distinguished: type A reactions which are predictable from the known pharmacology of the medical substance and which are dose-dependent, and type B reactions which are idiosyncratic and unpredictable. It is clear that this is a very crude distinction which is, however, useful from the point of view of discovering unknown adverse reactions as early as possible. During drug development, the efficacy and safety of the active substance is investigated in clinical trials in a relatively small selected homogenous patient population, during a limited period of time. As around 80% of the adverse drug reactions is estimated to be of type A, a large number of the potential adverse drug reactions is documented during the clinical phase. After regulatory review and approval, during which all available information is reviewed, the marketing phase starts. Through marketing, the product is available for the entire population which is obviously far more heterogeneous than the study population. In contrast to the limited timeframe available during the clinical drug development phase, the post-marketing phase continues until the drug is withdrawn from the market. As a consequence, previously unknown adverse drug reactions might come to light, especially those of type B. Therefore, the obligation for the marketing authorization holder, as well as for regulatory authorities, for the continuous evaluation of safety and efficacy during the post marketing phase of a drug, have been legally laid down