Sense and sensitisation: in vitro testing for hepatotoxic drug hypersensitivity.

Sense and sensitisation: in vitro testing for hepatotoxic drug hypersensitivity Over 30 years ago, small lymphocytes were observed to transform into larger, more granular cells and undergo proliferation in the presence of antigen in vitro. 1 This rapidly formed the basis of a laboratory test to detect sensitised lymphocytes. 2 In this system, mononuclear cells are separated from a patient's peripheral blood and cultured in medium in the presence of the suspected aller-gen. Monocytes are the major antigen presenting cells, although antigen presentation may also be performed, albeit to a lesser extent, by B cells. A positive result is most easily demonstrated by lymphocyte proliferation (usually measured by the uptake of tritiated thymidine) rather than by morphological transformation. The ratio of counts per minute in the presence of antigen compared with a control culture in its absence is expressed as the stimulation index. If this value is greater than a cut-oV level determined in a set of healthy controls, then the test is considered positive, and evidence of the presence of sensitised lymphocytes. These cells are predominantly long-lived memory T cells 3 identified by the surface expression of the CD45RO isoform and certain activation markers and adhesion molecules such as CD11a/CD18 (LFA-1), which accounts for their rapid reactivation on re-exposure to antigen. The diagnosis of drug related liver damage is essentially one of exclusion, supported by clinical features such as the known patterns of drug reactions, the temporal association between ingestion of the drug and the time to liver damage, and by the response to drug withdrawal. In practice, the diagnosis is relatively clear although where a patient is taking several drugs or where the clinical picture is atypical, diagnosis of a drug reaction may be problematic. Hence, a simple confirmatory laboratory test would be of considerable value not only in making a positive diagnosis of hepa-totoxic drug hypersensitivity but would also avoid potentially lethal inadvertent re-challenge with the drug, and unnecessary investigations. Unfortunately, the lym-phocyte proliferation test—although apparently straightforward—has been shown to be unreliable under these circumstances, with positive results demonstrated in as few as 30% of cases. 4 5 Indeed, on consideration of current theories of the mechanisms of drug induced immune mediated liver damage, it is remarkable that such a highly simplified in vitro test should ever be positive. Very few drugs are directly hepatotoxic and most require metabolism to induce liver damage. Metabolites of those that …