Idiosyncratic Toxicity

An idiosyncratic drug reaction is defined as hypersensitivity to a substance without a direct connection to the pharmacological target of the drug. Although the underlying mechanism is usually unknown, the toxicological response appears to involve direct hepatotoxicity and/or adverse immune reactions. Characteristics of an idiosyncratic drug reaction are as follows:

1. A low incidence (e.g., bromfenac toxicity is seen in 1 out of 20,000 patients).

2. No uniform response from patient to patient.

3. No clear dose/exposure-response relationship. The response is usually more pronounced after repetition of the treatment. An example is Stevens-Johnson syndrome caused by carba-mazepine. Note that some patients may be genetically predisposed to such drug reactions, such as those that might carry a specific human leukocyte antigen.

"It is now assumed that most idiosyncratic drug reactions are due to reactive metabolites, and yet most drugs form reactive metabolites to some degree, and we can not predict with

Continued any degree of certainty which drugs will be associated with a high incidence of idiosyncratic reactions". (Uetrecht 2002)

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