Quetiapine fumarate overdose: Clinical and pharmacokinetic lessons from extreme conditions

Background:: Although the new atypical antipsychotic, quetiapine fumarate, is growing in popularity over its progenitor, clozapine, clinical experienc e with overdose of this agent remains limited. Observation of an overdose s ituation provided a unique opportunity to define the safety, clinical effec ts, and pharmacokinetics of this medication more clearly Methods: A patient admitted immediately after ingesting an overdose of 30 t ablets of 100 mg of quetiapine was observed carefully to document effects o f the medication, These observations were compared with the only two other published cases of overdose, to the known pharmacology of the drug, and to serial measurements of serum drug concentrations obtained to document the t ime course of elimination of the drug. Results: Consistent with the two previously published cases, the main clini cal effects of overdose were hypotension, tachycardia, and somnolence as pr edicted by its known a-adrenergic receptor and histamine receptor blockade. These effects were managed with fluid resuscitation and supportive measure s. No cardiac arrhythmias other than tachycardia have been reported, but th e tachycardia was of an unexpectedly long duration in this case, Decline in serum quetiapine concentration followed a biexponential pattern with a ter minal elimination half-life of 22 hours. Unexpectedly low peak serum concen trations in three patients with overdose suggest that absorption is highly reduced, either by the effects of the overdose or by the activated charcoal administered. Conclusions: Quetiapine appears to have greater safety in overdose than tra ditional antipsychotic agents. Its toxicity is consistent with its receptor pharmacology Elevated serum concentrations associated with this overdose r emained above the limit of detection long enough to document a terminal eli mination half-life of 22 hours in this patient. This is much more consisten t with previously noted duration of clinical effects and detectable serum c oncentrations after overdose than the published half-life of 6 hours. Physi cians should be aware that any new drug that is active at low concentration s may have had its half-life underestimated during preclinical development because of the difficulty in detecting the drug after the distribution phas e has ended.