Effect of antipyrine coadministration on the kinetics of acetaminophen and lidocaine.

Pharmacokinetic interactions between antipyrine and acetaminophen were evaluated in 7 healthy volunteers. On 3 occasions subjects received: 1, antipyrine 1.0 g intravenously (i.v.); 2, acetaminophen 650 mg i.v.; 3, antipyrine 1.0 g and acetaminophen 650 mg i.v. simultaneously. Between Trials 1 and 3, antipyrine elimination t1/2 (17.2 vs 17.4 h), clearance (0.44 vs 0.43 ml.min-1.kg-1) and 24-h recovery of antipyrine and metabolites (313 vs 293 mg) did not differ significantly. Between Trials 2 and 3, acetaminophen VZ was reduced (1.14 vs 1.00 l.kg-1), t1/2 prolonged (2.7 vs 3.3 h), clearance reduced (4.8 vs 3.6 ml.min-1.kg-1), and fractional urinary recovery of acetaminophen glucuronide reduced. Eight additional subjects received 50 mg of lidocaine hydrochloride i.v. in the control state, and on a second occasion immediately after antipyrine 1.0 g given i.v. The two trials did not differ significantly in lidocaine VZ (2.6 vs 2.7 l.kg-1), t1/2 (2.0 vs 2.4 h) or clearance (15.0 vs 13.5 ml.min-1.kg-1). Although acetaminophen does not alter antipyrine kinetics, acute administration of antipyrine appears to impair acetaminophen clearance, possibly via inhibition of glucuronide formation. However, antipyrine has no significant effect on the kinetics of a single i.v. dose of lidocaine.