Tolfenamic acid is a potent CYP1A2 inhibitor in vitro but does not interact in vivo: correction for protein binding is needed for data interpretation.
OBJECTIVE
Our aim was to correlate the in vitro and in vivo CYP1A2 inhibition potential of tolfenamic acid, an NSAID highly (99.7%) bound to plasma proteins, to study the significance of protein binding of inhibitor in metabolic drug interactions.


METHODS
The effect of tolfenamic acid on CYP1A2 (phenacetin O-deethylation) was studied using human liver microsomes, with and without albumin (0-10 mg/ml). In a randomized, crossover study, 10 volunteers took 200 mg tolfenamic acid or placebo t.i.d. for 3 days. On day 2, a caffeine test was performed. On day 3, each ingested 4 mg of the CYP1A2 substrate tizanidine. Plasma tizanidine, its metabolites (M) and tolfenamic acid, and pharmacodynamic variables were measured.


RESULTS
Tolfenamic acid strongly inhibited phenacetin-O-deethylation in vitro (IC(50) 1.8 microM without albumin). Albumin decreased its inhibitory effect in a concentration-dependent manner; the IC(50) exceeded 100 microM with 10 mg/ml of albumin. Tolfenamic acid had no effect on the area under the concentration-time curve (AUC(0-oo)), peak concentration, time of peak concentration or half-life of tizanidine or M-3; only the AUC(0-oo) of secondary metabolite M-4 was slightly decreased (13%, P = 0.004). The caffeine test and the pharmacodynamic effects of tizanidine were unchanged.


CONCLUSIONS
Tolfenamic acid potently inhibits CYP1A2 in vitro when studied without albumin, but not in vivo. This apparent discrepancy is due to the high protein binding of tolfenamic acid. To avoid overestimation of the interaction potential, the inhibitory effect of highly albumin-bound compounds should also be studied in vitro with albumin, or their exact unbound plasma concentration should be used in predictions.
Karjalainen
Marjo J
MJ
Department of Clinical Pharmacology, University of Helsinki, Helsinki, Finland.
Neuvonen
Pertti J
PJ
Backman
Janne T
JT
D000328:Adult; D000465:Algorithms; D000704:Analysis of Variance; D000894:Anti-Inflammatory Agents, Non-Steroidal / Q000378:metabolism / Q000493:pharmacokinetics / Q000494:pharmacology; D019540:Area Under Curve; D018929:Cell Culture Techniques; D003000:Clonidine / Q000008:administration & dosage / Q000031:analogs & derivatives / Q000097:blood / Q000493:pharmacokinetics; D018592:Cross-Over Studies; D019388:Cytochrome P-450 CYP1A2 / Q000378:metabolism; D065609:Cytochrome P-450 CYP1A2 Inhibitors; D004305:Dose-Response Relationship, Drug; D006207:Half-Life; D006801:Humans; D007700:Kinetics; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism; D009125:Muscle Relaxants, Central / Q000008:administration & dosage / Q000378:metabolism / Q000493:pharmacokinetics; D011485:Protein Binding; D012709:Serum Albumin / Q000378:metabolism; D062367:ortho-Aminobenzoates / Q000378:metabolism / Q000493:pharmacokinetics / Q000494:pharmacology*
D000894:Anti-Inflammatory Agents, Non-Steroidal; D065609:Cytochrome P-450 CYP1A2 Inhibitors; D009125:Muscle Relaxants, Central; D012709:Serum Albumin; D062367:ortho-Aminobenzoates; C009500:tolfenamic acid; C023754:tizanidine; D019388:Cytochrome P-450 CYP1A2; D003000:Clonidine