Effects of age on the pharmacodynamics of naproxen in the rat

The pharmacodynamics of naproxen were evaluated in 5- and 24-month-old male Fischer 344 rats. Plasma naproxen concentrations and thromboxane B2 (TxB2) concentrations were measured as a function of time after intravenous administration of 25 mg/kg naproxen. Age-dependent alterations in naproxen pharmacokinetics were attributed to significant reductions in free plasma clearance (CLfree) and free steady-state volume of distribution (VSSfree), suggesting a decline in metabolic activity and naproxen binding to tissue components in aged rats, respectively. The time course of TxB2 production as a function of unbound naproxen concentrations was described by a sigmoid Emax model. Age had no significant effect on the pharmacodynamic parameter Emax, the maximum percent inhibition of TxB2 formation. Age also had no statistically significant effect on EC50, the drug concentration producing 50% of the maximum effect, however, average EC50 values were 35% higher in the aged rats. The duration of TxB2 inhibition was unaffected by age, possibly owing to similar relative decreases in receptor sensitivity (increased EC50) and increases in free naproxen concentrations (decreased free clearance and volume of distribution). Alternatively, the age-related changes in pharmacokinetic parameters were not of sufficient magnitude to produce a significant change in drug response, naproxen, pharmacokinetics, pharmacodynamics, age, rats, thromboxane B2, nonsteroidal anti-inflammatory drugs.