Acetanilide and paracetamol pharmacokinetics before and during phenytoin administration: genetic control of induction?

1 Steady state plasma concentrations (SSPCs) of acetanilide (AA) and its metabolite, paracetamol (PL), were studied in 27 healthy volunteer subjects before and at the end of an 11-day exposure to phenytoin (DPH). Plasma concentrations of DPH were estimated. Plasma concentrations of DPH varied from 5.1 to 20.4 microgram ml-1 (mean +/- s.e.mean 12.2 +/- 0.9). 2 The SSPC of AA before exposure to DPH varied from 0.06 to 0.67 microgram ml-1 (mean +/- s.e.mean 0.24 +/- 0.02), and following exposure from 0.03 to 0.47 microgram ml-1 (mean +/- s.e.mean 0.15 +/- 0.02). 3 The SSPC of PL before exposure to DPH varied from 1.2 to 4.4 microgram ml-1 (mean +/- s.e.mean 2.7 +/- 0.13), and following exposure from 1.1 to 3.8 microgram ml-1 (mean +/- s.e.mean 2.2 +/- 0.13). 4 SSPCs of AA and of PL decreased significantly during DPH administration (P less than 0.01 for AA, P less than 0.001 for PL). 5 Correlations were observed between the SSPCs of the drugs measured, suggesting a common influence on their kinetics. 6 Similarity was observed between the changes in plasma levels of AA and PL following DPH ingestion. There was, however, wide inter-subject variability in this regard. In some subjects no change was observed even though they had DPH demonstrable in their plasma. Consequently it may be speculated that the effects of DPH may be under genetic control.