Influence of urinary pH on pethidine kinetics in healthy volunteer subjects

The kinetics of intravenous pethidine (150 micrograms kg-1) were determined in 10 healthy Caucasian subjects under uncontrolled and controlled (acidic and alkaline) urinary pH. Although large variations in the 48 hr urinary recovery of pethidine and norpethidine (26.9 +/- 5.9% & 23.4 +/- 4.6%, 0.6 +/- 0.3% & 3.6 +/- 1.6%, 6.9 +/- 3.2% & 17.4 +/- 6.6%, respectively, under acidic, alkaline and uncontrolled urinary pH were induced by change in urinary pH, the terminal t1/2 (7-8 hr), the AUC and the plasma concentration-time profiles were not affected. Under all these conditions, the disappearance of pethidine from the plasma was described by a triexponential function. A 3-compartment open model with input into the central compartment and elimination from both the central and the fast accessible (metabolising) compartment was proposed to interpret pethidine distribution in the body. This model explains the complimentary elimination of pethidine by renal excretion and metabolism. Under alkaline urinary pH, the hydrolytic route predominates as recovery of pethidine and norpethidine in the urine is significantly lower under this condition. Acidification of the urine may increase body clearance of the unchanged drug due mainly to greater renal clearance, and this may be useful clinically to treat acute pethidine poisoning and when the complimentary hepatic metabolism is impaired.