Effect of urinary pH on the disposition of methadone in man.

The influence of urinary pH on the acute disposition of methadone in man was studied in five healthy volunteers. A cross-over experiment was performed in each subject. In the first experiment the subjects were treated with ammonium chloride (urinary pH approximately 5.2) and in the other the urine was made alkaline (pH approximately 7.8) by treatment with sodium hydrogen carbonate. d, l-Methadone-HCl 10 mg (M) was administered intramuscularly on each occasion and blood, saliva and urine levels of M were determined by mass fragmentography. Plasma half-lives, volumes of distribution and body clearances of M were calculated in both experiments. The plasma half-lives in the beta-phase were 19.5 +/- 3.6 h (acidic urine) and 42.1 +/- 8.8h (alkaline urine), respectively (p less than 0.001). The volumes of distribution were increased when the pretreatment was changed from ammonium chloride to sodium bicarbonate, namely from 3.51 +/- 0.41 l/kg to 5.24 +/- 0.83 l/kg (p less than 0.01). The body clearance decreased from 134 +/- 21 ml/min (acidic) to 91.9 +/- 9.1 ml/min (alkaline urine) (p less than 0.01). The ration M plasma/M RBC was about 2.3 and the elimination of M from RBCs was good agreement with the plasma kinetics of M under both experimental conditions. The salivary levels of M did not reflect the plasma kinetics and considerable variation was seen in the ratio M saliva/ M plasma (0.26-2.98). Thus, the present experiments demonstrate that pretreatment either with ammonium chloride or bicarbonate had profound effects on both the distribution and elimination kinetics of methadone.