Letter: Renal contribution to drug biotransformation

Quantitative determination of organ contribution to excretory metabolism

A method for the quantitative determination of organ contribution to in vivo excretory metabolism is described. By excretory metabolism, we mean metabolism of a compound by an organ with direct excretion. The technique separately and simultaneously quantifies clearance of circulating metabolite and clearance of circulating precursor by excretory metabolism. For xenobiotics, the method involves the simultaneous infusion of radiolabeled precursor and unlabeled metabolite. The method is dependent upon the achievement of steady-state plasma concentrations of precursor and metabolite and on the ability to accurately analyze, both chemically and radioactively, the precursor in plasma, and the metabolite in both plasma and excretory fluid. The technique is not compromised by in vivo conversion of metabolite to precursor. This method is suitable for the simultaneous quantitative determination of the contribution of several organs to the formation of any number of metabolites present in excretory fluids. The simultaneous contribution of excretory metabolism to the urinary and biliary elimination of 1-naphthol in the rat is presented.

In vivo quantification of renal glucuronide and sulfate conjugation of 1-naphthol and p-nitrophenol in the rat

The simultaneous in vivo renal sulfate and glucuronide conjugations of 1-naphthol (1-N) and p-nitrophenol (PNP) were determined in the rat. In mammals, 1-N and PNP are excreted almost entirely in the urine, mainly as the glucuronide and sulfate conjugates. In male Sprague-Dawley rats, greater than 98% of the infused [14C]1-N (1.0 mumole X min-1 X kg-1) or [14C]PNP (2.0 mumoles X min-1 X kg-1) recovered in urine was identified as the sulfate and glucuronide conjugates. Renal metabolism accounted for a minimum of 20% of the endogenously formed conjugates of either substrate excreted in the urine. The rat kidney formed the glucuronide and sulfate conjugates of PNP at equal rates, whereas the glucuronide: sulfate conjugate ratio for renally formed 1-N conjugates was 3:1. When the conjugates of either 1-N or PNP were infused systemically, in vivo hydrolysis contributed significantly to the amount of circulating parent phenol.

Kinetics of salicylate elimination by anephric patients

The objectives of this research were to determine the kinetics of salicylate elimination in anephric patients and particularly to establish if these patients form the major metabolite of salicylic acid, salicyluric acid, at a normal rate. This investigation was initiated because of conflicting reports concerning the contribution of the kidneys to the formation of salicyluric acid in man. Six patients, 20-44 yr old, three of whom were anatomically anephric while the other three were physiologically anephric, received an intravenous injection of 500 mg salicylic acid (as sodium salicylate)/1.73 m(2) body surface area on an interdialysis day. Serial blood samples were obtained for 12 or 16 h after injection and the plasma was assayed for salicylic acid, salicyluric acid, total protein, albumin, and creatinine. Detailed pharmacokinetic analysis based on an open, two-compartment linear model revealed no significant differences in apparent volume of distribution and apparent first-order distribution and elimination rate constants between the anephric patients and normal adult subjects. An estimate of salicyluric acid formation rate by the anephric patients, based on the initial rate of increase of salicylurate concentrations in plasma, indicates that the metabolite is formed at a normal rate. These results suggest that the kidneys do not contribute significantly to the formation of salicyluric acid from salicylic acid in man.