[ON PHARMACOKINETICS OF SULFONAMIDES UNDER PATHOLOGICAL CONDITIONS]

Inhibition of the conjugation of PABA with glycine in vitro by sulfamoyl benzoic acids, sulfonamides, and penicillins and its relation to tubular secretion

Elimination of drug molecules via tubular secretion is an important pharmacokinetic parameter especially for oral dosage forms where an extremely short half-life would prevent their application. We have studied the inhibition in vitro of the glycination of p-aminobenzoic acid (PABA) using rat liver microsomal preparations. The I50 values, the concentration of the inhibitor that leads to 50% inhibition of glycine conjugation of PABA as compared to the control, have been determined for sulfamoyl benzoic acids, sulfonamides, and penicillins. Statistically significant regression equations were derived explaining the observed variation in I50 values as a function of lipophilicity and steric bulk of the substituents for the combined set of sulfamoyl benzoic acids and sulfonamides (n = 33). For the penicillins studied, only steric effects seem to be important for the explanation of the I50 values. Finally, regression analysis and the use of neural networks allowed the classification of compounds by their in vitro I50 values as being tubular secreted or not. Therefore, it can be concluded that the inhibition in vitro of the glycine conjugation of PABA is a useful model for the estimation of tubular secretion and drug interaction potential of acidic drug molecules in this process in vivo.

Effects of methoxy groups in the NI-substituent of sulfonamides on the pathways of elimination in man. The acetylation-deacetylation equilibrium and mechanisms of renal excretion of sulfisomidine, sulfamethomidine and sulfadimethoxine

Sulfisomidine, sulfamethomidine, sulfadimethoxine and their corresponding N4-acetyl derivatives were administered to man. The percentages of acetylation and deacetylation and those of protein binding, the half-lives of elimination and the apparent and true renal clearance values were measured. No acetylation phenotype could be demonstrated in these compounds. Methoxy substitution in the NI-pyrimidine group of sulfisomidine affects predominantly the renal clearance value and mechanism of the parent compound but has no influence on the renal clearance of the N4-acetyl derivatives. The renal clearance value of sulfisomidine is 232 +/- 33 ml/min, of sulfamethomidine 21.60 +/- 16.4 ml/min and of sulfadimethoxine 10.87 +/- 10.44 ml/min. The renal clearance values of the corresponding N4-acetylsulfonamide derivatives are 314 +/- 91 ml/min, 342 +/- 63 ml/min and 202 +/- 65 ml/min respectively. Tubular reabsorption, caused by methoxy substitution in the NI-pyrimidine ring, lowers the rate of elimination and increases the half-life. The half-life of sulfisomidine is 8.5 +/- 0.5 h, of sulfamethomidine 27.8 +/- 5.3 h and of sufadimethoxine 34.6 +/- 10.4 h.

Protein binding of salicylate and quinidine in plasma from patients with renal failure, chronic liver disease and chronic respiratory insufficiency

The plasma protein binding of a representative acidic drug, salicylate, and a representative basic drug, quinidine, has been studied in patients with several diseases that are sometimes associated with uraemia or a change in serum albumin level. Decreased plasma protein binding of salicylate was observed in plasma from patients with uraemia and liver disease. Low albumin levels in these patients could only account inpart for the decreased binding. On the other hand, salicylate binding to plasma proteins appeared to be increased in patients with hypoxia. Decreased plasma protein binding of quinidine was observed in some patients with uraemia and in the majority of patients with liver disease.

The binding of drugs to plasma proteins from patients with poor renal function

Plasma from patients with renal failure has decreased binding of many drugs. The decreased binding of anionic drugs is pronounced and is apparently due to occupation of protein binding sites by non-dialysable endogenous acidic compounds. Organic bases seem to bind normally, particularly if hypoproteinaemia or hypoalbuminaemia are not present. This modification of drug binding to plasma proteind from uraemic patients must be considered when plasma concentrations of acidic drugs are measured.

Renal function, protein binding and pharmacological response to diazoxide

1 The effect of rapid (10s) injections of diazoxide was studied in ten hypertensive patients with varying degrees of impairment of renal function. 2 There was a significant correlation between the patient's plasma urea concentration and reduction in mean arterial blood pressure. Diazoxide was also shown to be less highly protein bound in patients with renal failure. 3 It is suggested that the explanation for the increased hypotensive effect of diazoxide observed in patients with reduced renal function is related to higher unbound drug concentrations.