Simultaneous determination of N(1)-acetyl sulfisoxazole and its metabolites, and relative bioavailability compare to sulfisoxazole in rats.

N(1)-acetyl sulfisoxazole (N1AS), a dihydropteroate synthase inhibitor is known to be biotransformed primarily to sulfisoxazole, partly to N(4)-acetyl sulfisoxazole (N4AS), and likely also to diacetyl sulfisoxazole (DAS) and other compounds. Although its clinical use has been limited due to urolithiasis, some countries still use the drug in combination with trimethoprim in cattle. A liquid chromatographic method using ultraviolet detection was developed for the simultaneous determination of four substances for the first time. Four analytes and sulfamethoxazole (IS) were separated on a reversed-phase column with gradient elution of a mobile phase. Because DAS and N1AS in plasma were changed very quickly into N4AS and sulfisoxazole, respectively, and esterase inhibitors (sodium fluoride and eserine) could not prevent the transformation, sulfisoxazole and N4AS were monitored in rat plasma following a single oral administration of N1AS and sulfisoxazole in five rats. The relative bioavailability of N1AS to sulfisoxazole was about two, indicating that a half-dose of N1AS would be equivalent to a dose of sulfisoxazole to achieve the same systemic exposure to sulfisoxazole.

Pharmacokinetics of sulfisoxazole in renal transplant patients.

We studied the elimination of sulfisoxazole in eight renal transplant patients. The patients received sulfisoxazole prophylactically for urinary tract infection commencing 7 days postoperatively. The renal elimination of sulfisoxazole (unbound renal clearance) was decreased in this patient population and was highly correlated with creatinine clearance. The unbound metabolic clearance and apparent unbound formation clearance of N4-acetyl sulfisoxazole did not differ from values found in healthy volunteers. The protein binding was marginally lower in this patient population than in healthy subjects after a single dose. The reduced binding was compatible with a reduced albumin concentration. In contrast to the situation for healthy subjects, the binding of sulfisoxazole decreased upon multiple dosing. This is probably due to a relatively higher sulfisoxazole and N4-acetyl sulfisoxazole-to-albumin ratio in this patient population than in healthy subjects. No complications of sulfisoxazole therapy were seen, although in three subjects concentration of the N4-acetyl sulfisoxazole in urine exceeded its theoretical solubility on a few occasions.

Influence of protein binding and metabolic interconversion on the disposition of sulfisoxazole and its N4-acetyl metabolite by the isolated perfused rat kidney.

The renal clearances of sulfisoxazole (SX) and N4-acetylsulfisoxazole (NSX) were studied in the isolated perfused rat kidney (IPK). Studies were conducted with conventional bovine serum albumin perfusates as well as with dextran perfusates to assess the influence of perfusate protein binding on the disposition of these compounds by the IPK. The results presented herein indicate that the disposition of sulfisoxazole by the IPK involves both metabolism and excretion. The metabolism of SX to NSX is reversible and is influenced by protein binding since metabolism increased with increased free fraction (Ff). The excretion of SX and NSX reflects a complex interaction of filtration, secretion, and reabsorption. A comparison of clearance values between kidneys perfused with bovine serum albumin perfusate (Ff 0.1) and dextran perfusate (Ff 1.0) suggests that tubular secretion of SX is a function of total (unbound plus bound) rather than free (unbound) drug in the perfusate.

Pharmacokinetics of sulfisoxazole in young and elderly subjects.

The pharmacokinetics of sulfisoxazole was studied in 6 elderly (age 63-75 years) and 7 young (age 22-37 years) healthy nonsmoking volunteers following the oral administration of 2 g of Gantrisin. The plasma levels of sulfisoxazole obtained in the postabsorption phase were higher in the elderly subjects. There was no significant variation between the two groups of volunteers in the absorption rate constant, Cmax, bioavailability, the fraction of the dose of sulfisoxazole excreted unchanged, the area under the plasma curve of the N4-acetyl conjugate formed, and in the apparent volume of distribution of the drug. The tmax value and plasma half-life of sulfisoxazole were significantly longer, and the total body and renal clearances of the drug decreased in the elderly subjects. Diminished renal function as estimated by the creatinine clearance and urinary flow may explain the slower elimination of sulfisoxazole in the elderly subjects. Therefore, advancing age should be considered as a factor in the adjustment of sulfisoxazole dosage.

Surgically affected sulfisoxazole pharmacokinetics in the morbidly obese.

Intestinal bypass surgery in 4 morbidly obese female (110-150 kg) had no permanent effect on the rate or amount of sulfisoxazole absorption. The loss of weight up to 44 per cent within an individual over a year's time had no significant effect on the apparent volumes of distribution or other pharmacokinetic parameters of sulfisoxazole and its N4-acetylsulfisoxazole metabolite. Dosing of this drug on a mg kg-1 basis is contraindicated. Renal clearances of sulfisoxazole were reasonably constant within a study but those of the N4-acetylsulfisoxazole decreased with time. Integrated pharmacokinetic models were applied to plasma and urine data to estimate the metabolic clearance of sulfisoxazole and the apparent volume of distribution of the N4-acetylsulfisoxazole. Sulfisoxazole solution is absorbed readily by primarily a zero order process after a short lag period, indicative of rate-determining gastric emptying. The classical Bratton-Marshall assays were compared with an HPLC assay of both drug and metabolite. There was greater confidence in plasma levels of the metabolite from the HPLC method.

"High-pressure" liquid chromatography of sulfisoxazole and N4-acetylsulfisoxazole in body fluids.

We describe a simple, rapid chromatographic method for separating and quantitatively determining sulfisoxazole and its N4-acetyl metabolite in plasma and urine. A 100-micro L sample of plasma or urine is combined with 200 micro L of a solution containing 12 mg/L of the internal standard, N4-acetylsulfamethoxazole, in absolute methanol and centrifuged to obtain a clear supernatant solution. This solution is then eluted through a 10-micron microparticulate column with a mobile phase of 32/68 (by vol) methanol/sodium acetate buffer (0.01 mol/L, pH 4.7), at a flow rate of 1.2 mL/min. The eluted sompounds are detected by their absorption at 254 nm. We calculated concentration from the peak-height ratios of sulfisoxazole or N4-acetylsulfisoxazole to N4-acetylsulfamethoxazole. The peak-height ratio was linear with concentration in the range 0.05--200 mg/L for both drug and metabolite in plasma and urine. Because this assay can be completed within 30 min of obtaining a blood or urine sample, it should be a valuable tool in clinical drug monitoring and pharmacokinetic studies.