Some consequences of drug choice and dosage regimen for patients with impaired kidney function

Clinical consequences of the biphasic elimination kinetics for the diuretic effect of furosemide and its acyl glucuronide in humans

This review discusses the possibility of whether furosemide acyl glucuronide, a metabolite of furosemide, contributes to the clinical effect of diuresis. First an analytical method (e.g. HPLC) must be available to measure both parent drug and furosemide acyl glucuronide. Then, with correctly treated plasma and urine samples (light protected, pH 5) from volunteers and furosemide-treated patients, the kinetic curves of both furosemide as well as its acyl glucuronide can be measured. The acyl glucuronide is formed in part by the kidney tubules and it is possible that the compound is pharmacologically active through inhibition of the Na+/2Cl-/K+ co-transport system; up to now the mechanism of action has been solely attributed to furosemide. The total body clearance of furosemide occurs by hepatic and renal glucuronidation (50%) and by renal excretion (50%). Enterohepatic cycling of furosemide acyl glucuronide, followed by hydrolysis, results in a second and slow elimination phase with a half-life of 20-30 h. This slow elimination phase coincides with a pharmacodynamic rebound phase of urine retention. After each dosage of furosemide, there is first a short stimulation of urine flow (4 h), which is followed by a 3-day recovery period of the body. The following clinical implications arise from study of the elimination kinetics of furosemide. Repetitive dosing must result in accumulation of the recovery period. Accumulation of furosemide and its acyl glucuronide in patients with end-stage renal failure results from infinite hepatic cycling. Impaired kidney function may result in impaired glucuronidation and diuresis. While kidney impairment normally requires a dose reduction for those compounds which are mainly eliminated by renal excretion, for diuretics, a dose increment is required in order to maintain a required level of diuresis. The full clinical impact of the accumulation of furosemide and its acyl glucuronide in patients with end-stage renal failure has to be determined.

Pharmacokinetics of cefradine, sulfamethoxazole and trimethoprim and their metabolites in a patient with peritonitis undergoing continuous ambulatory peritoneal dialysis

Cefradine and co-trimoxazole pharmacokinetics were studied in a patient with peritonitis that complicated continuous ambulatory peritoneal dialysis (CAPD). Concentrations in the plasma reached after oral administration of 500 mg cefradine four times daily and 400/80 mg co-trimoxazole four times daily were for cefradine 100 micrograms/ml, for trimethoprim 15 micrograms/ml, and for sulfamethoxazole 100 micrograms/ml, respectively. In the dialysate concentrations were reached of 35-70 micrograms/ml cefradine, 2-5 micrograms/ml trimethoprim and 8-17 micrograms/ml sulfamethoxazole. The values for sulfamethoxazole are regarded too low to be clinically effective. Half-lives, protein binding values and CAPD clearances are presented. Low CAPD clearances were obtained during the night and high values during the day. The dosage yielded too high plasma trimethoprim concentrations, while sulfamethoxazole dialysate concentrations were too low. It seems questionable therefore whether co-trimoxazole can be used orally for the treatment of CAPD peritonitis.

Pharmacokinetics, metabolism and renal excretion of sulfatroxazole and its 5-hydroxy- and N4-acetyl-metabolites in man

Hydroxylation is the predominant pathway of metabolism for sulfatroxazole in the body, accounting for 70 per cent of the dose. Fifteen per cent of the dose is acetylated unimodally and 10 per cent is excreted unchanged. The half-lives of sulfatroxazole and its metabolites 5-hydroxysulfatroxazole and N4-acetylsulfatroxazole are approximately 22 h after administration of sulfatroxazole. N4-acetylsulfatroxazole, taken as parent drug, is eliminated by renal excretion (92 per cent of the dose). The initial elimination half-life of N4-acetylsulfatroxazole is 4.5 h, which later increases to 70 h as the result of the acetylation-deacetylation equilibrium. Probenecid inhibits the renal excretion of the metabolites 5-hydroxy- and N4-acetylsulfatroxazole. Inhibition of the N4-acetyl metabolite favours the deacetylation, which results in an increase of the T 1/2 of sulfatroxazole from 20 to 30 h. The protein binding value of sulfatroxazole is 84 per cent, that of N4-acetylsulfatroxazole is 37 per cent. Sulfatroxazole is excreted renally by passive processes, while the metabolites are excreted by both passive and active processes.

Some pitfalls in selecting descriptive pharmacokinetic models

Some pitfalls in selecting pharmacokinetic models are enumerated. To calculate the pharmacokinetic parameters of a drug that exhibits a biphasic convex plasma concentration-time curve, a two-compartment model does not automatically have to be applied. When only the parent drug in plasma is considered, a two-compartment model seems to be most appropriate. However, when the kinetic behavior of the metabolite has to be taken into account, and when a metabolic equilibrium underlies the metabolic elimination, the two-compartment model may not be appropriate. Also, when calculating the kinetic parameters of a drug with a concave biphasic plasma concentration-time curve, a capacity-limited metabolic conversion is not the automatic explanation for this observation. Limitations in renal excretion and bioavailability may be the reasons for this behavior. Convex and concave biphasic plasma concentration-time curves are illustrated, using sulfonamides as test compounds.