Direct measurement of probenecid and its glucuronide conjugate by means of high pressure liquid chromatography in plasma and urine of humans

Probenecid with its phase-I metabolites, and phase-II glucuronide conjugate can be analysed by a gradient high pressure liquid chromatographic method. Probenecid glucuronide in plasma with pH 7.4 is not stable and declines to 10% of the original value within 6 h (t1/2 approximately 1 h). Probenecid glucuronide is stable in urine with pH 5.0, moderately unstable at pH 6.0 (t1/2 approximately 10 h), and unstable at pH 8.0 (t1/2 approximately 0.5 h). Probenecid glucuronide is stable in water and 0.01 mol/l phosphoric acid in the autosampler of the high pressure liquid chromatograph. The decrease in concentration in water is 5.5% during 9 h and 0% in diluted acid. Probenecid glucuronide and the phase-I metabolites were not detectable in plasma. The main compound in fresh urine is the phase-II conjugate probenecid glucuronide (62% of a 500 mg dose); the phase-I metabolites are present and only a trace of probenecid is present. The percentage of the dose of the phase-I metabolites varies between 5 and 10, while hardly any probenecid is excreted unchanged (0.33%).

Pharmacokinetics, N1-glucuronidation and N4-acetylation of sulfamethomidine in humans

Sulfamethomidine metabolism was studied in 6 volunteers. In humans, only N1-glucuronidation and N4-acetylation take place, leading to the final double conjugate N4-acetylsulfamethomidine N1-glucuronide. The N1-glucuronides were directly measured by high pressure liquid chromatography. Fast and slow acetylators show a similar half-life for sulfamethomidine (26 +/- 6 h) and its conjugates sulfamethomidine (26 +/- 6 h) and N4-acetylsulfamethomidine (36 +/- 16 h). Approximately 50-60% of the oral dose of sulfamethomidine is excreted in the urine, leaving 40-50% for excretion into bile and faeces. The main metabolite of sulfamethomidine is its N1-glucuronide, which accounts for 36 +/- 7% of the dose, followed by N4-acetylsulfamethomidine (16 +/- 8%). N1-glucuronidation results in a 75% decrease in protein binding of sulfamethomidine. N4-acetylsulfamethomidine and its N1-glucuronide showed the same high protein binding of 99%. The renal clearance of N4-acetylsulfamethomidine is 7.9 +/- 2.2 ml/min and approximately 20 times as high as that of the parent drug (0.46 +/- 0.16 ml/min). Total body clearance of sulfamethomidine is 4.5 +/- 0.9 ml/min and the volume of distribution in steady state 10.6 +/- 1.7 1. No measurable plasma concentrations of the N1-glucuronides from sulfamethomidine are found in plasma. This may be explained by renal glucuronidation after active tubular reabsorption.

Comparison of the metabolism of four sulphonamides between humans and pigs

Pigs are unable to form N1-glucuronides of sulphadimethoxine and sulphamethomidine, while humans are able to do so. Pigs and humans are able to oxidise sulphapyridine and form the O-glucuronide. The double conjugate N4-acetylsulphapyridine-O-glucuronide is formed in humans but not in pigs. Sulphadiazine is mainly acetylated in both humans and pigs. A hypothesis about N1-glucuronidation is presented.

Novel oxidative pathways of sulphapyridine and sulphadiazine by the turtle Pseudemys scripta elegans

The sulphonamides sulphapyridine and sulphadiazine show novel hydroxy metabolites in the turtle Pseudemys scripta elegans. In the excreta of the turtles the monohydroxy metabolites 4-hydroxy- and 5-hydroxysulphapyridine and the dihydroxy metabolite 4,5-dihydroxysulphapyridine were detected. Of sulphadiazine only dihydroxy metabolites 4,5- and 4,6-dihydroxysulphadiazine were detected. About 70-90% of the dose of sulphapyridine was recovered, while this figure varied between 48 and 69% for sulphadiazine.

O-Dealkylation and N4-acetylation of sulpha-2-monomethoxine by the turtle Pseudemys scripta elegans

Sulpha-2-monomethoxine is N4-acetylated to an extent of 12% of the dose by Pseudemys scripta elegans; 48% is excreted unchanged. No O-dealkylation of the 2-methoxy group takes place.

High pressure liquid chromatographic analysis and preliminary pharmacokinetics of sulfaphenazole and its N2-glucuronide and N4-acetyl metabolites in plasma and urine of man

A direct high pressure liquid chromatographic analysis of sulfaphenazole-N2-glucuronide in urine is described. After an oral dose of 439 mg of sulfaphenazole, 0% is excreted unchanged in the urine, less than 1% is excreted as N4-acetylsulfaphenazole. As N2-glucuronide 49.4% is excreted in one slow acetylator and 84.8% in one fast acetylator.

Pharmacokinetics, N1-glucuronidation and N4-acetylation of sulfadimethoxine in man

Sulfadimethoxine is metabolized by O-dealkylation, N4-acetylation and N1-glucuronidation. In man, only N1-glucuronidation and N4-acetylation takes place, leading to the final double conjugate N4-acetylsulfadimethoxine-N1-glucuronide. The N1-glucuronides are directly measured by high pressure liquid chromatography. When N4-acetylsulfadimethoxine is administered as parent drug, 30% of the dose is N1-glucuronidated and excreted. Fast acetylators show a shorter half-life for sulfadimethoxine than slow acetylators (27.8 +/- 4.2 h versus 36.3 +/- 5.4 h; P = 0.013), similarly the half-life of the N4-acetyl conjugate is also shorter in fast acetylators (41.3 +/- 5.2 h versus 53.5 +/- 8.5 h, P = 0.036). No measurable plasma concentrations of the N1-glucuronides from sulfadimethoxine are found in plasma. N1-glucuronidation results in a 75% decrease in protein binding of sulfadimethoxine. N4-acetylsulfadimethoxine and its N1-glucuronide showed the same high protein binding of 99%. Approximately 50-60% of the oral dose of sulfadimethoxine is excreted in the urine, leaving 40-50% for excretion into bile and faeces.

The role of plasma protein binding on the metabolism and renal excretion of sulphadimethoxine and its metabolite N4-acetylsulphadimethoxine in pigs

The effects of plasma protein binding on the elimination of sulphadimethoxine (SDM) were examined after intravenous administration of 6.25, 12.5, 25, 50, 100 and 150 mg/kg to pigs. At an early stage of the experiment, the animals were anaesthetised by inhalation of enflurane to obtain a more exact relationship between plasma concentration and the renal excretion. SDM and its acetylated conjugate, N4-acetylsulphadimethoxine (N4-SDM) were detected in plasma and urine of all animals, and the recovery of the doses was almost complete in two animals with negligible renal excretion of SDM. The percentages of plasma protein binding of SDM and N4-SDM were almost similar, and ranged from 30 to 95%, depending on the plasma concentration. The metabolic clearance of SDM by acetylation increased when the plasma protein binding decreased. These results suggested that the main elimination route of SDM in pigs is acetylation, and that the plasma protein binding can have a large effect on the elimination of SDM in pigs. The effect of plasma protein binding on the renal clearance of SDM was not so evident, because urine pH had a much greater effect on it. The deacetylation of N4-SDM was detected after 25 mg/kg intravenous administration of N4-SDM, which suggests that the metabolic clearance of SDM is part of an acetylation-deacetylation equilibrium. Saturation of the active tubular reabsorption of SDM and of the active tubular secretion of N4-SDM was also suggested after higher doses of SDM.

O-demethylation and N4-acetylation of sulfadimethoxine by the snail Cepaea hortensis

The snail Cepaea hortensis O-dealkylates sulfadimethoxine at the 2- and 6-position, the 2-position being favoured. Sulfadimethoxine is also acetylated, while no measurable acetylation of the hydroxy (= demethyl) metabolites took place.

Oxidation and acetylation of sulfamonomethoxine by the snail Cepaea hortensis

Sulfamonomethoxine is not O-demethylated in the snail Cepaea hortensis, but acetylated (15.2%) and oxidised (0.78%) at the 2 position of the pyrimidine nucleus.