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

Quantitative prediction of in vivo drug-drug interactions from in vitro data based on physiological pharmacokinetics: use of maximum unbound concentration of inhibitor at the inlet to the liver

PURPOSE

To assess the degree to which the maximum unbound concentration of inhibitor at the inlet to the liver (I(inlet,u,max), used in the prediction of drug-drug interactions, overestimates the unbound concentration in the liver.

METHODS

The estimated value of I(inlet,u,max) was compared with the unbound concentrations in systemic blood, liver, and inlet to the liver, obtained in a simulation study based on a physiological flow model. As an example, a tolbutamide/sulfaphenazole interaction was predicted taking the plasma concentration profile of the inhibitor into consideration.

RESULTS

The value of I(inlet,u,max) differed from the concentration in each compartment, depending on the intrinsic metabolic clearance in the liver, first-order absorption rate constant, non-hepatic clearance and liver-to-blood concentration ratio (Kp) of the inhibitor. The AUC of tolbutamide was predicted to increase 4-fold when co-administered with sulfaphenazole, which agreed well with in vivo observations and was comparable with the predictions based on a fixed value of I(inlet,u,max). The blood concentration of tolbutamide was predicted to increase when it was co-administered with as little as 1/100 of the clinical dose of sulfaphenazole.

CONCLUSIONS

Although I(inlet,u,max) overestimated the unbound concentration in the liver, the tolbutamide/sulfaphenazole interaction could be successfully predicted by using a fixed value of I(inlet,u,max) indicating that the unbound concentration of sulfaphenazole in the liver after its clinical dose is by far larger than the concentration to inhibit CYP2C9-mediated metabolism and that care should be taken when it is co-administered with drugs that are substrates of CYP2C9.

Epidural metabolism of articaine to its metabolite articainic acid in five patients after epidural administration of 600 mg articaine

The clinical pharmacokinetics, metabolism and renal excretion of articaine and its metabolite articainic acid have been investigated in man after epidural administration. (+/-)-Articaine and its metabolite (+/-)-articainic acid have different pharmacokinetic constants (P = 0.0079) except for lag-time (tlag; 0.06 min), first phase distribution of elimination (t 1/2 alpha; 0.49 +/- 0.21 h), and elimination half life (t 1/2 beta; 2.19 +/- 0.98 h), which are all the same for both compounds. The total body clearance of articaine (103 +/- 57 L h-1) is 10 times higher than that of the metabolite articainic acid (10.7 +/- 1.80 L h-1, P = 0.0079). With similar half-life (t 1/2 beta) values (2h), the volumes of distribution (V beta) are 10 times higher for the parent drug than for the metabolite ((329 +/- 212 L compared with 38.4 +/- 7.5 L, respectively; P = 0.0079). The difference between the areas under the curves for total plasma articainic acid and that formed in the plasma gives an indication of the percentage metabolism during epidural transfer (5.38 +/- 1.51%). This percentage of metabolism corresponds to a mean epidural transfer time of 5 min. The main compound in the urine is articainic acid (64.2 +/- 14.4%), followed by articainic acid glucuronide (13.4 +/- 4.97%) and the parent drug (1.45 +/- 0.77%). In total, 79.0 +/- 18.5% of the dose is recovered in the urine. The renal clearance of articaine is 22.5 +/- 13.9 mL min-1, whereas that of articainic acid is 119.6 +/- 30.1 mL min-1 (P < 0.0001). The apparent renal clearance of articainic acid glucuronide was 25.4 +/- 12.0 mL min-1. This value does not differ from that of the parent drug (P > 0.8). Articainic acid glucuronide is not present in plasma, but has an apparent renal clearance of 25 mL min-1. These results suggest that articainic acid is glucuronidated by the tubular cells and then excreted.

Isolation, identification and determination of sulfamethoxazole and its known metabolites in human plasma and urine by high-performance liquid chromatography

From human urine the following metabolites of sulfamethoxazole (S) were isolated by preparative HPLC: 5-methylhydroxysulfamethoxazole (SOH), N4-acetyl-5-methylhydroxysulfamethoxazole (N4SOH) and sulfamethoxazole-N1-glucuronide (Sgluc). The compounds were identified by NMR, mass spectrometry, infrared spectrometry, hydrolysis by beta-glucuronidase and ratio of capacity factors. The analysis of S and the metabolites N4-acetylsulfamethoxazole (N4), SOH, N4-hydroxysulfamethoxazole (N4OH), N4SOH, and Sgluc in human plasma and urine samples was performed with reversed-phase gradient HPLC with UV detection. In plasma, S and N4 could be detected in high concentrations, while the other metabolites were present in only minute concentrations. In urine, S and the metabolites and conjugates were present. The quantitation limit of the compounds in plasma are respectively: S and N4 0.10 micrograms/ml; N4SOH 0.13 micrograms/ml; N4OH 0.18 micrograms/ml; SOH 0.20 micrograms/ml; and Sgluc 0.39 microgram/ml. In urine the quantitation limits are: N4 and N4OH 1.4 micrograms/ml; S 1.5 micrograms/ml; N4SOH 1.9 micrograms/ml; SOH 3.5 micrograms/ml; and Sgluc 4.1 micrograms/ml. The method was applied to studies with healthy subjects and HIV positive patients.

Direct gradient reversed-phase HPLC analysis and preliminary pharmacokinetics of nalidixic acid, 7-hydroxymethylnalidixic acid, 7-carboxynalidixic acid, and their corresponding glucuronide conjugates in humans

A gradient reversed-phase high pressure liquid chromatographic analysis was developed for the direct measurement of nalidixic acid with its acyl glucuronide, 7-hydroxymethylnalidixic acid with its acyl and ether glucuronides, and 7-carboxynalidixic acid in human plasma and urine. The glucuronides and 7-carboxynalidixic acid were not present in plasma after an oral dose of 1,000 mg nalidixic acid. The acyl glucuronides of 7-carboxynalidixic acid were not present in plasma and urine. The acyl glucuronides are stable in urine at pH 5.0-5.5. The subject's urine must therefore be acidified by the oral intake of 4 x 1 g of ammonium chloride per day. With acidic urine, hardly any nalidixic acid was excreted unchanged (0.2%). It was excreted as acyl glucuronide (53.4% of dose), 7-hydroxymethyl-nalidixic acid (10.0%), the latter's acyl glucuronide (30.9%), and 7-carboxynalidixic acid (4.2%).

Contribution of the human kidney to the metabolic clearance of drugs

OBJECTIVE

To demonstrate that the human kidney is capable not only of filtering and secreting drugs and their metabolites, but also of carrying out conjugation reactions such as acyl glucuronidation, N-glucuronidation, and glycination.

DATA SOURCES

Plasma concentrations and renal excretion rates of drugs are measured and renal clearance is calculated in a series of selected pharmacokinetic studies in healthy human volunteers (some studies were conducted in the authors' laboratory and others were reported in the literature). BACKGROUND THEORY: It is generally agreed that the liver plays the dominant role in drug metabolism, and that the function of the kidneys is limited to excretion of parent drug and metabolites. This can be easily understood when a metabolite is present in both plasma and urine. When the metabolite is present in urine but is not measurable in plasma, then the possibility exists that the metabolite is formed by the kidneys.

RESULTS

"Simple" excretion by the kidneys is demonstrated for sulfatroxazole/sulfamethoxazole. Ether glucuronides of codeine are formed in the liver, and the resulting glucuronide is excreted by the kidneys. Possible formation of N1- and N2-glucuronides by the kidneys is demonstrated for sulfadimethoxine, sulfametomidine, and sulfaphenazole. Acyl glucuronidation of probenecid and nalidixic acid is carried out by the kidneys. The acyl glucuronidation of probenecid shows a capacity-limited formation/excretion rate of 46 mg/h, which is subject dependent. During this process, the acyl glucuronidation of co-administered nalidixic acid is reduced from 53 to 16 percent compared with that of nalidixic acid alone. Probenecid and its acyl glucuronidation do not inhibit the ether glucuronidation of codeine in the liver, but only interfere with the active tubular secretion process. The acyl glucuronidation of the nonsteroidal antiinflammatory drug naproxen and its metabolite, O-desmethylnaproxen, may be carried out by the liver and kidneys. Glycination of benzoic acid and salicylic acid is carried out in both the liver and kidneys.

CONCLUSIONS

It is difficult to recognize renal drug metabolism in the intact human body (in vivo); the glucuronides or conjugates must be measured via direct HPLC analysis. In cases where the metabolite is present in high concentrations in urine but not in blood, there may be an indication that the kidneys are responsible for the formation of the metabolite. Impaired kidney function not only affects renal excretion but may also affect renal metabolism.

Capacity-limited renal glucuronidation of probenecid by humans. A pilot Vmax-finding study

Probenecid shows dose-dependent pharmacokinetics. When in one volunteer the dose is increased from 250 to 1,500 mg orally, the t1/2 increased from 3 to 6 h. The Cmax was 14 micrograms/ml with a dosage of 250 mg, 31 micrograms/ml with 500 mg, 70 micrograms/ml with 1,000 mg and 120 micrograms/ml with 1,500 mg. The tmax remained 1 h for all four dosages. The AUC/dose ratio increased with the dose, indicating nonlinear elimination. The total body clearance declined from 64.5 ml/min for 250 mg to 26.0 ml/min for 1,500 mg. The renal clearance of probenecid remained constant, 0.6-0.8 ml/min. Protein binding of probenecid is high (91%) and independent of the dose. The phase I metabolites show lower protein binding values (34-59%). The protein binding of probenecid glucuronide in vitro (spiked plasma) is 75%. Probenecid is metabolized by cytochrome P-450 to three phase I metabolites. Each of the metabolites accounts for less than 10% of the dose administered; the percentage recovered in the urine is independent of the dose. The main metabolite probenecid glucuronide is only present in urine and not in plasma. The renal excretion rate--time profile of probenecid glucuronide shows a plateau value of approximately 700 micrograms/min (46 mg/h) with acidic urine pH. The duration of this plateau value depends on the dose: 2 h at 500 mg, 10 h at 1,000 mg and 20 h at 1,500 mg. It is demonstrated that probenecid glucuronide must be formed in the kidney during its passage of the tubule. The plateau value in the renal excretion rate of probenecid value reflects its Vmax of formation.

Direct-gradient high-performance liquid chromatographic analysis and preliminary pharmacokinetics of flumequine and flumequine acyl glucuronide in humans: effect of probenecid

A gradient high-performance liquid chromatographic analysis for the direct measurement of flumequine, with its acyl glucuronide, in plasma and urine of humans has been developed. In order to prevent hydrolysis and isomerization of flumequine acyl glucuronide, the samples were acidified by the oral intake of four 1.2-g amounts of ammonium chloride per day. In contrast to the acyl glucuronides of non-steroidal anti-inflammatory drugs, flumequine and its acyl glucuronide were stable in urine of pH 5.0-8.0. Flumequine acyl glucuronide is unstable at pH 1.5. In acidic urine (pH 5-6), almost no flumequine is excreted unchanged (1%): it is excreted chiefly as acyl glucuronide (84.2%). Probenecid co-medication reduces the renal excretion rate of flumequine acyl glucuronide from 662 to 447 micrograms/min (p = 0.00080), but not the percentage of glucuronidation.

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%).

Direct determination of codeine, norcodeine, morphine and normorphine with their corresponding O-glucuronide conjugates by high-performance liquid chromatography with electrochemical detection

A high-performance liquid chromatographic method has been developed for the detection, separation and measurement of codeine and its metabolites norcodeine, morphine and normorphine, with their glucuronide conjugates. The glucuronidase Escherichia coli type VIIA hydrolyses codeine-6-glucuronide completely and is used for the construction of the calibration curves of codeine-6-glucuronide. Enzymic hydrolysis of codeine-6-glucuronide depends on the specific activity of the glucuronidase applied. Examples are shown of a volunteer who is able to form morphine from codeine and one who is unable to do so.

Direct high pressure liquid chromatographic analysis and preliminary pharmacokinetics of enantiomers of oxazepam and temazepam with their corresponding glucuronide conjugates

Three high pressure liquid chromatographic systems for the separation of oxazepam, temazepam and their glucuronides (system A), the separation of their R,S glucuronide diastereomers (system B) and the chiral separation of the parent drugs (system C) are described. Preliminary pharmacokinetics of R,S-oxazepam and R,S-temazepam in a human volunteer reveal that the protein binding of the glucuronides is lower than that of the parent drugs, but that there is no difference in protein binding between the R-oxazepam/temazepam and S-oxazepam/temazepam and their corresponding glucuronides. The S-glucuronide is the main metabolite formed and excreted by man. The plasma ratio R/S-glucuronide is 1:1 for both oxazepam and temazepam. The renal clearance of R-temazepam, and S-temazepam are similar, and those of R-oxazepam and S-oxazepam tend to be different.