Recirculation of iopanoic acid after conjugation in the liver

Self-micellization of gemfibrozil 1-O-beta acyl glucuronide in aqueous solution

PURPOSE

Phase II metabolism involves the conjugation of a polar moiety, such as sulfate or glucuronic acid, to a (relatively) nonpolar xenobiotic. Although it might be expected that such conjugates may exhibit amphiphilic character (e.g., surface activity and potential to form micelles), no detailed study of the micellization characteristics of any drug-glucuronide conjugates has yet been reported. Therefore, the aim of this study was to investigate the solution behavior and amphiphilic characteristics of gemfibrozil 1-O-beta glucuronide (GG), a model drug-glucuronide conjugate.

METHODS

Crude GG was extracted from the urine of volunteers dosed with 600 mg of gemfibrozil, and this material was then purified by reversed-phase high-performance liquid chromatography to yield a white solid. The amphiphilic properties of GG within the bulk aqueous phase were studied by isothermal titration microcalorimetry and 1H-NMR spectrometry, whereas those at the aqueous/air interface were studied by surface tensiometry.

RESULTS

The results of each independent analytical technique were consistent with GG in aqueous solution exhibiting amphiphilic properties typical of a hydrophilic surfactant. The titration microcalorimetry and 1H-NMR spectrometry data were in excellent agreement with each other, yielding critical micellization concentrations (cmc) for GG in 0.1 M acetate buffer of 18.1 +/- 0.4 mM and 18.3 +/- 0.3 mM, respectively. The profile and results of the surface tension measurements were consistent with GG localizing at the aqueous/air interface.

CONCLUSIONS

These results confirm the hypothesis that a glucuronide conjugate of a relatively nonpolar xenobiotic, such as gemfibrozil, behaves as an amphiphile in aqueous solution. The implications of this observation include a likely basis for the previously observed concentration-dependence in the degradation rate of the acyl glucuronides of 2-phenylpropionic acid, as well as identifying a possible broader contributory effect to the structural dependencies in biliary choleresis of different glucuronide conjugates of xenobiotics.

Concentration and excretion of contrast agents during oral cholecystography as measured by computed tomography in dogs

Nine healthy mongrel dogs were given 2 consecutive doses of 1 of 3 cholecystographic contrast agents (iopanoic acid, sodium ipodate, and sodium tyropanoate), followed by daily computed tomograms (CT) and abdominal radiographs in a randomized crossover study in order to determine: (a) the maximum time for excretion of the contrast material from the gallbladder, (b) the maximum time for elimination of contrast material from the blood, and (c) the correlation between the density of the gallbladder on CT and the actual concentration of iodine in the gallbladder bile. In all 9 animals gallbladder opacification disappeared on CT within 4 days after administration of the contrast material. Plain abdominal radiographs did not show gallbladder opacification after 2 1/2 days. Daily blood iodine measurements showed that all of the contrast material was cleared from the blood within 7 days after administration. In 7 dogs CT imaging of the gallbladder was followed by percutaneous aspiration of bile from the gallbladder using CT guidance. There was a direct linear correlation between the actual concentration of iodine in the bile and the density of the gallbladder on CT (r = 0.925). This suggests that CT measurements can be used to determine the concentration of contrast agents in the gallbladder during oral cholecystography.

Thyroid, renal, and hepatic function tests following cholecystography with high-dose contrast agents

Serum biochemical tests were observed for about three weeks following oral cholecystography with fractionated high doses (6 g) of iopanoic acid (Telepaque) or sodium ipodate (Biloptin) in 24 and 29 patients, respectively. Both agents produced similar effects. No significant changes were seen in renal or hepatic function except for a mild increase in bilirubin on day 22. Serum urate decreased 10% on day 4, but the change was not significant. On days 4 and 11, there were significant increases in thyroid-stimulating hormone, thyroxine and free thyroxine index, and a moderate fall in triiodothyronine. Reverse triiodothyronine increased sharply on day 4. The pattern of changes observed suggests that these contrasts interfere with the extrathyroidal deiodination of iodothyronines. The temporary rise in thyroxine and free thyroxine index exceeded reference ranges in about half of all subjects, but they remained clinically euthyroid. Thyroid function tests should be interpreted with caution within three weeks of cholecystography.