Biliary elimination of indomethacin in man

Using a competitive counterflow assay to identify novel cationic substrates of OATP1B1 and OATP1B3

OATP1B1 and OATP1B3 are two drug transporters that mediate the uptake of multiple endo- and xenobiotics, including many drugs, into human hepatocytes. Numerous inhibitors have been identified, and for some of them, it is not clear whether they are also substrates. Historically radiolabeled substrates or LC-MS/MS methods were needed to test for transported substrates, both of which can be limiting in time and money. However, the competitive counterflow (CCF) assay originally described for OCT2 and, more recently, for OCT1, OATP2B1, and OATP1A2 does not require radiolabeled substrates or LC-MS/MS methods and, as a result, is a more cost-effective approach to identifying substrates of multidrug transporters. We used a CCF assay based on the stimulated efflux of the common model substrate estradiol-17β-glucuronide (E17βG) and tested 30 compounds for OATP1B1- and OATP1B3-mediated transport. Chinese Hamster Ovary (CHO) cells stably expressing OATP1B1 or OATP1B3 were preloaded with 10 nM [³H]-estradiol-17β-glucuronide. After the addition of known substrates like unlabeled estradiol-17β-glucuronide, estrone-3-sulfate, bromosulfophthalein, protoporphyrin X, rifampicin, and taurocholate to the outside of the preloaded CHO cells, we observed efflux of [³H]-estradiol-17β-glucuronide due to exchange with the added compounds. Of the tested 30 compounds, some organic cation transporter substrates like diphenhydramine, metformin, and salbutamol did not induce [³H]-estradiol-17β-glucuronide efflux, indicating that the two OATPs do not transport them. However, 22 (for OATP1B1) and 16 (for OATP1B3) of the tested compounds resulted in [³H]-estradiol-17β-glucuronide efflux, suggesting that they are OATP substrates. Among these compounds, we further tested clarithromycin, indomethacin, reserpine, and verapamil and confirmed that they are substrates of the two OATPs. These results demonstrate that the substrate spectrum of the well-characterized organic anion transporting polypeptides includes several organic cations. Furthermore, as for other drug uptake transporters, the CCF assay is an easy-to-use screening tool to identify novel OATP substrates.

In vitro drug interaction between diflunisal and indomethacin via glucuronidation in humans

It was reported that the plasma concentration of indomethacin was increased with concomitant oral dosages of diflunisal in humans. Both indomethacin and diflunisal are glucuronidated in humans. The effects of diflunisal on the indomethacin glucuronidation were thus investigated in vitro using human liver microsomes (HLM) and human intestine microsomes (HIM) in order to assess the drug-drug interaction. The glucuronidation of indomethacin in HLM showed atypical kinetics with Km and Ksi values of 210 and 89.5 microM, respectively, while HIM exhibited Michaelis-Menten kinetics with a Km value of 17.4 microM. Diflunisal inhibited the indomethacin glucuronidation in HLM with IC50 values ranging from 100 to 231 microM. In HIM, inhibition of the indomethacin glucuronidation by diflunisal was more potent with IC50 values of 15.2-48.7 microM. When the clinical dose of diflunisal (250 mg b.i.d.) is taken into consideration, it is expected that the diflunisal concentration in the intestine would be higher than the IC50 values for indomethacin glucuronidation in the intestine. These findings suggest that the clinical drug-drug interaction between diflunisal and indomethacin may be at least partly attributable to the inhibition of indomethacin glucuronidation by diflunisal in the intestine.

A possibility to predict the absorbability of poorly water-soluble drugs in humans based on rat intestinal permeability assessed by an in vitro chamber method

This paper describes a means to predict the absorbability of poorly water-soluble drugs in humans based on rat intestinal permeability assessed by the in vitro Ussing-type chamber method. We investigated the correlation between the apparent permeability coefficients (P(app)) of 10 water-soluble drugs obtained by the in vitro chamber method, in which the excised rat small intestinal tissue was used as the membrane, and the fractions absorbed (F(a)) in humans. Using this correlation, we predicted F(a) values of 5 poorly water-soluble drugs based on their P(app) obtained through our modified chamber method using an additive. For water-soluble drugs, a good correlation between P(app) and F(a') expressed by the equation: F(a) = 1 - exp(-P(app) x 1.51 x 10(5))(r(2) = 0.920), was found. The poorly water-soluble drugs used in the present study could be solubilized with 5% (final concentration) dimethylsulfoxide, and their P(app) could be obtained through our modified chamber method. For poorly water-soluble drugs whose dose:solubility ratio ranged from 2500 to 3500 ml, predicted F(a) values were favorably comparable with their F(a) values reported in humans in the literature. These results showed that the in vitro Ussing-type chamber method was a useful method for predicting the F(a) of poorly water-soluble drugs.

Clearance of indomethacin occurs predominantly by renal glucuronidation

In this report we describe the conditions of collection, storage and handling of urine samples, collected after oral dosing with indomethacin in man, in order to maintain the integrity of the labile glucuronide formed. We found that the body clearance occurs predominantly by renal metabolism, due to glucuronidation in the human kidney. These glucuronides may be converted to isomeric glucuronides and/or the parent compound indomethacin during the residence time in the bladder.

Pharmacokinetics of ketoprofen enantiomers in cholecystectomy patients: influence of probenecid

Ketoprofen (KT), a 2-arylpropionic acid nonsteroidal antiinflammatory drug, is administered as a racemate. Previous reports suggest stereoselective biliary excretion of KT enantiomers. This hypothesis was tested by administering 50 mg racemic KT to five patients who required bile drainage following cholecystectomy surgery. Subsequently, to study the influence of probenecid (PB), an inhibitor of KT renal elimination, on the biliary excretion, 1000 mg PB was administered 1.5 h before KT to the same patients. The unchanged and conjugated (as glucuronides) KT enantiomers were measured in plasma, urine and bile. In general, KT enantiomers had different plasma concentration-time curves. As compared to normal subjects, these patients had comparable AUCs and shorter t1/2s. Biliary concentrations of conjugated S-KT were greater than R-KT. Nevertheless, the total cumulative biliary excretion of conjugated KT did not exceed 2% of the dose ruling out this pathway as a significant route of KT elimination. There was a positive and significant correlation between the cumulative urinary excretion of conjugated KT enantiomers and creatinine clearance. Although PB did not influence the pattern of stereoselectivity of KT, it increased AUC and prolonged t1/2 of the enantiomers. While reducing cumulative urinary excretion, PB increased total biliary elimination of conjugated KT enantiomers. This, however, did not totally compensate for the reduced urinary excretion. It is suggested that the impaired conjugation of KT caused by PB administration may result in the augmentation of other, otherwise minor, metabolic pathways.

Pharmacokinetics of ketoprofen enantiomers in healthy subjects following single and multiple doses

Ketoprofen (KT; m-benzoylhydratropic acid), a 2-arylpropionic acid (2-APA) nonsteroidal anti-inflammatory drug (NSAID), is marketed and used as a racemic mixture. Although generally the activity of 2-APAs is suggested to be mainly due to the S-enantiomer, information on KT pharmacokinetics is based on measurement of total concentrations of S- and R-enantiomers. In this work, using a crossover fashion, the pharmacokinetics of KT enantiomers following single (50 mg, po) and then multiple (50 mg, q6h for 3 d) doses were delineated in eight healthy subjects. A sensitive stereospecific HPLC assay was used to measure KT enantiomers in plasma and urine, and conjugated KT enantiomers in urine. There were no significant differences between the pharmacokinetic indices calculated after single and multiple administration of KT. In plasma, small but significant differences were found between concentrations of the enantiomers (mean S:R ratios of 0.81 +/- 0.19 after single and 0.87 +/- 0.11 after repeated doses). Negligible amounts of unchanged KT enantiomers were found in urine. More than 80% of the given doses was found in urine as conjugated S- and R-KT, the predominant enantiomer being S-KT (mean S:R ratios of 1.19 +/- 0.05 after single and 1.17 +/- 0.05 after repeated doses). No significant difference between the elimination t1/2 of the enantiomers was observed. It is suggested that stereoselective conjugation followed by preferential biliary excretion of the conjugated R-KT enantiomer is responsible for the observed stereoselectivity in the pharmacokinetics of the drug.