ABSTRACT:The purpose of present study was to determine the intestinal absorption and metabolism of genistein and its analogs to better understand the mechanisms responsible for their low oral bioavailability. The Caco-2 cell culture model and a perfused rat intestinal model were used for the study. In both models, permeabilities of aglycones (e.g., genistein) were comparable to well absorbed compounds, such as testosterone and propranolol. In the Caco-2 model, permeabilities of aglycones were at least 5 times higher (p < 0.05) than their corresponding glycosides (e.g., genistin), and the vectorial transport of aglycones was similar (p > 0.05). In contrast, vectorial transport of glucosides favored excretion (p < 0.05). Limited hydrolysis of glycosides was observed in the Caco-2 model, which was completely inhibited (p < 0.05) by 20 mM gluconolactone, a broad specificity glycosidase inhibitor. In the perfused rat intestinal model, genistin was rapidly hydrolyzed (about 40% in 15 min) in the upper intestine but was not hydrolyzed at all in the colon. Aglycones were rapidly absorbed (P* eff > 1.5), and absorbed aglycones underwent extensive (40% maximum) phase II metabolism via glucuronidation and sulfation in the upper small intestine. Similar to the hydrolysis, recovery of conjugated genistein was also region-dependent, with jejunum having the highest and colon the lowest (p < 0.05). This difference in conjugate recovery could be due to the difference in the activities of enzymes or efflux transporters, and the results of studies tend to suggest that both of these factors were involved. In conclusion, genistein and its analogs are well absorbed in both intestinal models, and therefore, poor absorption is not the reason for its low bioavailability. On the other hand, extensive phase II metabolism in the intestine significantly contributes to its low bioavailability.

Despite concerns of nephrotoxicity, polymyxin antibiotics often remain the only susceptible agents for multidrug-resistant (MDR) Gram-negative bacteria. Colistin has been more commonly used clinically due to a perceived safety benefit. We compared the nephrotoxicity of colistin to polymyxin B. The in vitro cytotoxicity of colistin was compared to polymyxin B in two mammalian renal cell lines. To validate the clinical relevance of the findings, we evaluated adult patients with normal renal function who received a minimum of 72 h of polymyxin therapy in a multicenter study. The primary outcome was the prevalence of nephrotoxicity, as defined by the RIFLE (risk, injury, failure, loss, end-stage kidney disease) criteria. Colistin exhibited an in vitro cytotoxicity profile similar to polymyxin B. A total of 225 patients (121 receiving colistimethate, 104 receiving polymyxin B) were evaluated. Independent risk factors for colistimethate-associated nephrotoxicity included age (odds ratio [OR], 1.04; 95% confidence interval [CI], 1.00 to 1.07; P ‫؍‬ 0.03), duration of therapy (OR 1.08; 95% CI, 1.02 to 1.15; P ‫؍‬ 0.02), and daily dose by ideal body weight (OR 1.40; 95% CI, 1.05 to 1.88; P ‫؍‬ 0.02). In contrast, cystic fibrosis was found to be a protective factor in patients who received colistimethate (OR, 0.03; 95% CI, 0.001 to 0.79; P ‫؍‬ 0.04). In a matched analysis based on the risk factors identified (n ‫؍‬ 76), the prevalence of nephrotoxicity was higher with colistimethate than with polymyxin B (55.3% versus 21.1%; P ‫؍‬ 0.004). Polymyxin B was not found to be more nephrotoxic than colistin and may be the preferred polymyxin for MDR infections. A prospective study comparing the two polymyxins directly is warranted.

Phenolics including many polyphenols and flavonoids have the potentials to become chemoprevention and chemotherapy agents. However, poor bioavailability limits their biological effects in vivo. This paper reviews the factors that affect phenolics absorption and their bioavailabilities from the points of view of their physicochemical properties and disposition in the gastrointestinal tract. The up-to-date research data suggested that solubility and metabolism are the primary reasons that limit phenolic aglycones’ bioavailability although stability and poor permeation may also contribute to the poor bioavailabilities of the glycosides. Future investigations should further optimize phenolics’ bioavailabilities and realize their chemopreventive and chemotherapeutic effects in vivo.

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The present study aims to predict the regiospecific glucuronidation of three dihydroxyflavones and seven mono-hydroxyflavones in human liver and intestinal microsomes using recombinant UGT isoforms. Seven mono-hydroxyflavones (or HFs), 2′-, 3′-, 4′-, 3-, 5-, 6-, and 7-hydroxyflavone, and three di-hydroxyflavones (or diHFs), 3,7-dihydroxyflavone (3,7-diHF), 3,5-dihydroxyflavone (3,5-diHF) and 3,4′-dihydroxyflavone (3,4′-diHF) were chosen and rates were measured at 2.5, 10 and 35 μM. The results indicated that the position of glucuronidation of three diHFs could be determined by using the UV spectra of relevant HFs. The results also indicated that UGT1A1, UGT1A7, UGT1A8, UGT1A9, UGT1A10 and UGT2B7 are the most important six UGT isoforms for metabolizing the chosen flavones. Regardless of isoforms used, 3-HF was always metabolized the fastest whereas 5-HF was usually metabolized the slowest, probably due to the formation of an intra-molecular hydrogen bond between 4-carbonyl and 5-OH group. Relevant UGT isoformspecific metabolism rates generally correlated well with the rates of glucuronidation in human intestinal and liver microsomes at each of the three tested concentrations. In conclusion, the glucuronidation "fingerprint" of seven selected mono-hydroxyflavones was affected by UGT isoforms used, positions of the −OH group, and the substrate concentrations, and the rates of glucuronidation by important recombinant UGTs correlated well with those obtained using human liver and intestinal microsomes.