Check your microbiota when taking the drug

Potential conflict of interest: Nothing to report. See Article on page 282 In this edition of Hepatology, Yip's article entitled “The liver‐gut microbiota axis modulates hepatotoxicity of tacrine in the rat”1 elegantly uncovers the impact of the intestinal microbiota in the interindividual variation of tacrine‐induced hepatotoxicity. Tacrine is a reversible acetylcholinesterase inhibitor and was the first drug approved for the treatment of mild to moderate Alzheimer's dementia. One known side effect of the drug is hepatocellular damage, indicated by elevated serum aspartate aminotransferase activity, which is observed in about half of the patients on tacrine therapy. To date, the mechanism underlying tacrine‐induced hepatotoxicity cannot be explained, and tacrine was withdrawn from clinical use in 2013 for safety reasons.2 For this study, Yip and his colleagues treated male Lister hooded rats with tacrine and found that 40% of the rats developed hepatotoxicity, at almost the same rate as in humans. Rats with a strong response (3 times higher aspartate aminotransferase activity compared to the vehicle‐treated group) to tacrine treatment revealed an increased plasma concentration and a decreased clearance of the drug. Additionally, the amount of glucuronidated tacrine (tacrine‐N‐glucuronide) excreted in feces was higher in the rats responding to tacrine than in the nonresponder group. From these observations, the authors hypothesized that tacrine‐N‐glucuronide may undergo deglucuronidation catalyzed by microbial β‐glucuronidase in the intestine. Glucuronidation takes place in the liver, and the authors considered a possible contribution of this xenobiotic metabolic pathway; however, hepatic molecules involved in disposition and detoxification processes were not differently expressed between responder and nonresponder rats. To assess the crosstalk between host and gut microbiota, Yip et al. performed a comprehensive metabonomic approach for fecal and urinary metabolites. With this, they were able to show that metabolites from strong responders clustered together. In addition, the authors uncovered more than 100 unique metabolites in feces and urine associated with tacrine‐induced hepatotoxicity. Interestingly, half of those metabolites were of microbial or co‐metabolic origin, thus indicating that gut microbiota, at least in part, is involved in tacrine‐induced hepatotoxicity. One of the metabolites was lithocholic acid, a secondary bile acid processed from chenodeoxycholic acid by bacterial dehydroxylation in the intestine. This is of particular interest as lithocholic acid is well established as an hepatotoxicant.3 Other metabolites included fermentation products of aromatic amino acids and long‐chain unsaturated fatty acids, such as oleic acid and linoleic acid. The latter are well known for antibacterial activity by inhibition of bacterial fatty acid synthesis4 and are bactericidal to pathogenic microorganisms, such as Staphylococcus aureus, Helicobacter pylori, and mycobacteria.5 With this strong hint toward changes in the microbial community, the investigators used 16S ribosomal RNA sequencing to profile rat gut microbiota composition and shotgun sequencing to analyze differences in the functionality. These data showed an enrichment of β‐glucuronidase‐producing bacteria, like Bacteroides and Enterobacteriaceae, and an increase in β‐glucuronidase genes within the microbiota of strong responders versus nonresponding rats. Interestingly, these differences in microbiota functionality were not present at baseline; rather, increased intestinal β‐glucuronidase genes occurred following tacrine administration. How tacrine induces changes of the microbiota composition is not known. It will be very important to determine the underlying mechanism because functional microbiota changes do not occur in every animal receiving tacrine but only in the responders with increased hepatotoxicity. This shifts the attention from the liver to the microbiota as the determinant of an unpredictable side effect profile associated with tacrine and adds to the complexity of a personalized medicine approach. Finally, the authors present a validation study using pretreatment of rats with β‐glucuronidase or antibiotics (vancomycin and imipenem) prior to the administration of tacrine. With the co‐administration of β‐glucuronidase, the systemic exposure of tacrine and the number of rats revealing hepatotoxicity increased. After antibiotic pretreatment, the susceptibility to tacrine‐induced hepatotoxicity decreased to 35% based on alanine aminotransferase measurement.1 Built on those observations, the authors postulate that following excretion into bile, tacrine N‐glucuronide undergoes hydrolysis catalyzed by microbial β‐glucuronidase in the intestinal tract. This is facilitated by a tacrine‐associated increase in bacterial β‐glucuronidase activity in the intestine. The resulting product is free tacrine, which becomes available for reabsorption. Thus, the augmented enterohepatic recycling results in a higher systemic exposure of tacrine, leading to enhanced susceptibility to liver cell damage (Fig. 1).Figure 1: Gut microbiota and liver interaction during tacrine treatment. Oral‐administrated tacrine is absorbed in the intestine and reaches the liver by the portal vein. In the liver, tacrine undergoes biotransformation steps (hydroxylation and glucuronidation). Subsequently, water‐soluble tacrine‐N‐glucuronide is excreted through the bile into the intestine followed by ultimate fecal excretion or hydrolysis catalyzed by microbial β‐glucuronidase. In responder rats with tacrine‐induced hepatotoxicity, tacrine use is associated with changes in microbiota functionality toward bacteria genera with a higher abundance of β‐glucuronidase genes. The mechanism of this tacrine‐induced microbiotic shift is unknown and may directly be related to tacrine, its metabolites, or secondary effects, such as tacrine‐induced portal vein hypertension leading to hepatic ischemia. Increased hydrolysis of glucuronidated tacrine due to higher microbial β‐glucuronidase activity releases free tacrine in the intestine, which becomes available for reabsorption, and results in a higher systemic exposure of tacrine. Abbreviation: N‐Gluc, N‐glucuronide.Other studies have shown that microbial β‐glucuronidase inhibitors are capable of rescuing mice from drug toxicity (e.g., induced by irinotecan, a topoisomerase inhibitor for treatment of colorectal cancer6 and nonsteroidal anti‐inflammatory drugs, including diclofenac, indomethacin, and ketoprofen).7 Hence, β‐glucuronidase inhibitors might be an approach to support the results of this study and to minimize tacrine‐associated liver toxicity. Furthermore, it would be interesting to determine sex differences in tacrine‐induced microbiota changes and hepatotoxicity, especially as it is known that tacrine plasma levels are 50% higher in women than in men and that CYP1A1/2, which extensively metabolizes tacrine, shows reduced activity in women.8 This carefully and well‐designed study by Yip and coworkers has implications for other therapeutic drugs with large interindividual variation in pharmacokinetics and forms an important part of the bigger picture in the development of personalized medicine. There are further studies needed like this to develop a deeper understanding of which drugs are influenced by gut bacteria and where specific side effects are arising in order to make the next step toward personalized medicine, finding the optimal drugs and their doses based on the gut microbiota. Together with new techniques, which are working toward building a fingerprint of the gut microbiome,9 the results by Yip et al. greatly enhance the ability to design highly effective therapeutic interventions with reduced side effects.