ReviewThe gastrointestinal microbiota as a site for the biotransformation of drugs
Introduction
The oral route is the preferred route for drug administration with 84% of the fifty bestselling pharmaceutical products in the US and European markets given by mouth (Lennernäs and Abrahamsson, 2005). However, oral administration is arguably the most complex route of drug delivery. For an orally administered drug to be effective it must (a) dissolve in the fluids of the gastrointestinal (GI) tract (b) remain intact in the lumen (c) cross the epithelial membrane and (d) undergo minimal first-pass metabolism. Oral bioavailability is therefore a multi-factorial process dependent on the solubility, stability, permeability and metabolism of the drug molecule.
While much has been written on the role of dissolution, permeability and first-pass metabolism on oral drug bioavailability (Lindahl et al., 1997, Dressman et al., 1998, Amidon et al., 1995, Lennernäs and Abrahamsson, 2005, McConnell et al., 2008a), less attention has been paid to the stability of the drug in the intestinal lumen. Instability is often associated with pH and/or enzyme mediated degradation in the upper gut. These can be overcome by formulation approaches such as enteric coating. A major stability factor that is often overlooked is the effect of the microbiota in the gastrointestinal tract.
The gastrointestinal tract is populated with large numbers of bacteria that contribute to normal digestive function. Most of these bacteria reside in the large intestine and their primary function is to ferment carbohydrate and protein that escape digestion in the upper gut into absorbable energy. In addition, the metabolic reactions performed by these bacteria and their respective enzymes have the ability to metabolise drugs and other xenobiotics far more extensively than any other part of the body (Scheline, 1973, Abu Shamat, 1993, Mikov, 1994). Scheline has even suggested that the gastrointestinal microbiota has the ability to act as an organ with a metabolic potential at least equal to the liver (Scheline, 1973). There are, however, important differences between hepatic and bacterial metabolism. The liver is primarily responsible for metabolism via oxidation and conjugation producing polar high molecular weight metabolites, while the gastrointestinal microbiota is involved in reductive and hydrolytic reactions generating non-polar low molecular weight byproducts. Furthermore, all drugs that are delivered to, or absorbed into, the blood stream, are subject to hepatic metabolism. However, rate and extent of bacterial metabolism will be influenced by the amount of drug that reaches the distal gut.
The majority of drugs are rapidly and completely absorbed in the upper gut and have minimal contact with intestinal bacteria. This goes someway towards explaining why over the last forty years only thirty or so marketed drugs have been identified as substrates for intestinal bacteria. However, the biopharmaceutical complexity of new drug candidates is providing an increasing number of compounds that suffer from low solubility, low permeability or both (Davis, 2005). Drugs that display these properties will reach the lower confines of the gastrointestinal tract, presenting themselves to the host microbiota. Furthermore, drugs that are delivered via the intravenous route or that are fully absorbed from the upper parts of the gastrointestinal tract may still reach the lower gut by secretion or diffusion from the systemic circulation into the intestinal lumen, or may be excreted in the bile, possibly as conjugates following a recycling process known as enterohepatic recirculation. There is an increasing trend to develop modified release preparations (colon specific or extended release systems) to improve therapy via the oral route (Basit, 2005, Rubinstein, 2005, Ibekwe et al., 2006, Tiwari and Rajabi-Siahboomi, 2008, Ibekwe et al., 2008). In such cases, most if not all the entire drug load will be deposited in the large intestine, providing further opportunity for exposure to the microbiota. Drugs can also come in direct contact with bacteria via rectal administration in the form of suppositories or enemas. Given all this it is expected that opportunities for microbiota-mediated metabolism will increase and drug stability assessment in the presence of intestinal bacteria becomes of increasing importance.
The stability of a drug to the microbiota is clinically relevant: metabolism can render a drug pharmacologically active, inactive or toxic. An important example of the significance of this was seen in Japan in 1993 when sorivudine, a promising antiviral drug was introduced into the Japanese market. This was later discovered to be transformed by gut microbiota into (E)-5-(2-bromovinyl)uracil which can inhibit the metabolism of the anti-cancer drug 5-fluorouracil leading to toxic levels of this drug. Within forty days of reaching the Japanese market this bacterially-metabolized interaction was responsible for the death of eighteen patients who were co-administered sorivudine with oral 5-fluouracil prodrugs (Okuda et al., 1998). Sorivudine was withdrawn from the market a few weeks after these deaths. This highlights the importance of studying metabolism by the microbiota, and this has come to the attention of the pharmceutical industry. For example, AstraZeneca has now started to examine the stability of drugs in early development when relevant using an in vitro colonic model (based on an inoculum of human faecal bacteria). New molecules can be screened to assess whether there will be development issues with an extended release formulation option later in the programme. Significantly, of the fifty-one molecules examined since 2004, nineteen underwent degradation in the colonic in vitro model.
In light of the significant issue of bacterial drug metabolism, the purpose of this review is to describe the in vitro and in vivo methods used to assess drug metabolism in the presence of gastrointestinal bacteria and their relative merits. Detailed information on those drug molecules known to be susceptible to bacterial metabolism are also presented with a summary of the metabolic reactions involved. Knowledge of the individual drugs, and drug classes, should be useful for initial identification of chemical groups that are potential substrates: these data could be extrapolated to new drug molecules for further investigation.
Section snippets
The human gastrointestinal microbiota and its function
The term microflora has been used to describe microorganisms residing on body surfaces including the gastrointestinal tract. This is because these microorganisms were originally thought to be plants and were incorrectly classified as “flora”. Since this denomination is scientifically inaccurate and misleading, the term microbiota is preferred. We often consider the members of the human intestinal microbiota as key players in maintaining human health and well-being: they are implicated in
Models to study intestinal bacteria-related drug metabolism
Since the majority of gut bacteria are found in the large intestine this is the main site for biotransformation for endogenous and exogenous molecules. However, the inaccessibility of the human colon prevents the direct examination of the metabolic and ecological activity of the microbiota. Human and animal experiments to study this region are costly and have ethical drawbacks. Toxicity issues also render the use of human in vivo methodologies unviable in early drug development. The use of in
Drug metabolic reactions performed by the gastrointestinal microbiota
The following section highlights over 30 drug substrates for the gastrointestinal microbiota. A detailed understanding of the metabolic reactions involved and the types of studies to confirm a role for gut microbiota in metabolism is also presented (summarised in Table 4). The section is subdivided according to the class of reaction.
Conclusions
The human intestinal microbiota can have a major impact on drug metabolism and ultimately on oral bioavailability. More than 30 drugs that made it onto the market were subsequently identified as substrates for colonic bacteria. However, these examples are merely the tip of the iceberg as the next generation of drugs have a higher probability of being presented to the microbiota in the lower gut, either through poor solubility/permeability properties or formulation development strategies. The
References (140)
- et al.
Biliary excretion and intestinal metabolism of progesterone and estrogens in man
J. Steroid Biochem.
(1980) - et al.
Metabolism of 5′-ether prodrugs of 1-β-d-arabinofuranosyl-e-5(2-bromovinyl)uracil in rats
Biochem. Pharmacol.
(1993) - et al.
Colonic metabolism of ranitidine: implications for its delivery and absorption
Int. J. Pharm.
(2001) - et al.
Susceptibility of the H2-receptor antagonists cimetidine, famotidine and nizatidine, to metabolism by the gastrointestinal microflora
Int. J. Pharm.
(2002) - et al.
The use of formulation technology to assess regional gastrointestinal drug absorption in humans
Eur. J. Pharm. Sci.
(2004) - et al.
Fecal weight, colon cancer risk, and dietary intake of nonstarch polysaccharides (dietary fiber)
Gastroenterology
(1992) - et al.
The effect of meat protein and dietary fiber on colonic function and metabolism. II. Bacterial metabolites in feces and urine
Am. J. Clin. Nutr.
(1979) Formulation strategies for absorption windows
Drug Discov. Today
(2005)- et al.
Fermentation by gut microbiota cultured in a simulator of the human intestinal microbial ecosystem is improved by supplementing a soygerm powder
J. Nutr.
(2000) - et al.
Role of the intestinal flora in the acetylation of sulphasalazine metabolites
Biochem. Pharmacol.
(1987)
Beyond diversity: functional microbiomics of the human colon
Trends Microbiol.
Is p-aminobenzenesulphonamide the active agent in prontosil therapy?
Lancet
Aspects of in vitro and in vivo research approaches directed toward identifying probiotics and prebiotics for human use
J. Nutr.
Studies of intestinal microflora. IV. The microflora of ileostomy effluent: a unique microbial ecology
Gastroenterology
The bacterial degradation of chloramphenicol
Lancet
An investigation into the in vivo performance variability of pH responsive polymers for ileo-colonic drug delivery using gamma scintigraphy in humans
J. Pharm. Sci.
Metabolism of drugs and other xenobiotics in the gut lumen and wall
Pharmacol. Ther.
Role of the intestinal flora in the metabolism of misonidazole
Biochem. Pharmacol.
Acetamide—a metabolite of metronidazole formed by the intestinal flora
Biochem. Pharmacol.
Ecological and evolutionary forces shaping microbial diversity in the human intestine
Cell
Bacterial diversity in the human gut
Adv. Appl. Microbiol.
An in vivo comparison of intestinal pH and bacteria as physiological trigger mechanisms for colonic targeting in man
J. Control Release
Role of bile acids in colorectal carcinogenesis
Eur. J. Cancer
Factors affecting the enterohepatic circulation of oral contraceptive steroids
Am. J. Obstet. Gynecol.
Distribution studies of salicylazosulfapyridine and its metabolites
Gastroenterology
Pcr-dgge-based quantification of stability of the microbial community in a simulator of the human intestinal microbial ecosystem
FEMS Microbiol. Ecol.
The role of the gastrointestinal microflora in the metabolism of drugs
Int. J. Pharm.
Glyceryl trinitrate: metabolism by the intestinal flora
J. Pharm. Pharmacol.
Studies on mixed populations of human intestinal bacteria grown in single-stage and multistage continuous culture systems
Appl. Environ. Microbiol.
A theoretical basis for a biopharmaceutics drug classification: The correlation of in vitro drug product dissolution and in vivo bioavailability
Pharm. Res.
Sulfasalazine: I. An historical perspective
Am. J. Gastroenterol.
Advances in colonic drug delivery
Drugs
Effect of dietary fibre on stools and the transit-times, and its role in the causation of disease
Lancet
The demethylation of methamphetamine by intestinal microflora
J. Pharm. Pharmacol.
The size, pH and redox potential of the cecum in mice associated with various microbial floras
Proc. Soc. Exp. Biol. Med.
Studies of two novel sulfasalazine analogs, ipsalazide and balsalazide
Dig. Dis. Sci.
Immobilization of infant fecal microbiota and utilization in an in vitro colonic fermentation model
Microb. Ecol.
Methodological considerations for the study of bacterial metabolism
Passclaim—gut health and immunity
Eur. J. Nutr.
Role of intestinal bacteria in nutrient metabolism
JPEN J. Parenter. Enteral Nutr.
The gut microflora and its significance
Identification and functional characterization of arylamine n-acetyltransferases in eubacteria: evidence for highly selective acetylation of 5-aminosalicylic acid
J. Bacteriol.
The bacterial flora of the normal intestine
The significance of the gut flora in safety testing of food additives
Dissolution testing as a prognostic tool for oral drug absorption: immediate release dosage forms
Pharm. Res.
Diversity of the human intestinal microbial flora
Science
Colonic fermentation—in vitro and in vivo approaches to measurement
Sci. Aliment.
Lactulose in the treatment of chronic portal-systemic encephalopathy
N. Engl. J. Med.
Role of intestinal microflora in clonazepam metabolism in the rat
Xenobiotica
Die darmbakterien des neugeborenen und sauglings
Fortschr. Med.
Cited by (501)
The role of the gut microbiome in gastrointestinal cancers
2024, Cellular SignallingDoes gut brain axis has an impact on Parkinson's disease (PD)?
2024, Ageing Research ReviewsEnvironmental behaviors of emerging contaminants in freshwater ecosystem dominated by submerged plants: A review
2023, Environmental Research