Trial of Rifaximin in Probable Alzheimer's Disease

This study aims to improve cognition and function in patients with Alzheimer's Disease (AD) by administering the oral antibiotic, rifaximin. Rifaximin is a virtually non-absorbed antibiotic with the unique properties of altering gut microbiota and lowering blood ammonia levels. It is FDA approved for use in patients with hepatic encephalopathy. Rifaximin lowers blood ammonia by blocking gut bacterial RNA synthesis and also by increasing small bowel glutaminase. The Investigators hypothesize that rifaximin will improve cognition and function in AD patients by mechanisms such as lowering circulatory pro-inflammatory cytokines secreted by harmful gut bacteria or lowering blood ammonia.

Rifaximin is a relatively gut-specific antibiotic that interferes with by binding to the transcription subunit of bacterial . Because Rifaximin is absorbed poorly, most of the drug taken RNA polymerase orally stays in the gastrointestinal tract. It has been available since 2004 in the US and has orphan gastrointestinal tract drug status for hepatic encepalopathy. Rifaximin's relatively good safety profile and its ability to alter gut flora and lower blood ammonia have made it an attractive drug to treat hepatic encephalopathy or conditions such as traveller's diarrhea. The Investigators hypothesize that rifaximin will improve cognition and function in AD patients by mechanisms such as lowering circulatory pro-inflammatory cytokines secreted by harmful gut bacteria or lowering blood ammonia.

The Investigators will measure a variety of blood markers, such as pro-inflammatory and anti-inflammatory compounds, and analyze fecal microbiota in patients with AD before and after 3 months of rifaximin therapy. If patients exhibit measurable improvement in cognition and function, The Investigators will analyze our data to see if their improvement correlates with a shift towards anti-inflammatory species in the gut and a similar shift in the blood cytokine panel to favoring anti-inflammatory compounds.

Evidence supporting our hypothesis for this study is presented below.

Gut Microbiota Dysbiosis

The gut harbors 95% of the total human microbiome and is made up of more than 5,000 taxa. Gut species remain relatively constant throughout adulthood until the seventh decade, when it becomes less diverse, harboring higher numbers of Proteobacteria and lower numbers of Bifidobacteria. An imbalance in the gut bacterial species can weaken the intestinal barrier and create system wide inflammation via gut lymphoid tissue which comprises 70% - 80% of the immune system. Blood brain barrier permeability is also altered. The gut bacteria secrete pro-inflammatory compounds and neuroactive molecules that include serotonin, gamma-aminobutyric acid (GABA), catecholamines and acetylcholine which cross the blood brain barrier and cause brain inflammation and brain dysfunction.

A growing number of pro-inflammatory compounds secreted by gut bacteria and seen in patients with AD are being discovered. These include interleukin (IL) - 6, tumor necrosis - alpha and the inflammasome complex (NLRP3). A recent study revealed higher numbers of Escherichia and Shigella in the gut of amyloid positive AD patients when compared with healthy controls and this correlated with higher levels of circulating pro-inflammatory cytokines. The AD patients also had lower numbers of gut Eubacterium rectale and this correlated with lower levels of circulating anti-inflammatory compounds than seen in controls. Other researchers found that a preponderance of gut Bacteroides and Blautia and reduced numbers of SMB53 and Dialister correlated with elevated levels of brain amyloid Cerebrospinal Fluid (CSF) biomarkers. Interestingly, Clostridium tyrobutyricum and Bacteroides thetaiotaomicron, have been shown to actually increase the integrity of the blood brain barrier by enhancing expression of tight junction proteins and helping to maintain brain homeostasis. Recent evidence also suggests that neurotransmitters and neuropeptides secreted by bacteria in the gut activate vagal nerve ascending fibers, influencing brain function.

Ammonia Neurotoxicity

A second theory is that the blood brain barrier allows greater amounts of ammonia into the AD brain, triggering or worsening pre-existent changes. The possibility that ammonia is at least partly responsible for the pathologic changes seen in the AD brain was first proposed by Seiler in 1993, when he noted that some of the changes in the AD brain can also be seen in the brains of hyperammonemic patients. Arteriovenous sampling in early stage normoammonemic AD patients demonstrated higher endogenous cortical ammonia compared to young control subjects Low glutamine synthetase levels have been found to correlate with increased density of amyloid deposits in the AD brain.

The brain also receives exogenous ammonia, of which the gut is a major source. The aging colon contains a greater percentage of protein fermenting bacteria than is seen in the colon of younger patients.

Rifaximin lowers blood ammonia by altering fecal flora by blocking bacterial RNA synthesis and also by increasing small bowel glutaminase.

The Investigators hypothesize that Rifaximin will improve cognition and function in AD patients by lowering circulatory pro-inflammatory cytokines secreted by harmful gut bacteria and / or lowering blood ammonia levels. The subjects will be given rifaximin 550 mg orally twice daily for 3 months after evaluation to ensure they have no contraindications. Physician clinical and safety assessments, adverse events, as well as the ADAS-Cog11 will be administered at baseline and at the 3 month endpoint. Interim safety checks will occur via phone calls one week after baseline and then every 2 weeks till 3 month end point. Serum neuronal biomarkers, ammonia levels and pro-inflammatory and antiinflammatory compounds will also be measured at those times. Bodily fluids (Stool samples) will also be collected. Because of a small risk of developing C. difficile up to 2 months following the last administration of rifaximin, the subjects will be followed for an additional 2 months after the 3 month treatment ends.

This is an open label pilot study designed to provide preliminary evidence on the clinical efficacy of rifaximin in improving cognition in AD patients. The Investigators will be looking for improvement in test scores and changes in serum levels of neuronal markers, circulating cytokine pro-inflammatory compounds and colonic microbiota following treatment. The Investigators will also be collecting data regarding the safety of long term use of this non-absorbed antibiotic for this disease. This pilot may form the basis for a future larger randomized, double blind controlled study to further test this hypothesis.

The rifaximin dosage and route of administration are the same as that used to treat hepatic encephalopathy, for which the drug and dosage are FDA approved. The dose will remain the same throughout the 3 month study of rifaximin administration unless stopped for safety reasons. The age and study sample of mild to moderate probable AD is similar to that used in AD prior trials. The Investigators will select our sample to be medically stable and free of hepatic disease and not having had recent antibiotic therapy to minimize risk for C Difficile infection.

The end of the study will occur 5 months after baseline testing and first administration of the drug. While drug administration will conclude and endpoint testing will be done at 3 months, The Investigators will continue to follow each subject for an additional 2 months, as C. difficile infectious diarrhea can occur up until 2 months after stopping an antibiotic.