As infectious bacteria go, Clostridium difficile may be one of the most vexing for researchers, clinicians, and patients alike. It spreads from person to person by ingestion of the bacterium’s spores, which can not only remain viable for long periods of time outside of a human host, but can withstand most common disinfectants. Within the body, the spores can survive the acidity of the stomach, germinating in the intestines where the bacteria release toxins that wreak havoc on the bowel, causing severe abdominal pain and diarrhea. And while the proper regime of antibiotics usually eliminates the infection, residual spores can remain, and the bacteria can reemerge with a vengeance weeks or months later.
Recent estimates suggest that C. difficile infections (CDI) are on the rise, with up to 3 million cases in the United States each year, 1 and a third or more of CDI patients experience recurrence of the disease within the first month. Furthermore, recently evolved hypervirulent strains of C. difficile produce robust amounts of the disease-causing toxins, more spores, and additional surface proteins that help C. difficile persist in the gut and environment. Clearly, novel therapies are needed to combat the bacterium.
The case of Gertrude Smith (whose name was changed to protect her identity) a generally healthy 82-year-old grandmother living in Rhode Island, illustrates just how difficult treating CDI can be. She went to her doctor complaining of a bad cough, low fever, and chest pain. Her doctor prescribed a 10-day course of antibiotics, a decongestant, and bed rest for a presumed chest infection. But within a week, Smith had developed profuse, watery diarrhea, and she returned to her doctor for help.
She was eventually admitted to the ER, diagnosed with a C. difficile bacterial infection, and prescribed the recommended antibiotic treatment—500 milligrams of metronidazole three times a day for 10 days. Two weeks later, however, when she started a different antibiotic to address a persistent cough and chest pain, the diarrhea returned. Another cycle of metronidazole seemed to do the trick, until six months later, when her doctor prescribed yet another antibiotic for urinary symptoms, and she was once again hit with intolerable diarrhea. This time it was even more violent than before, and accompanied by acute abdominal pain. A double dose of traditional antibiotics temporarily quelled her symptoms, but four months later, the cycle started all over again. C. difficile was repeatedly attacking this otherwise healthy woman, with increasing intensity each time it struck.
Unfortunately, with rising rates of CDI in the United States and around the world, Smith’s experience is not that uncommon. Last year, CDI surpassed methicillin-resistant Staphylococcus aureus (MRSA) as the leading cause of hospital-acquired bacterial infection in the United States, and now also occurs in community settings with greater regularity, possibly transmitted through newly recognized sources such as raw and cooked food. 2 To make matters worse, as many as 40 percent of CDI patients experience a recurrence of the infection, 3 and patients who have already experienced more than one CDI episode have an estimated 50–65 percent chance of having another acute attack. 4 And while CDIs were once rarely reported in children, two reports in 2010 revealed that CDI rates among hospitalized children in the United States have doubled over the past decade. 5 , 6
Not only are C. difficile infections becoming more widespread, the symptoms appear to be getting more severe. Recent startling data from the United Kingdom showed that nearly 30 percent of patients over the age of 60 and more than 41 percent of patients over 90 died within 30 days of infection—significantly higher than previously estimated. 7 In the United States, mortality rates from CDI more than quadrupled from 1999 to 2004. 8 So just as MRSA was the bug of the 1990s, it is becoming clear that C. difficile can justly be considered the bug of the new millennium.
Recent work shows that the increase in CDIs has been accompanied by the appearance of particularly virulent strains of the pathogen. And although antibiotic resistance is not yet a major therapeutic issue with C. difficile, some strains have started to show resistance to quinolones (Cipro and Levaquin) and some macrolide antibiotics (Biaxin), 9 likely contributing to the selective pressures driving the evolution of the organism.
Fortunately, novel therapies are being developed to combat even the most pathogenic strains of C. difficile. New antibiotics, for example, have already shown encouraging potential in defeating this dreaded microbe and reducing high levels of recurrence, while complementary approaches focus on preventively fighting back even before patients are infected. Though still in early stages, preliminary results are promising, and some experts believe that a combination of antimicrobial, immunological, and biotherapeutic techniques may provide a breakthrough in the battle against CDI. Used appropriately—at the right times, and in the right patient populations—these new therapies should help reduce not only the incidence of the disease and the incessant recurrences seen with current therapies, but also alleviate the impact of CDI on human lives and economies.
One bad bacterium
CDIs are usually contracted by ingestion of C. difficile bacteria and their spores. While most of the bacteria die upon contact with the stomach’s acidic environment, the spores survive, passing on to the intestines where they wait for the right conditions to germinate. This often occurs following a course of antibiotic treatment, which can disturb the normal balance of gut flora, suppressing bacteria that are usually toxic to C. difficile.
Upon infection, C. difficile bacteria colonize the large intestine and produce at least two well-characterized toxins, simply called toxin A and toxin B, which attack the lining of the colon, causing abdominal pain and diarrhea. (See figure) A newly emerged pathogenic type of C. difficile, known as 027/NAP-1/BI, currently dominates the US landscape. It produces more robust amounts of toxins A and B compared to other strains, which destroy the epithelial cells lining the intestine by interfering with the cell structure. Not surprisingly, the host responds by producing antibodies that recognize and neutralize the toxins, as well as cytokines and other inflammatory molecules to eliminate the offending toxins and bacteria. However, the inflammation that results can cause further damage to the epithelial cells, which normally form a gastrointestinal barrier, resulting in passive leakage and active transport of watery fluids into the intestine. This fluid cannot be reabsorbed by the damaged cells, and diarrhea results. As the immune battle continues, cellular debris from dead epithelium, immune cells, and bacteria accumulates along the colon wall, creating patches of a pseudomembrane that prevent the flow of fluids that usually washes out the pathogens.
Additionally, many 027/NAP-1/BI strains of C. difficile produce more spores, which can contribute to the load of bacteria in the bowel of the patient and subsequently of spores in the environment, where they are passed onto other individuals. These so-called hypervirulent C. difficile strains are also hard to eradicate because highly conserved “sticky” surface proteins and other factors help them attach to gut surfaces. (See figure) Taken together, the robust production of two highly effective toxins, more spores, and multiple bacterial surface proteins that help C. difficile persist make this new “superbug” a formidable adversary that calls for creative new therapies and a rethinking of traditional clinical treatments.
New approaches for an old disease
The improved management of CDI has been a top priority for pharmaceutical and biotech companies over the past decade. Currently, oral vancomycin (Vancocin) is still the only one drug approved by the Food and Drug Administration (FDA) for the treatment of CDI, though doctors usually first prescribe the equally well-established antibiotic metronidazole (Flagyl). Not only has the overall incidence of CDI risen, but as many as 40 percent of CDI patients will experience an acute recurrence of the infection within a month of initial treatment with one of these two traditional antibiotics. This combination of the increasing threat of infection and the persistent risk of recurrence has prompted some companies to seek alternative treatments for the pathogen.
A new antibiotic called fidaxomicin that inhibits the enzyme RNA polymerase, produced by San Diego–based Optimer Pharmaceuticals, Inc., is in the late stages of development. This drug showed clinical cure rates of approximately 90 percent (comparable to vancomycin), and significantly reduced recurrence rates—by 47 percent compared to vancomycin—when given twice daily to more than 1,100 patients in two Phase III clinical trials. 10 , 11 Furthermore, fidaxomicin has a narrower spectrum of activity than vancomycin and metronidazole, meaning it is less likely to disturb the gut bacteria which normally help prevent invaders such as C. difficile from flourishing. Indeed, fidaxomicin has been shown to have a much muted effect on host flora. The drug, which last month received a unanimous recommendation for approval by an FDA advisory committee, was under review at the US agency and the European Medicines Agency when this article went to press. (Disclosure: Glenn S. Tillotson currently works for Optimer Pharmaceuticals, Inc.)
Other antimicrobial products in the queue include ramoplanin, an antibiotic that blocks bacterial cell wall synthesis, which is being developed by Florida–based Nanotherapeutics and is slated to begin Phase III trials later this year; and Massachusetts–based Cubist Pharmaceuticals’ drug CB-183,315, which disrupts the bacterial cell membrane function and is currently in Phase II clinical testing.
Another potential therapy for CDI takes a step outside the box of traditional medicine. Known as fecal bacteriotherapy, or fecal transplants, it involves the creation of a saline-diluted solution of fecal matter from a healthy donor, which is introduced into a CDI patient’s gastrointestinal tract using a catheter or enema. Based on reestablishing a normal, healthy gut flora, whose disturbance often allows the flourishing of C. difficile, the treatment has been used sporadically in North America for the last few decades. Recently, however, it has begun to gain acceptance among clinicians and patients, and a 2009 review of 100 Scandinavian cases reported an 89 percent cure rate among CDI patients. 12 Last October, epidemiologist Susy Hota of Toronto General Hospital began recruiting patients for the first North American randomized, controlled trial of the procedure, in combination with a 2-week regimen of oral vancomycin.
While these drugs and therapies hold promise as treatments for killing the bacteria after infection takes hold, other approaches are being developed to help protect at-risk patients before infection even begins. It is often flaws or weaknesses in the host’s immune system that enable C. difficile to become established more easily in the gut. As people age, their ability to make sufficient or appropriate antibodies goes down, which allows pathogens such as C. difficile to more easily evade the immune response. As the average age of CDI patients is more than 65 years, approaches that bolster immune function are being investigated to help quell the escalation in CDI.
Until recently, researchers did not know how to build a vaccine that would effectively target C. difficile. Rather than target the bacterium itself, researchers recognized that the body’s ability to protect against toxins A and B was of paramount importance in the fight, although the production of antibodies that block the adherence of C. difficile to the colon wall in the first place would be even better. There are now at least three approaches that aim to arm the immune system to fight CDI at various stages of the infection.
One such therapy is a vaccine being developed by Sanofi-Pasteur that uses neutralized forms of toxins A and B to bolster patients’ ability to make immunoglobulins that bind to and eliminate the disease-causing toxins. Phase I studies have shown that this vaccine, composed of a proprietary ratio of the modified toxins, was well tolerated and resulted in increased production of antibodies specific to the two toxins in healthy participants; however, the response rate was lower in those over 70 years old compared with those aged 25 years. The vaccine is currently being tested for the prevention of CDI recurrence in a Phase II trial in the United Kingdom and the United States. Last November, shortly after the FDA granted the vaccine a fast-track designation to expedite its development, the vaccine maker announced the initiation of a second Phase II trial—this one for primary prevention of CDI in at-risk individuals in the United States.
Merck & Co. also has a potential CDI treatment in its pipeline—a monoclonal antibody called MK-3415A that similarly targets toxins A and B. Last year, the pharmaceutical company reported that recurrence rates dropped by up to 72 percent in a Phase II trial of patients also taking metronidazole or vancomycin. 13 Based on those results, Merck had planned to follow up with Phase III studies “in the near term,” according to a company spokesperson, but has currently put the program on hold “in view of ongoing interactions with regulatory authorities.”
In the United States, recurrent C. difficile infections may cost nearly $10 billion in excess hospital expenses each year.
A third alternative approach to managing CDI is to prevent the pathogenic strain of C. difficile from colonizing the bowel in the first place—a strategy that should subsequently thwart the production of toxins and spores. Previous studies in hamsters showed that infection with strains of C. difficile that do not produce the two toxins, but in all other ways resemble the pathogenic C. difficile, can protect against further infection. Because these harmless bacteria presumably attach to the same bowel sites as the dangerous bacteria, they block all points of contact for the virulent variety. ViroPharma Incorporated has partnered with Dr. Dale Gerding, an infectious disease physician at Hines VA Hospital in Chicago, to evaluate this biotherapeutic approach. Phase I data suggest that this nontoxigenic C. difficile strain is well tolerated and colonizes the bowel of healthy volunteers, and the company plans to begin a Phase II trial this year. (Disclosure: Glenn S. Tillotson used to work for ViroPharma Incorporated.)
Few other infections have been the subject of so many varied therapeutic approaches. Those currently in development for the treatment of CDI will no doubt be key to beating the newly emerging superstrains of C. difficile. In addition, the use of new patient assessments that assist in selecting the right therapy for the right patient is of paramount importance for reducing disease recurrence.
Currently, doctors treat CDI patients on the basis of disease severity at the time the patient is seen, as per clinical guidelines, and prescribe either vancomycin or metronidazole, depending on the early signs and symptoms. But immediate clinical outcome is not always indicative of the long-term consequences of recurrent infections. Recent studies suggest that there may be clinical and historical risk factors that are related to a higher incidence of recurrence, including age, prior CDI, concomitant antibiotics, renal failure, and immunocompromised status. Elderly patients, for example, are more likely to have had recent antibiotics for unrelated infections, which can decrease host defenses and perturb the normal gut flora, reducing populations of C. difficile’s inhibitors and competitors in the gastrointestinal tract. Other age-related diseases, such as cancer, can also adversely affect host defenses, thus reducing the patient’s ability to fight off invaders, and often require therapies that disrupt the gut flora. Prior infection by C. difficile may also lead to an increased risk of recurrence because of residual spores left behind in the bowel, where they await the right environment for regermination. It is the recurrence of CDI that presents a significant burden to health care systems and patients. An 8-year-old estimate assesses the annual excess hospital costs in the United States at $3.2 billion, 14 and the costs now could be as high as $9.55 billion annually.
Almost a century ago Paul Ehrlich, the grandfather of chemotherapy, presciently warned of the emergence of organisms which would thwart our best medical efforts by developing means of resistance unless we “frapper fort et frapper vite,” roughly translated as “hit hard and hit fast.” When dealing with CDI, we must embrace this edict by choosing the right weapon for the right patient at the right time. Otherwise, Ehrlich’s predictions will continue to come true.
Glenn S. Tillotson used to work for ViroPharma Incorporated and now works for Optimer Pharmaceuticals, Inc., in San Diego, Calif. Gayatri Vedantam is an assistant professor in the Department of Veterinary Science and Microbiology at the University of Arizona in Tucson, as well as a research health scientist in the Southern Arizona VA Healthcare System. The authors wish to thank Joni Tillotson, a biology undergraduate student at Immaculata University, for her manuscript research and database compilation.
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