Drugs Repurposed To Tap Immune System In Fight Against Pathogens
By Deborah Borfitz
April 6, 2021 | Researchers at the University of California (UC), San Diego have proposed that the definition of antibiotic resistance be broadened to include treatment failure among patients on appropriate antibiotic therapy, which still encompasses the “great majority of unsuccessful outcomes in infectious disease,” says Victor Nizet, M.D., professor at the UC San Diego School of Medicine and faculty lead for the campus-wide Collaborative to Halt Antibiotic-Resistant Microbes initiative. “It doesn’t matter really what the result is in the test tube—what matters is how it works in the patient.”
Staphylococcus aureus, arguably the most important bacterial pathogen in causing serious human infections, has a roughly 20% mortality rate in bloodstream infections and sepsis due to its resistance to clearance by available antibiotic therapies, Nizet says. “We need to be asking if our approach to antibiotic therapy is comprehensive enough, if we have left no stone unturned in terms of eliminating the pathogen.”
His favored path is to focus on how pathogens like S. aureus interact with the immune system because it opens a wealth of therapeutic possibilities, such as trying to strip the infectious agent of its virulence. Rather than try to kill the bacteria or suppress its growth, as an antibiotic would, the puzzle Nizet has been working on is “how to make [a pathogen] less dangerous so the body can more easily handle it.”
The approach mimics that of vaccines, which “stimulate downstream immunity to future infections,” says Nizet. “We think we can do that in the setting of acute infections… [with] drugs that work on the host-pathogen equation.”
On that list are drugs such as FDA-approved ticagrelor (Brilinta), which reduces the tendency of platelets to clot the blood and is used to prevent heart attack recurrence. In addition to their role in blood coagulation, platelets have antimicrobial properties against S. aureus. Nizet and his UC San Diego colleagues recently demonstrated Brilinta could be repurposed to protect platelets from being damaged or cleared from the blood during staph sepsis (DOI: 10.1126/scitranslmed.abd6737). The FDA-approved anti-influenza sialidase inhibitor oseltamivir (Tamiflu) provided similar therapeutic benefit.
The researchers previously demonstrated how other marketed medicines increase the bactericidal capacity of immune cells, among them cholesterol-lowering statins (DOI: 10.1016/j.chom.2010.10.005) and the breast cancer drug tamoxifen (DOI: 10.1038/ncomms9369), Nizet shares. The common antibiotic azithromycin (Zithromax) was also shown to have activity in fighting superbugs that was not appreciated in standard laboratory testing (DOI: 10.1016/j.ebiom.2015.05.021).
Dropping The Ball
One key value of antibiotic susceptibility testing is that it provides an estimate of direct drug action against the bacteria that is the same the world over, says Nizet. But infection in patients is much more complex than what happens in a test tube. “First of all, the environment in the body is quite different in terms of its composition. The media that is used in the testing laboratory does not faithfully represent the physiological composition of our blood and serum and tissues.”
Additionally, he continues, “antibiotic susceptibility testing is completely agnostic to any element of immunity—by definition when you have a serious infection it is because your innate immune system has dropped the ball and now the bacteria have gotten deeper into the body than where they should be.” Many healthy people normally have staph on their skin and in their noses that never cause invasive disease.
Infection is a host-pathogen interaction “where the pathogen has gained the upper hand,” says Nizet. Perhaps a cut on the skin went untreated, or a viral infection or chronic medical problem was suppressing the host immune system, allowing the bacterial infection to set in.
Antibiotics—the mainstay of treatment for all infections—are selected on how well they kill the bug, an approach “divorced from the activity of the cells and defense molecules of the patient’s own immune system” that play a pivotal role in whether infection happens in the first place, Nizet says. “The evaluation… is the opposite of personalized medicine.”
Bacteria produce disease both by resisting immune clearance and producing harmful compounds, known as toxins, explains Nizet. One key disease-causing substance released by almost all strains of staph bacteria is the pore-forming alpha-toxin (α-toxin), which literally pokes holes in the host’s cell membranes. It has many targets, including skin cells, immune cells, and cells in the lining of the lung.
In their most recently published study in Science Translational Medicine, the UC San Diego research team found that α-toxin affects platelets (circulating immune cells) by damaging them as well as accelerating their clearance through the Ashwell-Morell receptor. The cellular receptor is one of the most abundant proteins present on the surface of liver cells and its job is to recognize aging platelets and blood proteins and remove them from circulation, Nizet says.
In healthy individuals, sugars known as sialic acid decorate the surface of virtually all cells in the body and many of the proteins circulating in the blood, continues Nizet. As cells age, the sialic acid falls off, typically through the action of enzymes called sialidases. Since sialic acid is often at the end of a sugar chain, this allows the Ashwell-Morell receptor to recognize the underlying galactose.
The making of platelets in the bone marrow (and other serum proteins by the liver) and their removal by the Ashwell-Morell receptor once they “go bald” of sialic acid is part of the body’s steady-state system that keeps the levels of each in the normal range, says Nizet. Platelets are replenished about every seven to nine days.
Researchers initially followed a cohort of 49 patients with staph bacteremia, about 20% of whom died despite receiving antibiotics that are active against the pathogen in the laboratory. One factor correlated to the bad outcome was a low platelet count (fewer than 100,000 per mm3 blood), Nizet says. “That was our first clue, and it was supported by other literature [indicating] a low platelet count may be a bad prognostic sign” among staph-infected patients.
Next, blood was taken from healthy individuals to learn which cell types were best at killing staph, he says. Surprisingly, it was not neutrophils, the main circulating white blood widely recognized as a key bacteria fighter. “Staph is quite resistant to neutrophil killing… but it was quite susceptible to killing by the platelets.”
An examination of patient samples in the lab revealed that the ones from patients with the low platelet counts tended to be the same ones generating high levels of the α-toxin, says Nizet. In a subsequent experiment, the researchers infected mice with mutant bacteria that lacked the toxin and saw that their platelet count was unaffected while in mice infected with the wild type bacteria the platelet count dropped.
“That implies that the toxin is the main factor that the bacteria is using to drive the platelet count down,” he says. Reducing the platelet count in mice with an antibody also made them much more susceptible to staph infection, “setting up this battle [where] the platelets are trying to clear the staph and the staph is trying to clear the platelets.”
Usually, the platelets win with the help of antibiotics and good doctors, Nizet says. But in some unfortunate patients, the platelet count begins to fall. When it gets too low, they no longer have a key fighter in the battle against the infection.
Many marketed drugs work on platelets, notes Nizet. Antiplatelet drugs, for example, are taken by heart attack and stroke survivors to “tone down” the clotting ability of platelets and prevent disease recurrence. Researchers’ curiosity if these drugs would alter the interaction of platelets with staph led to the discovery that the presence of Brilinta, a popular current member of the drug class, was associated with less damage to platelets and less activation of sialidases when mixed with staph bacteria or purified α-toxin.
Encouraged by this finding, they next gave mice doses of Brilinta equivalent to what humans would receive when being treated with the drug. When infected with staph, “their platelet count didn’t drop and they were more resistant to the staph infection,” he says. “Fewer of them died, and there was clearance of the staph from the blood and different organs of the body—even in the absence of antibiotics.”
To home in on the other component to the pathway, the researchers used knockout mice missing the Ashwell-Morell receptor and infected them with staph, Nizet continues. Once again, the platelet count in the mice did not drop and they were more resistant to the staph infection.
Ultimately, about 60% of mice treated with either Brilinta or Tamiflu survived 10 days following infection versus 20% of untreated mice, he says.
While it may not be a good idea to mess with the platelet homeostasis in healthy humans on a routine basis, it might be a reasonable approach “in the setting of a deadly staph infection where you have a 20% chance of not pulling though,” says Nizet. Tamiflu’s mechanism of action is to block the sialidase of the influenza virus to reduce the symptoms and duration of illness, he adds, and it was found to similarly protect the platelet count of mice during staph infection.
Potential Adjunctive Therapies
“The immune system is amazing and has a wide array of weapons to fight infection,” says Nizet. “That is why most individuals are healthy despite living with potentially disease-causing bacteria on their body and existing day in and day out in a microbial world.”
To better leverage all these natural abilities, the UC San Diego research team believes it is worthwhile to carefully evaluate all FDA-approved drugs to learn which biology-changing molecules might be useful as an adjunctive therapy by supporting the immune system in the context of infection, he says.
Infectious disease doctors would do well to follow the lead of oncologists who have pioneered the use of immunotherapies to improve patient outcomes, adds Nizet, noting how the approach helped Jimmy Carter live to be the oldest living ex-president in history despite having malignant melanoma that spread to his brain. The two fields have many similarities. The first chemotherapeutic drugs, like today’s antibiotics, were discovered by their ability to kill dividing cancer cells in a test tube.
But when chemotherapy drugs were given to patients, the drugs did not always work, and many patients experienced toxic side effects. Science is now starting to recognize that antibiotics have under-appreciated toxicities in that they destroy good bacteria in the human microbiome, Nizet says.
Throughout modern medical history, antibiotics have cured more infections than all other drug classes combined, but success has bred complacency, he says. Antibiotic resistance is a growing problem, and a subset of patients do not do well even on the right antibiotics.
Microbiome science, meanwhile, is exploding with multiple correlations seen between depleting the microbiome and adverse health outcomes—from allergies and chronic diseases like obesity and diabetes to the drug-resistant pathogen C. diff. “Sometimes the scars on our microbiome from too many antibiotics can have these adverse health effects,” says Nizet.
“We’re trying to move infectious disease therapy to more precision therapy,” including more specific antibiotics as well as treating infections via the immune system, he continues. “Supporting the immune cells in your blood [based on the infection at hand] is not going to affect the microbiome in your gut; there are no immune cells rushing into the gut attacking the normal flora.”
Some of the major pharmaceutical companies have already launched programs to study immune cell behavior and drugs in their portfolio that might be repurposed as adjunctive therapies against infective pathogens, Nizet says. “This problem of patients failing antibiotic therapy is only going to get worse unless we try to engage the immune system as an ally in our fight.”
Hundreds of medicines exist to dial down an overactive immune system, as with diseases like arthritis, multiple sclerosis, and asthma, he adds. “This is just the flipside of that same coin… accelerating the immune cell activity or protecting the immune cells of patients with severe infection who may not survive… at least until you get on top of the infection. We don’t view these as long-term therapies.”
One of the big challenges is financial, says Nizet. Antibiotic drugs are typically not blockbuster drugs like those used to treat a chronic disease or cancer. They only treat patients for a short period of time and are not supposed to cost much. Most drug companies have therefore avoided the infectious disease arena.
Outside of clinical trials specific to the immune-boosting potential of statins, he says, most studies involving repurposed drugs for infectious diseases have been investigator-initiated to address an unmet need. Exploring possible repurposing of a blockbuster drug is risky for pharmaceutical companies, both because of the smaller target audience and unwelcome information that may emerge to tarnish a product’s reputation.
That said, some companies are open to the idea of repurposing drugs initially explored in another area where they are no longer needed, so they might “debut” as an adjunctive therapy for an infectious disease, Nizet says. Although the pharma industry has significantly downsized its internal R&D, companies might have an eye on acquiring or working with smaller biotechs or academic groups.
When AstraZeneca was developing Brilinta as a drug for post-heart attack prophylaxis, he notes, fewer infections were seen in the treatment group. The curious finding was noted but not pursued in separate trials comparing patients with and without certain types of infections.
As mentioned in the recent Science Translational Medicine paper, certain antibiotics predicted to be ineffective based on laboratory testing do work when studied in the context of the immune system, Nizet adds. “I think we are prematurely discounting certain antibiotics because they did not perform well in a test tube… [outside] the chemistry of the body.”
The rescuing of prematurely discounted antibiotics due to “test tube resistance” is one area where product development practices are already starting to change by considering synergy with the immune system, says Nizet. “It’s a related topic, and we are also studying that.”