A particularly treacherous bacterium, Pseudomonas aeruginosa, is playing a TV-worthy game of “Survivor” with cystic fibrosis (CF) patients and researchers of their disease.
The microbe P. aeruginosa is ultimately guilty of visiting death on CF victims, at an average age of 32. But on the way to this fatal endpoint, the quick-change pathogen engages in a deadly duel of “now you see me, now you don’t” with clinical physiologist Richard Boucher, who heads the Cystic Fibrosis/Pulmonary Research and Treatment Center at the University of North Carolina in Chapel Hill.
Boucher is co-senior author of a paper in the February 2002 Journal of Clinical Investigation, titled, “Effects of mucus oxygen concentration in airway Pseudomonas infection of cystic fibrosis patients.”
“This paper has basically two thrusts,” Boucher told BioWorld Today. “The first helps us understand the mechanism by which CF accrues these terrible, unrelenting Pseudomonas infections. We know a lot about mutations in the CFTR [cystic fibrosis transmembrane regulator] gene. We know how the CFTR protein encoded by the mutated gene typically leads to misfolding a bent protein and the accompanying intracellular problems.
“What we haven’t known,” Boucher continued, “is how that problem in the cells lining the lung airways leads to the failure of normal lung defenses against the typical load of bacteria we all inhale every day. A number of theories are trying to link the problem in the CFTR protein in CF patients to their acquisition of chronic lung infection.
“We say that the CFTR protein is designed to control the amount of salt and water bathing airway surfaces. That liquid keeps the mucin the right thinness so the mucus can be cleared. It also acts as a lubricant on airway surfaces, to hustle mucus along by coughing. In CF,” Boucher went on, “there is a breakdown in normal lung defense against P. aeruginosa, as having too little liquid on airway surfaces. That’s the primary function missing in the mutant CFTR protein, which ultimately causes failure to clear mucous from the airways.
“Your lungs and mine,” Boucher explained, “are defended by a very thin layer a couple of millionths of an inch thick of mucus and water. The mucus is like flypaper, designed to trap bacteria. Then the whole clearance process moves up like an escalator, out of the lungs, nudged on by the hair-like cilia on the surface of our cells.”
Boucher summed up the events in CF patients: “They don’t have enough liquid to lubricate their airway surfaces, so they can’t clear mucus. The ubiquitous bacteria they inhale into their lungs are trapped in that sticky, stuck mucus.”
Fickle Pathogen Gives Up On Oxygen
P. aeruginosa starts life as an aerobic air-breathing microorganism. But in CF patients, the paper’s co-authors found, it changes its spots to anaerobic no longer beholden to oxygen.
“That pus-filled mucus in the CF lung,” Boucher noted, “is very, very low in oxygen. That’s because the infected CF airways are spending a lot of energy removing too much salt and water from the airway surfaces. So when they spend all that time pumping salt and water out of the airway secretions, that costs them energy. And the way the cells produce energy is by making ATP adenosine triphosphate which consumes oxygen. So cells lining the CF airways,” he added, “are stealing oxygen from the mucus. When colonies of bacteria hit the surface of this hypoxic mucus, a few of the microbes learn how to adapt to it. They actually do very well in that oxygen-starved environment. And, unfortunately, Pseudomonas aeruginosa, the bug that plagues CF patients, knows how to do this best.
“Part of that aerobic-to-anaerobic adaptation,” Boucher observed, “makes it very difficult to treat CF with antimicrobial agents. When the germs get into these stressful, low-oxygen conditions, they form impervious biofilms. To treat this disease at its root cause the initiating event you have to address strategies that restore the amount of salt and water on airway surfaces. We regard that as rebalancing the system.”
The co-authors used oxygen-sensing electrodes to directly measure how much oxygen was in this purulent mucus. They found that the bacteria weren’t on the surface of the lung, as they expected, but were buried deep in the mucus, where there was virtually no oxygen. “We did that,” Boucher recounted, “by direct measurement in people, putting oxygen-sensitive electrodes down into the mucus.
“This past year, we tested young adults with CF,” Boucher related, “actually going into their lungs with a bronchoscope that entered through the nose, on via the vocal cords, into the lungs. And we threaded down through the center bore of the bronchoscope an electrode that measured oxygen in the mucus plaques.”
The team discovered that culturing the bacteria in hospital labs instead of in actual clinical conditions yielded unexpected antibiotic consequences. “Certain antimicrobial agents that we use based on our hospital lab’s readout that these antibiotics should be good didn’t do as well in clinical practice as I hoped they would. The converse is also true. A class of antibiotics called macrolides were not predicted in hospital laboratories to be very useful against Pseudomonas. But our clinical experience looks as if they may be more useful than we would have expected, growing under anaerobic conditions rather than in oxygen-rich environments.”
Novel Candidate Drug In Phase I Trial
Among the new types of therapeutic compounds that Boucher and his co-authors are developing are sodium channel blockers to slow liquid absorption on airway surfaces, and nucleotide analogues that initiate mucus secretion. The latter candidate drugs, Boucher said, “are actually in Phase I clinical trials right now. We have licensed them out to a small biotech company called Inspire Pharmaceuticals Inc. in Durham, N.C.”
The first of these nationwide, eight-center trials, with 20 to 24 CF patients enrolled, began late last year. It is sponsored by Inspire, and based in the Cystic Fibrosis Foundation’s therapeutic trial network. “It’s a Phase I study,” Boucher pointed out, “so its major endpoint will be tolerability and safety. But people will look to see if the patients can breathe a bit better.”
A neighboring company, CyFi Pharmaceuticals, is focused on long-acting sodium channel blockers. A parallel firm to Inspire, it was co-founded by Boucher last year.