Before the advent of nitroglycerin 130-odd years ago, the only explosive in use was black gunpowder. The Swedish industrialist Alfred Nobel developed nitroglycerin, and its spin-off, dynamite, at a factory in Sweden. In the late 1860s, he noted two curious syndromes among his workers, who came in contact with the dynamite factory's nitroglycerin.
First, on Monday mornings, these laborers often complained of pounding headaches, which disappeared over the weekends.
Second, workers who suffered from angina pectoris - heart failure - often experienced relief from the anginal chest pains during the work week - but they recurred on weekends.
Local physicians were quick to realize that both effects were due to nitroglycerin's action in dilating and relaxing blood vessels. But this mechanism remained a mystery for another century. In the late 1970s and early 1980s, scientists determined that the vasodilator effect was caused by nitric oxide (NO), apparently generated from the nitroglycerin (NG) in vascular smooth muscle. And that's where things stood until the third millennium kicked in.
"It's been a 130-year-old mystery," observed research and clinical cardiologist Jonathan Stamler, at Duke University in Durham, N.C. "It's been the central question from a historical standpoint. It has this folklorish relationship to Alfred Nobel and the beginning of cardiovascular biology. The name Nobel has some connotations in science in general. He made his fortune off nitroglycerin.
"Nitroglycerin was among the first cardiovascular drugs ever given," Stamler pointed out. "Certainly one of the very few that's stood the test of time. So it's been one of the few remaining mysteries how this drug works, given that it does not release nitric oxide. But NG does relieve the chest pains of cardiovascular disease - the most common malady in the Western hemisphere.
"People who receive nitroglycerin in clinical practice," he went on, "are patients with ischemic coronary syndromes - ranging from stable angina to unstable coronary syndromes to myocardial infarctions. A patient can get a bottle of NG tablets - it costs pennies - to pop under the tongue to abort chest pains. But there are also dermal patches for more chronic use, or infusions of NG, provided at every coronary care clinic in the U.S."
NG Tolerance Broader Than Drug Resistance
Stamler, an investigator of the Howard Hughes Medical Institute at Duke, is senior author of a paper in today's Proceedings of the National Academy of Sciences (PNAS), dated June 11, 2002, but released online June 3. Its title: "Identification of the enzymatic mechanism of nitroglycerin bioactivation."
"The main finding of our PNAS paper," Stamler told BioWorld Today, "is discovery of the enzyme responsible for bioactivation of nitroglycerin, and the demonstration that inhibition of that enzyme is largely responsible for tolerance."
He explained: "Tolerance is the loss of efficacy of NG in all patients, so no patient can receive that drug continuously. That's why the way we give nitroglycerin today is to have drug holidays. Patients who take the drug often every day must have an NG-free interval. For six to eight hours in the day, they go unprotected from their angina.
"Tolerance specifically refers to a process wherein giving a drug induces resistance to its effect. In addition to this tolerance," he continued, "there is a lack of efficacy of NG with regard to hard endpoint data - like survival. Nitroglycerin may be toxic and bad for people. Whether that's caused by tolerance is not clear."
Stamler recounted how he and his co-authors cracked open the long-padlocked NG black box: "We reasoned that part of the problem was that NG operates at very low concentrations. Many studies seeking to elucidate pathways had been done with concentrations of NG much higher than are given to patients. So we worked with very low physiological concentrations of the drug. One resulting problem was that the blood vessels are hard to come by from the standpoint of an organ. You think of a liver, you have a lot of material. You think of a blood vessel, you have tiny bits of tissue.
"So we tried to find cells that mimic blood vessels by how they metabolize NG. We found that macrophages, which are immune system cells, resemble blood vessels, from the standpoint of their NG metabolism. And macrophages we could scale up in buckets. So that really was the big break for us. We scaled up the process by orders of magnitude, and then ran after an enzyme with very low amounts of the NG. We discovered the enzyme in macrophages, and confirmed that this was the enzyme in the blood vessel."
The team found that "it's a mitochondrial enzyme. What we learned is that how you make NO here is still a big question and problem," he said. "It seems as though we pay a big price for getting our nitric oxide out of nitroglycerin, and that price is a rusting of the system - by so-called oxidative stress. And that rust happens first and foremost in the mitochondria."
Patents Pending; Clinical Trials On Burner
"That, perhaps, gave a reason why nitroglycerin has not had such beneficial effects and may even have deleterious effects. What happens is that NG, when given at concentrations that are very, very low, are handled OK by the enzyme. Unfortunately, we don't know how to individualize that concentration to each patient. If you go up a little bit on the concentration, the NG inhibits the enzyme by oxidizing it. If the enzyme - formally named mitochondrial aldehyde dehydrogenase (mtALDH) - can't get rid of NG, it presumably starts to damage the mitochondria. So basically those drug-free intervals that reverse tolerance are really trying to repair the cell.
"There are many drugs we give to cardiac patients today in clinical practice," Stamler pointed out, "that affect the mtALDH enzyme and how NG works - including alcohol that patients drink, and other classes of drugs. So it became quite clear to us that there are alternative possibilities for getting NO activity out of systems, and avoiding tolerance as a way to improve efficacy.
"We have at least three new patents applied for, covering this concept," Stamler noted, "on new ways to avoid tolerance, new ways to give NG, and combinations of drugs with NG which would treat cardiac and other diseases, including stroke and brain disorders.
"Now," he concluded, "It's time to do some clinical trials of NG and mtALDH."