Basic science and knowledge about underlying mechanisms strengthen the case for “an aspirin a day keeps heart attacks away.” However, they are not sufficient to prove it. We saw earlier that tissue bioassay is better than whole animal experiments in isolating a process and uncovering its underlying mechanism, which is buried under myriad processes going on in a life animal. The advantage in discovery can become a disadvantage in applying its results. In isolating a process we ignore its interaction with other processes in the context of application. These interactions can generate side effects or even derail the process itself. In the test tube, aspirin inhibits COX1 in platelets and hence the formation of a prostaglandin that promotes blood clots; fine. In the body, the situation is far more complex. For instance, aspirin also inhibits COX2 in blood vessels and hence the formation of another prostaglandin that prevents blood clots. How would the two processes of opposing effects balance out? Another question, would taking aspirin years on end, even at low dosages, increase the risk of bleeding in the brain? These and many other questions involving the functioning of the body as a whole cannot be answered by test-tube experiments on individual processes. That is why governments require drugs to pass clinical trials in human subjects to prove their effectiveness and safety.
Blood clotting is a complex process. The blood contains, besides red and white blood cells, partial cells called platelets. The disc-like platelets are produced in the bone marrow and cannot reproduce themselves because they contain no nucleus. They usually lie dormant in the blood, awakened only by chemicals released by injured tissues or a tear in the artery’s plaque. These stimulants activate the COX1 enzyme in the platelets to produce a prostaglandin, which causes the platelets to stick together, triggering the cascade of reactions that result in clotting of blood. By inhibiting COX1 from synthesizing the prostaglandin, aspirin reduces the stickiness of platelets, hence the chance of forming blood clots. For this antiplatelet purpose aspirin is uniquely effective. All other aspirin-like drugs inhibit COX temporarily, aspirin alone inhibits it permanently. One dose of aspirin has antiplatelet effects that last through the platelet’s lifetime, about ten days.
No scientific discovery is solely the work of a single person, as acknowledged in Isaac Newton’s famous aphorism: “If I have seen further, it is by standing on ye shoulders of giants.” So it was with Vane. He was brought to aspirin in 1968 by Henry Collier, a pharmacologist who had worked on it for a decade. Collier had discovered that although both morphine and aspirin kill pain, they act by different principles. Morphine acts on the brain. Aspirin acts locally at the sites of injury. What local biochemical mechanisms underlie aspirin’s actions? Collier’s research was stymied, partly because his tools and techniques were rather blunt.
Collier experimented with whole animals such as guinea pigs and rabbits. He injected an animal with a pathology-inducing chemical and then a drug, observed the animal’s responses and analyzed its blood and tissues. By varying the pathogen and the drug, he hoped to tease out what acted on what and how. After numerous experiments, no pattern emerged. An animal’s body harbors millions of chemicals and hundreds of biochemical pathways. It is so complex a medium that therapeutic mechanisms are easily obscured. Furthermore, biopsy and blood analysis, which take time to perform, may not be able to capture fleeting biochemical reactions. Frustrated, Collier turned to Vane, an expert in bioassay. The relative successes of the two scientists illustrate the importance of experimental techniques and instruments in research.
Discovering mechanisms underlying disparate phenomena is the font of basic science. Science, especially biochemistry and molecular biology, advanced tremendously since aspirin made its debut. The castle of NSAIDs’ working principles was still intact, but siege engines were ready. The first to breach the wall in 1971, and would receive a Nobel Prize for it, was pharmacologist John Vane.
Most patients with AERD can be desensitized to aspirin: following the initial adverse reaction, repeating of the dose is tolerated by more that 50% of patients, and further incremental aspirin challenges lead to a tolerance (28). Once the patient tolerates 600 mg of aspirin he is considered “desensitized” and then can take aspirin on a daily basis indefinitely without further adverse respiratory reactions. Desensitization can be also achieved silently, for instance, without evoking initial adverse reaction providing the challenge starts with a sub threshold dose and then the dose is slowly increased in appropriate intervals (29). In order to maintain the tolerance a patient has to ingest aspirin on regular, usually daily basis – the tolerance state disappears after 2-5 days without aspirin with the full hypersensitivity returning after 7 days. Several protocols of desensitization have been proposed allowing for completing the procedure usually within 3 to 5 days. The standard protocol of desensitization is an extension of the oral aspirin challenge protocol and all the safety precautions recommended for the challenge should be employed.
Over the years, researchers have assembled a large library of how a kind of tissue reacts physically and chemically to various kinds of irritants. For instance, a tissue secrets a specific substance when it is exposed to a chemical known to cause inflammation in people, and that substance in turn causes another tissue to twitch. A bioassay test exposes a piece of partially known tissue to a novel environment and records the tissue’s reaction to figure out unknown characteristics of it or the environment. Vane had developed a powerful bioassay technique in which a sequence of tissues probed a chain of chemical reactions. When Collier approached him, he agreed to investigate what happened when he exposed tissues to pain-inducing chemicals, and what happened if he added aspirin to the chemicals.
To discover what a thing is good requires knowledge about relevant situations, which are often subtle and difficult. Lack of such knowledge partly explains why many chemicals sat on the shelf for decades before their therapeutic values were realized. This happened to aspirin’s rival Tylenol. Its active ingredient was synthesized in 1878, but had to wait until 1955 before being developed into a popular drug. Ever more revealing are the stories of antibacterial drugs. Sulfanilamide was synthesized in 1908, but it was the discovery of its therapeutic effectiveness in 1932 that won a Nobel Prize. Penicillin was discovered in 1928 and its therapeutic properties in 1939, and both discoveries were cited in the Nobel Prize. In Nobel Prizes such as these, the scientific community acknowledges the equal scientific importance of discovering and developing a drug. Unfortunately, this point is often overlooked in science studies, so that Hoffmann is often accorded with the credit for aspirin, to the neglect of Eichengrün and others in Bayer.
Enzymes are protein catalysts that speed up chemical reactions without being themselves used up in the reactions. An enzyme is a huge molecule with an active area that works somehow like a mold that accepts certain raw pieces and casts them into a final form. Imagine a mold that stamps a rod and a bowl into a spoon. Spoon production would be disrupted if someone throws a monkey range into the mold. Such a monkey range – an enzyme inhibitor – would make a desirable drug if it stops an enzyme from producing disease-inducing chemicals. Aspirin is an enzyme inhibitor. It suppresses the action of the enzyme COX, stops the production of prostaglandin, thus disrupting the pathways to pain, inflammation, elevated temperature, and stomach protection.
The mechanism of aspirin-hypersensitivity in asthmatic patients is not immunological but is related to pharmacological properties of ASA and other NSAIDs. As originally documented in 1975 by Andrew Szczeklik et al (7), only NSAIDs that are strong or at least moderate inhibitors of prostaglandins (more specifically, inhibitors of COX-1, an enzyme that converts arachidonic acid into prostaglandins, thromboxanes and prostacycline), can cause reactions in ASA-intolerant patients. It is postulated that inhibition of COX-1 by aspirin or other NSAIDs triggers a biochemical cascade which causes asthma. In fact, a local deficiency in prostaglandin E2 synthesis was found in nasal polyps, epithelial cells and bronchial fibroblasts from ASA-hypersensitive patients, suggesting a basal defect in this regulatory mechanism which may be further exacerbated by aspirin (8, 9).
With the concept of COX-inhibition, knowledge about aspirin changes from mere empiricism to theory guided research. Whereas an empirical fact is specific to a particular phenomenon, a concept is general and potentially applicable to other phenomena. COX enzyme is present in many parts of the body, including unexpected places such as colon tumors. The conceptual framework of COX inhibition suggests links between aspirin and phenomena hitherto deemed unrelated, thus enabling scientists to ask significant questions and direct their research efforts. Basic scientists can use NSAIDs as tools to probe the physiological effects of COX, for instance in the formation of cancer. Pharmaceutical firms can use COX enzyme in test tubes to screen for promising drugs. A conceptual framework that explains phenomena by their underlying mechanisms is not a last word but a scientific breakthrough. A final word closes the door on exploration, a breakthrough opens up a frontier of research.