Mechanisms of Action of Acetaminophen

Acetaminophen has antipyretic and moderate analgesic properties, but largely lacks anti-inflammatory activity. While its mechanism of action is not entirely understood, it is probably both an isoform nonspecific and partial cyclooxygenase (COX) inhibitor in humans at doses commonly taken for mild pain and pyrexia, such as 1000 mg. Although no inhibition of platelet aggregation is observed at this dosage, platelet thromboxane formation by COX is depressed by roughly 40%. Epidemiological studies suggest that at higher doses, 2000 mg and above, acetaminophen exhibits a gastrointestinal adverse effect profile indistinguishable from traditional, nonspecific NSAIDs. Thus, it is possible that maximal COX inhibition is achieved at higher doses. Interestingly, complete COX inhibition by non-selective COX inhibitors has the potential to antagonize the irreversible platelet inhibition induced by aspirin. In contrast to reversible inhibitors, aspirin acts by acetylation of a serine residue in the substrate binding channel of COX. For example, ibuprofen, a reversible and non-selective COX inhibitor, is thought to prevent aspirin from gaining access to this target site. This study investigates, whether COX inhibition by acetaminophen is dose dependent in humans and whether acetaminophen interacts with the irreversible COX inhibition by low dose aspirin. It addresses the dose-related effect of acetaminophen on COX activity and assesses potential pharmacological interactions with low dose aspirin in normal healthy volunteers. The primary hypothesis is that administrating acetaminophen before aspirin would antagonize the irreversible effects of aspirin, as assessed by the measurement of serum thromboxane B2 24 hrs after the administration of the first study drug on day 6 of combination therapy.

The second aim will determine the effects of acetaminophen on oxidant stress and cyclooxygenase activity in patients who smoke. While the structural interaction of acetaminophen with COX is unknown, it may inactivate the enzyme by a molecular mechanism different from other NSAIDs. Thus, acetaminophen, which is a good reducing agent, might act to reduce COX from its active, oxidized form. When uninhibited, the peroxidase component of this bifunctional enzyme oxidizes its catalytic center to generate a tyrosyl radical that is required for its activity. Indeed, some reducing agents have the capacity to prevent COX activation in vitro. If reduction were the basis for COX inhibition by acetaminophen in vivo, it would be expected to be less pronounced under conditions of high peroxide tone, as occurs in inflammation. Indeed, acetaminophen, which is a phenol derivative, may act as a free radical scavenging antioxidant like other phenolic compounds, such as vitamin E and has been shown to alleviate oxidative damage in model systems. This study explores the potential antioxidant effect of acetaminophen in smokers. Such individuals represent a human model of oxidant stress. Novel approaches to the quantitative assessment of free radical induced damage to lipids are applied, which are elevated in smokers. Additionally, it is determined whether COX inhibition by acetaminophen is conditioned by oxidant tone in vivo.

The third aim, will compare the degree of COX inhibition by acetaminophen with that of a thoroughly characterized nonselective COX inhibitor, ibuprofen.

Inclusion criteria

• Age between 18 - 55
• Subjects recruited for the "non-smoker group" must be in good health as based on medical history, physical examination, vital signs, and laboratory tests.
• Subjects recruited for the "smokers group" will be chronic smokers of at least 4 years duration, but no longer than 20 years duration, who smoke 11-20 cigarettes per day. Smokers must be otherwise healthy as described above.
• Female subjects of child bearing potential must be using a medically acceptable method of contraception (oral contraception, depo-provera injection, intrauterine device, condom with spermicide, diaphragm, cervical cap, progestin implant, abstinence, tubal ligation, oophorectomy, TAH) throughout the entire study period. All female subjects must consent to a urine pregnancy test at screening and just prior to the start of each treatment phase of the study, which must be negative at all time points.
• Subjects must be within 30% of their ideal body weight.

Exclusion criteria

• Female subjects who are pregnant or nursing a child.
• Subjects, who have received an experimental drug, used an experimental medical device within 30 days prior to screening, or who gave a blood donation of ≥ one pint within 8 weeks prior to screening.
• Subjects with any coagulation, bleeding or blood disorders.
• Subjects who are sensitive or allergic to acetaminophen and/or aspirin, as well as any of their components.
• Subjects with documented history of any gastrointestinal disorders, including bleeding ulcers.
• Subjects with any evidence of cancer.
• Subjects with a history of heart disease, including myocardial infarction, angina, coronary artery disease, any evidence of coronary artery stenosis, arrhythmias, heart failure, having had a coronary intervention or significant irregularities in the EKG.
• Subjects with history of peripheral artery disease (claudication, bypass surgery or stent placement in extremity.)
• Subjects with a history of stroke or transitory ischemic attacks.
• Subjects with renal, hepatic, respiratory, endocrine, metabolic, hematopoietic or neurological disorder.
• Subjects with a history of liver disease or abnormal liver function tests (>2x upper limit normal).
• Subjects with any abnormal laboratory value or physical finding that according to the investigator may interfere with interpretation of the study results, be indicative of an underlying disease state, or compromise the safety of a potential subject.
• Subjects who have had a history of drug or alcohol abuse within the last 6 months.