Cyclooxygenase Inhibitors, NSAIDs, Acetaminophen, and Glucocorticoids summary

Cyclooxygenase Isoforms and Physiological Roles

Cyclooxygenase (COX) catalyzes the synthesis of prostanoids, including prostaglandins ($PGE_2$ and $PGI_2$) and thromboxane $A_2$ ($TXA_2$), from arachidonic acid. COX-1, termed "good COX," is constitutively expressed in almost all tissues to maintain housekeeping functions. In the stomach, COX-1 promotes cytoprotective mucus, bicarbonate secretion, submucosal blood flow, and reduced acid secretion. In platelets, COX-1 stimulates $TXA_2$-mediated platelet aggregation. In the kidneys, COX-1 and COX-2 synthesize $PGE_2$ and $PGI_2$ to maintain renal perfusion.

COX-2, termed "bad COX," is induced primarily at sites of tissue injury to mediate inflammation and pain. COX-2 is also present in the brain (mediating fever and pain perception), blood vessels (promoting $PGI_2$-mediated vasodilation), kidneys (supporting renal function), and colon (promoting colorectal cancer). Inhibition of COX-1 leads to gastric ulceration, bleeding tendencies, and renal impairment, but provides protection against myocardial infarction (MI) and ischemic stroke through reduced platelet aggregation. Inhibition of COX-2 suppresses inflammation, alleviates pain, reduces fever, and protects against colorectal cancer, but can cause renal impairment and promote MI and stroke by suppressing vasodilation.

First-Generation NSAIDs and Aspirin

First-generation nonsteroidal anti-inflammatory drugs (NSAIDs) nonselectively inhibit both COX-1 and COX-2. Aspirin (acetylsalicylic acid or ASA) is the prototype derivative of salicylic acid. Unlike other NSAIDs, aspirin is an irreversible inhibitor of COX; its duration of action depends on the rate of tissue synthesis of new COX molecules. Aspirin is absorbed rapidly in the stomach and small intestine, with a half-life of $15\text{--}20\,minutes$ due to conversion to salicylic acid. Salicylate half-life is $2\,hours$ at low doses but exceeds $20\,hours$ at high therapeutic doses. Salicylate plasma levels below $100\,\mu g/mL$ occur with low doses, $150\text{--}300\,\mu g/mL$ with anti-inflammatory doses, while salicylism occurs above $200\,\mu g/mL$ and severe toxicity above $400\,\mu g/mL$. Excretion is urinary and pH-dependent, increasing fourfold when urine pH is raised from $6$ to 8$.\n\nAspirin suppresses inflammation, relieves mild to moderate pain (primarily peripheral via COX-2 inhibition and central actions), and lowers fever by inhibiting pyrogen-induced hypothalamic PGE_2$synthesis. Daily low-dose aspirin ($75\text{--}81\,mg/day$) irreversibly inhibits platelet COX-1 and$TXA_2$production for the$8\text{-}day$lifespan of platelets. Indications include ischemic stroke, transient ischemic attacks, acute MI, previous MI, chronic stable angina, unstable angina, and coronary angioplasty. Primary prevention of MI and stroke is recommended only when 10-year cardiovascular risk exceeds$10\% due to major risks of gastrointestinal hemorrhage.\n\nAdverse effects of aspirin include gastric distress, occult bleeding, gastric ulceration, and perforation caused by acid secretion, decreased mucus and bicarbonate, reduced blood flow, and direct mucosa irritation. Prophylaxis with a proton pump inhibitor (PPI), H_2$-receptor antagonist ($H_2RA$), or misoprostol is recommended for high-risk patients, along with testing for Helicobacter pylori infection. Aspirin is contraindicated in bleeding disorders, peptic ulcer disease, pregnancy (third trimester risk of premature closure of ductus arteriosus, postpartum hemorrhage), and children or teenagers under$18\,years$with influenza or chickenpox due to the risk of Reye syndrome. Salicylism manifests as tinnitus, sweating, headache, and dizziness, alongside complex acid-base disturbances (respiratory alkalosis followed by metabolic acidosis). Significant drug interactions occur with anticoagulants (warfarin, heparin), glucocorticoids, alcohol, non-aspirin NSAIDs (which block aspirin COX-1 access unless aspirin is taken$2\,hours prior), ACE inhibitors, ARBs, and vaccines.\n\n# Non-Aspirin First-Generation NSAIDs\n\nNon-aspirin first-generation NSAIDs competitively and reversibly inhibit COX-1 and COX-2. They provide anti-inflammatory, analgesic, and antipyretic actions equal to aspirin for conditions such as rheumatoid arthritis, osteoarthritis, bursitis, and dysmenorrhea. Unlike aspirin, these agents do not protect against MI and stroke; instead, they increase cardiovascular thrombotic risk by approximately 12\%$overall, with higher increases seen in indomethacin ($71\%$), sulindac ($41\%$), and meloxicam ($37\%$). Non-aspirin NSAIDs are contraindicated immediately prior to and for$14\,days following coronary artery bypass graft (CABG) surgery.\n\nIbuprofen (Advil, Motrin, Caldolor) is the prototype propionic acid derivative. It exhibits low adverse effect rates, causes less gastric bleeding than aspirin, and effectively suppresses uterine smooth muscle COX to relieve primary dysmenorrhea. It is also used to close ductus arteriosus in preterm infants. Rare severe cutaneous reactions such as Stevens-Johnson syndrome (SJS) may occur. Nonacetylated salicylates (magnesium salicylate, sodium salicylate, salsalate) cause minimal platelet aggregation suppression and present no MI or stroke protection. Salsalate acts as a prodrug releasing salicylate in the alkaline small intestine to lessen gastric irritation.\n\n# Second-Generation NSAIDs (Coxibs)\n\nSecond-generation NSAIDs, or coxibs, selectively inhibit COX-2 at therapeutic doses. Celecoxib (Celebrex) is indicated for osteoarthritis, rheumatoid arthritis, ankylosing spondylitis, juvenile idiopathic arthritis, acute pain, dysmenorrhea, and familial adenomatous polyposis. It achieves peak plasma levels in 3\,hours$, has$97\%$protein binding, undergoes hepatic metabolism, and exhibits an$11\text{-}hour half-life.\n\nSelective COX-2 inhibition provides anti-inflammatory and analgesic efficacy equivalent to naproxen while reducing gastroduodenal ulceration incidence compared to traditional NSAIDs short-term. However, celecoxib does not inhibit platelet COX-1 or aggregation, leaving platelet aggregation unimpeded while suppressing COX-2-mediated vasodilation in blood vessels. This imbalance significantly increases cardiovascular thrombotic events, MI, and stroke. Celecoxib can impair renal function (causing hypertension and edema), carries a risk of allergic reactions in patients with sulfonamide allergy due to its sulfur molecule, and is contraindicated during the third trimester of pregnancy. Celecoxib may also increase anticoagulant effects of warfarin and decrease diuretic effects of furosemide or antihypertensive effects of ACE inhibitors and ARBs.\n\n# Acetaminophen Pharmacology and Toxicity\n\nAcetaminophen (Tylenol) possesses analgesic and antipyretic properties equivalent to aspirin, but completely lacks anti-inflammatory, anti-rheumatic, or anti-platelet actions. Its action is limited to selective inhibition of cyclooxygenase in the central nervous system (CNS), with minimal peripheral COX inhibition. Consequently, it does not produce gastric ulceration, renal impairment, bleeding tendencies, or Reye syndrome. It is rapidly absorbed orally with a 2\text{-}hour plasma half-life.\n\nAcetaminophen undergoes hepatic metabolism via two pathways. The major pathway involves conjugation with glucuronic acid to form nontoxic metabolites. The minor pathway involves oxidation by cytochrome P450 enzymes into a highly reactive, toxic metabolite, N\text{-acetyl-p-benzoquinone imine}$. Under normal therapeutic conditions, glutathione rapidly converts this toxic metabolite into a nontoxic form. In overdose ($>4000\,mg/day$) or chronic alcohol use (which induces P450 and depletes glutathione), glutathione stores become depleted, causing toxic metabolite accumulation and severe hepatic necrosis. Acute intoxication manifests initially as nausea, vomiting, diarrhea, sweating, and abdominal discomfort, with overt indications of liver injury appearing$48\text{--}72\,hours post-ingestion.\n\n\n\nAcetaminophen toxicity is the leading cause of acute liver failure in the United States. Liver damage is minimized by administering acetylcysteine (Mucomyst, Acetadote) orally or intravenously, which substitutes for depleted glutathione. Acetylcysteine is 100\%$effective when given within$8\text{--}10\,hours$of overdose and offers significant protection up to$24\,hours$. Maximum daily dosage is$4000\,mg/day$, reduced to$2000\,mg/day$in undernourished individuals or regular consumers of$3$or more alcoholic drinks daily. Daily usage ($500\,mg/day or more) is associated with hypertension, and rare severe skin reactions (SJS, TEN, AGEP) can occur.\n\n# AHA Guidelines for Chronic Pain and Glucocorticoid Physiology\n\nThe American Heart Association (AHA) recommends a four-step care approach for managing musculoskeletal pain in patients with or at high risk for cardiovascular disease. Step 1 begins with non-drug measures (physical therapy, exercise, weight loss, orthotics, heat or cold). Step 2 initiates drug therapy with acetaminophen or aspirin, or short-term opioids or tramadol. Step 3 tries nonselective NSAIDs such as naproxen, ibuprofen, or nonacetylated salicylates. Step 4 utilizes the selective COX-2 inhibitor celecoxib as a last resort due to its high cardiovascular risk. Patients at high risk for thrombotic events in steps 2 through 4 should receive low-dose aspirin (81\,mg/day$) plus a PPI or$H_2RA$.

Glucocorticoid drugs are nearly identical to endogenous glucose-regulating steroids produced by the adrenal cortex under control of the hypothalamic-pituitary-adrenal (HPA) axis (hypothalamic CRH stimulates anterior pituitary ACTH release, promoting adrenal cortical cortisol synthesis). Physiologic effects (elicited by low doses) elevate blood glucose via gluconeogenesis, reduced peripheral utilization, and decreased muscle or adipose uptake. Glucocorticoids stimulate lipolysis, suppress protein synthesis, maintain vascular integrity, elevate circulating red blood cells and neutrophils, decrease counts of lymphocytes, eosinophils, basophils, and monocytes, and promote lung maturation in neonates. High pharmacologic doses are used to suppress inflammation and immune responses, but chronic high-dose therapy causes systemic toxicity and persistent suppression of the HPA feedback loop.