Naproxen

Indications

Naproxen is a widely used nonsteroidal anti-inflammatory drug (NSAID) indicated for the management of various inflammatory and pain-related conditions. These include rheumatoid arthritis, osteoarthritis, ankylosing spondylitis, polyarticular juvenile idiopathic arthritis, tendinitis, bursitis, acute gout, and primary dysmenorrhea. Additionally, it serves as a first-line therapy for osteoarthritis, acute gouty arthritis, dysmenorrhea, and musculoskeletal inflammation and pain due to its effective analgesic and anti-inflammatory properties.

Pharmacodynamics

As a non-selective NSAID, naproxen exhibits analgesic, anti-inflammatory, and antipyretic effects primarily through the inhibition of cyclo-oxygenase enzymes, leading to a reduction in prostaglandin synthesis in tissues and fluids such as synovial fluid, gastric mucosa, and blood. However, its use can lead to side effects, such as an increase in blood pressure, albeit less significant compared to other NSAIDs like ibuprofen. The risk of upper gastrointestinal bleeding is notably higher with NSAIDs, compounded by factors such as concurrent use of corticosteroids or anticoagulants, and a history of gastrointestinal ulcers.

Absorption

Naproxen is available both as a free acid and as a sodium salt, with slight differences in absorption rates at comparable doses (500 mg of naproxen equals 550 mg of naproxen sodium). Naproxen sodium reaches peak plasma concentration in about an hour, whereas the free acid form takes approximately two hours. These differences in onset should be considered for acute pain management, where a quicker onset of action is desirable. Various formulations such as immediate release, enteric-coated, and controlled release show similar Cmax values ranging from 94 mcg/mL to 97.4 mcg/mL. Naproxen is absorbed rapidly and completely, and while food can delay absorption, it does not affect the overall extent of absorption.

Metabolism

Naproxen undergoes extensive hepatic metabolism, involving both Phase I and Phase II processes. The initial step is demethylation mediated by cytochrome P450 enzymes CYP 1A2, 2C8, and 2C9. Subsequently, both naproxen and its metabolite, desmethylnaproxen, undergo Phase II metabolism. Naproxen forms acyl glucuronide, while desmethylnaproxen can generate both acyl and phenolic glucuronide products, catalyzed by various UDP-glucuronosyltransferase enzymes. Desmethylnaproxen is also subject to sulfation by sulfotransferase enzymes SULT 1A1, 1B1, and 1E1. This comprehensive metabolism is essential for its therapeutic efficacy and elimination from the body.

Mechanism of Action

Naproxen, like other non-selective NSAIDs, achieves its therapeutic effects by inhibiting the cyclooxygenase enzymes COX-1 and COX-2, which results in reduced synthesis of prostaglandins. Although both enzymes are involved in prostaglandin production, they have distinct roles. COX-1 is continually active and present in normal tissues, such as the stomach lining, whereas COX-2 is inducible and produces prostaglandins that are responsible for mediating pain, fever, and inflammation. The inhibition of COX-2 is primarily responsible for naproxen's antipyretic, analgesic, and anti-inflammatory benefits. However, the blocking of COX-1 is associated with adverse effects including gastrointestinal disturbances and renal toxicities.