Diflunisal (Dolobid, MK-647, CAS 22494-42-4, C₁₃H₈F₂O₃, MW 250.20 g/mol) is a difluorophenyl-substituted salicylic acid derivative NSAID developed by Merck and FDA-approved in 1982 for mild-to-moderate pain, osteoarthritis, and rheumatoid arthritis. Chemically, it is 5-(2,4-difluorophenyl)salicylic acid — the 2,4-difluorophenyl group is appended at the C5 position of the salicylic acid scaffold. This substitution delivers three decisive pharmacological improvements over aspirin (acetylsalicylic acid) and salicylic acid: (1) Extended half-life: 8-12 hours vs aspirin’s 0.25 hours (48× longer) and salicylic acid’s 2-4 hours. This results from fluorine-mediated metabolic stability (C-F bonds at the 2′ and 4′ positions block CYP450-mediated aromatic hydroxylation), high protein binding (>99% to albumin), and enterohepatic recirculation of glucuronide conjugates. BID dosing replaces aspirin’s q4-6h schedule. (2) Enhanced tissue penetration: logP ~4.3 vs aspirin’s ~2.3 — nearly 100-fold greater lipophilicity. This drives superior distribution into synovial fluid (concentrations exceeding plasma levels after repeated dosing) and other target tissues. (3) Distinct metabolic pathway: diflunisal is metabolized primarily via glucuronidation (UGT1A1, UGT1A9), NOT to salicylic acid. Aspirin is rapidly deacetylated (t_1/2 ~15 min) to salicylic acid, meaning aspirin’s pharmacology is largely salicylic acid pharmacology after the first hour. Diflunisal’s independent metabolic identity provides a distinct safety/efficacy profile — notably, less GI prostaglandin suppression at equi-analgesic doses and lower incidence of tinnitus (salicylism). The fluorine atoms act as metabolic blocking groups because: C-F bond dissociation energy (~485 kJ/mol) exceeds C-H (~460 kJ/mol), making enzymatic C-F cleavage energetically unfavorable, and fluorine’s high electronegativity (3.98 Pauling) deactivates the aromatic ring toward electrophilic oxidation by CYP450. Beyond its NSAID pharmacology, diflunisal has been repurposed as a TTR amyloidosis stabilizer — it binds the T4 site of the transthyretin tetramer, preventing amyloidogenic monomer release and fibril formation. UPOR Biotech supplies diflunisal pharmaceutical API at ≥99% HPLC purity under GMP with USP/EP compliance.

Diflunisal is a non-selective, reversible cyclooxygenase (COX) inhibitor that blocks both COX-1 and COX-2 enzymes, reducing the biosynthesis of pro-inflammatory and homeostatic prostanoids. The COX enzymes catalyze the committed step in prostaglandin synthesis: the bis-dioxygenation and cyclization of arachidonic acid (released from membrane phospholipids by phospholipase A₂) to prostaglandin G₂ (PGG₂), followed by peroxidation to PGH₂. COX-1 is constitutively expressed in most tissues and produces prostaglandins that mediate: gastric cytoprotection (PGE₂, PGI₂ — maintaining mucosal barrier integrity, bicarbonate secretion, and mucosal blood flow), renal blood flow autoregulation (PGE₂, PGI₂ — afferent arteriolar vasodilation), and platelet aggregation (TXA₂ — synthesized exclusively by COX-1 in platelets). COX-2 is induced at sites of inflammation by cytokines (IL-1β, TNF-α), growth factors, and bacterial endotoxins (LPS). COX-2-derived prostaglandins (predominantly PGE₂ and PGI₂) mediate the cardinal signs of inflammation: pain (sensitization of nociceptors to bradykinin and other algesic mediators), fever (PGE₂ action on hypothalamic thermoregulatory centers), swelling (increased vascular permeability), and erythema (vasodilation). By inhibiting both COX isoforms, diflunisal provides: analgesic and anti-inflammatory efficacy via COX-2 blockade at inflammatory sites, and antiplatelet activity via COX-1-dependent TXA₂ inhibition in platelets. The reversible binding kinetics distinguish diflunisal from aspirin, which irreversibly acetylates COX-1 at Ser529 (and COX-2 at Ser516), permanently inactivating the enzyme for the lifespan of the platelet (7-10 days). Diflunisal’s reversible inhibition means COX activity recovers as the drug is cleared, providing a more titratable, shorter-duration antiplatelet effect. The non-selective (COX-1 + COX-2) profile also carries the class-effect risks: GI mucosal injury (via COX-1-derived PGE₂ suppression), renal function impairment (particularly in volume-depleted or renin-dependent patients), and platelet dysfunction. These risks are inherent to all non-selective NSAIDs and require standard clinical risk assessment. The 2,4-difluorophenyl group enhances COX active-site binding through hydrophobic interactions within the cyclooxygenase channel, contributing to sustained target engagement across the 8-12 hour dosing interval.

Diflunisal represents one of the most successful examples of drug repurposing in modern pharmacology — transitioning from an NSAID analgesic to an orphan-disease therapeutic for transthyretin (TTR) amyloidosis. TTR amyloidosis (ATTR) is a progressive, fatal protein-misfolding disease in which the normally tetrameric transthyretin protein dissociates into monomers that undergo conformational rearrangement, misfold, and aggregate into cross-β-sheet amyloid fibrils. These fibrils deposit in the myocardium (ATTR-CM — cardiomyopathy, causing restrictive heart failure) and peripheral nerves (ATTR-PN — polyneuropathy, causing progressive sensorimotor dysfunction). The rate-limiting step in amyloidogenesis is tetramer dissociation into monomers; stabilizing the native tetrameric quaternary structure prevents monomer release and halts the amyloidogenic cascade. Diflunisal binds with high affinity to the thyroxine (T4) binding site within the TTR tetramer’s central channel. The 2,4-difluorophenyl group and the salicylic acid carboxylate occupy complementary subsites within the T4 binding pocket, forming hydrophobic contacts (difluorophenyl ring) and hydrogen bonds (carboxylate with Lys15 and Glu54 at the tetramer interface). This binding kinetically stabilizes the tetramer, raising the thermodynamic barrier to dissociation by approximately 2.5 kcal/mol. At clinically achievable plasma concentrations (100-200 µM achieved with 250 mg BID), diflunisal occupies >90% of T4 binding sites, effectively preventing monomer release. The landmark clinical trial (Berk et al., JAMA, 2013) was a randomized, double-blind, placebo-controlled study in 130 patients with familial amyloid polyneuropathy (FAP). Diflunisal 250 mg BID significantly reduced the rate of progression of neurological impairment (measured by the Neuropathy Impairment Score +7, NIS+7) compared to placebo over 2 years, with an acceptable safety profile. This trial established proof-of-concept for kinetic stabilization as a therapeutic strategy in TTR amyloidosis and directly informed the development of next-generation TTR stabilizers: tafamidis (Vyndaqel/Vyndamax, Pfizer, FDA-approved 2019 for ATTR-CM) and acoramidis (AG10, BridgeBio). Tafamidis binds the same T4 site with higher affinity and greater kinetic selectivity than diflunisal, but diflunisal remains in clinical use in regions where tafamidis is not available or affordable. Diflunisal’s TTR binding is pharmacologically independent of its COX-inhibitory activity — the TTR stabilization occurs at concentrations below those required for maximal COX inhibition, and structure-activity relationship (SAR) studies confirm that COX inhibition and TTR binding are mediated by distinct molecular recognition features. For research institutions investigating TTR amyloidosis, diflunisal serves as the prototypical small-molecule TTR stabilizer and a reference compound for kinetic stabilization assays. UPOR Biotech supplies research-grade diflunisal with full characterization (HPLC purity, endotoxin levels, residual solvent profile) for academic and clinical TTR amyloidosis stabilization studies.

Diflunisal exhibits a distinctive pharmacokinetic profile that sets it apart from all other salicylate-derived NSAIDs. The key parameters: Oral bioavailability >90% with rapid GI absorption (t_max 2-3 hours for standard tablets). Volume of distribution (V_d) ~0.1 L/kg — very small, reflecting >99% plasma protein binding (primarily to albumin, with a small contribution from α₁-acid glycoprotein). This extensive protein binding limits glomerular filtration of the free drug, contributing to the long half-life. Elimination half-life (t_1/2): 8-12 hours (mean ~10.5h at steady state with 250-500 mg BID). This is the longest half-life among all salicylate-derived NSAIDs: aspirin t_1/2 ~0.25h (15 min), salicylic acid t_1/2 2-4h, salsalate t_1/2 ~1h, and choline magnesium trisalicylate t_1/2 9-17h (the only comparable agent). The 8-12h half-life enables convenient twice-daily (BID) dosing with sustained therapeutic plasma concentrations across the 12-hour dosing interval. Three mechanisms converge to produce this extended half-life: (1) Fluorine-mediated metabolic stability — the C-F bonds at the 2′ and 4′ positions of the phenyl ring block CYP450-mediated aromatic hydroxylation at those sites. C-F bond dissociation energy (~485 kJ/mol) is significantly higher than C-H (~460 kJ/mol), making enzymatic C-F cleavage energetically infeasible, and fluorine’s electronegativity (3.98 Pauling scale) deactivates the aromatic ring toward electrophilic oxidation. This redirects metabolism exclusively to Phase II conjugation pathways. (2) Extensive enterohepatic recirculation — diflunisal is metabolized primarily via glucuronidation (UGT1A1, UGT1A9) to diflunisal phenolic glucuronide and diflunisal acyl glucuronide. These glucuronide conjugates are excreted in bile, hydrolyzed by intestinal β-glucuronidase back to the parent drug, and reabsorbed in the distal ileum and colon. This enterohepatic cycling contributes an estimated 20-30% to the overall AUC and extends the terminal elimination phase. (3) High protein binding and low V_d — >99% albumin binding restricts the drug largely to the vascular compartment, limiting distributional clearance and reducing the fraction of drug available for hepatocyte uptake and metabolism at any given time. Renal clearance of unchanged diflunisal is minimal (<2% of dose) because of extensive protein binding, but renal elimination of glucuronide conjugates is the primary excretion route. Renal clearance is pH-dependent: urinary alkalinization (e.g., with sodium bicarbonate or acetazolamide) increases diflunisal clearance by ion-trapping the weakly acidic drug (pKa ~3.0) in alkaline urine, reducing tubular reabsorption. Steady state is achieved within 3-5 days of BID dosing; at steady state, plasma concentrations are ~1.5-2× higher than after a single dose, consistent with the accumulation factor predicted from the half-life and dosing interval. Food effect: food delays absorption (t_max increases by 1-2 hours) but does not significantly affect total bioavailability (AUC). Drug interactions: diflunisal is not a significant inhibitor or inducer of major CYP450 isoforms. It can displace warfarin from albumin binding sites, potentially increasing INR; it may reduce the uricosuric effect of probenecid and sulfinpyrazone by competing for renal tubular secretion. Concomitant antacids (aluminum hydroxide) may reduce diflunisal absorption. The linear, predictable pharmacokinetics across the 250-500 mg dose range distinguish diflunisal from salicylic acid, which exhibits dose-dependent, saturable elimination kinetics (Michaelis-Menten) at anti-inflammatory doses, leading to disproportionate increases in plasma concentration with dose escalation.

UPOR Biotech provides a complete GMP-compliant diflunisal API documentation package designed for seamless integration into regulatory submissions (ANDA Module 3.2.S, NDA, MAA) and customer quality systems: (1) Certificate of Analysis (COA) per batch — assay (≥99.0% HPLC), full impurity profile with RRT/RRF values, water content (Karl Fischer), residual solvents, heavy metals, elemental impurities (ICH Q3D), and microbial panel (USP <61>/<62>). (2) Material Safety Data Sheet (MSDS/SDS) — GHS-compliant, 16-section format. (3) Signed and dated HPLC chromatogram with integration parameters, system suitability data, and Diflunisal Reference Standard (USP/EP) traceability. (4) GMP Compliance Certificate (ICH Q7). (5) ISO 9001:2015 Certificate. (6) USP/EP Monograph Compliance Statement with individual monograph parameter conformance verification. (7) FDA Facility Registration Number. (8) BSE/TSE-Free Statement. (9) Nitrosamine Risk Assessment Report per FDA Guidance (February 2024) and EMA CHMP Guidance (Article 5(3) of Regulation (EC) No 726/2004). (10) Elemental Impurities Compliance Statement (ICH Q3D) with risk assessment per USP <232>. (11) Residual Solvents Statement (ICH Q3C Class 3). (12) Stability Data Package — real-time (36-month, 25°C/60%RH) and accelerated (6-month, 40°C/75%RH) with trend analysis per ICH Q1A(R2) and Q1E. (13) Complete Lot Traceability Documentation from starting material through intermediate to finished API. (14) Genotoxic Impurity Assessment per ICH M7. DMF support (Type II) is available with a Letter of Authorization for ANDA cross-reference. HALAL and KOSHER certificates provided upon request. MOQ: 1 kg for commercial orders; 10 g for R&D characterization and method development samples. Free sample available for qualified B2B buyers — contact our sales team with your company profile, intended application, and target regulatory market. All documents are provided in English; certified translations available upon request for Spanish, Portuguese, Arabic, Japanese, and Korean regulatory submissions. Our facility undergoes regular customer and regulatory audits; audit reports and quality agreements are available upon execution of a Mutual Confidentiality Agreement (MCA). UPOR Biotech offers dedicated regulatory affairs support for ANDA filers, including DMF reference, CMC query responses, and post-approval change management. Every batch is shipped with complete documentation; electronic copies are provided within 24 hours of shipment confirmation.