5-(2,4-Difluorophenyl)salicylic acid, diflunisal (25), is the best compound, in terms of both efficacy and safety, from over 500 salicylates investigated in our laboratories. It is a chemically distinct, nonacetylating salicylic acid, more active than aspirin as an analgesic and antiinflammatory agent and superior in duration of action and therapeutic index. Some recent clinical and biochemical observations are briefly discussed.

I Diflunisal (MK-647; 5-(2,4-difluorophenyl)-salicylic acid) is a new analgesic anti-inflammatory agent discovered after an extensive chemical and pharmacological study from 1962-71. 2 In the search for a superior salicylate our objectives were higher potency, better tolerance, and a longer duration of action. 3 An evaluation of many available and newly synthesized salicylates, in the granuloma and carrageenan foot oedema assays, revealed the activity-enhancing trend of a hydrophobic group-for example, phenyl, at the carbon-5 position of salicylic acid. 4 The attachment of a 5-(4-fluorophenyl) group, previously found to enhance the potency of antiinflammatory (3,2-c)-pyrazole steroids and phenyl-a-propionic acids to acetyl salicylic acid yielded a clinical candidate flufenisal. As an analgesic, flufenisal is two times more potent than aspirin in man, but with a longer action; no distinct advantage in gastrointestinal tolerance has, however, been observed. 5 Further investigation of 5-heteroaryl salicylic acids, flufenisal congeners and their non-acylating carbonate esters identified diflunisal and 5-( l-pyrryl)-salicylic acid for subacute safety assessment. The O-acetyl group, commonly present in aspirin, benorylate and flufenisal, was purposely avoided in these two compounds for safety considerations. 6 Without an O-acetyl group, diflunisal cannot acetylate proteins and macro-molecules in vivo as aspirin does. In the prostaglandin synthetase assay in vitro, salicylic acid is much less active than aspirin. In contrast, the non-acetylated diflunisal and desacetyl flufenisal are both more active than flufenisal in vitro. A significant difference between aspirin and diflunisal in their biochemical mechanisms was noted. 7 On the basis of overall efficacy and tolerance data, diflunisal was finally chosen as a superior analgesic anti-inflammatory salicylate for clinical evaluation.

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