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The discovery by Woods (1940) that small amounts of p-aminobenzoic acid could block the antibacterial effects of the sulfonamides served to stimulate a number of investigations on sulfonamide p-aminobenzoic acid relationships. Woods's initial finding soon led to the observations that many derivatives of p-aminobenzoic acid also exhibited antisulfonamide activity. Keltch et al. (1941) demonstrated that local anesthetics which are esters of p-aminobenzoic acid are antagonistic to sulfonamide action whereas those anesthetics not derived from p-aminobenzoic acid are devoid of this activity. Not only is there antisulfonamide action by the p-aminobenzoates (procaine hydrochloride,2 etc.), but Raiziss, Severac, and Moetsch (1942) report that p-aminobenzamide exerts an antagonistic effect toward sulfanilamide and sulfapyridine in experimentally infected mice. An analagous effect has been described by Miller (1941) who notes that the bacteriostatic action of p-nitrobenzoate can be prevented by small amounts of sodium p-aminobenzoate. In view of the above findings and the knowledge that the antisulfonamide activity of p-aminobenzoic acid is decreased 10,000-fold by acetylation of the amino group (Woods, 1940), it appeared worth while to determine whether substitution of other groups in the N position of p-aminobenzoic acid and its derivatives would affect their antagonism toward sulfonamides. Accordingly, a series of compounds was prepared in which alkyl groups were substituted in the N position. This series included p-alkylaminobenzoic acid, its esters and amides, the structural formulae for which appear in table 1.

The In Vitro Effects of N-Substituted p -Aminobenzoic Acid Derivatives upon Sulfonamides