Antibacterial activity of phosphanilic acid, alone and in combination with trimethoprim.

We explored the antibacterial activity of phosphanilic acid (P), an analog of sulfanilic acid, alone and in combination with trimethoprim (T; TP, 1:5) with sulfamethoxazole (S) and co-trimoxazole, the combination of this sulfonamide with trimethoprim (TS, 1:5) as the reference. P resembled S in spectrum but, in addition, had significant activity against Pseudomonas aeruginosa. The overall frequency and degree of synergism with TP were lower than with co-trimoxazole. P, like S, was strongly affected by changes in inoculum size and was not bactericidal. P was well absorbed parenterally but not orally in mice. Despite low (but prolonged) blood levels, P, given orally to mice, was effective in treating infections caused by P. aeruginosa. However, against most experimental infections the therapeutic effectiveness of P, as well as that of TP, administered either intramuscularly or orally was unimpressive. Based on in vivo data, the therapeutic application of P or TP would appear to be limited.