Laboratory studies with a new broad-spectrum penicillin, pirbenicillin

Pirbenicillin {6-[d-2-phenyl-2(N-4-pyridylformimidoylaminoacetamido) -acetamido]-penicillanic acid} showed broad-spectrum antibacterial activity in vitro and also in the treatment of experimental infections after parenteral administration to mice. Against Pseudomonas aeruginosa, a three- to fourfold potency advantage over carbenicillin was seen both in vitro and in vivo. The in vitro antibacterial spectrum of pirbenicillin includes Escherichia coli, Serratia, Citrobacter, and Enterobacter isolates, against which it exhibited minimal inhibitory concentration values comparable to those of carbenicillin. However, mice infected with E. coli and Serratia were protected at doses of pirbenicillin that were two to four times lower than those required of carbenicillin. Pirbenicillin was more active than carbenicillin against gram-positive bacteria, especially Streptococcus faecalis. It was less active than carbenicillin against Proteus spp. and was inactive against ampicillin-resistant E. coli strains. Pirbenicillin was bactericidal at concentrations generally equal to or only two-fold higher than the minimal inhibitory concentration. With appropriately buffered media, pirbenicillin demonstrated eight- and fourfold better minimal bactericidal concentration values towards Pseudomonas isolates than those of carbenicillin and ticarcillin, respectively.