Mecillinam (FL 1060), a 6beta-amidinopenicillanic acid derivative: in vitro evaluation

Mecillinam (formerly called FL 1060), a novel 6beta-amidinopenicillanic acid derivative, showed markedly higher activity than ampicillin against clinical isolates of Enterobacteriaceae. The mode of action of mecillinam is different from that of ampicillin and consequently no intrinsic cross-resistance between the two antibiotics was observed. Mecillinam is inactivated by beta-lactamases but is generally more stable than ampicillin; thus an apparent cross-resistance exists for strong beta-lactamase producers. The protein binding was low and the activity was relatively independent of pH between 5.5 and 8.0. Slow-growing, insensitive variants of Escherichia coli could be selected but these were not stable. The frequency of insensitive variants was affected by the sodium chloride concentration in the medium.