Lactamase inhibitors: evolving compounds for evolving resistance targets

The many and diverse -lactamases produced by bacteria, particularly by Gram-negative pathogens, are increasingly posing a serious threat to the clinical utility of -lactams. First-generation inhibitors (clavulanic acid, sulbactam, tazobactam) focus on Ambler class A enzymes. However, recent structural upgrades of class A -lactamases (e.g. TEM, SHV) have extended their spectrum (extended-spectrum -lactamases and carbapenemases [Sme, NMC-A, IMI-1]) and have brought about the possibility of -lactamase-inhibitor resistance. Furthermore, the mobilisation and spread of originally chromosomal class C enzymes (CMY, MIR), the growing clinical importance of class B enzymes (IMP, VIM), the emergence of inhibitor-resistant, broad spectrum class D (OXA) enzymes and the co-existence of different classes of -lactamases in the same pathogen have spurred research toward universal inhibitors. A complicating issue is target accessibility in Gram-negative bacteria, particularly in Enterobacter, Acinetobacter, Pseudomonas, Stenotrophomonas and other organisms, which is necessary in order for the inhibitor to synergise with vulnerable -lactam antibiotics. Several new, broad-spectrum inhibitors have emerged: cephem sulfones and oxapenems are upgrades of penam sulfones and oxapenams, respectively, with cephem sulfones possibly extending their inhibition to class B metallo-enzymes; and boronates and phosphonates are designed de novo, based on common structural and mechanistic features of serine -lactamases.