Covalent inhibition of serine β-lactamases by novel hydroxamic acid derivatives

The effectiveness of β-lactam antibiotics is greatly limited by the ability of bacteria to produce β-lactamases. These enzymes catalyze the hydrolysis of β-lactams and thus loss of their antibiotic activity. The search for inhibitors of β-lactamases began soon after β-lactams were introduced into medical practice and continues today. Some time ago, we introduced a new class of covalent serine β-lactamase inhibitors, the O-aryloxycarbonyl hydroxamates, that inactivated these enzymes by a unique mechanism in which the active site became cross-linked. We describe in this paper some new variants of this class of inhibitor. First, we investigated compounds in which more polar hydroxamates were incorporated. These were generally not more active than the original compounds against representative class A and class C β-lactamases, but one of them, 1-(benzoyl)-O-(phenoxycarbonyl)-3-hydroxyurea, was significantly more stable in solution, thus revealing a useful platform for further design. Second, we describe a series of O-(arylphosphoryl) hydroxamates that are also irreversible inactivators of class A and class C β-lactamases, by phosphorylation of the enzyme, as revealed by mass spectra. These compounds did not, however, cross-link the enzyme active site. A striking feature of their structure-activity profile was that hydroxamate remained the leaving group on enzyme phosphorylation rather than aryloxide, even though the aryloxide was intrinsically the better leaving group, as indicated by pKa values and demonstrated by the products of hydrolysis in free solution. Model building suggested that this phenomenon arises from the relative affinity of the enzyme active site components for the two leaving groups. The results obtained for both groups of inhibitors are important for further optimization of these inhibitors.