Modified tunicamycins with reduced eukaryotic toxicity that enhance the antibacterial activity of β-lactams

Tunicamycins (TUN) are inhibitors of the UDP-HexNAc: polyprenol-P HexNAc-1-P transferase family of enzymes, which initiate the biosynthesis of bacterial peptidoglycan and catalyze the first step in eukaryotic protein N -glycosylation. The TUN are therefore general and potent toxins to both eukaryotes and prokaryotes. Screening a library of synthetic TUN against Bacillus and yeast identified TUN that are antibacterial, but have significantly reduced eukaryotic toxicity. One of these (Tun-15:0) differs from the native TUN control only by the lack of the conjugated double bond in the tunicaminyl N -acyl group. Tun-15:0 also showed reduced inhibition for protein N -glycosylation in a Pichia -based bioassay. Natural TUN was subsequently modified by chemically reducing the N -acyl double bond (TunR1) or both the N -acyl and uridyl double bonds (TunR2). TunR1 and TunR2 retain their antibacterial activity, but with considerably reduced eukaryotic toxicity. In protein N -glycosylation bioassays, TunR1 is a less potent inhibitor than native TUN and TunR2 is entirely inactive. Importantly, the less toxic TunR1 and TunR2 both enhance the antibacterial activity of β-lactams: oxacillin by 32- to 64-fold, comparable with native TUN, and with similar enhancements for methicillin and penicillin G. Hence, the modified TUNs, TunR1 and TunR2, are potentially important as less-toxic synergistic enhancers of the β-lactams.