Small-Molecular Adjuvants with Weak Membrane Perturbation Potentiate Antibiotics against Gram-Negative Superbugs

Combination therapy with membrane-targeting compounds is at the forefront because the bacterial membrane is an attractive target considering its role in various multidrug-resistant elements. However, this strategy is crippled by the toxicity associated with these agents. The structural requirements for optimum membrane perturbation and minimum toxicity have not been explored for membrane-targeting antibiotic potentiators or adjuvants. Here, we report the structural influence of different chemical moieties on membrane perturbation, activity, toxicity, and potentiating ability in norspermidine derivatives. It has been shown in this report that weak membrane perturbation, achieved by the incorporation of cyclic hydrophobic moieties, is an effective strategy to design antibiotic adjuvants with negligible in vitro toxicity and activity but good potentiating ability. Aryl or adamantane functionalized derivatives were found to be better resorts as opposed to the acyclic analogues, exhibiting as high as 4096-fold potentiation of multiple classes of antibiotics toward critical Gram-negative superbugs. The mechanism of potentiation was nonspecific, consisting of weak outer-membrane permeabilization, membrane depolarization, and efflux inhibition. This unique concept of "weakly perturbing the membrane" by incorporating cyclic hydrophobic moieties in a chemical design with free amine groups serves as a breakthrough for nontoxic membrane-perturbing adjuvants and has the potential to revitalize the effect of obsolete antibiotics to treat complicated Gram-negative bacterial infections.