HONG KONG – A structurally nanoengineered antimicrobial peptide polymer (SNAPP) showed great promise as an inexpensive but safe and effective antimicrobial, and may be a useful new weapon in the war against multidrug-resistant (MDR) gram-negative bacteria, an Australian study has found.

The University of Melbourne (UM) study was the first to show that a synthetic antimicrobial polymer had efficacy against a colistin and MDR (CMDR) strain of Acinetobacter baumannii, with no resistance acquisition being seen, including in the CMDR strain.

Gram-negatives such as A. baumannii, Klebsiella pneumoniae and Pseudomonas aeruginosa are responsible for most hospital-acquired infections, posing a major health threat, as they have acquired resistance to multiple antibiotics, due to the abuse and overuse of these agents.

Despite the increasingly serious threat, the pipeline for new antibiotics remains empty. That prompted the G20 group of world leaders at their recent meeting in Hangzhou, China, to call for steps to address industry underinvestment in research and development into new antibiotics and diagnostics. (See BioWorld Today, Sept. 8, 2016.)

This antibiotic developmental dearth is also due to gram-negative bacteria being harder to kill than gram-positives, as they possess additional defense mechanisms, in particular an outer membrane serving as an impermeable barrier.

ANTIMICROBIAL PEPTIDES

Unlike conventional antibiotics, which act on specific intracellular targets, antimicrobial peptides (AMPs) interact with microbial membranes through electrostatic interactions, specifically damaging bacterial morphology. As such, they are widely regarded as a promising potential solution to combat MDR, in particular MDR gram-negative pathogens.

However, AMPs per se are problematic. "AMPs can be effective in killing bacteria, but they are all very toxic to mammalian cells," lead researcher Greg Qiao, head of the Polymer Science Group and a professor and deputy head of the Department of Chemical and Biomolecular Engineering at UM, told BioWorld Today.

"Therefore, even though AMPs have been in development for some years, they have not been successful enough to become an effective alternative to antibiotics," said Qiao, who is also the assistant dean of research at the UM School of Engineering.

He further noted that AMPs, which usually comprise about 20 amino acid units, are produced using a peptide synthesizer, while "SNAPPs are made using a conventional polymerization method and are therefore much cheaper to make. SNAPPs are also much larger molecules than both AMPs and antibiotics."

In their study published In the Sept. 12, 2016, issue of Nature Microbiology, Qiao and his team demonstrated for the first time that two SNAPP molecules had excellent activity against all of the gram-negative bacteria tested, including MDR superbugs, with only low toxicity.

Moreover, comprehensive microscopy and bioassay analyses demonstrated that the antimicrobial activity of the SNAPPs was via a multimodal mechanism of cell wall death due to outer membrane destabilization, unregulated movement of ions across the cytoplasmic membrane, and induction of the apoptotic-like cell death pathway. The multimodal mechanism of action may account for why no resistance to the SNAPPs was observed, including in MDR bacteria.

Of particular significance, the SNAPPs exhibited highly effective submicromolar activity against nosocomial pathogens, in particular CMDR A. baumannii infections in vivo in a mouse model of peritonitis.

Colistin is a polymyxin antibiotic, a class that was considered the antibiotics of last resort for treating gram-negative infections in humans until recently. However, a gene conferring resistance to the polymyxins was recently reported in China and Denmark. (See BioWorld Today, Dec. 1, 2016, and Dec. 8, 2016.)

"We cultured our SNAPPs with A. baumannii at a sublethal concentration over 24 days, which is equivalent to 600 generations, and the bacteria did not develop resistance," Qiao said. "This is the first time that a peptide polymer, rather than an antibiotic, has been shown to be capable of killing drug-resistant superbugs in vivo, without killing the animal."

Indeed, SNAPPs "are much less toxic than the AMPs developed in the past, especially considering the fact that tested animals could survive the high dose we used to kill the superbugs."

While the reasons for this low toxicity remain unclear, "our best understanding is that the SNAPP is a much bigger [molecule than an AMP], which poses less toxicity. Also the large size may enable the SNAPP to cross the gram-negative bacterial protective lipopolysaccharide layer and disrupt the outer membrane," suggested Qiao.

"Having low toxicity is the first step toward the SNAPP entering human trials, but we would like to further reduce its toxicity to human cells without losing its superbug-killing ability in future research," he said.

"Therefore, in the near future, we will be further optimizing our formula to reduce its toxicity and hopefully develop a lead peptide polymer that could be ready for human trials, but I think we need at least five years before we can begin such trials."