Antimicrobial peptides are a class of natural molecules with unique anti-infective properties. While they've been viewed as a potential solution to the urgent problem of multidrug resistant infections, safety and stability issues have stymied their development for clinical use.
Now, scientists at Dana-Farber Cancer Institute say they have designed antimicrobial peptides (AMPs) that resolve a major safety challenge of current AMPs, which kill bacteria by penetrating their outer membranes but also damage the membranes of mammalian cells, including red blood cells and other normal tissues.
The researchers used chemical "staples" to create membrane-selective antimicrobial peptides, termed stapled AMPs (StAMPs), which are stable, potent, and punch holes in bacterial membranes to kill the germs while avoiding harm to the membranes of red blood cells and kidney cells. The team, headed by Loren D. Walensky, MD, PhD, demonstrated that one such StAMP cured the majority of mice infected with a highly drug-resistant Gram-negative bacterium, and did not cause toxic side effects. "We hope that our results will inspire the advancement of StAMPs as a novel class of antibiotics to combat multidrug resistant infections," said the investigators, reporting in Nature Biotechnology.
Resistance to existing antibiotics has become a serious, global threat to the point that treatment options have become very limited for some types of infections, especially with Gram-negative bacteria, which have outer membranes that shield them from many antibiotics. "By applying stapling technology, we hoped to tap into the full potential of antimicrobial peptides as a therapeutic alternative to traditional antibiotics," said Rida Mourtada, PhD, lead author of the study. Importantly, the investigators found that the bacteria could not develop resistance to StAMPs, likely because the target is the membrane itself rather than a particular molecule.
Antimicrobial peptides are naturally produced by all classes of organisms as a defense against invading microorganisms. They are found, for example, in plants, insects and vertebrates including humans. Their ability to kill a wide range of microorganisms without causing drug resistance has made antimicrobial peptides a prime candidate for clinical development. "However, the longstanding barrier to translating natural antimicrobial peptides has been the indiscriminate nature of membrane destruction, causing non-specific toxicity to normal tissues," said Walensky.
The antimicrobial peptides are made up of short chains of amino acids. They are linear but in the presence of membranes can fold into different secondary structures, some of which are called alpha-helices, a coiled form. Walensky and his colleagues have long studied the properties of helical peptides and have used chemical "staples" - made of hydrocarbon molecules - to brace peptides into stable conformations that recapitulate their biological properties. In their current research, they investigated how the properties of diverse stapled helical antimicrobial peptides determined their ability to penetrate bacterial membranes - and how they could be modified to avoid damaging normal cell membranes.
The particular antimicrobial peptide they worked with is called Magainin-II. Integrating laboratory experimentation and computer simulation, the scientists studied the effects of inserting chemical staples at various locations along the length of the peptide helix. Ultimately, they found that inserting two hydrocarbon staples at specific points along the helix yielded an antimicrobial peptide that was stable, resistant to being destroyed by enzymes, was a potent weapon against multidrug resistant Gram-negative bacteria, and caused little or no damage to red blood cells or kidney cells in culture.
Laboratory testing of this lead compound in panels of Gram-negative bacteria demonstrated that it was highly effective, even against the most drug-resistant bacterial isolates provided by the Centers for Disease Control and Prevention.
In mouse experiments, the StAMP proved capable of curing 75 percent of mice infected with a highly drug-resistant Gram-negative bacterium using only two doses of the compound, and caused no harmful effects.
The authors concluded that peptide stapling can be used to create antimicrobial peptides "that can kill even the most resistant Gram-negative bacteria and be administered internally without the toxic side effects that have long thwarted the clinical translation of AMPs."
The research was supported in part by National Institutes of Health grant R35CA197583 and a Leukemia and Lymphoma Society Scholar Award to Walensky and NIH grant R01GM101135.
Walensky is a scientific advisory board member and consultant for Aileron Therapeutics.
