Carbapenem-resistant Enterobacteriaceae (CRE) May Soon Be Treatable

Globally, members of the gram-negative Enterobacteriaceae family including Klebsiella pneumoniae, Escherichia coli, and Enterobacter spp. have become increasingly resistant to traditional carbapenem antibiotics and are referred to as carbapenem-resistant Enterobacteriaceae (CRE).¹ This represents a worldwide health crisis, as people with CRE infections are at an increased risk of death,^2,3 prompting the Centers for Disease Control and Prevention to classify CRE in their highest threat level.⁴

Although it is clear that effective treatment options for CRE are in high demand, "Identifying small molecules that successfully target gram-negative organisms, especially those displaying a multidrug resistance phenotype, is notoriously difficult..." according to the authors of a recent study published in ASSAY and Drug Development Technologies.⁵

Kenneth P. Smith, MD, and James E. Kirby, MD, from the department of pathology at the Beth Israel Deaconess Medical Center in Boston conducted experiments to first develop and then validate a high-throughput screen (HTS) that could detect two kinds of antimicrobial agents at the same time, those with the ability to restore carbapenem susceptibility to resistant strains (referred to as "adjunctives" in the publication), as well as those that target CRE directly.

To develop and test their screening method, the investigators selected K. pneumoniae BIDMC12A, which is considered to be a representative multidrug-resistant CRE strain. In their experiments, 11,698 known small molecule bioactive compounds were tested to determine which, if any, had the ability to inhibit the growth of K. pneumoniae BIDMC12A in both the presence (screen) and absence (counterscreen) of meropenem, which is considered to be a representative carbapenem antibiotic.

Through these experiments, Smith and Kirby detected 14 small molecule bioactive compounds that showed adjunctive activity capable of restoring carbapenem susceptibility to K. pneumoniae BIDMC12A. An additional 79 small molecule bioactive compounds were found to display a direct antimicrobial effect. These candidate molecules were then subjected to further scrutiny through a secondary screen to determine their inhibitory percentage. The results of this analysis winnowed down the list of potential candidates even further, as only one compound, triclosan, was found to have effective adjunctive activity. Additionally, four compounds, azidothymidine (AZT), NH125, diphenyleneiodonium chloride, and spectinomycin displayed direct and potent antimicrobial activity.

Because NH125 and diphenyleneiodonium chloride are not approved for human therapy, only AZT and spectinomycin were selected for additional testing against a collection of 103 representative Enterobacteriaceae strains comprising approximately half of all known CRE. Both were found to be effective against a significant percentage of CRE strains. These two agents, a nucleoside analog (AZT) and an aminoglycoside (spectinomycin), have been used to treat human immunodeficiency virus and gonorrhea, respectively. In describing their findings, Drs. Smith and Kirby stated, "These data identified AZT and spectinomycin as available agents warranting further study for potential treatment of multidrug-resistant CRE infection."

Drs. Smith and Kirby noted that the whole-cell bioassay developed for this study, "... demonstrated robust statistical performance..." Additionally, they asserted that the screen—counterscreen method used could identify three broad groups of adjunctives, those considered to be pure adjunctives that have no direct antimicrobial action in the absence of meropenem, mixed adjunctives that display enhanced antimicrobial activity in the presence of meropenem, and synergistic adjunctives that display enhanced antimicrobial activity in the presence of meropenem.

In a summation of their results, the investigators stated, "In this study, we describe HTS data and secondary analysis for >10,000 bioactive compounds. In addition to obtaining proof of principle for our approach, we identified several available agents that may prove useful in treating CRE infection." The investigators also stated that, "Intriguingly, data generated with known bioactive libraries highlighted classes of antimicrobials, such as nucleoside analogs and aminoglycosides, that maintain activity despite many potential barriers associated with multidrug resistance." The apparent success of the methodology employed in the study and the discovery of compounds with therapeutic potential against CRE suggests that CRE infection, which currently has almost no real therapeutic options, may indeed be treatable.

William Perlman, PhD, CMPP is a former research scientist currently working as a medical/scientific content development specialist. He earned his BA in Psychology from Johns Hopkins University, his PhD in Neuroscience at UCLA, and completed three years of postdoctoral fellowship in the Neuropathology Section of the Clinical Brain Disorders Branch of the National Institute of Mental Health.

References

1. Sanchez GV, Master RN, Karlowsky JA, Bordon JM. In vitro antimicrobial resistance of urinary Escherichia coli isolates among U.S. outpatients from 2000 to 2010. Antimicrob Agents Chemother 2012;56:2181—3.
2. Morrill HJ, Pogue JM, Kaye KS, LaPlante KL: Treatment options for carbapenem-resistant enterobacteriaceae infections. Open Forum Infect Dis 2015;2:ofv050.
3. Falagas ME, Tansarli GS, Karageorgopoulos DE, Vardakas KZ: Deaths attributable to carbapenem-resistant Enterobacteriaceae infections. Emerg Infect Dis 2014;20:1170—5.
4. Centers for Disease Control and Prevention: Antibiotic/antimicrobial resistance biggest threats. Available at: www.cdc.gov/drugresistance/biggest_threats.html. Accessed May 5, 2016.
5. Smith KP, Kirby JE. Validation of a High-Throughput Screening Assay for Identification of Adjunctive and Directly Acting Antimicrobials Targeting Carbapenem-Resistant Enterobacteriaceae. Assay Drug Dev Technol 2016;14:194—206.