Abstract
Most small molecules are unable to rapidly traverse the outer membrane of Gram-negative bacteria and accumulate inside these cells, making the discovery of much-needed drugs against these pathogens challenging. Current understanding of the physicochemical properties that dictate small-molecule accumulation in Gram-negative bacteria is largely based on retrospective analyses of antibacterial agents, which suggest that polarity and molecular weight are key factors. Here we assess the ability of over 180 diverse compounds to accumulate in Escherichia coli. Computational analysis of the results reveals major differences from the retrospective studies, namely that the small molecules that are most likely to accumulate contain an amine, are amphiphilic and rigid, and have low globularity. These guidelines were then applied to convert deoxynybomycin, a natural product that is active only against Gram-positive organisms, into an antibiotic with activity against a diverse panel of multi-drug-resistant Gram-negative pathogens. We anticipate that these findings will aid in the discovery and development of antibiotics against Gram-negative bacteria.
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Acknowledgements
We thank L. Li (Metabolomics Center, Roy J. Carver Biotechnology Center, UIUC) for LC–MS/MS analysis, L. Zechiedrich (Baylor College of Medicine) for the E. coli clinical isolates, C. Vanderpool (UIUC) for E. coli MG1655, and W. van der Donk (UIUC) for E. cloacae. We also thank K. Hull, S. Denmark, K. Morrison, R. Hicklin, H. Roth, B. Nakamura, A. Deets and A. Keyes for providing valuable compounds for the test set, and we thank M. Lambrecht for NMR expertise. This work was funded by the UIUC, including funds obtained through the Office of Technology Management Proof-of-Concept award. M.F.R. is a NSF predoctoral fellow. M.F.R., A.G., and R.L.S. are members of the NIH Chemistry-Biology Interface Training Grant (NRSA 1-T32-GM070421). A.P.R. is an NIH postdoctoral fellow. T.S. was supported by the Kao Corporation. Computer time was provided by the Texas Advanced Computing Center through Grant TG-CHE160050 funded by the NSF.
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P.J.H., M.F.R. and B.S.D. conceived the study. M.F.R. performed accumulation analyses. B.S.D. performed the computational analyses. M.F.R., B.S.D, A.P.R., A.G., T.S. and R.L.S. synthesized compounds in the test set. A.P.R. synthesized and tested DNM derivatives. P.J.H. supervised this research and wrote this manuscript with the assistance of M.F.R., B.S.D. and A.P.R.
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Extended data figures and tables
Extended Data Figure 1 ClogD7.4 and molecular weight do not correlate with accumulation of primary amines.
a, b, The accumulation of 68 primary amine containing compounds in E. coli MG1655 compared to ClogD7.4 (a) and molecular weight (b). High-accumulating controls (black x) = tetracycline, ciprofloxacin and chloramphenicol; low-accumulating controls (open circles) = novobiocin, erythromycin, rifampicin, clindamycin, mupirocin and fusidic acid, clindamycin and ampicillin. Statistical significance was determined by using a two-sample Welch’s t-test (one-tailed test, assuming unequal variance) relative to the negative controls. P values are relative to the average of the low-accumulating controls. *P < 0.05, **P < 0.01. Structures of all 68 compounds are shown in Supplementary Table 4. All experiments were performed in biological triplicate and s.e.m. values are provided in Supplementary Table 4.
Extended Data Figure 2 Training and performance of random forest classification model.
a, ROC plot comparing cross-validation methods in training classification model. b, Tuning plot optimizing for number of selected predictors. c, Confusion matrix of classification model using repeated tenfold cross-validation (n = 20). d, Relative importance of top 15 predictors. Predictors related to flexibility, shape, and amphiphilicity are highlighted in red.
Extended Data Figure 3 Accumulation of paired set of compounds containing one or two primary amines.
Error bars equal s.e.m. Statistical significance was determined by using a two-sample Student’s t-test (two-tailed test, assuming equal variance). *P < 0.05, **P < 0.01. All experiments were performed in biological triplicate.
Extended Data Figure 4 Additional scatter plots of small-molecule bacterial accumulators based on molecular shape descriptors.
a, Scatter plot of primary amines (compounds from Fig. 4a) according to rotatable bond count and average distance to the plane-of-best-fit (PBF)45. b, Scatter plot of primary amines (compounds from Fig. 4a) according to rotatable bond count and normalized principal moment of inertia (PMI1/molecular weight)47. c, Scatter plot of antibiotics (same compounds from Fig. 4b, plus β-lactams that are active against Gram-negative bacteria) according to rotatable bond count and PBF45. d, Scatter plot of antibiotics (same compounds from Fig. 4b, plus β-lactams that are active against Gram-negative bacteria) according to rotatable bond count and normalized principal moment of inertia (PMI1/molecular weight). e, Same plot as Fig. 4b, but this time the β-lactams that are active against Gram-negative bacteria are included. Structures of all antibiotics are in Supplementary Tables 6, 7.
Extended Data Figure 5 Mechanistic studies.
a, SDS–PAGE analysis of outer membrane proteins of E. coli BW25113 and E. coli BW25113 ∆ompR from the KEIO collection. E. coli outer membrane proteins were separated using SDS-12% PAGE, and visualized with Coomassie stain. Lane 1, protein ladder; lane 2, E. coli BW25113; lane 3, E. coli BW25113 ∆ompR. Representative of three replicates. b, Accumulation comparison of 12 compounds for E. coli ∆ompR vs parental strain E. coli BW25113. c, Small-molecule accumulation in protoplasts prepared from E. coli MG1655. Test compounds and known antibiotics are able to accumulate in the protoplast. ADP-HPD61,62, a non-cell permeable pyrophosphate, was used as a negative control. Compounds are indicated by the number assigned in the manuscript or the Supplementary Tables. For the compounds in the Supplementary Tables (whose structures are not shown in the manuscript), the structures are shown below the graph. Statistical significance was determined by using a two-sample Student’s t-test (two-tailed test, assuming equal variance). *P < 0.05, **P < 0.01, ***P < 0.001. All experiments were performed in biological triplicate. Error bars represent s.e.m.
Extended Data Figure 6
a, Distance of primary amine of compound 1 and constriction site residues in course of simulation. Stabilized distance at ~2 Å is indicative of hydrogen bonding. In repeated simulations molecules adopted similar pathways for traversing the porin while often hydrogen bonding with different hydrophilic residues. b, Displacement of Arg42 by compound 1 relative to initial coordinates was measured during course of simulation. c, Displacement of Arg42 by compound 13. d, Distance of primary amine of 6DNM-NH3 and constriction site residues in course of simulation. e, The displacement of Asp113 by 6DNM-NH3. f, Displacement of Asp113 by 6DNM.
Extended Data Figure 7 Snapshots of SMD simulation of compound translocation through OmpF.
a, Compound 1 is capable of making a key interaction with Asp113 (purple) that assists in movement past constriction site. This finding is in accord with previous reports of the importance of Asp113 in producing the cation selectivity of OmpF63,64. b, Same pose as a, viewed from above. c, Compound 13 makes no interactions with Asp113 and thus faces additional barriers to penetrance. d, Same pose as c, viewed from above. e, 6DNM-NH3 makes key H-bonding interactions with Asp113 (purple) that assists in the molecule’s passage through the constriction site. f, Same pose as e, viewed from above. g, 6DNM requires larger movement by peptide backbone in order to pass through constriction site. Asp113 is appreciably displaced by 6DNM. h, Same pose as g, viewed from above. In all simulations, the largest barrier to translocation is a series of stacked arginine residues (Arg 42, 82, and 132) that are depicted in blue.
Extended Data Figure 8
a, Gram-positive-only antibiotics with proper rotatable bond and globularity scores can be converted to broad-spectrum drugs via strategic placement of a primary amine, as demonstrated by ampicillin. However, for drugs that do not possess the proper flexibility and three-dimensionality parameters, such as erythromycin, addition of the amine does not broaden the antibacterial spectrum. b, Strategic placement of a primary amine (but not an acid or amide) on 6DNM leads to 6DNM-NH3, a broad-spectrum antibacterial. 6DNM and three derivatives were synthesized and evaluated against a panel of Gram-positive and Gram-negative organisms; ciprofloxacin was also evaluated. Flexibility (as measured by the number of rotatable bonds, RB), globularity (Glob), molecular weight, ClogD7.4, and E. coli accumulation (in nmol per 1012 CFUs) is provided. The s.e.m. for accumulation values are reported. MIC values were determined using the micro-dilution broth method as outlined by the Clinical and Laboratory Standards Institute and are listed in μg ml−1. All experiments were performed in biological triplicate.
Extended Data Figure 9 Some FDA-approved drugs or compounds in phase II or III clinical trials that are good candidates for derivative synthesis and expansion to broad-spectrum agents.
Placement of a primary amine at a position that does not alter interaction with the biological target is predicted to provide compounds that accumulate in and are active against Gram-negative bacteria.
Extended Data Figure 10 Primary amines are largely absent from many screening collections.
Comparison of the abundance of primary amines, secondary amines, tertiary amines, and carboxylic acids in the natural products subset of the ZINC database, the Molecular Libraries Small-molecule Repository – Natural Products (MLSMR-NP), and the Chembridge Microformat library.
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Supplementary Data
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Richter, M., Drown, B., Riley, A. et al. Predictive compound accumulation rules yield a broad-spectrum antibiotic. Nature 545, 299–304 (2017). https://doi.org/10.1038/nature22308
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DOI: https://doi.org/10.1038/nature22308
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