Studies on daptomycin lactam-based analogues.

Herein, we report the synthesis and antibacterial evaluation of a series of daptomycin lactam-based analogues. As compared with daptomycin, the daptomycin analogue with singly modified lactam has an eightfold increase in its minimum inhibitory concentration (MIC) against methicillin-resistant Staphylococcus aureus. Incorporating effective modifications found in previous daptomycin structure-activity relationship studies to produce lactam-based analogues with multiple modifications did not improve the antibacterial activity of the analogues. Instead, the antibacterial activity was greatly reduced when a rather rigid 4-(phenylethynyl)benzoyl group replaced the flexible n-decanoyl group. The fact that the lactam analogue with the 4-(phenylethynyl)benzoyl group did not exhibit the antibacterial activity comparable to the two respective singly modified analogues showed that the inactivity was probably due to the deviation from the active conformation. This series of lactam analogues may generate insights on the importance of studying the active conformation of daptomycin and how the structural modifications affect the active conformation.

Total synthesis and a systematic structure-activity relationship study of WAP-8294A2.

WAP-8294A2 is a cyclic peptide antibiotic with novel structure and excellent activity against Gram-positive pathogens. Herein, we report the total synthesis of complex macrocyclic peptide WAP-8294A2 (W1), ent-analogue W2, deoxy analogue W3 and de-methyl analogue W4 using a solid-phase synthetic route followed by a final stage solution-phase cyclization reaction. Exploitation of this process allowed the synthesis of eleven alanine-scanning analogues and eight lysine-scanning analogues. The antimicrobial activity of these analogues was evaluated in vitro against Gram-positive bacteria. Based on the MIC results, a primary systematic structure-activity relationship has been established.

Total Synthesis of Malacidin A by ß-Hydroxyaspartic Acid Ligation-Mediated Cyclization and Absolute Structure Establishment.

The development of novel antibiotics is critical to combating the growing emergence of drug-resistant pathogens. Malacidin A is a new member of the calcium-dependent antibiotic (CDAs) family with activity against antibiotic-resistant pathogens. Its mode of action is distinct from classical CDAs. However, the absolute structure of malacidin A has not been established. Herein, the total syntheses of malacidin A and its analogues are reported by a combination of Fmoc-based solid-phase peptide synthesis (SPPS) and ß-hydroxyaspartic acid ligation-mediated peptide cyclization. The total synthesis enabled us to establish the absolute configuration of malacidin A, which is in agreement with those for natural malacidin A confirmed by advanced Marfey's analysis in our study.

Synthesis and antibacterial studies of teixobactin analogues with non-isostere substitution of enduracididine.

Teixobactin is a structurally and mechanistically novel antimicrobial peptide with potent activities against Gram-positive pathogens. It contains l-allo-enduracididine (End) residue which is not readily accessible. In this report, we have used convergent Ser Ligation as the key step to prepare a series of teixobactin analogues with End being substituted with its non-isostere moieties. Among these analogues, compounds T16, T27 and T29 exhibited the best antimicrobial activities against different Gram-positive bacteria with MICs ranging from 0.25 to 1.0 µM. Structure-activity relationship is also established for further development of more promising teixobactin analogues.

Functional Characterization of CTX-M-14 and CTX-M-15 ß-Lactamases by In Vitro DNA Shuffling.

This work investigated the molecular events driving the evolution of the CTX-M-type ß-lactamases by the use of DNA shuffling of fragments of the blaCTX-M-14 and blaCTX-M-15 genes. Analysis of a total of 51 hybrid enzymes showed that enzymatic activity could be maintained in most cases, yet hybrids that were active possessed fewer amino acid substitutions than those that were inactive, suggesting that point mutations in the constructs rather than reshuffling of the fragments of the two target genes would more likely cause disruption of CTX-M activity. For example, the P67L and L261P changes in a CTX-M-14 fragment could completely abolish the activity of the enzyme on all antibiotics tested. Structural analysis showed that L216 was located in the active-site ß sheet and might interact with the adjacent hydrophobic residues to stabilize the active-site ß sheet and maintain the integrity of the enzyme active site. Likewise, a single amino acid substitution, E64K, was found to exhibit a significant suppressive effect on CTX-M-15 activity. Structural analysis showed that E64 might form a salt bridge with R44, disruption of which might affect CTX-M-15 activity. Further analysis of the structure-function relationship of a range of mutant enzymes confirmed that, as can be expected, unstable enzymes lose their activity and avoid selective events. These findings suggest that the distal pockets could also contribute to the activity of the enzymes and may be regarded as alternative targets for inhibitor development.

Synthesis and structure-activity relationship of teixobactin analogues via convergent Ser ligation.

Convergent Ser/Thr ligation has been used to prepare a series of teixobactin analogues (28 in total) to establish a structure-activity relationship of teixobactin. anti-bacterial evaluations of these synthetic analogues have revealed the critical amino acid residues and the sites tolerable of modifications. These studies will shed lights on the further development of teixobactin analogues with improved antibacterial activities.

Mutational Analysis of Quinolone Resistance Protein QnrVC7 Provides Novel Insights into the Structure-Activity Relationship of Qnr Proteins.

This study assessed the functional importance of residues located at the i(-2) position of face 4 of the tandem repeat loops of the quinolone resistance protein QnrVC7 through mutagenesis studies. The i(-2) position of face 4 on different coils required residues with different natures. Some substitutions reduced the protective activity of QnrVC7, while some of them increased it. These findings advanced our understanding on the detailed structural organization and functional requirements of Qnr proteins.

Detection of SARS-CoV-2 VOC-Omicron using commercial sample-to-answer real-time RT-PCR platforms and melting curve-based SNP assays.

Objectives: The World Health Organization (WHO) had designated the SARS-CoV-2 lineage B.1.1.529 as the new Variant of Concern Omicron (VOC-Omicron) on 26th November 20211. Real-time reverse transcription polymerase chain reaction (RT-PCR), single nucleotide polymorphisms (SNP) and whole genome sequencing (WGS) tests were widely employed to detect SARS-CoV-2 and its variant. Yet, the SARS-CoV-2 Omicron detection performance of commercial real-time RT-PCR platforms and SARS-CoV-2 spike SNP assays remain to be elucidated. Methods: In the first part of this study, we evaluated the VOC-Omicron detection performance of three commercial RT-PCR sample-to-answer platforms i.e. Roche cobas® 6800/8800, Roche cobas® Liat®, and Cepheid GeneXpert® systems. The detection performances were compared to one commercial conventional real-time RT-PCR assay (TIB MOLBIOL LightMix Modular SARS and Wuhan CoV E-gene) and one in-house real-time RT-PCR assay targeting RNA-dependent RNA polymerase (RdRP) gene of SARS-CoV-2 in the WHO COVID-19 Reference Laboratory at Public Health Laboratory Services Branch, Centre for Health Protection, Department of Health, The Government of the Hong Kong Special Administrative Region. In the second part of this study, we evaluated the SNP detection performance of four TIB MOLBIOL melting curve-based assays (1. Spike S371L/S373P, 2. Spike E484A, 3. Spike E484K and 4. Spike N501Y) in clinical samples obtained from hospitalized COVID-19 patients in Hong Kong. The SNP results were compared to whole genome sequences generated by Illumina platform. Results: The VOC-Omicron detection limits of three commercial sample-to-answer assays were tested to be ≤ 2.35 Log10 dC/ml. The detection performances of the sample-to-answer platforms were comparable to the two tested conventional real-time RT-PCR assays. The test sensitivities of TIB MOLBIOL VirSNiP SARS-CoV-2 Spike S371L/S373P assay and the Spike E484A assays were 100% and 96.6% respectively and the test specificities of both assays were 100%. An aberrant melting peak at Tm 42-44°C was observed when the specimens with Omicron variant were tested with the TIB MOLBIOL VirSNiP SARS-CoV-2 Spike E484K assay. Notably, the TIB MOLBIOL VirSNiP SARS-CoV-2 Spike N501Y assay failed to detect the spike N501Y mutation of Omicron variant in the tested specimens. Conclusions: The SARS-CoV-2 detection sensitivity of three commercial platforms, Roche cobas® 6800/8800, Roche cobas® Liat®, and Cepheid GeneXpert® systems were shown not to be impacted by the large number of mutations of VOC-Omicron. Also, the signature mutations i.e. Spike S371L/Spike S373P and Spike E484A in VOC-Omicron were correctly identified by the TIB MOLBIOL VirSNiP SARS-CoV-2 Spike S371L/S373P and VirSNiP SARS-CoV-2 Spike E484A assays. Unexpected findings including a shifted melting peak or absence of amplification curve/melting peak were observed when specimens with Omicron variant were tested with the TIB MOLBIOL VirSNiP SARS-CoV-2 Spike E484K assay and Spike N501Y assay, suggesting a potential alert for Omicron variant, prior confirmation by whole genome sequencing.

Convergent Synthesis of Calcium-Dependent Antibiotic CDA3a and Analogues with Improved Antibacterial Activity via Late-Stage Serine Ligation.

A convergent synthesis via the late-stage serine ligation of naturally occurring calcium-dependent antibiotic CDA3a and its analogues has been developed, which allowed us to readily synthesize the analogues with the variation on the lipid tail. Some analogues were found to show 100-500-fold higher antimicrobial activity than the natural compound CDA3a against drug resistant bacteria. This study will enhance our understanding of CDA3a and provide valuable antibacterial lead candidates for further development.

Establishing the Structure-Activity Relationship of Daptomycin.

Daptomycin is effective in treating infections caused by antibiotic-resistant Gram-positive pathogens, including methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococci (VRE), and vancomycin-resistant S. aureus (VRSA). Due to its distinct mechanism of action toward multidrug-resistant bacteria, daptomycin provides an attractive structural motif to generate new daptomycin-based antibiotics to combat the problem of bacterial resistance. In this study, we used the total synthesis method to produce daptomycin analogues with a variety in terms of types and sites of modifications. Five classes of daptomycin analogues were synthesized, and the antimicrobial activities of the analogues were analyzed by several biological assays. From this study, we established a comprehensive structure-activity relationship of daptomycin which will lay the foundation for the further development of daptomycin-based antibiotics.

Methylation of Daptomycin Leading to the Discovery of Kynomycin, a Cyclic Lipodepsipeptide Active against Resistant Pathogens.

Increased usage of daptomycin to treat infections caused by Gram-positive bacterial pathogens has resulted in emergence of resistant mutants. In a search for more effective daptomycin analogues through medicinal chemistry studies, we found that methylation at the nonproteinogenic amino acid kynurenine in daptomycin could result in significant enhancement of antibacterial activity. Termed "kynomycin," this new antibiotic exhibits higher antibacterial activity than daptomycin and is able to eradicate methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus (VRE) strains, including daptomycin-resistant strains. The improved antimicrobial activity of kynomycin was demonstrated in in vitro time-killing assay, in vivo wax worm model, and different mouse infection models. The increased antibacterial activity, improved pharmacokinetics, and lower cytotoxicity of kynomycin, compared to daptomycin, showed the promise of the future design and development of next-generation daptomycin-based antibiotics.

Total Synthesis and Structural Establishment/Revision of Antibiotics A54145.

A54145 is a family of antibacterial cyclic lipodepsipeptides structurally resembling daptomycin. Since its discovery in 1990, only the ambiguous structures of the methoxy-aspartic acid (MeO-Asp) and the hydroxy-asparagine (HO-Asn) have been reported. We have developed efficient routes to obtain the fully protected l-MeO-Asp and l-HO-Asn building blocks compatible with Fmoc-SPPS, and a total synthesis of A54145 that enabled us to establish its structure, consisting of l-3S-HO-Asn and l-3R-MeO-Asp, revising the wrongly proposed structure of l-3S-MeO-Asp.

Characterization of Protein Domain Function via in vitro DNA Shuffling.

We recently investigated the molecular events that drive evolution of the CTX-M-type ß-lactamases by DNA shuffling of fragments of the bla CTX-M-14 and bla CTX-M-15 genes. Analysis of a total of 51 hybrid enzymes showed that enzymatic activity could be maintained in most cases, yet the enzymatically active hybrids were found to possess much fewer amino acid substitutions than the few hybrids that became inactive, suggesting that point mutations in the constructs rather than reshuffling of the fragments of the two target genes would more likely cause disruption of CTX-M activity. Certain important residues that played important functional roles in mediating enzyme activity were identified. These findings suggest that DNA shuffling is an effective approach to identify and characterize important functional domains in bacterial proteins.

Total synthesis of teixobactin.

To cope with the global bacterial multidrug resistance, scientific communities have devoted significant efforts to develop novel antibiotics, particularly those with new modes of actions. Teixobactin, recently isolated from uncultured bacteria, is considered as a promising first-in-class drug candidate for clinical development. Herein, we report its total synthesis by a highly convergent Ser ligation approach and this strategy allows us to prepare several analogues of the natural product.