Metronidazole: an update on metabolism, structure-cytotoxicity and resistance mechanisms.

Metronidazole, a nitroimidazole, remains a front-line choice for treatment of infections related to inflammatory disorders of the gastrointestinal tract including colitis linked to Clostridium difficile. Despite >60 years of research, the metabolism of metronidazole and associated cytotoxicity is not definitively characterized. Nitroimidazoles are prodrugs that are reductively activated (the nitro group is reduced) under low oxygen tension, leading to imidazole fragmentation and cytotoxicity. It remains unclear if nitroimidazole reduction (activation) contributes to the cytotoxicity profile, or whether subsequent fragmentation of the imidazole ring and formed metabolites alone mediate cytotoxicity. A molecular mechanism underpinning high level (>256 mg/L) bacterial resistance to metronidazole also remains elusive. Considering the widespread use of metronidazole and other nitroimidazoles, this review was undertaken to emphasize the structure-cytotoxicity profile of the numerous metabolites of metronidazole in human and murine models and to examine conflicting reports regarding metabolite-DNA interactions. An alternative hypothesis, that DNA synthesis and repair of existing DNA is indirectly inhibited by metronidazole is proposed. Prokaryotic metabolism of metronidazole is detailed to discuss new resistance mechanisms. Additionally, the review contextualizes the history and current use of metronidazole, rates of metronidazole resistance including metronidazole MDR as well as the biosynthesis of azomycin, the natural precursor of metronidazole. Changes in the gastrointestinal microbiome and the host after metronidazole administration are also reviewed. Finally, novel nitroimidazoles and new antibiotic strategies are discussed.

Amino acid-linked porphyrin-nitroimidazole antibiotics targeting Porphyromonas gingivalis.

The periodontal pathogen Porphyromonas gingivalis requires porphyrin supplementation for growth. Previously, in order to inhibit P. gingivalis growth, we synthesised very effective 'Trojan horse' ester and amide-linked deuterporphyrin-nitroimidazole (DPIX-Nim) adducts that exploited this requirement to transport metronidazole-derived antibiotics with excellent antimicrobial selectivity and recognition by the HA2 porphyrin binding site. Herein, in the context of developing topical agents to target P. gingivalis, l-amino acids are incorporated into adducts as linkers to improve uptake. Ten 13- and 17-propionic amide regioisomers of l-amino acid-linked deuterporphyrin-nitroimidazole adducts were synthesised using a peptide coupling approach. DPIX-Lys regioisomers without attached nitroimidazole were also synthesised as comparison compounds. All the porphyrin adducts bound (Kd50 7 to 20 nM) to a recombinant HA2 receptor with similar binding affinity to haem, except the lysine-proline linked DPIX-Lys(Boc)Pro-Nim adducts (Kd50 300 nM) and the DPIX-Lys(Nim)-Nim adducts (Kd50 200 nM), both of which have large appended groups. DPIX-Lys(Boc)-Nim, DPIX-Lys(OH)-Nim, and DPIX-Pro-Nim adducts were shown to be very effective against P. gingivalis. DPIX-Lys(Boc)Pro-Nim adducts and DPIX-Lys(Nim)-Nim adducts showed weak activity. Importantly, DPIX-Lys(Boc)-Nim adducts were selective for P. gingivalis and, unlike metronidazole, did not kill a range of other anaerobic bacteria isolated from the human gastrointestinal tract.