Tetracyclines

Discovery

Tetracyclines are natural products, discovered from the fermentation of actinomycetes (soil bacteria). Benjamin Duggar identified the first tetracycline, called Chlortetracycline, a product of Streptomyces aureofaciens. It was used clinically in the late 1940s and marketed as Aureomycin (Grossman 2016). Scientists in Pfizer (New York) isolated a related compound, oxytetracycline, from S. rimosus, which was approved for clinical use in the 1950s and marketed as Terramycin (Chopra and Roberts, 2001). The name terramycin was possibly derived from the Latin root Terra - meaning soil (Nelson and Levy 2011). Additional molecules of this class of antibiotics were identified - e.g., tetracycline, and demeclocycline.

In addition to being used as antibiotics, tetracycline molecules have non-antibiotic uses - e.g., in treating inflammation-based diseases (Nelson and Leavy 2011). The US FDA approved the use of low doses of doxycycline to treat chronic gum inflammation. Since tetracycline antibiotics are relatively inexpensive, they have been used, both prophylactically and for treating human and animal infections. Tetracyclines are used at sub-therapeutic levels in animal feed as growth promoters (Chopra and Roberts, 2001)! Such practices have likely led to the evolution, selection, and spread of tetracycline resistance via various mechanisms.

Overview of Chemistry

Tetracyclines are a class of polyketide antibiotic that inhibits the 30S subunit of bacterial ribosomes (Chopra and Roberts, 2001). A linear fused tetracyclic nucleus (with rings designated A, B, C, and D) is present in all members of this class of antibiotics, to which various functional groups are attached. The absolute minimum core structure of this class of antibiotics is 6-deoxy-6-demethyltetracycline (Figure 1). Note, that the tetracycline molecule has additional methyl and hydroxyl groups at position 6.

The molecule has two regions or faces (Nelson and Levy, 2011; Fuoco, 2012) - (a) upper peripheral - marked with a shaded yellow region, and (b) lower peripheral - marked with shaded blue. The lower peripheral region has several hydrogen bonding donor/acceptor atoms that can coordinate metal ions and are invariable in this class of molecules. On the other hand, the upper peripheral region is less polar (Chopra and Roberts, 2001) and may be modified with various substitutions in other members of this class. Overall the antibiotic binds to rRNA via a combination of hydrogen bonds, metal coordinations (with Mg²⁺ atoms), pi-stacking, and van-der-Waals interactions.

Figure 1. 2D structure of the core of tetracycline antibiotics (6-deoxy-6-demethyltetracycline). The four rings in the structure are labeled A-D in red and the carbon atom numbers are marked with blue numbers.

Types

Most of the first generation of tetracyclines were naturally produced - e.g., chlortetracycline, oxytetracycline, and tetracycline (Rusu and Buta 2021). Through various substitutions in the upper peripheral region (especially at positions 7 and 9), the molecule's antibacterial and other properties were enhanced. Thus, the second generation of antibiotics, created semi-synthetically, include - e.g., doxycycline, minocycline. In the 1980s, research showed that adding longer/bulkier substitutions to the tetracycline core improved its activity. Thus, the third generation of semi-synthetically produced molecules in this class include - e.g., Tigecycline, omadacycline, and sarecycline. Even newer members of the family are synthetically produced - e.g., Eravacycline (Rusu and Buta 2021). The antibiotics in this class are more potent and can be expected to fight the tetracycline-resistant pathogens.

Resistance

Widespread use of tetracyclines has led to the development of bacterial resistance through plasmid-encoded tetracycline resistance genes (tet). These plasmids may be transferred horizontally through conjugated transposons and integrons across species/generations (Rusu and Buta 2021). The four main resistance mechanisms that appear include - (a) Efflux Pumps (e.g., tetA, tetB, tetK, tetL); (b) Ribosomal Protection (e.g., tetM, tetO genes); (c) Chemical Inactivation (e.g., tetX); and (d) rRNA Mutations (at specific positions where the antibiotic binds.

Learn more about tetracycline resistance.

References

Chopra, I., Roberts, M. (2001). Tetracycline antibiotics: mode of action, applications, molecular biology, and epidemiology of bacterial resistance. Microbiology and molecular biology reviews : MMBR, 65(2), 232–260. https://doi.org/10.1128/mmbr.65.2.232-260.2001

Fuoco, D. (2012) Classification Framework and Chemical Biology of Tetracycline-Structure-Based Drugs. Antibiotics (Basel). 1(1):1-13. https://doi.org/10.3390/antibiotics1010001

Grossman, T. H. (2016) Tetracycline Antibiotics and Resistance. Cold Spring Harb Perspect Med. 6(4):a025387. https://doi.org/10.1101/cshperspect.a025387

Nelson, M. L., Levy, S. B. (2011) The history of the tetracyclines. Ann N Y Acad Sci. 1241:17-32. https://doi.org/10.1111/j.1749-6632.2011.06354.x

Rusu, A., Buta, E. L. (2021) The Development of Third-Generation Tetracycline Antibiotics and New Perspectives. Pharmaceutics. 13(12):2085. https://doi.org/10.3390/pharmaceutics13122085