Azithromycin
Drug Name
Azithromycin is a semi-synthetic antibiotic that is a member of the macrolide class. It inhibits bacterial protein synthesis by binding in the 50S ribosomal subunit and blocking the egress of polypeptide chains. It is a broad-spectrum drug that is potent against gram-positive and gram-negative bacteria.
Table 1. Basic profile of azithromycin.
| Description | Broad-spectrum macrolide antibiotic |
| Target | Ribosome (50S subunit) |
| Generic | Azithromycin |
| Commercial Name | Zithromax |
| Combination Drug(s) | Rifabutin, ethambutol |
| Other Synonyms | N/A |
| IUPAC Name | (2R,3S,4R,5R,8R,10R,11R,12S,13S,14R)-11-[(2S,3R,4S,6R)-4-(dimethylamino)-3-hydroxy-6-methyloxan-2-yl]oxy-2-ethyl-3,4,10-trihydroxy-13-[(2R,4R,5S,6S)-5-hydroxy-4-methoxy-4,6-dimethyloxan-2-yl]oxy-3,5,6,8,10,12,14-heptamethyl-1-oxa-6-azacyclopentadecan-15-one |
| Ligand Code in PDB | ZIT |
| PDB Structure | 4v7y (Structure of the Thermus thermophilus 70S ribosome complexed with azithromycin) |
| ATC code | J01FA10 |
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Antibiotic Chemistry
The unique structural features of azithromycin allow it to bind in the ribosomal exit tunnel and provide the drug with potency against gram-positive and gram-negative bacteria. It contains a desosamine and cladinose sugar attached to a 15-membered lactone ring (Figure 2).
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| Figure 2. Chemical structure of azithromycin. Key atoms or chemical groups are colored and labeled. Structure created using ChemDraw. |
Drug Information
Table 2. Chemical and physical properties (DrugBank)
| Chemical Formula | C38H72N2O12 |
| Molecular Weight | 748.98 g/mol |
| Calculated Predicted Partition Coefficient: cLogP | 3.03 |
| Calculated Predicted Aqueous Solubility: cLogS | -3.2 |
| Solubility (in water) | 0.514 mg/mL |
| Predicted Topological Polar Surface Area (TPSA) | 180.08 Å2 |
Drug Target
The ribosome is the macromolecular machine on which proteins are synthesized. It is targeted by many classes of antibiotics that are approved by the US FDA, including the macrolides. Azithromycin positions itself in a section of the ribosomal exit tunnel located near the peptidyl transferase center in the 50S subunit. The drug blocks the exit tunnel and prevents the egress of polypeptide chains during translation.
Learn more about protein synthesis and the ribosome here.
Drug-Target Complex
Each ribosomal subunit is composed of protein chains and rRNA. X-ray crystallography of the Thermus thermophilus ribosome bound by azithromycin (shown in Figure 3) revealed that:
* The large subunit consists of 33 protein chains which are colored dark blue
* The large subunit consists of a 23S rRNA of 2,787 nucleotides which are colored cornflower blue
* The large subunit consists of a 5S rRNA of 122 nucleotides which are colored deep sky blue
* The small subunit consists of 21 protein chains which are colored violet-red
* The small subunit consists of a 16S rRNA of 1,522 nucleotides which are colored pink
Azithromycin binds in the ribosome exit tunnel near the peptidyl transferase center. It forms four interactions to stabilize its binding to the hydrophobic surface of the ribosomal exit tunnel. The first three interactions involve hydrophobic contacts between the lactone ring of the drug and 23S rRNA in the tunnel. Three methyl groups of the lactone ring interact with a hydrophobic surface formed by A2058, A2059, and U2611. A last interaction is made between the drug and the tunnel, where the 2’ hydroxyl of the desosamine sugar forms a hydrogen bond with the nitrogen base of A2058. All four interactions tether azithromycin in the ribosomal exit tunnel and are shown in Figure 3 (Bulkley et al., 2010).
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| Figure 3. The left image shows the 70S ribosome from T. thermophilus. The inset shows the set of interactions that azithromycin makes with the 23S rRNA (PDB ID: 4v7y, Bulkley et al., 2010). |
The drug blocks the lumen of the tunnel, preventing elongating polypeptides from moving through it. This leads to tRNA drop-off where tRNAs are prematurely released from the ribosome (Bulkley et al., 2010) (Figure 4).
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| Figure 4. Azithromycin binding (PDB ID: 4v7y, Bulkley et al., 2010). a) Shows the binding of azithromycin as seen looking through the tunnel. The drug is shown in a stick representation, while the 23S rRNA is shown in a surface representation. b) The drug and 23S rRNA are shown in a surface representation to highlight how it blocks the lumen of the exit tunnel. |
Pharmacologic Properties and Safety
Table 3. Pharmacokinetics: ADMET of azithromycin.
| Feature | Comment(s) | Source |
|---|---|---|
| Oral Bioavailability (%) | 37% | DrugBank |
| IC50 (μM) | 21.8 μM | (Eberl et al., 2007) |
| Ki (μM) | N/A | N/A |
| Half-Life (hrs) | 68 hours | DrugBank |
| Duration of Action | 2-5 hours | Drugs.com |
| Absorption Site | Gastrointestinal tract | DrugBank |
| Transporter(s) | N/A | N/A |
| Metabolism | In vivo and in vitro studies to assess metabolism have not been performed, though it is eliminated by the liver | DrugBank |
| Excretion | Biliary excretion as an unchanged drug is the primary route of elimination. About 6% is excreted unchanged in urine. | FDA |
| AMES Test (Carcinogenic Effect) | Probability 0.9133 (non-AMES toxic) | DrugBank |
| hERG Safety Test (Cardiac Effect) | Probability 0.9929 (weak inhibitor | DrugBank |
| Liver Toxicity | Azithromycin is linked to a low rate of asymptomatic, transient, and acute elevation in serum aminotransferases in 1%-2% of patients. The drug can also cause hepatocellular injury with symptoms and jaundice. Azithromycin is also associated with Stevens-Johnson syndrome, toxic epidermal necrosis, and erythema multiforme. | LiverTox |
Drug Interactions and Side Effects
Before starting treatment with azithromycin, patients should inform their healthcare provider if they have any of the following conditions:
- Liver disease
- Kidney disease
- Myasthenia gravis
- A heart rhythm disorder
- Long QT syndrome
- Low levels of potassium in the blood
Table 4. Drug interactions and side effects of azithromycin.
| Features | Comment(s) | Source |
|---|---|---|
| Total Number of Drug Interactions | 292 drugs | Drugs.com |
| Major Drug Interactions | 60 drugs (ex: bcg, live cholera vaccine, gatifloxacin) | Drugs.com |
| Alcohol/Food Interactions | No interactions with alcohol. Azithromycin can be taken with or without food. Patients should avoid magnesium- and aluminum-containing antacids while being treated with azithromycin. | FDA |
| Disease Interactions | Colitis (major), QT prolongation (major), Liver disease (moderate), Myasthenia gravis (moderate) | Drugs.com |
| On-Target Side Effects | The most commonly reported side effects are diarrhea/loose stools, nausea, and abdominal pain | Drugs.com |
| Off-Target Side Effects | N/A | N/A |
| CYP Interactions | CYP450 3A4 substrate | DrugBank |
Cases of Clostridium difficile associated diarrhea (CDAD) have been reported with the use of almost all antibacterial drugs, including azithromycin, and may vary in severity from mild diarrhea to fatal colitis. CDAD occurs because treatment with antibiotics changes the normal bacterial flora of the colon, which results in an overgrowth of C. difficile (FDA, 2017).
Regulatory Approvals/Commercial
Azithromycin was first synthesized in 1980 and gained US FDA approval in 1991. It is available as an oral suspension, tablet, capsule, intravenous injection, and ophthalmic solution. It is commonly used to treat infections in the lungs, sinuses, and skin. It is currently sold under several trade names including Zithromax and Sumamed (DrugBank). Azithromycin has become one of the most popular antibiotics in the world. In 2010, it was the most frequently prescribed antibiotic agent in the United States (Hicks et al., 2013).
Links
Table 5. Links to learn more about azithromycin
| Comprehensive Antibiotic Resistance Database (CARD) | ARO: 3000158 |
| DrugBank | DB00207 |
| Drugs.com | https://www.drugs.com/azithromycin.html |
| FDA – Zithromax | https://www.accessdata.fda.gov/drugsatfda_docs/label/2013/050710s039,050711s036,050784s023lbl.pdf |
| LiverTox: National Institutes of Health (NIH) | https://www.ncbi.nlm.nih.gov/books/NBK548434/ |
| PubChem CID | 447043 |
Learn about azithromycin resistance.
References
Azithromycin. Drugs.com. https://www.drugs.com/azithromycin.html
Azithromycin– DrugBank. Drugbank.ca. https://go.drugbank.com/drugs/DB00207
Azithromycin. PubChem. https://pubchem.ncbi.nlm.nih.gov/compound/447043
Bulkley, D., Innis, C. A., Blaha, G., Steitz, T. A. (2010). Revisiting the structures of several antibiotics bound to the bacterial ribosome. Proceedings of the National Academy of Sciences, 107(40), 17158-17163. https://doi.org/10.1073/pnas.1008685107 PDB ID: 4V7Y.
Eberl, S., Renner, B., Neubert, A., Reisig, M., Bachmakov, I., König, J., Dörje, F., Mürdter, T. E., Ackermann, A., Dormann, H., Gassmann, K. G., Hahn, E. G., Zierhut, S., Brune, K., Fromm, M. F. (2007) Role of p-glycoprotein inhibition for drug interactions: evidence from in vitro and pharmacoepidemiological studies. Clin Pharmacokinet. 46(12):1039-49. https://doi.org/10.2165/00003088-200746120-00004
Hicks, L. A., Taylor, T. H., Jr, Hunkler, R. J. (2013). U.S. outpatient antibiotic prescribing, 2010. The New England journal of medicine, 368(15), 1461–1462. https://doi.org/10.1056/NEJMc1212055
Jia, B., Raphenya, A. R., Alcock, B., Waglechner, N., Guo, P., Tsang, K. K., Lago, B. A., Dave, B. M., Pereira, S., Sharma, A. N., Doshi, S., Courtot, M., Lo, R., Williams, L. E., Frye, J. G., Elsayegh, T., Sardar, D. Westman, E. L., Pawlowski, A. C., Johnson, T. A., Brinkman, F. S., Wright, G. D., and McArthur, A. G. (2017) CARD 2017: Expansion and model-centric curation of the Comprehensive Antibiotic Resistance Database. Nucleic Acids Research 45, D566-573. https://doi.org/10.1093/nar/gkw1004
LiverTox – Clinical and Research Information on Drug-induced Liver Injury. National Institutes of Health. https://www.ncbi.nlm.nih.gov/books/NBK548434/
ZITHROMAX. (2013). (2010). Food and Drug Administration. https://www.accessdata.fda.gov/drugsatfda_docs/label/2013/050710s039,050711s036,050784s023lbl.pdf
March 2025, Steven Arnold, Helen Gao, Shuchismita Dutta; Reviewed by Drs. Albert Berghuis and Tolou Golkar
https://doi.org/10.2210/rcsb_pdb/GH/AMR/drugs/antibiotics/prot-syn/ribo/MCL/azithromycin
