Methicillin
Drug Name
Methicillin is a semi-synthetic bactericidal antibiotic. Derived from penicillin, it belongs to the class of β-lactams. It has narrow-spectrum activity against gram-positive bacteria (DrugBank). Although it was first approved in 1960 for use, methicillin has been discontinued and is no longer clinically used due to its ineffectiveness against resistant bacterial strains, such as methicillin-resistant Staphylococcus aureus (MRSA).
Table 1. Basic profile of methicillin.
| Description | Intravenously or intramuscularly administered, semi-synthetic narrow-spectrum antibacterial drug |
| Target(s) | Penicillin-binding proteins (PBPs) |
| Generic | Methicillin sodium |
| Commercial Name | Staphcillin |
| Combination Drug(s) | N/A |
| Other Synonyms | Meticillin, Methicillinum, Methycillin, Meticilina, Meticilline, Meticillinum, (2,6-Dimethoxyphenyl)penicillin |
| IUPAC Name | (2S,5R,6R)-6-(2,6-dimethoxybenzamido)-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid |
| Ligand Code in PDB | MII, 7EP (bound form) |
| PDB Structure | 1mwu (Structure of methicillin acyl-Penicillin binding protein 2a) |
| ATC code | J01CF03 |
|
|
Antibiotic Chemistry
As a penicillin antibiotic, methicillin (Figure 2) consists of a β-lactam ring (shown in pink) fused to a five-membered thiazolidine ring (shown in blue) in Figure 2. The sterically large ortho-dimethoxyphenyl side chain (shown in purple) makes methicillin a poor substrate for many β-lactamases and continues to be effective against pathogens (Stapleton and Taylor, 2002).
|
| Figure 2. 2D structure of methicillin highlighting functional moieties responsible for antibacterial activity. Structure created using ChemDraw. |
Drug Information
Table 2. Chemical and physical properties (DrugBank)
| Chemical Formula | C17H20N2O6S |
| Molecular Weight | 380.4 g/mol |
| Calculated Predicted Partition Coefficient: cLogP | 1.79 |
| Calculated Predicted Aqueous Solubility: cLogS | -3.1 |
| Solubility (in water) | 0.31 mg/mL |
| Predicted Topological Polar Surface Area (TPSA) | 105.17 Å2 |
Drug Target
Methicillin disrupts cell wall biosynthesis in bacteria by inhibiting enzymes known as penicillin-binding proteins (PBPs). This name originates from the ability of penicillin and other β-lactam antibiotics to bind to these proteins. Some of these PBP enzymes are responsible for catalyzing the final steps of the peptidoglycan synthesis pathway, which include polymerizing glycan strands and then cross-linking adjacent chains to form the characteristic mesh structure of peptidoglycan. On the other hand, other PBPs are involved in regulating peptidoglycan recycling and cell wall remodeling. Peptidoglycan, which is a polymer consisting of amino acids (peptido-) and sugars (-glycan), is the major component of the bacterial cell wall, and its mesh-like structure provides the cell wall with structure and rigidity.
Inhibition of the PBPs responsible for cross-linking results in a severely weakened cell wall, which then causes bacterial cell lysis and death. However, inhibition of those PBPs involved only in peptidoglycan remodeling is non-lethal to the bacteria.
Click here to learn more about PBPs.
Drug-Target Complex
There are several targets of methicillin, all PBPs. Binding to PBP2a, a determinant of methicillin resistance will be discussed here. The PBP2a protein is made of two domains:
* N-terminal domain - made of the N-terminal extension (residues 27-138) and non-penicillin-binding domain (residues 27-326)
* C-terminal domain
The active site of PBP2a is located in an extended groove, with the nucleophilic Ser403 and the backbone nitrogens of Ser403 and Thr600 forming the conserved oxyanion hole. Methicillin is able to inhibit PBP enzymes because its β-lactam ring (shown in Figure 3) is a structural mimic of the backbone of the D-Ala-D-Ala peptide bond, which allows the antibiotic to occupy the same binding site as the natural substrate. The substrate can no longer access the active site of the PBP once the antibiotic binds, thus inactivating the enzyme.
|
| Figure 3. Ribbon representation of PBP2a bound to methicillin (color-coded atomic stick figure: C-spring green, N-blue, O-red, S-yellow). The inset shows the covalent linkage that forms between methicillin and Ser403 (color-coded atomic stick figure: C-gold, O-red). (PDB ID: 1mwu; Lim and Strynadka, 2002). |
In the apo conformation, Ser 403 is in a poor position for nucleophilic attack. Upon methicillin binding, the strand β3 (residues 594–603) twists to accommodate the helix α2 N-terminus. This conformational change positions Ser403 for nucleophilic attack by methicillin (Figure 4) .
|
| Figure 4. (a) Ribbon representation of the active site in the apoenzyme structure of PBP2a (PDB ID: 1vqq; Lim and Strynadka, 2002). (b) Ribbon representation of the PBP2a active site after methicillin binding. (PDB ID: 1mwu; Lim and Strynadka, 2002). Color-coded atomic stick figure: C-khaki, O-red. |
Pharmacologic Properties and Safety
Table 3. Pharmacokinetics: ADMET of methicillin.
| Features | Comment(s) | Source |
|---|---|---|
| Absorption | Not absorbed following oral administration | DrugBank |
| IC50 (nM) N/A N/A | N/A | N/A |
| Ki (µM) | N/A | N/A |
| Half-life (hrs) | 25-60 minutes | DrugBank |
| Duration of Action | 4-6 hours | Drugs.com |
| Absorption Site | N/A | N/A |
| Transporter(s) | N/A | N/A |
| Metabolism | Hepatic (20-40%) | DrugBank |
| Excretion | Excreted unchanged in the urine. | PubChem |
| AMES Test (Carcinogenic Effect) | Probability 0.8607 (Non AMES toxic) | DrugBank |
| hERG Safety Test (Cardiac Effect) | Probability 0.9995 (weak inhibitor) | DrugBank |
| Liver Toxicity | Reversible, mild elevations of serum aminotransferase or alkaline phosphatase without jaundice are not uncommon in patients with severe penicillin hypersensitivity reactions | LiverTox |
Drug Interactions and Side Effects
Table 4. Drug interactions and side effects of methicillin.
| Features | Comment(s) | Source |
|---|---|---|
| Total Number of Drug Interactions | 30 drugs | Drugs.com |
| Major Drug Interaction(s) | bcg (Tice BCG, Tice BCG Vaccine); cholera vaccine, live; methotrexate; typhoid vaccine, live | Drugs.com |
| Alcohol/Food Interaction(s) | One gram of parenteral methicillin sodium contains about 60 to 71 mg of sodium. This sodium content should be taken into consideration when treating patients who have conditions, such as congestive heart failure, hypertension (high blood pressure), or fluid retention, which require sodium restricted diets. | Drugs.com |
| Disease Interaction(s) | Pseudomembranous colitis (major), Marrow toxicity (major), Renal Dysfunction (moderate), Cystic fibrosis (moderate) | Drugs.com |
| On-target Side Effects | Pain, swelling, and/or tenderness at the injection site, thrombophlebitis (inflammation of vein and swelling due to intravenous administration) | Drugs.com |
| Off-target Side Effects | Rash, diarrhea (mild), sore mouth or tongue, white patches in the mouth and/or on the tongue, vaginal itching and discharge, headache, dizziness, fever, anaphylaxis | Drugs.com |
| CYP Interactions | None | DrugBank |
Regulatory Approvals/Commercial
Methicillin received US FDA approval for clinical use on April 1st, 1971 under the name staphcillin. The antibiotic was indicated in the treatment of for the following (PubChem):
* Bone and joint infections
* Bacterial endocarditis
* Bacterial septicemia
* Sinusitis
* Skin and soft tissue infections
However, the product is no longer commercially available due to adverse side effects associated with it, such as acute interstitial nephritis, which has been reported in up to 17% of patients, and acute renal failure (Drugs.com). In addition, its ineffectiveness against MRSA and the fact that there are other similar and more stable antibiotics that can be used are also reasons for the discontinuation of methicillin.
Links
Table 5. Links to learn more about methicillin
| Comprehensive Antibiotic Resistance Database (CARD) | ARO: 0000015 |
| DrugBank | DB01603 |
| Drugs.com | https://www.drugs.com/drug-interactions/methicillin,staphcillin.html |
| LiverTox: National Institutes of Health (NIH) | https://www.ncbi.nlm.nih.gov/books/NBK548033/ |
| PubChem CID | 6087 |
Learn about methicillin resistance.
References
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., 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
Lim, D., Strynadka, N. C. (2002). Structural basis for the β lactam resistance of PBP2a from methicillin-resistant Staphylococcus aureus. Nat Struct Biol., 9, 870-6. https://doi.org/10.1038/nsb858
LiverTox - Clinical and Research Information on Drug-Induced Liver Injury. National Institutes of Health. https://www.ncbi.nlm.nih.gov/books/NBK548033/
Meticillin. PubChem. https://pubchem.ncbi.nlm.nih.gov/compound/Methicillin
Methicillin - Drugbank.ca https://www.drugbank.ca/drugs/DB01603
Staphcillin - Drugs.com https://www.drugs.com/drug-interactions/methicillin,staphcillin.html
Stapleton, P. D., Taylor, P. W. (2002) Methicillin resistance in Staphylococcus aureus: mechanisms and modulation. Sci Prog. 85, 57-72. https://doi.org/10.3184/003685002783238870
March 2025, Helen Gao, Gauri Patel, Shuchismita Dutta; Reviewed by Dr. Andrew Lovering
https://doi.org/10.2210/rcsb_pdb/GH/AMR/drugs/antibiotics/cellwall-biosynth/pbp/blm/methicillin
