【Pharmacology】Antibiotics part 2

Antibiotics

 Content:

1. Antibiotics classification
2. Mechanism of action antibiotics
3. Cell wall synthesis inhibitors
4. Cell membrane integrity disruptors ------------Part 1
5. Nucleic acid synthesis inhibitors
6. Protein synthesis inhibitors
7. Metabolic pathway inhibitors
8. Reference

Review: 【Pharmacology】Antibiotics part 1

5. Nucleic acid synthesis inhibitors

• living organisms store their genetic information in the form of DNA 
• the cell uses DNA to survive and replicate 
• but first things bacteria must convert their genetic information into functional molecules 
• this is done by using DNA as a template for the synthesis of RNA molecule 
• process known as transcription 
• RNA can then perform tasks directly or act as a blueprint for the synthesis of functional proteins
• process known as translation 

Drugs capable of inhibiting DNA or RNA synthesis 

• render the bacteria unable to create proteins and replicate 
• but not toxic to our own body's cells 
• bacterial enzymes that carry out synthesis of DNA and RNA happen to be different from ours
• allowing development of selectively toxic antibiotics the antimicrobial agents

a. Metronidazole

• prodrug that requires reductive activation of its nitro group upon diffusing across the cell membrane of anaerobic bacteria
• Metronidazole is converted by the bacterial enzyme pyruvate
• ferredoxin oxidoreductase into nitro radical anion
• this highly reactive radical is thought to be the main toxic agent 
• attacks DNA causing strand breaks and mutations
• lead to bacterial cell death

Side effects

• headaches 
• nausea
• metallic taste in the mouth
• unpleasant symptoms such as 
• vomiting
• flushing of the skin
• tachycardia
• shortness of breath
• because Metronidazole inhibits metabolism of alcohol in the body combining it with alcohol 

b. Quinolones 

• involves interactions with topoisomerases 
• topoisomerases are the enzymes responsible for the unwinding and unlinking of DNA
• In order for the bacterial cells to replicate tightly,
• coiled bacterial chromosome must unwind 
• to make the DNA code can be accessed and copied 
• two principal topoisomerases that perform this task
• DNA gyrase
• unwinds and relaxes supercoiled DNA
• topoisomerase IV 
• facilitates separation of the linked daughter DNA molecules 
• after replication is complete 
• both DNA gyrase and topoisomerase IV are the primary target of Quinolones 
• bind to these enzymes and effectively inhibit their function 
• blocks DNA synthesis and cell growth
• bacterial cell death
• vast majority of Quinolones in clinical use are so called Fluoroquinolones 
• have a fluorine atom attached to the central ring system that increases their antimicrobial activity 
• Examples of Fluoroquinolones:
• Ciprofloxacin
• Levofloxacin
• Moxifloxacin
• Norfloxacin
• Ofloxacin

Side effects

• nausea 
• vomiting
• diarrhea
• headache
• insomnia
• prolongation of the QT interval 
• tendon damage 
• peripheral neuropathy

c. Rifamycins

• interfere with transcription of bacterial DNA into RNA
• Rifamycins target enzyme responsible for translating DNA into RNA, called RNA polymerase 
• by combining with a protein of this bacterial DNA-dependent RNA polymerase 
• Rifamycins bring all synthesis of RNA to a halt 
• without RNA, the bacteria cannot make proteins
• cell death 
• Examples of Rifamycins:
• Rifampin
• Rifabutin
• Rifapentine

Side effects

• GI disturbances 
• flu-like symptoms 
• hepatotoxicity 
• discoloration of body fluids including urine saliva sweat and tears to red-orange color

6. Protein synthesis inhibitors

• bacterial genes are translated into proteins through RNA 
• the type of RNA that carries message from the DNA is called messenger RNA (mRNA)
• the protein synthesizing machine to which the message is carried to is called ribosome 
• the bacterial ribosome is composed of two subunits 
• the 30S into which mRNA feeds 
• the 50S which carries out catalytic functions the ribosome translates messenger RNA into protein by reading the nucleotide triplets known as codons 
• which specify amino acids that are required to make up specific proteins transfer RNA or tRNA
• brings the individual amino acids to the ribosomal aminoacyl site, known as the A-site
• form a peptide bond with a growing chain at the peptidyl site known as the P-site
• empty tRNA in the P-site is released
• ribosome moves to the next codon
• transferring tRNA with new polypeptide from the A-site to the P-site 
• this process is repeated until ribosome encounters a stop codon that signals the end of protein synthesis
• Protein synthesis inhibitors act at a specific site on the ribosome to inhibit different steps in the protein synthesis

a. Block bacterial protein synthesis 

can be divided into two groups:

• the 30S subunit inhibitors
• classes of antibiotics that bind to the 30S subunit:
• Aminoglycosides
• Tetracyclines
• Glycylcyclines
• the 50S subunit inhibitors 
• classes of antibiotics that bind to the 50S subunit:
• Amphenicols
• Macrolides
• Ketolides
• Lincosamides
• Streptogramins
• Oxazolidinones

Aminoglycosides 

• work primarily by binding to an area adjacent to the decoding site in the 30S subunit of the ribosome
• they interfere with the initiation of protein synthesis
• cause misreading of the genetic code
• leads to
• synthesis of nonfunctional proteins
• premature termination of protein synthesis 
• Drug:
• Neomycin
• Amikacin
• Gentamicin
• Streptomycin 
• Tobramycin 

Side effects

• serious toxicities including 
• ototoxicity 
• nephrotoxicity 
• in rare instances----neuromuscular blockade 

Tetracyclines and their derivatives Glycylcyclines 

• also bind to the 30S ribosomal subunit
• primary mode of action: 
• by blocking entry of aminoacyl tRNA molecules into the A-site of the ribosome 
• preventing introduction of new amino acids to the growing peptide chain 
• this action is usually inhibitory and reversible upon withdrawal of the drug 

Examples of Tetracycline antibiotics:

• Doxycycline
• Minocycline
• Tetracycline 

Example of Glycylcycline antibiotic:

• Tigecycline 

Side effects 

• GI disturbances 
• photosensitivity 
• hepatotoxicity 
• Tetracyclines have strong affinity for calcium
• discoloration of teeth
• inhibition of bone growth

Amphenicols 

• work primarily by binding to 50S ribosomal subunit
• they block the peptidyl transferase center that catalyzes peptide bond formation 
• prevents transfer of the elongating peptide chain to the newly attached aminoacyl tRNA 
• generally results in bacteriostatic effect 
• Example of Amphenicol antibiotic:
• Chloramphenicol 

Side effects 

(intravenous Chloramphenicol)

• gray baby syndrome 
• results from the inability of an infant's immature liver to metabolize Chloramphenicol 
• symptoms include 
• hypotension
• abdominal distension
• cyanosis 
• ultimately lead to death
• aplastic anemia
• rare
• occur weeks or even months after the treatment 

Macrolides and their close relatives Ketolides

• bind primarily to 50S ribosomal subunit near the peptidyl transferase center
• they block the peptide exit tunnel that the newly assembled polypeptides pass through on their way out of the ribosome 
• this results in inhibition of protein elongation process and bacteriostatic activity against most organisms 

Examples of Macrolide antibiotics: 

• Azithromycin
• Clarithromycin
• Erythromycin
• Fidaxomicin 

Example of Ketolide antibiotic: 

• Telithromycin 

Side effects

• nausea
• vomiting
• diarrhea
• ringing or buzzing in the ears 
• QT interval prolongation, lead to 
• ventricular arrhythmia 
• Torsades de pointes
• cholestatic hepatitis 
• associated only with the use of Erythromycin

Lincosamides and Streptogramins

• Macrolide binding site in the exit tunnel happens to overlap with the binding sites of Lincosamides and Streptogramins 
• inhibit primarily the translocation steps of protein synthesis

Example of Lincosamide antibiotic:

• Clindamycin

Example of Streptogramin antibiotic:

• Quinupristin/Dalfopristin 
• a combination of two Streptogramin antibiotics which act synergistically with Dalfopristin
• enhancing the binding of Quinupristin & inhibiting peptidyl transferase

Side effects 

Clindamycin:

• diarrhea
• nausea
• vomiting
• abdominal cramps 
• severe diarrhea and inflammation of the colon
• overgrowth of Clostridium
• caused by destruction of natural flora by Clindamycin
• caused by pseudomembranous colitis

Quinupristin/Dalfopristin:

• nausea
• vomiting
• diarrhea
• injection site reactions including pain, burning and irritation

Oxazolidinones 

• inhibit the first step of the synthesis 
• by binding to the a side on the 50S ribosomal subunit
• they prevent the initiation of complex formation 
• without functional initiation complex, bacteria can't synthesize proteins
• results in bacteriostatic or bactericidal effect depending on the species

Examples of Oxazolidinones:

• Linezolid
• Tedizolid

Side effects

• nausea
• vomiting
• diarrhea
• headache
• dizziness 
• bone marrow suppression optic 
• peripheral neuropathy seizures
• abnormal liver-function tests

7. Metabolic pathway inhibitors

• primary target: pathway that bacteria use to synthesize folic acid 
• Folic acid
• important vitamin that bacteria & humans 
• need to make nucleotides and some amino acids 
• Without folic acid, DNA replication and cellular growth would be disrupted

Humans who obtain folic acid from the diet bacteria

• make folic acid on their own bacteria synthesize folic acid by 
• taking para-aminobenzoic acid (PABA)
• adding a compound called pteridine 
• In the presence of enzyme,
• dihydropteroate synthase to form dihydropteroic acid
• they add glutamate to make dihydrofolic acid
• use an enzyme called dihydrofolate reductase to make tetrahydrofolic acid 
• tetrahydrofolic acid is the metabolically active form of folic acid which acts as a coenzyme for a number of key biochemical reactions 

Metabolic pathway inhibitors

• work by interfering with bacterial synthesis of tetrahydrofolic acid
• include 
• family of drugs called Sulfonamides or Sulfa drugs 
• drug called Trimethoprim

Sulfonamides

• act through competitive inhibition of dihydropteroate synthase
• due to their structural resemblance to para-aminobenzoic acid
• the enzyme that normally converts PABA to the precursor of folic acid combines with the Sulfonamide 
• instead the combination prevents tetrahydrofolic acid synthesis 
• stops the growth of the bacteria

Examples of Sulfonamide antibiotics: 

• inhibit folic acid synthesis
• Sulfamethoxazole 
• Sulfacetamide 

Trimethoprim

• targets the second key enzyme in the folic acid synthesis pathway: dihydrofolate reductase 
• as an analog of dihydrofolic acid
• competitively inhibits this enzyme
• effectively disrupting production of tetrahydrofolic acid 

Side effects 

Sulfonamides

• GI disturbances such as 
• nausea
• vomiting
• diarrhea
• photosensitivity
• renal stones 
• hepatotoxicity
• bone marrow suppression
• allergic reactions ranging from rash or hives to anaphylaxis and Stevens-Johnson syndrome

Trimethoprim

• little more modest and generally limited to 
• upset stomach
• nausea
• vomiting
• skin rashes

8. Reference

https://youtu.be/5HQmvQJWzNY