Antibacterials III

Overview of Antibacterial Drugs Targeting Protein Synthesis

• Objective of Presentation: To provide an overview of antibacterial drugs that target protein synthesis, including:
  
  
     - Selective toxicity to bacteria: These drugs are designed to target bacteria specifically, meaning they can kill or inhibit bacterial growth without harming human cells. This selectivity is crucial because it minimizes side effects in patients.
  
  
     - Advantages of developing multiple classes of Protein Synthesis inhibitors: Having various types of drugs allows for more options in treatment. Some bacteria may be resistant to one class of drug but susceptible to another. This diversity helps ensure effective treatment.
  
  
     - Prototypical drugs from different classes: Each class of protein synthesis inhibitors has a representative drug that helps illustrate its mechanism and effectiveness.
  
  
     - Important pharmacokinetic and pharmacodynamic properties: This refers to how the body absorbs, distributes, metabolizes, and excretes drugs (pharmacokinetics) as well as how the drug affects the body (pharmacodynamics). Understanding these properties helps doctors prescribe medications more effectively.
  
  
     - Mechanisms of acquired drug resistance: Over time, some bacteria can develop resistance to antibiotics, making them less effective. Knowing how this resistance occurs helps in developing new drugs and treatment strategies.
  
  
     - Major adverse effects or drug interactions with these agents: Side effects and interactions with other medications are essential considerations when prescribing antibiotics to ensure patient safety.

Basic Process of Bacterial Protein Synthesis

• Ribosomal Structure Comparison:
  
  
     - Bacteria: 50S and 30S ribosomal subunits: Bacteria have unique ribosomal subunits, allowing for the selective targeting of their protein synthesis.
  
  
     - Human Cells: Different ribosomal structure, allowing selective targeting of bacterial protein synthesis without affecting human protein synthesis: Human ribosomes differ in structure, which helps antibiotics differentiate between human and bacterial cells to target the bacteria effectively.

Initiation Phase

• Formation of Initiation Complex:
  
  
     - Small ribosomal subunit (30S) complexes with charged aminoacyl-tRNA (carrying Methionine) and messenger RNA (mRNA).: The process begins when the small subunit of the ribosome connects with the mRNA and a specific tRNA, which carries the first amino acid (Methionine).
  
  
     - Large subunit (50S) joins to form the complete initiation complex: After the small subunit is attached, the larger subunit joins, culminating in the formation of a complete ribosome ready to start protein synthesis.
  

• Drug Classes Targeting Initiation:
  
  
     - Aminoglycosides: These drugs work by binding to the 30S subunit, preventing the initiation complex from forming correctly.
  
  
     - Oxazolidinones: They inhibit the initiation process by binding to the 50S subunit, thus preventing the proper formation of the protein synthesis machinery.

Elongation Phase

• Binding of a Second Aminoacyl-tRNA:
  
  
     - Enzyme Peptidyl Transferase catalyzes the formation of a dipeptide (bond between Methionine and the incoming amino acid): During elongation, the ribosome helps link amino acids together to form a growing protein chain.
  
  
     - Ribosome translocates to the next codon on the mRNA, repeating the process until a complete polypeptide is formed: This movement along the mRNA is crucial for creating the correct sequence of amino acids.
  

• Drug Classes Targeting Elongation:
  
  
     - Tetracyclines: Inhibit tRNA binding at the ribosome, preventing new amino acids from being added.
  
  
     - Macrolides: These interfere with the formation of peptide bonds, which are necessary for linking amino acids.
  
  
     - Clindamycin: Similar to macrolides, it affects protein synthesis by binding to the ribosome.
  
  
     - Chloramphenicol: This drug has a unique mechanism that allows it to block peptide bond formation as well.

Mechanisms of Action

• Aminoglycosides:
  
  
     - Bind to the 30S ribosomal subunit, inhibiting initiation: The binding disrupts the formation of the initiation complex.
  
  
     - This leads to the production of abnormal proteins.
  

• Oxazolidinones:
  
  
     - Bind to the 50S ribosomal subunit, also inhibiting initiation: By preventing correct assembly at the start of protein synthesis, they effectively halt bacterial growth.
  

• Tetracyclines:
  
  
     - Bind to the ribosome, blocking aminoacyl-tRNA attachment: This blockage stops the addition of amino acids to the growing protein chain.
  

• Macrolides, Clindamycin, Chloramphenicol:
  
  
     - Bind to the Peptidyl Transferase Region impacting elongation and peptide bond formation: This action prevents the ribosome from effectively synthesizing proteins.

Resistance Considerations

• Mutation Mechanisms: Resistance mutations may affect one class of drugs without conferring resistance to others, due to differing binding sites: Bacteria can evolve, resulting in changes that make them resistant to certain drugs but not others.
  
  
   - Cross Resistance:
  
     - Potential between Clindamycin, Chloramphenicol, and Macrolides due to similar binding sites: Some bacteria may be resistant to multiple drugs if they target the same site.
  
  
   - General Activity: Most inhibitors are bacteriostatic except for Aminoglycosides, which are bactericidal: Bacteriostatic means they prevent bacterial growth, while bactericidal means they kill bacteria.

Aminoglycosides

Example

• Prototypical Drug: Gentamicin (trade name: Garamycin, not needed for exam).: Gentamicin is commonly used and serves as a representative example of aminoglycoside drugs.

Characteristics

• Spectrum: Extended activity against aerobic Gram-positive and Gram-negative bacteria (ineffective against anaerobes due to lack of required transporter): It is effective against a wide range of bacteria that require oxygen for growth, but it does not work against bacteria that thrive in low-oxygen environments, known as anaerobes.
  
  
   - Administration: IV or intramuscular injections due to poor gastrointestinal absorption: Gentamicin must be given directly into the bloodstream or muscles because it does not absorb well when taken by mouth.

Synergistic Effects

• Combined use with cell wall inhibitors (e.g., beta-lactams or vancomycin):
  
  
     - Enhances efficacy due to improved cellular penetration: Using aminoglycosides with other antibiotics helps them enter bacterial cells more effectively, increasing their overall effectiveness.
  
  
     - Damaged cell wall allows greater aminoglycoside access, while aminoglycosides produce defective proteins that impair cell wall synthesis leading to synergism: This means that not only do the drugs help each other but also make it harder for bacteria to survive.

Resistance Mechanisms

• Enzyme Modification: Bacteria produce modifying enzymes, such as:
  
  
     - Acetylation
  
  
     - Phosphorylation
  
  
     - Adenylation: These processes change the aminoglycosides, preventing them from binding effectively to their targets in the ribosomes.
  
  
   - Resulting modifications prevent drug binding: Resistance lowers the effectiveness of the drug.

Adverse Effects

• Renal Toxicity:
  
  
     - Occurs with prolonged use (exceeding 3 days), typically reversible: Long-term use can harm the kidneys, but this damage may recover after treatment stops.
  
  
   - Ototoxicity:
  
  
     - Results from drug accumulation in the inner ear, leading to irreversible hearing loss: This side effect can be severe and permanent, making it crucial to monitor dosage.
  
  
   - Neuromuscular Paralysis:
  
  
     - Effects on acetylcholine at the neuromuscular junction can exacerbate conditions like Myasthenia Gravis: This paralysis may worsen certain pre-existing conditions affecting muscle control.

Post-Antibiotic Effect

• Definition: Continued inhibition of bacterial growth despite drug levels falling below the minimum inhibitory concentration (MIC): This means that even when there is not enough drug to kill bacteria, it can still stop their growth for a while.
  
  

• Therapeutic Application: Allows modifications to dosing regimens to minimize toxicity while maintaining efficacy: Doctors can adjust how they give the medication to keep it as effective as possible without causing too much harm.
  
  
   - Example Dosing:
  
  
     - Frequent low-dose regimen keeps drug levels above toxicity threshold, risking renal toxicity: Taking smaller doses more often can help maintain effectiveness but may lead to kidney issues.
  
  
     - Large bolus doses once daily could drop below toxicity threshold to allow renal recovery, while still effectively inhibiting bacterial growth: Giving a larger dose less frequently might help protect kidney function while still fighting the infection.

Oxazolidinones

Example

• Prototypical Drug: Linezolid: An example of a drug from this class that is used to treat infections caused by resistant bacteria.

Mechanism

• Mechanism of Action: Binds to a distinct site on the ribosome, inhibiting initiation of protein synthesis: Linezolid specifically targets the ribosome in a way different from other antibiotics, making it effective against certain resistant bacteria.

• Characteristic: Primarily bacteriostatic, not bactericidal: Like most oxazolidinones, Linezolid stops bacteria from growing but does not necessarily kill them outright.

Spectrum of Action

• Targets mainly Gram-positive organisms, effective against resistant infections (e.g., Vancomycin-resistant Enterococcus faecium): This means it works well on a specific group of bacteria that are often harder to treat with other antibiotics.

Adverse Effects

• Can cause myelosuppression, reducing white blood cell counts, posing risk in immunocompromised patients: This side effect can weaken the immune system, making infections harder to fight, particularly in vulnerable populations.