PMP201 - Antibiotics Class & Mechanisms Part 3

PMP201 - Infection ISU: Antibiotics - Drug Classes and Mechanisms - Part 3

• Course: Infection Immunology
• Date: 6th November 2024
• Lecturer: Dr. Giulio Nannetti
• Topic: Antibiotics - Drug Classes and Mechanisms - Part 3

Previous & Complementary Knowledge

• Health, Disease, and Patient (PMP101): 2023/24
• Microbiology Lectures: Bacteria composition
• Bacterial Growth
• Patient-Centred Learning I (PMP201): 2024/25
• Antibiotics Overview 1: Dr. Guirguis
• Antibiotics Overview 2 & 3: Dr. Guirguis
• Antibiotic Chemistry: Dr. Padalino

Learning Outcomes

• Outline antibacterial mechanism, effect, and properties of the main types of antimetabolite antibiotics
• Outline antibacterial mechanism, effect, and properties of inhibitors of nucleic acids synthesis
• Outline antibacterial mechanism, effect, and properties of antibiotics acting with an alternative mechanism of action
• Identify therapeutic agents for tuberculosis

Mechanisms of Action - RECAP

• 1. Inhibition of cell wall synthesis:
  • Beta-lactams (penicillins, cephalosporins, monobactams, carbapenems)
  • Glycopeptides (vancomycin)
• 4. Inhibition of nucleic acids synthesis:
  • Fluoroquinolones (ciprofloxacin, levofloxacin, moxifloxacin)
  • Rifamycins (rifampin)
• 2. Inhibition of protein synthesis:
  • Aminoglycosides
  • Tetracyclines
  • Macrolides
  • Lincosamides
  • Chloramphenicol
  • Oxazolidinones
• 3. Acting as antimetabolites:
  • Sulfonamides
  • Trimethoprim
  • Mycolic acid synthesis inhibitors (isoniazid)

1- Inhibitors of Cell Wall Synthesis - RECAP

• Cell wall inhibitors:
  • β-lactams, penicillins, cephalosporins, monobactams
  • Carbapenems
  • Glycopeptides
  • Vancomycin

β-Lactam and Non β-Lactam Classes Inhibiting Cell Wall Synthesis - RECAP

• β-lactams (bactericidal):
  • Target transpeptidase enzymes (mimicking their substrate).
  • Structurally diverse chemical classes (with a β-lactam ring)
  • Penicillins
  • Cephalosporins
  • Carbapenems
  • Monobactams
• Non β-lactams (Glycopeptide): (Vancomycin) (bactericidal)
  • No β-lactam ring
  • Target transpeptidase's substrate
  • Shared mechanism of action

Patient Centred Integration

• Human Biology
• Clinical Pharmacy
• Cellular & Molecular Bioscience
• Pharmacology & Therapeutics
• Pharmaceutical Chemistry
• Pharmacy Practice
• Pharmaceutics

3) Antibiotic Classes Acting as Antimetabolites

• Inhibit bacterial metabolic pathways (eukaryotic cells have different pathways)
• Selective toxicity
• Target key enzymatic steps in folic acid synthesis (essential for nucleotide production)
• Bacteriostatic effect

3a) Sulphonamides (Sulfamethoxazole)

• Mechanism:
  • Folic acid is key for producing purines (nucleotides)
  • Bacteria cannot absorb folic acid externally
  • Bacteria synthesize folic acid starting from PABA
  • Sulphonamides compete with PABA for the first enzyme: DHPS
• In human bodies: Folic acid (Vitamin B9) obtained through the diet

3b) Trimethoprim

• Mechanism:
  • DHFR (dihydrofolate reductase) targeted
  • Present in human cells (for folic acid activation)
  • 100,000x greater affinity for bacterial DHFR over human counterpart

3a+b) Sulphonamides + Trimethoprim

• Sequential blocking mechanism: Both block folic acid synthesis at various steps.
• Synergistic effect: Combined action potentiates antibacterial effect
• Bacteriostatic effect: Inhibits bacterial growth.
• Spectrum: Broad-spectrum (Gram-positive & Gram-negative)

3a+b) Sulphonamides + Trimethoprim (Co-trimoxazole) - Side Effects & Cautions

• Side effects:
  • Folic Acid Deficiency (esp. pregnant women)
  • Hyperkalaemia (high potassium, renal monitoring)
  • Hypersensitivity; rash and anaphylaxis
• Contraindications:
  • First trimester of pregnancy
  • Patients with blood dyscrasias (e.g., decreased blood cell counts)
  • Acute porphyria
  • Elderly and neonates
  • Increased risk of folate deficiency

4) Inhibition of Nucleic Acids Synthesis

• Inhibitors of DNA synthesis:
  • Fluoroquinolones (ciprofloxacin, levofloxacin, moxifloxacin)
• Inhibitors of RNA synthesis:
  • Rifamycins (rifampin)

4) Nucleic Acid Synthesis Inhibition

• Fluoroquinolones: Target DNA topoisomerases, crucial for replication
• Rifamycins: Inhibit RNA polymerase, disrupting mRNA transcription.
• Bactericidal effect
• Lower selective toxicity due to similarities in bacterial and eukaryotic processes.

4a) Fluoroquinolones

• Mechanism:
  • Inhibit bacterial topoisomerases (DNA replication disruption).
  • DNA gyrase inhibition in Gram-negative bacteria.
  • Topoisomerase IV inhibition in some Gram-positive bacteria (at higher doses)
• Spectrum:
  • Broad-spectrum (including H. influenzae, P. aeruginosa).
• Uses:
  • Serious RTIs (CAP), skin/soft tissue infections, UTIs
• Side effects:
  • GI distress
  • Tendonitis/rupture (-2%)
  • Muscle weakness/joint pain
  • QT prolongation (rare)/aortic aneurysm (very rare)
  • Seizures (rare) /peripheral neuropathy
• Cautions / Contraindications:
  • History of tendon damage
  • Corticosteroid use
  • QT prolongation risk factors
  • Epilepsy, psychiatric disorders
  • Renal impairment
  • Exposure to sunlight

4b) Rifamycins (Rifampicin)

• Mechanism:
  • Inhibit bacterial mRNA synthesis by blocking the bacterial RNA polymerase.
• Bactericidal effect
• Uses:
  • Tuberculosis, meningitis (N. meningitidis/H. influenzae), related infections
• Side effects:
  • GI distress
  • Minor hepatotoxicity
  • Body fluid discoloration (urine/sweat turns orange - harmless)
  • Many drug interactions (cytochrome P450 inducer)
• Contraindications:
  • Patients with acute porphyrias

5) Alternative Mechanisms

• Nitroimidazoles: (Metronidazole, Nitrofurantoin)
  • Generate reactive free radicals, disrupting DNA and other bacterial components
  • Bactericidal effect
  • Spectrum: Mainly against anaerobic bacteria & some Gram-positives(e.g., E. coli)
• Uses:
  • Treating anaerobic bacterial infections, H. pylori eradication; UTIs (Nitrofurantoin is a significant component).

5a) Nitroimidazoles (Metronidazole)

• Mechanism: Generating free radicals in anaerobic bacteria.
• Spectrum: Only anaerobes (including protozoa). Requires low redox potential. Aerobic bacteria have high redox potential.
• Uses: Treating anaerobic infections, H. pylori eradication.

5b) Nitrofurantoin

• Mechanism: Generates reactive free radicals, interfering with RNA, DNA, and protein synthesis.
• Spectrum: Effective against most Gram-positive & some Gram-negative bacteria (E. coli)
• Uses: Treating and preventing uncomplicated acute UTIs.

Treatment for Tuberculosis (TB)

• 10.6 million new TB cases and 1.6 million deaths due to TB reported in 2021 globally.
• TB caused by Mycobacterium tuberculosis organisms.
• Infection can be pulmonary (lungs) or extrapulmonary (other organs).
• TB can occur in latent (asyptomatic) or active (symptomatic) forms.
• Microbiologically diagnosed using acid-fast staining (lipid-rich cell wall) or auramine fluorescent staining (mycolic acid).

Mycobacterium tuberculosis

• Gram-positive and aerobic (oxygen requirement)
• Complex and unique cell wall: Thick, hydrophobic, waxy (lipids) – preventing antibiotics entry
• Virulence factors: Survive and replicate within macrophages.
• Therapeutic target: Cell wall components (e.g., mycolic acids, arabinogalactan).

Cell Wall of Mycobacteria

• Unique components: Glycolipids (TDM, GPL, PDIM), mycolic acids, arabinogalactan, peptidoglycan
• Thick, hydrophobic barrier: Impedes antibiotic penetration.
• The nature of the cell wall renders mycobacterium highly resistant to traditional antibiotics

Treatment for Tuberculosis (TB) - First-Line Drugs (RIPE)

• Rifampicin (RNA polymerase inhibitor)
• Isoniazid (inhibitor of mycolic acid synthesis)
• Pyrazinamide (interferes with fatty acid synthesis)
• Ethambutol (interferes with the synthesis of arabinogalactans)

Treatment for Tuberculosis (TB) - Drug Resistance

• Newly diagnosed TB: RIPE (6 months) and subsequent continuation (RI), 4 additional months.
• Re-treatment of TB: RIPE and streptomycin (2 months) & a continuation with ethambutol (5 months)
• Drug-Resistant TB (MDR-TB, XDR-TB): Rifampicin and Isoniazid resistance, and other second-line drug resistances. Treatment is tailored for resistance; Supervision is emphasized to meet patient compliance.

Quizzes

• Trimethoprim Target: Folic acid synthesis
• Not a First Line TB Drug: Amoxicillin