Antimicrobial drugs notes

Foundations of Chemotherapy and Antimicrobial Discovery

• Broad Definition of Chemotherapy

  • Chemotherapy is not limited to cancer treatment; it involves the use of drugs that target specific biological entities.

  • Specific targets include:

  • Cancerous cells.

  • Cancerous tissues.

  • Infectious microorganisms.

• History of Antimicrobial Discovery

  • Paul Ehrlich: Influential in the discovery of Compound $606$, an antimicrobial agent that served as an effective treatment for syphilis.

  • Alexander Fleming: Discovered penicillin in $1928$, the first naturally produced antimicrobial substance.

  • Howard Florey and Ernst Chain: Credited with discovering methods to scale up penicillin production and purify the drug. They demonstrated its efficacy in animal and human trials during the early $1940\text{s}$.

General Principles of Antimicrobial Therapy

• Bacteriostatic vs. Bactericidal Classification

  • Bacteriostatic Drugs: Cause a reversible inhibition of bacterial growth. Bacterial growth can restart once the drug is eliminated.

  • Bactericidal Drugs: Kill the target bacteria directly.

  • Selection Criteria: The choice between bacteriostatic and bactericidal drugs is based on the type of infection and the immune status of the patient.

• Clinical Implications of Drug Spectrum

  • Broad-Spectrum Antimicrobials: Effective against a wide range of bacteria. Their use may lead to the development of a superinfection, where a secondary infection occurs after the protective microbiota are killed.

• Pharmacokinetics: Drug Concentration and Administration Route

  • Intravenous (IV) Administration: Drug concentration in the plasma peaks very quickly following administration and then gradually decreases ($t_1$ through $t_4$).

  • Oral or Intramuscular (IM) Administration: It takes significantly longer for the drug concentration to reach its peak compared to IV dosing.

Mechanisms of Antibacterial Action

• Inhibition of Cell Wall Biosynthesis

  • Targets: Penicillin-binding proteins (PBPs), peptidoglycan subunits, and peptidoglycan subunit transport.

  • Drug Classes:

  • $\beta$-lactams: Includes penicillins, cephalosporins, monobactams, and carbapenems.

  • Glycopeptides: (e.g., Vancomycin).

  • Bacitracin.

• Inhibition of Protein Biosynthesis

  • $30S$ Ribosomal Subunit Targets: Aminoglycosides (cause mismatches between codons and anticodons) and Tetracyclines (block tRNA binding).

  • $50S$ Ribosomal Subunit Targets: Macrolides, lincosamides, chloramphenicol (blocks peptide bond formation), and oxazolidinones.

• Disruption of Plasma Membranes

  • Targets: Lipopolysaccharide, inner and outer membranes.

  • Drugs: Polymyxin B, colistin, and daptomycin.

• Inhibition of Nucleic Acid Synthesis

  • RNA Target: Rifamycins (inhibit RNA polymerase activity).

  • DNA Target: Fluoroquinolones (inhibit DNA gyrase).

• Antimetabolite Actions

  • Folic Acid Synthesis Enzyme: Sulfonamides and trimethoprim.

  • Mycolic Acid Synthesis Enzyme: Isonicotinic acid hydrazide (Isoniazid).

  • Mycobacterial ATP Synthase Inhibitor: Diarylquinoline.

Detailed Breakdown: Cell Wall Synthesis Inhibitors

• $\beta$-Lactam Structure and Function

  • All $\beta$-lactams contain a $\beta$-lactam ring, which is the site of attack by inactivating $\beta$-lactamase enzymes.

  • Chemical modifications to $R$ groups in penicillins provide increased spectrum of activity, acid stability, and resistance to $\beta$-lactamase.

• Penicillin Group

  • Penicillin G and V: Natural; narrow-spectrum against gram-positive and few gram-negative bacteria.

  • Ampicillin and Amoxicillin: Semisynthetic; narrow-spectrum against gram-positives but with an increased gram-negative spectrum.

  • Methicillin: Semisynthetic; narrow-spectrum against gram-positive bacteria only (includes strains producing penicillinase).

• Cephalosporins

  • Cephalosporin C: Natural; narrow-spectrum similar to penicillin but with increased gram-negative activity.

  • First-generation: Semisynthetic; similar to cephalosporin C.

  • Second-generation: Semisynthetic; increased gram-negative spectrum.

  • Third- and Fourth-generation: Semisynthetic; broad-spectrum against gram-positive and gram-negative, including some $\beta$-lactamase producers.

  • Fifth-generation: Semisynthetic; broad-spectrum, including activity against MRSA.

• Other Cell Wall Inhibitors

  • Monobactams (Aztreonam): Semisynthetic; narrow-spectrum against gram-negative bacteria.

  • Carbapenems (Imipenem, Meropenem, Doripenem): Semisynthetic; broadest spectrum of $\beta$-lactams against gram-positive and gram-negative bacteria.

  • Glycopeptides (Vancomycin): Natural; large molecules that bind to the peptide chain of peptidoglycan subunits, blocking transglycosylation and transpeptidation. Narrow-spectrum against gram-positives, including multidrug-resistant strains.

  • Bacitracin: Natural; blocks transport of peptidoglycan subunits across the cytoplasmic membrane. Broad-spectrum.

Protein Synthesis Inhibitors

• $30S$ Subunit Inhibitors

  • Aminoglycosides (Streptomycin, Gentamicin, Neomycin, Kanamycin): Bactericidal; broad-spectrum. Lead to production of faulty proteins that disrupt the cytoplasmic membrane.

  • Tetracyclines (Tetracycline, Doxycycline, Tigecycline): Bacteriostatic; broad-spectrum. Blocks association of tRNAs with the ribosome.

• $50S$ Subunit Inhibitors

  • Macrolides (Erythromycin, Azithromycin, Telithromycin): Bacteriostatic; broad-spectrum. Blocks peptide bond formation.

  • Lincosamides (Lincomycin, Clindamycin): Bacteriostatic; narrow-spectrum. Blocks peptide bond formation.

  • Chloramphenicol: Bacteriostatic; broad-spectrum. Blocks peptide bond formation.

  • Oxazolidinones (Linezolid): Bacteriostatic; broad-spectrum. Interferes with the formation of the initiation complex between $50S$ and $30S$ subunits.

Membrane and Nucleic Acid Inhibitors

• Membrane Function Inhibitors

  • Polymyxins (Polymyxin B and Colistin/Polymyxin E): Interact with lipopolysaccharide in gram-negative bacteria; narrow-spectrum.

  • Polymyxin B: Used in topical wound preparations.

  • Colistin: Used for bowel decontamination or IV for serious systemic multidrug-resistant infections.

  • Lipopeptides (Daptomycin): Inserts into the cytoplasmic membrane of gram-positive bacteria. Used for MRSA and complicated skin infections.

• Nucleic Acid Synthesis Inhibitors

  • Rifamycins (Rifampin): Blocks transcription by inhibiting bacterial RNA polymerase. Used in combination therapy for tuberculosis.

  • Fluoroquinolones (Ciprofloxacin, Ofloxacin, Moxifloxacin): Inhibits DNA gyrase to block DNA replication. Broad-spectrum for skin and systemic infections.

Antimetabolite Drugs

• Folic Acid Pathways

  • Sulfonamides (Sulfamethoxazole) and Sulfones (Dapsone): Inhibit the enzyme involved in dihydrofolic acid production; broad-spectrum.

  • Trimethoprim: Inhibits the enzyme involved in tetrahydrofolic acid production; broad-spectrum.

• Specific Pathways

  • Isoniazid: Interferes with mycolic acid synthesis; narrow-spectrum against Mycobacterium spp. including M. tuberculosis.

Antifungal, Antiprotozoan, and Antihelminthic Drugs

• Antifungal Mechanisms

  • Ergosterol Synthesis Inhibition: Imidazoles (Miconazole, Ketoconazole, Clotrimazole), Triazoles (Fluonazole), and Allylamines (Terbinafine).

  • Ergosterol Binding (Pore formation): Polyenes (Nystatin, Amphotericin B).

  • Cell Wall Synthesis Inhibition: Echinocandins (Caspofungin) and Nikkomycin Z.

  • Microtubule/Division Inhibition: Griseofulvin.

• Antiprotozoan Mechanisms

  • Electron Transport Inhibition: Atovaquone.

  • Folic Acid Synthesis Inhibition: Proguanil, Sulfadiazine, Pyrimethamine.

  • Reactive Oxygen Species Production: Artemisinin.

  • Heme Detoxification Inhibition: Chloroquine, Quinolines (Mepacrine, Mefloquine).

  • DNA Synthesis Inhibition: Nitroimidazoles (Metronidazole, Tinidazole), Pentamidine.

• Antihelminthic Mechanisms

  • Microtubule Formation Inhibition: Benzimidazoles (Mebendazole, Albendazole).

  • Neuronal Transmission Blockade: Avermectins (Ivermectin).

  • ATP Production Inhibition: Niclosamide.

  • Calcium Influx Induction: Praziquantel.

  • RNA Synthesis Inhibition: Thioxanthenones (Lucanthone, Hycanthone, Oxamniquine).

Antiviral Drugs

• Nucleoside Analogues (Nucleic Acid Synthesis Inhibition):

  • Acyclovir, Vidarabine (Herpes).

  • Azidothymidine/AZT (HIV).

  • Ribavirin, Sofosbuvir (Hepatitis C).

• Other Mechanisms:

  • Non-nucleoside Noncompetitive Inhibition: Etravirine (HIV).

  • Inhibit escape from endosomes: Amantadine, Rimantadine (Influenza).

  • Neuraminidase Inhibition: Oseltamivir, Zanamivir, Peramivir (Influenza).

  • Viral Uncoating Inhibition: Pleconaril (Enterovirus).

  • Protease Inhibition: Ritonavir (HIV), Simeprevir (Hepatitis C).

  • Integrase Inhibition: Raltegravir (HIV).

  • Membrane Fusion Inhibition: Enfuvirtide (HIV).

Antimicrobial Resistance Strategies

• Microbes develop resistance via several strategies:

  • Efflux pumps.

  • Inactivation of enzymes (e.g., $\beta$-lactamase).

  • Blocked penetration.

  • Target modification.

  • Target overproduction.

  • Target mimicry.

  • Enzymatic bypass.

Foundations of Chemotherapy and Antimicrobial Discovery

• Broad Definition of Chemotherapy
    Chemotherapy is not limited to cancer treatment; it involves the use of drugs that target specific biological entities, aiming to eliminate or inhibit growth of harmful organisms.

  • Specific Targets Include:

  • Cancerous cells.

  • Cancerous tissues.

  • Infectious microorganisms including bacteria, viruses, fungi, and parasites.

• History of Antimicrobial Discovery

  • Paul Ehrlich: Influential in the discovery of Compound $606$, an antimicrobial agent that served as an effective treatment for syphilis, signifying the beginning of modern chemotherapy.

  • Alexander Fleming: Discovered penicillin in $1928$, the first naturally produced antimicrobial substance, leading to the era of antibiotics.

  • Howard Florey and Ernst Chain: Credited with discovering methods to scale up penicillin production and purify the drug. They demonstrated its efficacy in animal and human trials during the early $1940\text{s}$, which transformed the treatment of bacterial infections.

General Principles of Antimicrobial Therapy

• Bacteriostatic vs. Bactericidal Classification

  • Bacteriostatic Drugs: Cause a reversible inhibition of bacterial growth. Bacterial growth can restart once the drug is eliminated. Ideal for patients with a functioning immune system.

  • Bactericidal Drugs: Kill the target bacteria directly. Preferred in severe infections, especially when host defenses are compromised.

  • Selection Criteria: The choice between bacteriostatic and bactericidal drugs is based on the type of infection (e.g., bacterial vs. viral), the site of infection, and the immune status of the patient to ensure optimal therapeutic outcomes.

• Clinical Implications of Drug Spectrum

  • Broad-Spectrum Antimicrobials: Effective against a wide range of bacteria. Their use may lead to the development of a superinfection, where a secondary infection occurs after the protective microbiota are killed, necessitating careful monitoring and potentially targeted therapy.

• Pharmacokinetics: Drug Concentration and Administration Route

  • Intravenous (IV) Administration: Drug concentration in the plasma peaks very quickly following administration and then gradually decreases ($t\text{ }1$ through $t\text{ }4$), offering rapid therapeutic effects but requires skilled administration.

  • Oral or Intramuscular (IM) Administration: It takes significantly longer for the drug concentration to reach its peak compared to IV dosing, but may increase patient comfort and compliance.

Mechanisms of Antibacterial Action

• Inhibition of Cell Wall Biosynthesis

  • Targets: Penicillin-binding proteins (PBPs), peptidoglycan subunits, and peptidoglycan subunit transport.

  • Drug Classes:
    

    • $\beta$-lactams: Includes penicillins, cephalosporins, monobactams, and carbapenems.

    • Glycopeptides: (e.g., Vancomycin).

    • Bacitracin.

• Inhibition of Protein Biosynthesis

  • $30S$ Ribosomal Subunit Targets: Aminoglycosides (cause mismatches between codons and anticodons) and Tetracyclines (block tRNA binding).

  • $50S$ Ribosomal Subunit Targets: Macrolides, lincosamides, chloramphenicol (blocks peptide bond formation), and oxazolidinones, demonstrating varying mechanisms that can be exploited in therapy.

• Disruption of Plasma Membranes

  • Targets: Lipopolysaccharide, inner and outer membranes.

  • Drugs: Polymyxin B, colistin, and daptomycin are examples of agents used particularly against resistant strains.

• Inhibition of Nucleic Acid Synthesis

  • RNA Target: Rifamycins (inhibit RNA polymerase activity), primarily used in tuberculosis therapy.

  • DNA Target: Fluoroquinolones (inhibit DNA gyrase), essential for DNA replication, hence widely utilized in various infections.

• Antimetabolite Actions

  • Folic Acid Synthesis Enzyme: Sulfonamides and trimethoprim are notable examples that interfere with bacterial metabolism, underscoring the significance of metabolic pathways as targets.

  • Mycolic Acid Synthesis Enzyme: Isonicotinic acid hydrazide (Isoniazid) is specifically important in treating tuberculosis.

Detailed Breakdown: Cell Wall Synthesis Inhibitors

• $\beta$-Lactam Structure and Function
  

  • All $\beta$-lactams contain a $\beta$-lactam ring, which is the site of attack by inactivating $\beta$-lactamase enzymes, presenting a significant challenge in resistant infections.

  • Chemical modifications to $R$ groups in penicillins provide increased spectrum of activity, acid stability, and resistance to $\beta$-lactamase, highlighting the role of pharmacology in combating resistance.

Protein Synthesis Inhibitors

• $30S$ Subunit Inhibitors

  • Aminoglycosides (Streptomycin, Gentamicin, Neomycin, Kanamycin): Bactericidal; broad-spectrum, leading to production of faulty proteins that disrupt the cytoplasmic membrane, emphasizing the importance of understanding bacterial ribosomal structures.

  • Tetracyclines (Tetracycline, Doxycycline, Tigecycline): Bacteriostatic; broad-spectrum, effective against a variety of gram-positive and gram-negative bacteria.

• $50S$ Subunit Inhibitors

  • Macrolides (Erythromycin, Azithromycin, Telithromycin): Bacteriostatic; broad-spectrum, significant for community-acquired infections.

  • Chloramphenicol: Bacteriostatic; broad-spectrum, often used as a last resort due to potential side effects.

Antimicrobial Resistance Strategies

• Microbes develop resistance through several mechanisms:
  

  • Efflux pumps that expel drugs from the cell.

  • Inactivation of enzymes (e.g., $\beta$-lactamase) that break down drugs.

  • Blocked penetration into the bacterial cell membrane.

  • Target modification that reduces drug binding affinity.

  • Target overproduction, creating competition with the drug.

  • Target mimicry, using similar structures to evade detection.

  • Enzymatic bypass of inhibited pathways.
      Understanding these mechanisms is crucial for developing new therapies and combating resistance in clinical settings.