Targeting Endogenous Metabolism with Pro-Drugs and the Development of Antibiotic Resistance

Assignment Brief

You must choose ONLY ONE question out of THREE.

Each question is worth 100 marks.

In your answers, where appropriate:

Use flow charts or diagrams

Give named examples

1. Define synergism, antagonism and indifference in anti-bacterial chemotherapy with specific case examples (20%). Write an essay on the role of whole-cell phenotypic evaluation methods in antibiotic discovery with special reference to HT-SPOTi (80%).

2. Give details of how a named pro-drug targets endogenous metabolism in bacteria (20%). Discuss how antibiotic resistant strains develop in bacteria, and discuss the significance of the permeability barrier and active drug efflux in this process (80%).

3. Comment on antimicrobial drug resistance as a global health challenge (20%). Write an essay on validating arylamine N-acetyl transferase (NAT) as a novel therapeutic target in Mycobacterium tuberculosis (80%).

Sample Answer

Targeting Endogenous Metabolism with Pro-Drugs and the Development of Antibiotic Resistance

Introduction

Antibiotic resistance remains one of the most significant challenges in modern medicine, threatening the efficacy of therapies against bacterial infections worldwide. The strategic use of pro-drugs offers a unique opportunity to target bacterial metabolism selectively, enhancing therapeutic outcomes while minimizing toxicity to host cells. This essay examines the mechanism by which the pro-drug isoniazid acts on Mycobacterium tuberculosis metabolism, explores how bacterial strains develop resistance, and evaluates the role of the permeability barrier and active drug efflux in the development of resistance.

Isoniazid as a Pro-Drug Targeting Endogenous Metabolism

Isoniazid (INH) is a first-line anti-tuberculosis agent that requires metabolic activation within M. tuberculosis to exert its bactericidal effect. As a pro-drug, isoniazid is inactive until converted by the bacterial enzyme catalase-peroxidase (KatG) into its active form. Once activated, INH forms adducts with NAD+, inhibiting the enzymes InhA and KasA, which are essential for mycolic acid synthesis in the bacterial cell wall. Mycolic acids are critical for the integrity and impermeability of the mycobacterial cell envelope; disruption of their synthesis compromises cell viability and increases susceptibility to host immune responses.

The selective activation of INH by KatG ensures that the drug primarily targets M. tuberculosis without affecting host cellular processes. This pro-drug strategy exemplifies how targeting endogenous bacterial metabolism can provide both specificity and potency in antimicrobial therapy.

Development of Antibiotic Resistance in Bacteria

Antibiotic resistance can arise through a combination of genetic mutations and selective pressures imposed by drug exposure. In the case of isoniazid, resistance primarily occurs via mutations in the katG gene, which reduce or eliminate enzymatic activation of the pro-drug. Mutations in the inhA gene or its promoter region can also confer resistance by decreasing the drug’s binding affinity for its enzymatic targets.

Resistance mechanisms are not limited to target modification. Bacteria can develop resistance through horizontal gene transfer, acquiring genes that encode antibiotic-degrading enzymes, altering metabolic pathways to bypass inhibited reactions, or modifying regulatory networks to reduce drug susceptibility. Overuse and misuse of antibiotics accelerate these processes, contributing to the global spread of resistant strains.

Permeability Barrier and Its Role in Resistance

The bacterial cell envelope acts as a natural barrier to antibiotics, particularly in Gram-negative organisms and mycobacteria with lipid-rich cell walls. The permeability barrier limits the diffusion of hydrophilic and hydrophobic molecules, preventing adequate intracellular drug concentrations. Mycobacteria, for example, possess a thick, waxy outer membrane enriched with mycolic acids, which inherently limits drug penetration.

Alterations to this barrier, whether by increased production of cell wall components or modification of porin channels, further reduce drug uptake and contribute to resistance. In clinical isolates of M. tuberculosis, changes in the cell wall composition are associated with reduced susceptibility to multiple antibiotics, complicating treatment regimens.

Active Drug Efflux in Antibiotic Resistance

Active efflux pumps are transport proteins that expel antibiotics from the bacterial cytoplasm, lowering intracellular drug concentrations to sub-lethal levels. Efflux mechanisms can confer multidrug resistance, as a single pump often targets structurally unrelated drugs.

For isoniazid, efflux is not the primary resistance mechanism; however, in other pro-drugs and antibiotics, efflux systems such as AcrAB-TolC in Gram-negative bacteria or Tap in mycobacteria play a crucial role in survival under antibiotic pressure. Upregulation of efflux pumps can occur through mutations in regulatory genes, environmental stress, or exposure to sub-inhibitory antibiotic concentrations.

The interplay between reduced permeability and active efflux creates a formidable barrier to effective antibiotic therapy, necessitating higher drug doses or combination therapy to overcome resistance.

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