Mechanism of Drug Resistance in Anti-Tuberculosis (ATT) Drugs

I. Introduction

II. General Mechanisms of Resistance (Applicable to All ATT Drugs)

A. Traditional (Classical) Mechanism — Mutant Selection

• Erratic / incomplete therapy kills susceptible bacilli but allows pre-existing DR mutants to replicate
• Probability of simultaneous resistance to two drugs is infinitesimally small (product of individual mutation rates), explaining why combination therapy prevents MDR
• Programme-related factors: drug shortages, poor DST, non-standardised regimens, inadequate training
• Patient-related factors: non-adherence, default, irregular treatment

B. Pharmacokinetic (PK) Variability — Newer Understanding

• Drug resistance can develop even with excellent adherence (Sharma & Mohan, Chapter 42)
• PK mismatch: drugs with long half-lives (bedaquiline, clofazimine) may act as functional monotherapy during washout periods
• Sub-MIC (minimum inhibitory concentration) drug concentrations in hollow fibre system studies strongly correlate with resistance amplification
• Between-person variability in drug absorption/metabolism drives site-specific sub-therapeutic levels

C. Induction of Efflux Pumps

• Sub-therapeutic drug concentrations induce multi-drug efflux pumps early in treatment
• Efflux pumps confer low-level resistance/tolerance, protect bacilli during several replication rounds
• Act as a "gateway" for eventual high-level chromosomal mutations — termed the "antibiotic resistance arrow of time"
• Co-existence of efflux pumps and genetic mutations is well documented

D. Differential Drug Penetration into Lung Micro-compartments

• Different susceptibility profiles (hetero-resistance) found in isolates from different lesions in the same patient (cavity wall vs. peri-fibrotic margin vs. normal lung)
• Immunopathology in the lung drives within-person PK variability
• Granulomas and caseous cavities may act as pharmacological sanctuaries with sub-MIC drug concentrations

E. Genotype and Compensatory Mutations

• Certain Mtb strains have higher in vivo mutation rates (dysfunctional DNA repair)
• Resistance-encoding mutations associate with compensatory mutations elsewhere in the Mtb genome (WGS studies)
• Compensatory mutations may restore fitness cost of DR mutations, allowing efficient transmission
• May alter mycobacterial antigen specificity, affecting T-cell immune responses

F. Primary (Person-to-Person) Transmission

• In high-burden settings (e.g., South Africa, China), 80–90% of MDR-TB cases result from primary transmission rather than acquired resistance
• A minority of "super-spreaders" (<10–20% of patients) account for most transmission
• XDR-TB outbreaks (e.g., Tugela Ferry) are primarily transmission-driven, confirmed by WGS

III. Drug-Specific Mechanisms of Resistance

1. Isoniazid (INH)

2. Rifampicin (RIF)

3. Pyrazinamide (PZA)

4. Ethambutol (EMB)

5. Streptomycin (SM)

6. Second-line Injectable Agents (Amikacin, Kanamycin, Capreomycin)

7. Fluoroquinolones (Ofloxacin, Moxifloxacin, Levofloxacin)

8. Ethionamide (Eto)

IV. Types of Drug Resistance

V. The Bacteriological Basis (Why Combination Therapy Prevents Resistance)

• In a cavity (bacterial load ~10⁸), spontaneous INH-resistant mutants occur at rate 1/10⁸ → ~1 resistant organism
• Spontaneous RIF-resistant mutants at rate 1/10⁸ → ~1 resistant organism
• Probability of simultaneous INH + RIF resistance = 1/10⁸ × 1/10⁸ = 1/10¹⁶ — clinically negligible
• Hence, combination therapy prevents emergence of resistance by multiple independent drug mechanisms acting simultaneously

VI. Clinical Implications

1. Universal DST — every TB case should undergo DST for at least rifampicin resistance (Xpert MTB/RIF)
2. Line Probe Assays (LPA) — MTBDRplus detects katG and inhA mutations; guides dosing (high-dose INH for inhA mutations)
3. Therapeutic Drug Monitoring (TDM) — to address PK variability and sub-therapeutic levels
4. Pharmacogenomics — identifying patient genotypes with rapid drug metabolism mandating TDM
5. High-dose INH — overcomes inhA-mediated low-level resistance
6. Avoid functional monotherapy — especially with long-half-life drugs (bedaquiline, clofazimine)
7. Whole Genome Sequencing (WGS) — identifies resistance mutations, compensatory mutations, and tracks outbreaks

VII. Summary Table — Key Resistance Mutations

• Chapter 42 (Dheda et al.): Pathogenesis of DR-TB — Table 42.2, Figure 42.3 (Pathogenesis diagram), pp. 579–607
• Chapter 44 (Singla et al.): Table 44.2 — Mechanism of action and genes involved in resistance for first-line drugs, p. 622
• Chapter 43 (Paramasivan, Balasangameshwara): ATT drug resistance surveillance, pp. 609–620

Recent Evidence Note: A 2025 review (Sikandar & Xing, Microb Pathog 2025; PMID: 40609770) confirms the above mechanisms and highlights emerging resistance to newer agents (bedaquiline: atpE mutations; linezolid: rrl, rplC mutations), consistent with Sharma & Mohan's framework.