Characterization of the Synergistic Antibacterial Effect of Verapamil on Bacterial Isolates From Cancer Patients

Antibiotic Resistance, Multi Drug Resistant Organisms

Verapamil, Antibiotic Adjuvant, Efflux Pump Inhibitor, Antimicrobial Synergy, Checkerboard Assay, Fractional Inhibitory Concentration Index (FICI), Minimum Inhibitory Concentration (MIC), MDR / XDR / PDR Bacteria, Cancer Patient Isolates

Multidrug-resistant (MDR) bacteria represent a significant global health challenge, particularly among immunocompromised populations such as cancer patients undergoing chemotherapy. These patients are highly susceptible to severe infections due to weakened immune defenses, often necessitating the use of broad-spectrum or combination antibiotic therapy. Combination regimens may enhance treatment efficacy through synergistic effects, helping to overcome bacterial resistance mechanisms and improve clinical outcomes. In recent years, there has been growing interest in the use of non-antibiotic drugs as adjunctive agents to enhance antimicrobial activity. These agents, often referred to as antibiotic adjuvants or resistance modifiers, may improve antibiotic effectiveness through mechanisms such as inhibition of bacterial efflux pumps, disruption of biofilm formation, or interference with resistance pathways. Verapamil, a widely used calcium channel blocker, has demonstrated potential antimicrobial and resistance-modifying properties. Experimental evidence suggests that verapamil can inhibit bacterial efflux pumps, thereby increasing intracellular concentrations of antibiotics and enhancing their activity against resistant organisms. This study aims to evaluate the in vitro synergistic antibacterial activity of verapamil in combination with selected antibiotics against MDR, extensively drug-resistant (XDR), and pandrug-resistant (PDR) bacterial isolates obtained from cancer patients. Standard microbiological methods will be used to determine antimicrobial susceptibility and minimum inhibitory concentrations, while combination effects will be assessed using established synergy testing approaches. The findings of this study may contribute to identifying novel, cost-effective strategies to combat antimicrobial resistance through drug repurposing and optimization of existing antibiotic therapies.

This study evaluates the potential synergistic antibacterial effect of verapamil in combination with selected antibiotics against multidrug-resistant (MDR), extensively drug-resistant (XDR), and pandrug-resistant (PDR) bacterial isolates obtained from cancer patients. 1. Study Samples Clinical specimens will be collected as part of routine diagnostic care from cancer patients. Specimen types may include: Blood Urine Respiratory samples Wound swabs Other relevant clinical specimens Only non-duplicate bacterial isolates will be included. Only isolates classified as MDR, XDR, or PDR will be selected for further analysis. 2. Isolation of Bacterial Isolates Specimens will be cultured using standard microbiological techniques. Culture media will include: Blood agar MacConkey agar Plates will be incubated under appropriate conditions (temperature and atmosphere) according to standard laboratory protocols. 3. Identification of Bacteria Bacterial isolates will be identified using: Colony morphology Gram staining Biochemical tests Where available, automated identification systems will be used. Identification procedures will follow standard microbiological guidelines. 4. Antimicrobial Susceptibility Testing (AST) AST will be performed according to: Clinical and Laboratory Standards Institute (CLSI), 2025 guidelines Methods include: Kirby-Bauer disk diffusion method Automated susceptibility testing systems (if available) Results will be interpreted using CLSI breakpoints. Quality control will be ensured using standard reference strains. 5. Preparation of Verapamil Verapamil stock solution will be prepared: Under aseptic conditions Using sterile distilled water or appropriate solvent Serial dilutions will be prepared to achieve required working concentrations. 6. Determination of Minimum Inhibitory Concentrations (MICs) MICs for: Selected antibiotics Verapamil Will be determined using: Broth microdilution method Procedures include: Preparation of serial twofold dilutions Standardization of bacterial inoculum (e.g., 0.5 McFarland) Incubation under appropriate conditions MIC values will be recorded as the lowest concentration inhibiting visible growth. 7. Assessment of Synergistic Activity Synergy testing will be performed using: Checkerboard assay Interaction between verapamil and antibiotics will be evaluated by calculating: Fractional Inhibitory Concentration Index (FICI) Interpretation of FICI: ≤ 0.5 → Synergistic \> 0.5 - 1 → Additive \> 1 - 4 → Indifferent \> 4 → Antagonistic 8. Additional/Confirmatory Testing Where applicable, additional methods may be used: Disk diffusion-based combination testing These methods will support and validate synergy findings. 9. Exploratory Analysis If significant synergy is observed: Further analysis may be conducted to investigate resistance mechanisms This may include: Evaluation of changes in resistance patterns Analysis of resistance-related gene expression (if feasible) 10. Quality Control and Laboratory Standards All procedures will follow: Standard microbiological protocols Institutional biosafety regulations Quality assurance measures will be applied to ensure: Accuracy Reproducibility Reliability of results

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