Evaluation of synergistic antibacterial activity and cytotoxicity of [W<sub>4</sub>KR<sub>5</sub>]-Curcumin conjugates against drug-resistant pathogens

The rise of antibiotic resistance necessitates new treatment strategies. Although curcumin exhibits antibacterial properties, its poor solubility and bioavailability limit its therapeutic use. This study investigates the antibacterial potential of cyclic peptide [W4KR5], curcumin derivatives, and their new conjugates to overcome curcumin’s limitations. Minimum inhibitory concentration (MIC) assays revealed that [W4KR5] had moderate activity (7.8–31.3 µg/mL), while curcumin alone was less effective (200–225 µg/mL). Curcumin derivatives showed improved activity (MIC ~50 µg/mL), and conjugates of [W4KR5]-curcumin had MICs of 31.3–62.5 µg/mL. Physical mixtures of [W4KR5] with curcumin derivatives demonstrated enhanced activity (MIC = 7.8–31.3 µg/mL) with partial synergy or additive effects (FICI = 0.75–1.5) against S. aureus, E. coli, P. aeruginosa, and K. pneumoniae. In melanoma cells (SK-MEL-28), curcumin derivatives reduced viability to ~55% at 500 µg/mL, while [W4KR5] alone maintained >65% viability. Normal skin cells (Hs-895-Sk) showed >97% viability across all treatments. Hemolysis assays revealed that conjugation reduced [W4KR5]’s hemolytic activity from 34.9% to 15–21%. Time-kill studies in drug-resistant bacterial strains showed that physical mixtures achieved ≥3 log₁₀ reductions within 2–12 hours. These results support the potential of [W4KR5]-curcumin combinations to enhance antibacterial efficacy and reduce cytotoxicity, making them promising candidates for further development.