DESIGN, SYNTHESIS, SPECTROSCOPIC CHARACTERIZATION, AND IN VITRO ANTIBACTERIAL EVALUATION OF STRUCTURALLY MODIFIED SULFAMETHOXAZOLE DERIVATIVES: A STRUCTURE–ACTIVITY RELATIONSHIP STUDY

Background: The increasing prevalence of antimicrobial resistance (AMR) has become a major global health concern, reducing the effectiveness of existing antibacterial agents and necessitating the development of novel therapeutic alternatives. Sulfamethoxazole, a clinically important sulfonamide antibiotic, inhibits bacterial folate biosynthesis but has experienced diminished efficacy due to the emergence of resistant bacterial strains. Structural modification of the sulfamethoxazole scaffold represents a promising medicinal chemistry strategy for enhancing antibacterial activity and overcoming resistance. Objective: The present study aimed to design, synthesize, spectroscopically characterize, and evaluate the in vitro antibacterial activity of structurally modified sulfamethoxazole derivatives. In addition, the study sought to establish structure–activity relationships (SAR) to identify structural features associated with improved antibacterial efficacy. Methods: Four structurally modified sulfamethoxazole derivatives, namely a Schiff base (SMX-1), an N-acyl derivative (SMX-2), a hydrazone derivative (SMX-3), and a Mannich base (SMX-4), were synthesized using conventional synthetic procedures and purified by recrystallization. Structural characterization was performed using Fourier-transform infrared (FT-IR) spectroscopy, proton nuclear magnetic resonance (^1H NMR) spectroscopy, electrospray ionization mass spectrometry (ESI-MS), UV–Visible spectroscopy, elemental analysis, melting point determination, and thin-layer chromatography (TLC). The antibacterial activity of the synthesized compounds was evaluated in vitro against representative Gram-positive and Gram-negative bacterial strains using agar well diffusion, minimum inhibitory concentration (MIC), and minimum bactericidal concentration (MBC) assays. Sulfamethoxazole was used as the reference antibacterial agent, and SAR analysis was performed to correlate structural modifications with biological activity. Results: All synthesized derivatives were successfully obtained and confirmed by spectroscopic and analytical characterization. Comparative antibacterial evaluation demonstrated that structural modification of the sulfamethoxazole nucleus influenced antibacterial activity, with several derivatives exhibiting enhanced inhibitory effects compared with the parent drug. Variations in antibacterial potency were associated with the nature of the introduced aromatic and heterocyclic substituents. SAR analysis suggested that appropriate structural modifications enhanced molecular interactions with bacterial folate biosynthesis targets, thereby improving antibacterial efficacy. Conclusion: The present investigation demonstrates that rational structural modification of sulfamethoxazole is an effective approach for developing novel antibacterial agents with improved in vitro activity. The synthesized derivatives exhibited promising antibacterial potential and provide a scientific basis for further molecular docking, toxicity studies, pharmacokinetic evaluation, and in vivo investigations toward the development of next-generation sulfonamide-based antibacterial therapeutics.