Study of Peptide Amphiphiles as Antimicrobial Agents

The global antimicrobial resistance (AMR) crisis presents a severe public health threat, with drug-resistant infections now the third leading cause of mortality worldwide. This escalating problem, driven by the rapid proliferation of multidrug-resistant organisms (MDROs) like the "ESKAPE" pathogens, severely limits treatment options and imposes substantial economic burdens. The crisis is further exacerbated by a limited pipeline of new antimicrobial agents, as most recently approved drugs are modifications of existing classes rather than novel compounds. Peptide amphiphiles (PAs) offer a promising alternative due to their membrane-targeting mechanism, which reduces the likelihood of resistance development. These molecules are amphiphilic and positively charged, allowing them to interact with and disrupt microbial cell membranes. This non-specific, physical disruption makes it significantly harder for bacteria to evolve resistance compared to conventional antibiotics. Beyond direct killing, PAs can also disrupt bacterial biofilms. Chapter 2 detailed a comprehensive structure-activity relationship (SAR) study of 33 PAs, elucidating the physicochemical properties governing their antimicrobial efficacy and specificity. PCA revealed that zeta potential was critical for Gram-positive MRSA JE2, while hydrophobicity (LogP) was more important for Gram-negative E. coli K12 and P. aeruginosa; for A. baumannii, both factors were equally crucial. PAs forming fibrous nanostructures exhibited lower activity compared to spherical micelles, due to strong intermolecular cohesion. Mechanistic studies confirmed PA-induced membrane permeability and depolarization, leading to visible structural damage. In vitro cytotoxicity and in vivo studies in Galleria mellonella showed promising profiles for selected PAs. Chapter 3 explored the additive effect of PA 25 in combination with naphthylthiazole 1-81 against MRSA JE2. The 16:1 ratio (PA 25:1-81) showed an additive effect, suggesting PA-mediated membrane disruption facilitated 1-81's entry. The combination demonstrated significant anti-biofilm activity, inhibiting formation and disrupting mature biofilms. Gene expression analysis indicated modulation of key virulence and biofilm-related genes (e.g., agrA, saeR, fib). A 21-day multi-passage resistance study showed low resistance induction. The combination significantly reduced in vitro hemolytic activity of PA 25. In vivo studies confirmed robust antimicrobial protection, comparable to or surpassing vancomycin. In conclusion, this thesis successfully demonstrated PAs as effective antimicrobial agents, individually and in combination, against resistant bacterial strains. The detailed SAR provided critical insights for rational design. The consistent membrane-disrupting mode of action reduces resistance development, a significant advantage in the AMR crisis. The synergistic potential of PAs with small molecules enhanced efficacy and improved the therapeutic index by reducing toxicity. Anti-biofilm activity and low resistance induction are particularly encouraging. Future work includes molecular dynamics simulations, target identification for small molecules, broader gene expression analysis, and optimizing design principles for species-specificity, aiming to accelerate clinical translation. In Chapter 4, the conclusion and future directions are described.