Optimization of oncocin for antibacterial activity using a SPOT synthesis approach: Extending the pathogen spectrum to Staphylococcus aureus

The identification of lead molecules against multidrug-resistant bacteria ensuing the development of novel antimicrobial drugs is an urgent task. Proline-rich antimicrobial peptides are highly active in vitro and in vivo, but only against a few Gram-negative human pathogens, with rather weak activities against Pseudomonas aeruginosa and Staphylococcus aureus. This reduced level of efficacy could be related to inadequate uptake mechanisms or structural differences of the intracellular target proteins, i.e., the 70S ribosome or chaperone DnaK. Here we synthesized peptide arrays on cellulose membranes using cleavable linkers to release the free individual peptides for further antimicrobial tests. Thus, a library of singly substituted oncocin analogs was produced by replacing each residue by all other 19 canonical amino acids yielding a set of 361 individual peptides to be evaluated against a luminescent P. aeruginosa strain. Thirteen substitutions appeared promising and their improved antibacterial activities were confirmed for different bacteria after larger scale synthesis of these analogs. By combining two favorable substitutions into one peptide, we finally obtained an oncocin analog that was ten times more active against P. aeruginosa and even 100-fold more active against S. aureus than the original oncocin, providing minimal inhibitory concentrations of 4-8 and 0.5 μg/mL, respectively.