Synthesis and Characterization of Triphenyl Phosphonium-Modified Triterpenoids with Never Reported Antibacterial Effects Against Clinically Relevant Gram-positive Superbugs

The rapid adaptation of microorganisms to antibiotics, including those previously re-garded as last-resort choices, led to intractable superbugs among both Gram-positive and Gram-negative species, causing increasing antibiotic resistance (AR). AR triggers the worldwide propagation of tenacious to lethal infections, which need urgent development of novel antibacterial agents, active also on worrying superbugs. To this end, totally inac-tive betulin (BET) and betulinic acid (BA), as well as ursolic acid (UA) already active on bacteria Gram-positive, have been chemically modified achieving derivatives 1-7. Com-pounds 1 and 4-7 contained the triphenyl phosphonium (TPP) group, promoting antibac-terial effects, while 2 and 3 did not. 1-7 and all synthetic intermediates were characterized by, chemometric-assisted FTIR and NMR spectroscopy, as well as by other analytical techniques, which confirmed their structure and high purity. Determinations of the min-imum inhibitory concentration values (MICs) of 1-7, BET, BA and UA using a selection of Gram-positive and Gram-negative clinically isolated superbugs, evidenced that com-pounds 4-7 had potent antibacterial effects against Gram-positive strains, higher than those reported so far for other BET, BA and UA derivatives, mainly considering the com-plex pattern of resistance of isolates used here and their clinical source. For the first time, due to the use of TPP, a real activity (MICs 2-16 µg/mL) was conferred to inactive BET and BA (original MICs > 1024 and 256 µg/mL). Moreover, the antibacterial effects of UA were 16 and 32-fold improved against MRSE and MRSA (MICs = 2 vs. 23 and 64 μg/mL). These early, but very promising microbiologic results, pave the way for further experiments with the best performant compounds 5 and 7 (MICs = 2 μg/mL) on an enlarged number of Gram-positive isolates, to assess their time-killing curves, to evaluate their cytotoxicity on eukaryotic cells and to assess their possible antibiofilm activity.