Background: Conjugation of triterpenoids such as betulinic acid 1 with the Triphenylphosphonium
(TPP) group is a powerful approach to generating medicinal compounds. Their development proposes structure
optimization in respect of availability and activity towards target cells and organelles. Selection of 1 or its precursor
betulonic acid 2 and the optimal linker is of particular importance for drug candidate identification among
the TPP-triterpenoid conjugates.
Objective: In this study, new C-28-TPP conjugated derivatives of 1 and 2 with the alkyl/alkoxyalkyl linkers of
variable length were synthesized and compared regarding their anticancer, antibacterial, and mitochondriatargeted
Methods: The TPP conjugates of 1 and 2 [6a-f, 7a-f] were synthesized by the reaction of halogenalkyl esters
[3a-f, 4a-f, 5] with triphenylphosphine in acetonitrile upon heating. Cytotoxicity (MTT assay), antibacterial
activity (microdilution assay), and mitochondrial effects (flow cytofluorometry) were studied.
Results: Conjugation with the TPP group greatly increased the cytotoxicity of the triterpenoids up to 30 times.
The conjugates were up to 10-17 times more active against MCF-7 (IC50 = 0.17μM, 72h, 6c) and PC-3 (IC50 =
0.14μM, 72h, 6a) cancer cells than for human skin fibroblasts. The enhanced antibacterial (bactericidal) activity
of the TPP-triterpenoid conjugates with MIC for Gram-positive bacteria as low as 2μM (6a, 7a) was for the first
time revealed. The conjugates were found to effectively inhibit fluorescence of 2′,7′-dichlorofluorescin probe in the
cytosol upon oxidation, decrease transmembrane potential, and increase superoxide radical level in mitochondria.
Conclusion: Relationships between the effects and structure of the TPP-triterpenoid conjugates were evaluated
and discussed. Based on the results, 6a can be selected for further preclinical investigation as a potential