Synthesis and molecular-based analysis of betulin derivatives as strong multidrug reversal agents targeting P-glycoprotein

Chemotherapy is a powerful means of cancer treatment but its efficacy is compromised by the emergence of multidrug resistance (MDR), mainly linked to the efflux transporter P-glycoprotein (P-gp). Based on the chemical structure of the P-gp reverser betulin identified in our previous work, a series of analogs were designed, synthesized and evaluated as a source of novel inhibitors. Compounds 6g and 6i efficiently inhibited rhodamine 123 efflux in the P-gp overexpressed leukemia cells, Lucena 1, at concentrations of 0.19 and 0.39 µM, respectively, and increased the intracellular accumulation of doxorubicin at the submicromolar concentration of 0.098 µM. Compounds 6g and 6i were able to restore the sensitivity of Lucena 1 to Dox at 0.024 and 0.19 µM, respectively. Structure-activity relationship analysis and molecular modeling revealed important information about the structural features conferring activity. All the active compounds fitted in a specific region involving mainly transmembrane helices (TMH) 4–6 from one homologous half and TMH 7 and 12 from the other, also showing close contacts with TMH 6 and 12. Compounds that bound preferentially to another region were inactive, regardless of their free energy of binding. It should be noted that compounds 6g and 6i were devoid of toxic effects against peripheral blood mononuclear normal cells and erythrocytes. The data obtained indicates that both compounds might be proposed as scaffolds for obtaining promising P-gp inhibitors for overcoming MDR.