Previously, we reported
sipholenol A, a sipholane
triterpenoid from the Red Sea sponge Callyspongia siphonella, as a potent reversal of multidrug resistance (MDR) in
cancer cells that overexpressed
P-glycoprotein (P-gp). Through extensive screening of several related sipholane
triterpenoids that have been isolated from the same sponge, we identified sipholenone E, sipholenol L and siphonellinol D as potent reversals of MDR in
cancer cells. These compounds enhanced the cytotoxicity of several P-gp substrate anticancer drugs, including
colchicine,
vinblastine and
paclitaxel, and significantly reversed the MDR-phenotype in P-gp-overexpressing MDR
cancer cells KB-C2 in a dose-dependent manner. Moreover, these three sipholanes had no effect on the response to
cytotoxic agents in cells lacking P-gp expression or expressing
MRP1 (ABCC1) or MRP7 (ABCC10) or
breast cancer resistance
protein (BCRP/ABCG2). All three sipholanes (IC(50) >50 μM) were not toxic to all the cell lines that were used. [(3)H]-
Paclitaxel accumulation and efflux studies demonstrated that all three
triterpenoids time-dependently increased the intracellular accumulation of [(3)H]-
paclitaxel by directly inhibiting P-gp-mediated
drug efflux. Sipholanes also inhibited
calcein-AM transport from P-gp-overexpressing cells. The Western blot analysis revealed that these three
triterpenoids did not alter the expression of P-gp. However, they stimulated P-gp
ATPase activity in a concentration-dependent manner and inhibited the photolabeling of this transporter with its transport substrate [(125)I]-iodoarylazidoprazosin. In silico molecular docking aided the virtual identification of
ligand binding sites of these compounds. In conclusion, sipholane
triterpenoids efficiently inhibit the function of P-gp through direct interactions and may represent potential reversal agents for the treatment of MDR.