Silibinin derivatives as anti-prostate cancer agents: Synthesis and cell-based evaluations

Abstract

This study aims to systematically explore the alkylation effect of 7-OH in silibinin and 2,3-dehydrosilibinin on the antiproliferative potency toward three prostate cancer cell lines. Eight 7-O-alkylsilibinins, eight 7-O-alkyl-2,3-dehydrosilibinins, and eight 3,7-O-dialkyl-2,3-dehydrosilibinins have been synthesized from commercially available silibinin for the in vitro cell-based evaluation. The WST-1 cell proliferation assay indicates that nineteen out of twenty-four silibinin derivatives have significantly improved antiproliferative potency when compared with silibinin. 7-O-Methylsilibinin (2) and 7-O-ethylsilibinin (3) have been identified as the most potent compounds with 98- and 123-fold enhanced potency against LNCaP human androgen-dependent prostate cancer cell line. Among 2,3-dehydrosilibinin derivatives, 7-O-methyl-2,3-dehydrosilibinin (10) and 7-O-ethyl-2,3-dehydrosilibinin (11) have been identified as the optimal compounds with the highest potency towards both androgen-dependent LNCaP and androgen-independent PC-3 prostate cancer cell lines. 7-O-Ethyl-2,3-dehydrosilibinin (11) was demonstrated to arrest PC-3 cell cycle at the G0/G1 phase and to induce PC-3 cell apoptosis. The findings in this study suggest that antiproliferative potency of silibinin and 2,3-dehydrosilibinin can be appreciably enhanced through suitable chemical modifications on the phenolic hydroxyl group at C-7 and that introduction of a chemical moiety with the potential to improve bioavailability through a linker to 7-OH in silibinin and 2,3-dehydrosilibinin would be a feasible strategy for the development of silibinin derivatives as anti-prostate cancer agents.

Keywords: Anti-proliferative activity; Cell apoptosis; Prostate cancer; Silibinin derivatives; Structure–activity relationship.

Figure 1

Key HMBC correlations in derivatives 4, 13, and 24

Figure 3

Cell cycle analysis of PC-3 cells. PC-3 cancer cells were untreated or treated with 7-O-ethyl-2,3-dehydrosilibinin (11). Cells were harvested after 24 and 30 hours, fixed, stained, and analyzed for DNA content. The distribution and percentage of cells in G1/G0, and G2 phase of the cell cycle are indicated.

Figure 4

Evolution of viable, apoptotic, and necrotic PC-3 cells populations in response to increasing dosages of 7-O-ethyl-2,3-dehydrosilibinin (11).

Scheme 1

Synthesis of silibinin derivatives (2-25). Reactants and conditions: (i) RX, K₂CO₃, DMF (or acetone); (ii) (a) Air, CH₃COOK, DMF, 60 °C, 6 h; (b) RX, K₂CO₃, DMF