Synthesis of structurally diverse benzosuberene analogues and their biological evaluation as anti-cancer agents

Abstract

Diversely functionalized, fused aryl-alkyl ring systems hold a prominent position as well-established molecular frameworks for a variety of anti-cancer agents. The benzosuberene (6,7 fused, also referred to as dihydro-5H-benzo[7]annulene and benzocycloheptene) ring system has emerged as a valuable molecular core component for the development of inhibitors of tubulin assembly, which function as antiproliferative anti-cancer agents and, in certain cases, as vascular disrupting agents (VDAs). Both a phenolic-based analogue (known as KGP18, compound 39) and its corresponding amine-based congener (referred to as KGP156, compound 45), which demonstrate strong inhibition of tubulin assembly (low micromolar range) and potent cytotoxicity (picomolar range for KGP18 and nanomolar range for KGP156) are noteworthy examples of such benzosuberene-based compounds. In order to extend the structure-activity relationship (SAR) knowledge base related to benzosuberene anti-cancer agents, a series of eleven analogues (including KGP18) were prepared in which the methoxylation pattern on the pendant aryl ring as well as functional group incorporation on the fused aryl ring were varied. The synthetic approach to these compounds featured a sequential Wittig olefination, reduction, Eaton's reagent-mediated cyclization strategy to achieve the core benzosuberone intermediate, and represented a higher-yielding synthesis of KGP18 (which we prepared previously through a ring-expansion strategy). Incorporation of a fluorine or chlorine atom at the 1-position of the fused aryl ring or replacement of one of the methoxy groups with hydrogen (on the pendant aryl ring of KGP18) led to benzosuberene analogues that were both strongly inhibitory against tubulin assembly (IC50 approximately 1.0 M) and strongly cytotoxic against selected human cancer cell lines (for example, GI50 = 5.47 nM against NCI-H460 cells with fluorobenzosuberene analogue 37). A water-soluble phosphate prodrug salt of KGP18 (referred to as KGP265, compound 44) and a water-soluble serinamide salt (compound 48) of KGP156 were also synthesized and evaluated in this study.

Structural interrogation of benzosuberene-based inhibitors of tubulin polymerization

“…We previously demonstrated the regioselective demethylation of compound 13 . 33,48 Protection of the phenolic moieties as their corresponding tert -butyldimethylsilyl (TBS) ethers followed by nucleophilic addition with an appropriately substituted lithiated aryl ring produced tertiary alcohols 44-50 , which were subsequently dehydrated to yield TBS protected benzosuberene analogues 51-57 (Scheme 3). Removal of the TBS protecting groups upon treatment with TBAF resulted in benzosuberene analogues 64-70 (Scheme 4).…”

“…Representative small-molecule inhibitors of tubulin polymerization: colchicine, combretastatins ( CA4, CA1 ), 20–21 dihydronaphthalene analogue ( OXi6196 ), 30–32 benzosuberene analogues ( KGP18 and KGP156) , 30,33,34 indole analogue ( OXi8006 ) 35 and benzo[ b ]furan analogue ( BNC105 ). 36 …”

“…Two benzosuberene analogues (referred to as KGP18 30,33 and its amino congener KGP156 31,34 ) are especially promising anticancer agents based, in part, on their pronounced cytotoxicity against human cancer cell lines and their efficacy as inhibitors of tubulin polymerization. Our previous studies in this area 30,33,34 resulted in two separate synthetic strategies towards the pendant 9-aryl, fused six-seven ring system present in the benzosuberene analogues KGP18 and KGP156 . These studies included a variety of functional group modifications designed to probe structural diversity as it relates to biological function.…”

Design, synthesis, and biological evaluation of water-soluble amino acid prodrug conjugates derived from combretastatin, dihydronaphthalene, and benzosuberene-based parent vascular disrupting agents

“…40,45–48 These compounds have emerged as potential pre-clinical candidates due to their robust in vitro cytotoxicity (sub-nanomolar to picomolar GI 50 values) against selected human cancer cell lines and strong tubulin inhibitory activities. 45–47 Certain of these dihydronaphthalene analogues have subsequently been reported by another group. 49 In our previous work, 46 we also demonstrated robust tubule disruption [in a human umbilical vein endothelial cell (HUVEC) tube disruption assay] and cell rounding capability of KGP156, which is one of the parent compounds for several of the prodrugs designed and synthesized in this study.…”

Synthesis and biological evaluation of benzimidazole–oxindole conjugates as microtubule-targeting agents

“…As the key structural basis of microtubule, tubulin is considered as a highly attractive target for anticancer therapy [4][5][6][7][8][9][10][11][12]. Several tubulin inhibitors have been approved as the first line chemotherapeutic agents for different types of human cancer [13,14].…”