Tetrahydroquinoline units in flexible heteroarotinoids (Flex-Hets) convey anti-cancer properties in A2780 ovarian cancer cells

SHetA2 (NSC 721689), our lead Flex-Het anti-cancer agent, consists of a thiochroman (Ring A) and a 4-nitrophenyl (Ring B) linked by a thiourea bridge. In this work, several series of new analogs having a tetrahydroquinoline (THQ, Ring A) unit connected by a urea or thiourea linker to a 4-substituted phenyl (Ring B) have been prepared and evaluated relative to SHetA2 in terms of binding affinity with mortalin and inhibition of A2780 ovarian cancer cells. Six of the derivatives equaled or exceeded the efficacy shown by SHetA2. Compounds 1a-d (series 1), lacking a methyl on the Ring A nitrogen and the gem-dimethyls on the adjacent carbon, showed only weak activity. Salt 2, the quaternized N,N-dimethyl iodide salt analog of 1a, also possessed very modest growth inhibition in the cell line studied. Series 3 compounds, which had a C3 ketone and an N-methyl replacing the sulfur in Ring A, were most successful. Compound 3a [Ring A = 1,2,2,4,4-pentamethyl-3-oxo-1,2,3,4-tetrahydroquinolin-6-yl; urea linker; Ring B = 4-nitrophenyl] had slightly lower potency (IC₅₀ 3.8 μM), but better efficacy (94.8%) than SHetA2 (IC₅₀ 3.17 μM, efficacy 84.3%). In addition, 3c and 3d [urea and thiourea linkers, respectively; Ring B = 4-(trifluoromethyl)phenyl] and 3e and 3f [urea and thiourea linkers, respectively; Ring B = 4-(trifluoromethoxy)phenyl] were also evaluated since these agents possessed electron-withdrawing groups with H-bonding capability. All displayed good activity. Compounds 3c and 3e showed improvement in both potency and efficacy compared to SHetA2. In general, when the linker group between Rings A and B was a urea, efficacy values slightly exceeded those with a thiourea linker in the carbonyl-containing THQ systems 3a-g. In contrast, when Ring A possessed the 1,2,2,4,4-pentamethyl-3-hydroxytetrahydroquinolin-6-yl unit (4a-f, series 4), very modest potency and efficacy was observed. Model compound 5, an exact N-methyl THQ analog of SHetA2, demonstrated less potency (IC₅₀ 4.5 μM), but improved efficacy (91.7%). Modeling studies were performed to rationalize the observed results.