Synthesis, structure-activity relationships, and RARγ-ligand interactions of nitrogen heteroarotinoids

Three heteroarotinoids containing a nitrogen atom in the first ring and a C-O linking group between the two aryl rings were synthesized and evaluated for RAR and RXR retinoid receptor transactivation, tumor cell growth inhibition, and transglutaminase (TCase) induction. Ethyl 4-(N,4,4-trimethyl-1,2,3,4-tetrahydroquinolinyl)benzoate (1) contained an N-CH3 group and activated all retinoid receptors except for RAR gamma. Inceasing the hydrophobicity around the rings with analogues ethyl 4-(N,4,4,7-tetramethyl-1,2,3,4-tetrahydroquinolin-6-oyloxy)benzoate (2) [7-methyl group added] and ethyl 4-(4,4-dimethyl-N-isopropyl-1,2,3,4-tetrahydroquinolin-6-oyloxy)benzoate (3) [NCH(CH3)2 group at C-4] increased the potency and specificity for RAR alpha, RAR beta, and RXR alpha, compared to 1, but had little effect on RXR beta and RXR gamma activation. Although 1 and 3 were unable to activate RAR gamma, 2 did activate this receptor with efficacy and high potency equal to that of 9-cis-retinoic acid (9-c-RA). All three heteroarotinoids exhibited 5-8-fold greater specificities for RAR beta over RAR alpha. In addition, esters 1-3 inhibited the growth of two cell lines each derived from cervix, vulvar, ovarian, and head/neck tumors with similar efficiencies to that of 9-c-RA through a mechanism independent of apoptosis. The vulvar cell lines were the most sensitive, and the ovarian lines were the least sensitive. Ester 2 was similar to 1 and 3 except that 2 was a much more potent growth inhibitor of the two vulvar cell lines, which is consistent with strong RAR gamma activation by 2 (but not by 1 and 3) and the high levels of RAR gamma expression in skin. All three heteroarotinoids induced production of TGase, a marker of retinoid activity in human erythroleukemic cells. Esters 2 and 3 were the more potent TGase activators than 1, in agreement with the stronger activation of the RAR receptors by 2 and 3. The biological activities of these agents, and the RAR gamma potency of 2 in particular, demonstrate the promise of these compounds as pharmaceutics for cancer and skin disorders.