Methyl Substitution of a Rexinoid Agonist Improves Potency and Reveals Site of Lipid Toxicity

Abstract

(2E,4E,6Z,8E)-8-(3′,4′-Dihydro-1′(2′H)-naphthalen-1′-ylidene)-3,7-dimethyl-2,4,6-octatrienoic acid, 9cUAB30, is a selective rexinoid that displays substantial chemopreventive capacity with little toxicity. 4-Methyl-UAB30, an analogue of 9cUAB30, is a potent RXR agonist but caused increased lipid biosynthesis unlike 9cUAB30. To evaluate how methyl substitution influenced potency and lipid biosynthesis, we synthesized four 9cUAB30 homologues with methyl substitutions at the 5-, 6-, 7-, or 8-position of the tetralone ring. The syntheses and biological evaluations of these new analogues are reported here along with the X-ray crystal structures of each homologue bound to the ligand binding domain of hRXRα. We demonstrate that each homologue of 9cUAB30 is a more potent agonist, but only the 7-methyl-9cUAB30 caused severe hyperlipidemia in rats. On the basis of the X-ray crystal structures of these new rexinoids and bexarotene (Targretin) bound to hRXRα-LBD, we reveal that each rexinoid, which induced hyperlipidemia, had methyl groups that interacted with helix 7 residues of the LBD.

Conformationally Defined Rexinoids and Their Efficacy in the Prevention of Mammary Cancers

“…11 For Class II rexinoid 4 , the methyl group at carbon-7 interacted strongly with Phe346 and Val349 of helix 7. The methyl groups on the reduced naphthalene ring of bexarotene interacted with same residues on helix 7.…”

“…10, 11 Rexinoid 1 was synthesized using commercially available racemic 4-methyl-tetralone. Rexinoid 6 was synthesized utilizing the same methodology starting from benzosuberone rather than tetralones.…”

“…1, 10, 11 Using this method, the K d values for the dissociation of either 9-cis-retinoic acid or UAB30 from its complex with the hRXRα-LBD homodimer were determined to be 14 and 33 nM, respectively (Table 1). 1 Fluorescence titrations of rexinoids 1 – 5 demonstrated each methyl homolog was a better binder to hRXRα-LBD than UAB30 (Table 1).…”

A review of the molecular design and biological activities of RXR agonists

“…These groups displayed the van der Waals interactions with some residues of helix 7 (F346 and V349), revealing the site of lipid toxicity. Notably, these interactions are missing in the crystal structure of 9cUAB30 ( 8, Figure B) . The study demonstrated that the addition of a single methyl group increases the agonist activity nearly 10‐fold when compared with 8 , presumably due to additional van del Waals forces within the RXR binding pocket.…”

“…Atigadda et al synthesized four 9cUAB30 homologues with methyl substitutions at the 4‐, 5‐, 6‐, 7‐, or 8‐position of the tetralone ring 9 to 13 to evaluate the influence of a methyl group in potency and lipid toxicity. Substitution at those positions led to more potent agonists than 8 (Table ).…”