CONFORMATIONALLY DEFINED 6-S-TRANS-RETINOIC ACID ANALOGS .3. STRUCTURE-ACTIVITY-RELATIONSHIPS FOR NUCLEAR RECEPTOR-BINDING, TRANSCRIPTIONALACTIVITY, AND CANCER CHEMOPREVENTIVE ACTIVITY

We recently demonstrated that conformationally defined 6-s-trans-retin oic acid (RA) analogs were effective in the prevention of skin papillo mas (Vaezi et al. J. Med. Chem. 1994, 37, 4499-4507) and selective ago nists for nuclear receptor binding and activation (Alam et al. J. Med. Chem. 1995, 38, 2302-2310). In order to probe important structure-act ivity relationships, we evaluated a homologous series of four 6-s-tran s-retinoids that are ohexen-1'-ylidene)-3,7-dimethyl-2,4,6-octatrienoi c acids with different substituents at 2' (R(2)) and 3' (R(1)) positio ns on the cyclohexene ring. UAB1 (R(1) = R(2) = H), UAB4 (R(1) = R(2) = Me), UAB7 (R(1) = Me, R(2) = iPr), and UAB8 (R(1) = Et, R(2) = iPr) contain alkyl R groups that mimic, to different extents, portions of t he trimethylcyclohexenyl ring of RA. Both 9Z- and all-E-isomers of the se retinoids were evaluated in binding assays for cellular retinoic ac id-binding proteins (CRABP-I and CRABP-II), a nuclear retinoic acid re ceptor (RAR alpha), and a nuclear retinoid X receptor (RXR alpha). The all E-isomers of UAB retinoids bound tightly to CRABPs and RAR alpha, the binding affinity of the all-E-isomer increased systematically fro m UAB1 to UAB8, and binding for the latter was comparable to that of a ll-E-RA. In contrast to RA, the (9Z)-UAB retinoids were at least 200-f old less active than the all-E-isomers in binding to RAR alpha. The (9 Z)-UAB isomers exhibited increasingly stronger binding to RXR alpha, a nd (9Z)-UAB8 was nearly as effective as (9Z)-RA in binding affinity. T he retinoids were also evaluated in gene expression assays mediated by RAR alpha and RXR alpha homodimers or RAR alpha/RXR alpha heterodimer s. Consistent with the binding affinities, the (all-E)-UAB retinoids a ctivated gene transciption mediated by RAR alpha homodimers or RAR alp ha/RXR alpha heterodimers, while the (9Z)-UAB isomers activated only t he RXR alpha homodimer-mediated transcription. The all-E- and 9Z-isome rs of the UAB retinoids were further evaluated for their capacity to p revent the induction of mouse skin papillomas. When compared to RA, on ly the (all-E)-UAB retinoids containing bulky R(1) and R(2) groups wer e effective in this chemoprevention assay. (9Z)-RA displayed equal cap acity as RA to prevent papillomas, while the 9Z-isomers of the UAB ret inoids were much less effective. Taken together, these studies demonst rate that the cyclohexenyl ring substituents of 6-s-trans-UAB retinoid s are important for their biological activities and that the chemoprev entive effect of the all-E-isomers of these retinoids correlates well with their capacity to bind to RARs and activate RAR/RXR-mediated tran scription.