RXR HOMODIMER FORMATION AND BRIDGED BICYCLIC AROMATIC COMPOUNDS AND THEIR USE IN MODULATING GENE EXPRESSION

Abstract: The invention provides a method of screening a substance for the ability to affect the formation of a retinoid X receptor homodimer comprising combining the substance and a solution containing retinoid X receptors and determining the presence of homodimer formation. The screening method can be used to determine compounds which selectively activate homodimer formation and heterodimer formation. Also provided is a method of screening a substance for an effect on a retinoid X receptor homodimer's ability to bind DNA comprising combining the substance with the homodimer and determining the effect of the compound on the homodimer's ability to bind DNA. Finally, the invention provides methods of activating retinoid X receptor homodimer formation. Bridged bicyclic aromatic compounds are provided having structure (I); wherein R<1>, R<2>, R<3>, R<4>, R<5> and n are as defined herein. The compounds are useful for modulating gene expression of retinoic acid receptors, vitamin D receptors and thyroid receptors. Pharmaceutical compositions and methods for modulating gene expression are provided as well.

Description: RXR HOMODIMER FORMATION AND BRIDGED BICYCLIC AROMATIC COMPOUNDS AND THEIR USE IN MODULATING GENE EXPRESSION TECHNICAL FIELD This invention relates generally to the regulation of gene expression by retinoid receptors, and more particularly relates to the retinoid x receptor and novel bridged bicyclic aromatic compounds that are useful in modulating gene expression by retinoic acid receptors, retinoid X receptors, vitamin D receptors and thyroid receptors. BACKGROUND ART The vitamin A metabolite all-trans-retinoic acid (RA) and its natural and synthetic derivatives (retinoids) exert a broad range of biological effects 1,2 . Clinically, retinoids are important therapeutics in the treatment of skin diseases and cancers 3'6 . Understanding how the multitude of retinoid actions can be mediated at the molecular level has been greatly enhanced by the cloning and characterization of specific nuclear receptors, the retinoic acid receptors (RARs) 7'12 and the retinoid X receptors (RXRs) 13'17 . RARs and RXRs are part of the steroid/thyroid hormone receptor superfamily 18,19 . Both types of receptors are encoded by three distinct genes, a, β, and y, from which, in the case of RARs, multiple isoforms can be generated 20'22 . Interestingly, while RARs are specific to vertebrates, the RXRs have been well conserved from Drosophila to man 17,23 . Despite the considerable advances in the understanding of the molecular mechanisms of retinoid receptor action in recent years, a central question of whether distinct molecular pathways for naturally occurring retinoids exist has not yet been answered. The recent observation that the RA stereoisomer 9-cis-RA binds with high affinity to RXRs 23,24 suggested a retinoid response pathway distinct from that of all-trans-RA. However, it was almost simultaneously discovered by several laboratories that RARs require interaction with auxiliary receptors for effective DNA binding and function and that RXRs are such auxiliary receptors 15,16,26"29 . Hence, RARs appear to function effectively only as heterodimeric RAR/RXR complexes, or in combination with comparable auxiliary proteins that still need to be identified. Similarly, RXRs were shown to require RARs, thyroid hormone receptors (TRs), or Vitamin D 3 receptors (VDRs) for effective DNA binding 15,16,26'29 . Thus, from these DNA binding studies, RXRs appeared to be able to function predominantly if not exclusively as auxiliary receptors, thereby playing a crucial role in generating a high degree of diversity and specificity of transcriptional controls and mediating the highly pleiotropic effects of different hormones by increasing DNA affinity and specificity for at least 3 different classes of ligand- activated receptors. Contrary to these findings, the present invention provides that RXRs form homodimers. The invention provides that these homodimers effectively bind to,specific response elements in the absence of auxiliary receptors and their DNA binding specificity is distinct from that of the RXR containing heterodimers. The invention demonstrates a novel mechanism for retinoid action by which a ligand induced-homodimer mediates a distinct retinoid response pathway. Additionally, ligands are provided which selectively activate RXR homodimer formation. The present invention also provides a new class of retinoids in the form of bridged bicyclic aromatic compounds as will be described in detail herein. These new compounds are useful for regulating and/or eliciting selective gene expression by receptors in the retinoic acid family, i.e., RARs, RXRs, vitamin D receptors (VDRs), and thyroid hormone receptors (THRs). While not wishing to be bound by theory, it is postulated that the presently disclosed and claimed compounds are effective in modulating gene expression by virtue of their capability of interacting with a receptor protein that binds to the aforementioned receptors. The compounds are also believed to be useful in modulating gene expression by inducing formation of RXR homodimers in addition to RAR-RXR, VDR-RXR and THR- RXR heterodimers. The novel compounds are thus useful for controlling cellular processes that are regulated by thyroid hormone, vitamin D, and retinoids such as 9-cis-retinoic acid, the natural ligand for RXR. Thus, acne, leukemia, psoriasis, and skin aging, all of which are regulated by retinoic acid, may be treated using the compounds of the invention, as may bone calcification, regulated by vitamin D, and energy levels, regulated by thyroid hormone. Unlike the synthetic retinoids of the prior art, the present compounds are believed to provide for substantially reduced side effects and teratogenicity. SUMMARY OF THE INVENTION The invention provides a method of screening a substance for the ability to affect the formation of a retinoid X receptor homodimer comprising combining the substance and a solution containing retinoid X receptors and determining the presence of homodimer formation. Also provided is a method of screening a substance for an effect on a retinoid X receptor homodimer's ability to bind DNA comprising combining the substance with the homodimer and determining the effect of the compound on the homodimer's ability to bind DNA. A method of inhibiting an activity of a retinoid X receptor heterodimer comprising increasing the formation of a retinoid X receptor homodimer, thereby preventing the retinoid X receptor from forming a heterodimer and preventing the resulting heterodimer activity is also provided. A method of inhibiting an activity of a retinoid X receptor homodimer is also provided. A method of determining an increased probability of a pathology associated with retinoid X receptor homodimer formation and treating such pathology are further provided. In addition, a method of screening a response element for binding with a retinoid X receptor homodimer is provided. Finally, the invention provides methods of activating retinoid X receptor homodimer formation. Additionally, it is a primary object of the invention to address the above-mentioned need in the art by providing novel compounds useful to modulate gene expression by a receptor in the retinoic acid family of receptors. It is another object of the invention to provide novel compounds useful to control cell differen¬ tiation processes regulated by thyroid hormone, vitamin D, and/or retinoids such as 9-cis-retinoic acid. It is still another object of the invention to provide novel compounds in the form of bridged, bicyclic aromatic structures. It is yet another object of the invention to provide pharmaceutical compositions containing one or more of the novel compounds. It is a further object of the invention to provide a method for modulating gene expression of a receptor selected from the group consisting of retinoic acid receptors, retinoid X receptors, vitamin D receptors, and thyroid receptors. It is still a further object of the invention to provide a method for controlling cell differentiation processes regulated by thyroid hormone, vitamin D, and/or retinoids such as 9-cis-retinoic acid. Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. In one aspect, then, the invention relates to novel compounds having the structural formula (I) R 1 in which: R 1 is selected from the group consisting of lower alkyl and adamantyl; R 2 is -0-R 6 or -S-R 6 where R 6 is lower alkyl; or where R 1 is ortho to R 2 , R 1 and R 2 may be linked together to form a 5- or 6-membered cycloalkylene ring, either unsubstituted or substituted with 1 to 4 lower alkyl groups, and optionally containing 1 or 2 heterocyclic atoms selected from the group consisting of 0, S and NR where R is hydrogen or lower alkyl, preferably adjacent to the aromatic ring; R 3 is selected from the group consisting of carbonyl, in which X 1 and X 2 are independently selected from the group consisting of 0, S and methylene, wherein at least one of X 1 and X 2 is 0 or S, or wherein one of X 1 and X 2 is NR, and the other is methylene, is 2 or 3, R 6 , R 7 , R 8 and R 9 are independently hydrogen or lower alkyl, with the proviso that when n is 0, R 6 and R 7 are not both hydrogen and R 8 and R 9 are not both hydrogen, or R 8 and R 9 may be linked together to form a cycloalkylene ring containing 3 to 6 carbon atoms, and * represents the point of attachment of the R 3 substituent to the remainder of the molecule; and R* is selected from the group consisting of in which R 10 is hydrogen or methyl, 1 is 0 or 1, and ** represents the point of attachment of the R* substituent to the remainder of the molecule, the R 5 are independently selected from the group consisting of lower alkyl and lower alkoxy; and n i s 0, 1, 2 or 3, with the proviso that if n is 0, R 3 is other than carbonyl, *C=CH 2 or CH 2 . The invention also encompasses pharmaceutically acceptable esters, amides and salts of such compounds, as will be explained in detail, infra. In other aspects, the invention relates to pharmaceutical compositions containing the aforementioned compounds and to methods of using the compounds to modulate selective gene expression by a receptor in the retinoic acid family of receptors. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows 9-cis-retinoic acid induces RXR homodimer binding on TREpal . (a) In vitro synthesized RXRαr was incubated either with (+) or without (-) indicated hormones (10" 7 M 9-cis-RA: 10" 6 M RA; 10 '6 M T 3 ) in the presence or absence of in vitro synthesized TRα or RARβ for 30 min at room temperature. After this preincubation, the reaction mixtures were analyzed by gel retardation assay using 32 P-labeled TREpal as probe. Lane 1 represents the nonspecific binding of unprogram ed reticulocyte lysate. Open triangles indicate the nonspecific complex observed with unprogrammed reticulocyte lysate. Solid triangles indicate the specific TRσ-RXR heterodimer binding. Arrows indicate specific RXRσ homodimer binding. The RXRσ/RARβ heterodimer migrates at the same position as the RXRσ homodimer. For comparison, the effect of 9-cis-RA on RARβ binding is shown. (b) To determine that 9-cis-RA induced DNA binding complex contains RXR protein, Flag-RXRσ (F-RXR), an RXRσ derivative that contains an eight-amino-acid epitope (Flag) at its amino terminal end which can be recognized by a specific anti-Flag monoclonal antibody, was constructed. In vitro synthesized F-RXRσ was incubated either with (+) or without (-) 10 "7 M 9-cis-RA in the presence of either specific anti-Flag antibody (σF) or nonspecific preimmune serum (NI) for 30 min at room temperature. The effect of anti-Flag antibody on F-RXRσ binding in the presence of 9-cis-RA was then analyzed by gel retardation assay using 32 P-labeled TREpal as a probe. Lane 1 represents the nonspecific binding of unprogrammed reticulocyte lysate (open triangles). Arrows indicate the specific F-RXRσ homodimer and RAR-RXR heterodimer binding. Diamonds indicate the anti-Flag antibody up-shifted F-RXR homodimer. Figure 2 shows the characterization of 9-cis-RA induced RXR homodimer on TREpal . (a) Cooperative binding of 9-cis-RA induced RXRσ homodimer. Formation of RXR-DNA complex at different receptor concentrations in the absence or presence of 10" 7 M 9-cis-RA was analyzed by gel retardation assay using labeled TREpal as probe. Open triangle indicates the nonspecific binding of an unprogrammed reticulocyte lysate. Arrows indicate the specific RXR binding complex. (b) Quantitation of the RXR binding complex at different receptor concentrations in the presence of 9-cis-RA. Gel slices containing RXR binding complex in the presence of 9-cis-RA shown in Figure 2(a) were excised and counted in a scintillation counter and plotted. (c) 9-cis-RA concentration-dependent binding of RXR homodimer on TREpal. Equal amounts of in vitro synthesized RXR protein were incubated with indicated concentrations of 9-cis-RA. The reaction mixtures were then analyzed by gel retardation assay using labeled TREpal as a probe. Open triangles indicate the nonspecific binding of unprogrammed reticulocyte lysate. Arrows indicate the specific RXR binding complex. Figure 3 shows 9-cis-RA induces RXR homodimer binding on RXR-specific response elements. (a) Nuclear receptor binding elements used in this study. These oligonucleotides were synthesized with appropriate restriction sites at both ends as indicated by the small letters. Sequences that are closely related to the AGG/TTCA motif are indicated by arrows. (b) The effect of 9-cis-RA on RXR homodimer binding of ApoAI-RARE or CRBPII-RARE was analyzed essentially as described in Fig. la. Lane 1 represents the nonspecific binding of unprogrammed reticulocyte lysate, which are indicated by the open triangles. Solid triangles indicate the RAR/RXR heterodimer complex. Arrows indicate the specific RXR binding complex. Figure 4 shows response element specific binding of RXR homodimer. The effect of 9-cis-RA on RXR binding on RA specific response elements (a), T 3 specifip response elements (b), or estrogen specific response element (c) was analyzed by gel retardation assays as described in Figure la. For comparison, the binding of RXR/RAR heterodimer (a), RXR/TR heterodimer (b) or estrogen receptor (c) is shown. Open triangles indicate the nonspecific binding of unprogrammed reticulocyte lysate. Solid triangle indicates the RAR/RXR heterodimer complex (a), TR/RXR heterodimer complex (b) or ER complexes (c). Figure 5 shows RXR homodimerization occurs in solution. 35 S-labeled in vitro synthesized RXRσ proteins were incubated with partially purified bacterially expressed Flag-RXR (F-RXR) (+) or similarly prepared glutathione transferase control protein (-) either in the presence or absence of response elements or chemical cross- linker DSP as indicated. After incubation, either anti-Flag antibody (F) or nonspecific preimmune serum (NI) was added. 10 "7 M 9-cis-RA was maintained during working process. The immune complexes were washed in the presence of 10 *7 M-cis-RA. boiled in SDS sample buffer and separated on a 10% SDS-PAGE. The 35 S-labeled in vitro synthesized RXRσ protein is shown in the right panel. Figure 6 shows transcriptional activation of RXR and RARσ: RXR heterodimers by 9-cis-RA on natural response elements. CV-1 cells were cotransfected with 100 ng of the reporter plasmids (a) TREpal-tk- CAT (b) 0RARE-tk-CAT (c) ApoAI-RARE-tk-CAT and (d) CRBPI-RARE-tk-CAT and 5 ng of empty pECE expression vector, pECE-RXRα, pECE RARσ or combination of both as indicated. Transfected cells were treated with no hormone (open bars), 10 "7 M RA (shadowed bars) or 10" 7 M 9-cis-RA (dark shadowed bars). The results of a representative experiment performed in duplicate are shown. Figure 7 shows RXR -dependent transactivation of reporter constructs (a) TREpal-tk-CAT (10) or (b) CRBPII-tk-CAT (10) by 9-cis-RA or retinoids SR11203, SR11217, SR11234, SR11235, SR11236, and SR11237. Results of a representative experiment carried out four times are shown. In four independent experiments, induction profiles did not vary significantly. CAT activity was normalized for transfection and harvesting efficiency by measuring the enzymatic activity derived from the cotransfected β-galactosidase expression plasmid (pCHHO, Pharmacia). DETAILED DESCRIPTION OF THE INVENTION Definitions and Nomenclature: Before the present compounds, compositions and methods are disclosed and described, it is to be understood that this invention is not limited to specific synthetic methods, specific pharmaceutical carriers, or to particular pharmaceutical formulations or administration regimens, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. It must be noted that, as used in the specification and the appended claims, the singular forms "a," "an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a bicyclic aromatic compound" includes mixtures of bicyclic aromatic compounds, reference to "a pharmaceutical carrier" includes mixtures of two or more such carriers, and the like. In this specification and in the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings: The term "alkyl" as used herein refers to a branched or unbranched saturated hydrocarbon group of 1 to 24 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t- butyl, octyl, decyl, tetradecyl , hexadecyl, eicosyl, tetracosyl and the like. Preferred alkyl groups herein contain 1 to 12 carbon atoms. The term "lower alkyl" intends an alkyl group of one to six carbon atoms, preferably one to four carbon atoms. The term "cycloalkyl" intends a cyclic alkyl group of three to eight, preferably five or six, carbon atoms. The term "alkoxy" as used herein intends an alkyl group bound through a single, terminal ether linkage; that is, an "alkoxy" group may be defined as -OR where R is alkyl as defined above. A "lower alkoxy" group intends an alkoxy group containing one to six, more preferably one to four, carbon atoms. The term "alkylene" as used herein refers to a di- functional saturated branched or unbranched hydrocarbon chain containing from 1 to 24 carbon atoms, and includes, for example, methylene (-CH 2 -), ethylene (-CH 2 -CH 2 -), propylene (-CH 2 -CH 2 -CH 2 -), 2- methylpropylene [-CH 2 -CH(CH 3 )-CH 2 -], hexylene [-(CH 2 ) 6 -] and the li e. "Lower alkylene" refers to an alkylene group of 1 to 6, more preferably 1 to 4, carbon atoms. The term "cycloalkylene" as used herein refers to a cyclic alkylene group, typically a 5- or 6-membered ring. The term "alkene" as used herein intends a mono- unsaturated or di-unsaturated hyd.rocarbon group of 2 to 24 carbon atoms. Preferred groups within this class contain 2 to 12 carbon atoms. Asymmetric structures such as (AB)C=C(CD) are intended to include both the E and Z isomers. This may be presumed in structural formulae herein wherein an asymmetric alkene is present, or it may be explicitly indicated by the bond symbol . "Optional" or "optionally" means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or cir- cumstance occurs and instances where it does not. For example, the phrase "optionally substituted phenyl" means that the phenyl may or may not be substituted and that the description includes both unsubstituted phenyl and phenyl where there is substitution. By the term "effective amount" of a compound as provided herein is meant a nontoxic but sufficient amount of the compound to provide the desired regulation of gene expression. As will be pointed out below, the exact amount required will vary from subject to subject, depending on the species, age, and general condition of the subject, the severity of the disease that is being treated, the particular bicyclic compound used, its mode of administration, and the like. Thus, it is not possible to specify an exact "effective amount." However, an appropriate effective amount may be determined by one of ordinary skill in the art using only routine experimentation. By "pharmaceutically acceptable" is meant a material that is not biologically or otherwise undesirable, i.e., the material may be administered to an individual along with the selected bicyclic compound without causing any undesirable biological effects or interacting in a deleterious manner with any of the other components of the pharmaceutical composition in which it is contained. "Eliciting," "modulating" or "regulating" selective gene expression is intended to mean that a compound is capable of acting as an activator or an antagonist of gene expression by a particular receptor, i.e., a receptor in the retinoic acid family. The "retinoic acid family" of receptors, also termed "retinoid receptors," is intended to encompass retinoic acid receptors, retinoid X receptors, vitamin D receptors and thyroid hormone receptors. The aforementioned three groups of receptors may also be loosely referred to herein as "retinoic acid receptors," i.e., such that the term includes retinoid X receptors, vitamin D receptors and thyroid hormone receptors in addition to retinoic acid receptors themselves. "Bridged, bicyclic aromatic compounds" intends all compounds encompassed by the structure of formula (I). These compounds may also be termed "retinoids," with that term further intended to include such species as retinoic acid and 9-cis retinoic acid. The invention provides a method of screening a substance for the ability to affect the formation of an RXR homodimer comprising combining the substance and a solution containing RXRs and determining the presence of a homodimer formation. The presence of homodimer formation can, for example, be determined by detecting the activation of transcription by the RXR homodimer or by coprecipitation. The affect can be the induction of homodimer formation, for example, an activity similar to that activated by 9-cis-RA or an activity which selectively activates homodimer formation over heterodimer formation. By "selectively activates" is meant a compound which activates homodimer formation but does not significantly activate heterodimers. Alternatively, "selectively activates" includes compounds which activate heterodimer formation but do not significantly activate homodimer formation. "Significant activation" includes activation that is sufficient to cause a harmful pharmological effect on the subject. The affect can also be the inhibition of homodimer formation. Examples of inhibition include a substance which competes for 9-cis-RA binding to the receptor but itself does not activate or induce dimerization or which binds 9-cis-RA to block its activity. Such screening of substances is routinely carried out given the subject discovery of homodimer formation. In particular assays set forth below can generally be used for screening by merely substituting the substance of interest for 9-cis-RA. A good starting point for screening such "substances" is the activity of 9-cis-RA described herein. The substituents on 9-cis-RA can be varied to make 9-cis-RA analogs and screened in the method to determine any increase or decrease in homodimer formation. However, any substance can be screened in this assay to determine any affect on homodimer formation. Such compounds can then be used to promote homodimer formation and gene transcription in a cell. A cell as used herein includes cells found either in vitro or in vivo. Thus, the compounds can be administered to a human subject to effect RXR homodimer formation and promote transcription of a gene activated by an RXR homodimer. Such compounds are set forth in the Examples. The data set forth herein utilizes RXRσ. However, given the high homology between RXRσ, β and γ t each protein should form homodimers and have the activity described for RXRσ homodimers. Relatedly, homodimers can form between different RXRs. For example, homodimers can form between RXRσ and RXRβ or between RXRβ and RXRp or between RXRσ and RXRy. The activity of these homodimers can be confirmed using the methods set forth herein. The invention also provides a method of screening a substance for an effect on an RXR homodimer's ability to bind DNA comprising combining the substance with the homodimer and determining the effect of the compound on the homodimer's ability to bind DNA. For example, compounds which might bind the homodimer or bind the DNA response element recognized by an RXR homodimer can be screened in this method. The invention further provides a method of inhibiting an activity of an RXR-containing heterodimer comprising increasing the formation of an RXR homodimer, thereby preventing the RXR from forming a heterodimer and preventing the resulting heterodimer activity. The activity can be any activity but is generally the activation or repression of transcription. The activity can be blocked, for example, by utilizing RXRs to form homodimers which otherwise would be available to form heterodimers. Since the number of heterodimers are decreased, the activity of the heterodimers is decreased. In one example, the RXR heterodimer is comprised of thyroid hormone receptor and RXR. The activity of the RXR/TR heterodimer was decreased. Other heterodimers can be tested using standard methods given the teaching set forth herein. The invention also provides a method of inhibiting an activity of an RXR receptor homodimer comprising preventing the formation of the RXR homodimer. Such inhibition can be obtained, for example, by inhibiting 9-cis-RA or the transcription or activation by 9-cis-RA. The activity inhibited is generally the activation or repression of transcription. The invention also provides a method of inhibiting an activity of an RXR homodimer comprising preventing the binding of the RXR homodimer to its response element. For example, the activity of a receptor which competes for the same response element can be promoted. In general, the activity which is inhibited is the activation or repression of transcription. The invention still further provides a method of determining an increased probability of a pathology associated with RXR homodimer formation comprising detecting a modulation of RXR homodimer formation in the subject when compared to a normal subject. The modulation can be an increase or a decrease in homodimer formation. Such a modulation can result, for example, from a mutated RXR. The decrease can be detected by an assay for the homodimers in a sample or by detecting mutations known to decrease homodimer formation. Relatedly, the invention provides a method of treating a pathology associated with RXR homodimer formation in a subject comprising modulating homodimer formation in the subject. The modulation can be an increase or a decrease depending on the pathology. Such an increase can be accomplished, for example, utilizing compounds which promote RXR transcription. The pathology can be associated with the skin, e.g., acne and psoriasis. In addition, the pathology can be a cancer. The exact amount of such compounds required will vary from subject to subject, depending on the species, age, and general condition of the subject, the severity of the disease that is being treated, the particular compound used, its mode of administration, and the like. Thus, it is not possible to specify an exact activity promoting amount. However, an appropriate amount may be determined by one of ordinary skill in the art using only routine experimentation given the teachings herein. The invention also provides a purified RXR homodimer. By "purified" is meant free of at least some of the cellular components associated with RXR homodimers in a natural environment. The invention further provides a method of screening a response element for binding with a RXR homodimer comprising combining the response element with the RXR homodimer and detecting the presence of binding. The presence of binding can be determined by a number of standard methods. In one method, binding is detected by the transcriptional activation of a marker which is operably linked to the response element. By "operably linked" is meant the marker can be transcribed in the presence of the transcriptional activator. The invention grows out of our study of the effects of the natural vitamin A derivative 9-cis-RA on retinoid receptor DNA binding and transcriptional activation. In contrast to al1-trans.-RA, the 9- cis analog dramatically enhances RXRσ binding at 10 "9 to 10" 8 M concentrations to several RXR-specific RAREs but not to natural TREs or the ERE. The effect is specific to RXR since 9-cis-RA did not induce binding of RARσ, β or y to response elements (Fig. 1, Fig. 3). Judging from the migration pattern in the gel shift assays, we assume that 9-cis-RA induces homodimer formation, although a larger complex containing an RXR trimer or tetramer can occur (particularly in the case of the CRBPII-RARE). Such trimer or tetramer formation can be tested using the methods set forth, herein. RXRσ homodimers exert response element specificity distinct from heterodimers. The rCRBPI response element did not interact with RXR homodimers, while the CRBPII response element, the only natural RARE identified so far that contains perfect repeats, was a strong binder of 9-cis-RA induced RXRσ homodimers. It has been shown previously that this response element is well activated by RXRα 24,30 . Although this response element is also bound effectively by the RXRσ-RARff heterodimer (Fig. 3) 27,29 , the heterodimer appears to have a repressor function 30 . The results obtained with the transcriptional activation studies agree well with the DNA binding studies although CV-1 cells, like all other mammalian tissue culture cells tested, contain endogenous retinoid receptors that can partially obscure effects. Nonetheless, response elements that strongly bind 9-cis-RA-RXRq homodimers also responded strongly to cotransfected RXRσ in the presence of 9-cis-RA, whereas response elements that did not bind well to RXRα homodimers, like the rCRBPI-RARE or the MHC-TRE, could not be activated by RXRσ alone. It is generally believed that the dimerization - homodimerization or heterodimerization - of nuclear hormone receptors is critical for high affinity interaction of the receptors with their cognate response elements. RXRs exist mainly as monomer in solution 16 and require high concentrations or the presence of RARs, TRs or VDR to display effective DNA binding activity »- 15 - 16 - 26 - 30 . The observation of the enhanced cooperative RXR DNA binding activity in the presence of 9-cis-RA demonstrates that 9-cis-RA induced he formation of RXR homodimers which have an increased affinity for DNA. Thus, binding of 9-cis-RA to RXR caninduce a conformational change, which allows homodimerization to occur. It is interesting that although 9-cis-RA and RA can bind to RAR, 24,25 they do not induce RAR homodimer formation. RXRσ homodimer formation can occur in solution in the absence of DNA. Thus, when 9-cis-RA becomes available to cells, the equilibrium between monomeric and dimeric receptors is changed and an additional species, the RXR homodimer can be formed, allowing for novel response pathways. The concept of ligand-induced homodimer binding as observed by in vitro gel shift assay has not been previously observed for nuclear receptors with the exception of a mutated estrogen receptor (ER-val-400) 44,45 . For the related TRs, a ligand effect has been reported on homodimer binding, 46,47 however the ligand (T3) reduced homodimer response element interaction. Since the carboxy terminal half of TRs and RARs encodes both ligand as well as dimerization functions 36,46,48 , the strong effect of the ligand on dimerization as observed here is not completely surprising. However, the specificity of the effect is quite dramatic since only homodimer but not heterodimer formation appears to be affected. An overall picture emerges where the carboxy terminal region of a receptor through its intermixed domains (that also includes a transcriptional activation region) 49 allows for multiple activities of individual receptors that may also include interactions with other regulatory proteins 50 . The data presented in this application clearly demonstrate the central role of the RXRs, having dual functions that allow them to act as auxiliary receptors for three classes of hormone receptors, the RARs, TRs and VDRs through heterodimerization. The two functions of RXR - homodimerization and heterodimerization - represent two distinct transcriptional regulatory controls that can be expected to affect distinct physiological processes. Thus, 9-cis-RA can have therapeutic properties distinct from that of all-trans-RA. The novel compounds provided herein are those defined by structural formula (I) above. Preferred compounds within this generic structure include where R" is selected from the group consisting of 0, S, (CH 3 ) 2 C and CH 2 , and R 12 is hydrogen or methyl. Particularly preferred compounds within this group are as shown in structural formula (II). Preferred R 3 substituents, when R 3 has the general structure R 6 R 7 \ / C II c are selected from the group consisting of CH 3 CH 3 CH, H CH- CH 2 CH 3 \ / \ / \ / CH 2 CH 2 H CH 2 CH 3 CH 2 CH 3 CH 2 CH 3 Preferred R 3 substi tuents , when R 3 has the general structure ,8 1 \ / C * are selected from the group consisting of CH CH-? CHΛ ~— CH CHΛ —— CHCH-7 \ / \ / \ / C > C ) c > * CH 2 25 35 and Examples of specific and particularly preferred compounds within the class of compounds defined by formula (II) include: 10 20 and The invention also encompasses pharmaceutically acceptable nontoxic ester, amide and salt derivatives of those compounds of formula (I) containing a carboxylic acid moiety. Pharmaceutically acceptable salts are prepared by treating the free acid with an appropriate amount of pharmaceutically acceptable base. Representative pharmaceutically acceptable bases are ammonium hydroxide, sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, magnesium hydroxide, ferrous hydroxide, zinc hydroxide, copper hydroxide, aluminum hydroxide, ferric hydroxide, isopropylamine, trimethylamine, diethylamine, triethylamine, tripropylamine, ethanolamine, 2-dimethylaminoethanol, 2-diethylaminoethanol, lysine, arginine, histidine, and the like. The reaction is conducted in water, alone or in combination with an inert, water-miscible organic solvent, at a temperature of from about 0°C to about 100°C, preferably at room temperature. The molar ratio of compounds of structural formula (I) to base used are chosen to provide the ratio desired for any particular salts. For preparing, for example, the ammonium salts of the free acid starting material—a particular preferred embodiment herein—the starting material can be treated with approximately one equivalent of pharmaceutically acceptable base to yield a neutral salt. When calcium salts are prepared, approximately one-half a molar equivalent of base is used to yield a neutral salt, while for aluminum salts, approximately one- third a molar equivalent of base will be used. Ester derivatives are typically prepared as precursors to the acid form of the compounds—as illustrated in the Examples below-- and accordingly may serve as prodrugs. Generally, these derivatives will be lower alkyl esters such as acetate, propionate, and the like. Amide derivatives -(C0)NH 2 , -(CO)NHR and -(C0)NR 2 , where R is lower alkyl, may be prepared by reaction of the carboxylic acid-containing compound with ammonia or a substituted amine (as illustrated in Example VII below). Synthetic Methods: The compounds of the invention may be readily synthesized using techniques generally known to synthetic organic chemists. Suitable experimental methods for making and derivatizing bridged bicyclic aromatic compounds are described, for example, in the Maignan et al. patents summarized above, the disclosures of which patents are hereby incorporated by reference. Methods for making specific and preferred compounds of the present invention are described in detail in Examples III - XVII below. Utility and Administration: The compounds of the invention defined by structural formula (I), including the pharmacologically acceptable esters, amides or salts thereof, are useful to elicit and/or regulate selective gene expression by receptors in the retinoic acid family and to control cell differentiation processes regulated by retinoids, vitamin D and/or thyroid hormone. As noted above, the compounds of the invention are thus useful for treating acne, leukemia, psoriasis, skin aging, bone calcification and energy levels, as well as other indications related to cellular processes regulated by retinoic acid, vitamin D, thyroid hormone and 9-cis-retinoic acid. The compounds of the invention may be conveniently formu¬ lated into pharmaceutical compositions composed of one or more of the compounds in association with a pharmaceutically acceptable carrier. See, e.g., Remington's Pharmaceutical Sciences, latest edition, by E.W. Martin (Mack Publ. Co., Easton PA) discloses typical carriers and conventional methods of preparing pharmaceutical compositions that may be used in conjunction with the preparation of formulations of the inventive compounds. The compounds may be administered orally, parenterally (e.g., intravenously), by intramuscular injection, by intraperitoneal injection, topically, transdermally, or the like, although oral or topical administration is typically preferred. The amount of active compound administered will, of course, be dependent on the subject being treated, the subject's weight, the manner of administration and the judgment of the prescribing physician. Generally, however, dosage will approximate that which is typical for the administration of retinoic acid, and will preferably be in the range of about 2 μg/kg/day to 2 g/kg/day. Depending on the intended mode of administration, the pharmaceutical compositions may be in the form of solid, semi-solid or liquid dosage forms, such as, for example, tablets, suppositories, pills, capsules, powders, liquids, suspensions, lotions, creams, gels, or the like, preferably in unit dosage form suitable for single administration of a precise dosage. The compositions will include, as noted above, an effective amount of the selected drug in combination with a pharmaceutically acceptable carrier and, in addition, may include other medicinal agents, pharmaceutical agents, carriers, adjuvants, diluents, etc. For solid compositions, conventional nontoxic solid carriers include, for example, pharmaceutical grades of mannitol, lactose, starch, magnesium stearate, sodium saccharin, talc, cellulose, glucose, sucrose, magnesium carbonate, and the like. Liquid pharmaceutically administrable compositions can, for example, be prepared by dissolving, dispersing, etc., an active compound as described herein and optional pharmaceutical adjuvants in an excipi- ent, such as, for example, water, saline, aqueous dextrose, glycerol, ethanol, and the like, to thereby form a solution or suspension. If desired, the pharmaceutical composition to be administered may also contain minor amounts of nontoxic auxiliary substances such as wetting or emulsifying agents, pH buffering agents and the like, for example, sodium acetate, sorbitan monolaurate, triethanolamine sodium acetate, triethanolamine oleate, etc. Actual methods of preparing such dosage forms are known, or will be apparent, to those skilled in this art; for example, see Remington's Pharmaceutical Sciences, referenced above. For oral administration, fine powders or granules may contain diluting, dispersing, and/or surface active agents, and may be presented in water or in a syrup, in capsules or sachets in the dry state, or in a nonaqueous solution or suspension wherein suspending agents may be included, in tablets wherein binders and lubricants may be included, or in a suspension in water or a syrup. Where desirable or necessary, flavoring, preserving, suspending, thickening, or emulsifying agents may be included. Tablets and granules are preferred oral administration forms, and these may be coated. Parenteral administration, if used, is generally charac¬ terized by injection. Injectables can be prepared in conventional forms, either as liquid solutions or suspensions, solid forms suitable for solution or suspension in liquid prior to injection, or as emulsions. A more recently revised approach for parenteral administration involves use of a slow release or sustained release system, such that a constant level of dosage is maintained. See, e.g., U.S. Patent No.3,710,795, wiich is incorporated by reference herein. Experimental The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the methods, compositions, and compounds claimed herein are made and evaluated, and are not intended to limit the scope of what the inventors regard as their invention. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.) but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in °C and pressure is at or near atmospheric. In describing the location of groups and substituents, the following numbering system will be employed throughout the examples: EXAMPLE I Identification of homodimer formation 9-cis-RA induces RXR homodimer binding on the TREpal Although RXRs have been shown to bind RA response elements (RAREs) when used at high concentrations, 28,30,31 more recent investigations revealed that RXR exists mainly as monomers in solution 10 and that effective DNA interaction requires heterodimer formation with RARs or TRs or VDR 15,16,26"29 . Binding of the heterodimers to a variety of response elements was found to be ligand independent 32 . The newly discovered natural RA isomer, 9-cis-RA. has been reported to be an effective activator of RXRs in Drosophila Schneider cells that are known to contain neither RAR nor TRs 17,24 . If 9-cis-RA is indeed a true ligand for RXRs, one might expect that this ligand modulates RXR response element interaction. We therefore investigated the effect of 9-cis-RA on RXRσ binding to the palindromic TRE (TREpal), an RXR responsive element 14 , in the absence and presence of coreceptors (TRs and RARs). The cloning of the receptor cDNAs for RXRσ, RARβ, or TRσ into the pBluescript (Stratagene, San Diego, CA) have been described previously 26 . Flag-RXRσ was constructed as described previously 33 by ligation of a double-stranded oligonucleotide containing an ATG codon and a DNA sequence encoding Flag (Arg-Tyr-Lys-Asp-Asp-Asp-Asp-Lys) [SEQ ID N0:1] to the N-terminus of RXRσ. The fusion product was then cloned into pBluescript. The synthesis of receptor proteins using in vitro transcription/translation system and gel retardation assays using synthesized receptor proteins and the double-stranded TREpal 34 were as described 33 . 9-cis-RA (m.p. 184-187°C) was prepared from 9- cj -retinal by a two-step sequence of Mn0 2 oxidation in the presence of HOAc-MeOH 35 to give the methyl ester (69%) followed by hydrolysis (80%) in 0.5 N KOH in 25% aq. MeOH and crystallization (MeOH); HPLC (Novapak C 18 , 32% MeCN, 27% MeCH, 16% I-PrOH, 24% H 2 0, 1% HOAc, 1.9 mL/min, 260nM) t R 15.4 min (100%). To analyze the effect of hormones, the receptor proteins were incubated with appropriate concentrations of hormone at room temperature for 30 min before performing the DNA binding assay. When anti-Flag antibody (Immunex, Seattle, WA) was used, 1 μλ of the antiserum was incubated with receptor protein for another 30 min at room temperature before performing the DNA binding assay. While in the absence of ligand, RXR alone did not bind effectively to the TREpal and required TR or RAR for response element interaction, binding of RXR was dramatically increased in the presence of 9-cis-RA (Fig. la) and did not require TRs or RARs. The RXR- specific band observed in the presence of 9-cis-RA was as prominent as the bands obtained with the heterodimeric TR-RXR and RAR-RXR complexes. The RXR complex migrated more slowly than the TR-RXR complex at a position very similar to that of the RAR-RXR heterodimer- TREpal complex. These data therefore demonstrate that 9-cis-RA induces RXR homodimer formation. Due to migration of the RAR-RXR complex at the same position as the 9-cis-RA induced RXR homodimer complex, we were unable to determine in this experiment whether both complexes were formed. Remarkably, all-trans-RA when added at a concentration of 10 "6 M, also induced to some degree RXRσ homodimer binding whereas T 3 did not. Although the RA effect could be observed with several freshly prepared RA solutions, it is not clear at this point whether this homodimer formation in the presence of RA is due to 9-cis-RA impurities in the all-trans-RA solutions used here, or is a direct effect of all-trans-RA (see below). Interestingly, although it was reported that 9-cis-RA is capable of binding to RAR 24 , unlike its effect on RXR, 9-cis-RA was not found to induce RAR homodimer binding to the TREpal . To provide further evidence that the observed complex in the presence of 9-cis-RA indeed was an RXR complex, we performed the DNA binding experiment with Flag-RXR, a derivative that carries an 8 amino acid aminoterminal epitope recognized specifically by anti-Flag (σF) antibody 33 . As shown in Fig. lb, σF supershifted the 9-cis-RA induced complexes, while a nonspecific antibody did not. This proves that the complex observed with the TREpal in the presence of 9-cis-RA indeed contained RXR protein. To examine how dependent the 9-cis-RA induced homodimer formation is on the concentration of RXR protein, increasing concentrations of in vitro translated RXR protein were mixed with the labeled TREpal in the presence or absence of 9-cis-RA (Fig. 2a). When these mixtures were analyzed by the gel retardation assay, a strong cooperative effect in homodimeric DNA binding was seen, positively dependent on the RXRσ protein concentration (Fig. 2a,b). Although slight binding of RXR can be observed when high concentrations of RXR were used, 9-cis-RA was required for efficient complex formation at all receptor concentrations used. We further determined the concentrations of 9-cis-RA required for homodimer complex formation at all receptor concentrations used. We further determined the concentrations of 9-cis-RA required for homodimer complex formation and observed a significant effect already at 10' 9 M while optimal binding was seen at 10 "8 M (Fig. 2c). Thus, effective RXR homodimer DNA interaction is dependent on RXR protein concentration and can occur at low levels of 9-cis-RA. 9-cis-RA induced homodimeric interaction with specific response elements RXR containing heterodimers have a highly specific interaction with various natural response elements in that TR-RXR heterodimers only bind strongly to TREs but not to RAREs, whereas the opposite is true for RAR-RXR heterodimers 15,32 . We examined the sequence requirement of DNA binding of 9-cis-RA induced RXR homodimer using a number of natural and synthetic response elements (Fig. 3a). Gel retardation assays using in vitro synthesized receptor protein are described in the Fig. 1 legend. The following oligonucleotides and their complements were used as probes in Figs. 3 and 4. ApoAI-RARE, a direct repeat response element with 2 bp spacer 31 , gatcAGGGCAGGGGTCAAGGGTTCAGTgatc [SEQ ID N0:2]; CRBPII-RARE, a direct repeat RXR-specific response element with 1 bp spacer 30 , gatcCAGGTCACAGGTCACAGGTCACAGTTCAAgatc [SEQ ID N0:3]; BRARE, a direct repeat of RA response element present in RARβ promoter 36,37 , gatctGTAGGGTTCACCGAAAGTTCACTCagatc [SEQ ID N0:4]; CRBPI-RARE, a direct repeat RA specific response element present in rat CRBPI promoter 38 , gatccAGGTCAAAAAGTCAGgatc [SEQ ID N0:5]; MHC-TRE, a direct repeat T 3 specific response element present in rat σ-myosin heavy chain gene 40 , gatcCTGGAGGTGACAGGAGGACAGCgatc [SEQ ID N0:6]; ME-TRE, a direct repeat T 3 specific response element present in the rat malic enzyme gene 41 , gatcCAGGACGTTGGGGTTAGGGGAGGACAGTGGgatc [SEQ ID N0:7]; DR-4, an idealized direct repeat T 3 specific response element with 4 bp spacer 39 , gatcTCAGGTCATCCTCAGGTCAgatc [SEQ ID N0:8]; DR-5, an idealized direct repeat RA specific response element with 5 bp spacer 39 , gatcTCAGGTCATCCTCAGGTCAgatc [SEQ ID N0:9]; ERE, a perfect palindromic ER response element 42 , gatcTCAGGTCACTGTGACCTGAgatc [SEQ ID NO:10]. The sequence of TREpal [SEQ ID NO:11] is shown for comparison. ApoAI-RARE (a direct repeat response element that contains a 2 bp spacer), which has been suggested to be RXR-specific 31 , resulted in a strong RXR complex in the presence of 9-cis-RA and to a lesser degree with RA (10 "6 M). The RXR-RAR heterodimer also bound effectively to this response element. Since the heterodimer complex migrated at the same position as RXR homodimers, the effect of 9-cis- RA on RXR-RAR heterodimers cannot be clearly determined. RAR homodimer binding was not induced by 9-cis-RA (Fig. 3b). When we investigated 9-cis-RA induced RXR binding to another RXR responsive element 30 , the CRBPII-RARE, we observed a complex that migrated more slowly than the heterodimer (in the absence of ligand), while in the presence of 9-cis-RA and RAR, both homodimer and heterodimer binding appeared to be reduced in their intensity (Fig. 3); a similar effect was seen at high RA concentrations or when both 9-cis-RA and RA were present. 9-cis-RA also induced RXR interaction with the BRARE (Fig. 4a), the RAR response element from the human RARβ promoter 36,37 that contains a 5 bp spacer. However, the RXR homodimer band was considerably weaker than the RAR-RXR heterodimer band at the protein concentrations used. Interestingly, another natural RARE derived from the rat CRBPI promoter 38 , that like the ApoAI-RARE contains a 2 bp spacer, did not show any binding of RXR in the presence of 9-cis-RA (Fig. 4a), indicating that the actual sequence of the repeated core motif is critical for RXR homodimer binding. Similarly, the DR-5- RARE, a perfect repeat element derived from the β-RARE 39 did not exhibit interaction with RXR in the presence of 9-cis-RA. while it interacted strongly with RXR when RARE was present (Fig. 4a). To examine whether RXR homodimer binding is specific to certain RAREs, we also performed gel shift experiments with the T 3 response elements from the rat σ-myosin heavy chain promoter (MHC- TRE) 40 , the rat malic enzyme (ME-TRE) 41 and the perfect repeat DR-4 39 . In all three cases, specific binding of RXR in the presence or absence of 9-cis-RA could not be observed, while all three response elements bound effectively TR/RXR heterodimers (Fig. 4b), consistent with the notion that these elements are not induced by retinoids. Similarly, the perfect palindromic ERE 42 also did not interact with RXR homodimers (Fig. 4c). 9-cis-RA induced homodimer formation occurs in the absence of DNA It was reported the RXR exists mainly as monomer in solution 16 . An important question is whether 9-cis-RA induced RXR homodimers, like the RXR containing heterodimers, can form in solution in the absence of DNA. To address this question we took advantage of the Flag-RXR derivative that can be specifically precipitated with anti-Flag antibody while RXR wild type cannot. Flag-RXRtf was cloned in frame in the expression vector pGex 2T (Pharmacia) and was expressed in bacteria using the procedure provided by the manufacturer. Protein was partially purified on a prepacked glutathione sepharose 4B column (Pharmacia) and tested for its function by gel retardation assays and western blotting using anti-Flag antibody. Immunocoprecipitation assay was performed essentially as described 26 . Briefly, 10 μl of 35 S-labeled in vitro synthesized RXRσ protein was incubated with 5 μl (approximately 0.1 μg) of partially purified bacterially expressed Flag-RXRσ fusion protein or similarly prepared glutathione transferase control protein in 100 μl buffer containing 10 "7 M 9-cis-RA. 50 mM KCI and 10% glycerol for 30 min at room temperature. When assayed in the presence of chemical cross-linker or oligonucleotides, we added 2 μl of 100 mM dithiobis succinimidylpropionate (DSP) dissolved in DMSO or 10 ng of oligonucleotide and continued the incubation at room temperature for 15 min. The reaction mixtures were then incubated with 1 μl of anti- Flag antibody or nonspecific preimmune serum for 2 h on ice. Immune complexes were precipitated by adding 50 μl of protein-A-sepharose slurry and mixing continuously in the cold room for 1 h. The immune complexes were washed extensively with cold NET-N buffer (20 mM Tris, pH 8.0, 100 mM NaCl, 1 mM DTT, 0.5% NP-40) containing 10' 7 M 9-cis-RA. boiled in SDS sample buffer and resolved by SDS-polyacrylamide gel electrophoresis. The gel was fixed, dried and visualized by autoradiography. When we mixed Flag-RXR with in vitro labeled 35 S-RXR protein, the labeled RXR could be coprecipitated in the presence of anti-Flag antibody but not in the presence of nonspecific serum (Fig. 5). Coprecipitation efficiency was slightly increased in the presence of the ApoAI-RARE but not in the presence of the MHC-TRE. In addition, incubation with a crosslinker (DSP) further enhanced coprecipitation of the labeled RXR. In all cases, specific coprecipitation was only observed in the presence of 9-cis-RA. These data, therefore, give strong support to the assumption that 9-cis-RA induced RXR homodimer formation occurs in solution and does not require RXR-DNA interaction. Response element specific transcriptional activation by 9-cis-RA and RXRσ To investigate whether 9-cis-RA/RXRσ homodimer response element interaction can be correlated with transcriptional activation of such response elements by the 9-cis-RA/RXR complex, we carried out a series of transient transfection assays in CV-1 cells, where we cotransfected receptor expression vectors with CAT reporter constructs that carried various response elements upstream of the tk promoter. CV-1 cells were transiently transfected Using a modified calcium phosphate precipitation procedure as described previously 43 . CAT activity was normalized for transfection efficiency by measuring the enzymatic activity derived from the cotransfected β-galactosidase expression plasmid (pCHHO, Pharmacia). The transfected cells were grown in the absence or presence of 10 *7 M 9-cis-RA or all-trans-RA. With the TREpal containing reporter, we observed strong activation by RXRσ in the presence of 9-cis-RA and little activation when RA was added. Activation could be further enhanced by cotransfection of RARσ. In this case, however, RA also functioned as an effective activator although not as efficiently as 9-cis-RA. Overall, activation by 9-cis-RA in the presence of RARσ and RXRσ was approximately twice as strong as seen with RXRα alone, consistent with the DNA binding data where binding was observed by both the heterodimer and the homodimer in the presence of 9-cis-RA when both receptors were present (Fig. 6a). When we examined the BRARE (Fig. 6b), we observed that this response element was highly activated by endogenous CV-1 cell receptors consistent with previous observations 37,42 , such that further activation by low concentrations of cotransfected receptors could not be observed. Interestingly, however, 9-cis-RA was a more potent activator at 10' 7 M than RA. These data thus indicate that CV-1 cells contain endogenous retinoid receptor activity that is particularly active on the BRARE and is responsive to 9-cis-RA. The ApoAI element containing reporter was also very effectively activated by RXR in the presence of 9-cis-RA (Fig. 6c) while RA did not induce above the level obtained in the absence of RXRσ. Similar to the TREpal, maximal activation was seen when both receptors RXRσ and RARσ were cotransfected. Under these conditions RA also led to a strong activation. In contrast, the CRBPI element, where we did not observe DNA binding by RXR in the presence of 9-cis- RA, also was not activated by RXR and 9-cis-RA in the transient transfection studies (Fig. 6d) while RARσ alone led to significant activation that was mostly 9-cis-RA dependent. The heterodimer RARσRXRσ allowed maximal activation in the presence of 9-cis-RA. Not unexpectedly, no induction by RXRσ and 9-cis-RA was observed on the MHC-TRE, the ME-TRE or the ERE. These in vivo analyses showed a very significant correlation to the results obtained with the in vitro DNA binding studies, in that strong activation by RXRσ in the presence of 9-cis-RA is only observed on the response elements that strongly interact with the 9-cis-RA induced RXRσ homodimer. 9-cis Retinoic acid inhibits activation by TR/RXR heterodimer We have observed that a CAT reported gene that is activated by a thyroid hormone receptor/RXR heterodimer in the presence of thyroid hormone (T 3 ) can be inhibited by adding 9-cis-RA. This type of inhibition is most easily measured by using a transfection assay. EXAMPLE II Identification and selection of compounds which induce RXR activity We used the TREpal-tk-reporter gene in a transient transfection assay essentially as described 51 to evaluate compounds for induction of RXR activity. Briefly, CV-1 cells or Hep G2 cells were grown in DME medium supplemented with 10% fetal calf serum. Cells were plated at 1.0 x 10 s per well in a 24-well plate 16-24 h before transfection. In general, 100 ng of reporter plasmid, 150 ng of β-galactosidase expression vector (pCHHO, Pharmacia), and variable amounts of receptor expression vector were mixed with carrier DNA (pBluescript) to 1,000 ng of total DNA per well. Chloramphenicol actyl transferase (CAT) activity was normalized for transfection efficiency by the corresponding β-galactosidase activity as previously described 52 . As shown in Example I, the TREpal represents a response element that is activated by both RAR/RXR heterodimers and RXR homodimers. When the RXR expression vector is cotransfected with the TREpal-tk-reporter gene into CV-1 cells, all-trans-RA does not efficiently activate the reporter, whereas 9-cis-RA does. Evaluation of a series of retinoids indicated, that several showed activity with RXR. The pharmacophoric elements of these structures were then combined and further modified to produce a subset of retinoids whose activation profiles for RXR were similar to that of 9-cis-RA. Induction curves for several retinoids active with RXR are shown in Fig. 7a. Interestingly, while none of the active compounds revealed activity at 10 "8 M, all showed activities similar to 9-cis-RA at 10 *7 M. We next used a reporter gene carrying the RARE 30 of the cytoplasmic retinol binding protein II (CRBPII), a response element that is only activated by RXR homodimers, but not by RAR/RXR heterodimers. The induction profiles obtained with this response element were similar to the TREpal responses (Fig. 7b). Thus, the synthetic retinoids SR11203, SR11217, SRU234, SR11235, SR11236, and SR11237 appeared to be effective activators of RXRσ. The structures of the compounds are as follows: The activity rankings for this series of retinoids were the same for both the TREpal and CRBPII reporter genes. The ketal SR11237 was the most active, followed by the isopropylidenyl retinoid SR11217, the hemithioketal SR11235 and the thioketal SR11234. The dithiane SR11203 and dioxane SR11236 had the lowest activity. Conformational analysis indicated that these retinoids had spatial orientations of the lipophilic head and carboxyl terminus that were similar to those of 9-cis-RA and that activity could be related to the length and volume of the substituent group (CRR') linking the tetrahydronaphthalene and phenyl ring systems. Given the showing in Example I that 9-cis-RA specifically activates RXRσ by inducing RXRσ homodimer formation, we investigated the retinoid-induced RXR homodimer binding to the TREpal using a gel retardation assay. Briefly, gel retardation assays were carried out essentially as described previously 33 . In vitro translated receptor receptor protein (1 to 5 ml depending on the translation efficiency) was incubated with the 32 P-labeled oligonucleotides in a 20-ml reaction mixture containing 10 mM Hepes buffer, pH 7.9, 50 mM KCI , 1 mM DTT, 2.5 mM MgCl, 10% glycerol, and 1 mg of poly(dl-dC) at 25°C for 20 minutes. The reaction mixture was then loaded on a 5% nondenaturing polyacrylamide gel containing 0.5 x TBE (1 x TBE = 0.089 M Tris-borate, 0.089 M boric acid, and 0.002 M EDTA) . In the absence of 9-cis-RA. RXR did not bind to this response element. Retinoids SR11217, and SR11237 induced RXR homodimer binding to the response element in a concentration-dependent manner. Retinoid 11203, which behaved as a weak activator in the transient transfection assays, also induced only weak RXR binding. SR11231 which did not activate the RXR homodimer was also not able to induce RXR homodimer binding. Similar results were obtained with the CRBPII-RARE and the ApoAI-RARE. We have thus defined here a class of synthetic retinoids that activate RXRσ by inducing homodimer formation and binding to DNA. An important question was whether these RXR-active compounds, like 9-cis-RA. would also activate RAR/RXR heterodimers or whether they would be truly RXR selective. To analyze this, we used four different reporter constructs carrying either the i) rat cytoplasmic retinol binding protei.n I (CRBPI) gene RARE 38 that is only bound and activated by RAR/RXR heterodimers; ii) the RARB2 gene promoter RARE 36,37 , which is most effectively bound by heterodimers but also activated to some degree by RXR homodimers; iii) the CRBPII-RARE, which is activated only by RXR homodimers, and on which RAR represses RXR activity 30 ; iv) the apolipoprotein Al (apoAI) gene RARE 31 that is bound and activated by RAR/RXR heterodimers as well as by RXR homodimers. The four different reporter constructs were cotransfected with RARσ, RARβ, RXRσ, or with RXRσ and RARσ together 51 . The retinoids were analyzed at a concentration of 5 x 10 "7 M (a dose shown to yield almost full induction (Fig. 7)). CV-1 cells were cotransfected with 100 ng reporter plasmid a) CRBPI-tk-CAT, b) BRARE-tk-CAT, c) CRBPII-tk-CAT, and d) apoAI-tk-CAT. Retinoids were applied at 5 x 10" 7 M. Results of a representative experiment are shown. The RXR-specific retinoids behaved strikingly different from 9-cis-RA (or RA) in that they only activated RXR homodimers but not RAR/RXR heterodimers. As with 9-cis-RA. both SR11217 and SR11237 were strong activators of the CRBPII-RARE (i.e. the RARE that is significantly activated only by the RXR homodimer). However, in contrast to 9-cis-RA. they did not induce the CRBPI-RARE that is activated only by the RAR/RXR heterodimer. Thus, while SR11217 and SR11237 behaved very similarly to 9-cis-RA on the CRBPII-RARE, they showed no response on the CRBPI-RARE, where 9-cis-RA is the optimal activator. The BRARE was slightly activated by SR11217 and SR11237, consistent with the relatively low affinity of RXR homodimers for this response element. The apoAI-RARE was most effectively activated by RAR/RXR heterodimers in the presence of 9-cis-RA. In addition to the activity found in CV-1 cells, a significant and RXR-specific activation by retinoids SR11217 and SR11237 was also seen in various other cell lines, including Hep G2 cells, where a particular high response was seen. RARσ and RARβ, when cotransfected alone, were not activated significantly by any of the synthetic retinoids on any of the response elements tested. Similar negative results were obtained for RARy. RARσ and β are assumed to form heterodimers with endogenous RXR-like proteins in CV-1 cells, thus these heterodimers are also unresponsive to the synthetic retinoids. Our data demonstrate that we have identified a novel class of retinoids that specifically induces RXR homodimer formation and that activates RXR homodimers on specific response elements but not RAR/RXR heterodimers. These retinoids allow the specific activation of RXR-selective response pathways, while not inducing RAR-dependent response pathways. These retinoids provide a much more restricted physiological response than RA isomers or other retinoids presently used. Exampl e I I I (a.) Methyl 4-[(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl- 2-naphthalenyl)carbonyl]benzoate (3): To a suspension of aluminum chloride (1.13 g, 8.5 mmol) in 1.5 mL of 1,2-dichloroethane at 0°C under argon was added a solution of l,2,3,4-tetrahydro-l,l,4,4-tetramethylnaphthalene 1 (1.45 g, 7.7 mmol) (Kagechika, H., et al., J. Med. Chem. 31:2182 (1988)) and 4-carbomethoxybenzoyl chloride 2 (1.56 g, 7.9 mmol) (4-carbo- methoxybenzoyl chloride 2 was obtained from mono-methyl terephthalate, which is readily available from Aldrich, in one step (S0C1 2 , DMF)) in 6 mL of 1,2-dichloroethane. The resulting solution was brought to room temperature and stirred thereafter for 16 h. The reaction mixture was poured onto ice water and extracted with 40% ethyl acetate/hexane. The combined organic layers were washed with saturated aqueous NaHC0 3 and brine. The solution was dried over anhydrous MgS0 4 , filtered and concentrated to afford an orange solid (4.5 g). Flash chromatography (50% diehloromethane/hexane) yielded the desired product 3 as a pale yellow solid (2.07 g). Recrystallization from diehloromethane/hexane afforded the desired product 3 as a white, crystalline solid (1.96 g, 50%): m.p. 146-148oC; R f 0.14 (50% CH 2 Cl 2 /hexane). The structure of the product was also confirmed using IR, X H NMR and mass spectroscopy. (b.) [2(5,6,7,8-Tetrahydro-5,5,8,8-tetramethylnaphthalen- 2-yl)-2-(4-carbomethoxyphenyl)]-l,3-dithiane (4): To a solution of the keto-ester 3 (97 mg, 0.277 mmol) in 3 mL of chloroform at 0°C under argon was added a solution of 1,3-propanedithiol (33 μL, 36 mg, 0.332 mmol) followed by boron trifluoride etherate (17 μL, 0.140 mmol). The resulting mixture was stirred at 0oC for 1 h and then warmed to room temperature overnight. The reaction mixture was quenched by pouring into saturated aqueous Na 2 C0 3 , and extracted with dichloromethane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford a yellow solid (0.108 g). Recrystallization from ethyl acetate/hexane afforded the desired dithiane 4 as a white, crystalline solid (0.087 g, 71%): m.p. 195-197oC; R f 0.32 (50% CH 2 Cl 2 /hexane) . The structure of the product was also confirmed using IR, H NMR and mass spectroscopy. (c.) [2-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethylnaphthalen- 2-yl)-2-(4-carboxyphenyl)]-l,3-dithiane (5): To a suspension of the ester 4 (85 mg, 0.193 mmol) in 75% aqueous methanol (2 mL) was added 0.024 g of potassium hydroxide and the mixture was stirred at 50°C for 2 h during which time the material dissolved. The solution was cooled to room temperature, acidified with 1 N aqueous hydrochloric acid, and then extracted with 80% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford a white solid. Recrystallization from benzene/hexane afforded the desired acid 5 as a white, powder (0.076 g, 92%): m.p. 229-231oC. The structure of the product was also confirmed using IR, J H NMR and mass spectroscopy. Exampl e IV (a.) [2-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl- 2-naphthalenyl)-2-(4-carbomethoxyphenyl)]-1,3-dithiolane (6): To a solution of the keto-ester 3 (80 mg, 0.228 mmol) in dichloromethane (2 mL) at 0oC under argon was added a solution of ethanedithiol (26 mg, 0.27 mmol) in dichloromethane (0.5 mL) followed by boron trifluoride etherate (0.04 mL, 0.3 mmol). The resulting mixture was stirred at 0°c for 1 h and then warmed to room temperature overnight. The reaction mixture was quenched by pouring into saturated aqueous Na 2 C0 3 , and extracted with 40% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford a solid. Flash chromatography (30; 40% CH 2 Cl 2 /hexane) yielded the desired dithiane 6 as a white solid (0.088 g, 90%): m.p. 105-107oC; R f 0.33 (50% CH 2 Cl 2 /hexane) . The structure of the product was also confirmed using IR, J H NMR and mass spectroscopy. (b.) [2-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl- 2-naphthalenyl)-2-(4-carboxyphenyl)]-1,3-dithiolane (7): To a suspension of the ester 6 (85 mg, 0.199 mmol) in 75% aqueous methanol (3 mL) was added one pellet of potassium hydroxide (0.11 g) and the mixture was stirred at 70oC for 1 h during which time the material dissolved. The solution was cooled to room temperature, acidified with 1 N aqueous hydrochloric acid, and then extracted with 80% ethyl acetate/hexane. The combined organic layers when dried over anhydrous MgS0 4 , filtered, and concentrated to afford a white solid. Recrystallization from diehloromethane-hexane afforded the desired acid 7 as a white powder (0.064 g, 79%): m.p. 218-221oC. The structure of the product was also confirmed using IR, X H NMR and mass spectroscopy. Example V e (a.) [2-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl- 2-naphthalenyl)-2-(4-carbomethoxyphenyl)]-l,3-dioxolane (8): To a solution of keto-ester 3 (80 mg, 0.228) in 1 mL of benzene was added ethylene glycol (1 L), l,2-bis(trimethylsilyloxy)ethane (2 mL) and a catalytic amount of JJ-TSOH. The reaction mixture was heated at reflux for 4 h and then cooled to room temperature. The solution was poured into saturated aqueous NaHC0 3 and extracted with 40% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford a solid. Flash chromatography (50% CH 2 Cl 2 /hexane) yielded the desired ketal 8 as a white solid (0.082 g, 91%): m.p. 145-147oC; R f 0.16 (50% CH 2 Cl 2 /hexane). The structure of the product was also confirmed using IR, l W NMR and mass spectroscopy. (b.) [2-(5,6,7,8-Tetrahydro r 5,5,8,8-tetramethyl-2-naph- thalenyl)-2-(4-carboxyphenyl)]-l,3-dioxolane ammonium salt (10): To a suspension of the ester 8 (50 mg, 0.127 mmol) in 75% aqueous methanol (2 mL) was added one pellet of potassium hydroxide (0.1 g), and the reaction mixture was stirred at 70oC for 1 h during which time the material dissolved. The solution was cooled to room temperature, acidified with 1 N aqueous hydrochloric acid, and then extracted with 80% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford a white solid 9. The acid 9 was dissolved in dichloromethane (4 mL) under argon, and ammonia gas was condensed into the solution, which was stirred for 5 min at -33°C. The solution was warmed to room temperature for 20 min to evaporate the ammonia and concentrated to afford the ammonium salt .10 as a white powder (47 mg, 93%): m.p. 259-261°C. The structure of the product was also confirmed using IR, l NMR and mass spectroscopy. Exampl e VI 12 (a.) [2-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl- 2-naphthalenyl)-2-(4-carbomethoxyphenyl)]-1,3oxathiolane (H): To a solution of keto-ester 3 (88 mg, 0.251) in 2 ml of benzene was added 2-mercaptoethanol (1 mL), and a catalytic amount of 2-Ts0H. The reaction mixture was heated at reflux overnight and then cooled to room temperature. The solution was poured into saturated aqueous NaHC0 3 and extracted with 40% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford a solid. Flash chromatography (50% CH 2 Cl 2 /hexane) yielded the desired ketal H as a white solid (0.09 g, 87%): m.p. 122-124oC; R f 0.24 (50% CH 2 Cl 2 /hexane) . The structure of the product.was also confirmed using IR, W NMR and mass spectroscopy. (b.) [2-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl- 2-naphthalenyl)-2-(4-carboxyphenyl)]-l,3-oxathiolane (12): To a suspension of the ester ϋ (64 mg, 0.156 mmol) in 753 aqueous methanol (3 mL) was added one pellet of potassium hydroxide (0.12 g), and the reaction mixture was stirred at 75oC for 1 h during which time the material dissolved. The solution was cooled to room temperature, acidified with 1 N aqueous hydrochloric acid, and then extracted with 80% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford a white solid. Recrystallization from diehloromethane-hexane afford the desired acid 12 as a white powder (0.06 g, 97%): m.p. 216-217.5oC. The structure of the product was also confirmed using IR, J H NMR and mass spectroscopy. Exampl e VI I y 15 (a.) [2-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl- 2-naphthalenyl)-2-(4-carbomethoxyphenyl)]-1,3-dioxane Q3): To a solution of keto-ester 3 (150 mg, 0.428 mmol) in 5 ml of benzene was added 1,3-propanediol (1.5 mL), and a catalytic amount of p_-Ts0H. The reaction mixture was heated at reflux overnight and then cooled to room temperature. The solution was poured into saturated aqueous NaHC0 3 and extracted with 40% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford a solid. Flash chromatography (50% CH 2 Cl 2 /hexane) yielded the desired ketal 13 as a white solid (0.164 g, 94%): m.p. 157-159oC; R f 0.24 (5% ethyl acetate/hexane). The structure of the product was also confirmed using IR, l H NMR and mass spectroscopy. (b.) [2-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl-2-naph- thalenyl)-2-(4-carboxyphenyl)]-l,3-dioxane ammonium salt (15): To a suspension of the ester 13 (0.1 g, 0.245 mmol) in 75% aqueous methanol (3 mL) was added one pellet of potassium hydroxide (0.12 g) . The reaction mixture was stirred at 80°C for 30 min during which time the material dissolved. The solution was cooled to room temperature, acidified with 1 N aqueous hydrochloric acid, and then extracted with 80% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford a white solid 14. The acid 14 was dissolved in dichloromethane (4 mL) under argon. Ammonia gas was condensed into the flask and the mixture was stirred for 5 min at -33oC. The solution was warmed to room temperature for 20 min to evaporate ammonia and concentrated to afford the ammonium salt 15 as a white powder (0.238 g, 97%): m.p. 228-230oC. The structure of the product was also confirmed using IR, Hi NMR and mass spectroscopy. Example VIII CO j Me , j EI ? Zn . CH 2 _ 2 . C f iH. . 60 2 ) (a.) Methyl 4-[l-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl- 2-naphthalenyl)-l-ethenyl]benzoate Q6): To a suspension of methyltriphenylphosphoniurn bromide (0.78 g, 2.18 mmol) in 5 mL of benzene under argon at room temperature was added a 0.5 M solution of potassium hexamethyldisilazide in toluene (4.4 mL, 2.2 mmol), and the yellow solution was stirred for 5 min. A solution of keto-ester 3 (0.51 g, 1.455 mmol) in 7 L of benzene was added, and the orange solution was stirred for 1 h at room temperature. The reaction mixture was poured into saturated aqueous NaHC0 3 and extracted with 40% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered through a plug of silica gel, and concentrated to afford a solid. Flash chromatography (30% diehloromethane/hexane) yielded the desired product 16 as a white solid (0.405 g, 80%): m.p. U7-118oC; R f 0.2 (25% CH 2 Cl 2 /hexane). The structure of the product was also confirmed using IR, l ti NMR and mass spectroscopy. (b.) [l-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl- 2-naphthalenyl)-l-(4-carbomethoxyphenyl)]-cyclopropane (17): To a solution of ethene 16 (0.130 g, 0.373 mmol) in 10 mL of benzene under argon at room temperature was added a 1 M solution of diethylzinc in hexane (5.6 mL, 5.6 mmol), and the reaction mixture was heated to 60oC. Diiodomethane (0.48 mL, 6.0 mmol) in 2 mL of benzene was added dropwise for 5 min. The,reaction mixture was cooled to room temperature and oxygen was bubbled through for 3 h. The cloudy solution was diluted with 40% ethyl acetate/hexane and washed with aqueous hydrochloric acid, water and saturated aqueous NaHC0 3 . The organic layer was dried over anhydrous MgS0 4 , filtered, and concentrated to afford a solid. Flash chromatography (30%; 40% CH 2 Cl 2 /hexane) yielded the desired product 17 as a white solid (0.08 g, 59%): m.p. 100-102oC; R f 0.36 (50% CH 2 Cl 2 /hexane). The structure of the product was also confirmed using IR, l NMR and mass spectroscopy. (c.) [l-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl- 2-naphthalenyl)-1-(4-carboxyphenyl)]cyclopropane Q8): To a suspension of the ester 17 (60 mg, 0.166 mmol) in 75% aqueous methanol (2 mL) was added one pellet of potassium hydroxide (0.12 g), and the reaction mixture was stirred at 70°C for 1 h during which time the material dissolved. The solution was cooled to room temperature, acidified with 1 N aqueous hydrochloric acid, and then extracted with 80% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford a white solid. Recrystallization from diehloromethane-hexane afforded the desired acid 18 as a white powder (0.055 g, 95%): m.p. 333-335oC. The structure of the product was also confirmed using IR, J H NMR and mass spectroscopy. Exampl e IX 1 ) KOH . 60 • - 70 ° C UeOH/H 2 0 (3:1) 2 ) H 3 0 20 (a.) Methyl 4-[l-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl- 2-naphthalenyl)-2-methyl-1-propenyl]benzoate (19): To a suspension of isopropyltriphenylphosphoniurn iodide (0.35 g, 0.807 mmol) in 3 mL of benzene under argon at room temperature was added a 0.5 M solution of potassium hexamethyl- disilazide in toluene (1.8 mL, 0.89 mmol), and the red solution was stirred for 5 min. A solution of keto-ester 3 (0.169 g, 0.481 mmol) in 3 mL of benzene was added, and the red solution was heated to HOoC, while approximately 4 mL of benzene was distilled out. After 1 h, the reaction mixture was diluted with 40% ethyl acetate/hexane and washed with saturated aqueous NaHC0 3 and brine. The organic layer was dried over anhydrous MgS0 4 , filtered through a plug of silica gel, and concentrated to afford a solid. Flash chromatography (40% diehloro¬ methane/hexane) yielded the desired product 19 as a white powder (0.128 g, 71%): R f 0.44 (50% CH 2 Cl 2 /hexane) . The structure of the product was also confirmed using IR, J H NMR and mass spectroscopy. (b.) 4-[l-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl- 2-naphthalenyl)-2-methyl-1-propenyl]benzoic acid (20): To a suspension of the ester 19 (0.115 g, 0.304 mmol) in 75% aqueous methanol (3 mL) was added one pellet of potassium hydroxide (0.12 g), The mixture was stirred at 75©C for 1 h during which time the material dissolved. The solution was cooled to room temperature, acidified with 1 N aqueous hydrochloric acid, and then extracted with 80% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford the desired acid 20 as a white powder (0.11 g, 99%): m.p. 204-206oC. The structure of the product was also confirmed using IR, ! H NMR and mass spectroscopy. Example X (a.) Methyl 4-[(4,4-dimethyl-3,4-dihydro-2H-l-benzopyran- 6-yl)carbonyl]benzoate (22) (Maignan, J., et al., BE 1,000,195): To a suspension of aluminum chloride (1.6 g, 12 mmol) in 1 mL of 1,2-dichloroethane under argon at room temperature was added a solution of 4,4-dimethyl-3,4-dihydro-2H-l-benzopyran 21 (1.5 g, 9.25 mmol) (Dawson, M.I., et al., J. Med. Chem. 27:1516-1531 (1984)) and 4-carbomethoxybenzoyl chloride 2 (1.79 g, 9 mmol) in 9 mL of 1,2-dichloroethane. The reaction mixture was stirred overnight, poured onto ice-water and extracted with 40% ethyl acetate/hexane. The combined organic layers were washed with saturated aqueous NaHC0 3 and brine. The solution was dried over anhydrous MgS0 4 , filtered, and concentrated to afford a yellow solid (3.24 g) . Flash chromatography (80% diehloromethane/hexane) yielded the desired product 22 as a white powder (1.42 g, 49%): m.p. 129-131oC; R f 0.26 (CH 2 C1 2 ). The structure of the product was also confirmed using IR, J H NMR and mass spectroscopy. (b.) [2-(4,4-Dimethyl-3,4-dihydro-2H-l-benzopyran-6-yl)-2- (4-carbomethoxyphenyl)]-1,3-dithiane (23): To a solution of the keto-ester 22 (0.152 g, 0.469 mmol) in dichloromethane (3 L) at 0°C under argon was added 1,3-propane- dithiol (0.061 g, 0.563 mmol), followed by boron trifluoride etherate (0.07 L, 0.57 mmol). The resulting mixture was stirred at 0oC for 1 h and at ambient temperature overnight. The reaction mixture was quenched by pouring into saturated aqueous Na 2 C0 3 , and then extracted with 40% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford an oil. Flash chromatography (80% diehloromethane/hexane) yielded the desired dithiane 23 as a white solid (0.175 g, 90%): m.p. 103-105oC; R f 0.2 (50% CH 2 Cl 2 /hexane). The structure of the product was also confirmed using IR, *H NMR and mass spectroscopy. (c.) [2-(4,4-Dimethyl-3,4-d_ihydro-2H-l-benzopyran-6-yl)- 2-(4-carboxyphenyl)]-1,3-dithiane (24): To a suspension of the dithiane 23 (0.145 g, 0.349 mmol) in 75% aqueous methanol (4 mL) was added one pellet of potassium hydroxide (0.106 g). The reaction mixture was stirred at 70°C for 1 h during which time the material dissolved. The solution was cooled to room temperature, acidified with 1 N aqueous hydrochloric acid, and then extracted with 80% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford a white solid. Recrystallization from diehloromethane-hexane afforded the desired acid 24 as a white powder (0.127 g, 90%): m.p. 204-205oC. The structure of the product was also confirmed using IR, - NMR and mass spectroscopy. Exampl e XI 28 27 (a.) Methyl 4-[(4,4-dimethyl-3,4-dihydro-2H-l-benzothiopyran- 6-yl)carbonyl]benzoate (26): To a suspension of aluminum chloride (1.65 g, 9.25 mmol) in 1 mL of 1,2-dichloroethane under argon at room temperature was added a solution of 4,4-dimethyl-3,4-dihydro-2H-l-benzothiopyran 25 (1.65 g, 9.25 mmol) ( augh, K.M., et al., J. Med. Chem. 28:116-124 (1985)) and 4-carbomethoxybenzoyl chloride 2 (1.8 g, 9.06 mmol) in 9 mL of 1,2-dichloroethane. The reaction mixture was stirred overnight and poured onto ice water and extracted with 40% ethyl acetate/hexane. The combined organic layers were washed with saturated aqueous NaHC0 3 and brine. The solution was dried over anhydrous MgS0 4 , filtered and concentrated to afford a green solid (2.1 g). Flash chromatography (80% diehloromethane/hexane) yielded the desired product 26 as a white powder (1.23 g, 40%): m.p. 118-120oC; R f 0.35 (CH 2 C1 2 ). The structure of the product was also confirmed using IR, l ti NMR and mass spectroscopy. (b.) [2-(4,4-Dimethyl-3,4-dihydro-2H-l-benzothiopyran-6-yl)- 2-(4-carbomethoxyphenyl)]-l,3-dithiane (27): To a solution of the keto-ester 26 (0.12 g, 0.353 mmol) in dichloromethane (3 L) at 0©C under argon was added 1,3-propandithiol (0.06 mL, 0.53 mmol) followed by boron trifluoride etherate (0.07 mL, 0.57 mmol). The resulting mixture was stirred at 0°C for 1 h and then warmed to room temperature overnight. The reaction mixture was quenched by pouring into saturated aqueous Na 2 C0 3 , and extracted with 40% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford an oil. Flash chromatography (80% CH 2 Cl 2 /hexane) yielded the desired dithiane 27 as a white solid (0.145 g, 96%): m.p. 164-166°C; R f 0.27 (50% CH 2 Cl 2 /hexane). The structure of the product was also confirmed using IR, J H NMR and mass spectroscopy. (c.) [2-(4,4-Dimethyl-3,4-dihydro-2H-l-benzothiopyran-6-yl)~ 2-(4-carboxyphenyl)]-l,3-dithiane (28): To a suspension of the dithiane 27 (0.1 g, 0.232 mmol) in 75% aqueous methanol (4 L) was added one pellet of potassium hydroxide (0.12 g). The reaction mixture was stirred at 75°C for 1 h during which time the material dissolved. The solution was cooled to room temperature, acidified with 1 N aqueous hydrochloric acid, and then extracted with 80% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford a white solid. Recrystallization from diehloromethane/hexane afforded the desired acid 28 as a white powder (0.089 g, 92%): m.p. 211-212.5oC. The structure of the product was also confirmed using IR, Hi NMR and mass spectroscopy. Example XII 31 30 (a.) Methyl [4-(5,6,7,8-tetrahydro-3,5,5,8,8-pentamethyl- 2-naphthalenyl)carbonyl]benzoate (30): To a suspension of aluminum chloride (1.10 g, 8.25 mmol) in 30 L of 1,2-dichloroethane under argon at room temperature was added a solution of l,2,3,4-tetrahydro-l,l,4,4,6-pentamethyl- naphthalene 29 (1.52 g, 7.5 mmol) (Kagechika, H., et al., J. Med. Chem. 31:2182 (1988)) and 4-carbomethoxybenzoyl chloride 2 (1.57 g, 7.87 mmol) in 15 mL of 1,2-dichloroethane. The reaction mixture was stirred overnight and poured onto ice water and extracted with 40% ethyl acetate/hexane. The combined organic layers were washed with saturated aqueous NaHC0 3 and brine. The solution was dried over anhydrous MgS0 4 , filtered and concentrated to afford a brown solid (2.56 g). Flash chromatography (60% diehloromethane/hexane) yielded the desired product 30 as a white, crystalline solid (1.733 g, 64 %): m.p. 146-149oC; R f 0.11 (50% CH 2 Cl 2 /hexane) . The structure of the product was also confirmed using IR, X H NMR and mass spectroscopy. (b.) [4-(5,6,7,8-Tetrahydro-3,5,5,8,8-pentamethyl- 2-naphthalenyl)carbonyl]benzoic acid (31): To a suspension of the ester 30 (0.120 g, 0.329 mmol) in 75% aqueous methanol (2 mL) was added potassium hydroxide (0.055 g) . The reaction mixture was stirred at 60oC for 1 h during which time the material dissolved. The solution was cooled to room temperature, acidified with 1 N aqueous hydrochloric acid, and then extracted with 80% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford a white solid (0.109 g). Recrystallization from benzene/hexane afforded 31 as a white, crystalline solid (0.102 g, 89%): m.p. 209-212oC. The structure of the product was also confirmed using IR, l NMR and mass spectroscopy. Exampl e XI I I 33 (a.) Methyl 4-[l-(5,6,7,8-tetrahydro-3,5,5,8,8-pentamethyl- 2-naphthalenyl)-l-ethenyl]benzoate (32): To a suspension of methyltriphenylphosphoniurn bromide (0.196 g, 0.55 mmol) in 1 mL of benzene under argon at room temperature was added a 0.5 M solution of potassium hexa ethyl- disilazide in toluene (1.2 L, 0.6 mmol), and the yellow solution was stirred for 5 min. A solution of keto-ester 30 (0.1 g, 0.274 mmol) in 1.5 mL of benzene was added, and the orange solution was stirred for 3 h at room temperature. The reaction mixture was filtered through a plug of silica gel with 40% ethyl acetate/hexane. The filtrate was concentrated to afford a solid. Flash chromatography (30%; 40% dichloromethane/ hexane) yielded the desired product 32 as a white solid (0.077 g, 78%): m.p. 167-168°C; R f 0.4 (50% dichloro- methane/hexane) . The structure of the product was also confirmed using IR, *H NMR and mass spectroscopy. (b.) [4-[1-(5,6,7,8-Tetrahydro-3,5,5,8,8-pentamethyl- 2-naphthalenyl)-l-ethenyl]benzoic acid (33): To a suspension of the ester 32 (0.058 g, 0.16 mmol) in 75% aqueous methanol (2 mL) was added one pellet of potassium hydroxide (0.1 g). The mixture was stirred at 70oC for 1 h during which time the material dissolved. The solution was cooled to room temperature, acidified with 1 N aqueous hydrochloric acid, and then extracted with 80% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford a white solid. Recrystallization from diehloromethane/hexane afforded the desired acid 33 as a white, crystalline solid (42 mg, 91%): m.p. 230-231oC. The structure of the product was also confirmed using IR, J H NMR and mass spectroscopy. Example XIV Hθ 2 C0 2 Et 38 (a.) 5-Carboethoxythiophen-2-carboxylic acid (35): To a solution of diisopropylamine (3.6 mL, 25.75 m ol) in 10 mL of THF at -78°C under argon was added a 1.6 M solution of n-butyllithium in hexane (16.1 mL, 25.75 mmol). The mixture was stirred for 15 min, and a solution of 2-thiophenecarboxylic acid 34 (1.5 g, 11.705 mmol) (2-thiophenecarboxylic acid 34 is readily available from Aldrich) in 5 mL of THF was added slowly. The mixture was stirred for 15 min, and ethyl chloroformate (2.7 mL, 28.33 mmol) was added. The mixture was stirred for 30 min at -78oC, and warmed to OoC for another 30 min. The reaction mixture was poured into saturated aqueous NaHC0 3 , and washed with 80% ethyl acetate/hexane. The aqueous layer was acidified with acetic acid and extracted with 80% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford a yellow solid. Flash chromatography yielded the desired acid 35 (1.76 g, 75%) as a white solid: m.p. >300oC. The structure of the product was also confirmed using IR, Hi NMR and mass spectroscopy. (b.) 5-Carboethoxythiophen-2-carbonyl chloride (36): To a suspension of acid 35 (0.64 g, 3.2 mmol) in dichloromethane (20 mL) under argon at room temperature was added a 2.0 M solution of (C0C1) 2 in dichloromethane (2.4 mL, 4.8 mmol) and two drops of DMF, and stirred overnight. Excess (C0C1) 2 and dichloromethane were removed at reduced pressure, and the viscous, light, yellow product was dried overnight to afford the desired benzoyl chloride 36 as a yellow solid. The structure of the product was also confirmed using IR and ! H NMR spectroscopy. (c.) Ethyl 5-[(5,6,7,8-tetrahydro-3,5,5,8,8-pentamethyl- 2-naphthalenyl)carbonyl]thiophen-2-carboxylate (37): To a suspension of aluminum chloride (0.5 g, 3.75 mmol) in 1 L of 1,2-dichloroethane under argon at room temperature was added a solution of l,2,3,4-tetrahydro-l,l,4,4,6-pentamethylnaphthalene 29 (0.712 g, 3.52 mmol) and 36 (0.7 g, 3.2 mmol) in 3.5 mL of 1,2-dichloroethane dropwise. The reaction mixture was stirred overnight, and then poured onto ice water, and extracted with 40% ethyl acetate/hexane. The combined organic layers were washed with saturated aqueous NaHC0 3 and brine. The solution was dried over anhydrous MgS0 4 , filtered and concentrated to afford a yellow solid (1.5 g). Flash chromatography (50% diehloromethane/hexane) yielded a light-yellow solid. Recrystallization from diehloromethane/hexane afforded 37 as a white crystalline solid (0.62 g, 50%): m.p. lll-112oC; R f 0.2 (50% CH 2 Cl 2 /hexane) . The structure of the product was also confirmed using IR, Hi NMR and mass spectroscopy. (d.) 5-[(5,6,7,8-tetrahydro-3,5,5,8,8-pentamethyl- 2-naphthalenyl)carbonyl]thiophen-2-carboxylic acid (38): To a suspension of the ester 3_7 (50 mg, 0.13 mmol) in 75% aqueous methanol (3 mL) was added one pellet of potassium hydroxide (0.1 g). The reaction mixture was stirred at 70°C for 1.5 h during which time the material dissolved. The solution was cooled to room temperature, acidified with 1 N aqueous hydrochloric acid, and then extracted with 80% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford a white solid (46 mg). Recrystallization from methanol afforded the desired acid 38 as a white crystalline solid (42 mg, 91%): m.p. 214-215oC. The structure of the product was also confirmed using IR, Hi NMR and mass spectroscopy. Exampl e XV 0 ethylene gylcot . p-TsOH. TMSO _ benzene OTMS , reflux (a.) 2-Acetyl-3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydro- naphthalene (39): To a suspension of aluminum chloride (0.96 g, 7.17 mmol) in 1 mL of 1,2-dichloroethane under argon at room temperature was added a solution of l,2,3,4-tetrahydro-l,l,4,4,6-pentamethyl- naphthalene 29 (1.2 g, 5.93 mmol) and acetyl chloride (0.51 g, 6.52 mmol) in 9 L of 1,2-dichloroethane. The reaction mixture was stirred for 1 h and then was poured onto ice water and extracted with 40% ethyl acetate/hexane. The combined organic layers were washed with saturated aqueous NaHC0 3 and brine. The solution was dried over anhydrous MgS0 4 , filtered through a plug of silica gel, and concentrated to afford a white solid 39 (1.45 g, 99%): m.p. 54-57°C; R f 0.62 (10% ethyl acetate/hexane). The structure of the product was also confirmed using IR, Hi NMR and mass spectroscopy. (b.) Ethyl (E)-4-hydroxy-3-methyl-6-oxo-6-(3,5,5,8,8- pentamethyl-5,6,7,8-tetrahydro-2-naphthalenyl)-2-hexenoate (ϋ): To a solution of diisopropylamine (0.5 mL, 3.52 mmol) in 7 mL of THF at -78°C under argon was added a 1.6 M solution of n-butyllithium in hexane (2.2 mL, 3.52 mmol). The mixture was stirred for 25 min, and a solution of ketone 39 (0.78 g, 3.2 mmol) in 4 mL of THF was added slowly. The mixture, was stirred for 20 min, and a solution of ethyl (E)-3-formyl-2-butenoate 40 (0.45 g, 3.2 mmol) (ethyl (E)-3-formyl-2-butenoate 40 is readily available from Fluka) in 3 mL of THF was added slowly. The mixture was stirred for 35 min at -78oC, poured into saturated aqueous NH 4 C1 , and extracted with 40% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford a light-yellow solid. Flash chromatography yielded the desired alcohol 41 as a white crystalline solid (0.98 g, 80%): m.p. 126-128°C; R f 0.13 (10% ethyl acetate/hexane). The structure of the product was also confirmed using IR, l ti NMR and mass spectroscopy. (c.) Ethyl (2E.4E)-6-0X0-6-(3,5,5,8,8-pentamethyl-5,6,7,8- tetrahydro-2- naphthalenyl)-2,4-hexadienoate (42): To a solution of 41 (0.33 g, 0.85 mmol) in 8 mL of THF at OoC was added triethylamine (0.5 mL, 4 mmol), and a solution of methylsulfonyl chloride (0.106 g, 0.93 mmol) in 2 mL of THF slowly. The mixture was stirred for 1 h at 0©C and warmed to room temperature for 30 min. The mixture was filtered through a plug of silica gel with 10% ethyl acetate/hexane. The filtrate was concentrated to afford 42 as a light-yellow solid. Recrystallization from diehloromethane/hexane afforded the desired acid 42 as a yellow powder (0.3 g, 95%): m.p. 101-102oC; R f 0.42 (10% ethyl acetate/hexane). The structure of the product was also confirmed using IR, l H NMR and mass spectroscopy. (d.) [2-(5,6,7,8-Tetrahydro-3,5,5,8,8-pentamethyl-2-naph- thaienyl)-2-(4-carboethoxy-2E,4E-3-methylbutadienyl)]-1,3-dioxolane (43): To a solution of keto-ester 42 (0.12 g, 0.33) in 2 mL of benzene was added ethylene gylcol (0.8 mL), l,2-bis(trimethylsilyloxy)ethane (2 mL) and a catalytic amount of β-Ts0H. The reaction mixture was heated at reflux for 2 days and then cooled to room temperature. The solution was poured into saturated aqueous NaHC0 3 and extracted with 40% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford a solid. Flash chromatography (5% ethyl acetate/hexane) yielded the desired ketal 43 as a colorless oil (0.109 g, 80 %).: R f 0.43 (10% ethyl acetate/hexane). The structure of the product was also confirmed using IR, Hi NMR and mass spectroscopy. (e.) [2-(5,6,7,8-Tetrahydro-3,5,5,8,8-pentamethyl- 2-naphthalenyl)-2-(4-carboxy-2E,4E-3-methylbutadienyl)]-1,3-dioxolane (44): To a solution of the ester 43 (30 mg, 0.073 rnrnol) in 75% aqueous methanol (2 mL) was added one pellet of potassium hydroxide (0.1 g). The reaction mixture was stirred at 60°C for 0.5 h during which time the material dissolved. The solution was cooled to room temperature, acidified with 1 N aqueous hydrochloric acid, and then extracted with 80% ethyl acetate/hexane. The combined organic layers were dried over anhydrous MgS0 4 , filtered, and concentrated to afford a white solid. Recrystallization from diehloromethane/hexane afforded the desired acid 44 as a white, crystalline solid (27 mg, 96 %): m.p. 189-190©C. The structure of the product was also confirmed using IR, Hi NMR and mass spectroscopy. Exampl e XVI (E) and (Z)-4-[l-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl- 2-naphthalenyl)propen-l-yl]benzoic acid (46 and 47): To a suspension of ethyltriphenylphosphoniurn bromide (160 mg, 0.43 mmol) in 1 mL of THF under argon at 0©C was added a 0.5 M solution of potassium hexamethyldisilazide in toluene (0.95 mL, 0.47 mmol), and the reaction mixture was stirred for 5 min. A solution of keto-ester 3 (100 mg, 0.28 mmol) in 0.8 mL of THF was added, and the orange solution was stirred at room temperature for 3 h. The reaction mixture was diluted with 20% ethyl acetate/hexane and filtered through a plug of silica gel with 20% ethyl acetate/hexane. The solution was concentrated to afford a yellow gum. Chromatography (38% diehloromethane/hexane) yielded the mixture of 45 as a pale-yellow gum (51 mg, 50%): R f 0.43, 0.47 (40% CH 2 Cl 2 /hexane) . Preparative HPLC (Waters Radialpak Novapak silica, 8 mm x 10 cm, 2% ether/hexane, 1.0 mL/min, 260 nm) gave a colorless gum 45a (20 mg, t r = 9.8 min) and a white solid 45b (25 mg, t r = 10.8 min). To a suspension of the ester 45a (20 mg) in 0.5 mL of ethanol was added a 40% solution of aqueous potassium hydroxide (0.2 g), and the reaction mixture was stirred at 70©C under argon for 2 h during which time the material dissolved. The solution was then concentrated in an argon stream. The residue was cooled to room temperature, acidified with 1 N aqueous sulfuric acid to pH 2-3, and then filtered. The precipitate was repeatedly washed with water (6 x 1 mL) and dried to afford a white powder (20 mg). Recrystallization from ethyl acetate/hexane afforded the acid 46 as a white powder (16 mg, 16% overall yield): m.p. 212oC. The structure of the product was also confirmed using IR, J H NMR and mass spectroscopy. The regiochemistry of 46 was confirmed by l H nOe NMR spectroscopy. The ester 45b was hydrolyzed as above to give 25 mg of pale-yellow solid. Recrystallization from ethyl acetate afforded the acid 47 as a pale-yellow powder (21 mg, 20% overall yield): m.p. 229°C. The structure of the product was also confirmed using IR, *H NMR and mass spectroscopy. The regiochemistry of 47 was confirmed by J H nOe NMR spectroscopy. Exampl e XVI I H 2 . 5% Pd onchaιcαιl, EtOH. ii (a.) 4-[l-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl-2- naphthalenyl)-l-ethenyl]benzoic acid (48): To a suspension of the ester 16 (94 mg, 0.27 mmol) in 75% aqueous methanol (2 mL) was added potassium hydroxide (0.045 g), and the reaction mixture was stirred at 50°C for 14 h during which time the material dissolved. The solution was cooled to room temperature, acidified with 2 N aqueous hydrochloric acid, and then extracted with ether. The combined organic layers were washed with water and brine. The organic solution was then dried over anhydrous MgS0 4 , filtered and concentrated to afford a white solid. Recrystallization from benzene- hexane afforded the desired acid 48 as a white crystalline solid (0.074 g, 82%): m.p. 201-204oD. The structure of the product was also confirmed using IR, Hi NMR and mass spectroscopy. (b.) 4-[l-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl-2- naphthalenyl)-l-ethyl]benzoic acid (49): The olefin 48 (0.0105 g, 0.0314 mmol) was hydrogenated over 5% palladium on charcoal (1 mg) in 0.5 mL of ethanol at room temperature and atmospheric pressure. After one equivalent (0.7 mL) of hydrogen was taken up, the catalyst was removed by filtration through a small Celite pad. The solvent was removed in vacuo to give the crude acid as a white solid (0.019 g) . Recrystallization from benzene-hexane afforded the desired acid 49 as a white crystalline solid (0.0078 g, 74%): m.p. I86-I880C. The structure of the product was also confirmed using IR, Hi NMR and mass spectroscopy. The preceding examples are intended to illustrate but not limit the invention. While they are typical of those that might be used, other procedures known to those skilled in the art may be alternatively employed. Throughout this application various publications are referenced by numbers. Following is a complete citation to the publications. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains. REFERENCES 1. Lotan, R. Biochem. Biophvs. Acta 605, 33-91 (1981). 2. Roberts, A.B., & Sporn, M.B. In: The Retinoids (M.B. Sporn, A.B. Roberts, D.S. Goodman, eds). Academic Press, Florida pp. 209-286 (1984) . 3. Knudson, D.D. J. Invest. Dermatol . 62,288 (1974). 4. Weiss, J.S., Ellis, C.N., Headington, J.T., Tincoff, T., Hamilton, T.A., & Voorhees, J.J. 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Ha ada, K., Gleason, S.L., Levi, B-Z., Hirschfeld, S., Appella, E., & Ozato, K. Proc. Natl. Acad. Sci.. USA 86, 8289-8293 (1989). 14. Mangelsdorf, D. J., Ong, E. S., Dyck, J. A., & Evans, R. M. Nature 345, 224-229 (1990). 15. Yu, V.C., Delsert, C, Andersen, B., Holloway, J.M., Devary, O.V., Naar, A.M., Kim, S.Y., Boutin, J-M., Glass, C.K., & Rosenfeld, M.G. Cell 67, 1251-1266 (1991). 16. Leid, M., Kastner, P., Lyons, R., Nakshatri, H., Saunders, M., Zacharewski, T., Chen, J-Y., Staub, A., Gamier, J-M., Mader, S., & Chambon, P. Cell 68, 377-395 (1992). 17. Mangelsdorf, D.J., Borgmeyer, U., Heyman, R.A., Zhou, J.Y., Ong, E.S., Oro, A.E., Kakizuka, A., & Evans, R.M. Genes & Dev. 6, 329-344 (1992). 18. Evans, R.M. Science 240, 889-895 (1988). 19. Green, S., & Chambon, P. Trends Genet. 4, 309-314 (1988). 20. Lehmann, J.M., Hoffman, B., & Pfahl, M. Nucl . Acids Res. 19, 573- 578 (1991). 21. Leroy, P., Krust, A., Zelent, A., Mendelsohn, C, Gamier, J-M., Kastner, P., Dierich, A., & Chambon, P. EMBO J. 10, 59-69 (1991). 22. Zelent, A., Mendelsohn, C, Kastner, P., Krust, A., Gamier, J-M,. Ruffenach, F., Leroy, P., & Chambon, P. EMBO J. 10, 71-81 (1991). 23. Oro, A.E., McKeown, M., & Evans, R.M. Nature 347, 298-301 (1990). 24. Hey an, R., Mangelsdorf, D.J., Dyck, J.A., Stein, R.B., Eichele, G., Evans, R.M., & Thaller, C. Cell 68, 397-406 (1992). 25. Levin, A.A., Sturzenbecker, L.J., Kazmer, S., Bosakowski, T., Huselton, C, Allenby, G., Speck, J., Kratzeisen, C, Rosenberger, M., Lovey, A., & Grippo, J.F. Nature 355, 359-361 (1992). 26. Zhang, X-k., Hoffmann, B., Tran, P., Graupner, G., & Pfahl, M. Nature 355, 441-446 (1992). 27. Kliewer, S.A., Umesono, K., Mangelsdorf, D.J., & Evans, R.M. Nature 355, 446-449 (1992). 28. Marks, M.S., Hallenbeck, P.L., Nagata, T., Segars, J.H., Apella, E., Nikodem, V.M., & Ozato, K. EMBO J. 11, 1419-1435 (1992). 29. Bugge, T.H., Pohl, J., Lonnoy, 0., & Stunnenberg, H.G. EMBO J. 11, 1409-1418 (1992). 30. Mangelsdorf, D.J., Umesono, K., Kliewer, S.A., Borgmeyer, U., Ong, E.S., & Evans, R.M. Cell 66, 555-561 (1991). 31. Rottman, J.N., Widom, R.L., Nadal-Ginard, B., Mahdavi , V., & Karathanasis, S.K. Mol. Cell. Biol. 11, 3814-3820 (1991). 32. Hermann, T., Hoffmann, B., Zhang, X-k., Tran, P. & Pfahl, M. Mol. Endrinol . (accepted for publication). 33. Zhang, X-k., Tran, P., & Pfahl, M. Mol. Endocrinol. 5, 1909-1920 (1991b). 34. Glass, C.K., Holloway, J.M., Devary, O.V., & Rosenfeld, M.G. Cell 54, 313-323 (1988). 36. de Thέ, H., Vivanco-Ruiz, M.M., Tiollais, P., Stunnenberg, H., & Dejean, A. Nature 343, 177-180 (1990). 37. Hoffmann, B., Lehmann, J.M., Zhang, X-k., Hermann, T., Graupner, G., & Pfahl, M. Mol. Endocrinol. 4, 1734-1743 (1990). 38. Husmann, M.B., Hoffmann, B., Stump, D.G., Chytil, F., & Pfahl, M. BBRC in press (1992). 39. Umesono, K., Murakami, K.K., Thompson, CC, & Evans, R.M. Cell 65, 1255-1266 (1991). 40. Flink, I.L., & Morkin, E. J. Biol. Chem. 265, 11233-11237 (1990). 41. Desvergne, B., Petty, K.J., & Nikodem, V.M. J. Biol. Chem. 266, 1008-1013 (1991). 42. Klein-Hitpass, L., Schorpp, M., Wagner, U., & Ryffel, G.U. Cell 46, 1053-1061 (1986). 43. Husmann, M., Lehmann, J., Hoffmann, B., Hermann, T., Tzukerman, M., & Pfahl, M. Mol. Cell. Biol. 11, 4097-4103 (1991). 44. Tora, L., White, J., Brou, C, Tasset, D., Webster, N., Scheer, E., & Chambon, P. Cell 59, 477-487 (1989). 45. Tzukerman, M., Zhang, X-k., Wills, K.N., Graupner, G., Hermann, T., & Pfahl, M. New Biol. 2, 613-620 (1990). 46. Zhang, X-k., Wills, K.N., Graupner, G., Tzukerman, M., Hermann, T., & Pfahl, M. New Biol 3, 1-14 (1991a). 47. Yen, P.M., Darling, D.S., Carter, R.L., Forgione, M., Umeda, P.K., & Chin, W.W. J. Biol. Chem. 267, 3565-3568 (1992). 48. Forman, B.M., & Samuels, H.H. Mol. Endocrinol. 4, 1293-1301 (1990). 49. Zenke, M., Munoz, A., Sap, J., Vennstrom, B., & Beug, H. 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SEQUENCE LISTING (1) GENERAL INFORMATION: (A) NAME: The Government of the United States of America, as represented by The Secretary (B) STREET: 6011 Executive Blvd., Suite 325 (C) CITY: Rockville (D) STATE: Maryland (E) COUNTRY: United States of America (F) POSTAL CODE (ZIP): 20852 (G) TELEPHONE: (301) 496-7056 (H) TELEFAX: (301) 402-0220 (I) TELEX: None (ii) TITLE OF INVENTION: RXR HOMODIMER FORMATION (iii) NUMBER OF SEQUENCES: 11 (iv) COMPUTER READABLE FORM: , (A) MEDIUM TYPE: Floppy disk (B) COMPUTER: IBM PC compatible (C) OPERATING SYSTEM: PC-DOS/MS-DOS (D) SOFTWARE: Patentin Release #1.0, Version #1.25 (vi) CURRENT APPLICATION DATA: (A) APPLICATION NUMBER: NOT YET ASSIGNED (B) FILING DATE: (vii) PRIOR APPLICATION DATA: (A) APPLICATION NUMBER: US/07/901/719 (B) FILING DATE: 16-JUN-1992 (2) INFORMATION FOR SEQ ID N0:1: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 8 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: peptide (xi) SEQUENCE DESCRIPTION: SEQ ID N0:1: Arg Tyr Lys Asp Asp Asp Asp Lys 1 5 (2) INFORMATION FOR SEQ ID N0:2 : (i ) SEQUENCE CHARACTERISTICS: (A) LENGTH: 31 base pai rs (B) TYPE: nucl ei c aci d (C) STRANDEDNESS: si ngl e (D) TOPOLOGY: l i near (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID N0:2: GATCAGGGCA GGGGTCAAGG GTTCAGTGAT C 31 (2) INFORMATION FOR SEQ ID NO:3: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 37 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single, (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID N0:3: GATCCAGGTC ACAGGTCACA GGTCACAGTT CAAGATC 37 (2) INFORMATION FOR SEQ ID N0:4: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 34 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID N0:4: GATCTGTAGG GTTCACCGAA AGTTCACTCA GATC 34 (2) INFORMATION FOR SEQ ID N0:5: 3 (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 24 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID N0:5: GATCCAGGTC AAAAAGTCAG GATC 24 (2) INFORMATION FOR SEQ ID N0:6: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 30 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID N0:6: GATCCTGGAG GTGACAGGAG GACAGCGATC 30 (2) INFORMATION FOR SEQ ID N0:7: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 38 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID N0:7: GATCCAGGAC GTTGGGGTTA GGGGAGGACA GTGGGATC 38 (2) INFORMATION FOR SEQ ID N0:8: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 27 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID N0:8: GATCTCAGGT CATCCTCAGG TCAGATC 27 (2) INFORMATION FOR SEQ ID NO:9: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 27 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID N0:9: GATCTCAGGT CATCCTCAGG TCAGATC 27 (2) INFORMATION FOR SEQ ID NO:10: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 27 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID NO:10: GATCTCAGGT CACTGTGACC TGAGATC 27 (2) INFORMATION FOR SEQ ID NO:11: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 24 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID NO:11: GATCTCAGGT CATGACCTGA GATC 24

Claims: What is claimed is: 1. A method of screening a substance for the ability to affect the formation of a retinoid X receptor homodimer comprising combining the substance and a solution containing retinoid X receptors and determining the presence of homodimer formation. 2. The method of claim 1, wherein the presence of homodimer formation is determined by detecting the activation of transcription by the retinoid X receptor homodimer. 3. The method of claim 1, wherein the presence of homodimer formation is determined by coprecipitation. 4. The method of claim 1, wherein the effect is the induction of homodimer formation. 5. The method of claim 1, wherein the effect is the inhibition of homodimer formation. 6. The method of claim 1, wherein the retinoid X receptor is retinoid X receptor a. 7. The method of claim 4, further comprising determining if the substance induces a retinoid X receptor heterodimer and selecting the substance which selectively induces the formation of a retinoid X receptor homodimer. 8. A method of screening a substance for the ability to selectively induce the formation of a retinoid X receptor heterodimer over a retinoid X receptor homodimer comprising: a) determining if the substance induces retinoid X receptor homodimer formation; b) determining if the substance induces retinoid X receptor heterodimer formation; and c) selecting the compound which selectively induces the formation of a retinoid X receptor heterodimer over a retinoid X receptor homodimer. 9. A method of screening a substance for an affect on a retinoid X receptor homodimer's ability to bind DNA comprising combining the substance with the homodimer and determining the affect of the compound on the homodimer's ability to bind DNA. 10. A method of screening a response element for binding with a retinoid X receptor homodimer comprising combining the response element with the retinoid X receptor homodimer and detecting the presence of binding. 11. The method of claim 10, wherein the presence of binding is detected by the transcriptional activation of a marker which is operably linked to the response element. 12. A purified retinoid X receptor homodimer. 13. A bicyclic aromatic compound having the structural formula R 1 wherein: R 1 is selected from the group consisting of lower alkyl and adamantyl; R 2 is -0-R 6 or -S-R 6 where R 6 is lower alkyl; or where R 1 is ortho to R 2 , R 1 and R 2 may be linked together to form a 5- or 6-membered cycloalkylene ring, either unsubstituted or substituted with 1 to 4 lower alkyl groups, and optionally containing 1 or 2 heterocyclic atoms selected from the group consisting of 0, S and NR where R is hydrogen or lower alkyl; R 3 is selected from the group consisting of carbonyl, ( CH 2 \ / \ \ / \ / \ / c c σnd + * in which X 1 and X 2 are independently selected from the group consisting of 0, S and methylene, wherein at least one of X 1 and X 2 is 0 or S, or wherein one of X 1 and X 2 is NR and the other is methylene, m is 2 or 3, R δ , R 7 , R 8 and R 9 are independently hydrogen or lower alkyl, with the proviso that when n is 0, R 6 and R 7 are not both hydrogen and R 8 and R 9 are not both hydrogen, or R 8 and R 9 may be linked together to form a cycloalkylene ring containing 3 to 6 carbon atoms, and * represents the point of attachment of the R 3 substituent to the remainder of the molecule; and R* is selected from the group consisting of in which R 10 is hydrogen or methyl, 1 is 0 or 1, and ** represents the point of attachment of the R 4 substituent to the remainder of the molecule, the R 5 are independently selected from the group consisting of lower alkyl and lower alkoxy; and n is 0, 1, 2 or 3, with the proviso that if n is 0, R 3 is other than carbonyl, *C=CH 2 or CH 2 , and pharmaceutically acceptable esters, amides and salts of the compound. 14. The compound of claim 13, having a structure selected from the group consisting of where R 11 is selected from the group consisting of 0, S, (CH 3 ) 2 C and CH 2 , and R 12 is hydrogen or methyl. 15. The compound of claim 14, having the structural formula (II). 16. The compound of claim 15, wherein n is 0. 17. The compound of claim 16, wherein R 4 is (COOH) 1 and 1 is 1. 18. The compound of claim 17, wherein R 4 is 19. The compound of claim 17, wherein R 4 is 20. The compound of claim 15, wherein R 3 has the structural formula < CH 2 >m / \ 2 \ / c 21. The compound of cl aim 17, wherein R 3 has the structural formul a 22. The compound of cl aim 15, wherein R 3 has the structural formul a R 6 R 7 C * 23. The compound of claim 17, wherein R 3 has the structural formula R 6 R 7 C * 24. The compound of claim 23, wherein R 3 is selected from the group consisting of CH, CH, CH, H CH- CH 2 CH 3 \ / \ / \ / C C c + + * CH.p CHΛ H CHΛCH- J CHΛCH- J CH CH- 7 \ / \ / c \ / c » c αnd c 25. The compound of claim 15, wherein R 3 has the structural formula R 8 R 9 \ / C 26. The compound of claim 17, wherein R 3 has the structural formula 27. The compound of claim 26, wherein R 3 is selected from the group consisting of CH 3 CH 3 CH 2 — CH 2 CH 2 — CHCH j \ / \ / \ / 28. The compound of claim 15, having the structural formula 29. The compound of claim 15, having the structural formula COOH 30. The compound of claim 15, having the structural formula 31. The compound of claim 15, having the structural formula 32. The compound of claim 15, having the structural formula 33. The compound of claim 15, having the structural formula COOH wherein X 1 and X 2 are independently selected from the group consisting of 0 and S, and the ammonium salt of the compound. 34. The compound of claim 15, having the structural formula wherein X 1 and X 2 are independently selected from the group consisting of 0 and S, and the ammonium salt of.the compound. 35. The compound of claim 15, having the structural formula 36. The compound of claim 15, having the structural formula COOH 37. The compound of claim 15, having the structural formula wherein R 1 is 0 or S. 38. The compound of claim 15, having the structural- formula and the ammonium salt of the compound. 39. The compound of claim 15, having the structural formula 40. The compound of claim 15, having the structural formula COOH 41. The compound of claim 15, having the structural formula 42. The compound of claim 15, having the structural formula 43. A method of inhibiting an activity of a retinoid X receptor heterodimer comprising inducing the formation of a retinoid X receptor homodimer, thereby preventing the retinoid X receptor from forming a heterodimer and preventing the resulting heterodimer activity. 44. The method of claim 43, wherein the activity is the activation or repression of transcription. 45. The method of claim 43, wherein the retinoid X receptor heterodimer is thyroid hormone receptor and retinoid X receptor. 46. A method of promoting the transcription of a gene activated by a retinoid X receptor homodimer in a cell comprising contacting the cell with an amount of a synthetic compound capable of promoting the formation of retinoid X receptor homodimers. 47. The method of claim 46, wherein the compound has the structural formula i —V 48. The method of claim 46, wherein the compound has the structural formula 49. A method of inhibiting an activity of a retinoid X receptor homodimer comprising preventing the formation of the retinoid X receptor homodimer. 50. The activity of claim 49, wherein the activity is the activation or repression of transcription. 51. A method of inhibiting an activity of a retinoid X receptor homodimer comprising preventing the binding of the retinoid X receptor homodimer to its response element. 52. The method of claim 51, wherein the activity is the activation or repression of transcription. 53. A method of determining an increased probability of a pathology associated with retinoid X receptor homodimer formation comprising detecting a decrease of retinoid X receptor homodimer formation in the subject when compared to a normal subject. 54. A method of treating a pathology associated with retinoid X receptor homodimers in a subject comprising inducing retinoid X receptor homodimer formation in the subject, thereby increasing the expression of a gene which can be activated by a retinoid X receptor homodimer. 55. The method of claim 54, wherein retinoid X receptor homodimer formation is induced by the addition of a compound having the structural formula 56. The method of claim 54, wherein retinoid X receptor homodimer formation is induced by the addition of a compound having the structural formula 57. The method of claim 54, wherein the pathology is associated with the skin. 58. The method of claim 54, wherein the skin pathology is selected from the group consisting of acne and psoriasis. 59. The method of claim 54, wherein the pathology is a cancer. 60. The method of claim 54, wherein the gene is the apolipoprotein Al gene. 61. A method of selectively activating retinoid X receptor homodimer formation in a cell comprising adding to the cell a homodimer formation specific ligand, thereby selectively activating the retinoid X receptor homodimer formation in the cell. 62. The method of claim 61, wherein the compound has the structural formula 63. The method of claim 61, wherein the compound has the structural formula 64. A method of promoting retinoid X receptor homodimer formation in a cell comprising the expression of 9-cis-RA in the cell. 65. A pharmaceutical composition for control of cellular processes regulated by retinoic acid, vitamin D, or thyroid hormone, comprising an effective regulating amount of the compound of claim 13 in combination with a pharmaceutically acceptable carrier. 66. A pharmaceutical composition for control of cellular processes regulated by retinoic acid, vitamin D, or thyroid hormone, comprising an effective regulating amount of the compound of claim 15 in combination with a pharmaceutically acceptable carrier. 67. A method for modulating gene expression, in a subject, by a receptor selected from the group consisting of retinoic acid receptors, retinoid X receptors, vitamin D receptors and thyroid receptors, comprising administering to the subject an effective modulating amount of the compound of claim 13 or a pharmaceutical composition thereof. 68. A method for modulating gene expression, in a subject, by a receptor selected from the group consisting of retinoic acid receptors, retinoid X receptors, vitamin D receptors and thyroid receptors, comprising administering to the subject an effective modulating amount of the compound of claim 15 or a pharmaceutical composition thereof. 69. A method for treating a subject afflicted with a disease caused by malfunction of cell differentiation processes regulated by retinoids, thyroid hormone, or vitamin D, comprising administering to the subject a therapeutically effective amount of the compound of claim 13 or a pharmaceutical composition thereof. 70. A method for treating a subject afflicted with a disease caused by malfunction of cell differentiation processes regulated by retinoids, thyroid hormone, or vitamin D, comprising administering to the subject a therapeutically effective amount of the compound of claim 15 or a pharmaceutical composition thereof.

Inventors: CAMERION JAMES F, DAWSON MARCIA I, HOBBS PETER D, JONG LING, LEHMANN JURGEN M, PFAHL MAGNUS, ZHANG XIAO-KUN, CAMERION, JAMES, F., DAWSON, MARCIA, I., HOBBS, PETER, D., JONG, LING, LEHMANN, JURGEN, M., PFAHL, MAGNUS, ZHANG, XIAO-KUN
Applicants: JOLLA CANCER RES FOUND, STANFORD RES INST INT, LA JOLLA CANCER RESEARCH FOUNDATION, SRI INTERNATIONAL
Application year: 1993
Main classification: G01N33
IPC codes: A61K31, A61P3, A61P5, A61P17, A61P31, A61P35, A61P37, A61P43, C07C63, C07C65, C07C69, C07C233, C07D311, C07D317, C07D319, C07D327, C07D333, C07D339, C07D409, C07K14, C12N15, G01N33