Heat shock protein 90 (Hsp90) is a molecular chaperone that plays a key role in the conformational maturation of oncogenic signalling proteins, including HER-2/ErbB2, Akt, Raf-1, Bcr-Abl and mutated p53. Hsp90 inhibitors bind to Hsp90, and induce the proteasomal degradation of Hsp90 client proteins. Although Hsp90 is highly expressed in most cells, Hsp90 inhibitors selectively kill cancer cells compared to normal cells, and the Hsp90 inhibitor 17-allylaminogeldanamycin (17-AAG) is currently in phase I clinical trials. However, the molecular basis of the tumour selectivity of Hsp90 inhibitors is unknown. Here we report that Hsp90 derived from tumour cells has a 100-fold higher binding affinity for 17-AAG than does Hsp90 from normal cells. Tumour Hsp90 is present entirely in multi-chaperone complexes with high ATPase activity, whereas Hsp90 from normal tissues is in a latent, uncomplexed state. In vitro reconstitution of chaperone complexes with Hsp90 resulted in increased binding affinity to 17-AAG, and increased ATPase activity. These results suggest that tumour cells contain Hsp90 complexes in an activated, high-affinity conformation that facilitates malignant progression, and that may represent a unique target for cancer therapeutics.

Retinoic acid receptors (RAR), thyroid hormone receptors (TR), peroxisome proliferator activated receptors (PPARs) and the orphan receptor, LXR, bind preferentially to DNA as heterodimers with a common partner, retinoid X receptor (RXR), to regulate transcription. We investigated whether RXR-selective agonists replicate the activity of ligands for several of these receptors? We demonstrate here that RXR-selective ligands (referred to as rexinoids) function as RXR heterodimer-selective agonists, activating RXR: PPARgamma and RXR:LXR dimers but not RXR:RAR or RXR:TR heterodimers. Because PPARgamma is a target for antidiabetic agents, we investigated whether RXR ligands could alter insulin and glucose signalling. In mouse models of noninsulin-dependent diabetes mellitus (NIDDM) and obesity, RXR agonists function as insulin sensitizers and can decrease hyperglycaemia, hypertriglyceridaemia and hyperinsulinaemia. This antidiabetic activity can be further enhanced by combination treatment with PPARgamma agonists, such as thiazolidinediones. These data suggest that the RXR:PPARgamma heterodimer is a single-function complex serving as a molecular target for treatment of insulin resistance. Activation of the RXR:PPARgamma dimer with rexinoids may provide a new and effective treatment for NIDDM.

Retinoic acid receptors (RARs) and retinoid X receptors (RXRs) regulate transcription by binding to response elements in target genes that generally consist of two direct repeat half-sites of consensus sequence AGGTCA (ref. 1). RAR/RXR heterodimers activate transcription in response to all-trans or 9-cis retinoic acid by binding to direct repeats spaced by five base pairs (DR5 elements), such that RAR occupies the downstream half-site. RXR homodimers activate transcription in response to 9-cis retinoic acid by binding to direct repeats spaced by one base pair (DR1 elements). Although RXR/RAR heterodimers bind to DR1 elements with higher affinity than RXR homodimers, in most contexts they are unable to activate transcription in response to either all-trans or 9-cis retinoic acid. As a result, RARs inhibit RXR-dependent transcription from these sites. We report that the switching of the RAR from an activator to an inhibitor of retinoid-dependent transcription requires that it be bound to the upstream half-site of DR1 elements and that it allosterically block the binding of ligand to the RXR.

Two series of potent retinoid X receptor (RXR)-selective compounds were designed and synthesized based upon recent observation that (E)-4-[2-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-2-naphthalenyl)-1- propenyl]benzoic acid (TTNBP) binds and transactivates only the retinoic acid receptor (RAR) subtypes whereas (E)-4-[2-(3,5,5,8,8-pentamethyl-5,6,7,8- tetrahydro-2-naphthalenyl)-1-propenyl]benzoic acid (3-methyl TTNPB) binds and transactivates both the RAR and RXR subfamilies. Addition of functional groups such as methyl, chloro, bromo, or ethyl to the 3 position of the tetrahydronaphthalene moiety of 4-[(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-2-naphthyl)carbonyl]benzoic acid (5a) and 4-[1-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-2- naphthyl)ethenyl]benzoic acid (6a) results in compounds which elicit potent and selective activation of the RXR class. Such RXR-selective compounds offer pharmacological tools for elucidating the biological role of the individual retinoid receptors with which they interact. Activation profiles in cotransfection and competitive binding assays as well as molecular modeling calculations demonstrate critical structural determinants that confer selectivity for members of the RXR subfamily. The most potent compound of these series, 4-[1-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydro-2-naphthyl)ethenyl]ben zoi c acid (6b), is the first RXR-selective retinoid (designated as LGD1069) to enter clinical trials for cancer indications.

The molecular chaperone heat-shock protein 90 (Hsp90) is involved in the stabilization and conformational maturation of many signaling proteins that are deregulated in cancers. Hsp90 inhibition results in the proteasomal degradation of these client proteins and leads to potent antitumor activity. The Hsp90 inhibitor 17-allylaminogeldanamycin (17-AAG) is presently in clinical trials. Recent work has identified the role of Hsp90 in multiple signal transduction pathways and revealed that the molecular mechanism of tumor selectivity by Hsp90 inhibitors is the result of an activated, high-affinity conformation of Hsp90 in tumors. This review discusses these recent advances in the understanding of tumor Hsp90 for the treatment and diagnosis of cancer. In addition, the role of Hsp90 in non-oncological diseases will also be discussed.

Structural modifications of the retinoid X receptor (RXR) selective compound 4-[1-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydro-2- naphthyl)ethenyl]benzoic acid (LGD1069), which is currently in phase I/IIA clinical trials for cancer and dermatological indications, have resulted in the identification of increasingly potent retinoids with > 1000-fold selectivity for the RXRs. This paper describes the design and preparation of a series of RXR selective retinoids as well as the biological data obtained from cotransfection and competitive binding assays which were used to evaluate their potency and selectivity. The most potent and selective of the analogs is 6-[1-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydronaphthalen-2- yl)cyclopropyl]nicotinic acid (12d; LG100268). This compound has proven useful for investigating RXR dependent biological pathways including the induction of programmed cell death (PCD) and transglutaminase (TGase) activity. Our studies indicate that the induction of PCD and TGase in human leukemic myeloid cells is dependent upon activation of RXR-mediated pathways.

BACKGROUND AND PURPOSESphingosine1-phosphate (S1P) receptors mediate multiple events including lymphocyte trafficking, cardiac function, and endothelial barrier integrity. Stimulation of S1P 1 receptors sequesters lymphocyte subsets in peripheral lymphoid organs, preventing their trafficking to inflamed tissue sites, modulating immunity. Targeting S1P receptors for treating autoimmune disease has been established in clinical studies with the non-selective S1P modulator, FTY720 (fingolimod, Gilenya ™ ). The purpose of this study was to assess RPC1063 for its therapeutic utility in autoimmune diseases. EXPERIMENTAL APPROACHThe specificity and potency of RPC1063 (ozanimod) was evaluated for all five S1P receptors, and its effect on cell surface S1P 1 receptor expression, was characterized in vitro. The oral pharmacokinetic (PK) parameters and pharmacodynamic effects were established in rodents, and its activity in three models of autoimmune disease (experimental autoimmune encephalitis, 2,4,6-trinitrobenzenesulfonic acid colitis and CD4 + CD45RB hi T cell adoptive transfer colitis) was assessed. KEY RESULTSRPC1063 was specific for S1P 1 and S1P 5 receptors, induced S1P 1 receptor internalization and induced a reversible reduction in circulating B and CCR7 + T lymphocytes in vivo. RPC1063 showed high oral bioavailability and volume of distribution, and a circulatory half-life that supports once daily dosing. Oral RPC1063 reduced inflammation and disease parameters in all three autoimmune disease models. CONCLUSIONS AND IMPLICATIONSS1P receptor selectivity, favourable PK properties and efficacy in three distinct disease models supports the clinical development of RPC1063 for the treatment of relapsing multiple sclerosis and inflammatory bowel disease, differentiates RPC1063 from other S1P receptor agonists, and could result in improved safety outcomes in the clinic.

Inhibition of heat shock protein 90 (Hsp90) results in the degradation of oncoproteins that drive malignant progression, inducing cell death, making Hsp90 a target of substantial interest for cancer therapy. BIIB021 is a novel, fully synthetic inhibitor of Hsp90 that binds competitively with geldanamycin in the ATP-binding pocket of Hsp90. In tumor cells, BIIB021 induced the degradation of Hsp90 client proteins including HER-2, AKT, and Raf-1 and up-regulated expression of the heat shock proteins Hsp70 and Hsp27. BIIB021 treatment resulted in growth inhibition and cell death in cell lines from a variety of tumor types at nanomolar concentrations. Oral administration of BIIB021 led to the degradation of Hsp90 client proteins measured in tumor tissue and resulted in the inhibition of tumor growth in several human tumor xenograft models. Studies to investigate the antitumor effects of BIIB021 showed activity on both daily and intermittent dosing schedules, providing dose schedule flexibility for clinical studies. Assays measuring the HER-2 protein in tumor tissue and the HER-2 extracellular domain in plasma were used to show interdiction of the Hsp90 pathway and utility as potential biomarkers in clinical trials for BIIB021. Together, these data show that BIIB021 is a promising new oral inhibitor of Hsp90 with antitumor activity in preclinical models. [Mol Cancer Ther 2009;8(4):921-9] IntroductionHeat shock protein 90 (Hsp90) is a widely expressed molecular chaperone that functions in the maturation and stabilization of cellular proteins (1-3). Hsp90, in complex with other cochaperone proteins, catalyzes the conformational changes of client proteins via its ATPase activity (4). The activity of Hsp90 maintains a variety of client proteins in their active conformation (5). Hsp90 also plays an important role in the regulation of several key oncogenic signaling proteins (6-8) and steroid receptors (9). Mutated proteins are particularly dependent on Hsp90 for the maintenance of the active conformation (2, 3).Ansamycin drugs such as geldanamycin bind in the ATPbinding site in the NH 2 terminus of Hsp90 (6, 10). This binding inhibits the chaperone activity of Hsp90 and results in proteasomal degradation of the client proteins (5, 11-13). Because tumor cells rely on the activity of client proteins for cell proliferation and survival, drug-induced client protein degradation leads to cytostasis and/or selective cell killing of tumor cell in vitro and in vivo (14-16).The semisynthetic Hsp90 inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG) is currently in clinical trials for cancer (17)(18)(19). However, 17-AAG is expensive to prepare and difficult to formulate. The problematic nature of the formulations may well contribute to the dose-limiting toxicity observed with this compound. 17-AAG is also susceptible to metabolism by NQO1/DT-diaphorase enzymes (20) and to efflux by P-glycoprotein (21). The identification of a synthetic Hsp90 inhibitor would be of great therapeutic interest as it would circumvent t...