AR Structural Variants and Prostate Cancer.

Therapeutic interventions for advanced castration-resistant prostate cancer (CRPC) are focused on inhibiting the androgen receptor (AR) through targeting of its C-terminal ligand binding domain (LBD). However, a significant subset of CRPC patients demonstrate primary resistance to androgen deprivation and anti-androgen therapies, suggesting that other targets, outside of the AR, might be pertinent to the cancer progression. One explanation is the expression of androgen receptor splice variants (AR-Vs). So far, more than 20 AR-Vs have been identified from both prostate cancer cell lines and prostate cancer tissue biopsies. Most of the AR-Vs have a conserved N-terminal domain, but lack the LBD, yet retain the ability to bind DNA and activate downstream signaling. Although it remains unclear whether AR-Vs are principal divers or mere bystanders of CRPC progression, inhibiting AR-Vs, through drugs that target the AR transactivation function outside of the LBD, has been a major emphasis for next generation therapeutics in prostate cancer. This book chapter is dedicated to the role of AR variants and their clinical importance. We will review the initial discovery of AR-Vs, their regulation and prevalence, as well as their biological function in prostate cancer. We will provide an overview of the role of AR-Vs in the development of metastatic CRPC and in promoting clinical treatment failures. Lastly, we will present an introduction to the therapeutic approaches towards developing AR-V-targeted therapies including the continuing progress, the old challenges, and the new prospects.

SAR439859, a Novel Selective Estrogen Receptor Degrader (SERD), Demonstrates Effective and Broad Antitumor Activity in Wild-Type and Mutant ER-Positive Breast Cancer Models.

Primary treatment for estrogen receptor-positive (ER+) breast cancer is endocrine therapy. However, substantial evidence indicates a continued role for ER signaling in tumor progression. Selective estrogen receptor degraders (SERD), such as fulvestrant, induce effective ER signaling inhibition, although clinical studies with fulvestrant report insufficient blockade of ER signaling, possibly due to suboptimal pharmaceutical properties. Furthermore, activating mutations in the ER have emerged as a resistance mechanism to current endocrine therapies. New oral SERDs with improved drug properties are under clinical investigation, but the biological profile that could translate to improved therapeutic benefit remains unclear. Here, we describe the discovery of SAR439859, a novel, orally bioavailable SERD with potent antagonist and degradation activities against both wild-type and mutant Y537S ER. Driven by its fluoropropyl pyrrolidinyl side chain, SAR439859 has demonstrated broader and superior ER antagonist and degrader activities across a large panel of ER+ cells, compared with other SERDs characterized by a cinnamic acid side chain, including improved inhibition of ER signaling and tumor cell growth. Similarly, in vivo treatment with SAR439859 demonstrated significant tumor regression in ER+ breast cancer models, including MCF7-ESR1 wild-type and mutant-Y537S mouse tumors, and HCI013, a patient-derived tamoxifen-resistant xenograft tumor. These findings indicate that SAR439859 may provide therapeutic benefit to patients with ER+ breast cancer, including those who have resistance to endocrine therapy with both wild-type and mutant ER.

Translational strategy using multiple nuclear imaging biomarkers to evaluate target engagement and early therapeutic efficacy of SAR439859, a novel selective estrogen receptor degrader.

PURPOSE: Preclinical in vivo nuclear imaging of mice offers an enabling perspective to evaluate drug efficacy at optimal dose and schedule. In this study, we interrogated sufficient estrogen receptor occupancy and degradation for the selective estrogen receptor degrader (SERD) compound SAR439859 using molecular imaging and histological techniques. MATERIAL AND METHODS: [18F]FluoroEstradiol positron emission tomography (FES-PET), [18F]FluoroDeoxyGlucose (FDG) PET, and [18F]FluoroThymidine (FLT) PET were investigated as early pharmacodynamic, tumor metabolism, and tumor proliferation imaging biomarkers, respectively, in mice bearing subcutaneous MCF7-Y537S mutant ERα+ breast cancer model treated with the SERD agent SAR439859. ER expression and proliferation index Ki-67 were assessed by immunohistochemistry (IHC). The combination of palbociclib CDK 4/6 inhibitor with SAR439859 was tested for its potential synergistic effect on anti-tumor activity. RESULTS: After repeated SAR439859 oral administration over 4 days, FES tumoral uptake (SUVmean) decreases compared to baseline by 35, 57, and 55% for the 25 mg/kg qd, 12.5 mg/kg bid and 5 mg/kg bid treatment groups, respectively. FES tumor uptake following SAR439859 treatment at different doses correlates with immunohistochemical scoring for ERα expression. No significant difference in FDG uptake is observed after SAR439859 treatments over 3 days. FLT accumulation in tumor is significantly decreased when palbociclib is combined to SAR439859 (- 64%) but not different from the group dosed with palbociclib alone (- 46%). The impact on proliferation is corroborated by Ki-67 IHC data for both groups of treatment. CONCLUSIONS: In our preclinical studies, dose-dependent inhibition of FES tumoral uptake confirmed target engagement of SAR439859 to ERα. FES-PET thus appears as a relevant imaging biomarker for measuring non-invasively the impact of SAR439859 on tumor estrogen receptor occupancy. This study further validates the use of FLT-PET to directly visualize the anti-proliferative tumor effect of the palbociclib CDK 4/6 inhibitor alone and in combination with SAR439859.

Discovery of 6-(2,4-Dichlorophenyl)-5-[4-[(3S)-1-(3-fluoropropyl)pyrrolidin-3-yl]oxyphenyl]-8,9-dihydro-7H-benzo[7]annulene-2-carboxylic acid (SAR439859), a Potent and Selective Estrogen Receptor Degrader (SERD) for the Treatment of Estrogen-Receptor-Positive Breast Cancer.

More than 75% of breast cancers are estrogen receptor alpha (ERα) positive (ER+), and resistance to current hormone therapies occurs in one-third of ER+ patients. Tumor resistance is still ERα-dependent, but mutations usually confer constitutive activation to the hormone receptor, rendering ERα modulator drugs such as tamoxifen and aromatase inhibitors ineffective. Fulvestrant is a potent selective estrogen receptor degrader (SERD), which degrades the ERα receptor in drug-resistant tumors and has been approved for the treatment of hormone-receptor-positive metastatic breast cancer following antiestrogen therapy. However, fulvestrant shows poor pharmacokinetic properties in human, low solubility, weak permeation, and high metabolism, limiting its administration to inconvenient intramuscular injections. This Drug Annotation describes the identification and optimization of a new series of potent orally available SERDs, which led to the discovery of 6-(2,4-dichlorophenyl)-5-[4-[(3S)-1-(3-fluoropropyl)pyrrolidin-3-yl]oxyphenyl]-8,9-dihydro-7H-benzo[7]annulene-2-carboxylic acid (43d), showing promising antitumor activity in breast cancer mice xenograft models and whose properties warranted clinical evaluation.

Six Weeks of Daily Abaloparatide Treatment Increased Vertebral and Femoral Bone Mineral Density, Microarchitecture and Strength in Ovariectomized Osteopenic Rats.

Abaloparatide is a novel, potent and selective activator of parathyroid hormone receptor 1 (PTHR1) under clinical development for the treatment of osteoporosis. We assessed the effect of 6 weeks of abaloparatide on bone mass, microarchitecture, quality and strength in ovariectomized (OVX) rats. After 8 weeks of post-surgical bone depletion (baseline), OVX rats (n = 20-21/group) received daily subcutaneous vehicle (OVX-Veh) or abaloparatide at 5 or 20 µg/kg. Sham-operated control rats (n = 24) received vehicle. Areal bone mineral density (aBMD) of the lumbar spine (L4), total femur and femur diaphysis was measured at baseline and after 6 weeks of treatment. Femur and vertebral bone architecture and mechanical properties were assessed at the end of the treatment phase. At baseline, OVX-Veh rats exhibited significantly lower aBMD relative to Sham controls. Treatment of OVX rats with abaloparatide at 5 or 20 µg/kg/day increased aBMD dose-dependently in the lumbar spine, total femur and femur diaphysis to levels exceeding OVX-Veh or Sham controls. The abaloparatide 5 and 20 µg/kg groups had improved trabecular microarchitecture relative to OVX vehicle, with trabecular BV/TV exceeding OVX-Veh control values by 57 and 78 % (respectively) at the lumbar spine, and by 145 and 270 % at the distal femur. Femur diaphyseal cortical volume and thickness were significantly greater in the abaloparatide 20 µg/kg group relative to OVX vehicle or Sham controls. Bone strength parameters of the femur diaphysis, femur neck and L4 vertebra were significantly improved in the OVX-ABL groups relative to OVX-Veh controls. Bone mass-strength relationships and estimated intrinsic strength properties suggested maintained or improved bone quality with abaloparatide. These data demonstrate skeletal restoration via abaloparatide treatment of osteopenic OVX rats, in association with improved trabecular microarchitecture, cortical geometry and bone strength at sites that have clinical relevance in patients with osteoporosis.

RAD1901: a novel, orally bioavailable selective estrogen receptor degrader that demonstrates antitumor activity in breast cancer xenograft models.

Agents that inhibit estrogen production, such as aromatase inhibitors or those that directly block estrogen receptor (ER) activity, such as selective estrogen receptor modulators and selective estrogen receptor degraders, are routinely used in the treatment of ER-positive breast cancers. However, although initial treatment with these agents is often successful, many women eventually relapse with drug-resistant breast cancers. To overcome some of the challenges associated with current endocrine therapies and to combat the development of resistance, there is a need for more durable and more effective ER-targeted therapies. Here we describe and characterize a novel, orally bioavailable small-molecule selective estrogen receptor degrader, RAD1901, and evaluate its therapeutic potential for the treatment of breast cancer. RAD1901 selectively binds to and degrades the ER and is a potent antagonist of ER-positive breast cancer cell proliferation. Importantly, RAD1901 produced a robust and profound inhibition of tumor growth in MCF-7 xenograft models. In an intracranial MCF-7 model, RAD1901-treated animals survived longer than those treated with either control or fulvestrant, suggesting the potential benefit of RAD1901 in the treatment of ER-positive breast cancer that has metastasized to the brain. Finally, RAD1901 preserved ovariectomy-induced bone loss and prevented the uterotropic effects of E2, suggesting that it may act selectively as an agonist in bone but as an antagonist in breast and uterine tissues. RAD1901 is currently under clinical study in postmenopausal women with ER-positive advanced breast cancer.

Design, Synthesis, and Preclinical Characterization of the Selective Androgen Receptor Modulator (SARM) RAD140.

This report describes the discovery of RAD140, a potent, orally bioavailable, nonsteroidal selective androgen receptor modulator (SARM). The characterization of RAD140 in several preclinical models of anabolic androgen action is also described.

Synthesis of potent, substituted carbazoles as selective androgen receptor modulators (SARMs).

The synthesis and in vitro binding affinity for a novel series of potent androgen receptor modulators is described. One of the more potent compounds (17, RAD35010) was further characterized in vivo where it restored levator ani weight in castrated male rats to near sham level while having no significant effect on prostate weight.