Kam W. Siu

PROTAC-induced BET protein degradation as a therapy for castration-resistant prostate cancer

Significance We describe the development of a small molecule that mediates the degradation of bromodomain and extra-terminal (BET) proteins and its application in the treatment of castration-resistant prostate cancer (CRPC). Few therapeutic options exist to treat CRPC, especially CRPC tumors expressing constitutively active androgen receptor (AR) splice variants that lack the ligand-binding domain and can effect androgen-independent transactivation of target genes. Importantly, we demonstrate that targeted degradation of BET proteins using proteolysis-targeting chimera (PROTAC) technology causes cell death in cultured prostate cancer cells and results in tumor growth inhibition or regression in mouse models of CRPC, including models that express high levels of AR splice variant 7. Our work thus contains a significant potential therapeutic advance in the treatment of this cancer. Prostate cancer has the second highest incidence among cancers in men worldwide and is the second leading cause of cancer deaths of men in the United States. Although androgen deprivation can initially lead to remission, the disease often progresses to castration-resistant prostate cancer (CRPC), which is still reliant on androgen receptor (AR) signaling and is associated with a poor prognosis. Some success against CRPC has been achieved by drugs that target AR signaling, but secondary resistance invariably emerges, and new therapies are urgently needed. Recently, inhibitors of bromodomain and extra-terminal (BET) family proteins have shown growth-inhibitory activity in preclinical models of CRPC. Here, we demonstrate that ARV-771, a small-molecule pan-BET degrader based on proteolysis-targeting chimera (PROTAC) technology, demonstrates dramatically improved efficacy in cellular models of CRPC as compared with BET inhibition. Unlike BET inhibitors, ARV-771 results in suppression of both AR signaling and AR levels and leads to tumor regression in a CRPC mouse xenograft model. This study is, to our knowledge, the first to demonstrate efficacy with a small-molecule BET degrader in a solid-tumor malignancy and potentially represents an important therapeutic advance in the treatment of CRPC.

A highly effective one-pot synthesis of quinolines from o-nitroarylcarbaldehydes.

A highly effective one-pot Friedländer quinoline synthesis using inexpensive reagents has been developed. o-Nitroarylcarbaldehydes were reduced to o-aminoarylcarbaldehydes with iron in the presence of catalytic HCl (aq.) and subsequently condensed in situ with aldehydes or ketones to form mono- or di-substituted quinolines in high yields (66-100%).

Identification and Characterization of Von Hippel-Lindau-Recruiting Proteolysis Targeting Chimeras (PROTACs) of TANK-Binding Kinase 1

Proteolysis targeting chimeras (PROTACs) are bifunctional molecules that recruit an E3 ligase to a target protein to facilitate ubiquitination and subsequent degradation of that protein. While the field of targeted degraders is still relatively young, the potential for this modality to become a differentiated and therapeutic reality is strong, such that both academic and pharmaceutical institutions are now entering this interesting area of research. In this article, we describe a broadly applicable process for identifying degrader hits based on the serine/threonine kinase TANK-binding kinase 1 (TBK1) and have generalized the key structural elements associated with degradation activities. Compound 3i is a potent hit (TBK1 DC50 = 12 nM, Dmax = 96%) with excellent selectivity against a related kinase IKKε, which was further used as a chemical tool to assess TBK1 as a target in mutant K-Ras cancer cells.

Discovery and optimization of 7-aminofuro[2,3-c]pyridine inhibitors of TAK1.

The discovery and potency optimization of a series of 7-aminofuro[2,3-c]pyridine inhibitors of TAK1 is described. Micromolar hits taken from high-throughput screening were optimized for biochemical and cellular mechanistic potency to ~10nM, as exemplified by compound 12az. Application of structure-based drug design aided by co-crystal structures of TAK1 with inhibitors significantly shortened the number of iterations required for the optimization.

Potent and selective cyclohexyl-derived imidazopyrazine insulin-like growth factor 1 receptor inhibitors with in vivo efficacy

Preclinical and emerging clinical evidence suggests that inhibiting insulin-like growth factor 1 receptor (IGF-1R) signaling may offer a promising therapeutic strategy for the treatment of several types of cancer. This Letter describes the medicinal chemistry effort towards a series of 8-amino-imidazo[1,5-a]pyrazine derived inhibitors of IGF-1R which features a substituted quinoline moiety at the C1 position and a cyclohexyl linking moiety at the C3 position. Lead optimization efforts which included the optimization of structure-activity relationships and drug metabolism and pharmacokinetic properties led to the identification of compound 9m, a potent, selective and orally bioavailable inhibitor of IGF-1R with in vivo efficacy in an IGF-driven mouse xenograft model.

Discovery of novel insulin-like growth factor-1 receptor inhibitors with unique time-dependent binding kinetics.

This letter describes a series of small molecule inhibitors of IGF-1R with unique time-dependent binding kinetics and slow off-rates. Structure-activity and structure-kinetic relationships were elucidated and guided further optimizations within the series, culminating in compound 2. With an IGF-1R dissociative half-life (t 1/2) of >100 h, compound 2 demonstrated significant and extended PD effects in conjunction with tumor growth inhibition in xenograft models at a remarkably low and intermittent dose, which correlated with the observed in vitro slow off-rate properties.

Discovery of potent, selective and orally bioavailable imidazo[1,5-a]pyrazine derived ACK1 inhibitors

This Letter describes the medicinal chemistry effort towards a series of novel imidazo[1,5-a]pyrazine derived inhibitors of ACK1. Virtual screening led to the discovery of the initial hit, and subsequent exploration of structure-activity relationships and optimization of drug metabolism and pharmacokinetic properties led to the identification of potent, selective and orally bioavailable ACK1 inhibitors.

Discovery of an Orally Efficacious Imidazo[5,1-f][1,2,4]triazine Dual Inhibitor of IGF-1R and IR.

This report describes the investigation of a series of 5,7-disubstituted imidazo[5,1-f][1,2,4]triazine inhibitors of insulin-like growth factor-1 receptor (IGF-1R) and insulin receptor (IR). Structure-activity relationship exploration and optimization leading to the identification, characterization, and pharmacological activity of compound 9b, a potent, selective, well-tolerated, and orally bioavailable dual inhibitor of IGF-1R and IR with in vivo efficacy in tumor xenograft models, is discussed.

Abstract PR08: ARV-330: An androgen receptor PROTAC degrader for prostate cancer

Patients with prostate cancer who progress on therapy often have enhanced Androgen Receptor (AR) signaling due to several mechanisms: increased androgen production, increased AR expression, and/or specific AR mutations that render current therapies ineffective. A novel approach to block AR signaling is to specifically target AR for degradation. To do this, we have created AR PROTACs (PROtein-TArgeting Chimeras), bi-functional molecules that have an AR binding moiety on one end and an E3 ligase-recruiting element on the other end, which leads to AR ubiquitination and degradation. We have applied this technology to determine whether it could address mechanisms of resistance to current therapy in prostate cancer models. Our lead AR PROTAC, ARV-330, degrades AR in LNCaP and VCaP cells with 50% degradation concentrations (DC50s) 80% after sc injection. Treatment of mice with ARV-330, at doses ranging from 0.3 to 10 mg/kg, resulted in reduction of AR protein levels and prostate involution in normal mice and, in mice implanted with VCaP tumors, reduction in plasma PSA and blockade of tumor growth. In summary, the AR PROTAC ARV-330 removes AR from prostate cancer cells in a potent manner and produces therapeutic effects as a result. This cellular efficacy has translated into biomarker activity and efficacy in animal models, and ARV-330 is now in preclinical development. Thus, targeted degradation of AR may provide a novel mechanism for providing efficacious therapy for patients with prostate cancer for whom current therapies have failed. Citation Format: James D. Winkler, Meizhong Jin, Andy P. Crew, AnnMarie K. Rossi, Ryan R. Willard, Hanqing Dong, Kam Siu, Jing Wang, Deborah A. Gordon, Xin Chen, Caterina Ferraro, Craig M. Crews, Kevin Coleman, Taavi K. Neklesa. ARV-330: An androgen receptor PROTAC degrader for prostate cancer. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr PR08.

Abstract LB-097: Targeted degradation of the androgen receptor in prostate cancer

Progression of prostate cancer in patients treated with anti-androgen therapy usually involves several mechanisms of enhanced Androgen Receptor (AR) signaling, including increased intratumoral androgen synthesis, increased AR expression and AR mutations. We have developed a protein degradation technology called PROTACs (PROteolysis TArgeting Chimera), which uses bi-functional molecules that simultaneously bind a target of choice and an E3 ligase. PROTACs, via induced proximity, cause ubiquitination and degradation of the targeted, pathological protein. As opposed to traditional target inhibition, which is a competitive process, degradation is a progressive process. As such, it is less susceptible to increases in endogenous ligand, target expression, or mutations in the target. Thus this technology seems ideal for addressing the mechanisms of AR resistance in patients with prostate cancer. AR PROTACs were shown to degrade AR in LNCaP and VCaP cells, with low nM to pM potency, and had a >85% reduction in AR concentration (Dmax). Degradation was rapid, with 50% of AR lost within 15 minutes and maximal degradation observed by 4 hours. The degradation process in cells was specific, as the PROTAC activity can be competed with excess E3 ligand and PROTACs with an inactive epimer for E3 ligase binding did not degrade AR. AR PROTACs induced rapid apoptosis and cell death in VCaP cells. In LNCap and VCaP cell systems, AR PROTACs were anti-proliferative under conditions in which enzalutamide was inactive, such as increasing concentrations of the AR agonist R1881 and cells containing the ARF876L mutation. AR PROTACs typically exhibited good pharmacokinetic properties, with t1/2 values of several hours and bioavailability of >50% after ip or sc injection. In mice, AR PROTACs demonstrate in vivo activity, including reduction of AR protein levels, prostate involution and tumor growth inhibition. In summary, PROTACs designed to degrade AR are potent, specific, active in vitro and in vivo, and have cellular efficacy superior to enzalutamide. Targeted degradation of AR may provide a novel mechanism for providing efficacious therapy for patients with prostate cancer for whom current therapies have failed. Citation Format: Meizhong Jin, James D. Winkler, Kevin Coleman, Andrew P. Crew, AnnMarie K. Rossi, Ryan R. Willard, Hanqing Dong, Kam Siu, Jing Wang, Deborah A. Gordon, Xin Chen, Caterina Ferraro, Craig M. Crews, Taavi K. Neklesa. Targeted degradation of the androgen receptor in prostate cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-097. doi:10.1158/1538-7445.AM2015-LB-097