Novel mechanism-based therapeutics for androgen axis blockade in castration-resistant prostate cancer

Abstract

PURPOSE OF REVIEW

Understanding the mechanisms by which castration-resistant prostate cancer progresses provides an opportunity to identify novel therapeutic strategies to treat this disease. This understanding has led to approaches to attack prostate cancer’s androgen axis in unique ways. This review will examine the classes of novel therapies for androgen axis blockade in castration-resistant prostate cancer, with a particular focus on the unique characteristics of drugs in various stages of clinical development.

RECENT FINDINGS

The success of abiraterone and enzalutamide has stimulated multiple investigations into novel approaches to attack the androgen-signaling pathway. Drugs under development include cytochrome P17 inhibitors with 17,20-lyase specificity, androgen receptor antagonists that are active against mutated and constitutively-active splice variant forms of the protein, androgen receptor degraders, and bromodomain/BET inhibitors that prevent chromatin binding of activated receptors. The clinical development of several of these experimental agents is reviewed.

SUMMARY

Given the unique mechanisms of action for drugs in development, and the possibility that the novel agents may be active in the setting of common resistance mechanisms, treatment options for patients are likely to expand greatly in the coming years. Future studies should prioritize combinations of agents with unique mechanisms of action to optimize outcomes for patients, and should rely on precision-medicine approaches to target known molecular alteration.

1 Introduction

Novel approaches for targeting the androgen-signaling pathway have led to new and promising treatments for patients with castration-resistant prostate cancer (CRPC) in recent years. While nearly all patients have an initial response to androgen deprivation therapy (ADT), most tumors adapt and continue to use the androgen-signaling pathway for growth despite castrate levels of testosterone (typically defined as serum testosterone concentrations <50 ng/dl). It is in this space that drugs designed to attack the androgen-signaling pathway using unique mechanisms are leading to new treatments for patients. Multiple compounds are in clinical development, including drugs that are already FDA-approved based upon extended survival for patients with CRPC. This review will examine novel compounds and approaches for treating prostate cancer, with a focus on their mechanisms of androgen signaling blockade.

1.1 Androgen signaling in prostate cancer

Androgen signaling is critical to prostate cancer growth and progression. Much work has focused on the mechanisms by which the androgen-signaling axis adapts to castrate levels of testosterone (1), and knowledge of these adaptive resistance mechanisms informs strategies for novel targeted agents for CRPC (2, 3). The androgen receptor (AR) can be overexpressed due to gene amplification and stabilization of mRNA, allowing for AR signaling to occur despite low levels of testosterone. Mutations in AR can result in receptors that can bind to other ligands besides testosterone, and mutations convert the action of antiandrogens from antagonist into agonists. Splice variants of the AR mRNA can be produced that allow for ligand-independent signaling. Outside of changes in AR, intratumoral testosterone production to restore effective testosterone levels to non-castrate levels can occur through upregulation and overexpression of enzymes needed to produce testosterone from steroid precursors. These principles of sustained androgen axis signaling in CRPC have been recently reviewed(4).

2 Androgen synthesis inhibitors

Medical or surgical castrating therapies effectively eliminate testosterone originating from the testes. However, androgens are also synthesized to a lesser degree in adrenal tissue, as well as intratumorally (5). Historically, treatments administered to patients with CRPC that suppress adrenal synthesis of androgens have led to tumor responses. Agents that have been used previously include corticosteroids (through adrenal suppression) (6) and high-dose ketoconazole (through inhibition of adrenal enzymes involved in androgen biosynthesis) (7). While ketoconazole and corticosteroids are blunt tools given their relatively non-specific modes of action, their modest success has led to investigation of more elegant compounds to use clinically in prostate cancer.

2.1 Abiraterone

The discovery and FDA-approval of abiraterone as the first hormonal agent to increase survival for patients with CRPC serves as a proof-of-concept that targeting the androgen-signaling pathway in CRPC is fruitful. Abiraterone is an orally bioavailable small molecule steroid that is a potent inhibitor of cytochrome P17 (CYP17), an enzyme critical in biosynthesis of steroid hormones. The pharmacodynamic studies of abiraterone demonstrated that testosterone levels were suppressed to undetectable levels (<0.1 ng/dl) (8), thus achieving profound suppression of testosterone compared to traditional ADT alone.

Abiraterone functions by interference with steroid metabolism (9). Normally in the adrenal glands, ACTH stimulates metabolism of the steroid precursor pregnenolone. Pregnenolone can be further metabolized to aldosterone or to 17OH-pregnenolone, a common precursor for cortisol and testosterone. The action of 17α-hydroxylase converts pregnenolone to 17OH-pregnenolone, and 17,20-lyase further converts this product to dehydroepiandrostenedione (DHEA). DHEA is subsequently converted to an intermediary and finally testosterone. Abiraterone is a potent inhibitor of the 17α-hydroxylase and 17,20-lyase enzymatic functions of CYP17. Recent preclinical work has also identified Δ4-abiraterone, an active metabolite of abiraterone, that further inhibits 3β-HSD, CYP17A1, and 5α-reductase (10). In the presence of ACTH stimulation and abiraterone, pregnenolone is shunted to mineralocorticoid synthesis. Abiraterone used without replacement corticosteroids to suppress ACTH results in a syndrome of mineralocorticoid excess. Abiraterone thus was studied in conjunction with corticosteroids in its clinical development.

Abiraterone, in combination with prednisone, was approved by the FDA in 2011 after a pivotal phase III trial demonstrated improved survival compared to prednisone alone (11). In that study, 1195 patients with metastatic CRPC with disease progression after docetaxel chemotherapy were randomized to abiraterone 1000mg daily and prednisone 5mg twice daily versus placebo and prednisone (12). Patients receiving abiraterone had superior overall survival (14.8 vs. 10.9 months), superior progression-free survival (5.6 vs 3.6 months), and superior >50% PSA response rates (29% vs 6%). A similarly designed study in chemotherapy-naïve patients with metastatic CRPC showed benefit as well, with a median overall survival difference of 4.3 months for abiraterone plus prednisone compared to placebo plus prednisone (34.7 vs 30.3 months) (13, 14). Despite the co-administration of prednisone with abiraterone, adverse events were more common in the abiraterone cohorts—in particular mineralocorticoid excess-related side effects. These adverse events included hypokalemia, hypervolemia, hypertension, cardiac arrhythmias, and liver-function abnormalities. Abiraterone currently holds FDA-indications for patients with CRPC either before or after docetaxel chemotherapy.

2.2 Orteronel

Orteronel (TAK-700) is a non-steroidal androgen synthesis inhibitor that was developed for the treatment of CRPC. Orteronel, like abiraterone, is a CYP17A inhibitor (See Figure 1 for chemical structures of several of the androgen synthesis inhibitors in development). It selectively inhibits 17,20-lyase activity and weakly inhibits 17α-hydroxylase (15). This differential selectivity of orteronel has a theoretical advantage of inhibiting testosterone biosynthesis while minimizing interference with cortisol production to reduce risk of mineralocorticoid-related toxicities. In the presence of orteronel, 17OH-pregnenolone conversion to DHEA is minimized, thus resulting testosterone suppression to near-undetectable levels (< 0.2 ng/dl) (16). Promising preclinical and early-phase studies led to two phase III investigations of orteronel. In a study of 1099 patients with metastatic CPRC with progression after docetaxel chemotherapy, orteronel 400mg daily plus prednisone 5mg twice daily was compared to placebo plus prednisone (17). Although orteronel demonstrated statistically improved secondary endpoints including prolonged radiographic progression-free survival, PSA response rates, and PSA progression-free survival; overall survival was not statistically improved (17.0 vs 15.2 months). A second study of orteronel in the chemotherapy-naïve CRPC setting similarly failed to meet an endpoint of improved overall survival (18). In that study, 1560 patients were randomized to receive orteronel plus prednisone versus placebo plus prednisone. Those patients receiving orteronel benefited in terms of radiographic progression-free survival (13.8 vs 8.6 months), but overall survival was not statistically improved (31.4 vs 29.5 months). While mineralocorticoid side effects were not noticeably increased compared to the studies of abiraterone, more increases in amylase, lipase, and clinical pancreatitis were observed with orteronel. It was speculated that tolerability of orteronel was poor, as evidenced by a high percentage of drug discontinuation. There were also regional differences in survival based upon post-hoc analysis, where locations with less available of other novel life-prolonging therapies demonstrated a benefit. Nonetheless, further clinical development for orteronel in CRPC is not being pursued, although orteronel continues to be investigated in other settings. Orteronel at a dose of 600mg—without prednisone—is included as part of a cooperative group trial as first-line systemic therapy in conjunction with ADT for newly-diagnosed metastatic prostate cancer (NCT01809691).

Figure 1.

Structures of selected androgen synthesis inhibitors in development.

2.3 Galeterone

Galeterone (TOK-001) is a steroidal compound in clinical development for CRPC. Similarly to abiraterone and orteronel, galeterone inhibits CYP17 interfering with androgen biosynthesis, with more potent action against 17,20-lyase (19). Preclinical data of galeterone has also suggested multiple other therapeutic effects, including antagonizing AR and promoting its degradation at the protein level (20). Galeterone may have activity in decreasing AR-V7 splice variant levels by targeting them for proteosomal degration after ubiquination (21). Activity against AR-V7–positive prostate cancer would provide a distinct advantage over abiraterone, given the emerging data regarding AR-V7 and abiraterone resistance (22, 23). Phase I and II trials testing galeterone in CRPC have been recently published (24). These trials established a formulation and dose for galeterone that is being pursued in further clinical study, specifically 2550mg in a spray-dry dispersion tablet once daily. Galeterone was not co-administered with corticosteroids, and there were no increased adverse events related to mineralocorticoid excess. Testosterone levels were lowered to a median of 2 ng/dl in the phase II study, without significant change in cortisol levels. There was evidence of anti-tumor activity, based upon PSA responses seen with increasing doses of drug. A phase III trial of galeterone versus enzalutamide in a population of patients with CRPC and circulating tumor cell that express AR-V7 is currently underway (see Table 1 for summary of pending clinical trials) (25).

Table 1.

Selected ongoing clinical trials of investigational agents with novel mechanisms of action in CRPC.

Androgen Synthesis Inhibitors	Galeterone	NCT02438007:	ARMOR3-SV: A Phase 3, Randomized, Open Label, Multi-Center, Controlled Study of Galeterone Compared to Enzalutamide in Men Expressing Androgen Receptor Splice Variant-7 mRNA (AR-V7) Metastatic (M1) Castrate Resistant Prostate Cancer (CRPC)
		NCT01709734:	ARMOR2: A 2 Part, Phase 2 Trial of Galeterone in the Treatment of Castration Resistant Prostate Cancer
	VT-464 (Seviteronel)	NCT02012920:	A Phase 1/2 Open-Label, Multiple-Dose Study to Evaluate the Safety, Tolerability, Pharmacokinetics, and Pharmacodynamics of VT-464 in Patients With Castration-Resistant Prostate Cancer
		NCT02361086:	A Phase 1/2 Open-Label, Multiple-Dose Study to Evaluate the Safety, Tolerability, Pharmacokinetics, and Pharmacodynamics of Once-Daily VT-464 in Patients With Castration-Resistant Prostate Cancer
		NCT02130700:	A Phase 2 Open-Label Study to Evaluate the Efficacy and Safety of VT-464 in Patients With Metastatic Castration Resistant Prostate Cancer Who Have Previously Been Treated With Enzalutamide
		NCT02445976:	A Single-arm, Phase 2 Study to Evaluate the Safety and Efficacy of VT-464 in Patients With Castration-Resistant Prostate Cancer Progressing on Enzalutamide or Abiraterone.
	CFG920	NCT01647789:	A Phase I/II Multicenter, Open-label Dose Finding Study of Oral CFG920 in Patients With Metastatic Castration-resistant Prostate Cancer
AR antagonists targeting the LBD	Apalutamide	NCT01946204:	SPARTAN: A Multicenter, Randomized, Double-Blind, Placebo-Controlled, Phase III Study of ARN-509 in Men With Non-Metastatic (M0) Castration-Resistant Prostate Cancer
		NCT02106507:	Phase 1b Study of ARN-509 Plus Everolimus in Men With Progressive Metastatic Castration-Resistant Prostate Cancer After Treatment With Abiraterone Acetate
		NCT01792687:	Phase Ib, Open-label Study to Assess the Safety, Tolerability, Pharmacokinetics, and Preliminary Anti-tumor Activity of Ascending Doses of ARN-509 in Combination With Abiraterone Acetate in Patients With Metastatic Castrate Resistant Prostate Cancer (CRPC)
		NCT02123758:	A Drug-Drug Interaction, Safety and Efficacy Study With ARN-509 and Abiraterone Acetate in Subjects With Metastatic Castration-Resistant Prostate Cancer
		NCT02257736:	A Phase 3 Randomized, Placebo-controlled Double-blind Study of ARN-509 in Combination With Abiraterone Acetate and Prednisone Versus Abiraterone Acetate and Prednisone in Subjects With Chemotherapy-naive Metastatic Castration-resistant Prostate Cancer (mCRPC)
	ODM-201	NCT02200614:	ARAMIS: A Multinational, Randomized, Double-Blind, Placebo-Controlled, Phase III Efficacy and Safety Study of ODM-201 in Men with High-Risk Non-Metastatic Castration-Resistant Prostate Cancer
	ODM-204	NCT02344017:	Safety and Pharmacokinetics of ODM-204 in Patients With Metastatic Castration-Resistant Prostate Cancer (CRPC): Open, Non-Randomised, Uncontrolled, Multicentre, Dose Escalation, First-in-man Study With a Dose Expansion
	TAS3681	NCT02566772:	A Phase 1, Open-Label, Non-Randomized, Safety, Tolerability and Pharmacokinetic Study of TAS3681 in Patients With Metastatic Castration Resistant Prostate Cancer
AR antagonists targeting the NTD	EPI-506	NCT02606123:	A Phase 1/2 Open-Label Study to Assess the Safety, Pharmacokinetics, and Anti-Tumor Activity of Oral EPI-506 in Patients With Metastatic Castration-Resistant Prostate Cancer
AR agonists	High-dose testosterone	NCT02286921:	TRANSFORMER: A Randomized Phase II Study Comparing Bipolar Androgen Therapy vs. Enzalutamide in Asymptomatic Men With Castration Resistant Metastatic Prostate Cancer
		NCT02090114:	RESTORE: A Phase II Study to Determine Sequential Response to Bipolar Androgen Therapy (BAT) Followed by Enzalutamide or Abiraterone Post-BAT in Men With Prostate Cancer Progressing on Combined Androgen Ablative Therapies
BET/Bromodomain inhibitors	GSK525762	NCT01587703:	A Phase I/II Open-Label, Dose Escalation Study to Investigate the Safety, Pharmacokinetics, Pharmacodynamics, and Clinical Activity of GSK525762 in Subjects With NUT Midline Carcinoma (NMC) and Other Cancers
	OTX015	NCT02259114:	A Phase IB Trial With OTX015, a Small Molecule Inhibitor of the Bromodomain and Extra-Terminal (BET) Proteins, in Patients With Selected Advanced Solid Tumors
	GS-5829	NCT02607228:	A Phase 1b/2 Study to Evaluate the Safety, Tolerability, Pharmacokinetics, and Pharmacodynamics of GS-5829 as a Single Agent and In Combination With Enzalutamide in Subjects With Metastatic Castrate-Resistant Prostate Cancer

2.4 VT-464 (Seviteronel)

VT-464 is a 17,20-lyase–selective non-steroidal CYP17 inhibitor. Preclinical studies of VT-464 in prostate cancer xenograft models of prostate cancer cell lines demonstrated suppression of testosterone and AR activity (26). AR antagonism activity was also seen in prostate cell lines, including those with AR point mutations (T878A or F877L) that render them resistant to other antiandrogens. In this preclinical study, VT-464 was suggested to have more profound androgen suppression and AR antagonism than abiraterone. Multiple early-phase clinical trials have opened to test VT-464 in patients with CRPC, including those that have progressed after other novel targeted agents including enzalutamide and abiraterone. Phase I/II results testing VT-464 in patients with CRPC have been reported in abstract form (27). No significant mineralocorticoid side effects were noted even in the absence of prednisone co-administration, and the most common side effects included dizziness, tremors, and vasovagal syncope. Objective tumor responses (3 of 23 evaluable patients) were observed in the early phase trials, including in patients who had progressed after enzalutamide and taxane chemotherapy (28). This drug has recently received FDA fast-track designation for the treatment of CRPC patients who have failed abiraterone and/or enzalutamide.

2.5 CFG920

CFG920 is a non-steroidal inhibitor of CYP17 and CYP11B2 (29). The inhibition activity on CYP11B2 interferes with aldosterone biosynthesis, which theoretically reduces the risk of mineralocorticoid side effects. A phase I/II study of CFG920, without prednisone, in patients with metastatic CRPC is currently ongoing.

2.6 EN3356

EN3356 is a non-steroidal inhibitor of CYP17 17-20-lyase. As discussed with orteronel, galeterone, and VT464, the selectivity for inhibition of the lyase activity of CYP17 has the potential to spare cortisol biosynthesis and minimize mineralocorticoid side effects (30). Early-phase trials with this agent have been announced, but no clinical data is yet available.

2.7 ASP9521

Precursors hormones DHEA and androstenedione are converted to androstenediol and testosterone, respectively, through the action of 17β-hydroxysteroid dehydrogenase (also known as aldo-keto reductase 1C3 [AKR1C3]). ASP9521 is a first-in-class small molecule selective inhibitor of AKR1C3 that had been undergoing development for CRPC. Preclinical studies demonstrated significant reduction in intratumoral testosterone and PSA in xenograft prostate cancer models (31). Based upon these promising data, a phase I trial was conducted in 13 patients with CRPC with previous treatment with chemotherapy (32). However, the trial failed to demonstrate evidence of clinical activity (through lack of radiographic responses, PSA responses, nor reduction in circulating tumor cells). Investigators speculated that rapid resistance and biosynthesis of testosterone bypassing AKR1C3 may have accounted for the lack of activity. No further clinical development of ASP9521 has been announced, although other AKR1C3 inhibitors are in development (33).

3 AR Antagonists – Targeting the Ligand Binding Domain (LBD)

The approach of using drugs that antagonize the AR has been employed in both castration-sensitive and castration-resistant disease. Historically, “first-generation” antiandrogens (e.g. bicalutamide, nilutamide, flutamide) were added to ADT to achieve a more complete androgen blockade in hormone-sensitive disease (34). Responses can also be observed when antiandrogens are used in the setting of progression despite castrate levels of testosterone (35). More recently, highly potent AR antagonists have been developed that have shown significant efficacy in CRPC.

3.1 Enzalutamide

Enzalutamide is a non-steroidal compound that potently antagonizes AR. The objective of the preclinical development of this drug was to identify a compound that would maintain anti-androgen activity in the face of AR overexpression (36). In addition, investigators sought to identify a pure antagonist of AR without agonistic activity. First-generation anti-androgens are weak partial agonists of AR, which can paradoxically cause tumor growth in certain clinical settings (35). In preclinical studies, enzalutamide was shown to bind AR with high affinity, reduce its nuclear translocation, prevent binding to androgen response elements, and prevent recruitment of coactivators. Phase I/II trials identified common side effects to be fatigue, nausea and anorexia (37).

The efficacy of enzalutamide was confirmed in two phase III trials in men with metastatic CRPC. In the first trial, 1199 patients with progressive disease after chemotherapy were randomized to 160mg of enzalutamide daily versus placebo (38). The median overall survival in patients receiving enzalutamide was significantly improved by 4.8 months (18.4 vs 13.6 months). Patients receiving the enzalutamide also had superior progression-free survival, response rates, and quality-of-life. In the second phase III trial, enzalutamide was tested in men with metastatic CRPC prior to chemotherapy (39). In that trial, 1717 patients were treated with enzalutamide or placebo, and those receiving enzalutamide had superior overall survival with a hazard ratio of 0.71. The co-primary endpoint of the study was radiographic progression-free survival, and this objective was overwhelmingly met with a hazard ratio of 0.35. The vast majority of patients (87%) entered into these studies had experienced progression after prior treatment with first-generation antiandrogens, thus confirming that enzalutamide is effective in sequence after prior antiandrogen therapy. Similar to abiraterone, enzalutamide is FDA-approved for the treatment of metastatic CRPC either before or after chemotherapy (40). Finally, two new studies have shown that enzalutamide is far superior to bicalutamide when tested head-to-head in the CRPC setting (41, 42).

3.2 Apalutamide

Apalutamide (ARN-509) is an AR antagonist in development for treatment of CRPC (see Figure 2 for chemical structures of several of the antiandrogens in development). Similar to enzalutamide, apalutamide binds AR with high affinity and minimal-to-no agonist activity (43). In prostate cancer cell lines, aptalumide prevented AR nuclear localization and binding to androgen response elements. In xenograft prostate cancer models, a low dose of apalutamide was necessary to achieve maximal response, leading investigators to speculate that the therapeutic index for apalutamide may be superior to other next-generation antiandrogens. One specific concern in the clinical development of enzalutamide was the induction of seizures in patients with a predisposition, which is thought to be secondary to binding of GABA-A receptors in the central nervous system (CNS). With a lower efficacious dose—and subsequently lower levels of drug present in the CNS—a potential advantage of apalutamide is reduced risk of seizures.

Figure 2.

Structures of selected androgen receptor antagonists in development.

The first-in-human phase I study of apalutamide was published in 2013, and multiple phase II and III investigations are pending. In the phase I study, no maximum tolerated dose was identified, and grade 1–2 fatigue was commonly reported across dose ranges (44). An optimal biologic dose of 240mg was chosen for phase II investigations based upon data including drug efficacy and correlative studies of testosterone update. The correlative study utilized 16β-[18F] fluoro-α-dihydrotestosterone (F-DHT)–positron emission tomography (PET)/CT scans as an investigational study to provide a measurement the suppression of tumoral AR uptake in response to apalutamide. A dose of at least 120mg produced maximum suppression of PET uptake, and the dose of 240mg produced steady-state plasma levels that were predicted to have anti-tumor efficacy based upon preclinical models.

The phase II portion of this study has been presented in abstract form (45). In that study, patients with CRPC received 240mg of apalutamide daily, with divisions into cohorts based upon whether they had non-metastatic disease, treatment-naïve metastatic disease, or metastatic disease with prior abiraterone treatment. When used as a first-line therapy for metastatic CRPC, apalutamide produced >50% PSA responses in the vast majority of patients (88%, n=25). For patients with prior abiraterone treatment, PSA responses were observed at a much more modest rate (24%, n=21). Of note, inclusion in the abiraterone cohort required that patients had been treated with abiraterone for at least 6 months before progression. The pivotal phase III study of apalutamide, called SPARTAN, is currently randomizing men with non-metastatic CRPC (2:1) to apalutamide versus placebo (46); the primary endpoint of this trial is radiographic metastasis-free survival.

In addition, apalutamide is being investigated in combination with other AR-directed therapies for CRPC. A phase Ib study of apalutamide plus abiraterone has been presented (47). This study combined standard doses of abiraterone (1000mg daily), prednisone (5mg twice daily), and apalutamide (240mg daily), and this combination was well tolerated. PSA responses were observed in patients with the combination, including in patients with progression after treatment with abiraterone and/or enzalutamide.

3.3 ODM-201

ODM-201 is another AR antagonist in development for CRPC. It functions in a similar manner to enzalutamide and apalutamide—specifically by binding to AR, preventing nuclear translocation, and preventing DNA binding (48). ODM-201 has features that set it apart from other next-generation anti-androgens. First, ODM-201 does not significantly cross the blood-brain barrier, which may reduce risk of seizures by minimizing binding to GABA-A receptors in the CNS. In addition, by not crossing the blood-brain barrier, ODM-201 does not induce production of endogenous testosterone. ODM-201 also has in vitro activity against mutated androgen receptors, including F877L, W742L, and T878A. The activity against F877L-mutated androgen receptors is particularly notable as this mutation renders resistance to enzalutamide and apalutamide.

In phase I investigations, ODM-201 was well-tolerated with no maximum tolerated dose identified (49). Common grade I/II adverse events included fatigue, arthralgias, and constipation. In the phase II portion of the study, patients without prior abiraterone exposure (including those with prior chemotherapy) showed promising preliminary efficacy based upon PSA and objective responses notes. However, no significant signal of activity was seen in patients with prior treatment with abiraterone (1 of 17 patients with >50% PSA response). The dose of 1200mg daily (in two divided doses) is being pursued in a phase III trial (ARAMIS) for patients with non-metastatic CRPC (50); the primary endpoint here is also radiographic metastasis-free survival.

3.4 ODM-204

ODM-204 is non-steroidal antiandrogen that also inhibits CYP17. In preclinical studies presented in abstract form, ODM-204 antagonized AR as well as androgen receptors with common mutations in CRPC including F877L, W742L, and T878A (51). In addition, ODM-204 has in vitro and in vivo inhibitory activity against CYP17A1, demonstrating the ability to suppress testosterone to undetectable levels when used in conjunction with LHRH agonists. A clinical trials of this compound in patients with metastatic CRPC is underway (NCT02344017). Of note, the announced trial is co-administering ODM-204 with prednisone, analogous to abiraterone.

3.5 AZD-3514

AZD-3514 is a novel antiandrogen with a potentially unique mechanisms of action. In preclinical studies, AZD-3514 was demonstrated to bind AR and inhibit its translocation to the nucleus (52). In addition, the compound reduced the rate of synthesis of AR in cell lines resulting in AR downregulation (53). Given this mechanism of AR downregulation, AZD-3514 has been designated as both an anti-androgen and selective AR down-regulator. Phase I studies on AZD-3514 have been published (54). Dose escalation of the compound demonstrated that nausea and vomiting were common (79% of patients with grade I or II). In addition, PK properties of the drug prevented achievement of plasma steady-state levels that were predictive of anti-tumor efficacy. PSA and soft tissue responses were seen in some patients; however, activity in patients with prior progression after abiraterone was minimal. Therefore, further clinical development of this agent is not being pursued.

3.6 TAS3681

TAS3681 is another agent in development that potentially possessed both anti-androgen and AR downregulation properties. TAS3681 exhibits in vitro and in vivo activity in prostate cancer cell lines, including those expressing wild-type and mutated AR (F877L) (55). Preclinical studies demonstrated that TAS3681 prevents nuclear translocation of the AR after binding, and the drug reduces AR levels for both full-length and AR-variant receptors. The potential activity against AR-V7–positive lines, due to AR downregulation, has led to announcement of a phase I study in patients with mCRPC (NCT02566772).

4 AR Antagonists – Targeting the N-Terminal Domain (NTD)

4.1 EPI-001 and EPI-506

The AR antagonists that have been previously discussed all bind the ligand-binding domain at the C-terminal of the AR protein. However, the N-terminal domain is essential for the transcriptional activity of AR (56). By targeting the N-terminal domain—instead of the ligand-binding domain of the C-terminus—a drug may be able to interfere with androgen signaling, even after the development of resistance that typically allows for ligand-independent AR signaling including signaling mediated by AR splice variants. EPI-001 is small molecule that specifically binds and inhibits the N-terminus of AR. Preclinical work demonstrated that EPI-001 prevented transactivation of AR (57). EPI-001 was also effective in interfering with signaling in prostate cancer cells with constitutively active splice variants (58). Its efficacy against AR splice variants, which is emerging as an important clinical resistance mechanism to anti-androgen therapy, may set N-terminal domain–targeted agents apart from other antiandrogens. Clinical trials are underway testing the lead agents EPI-506, a related compound to EPI-001, in patients with CRPC who have progression after enzalutamide and/or abiraterone (59).

5 AR agonists

5.1 High-dose testosterone therapy

While the basis for hormonal therapy for prostate cancer has been aimed at lowering testosterone to interfere with the androgen-signaling pathway, a recent approach being trialed uses supraphysiologic androgen therapy in CRPC. Laboratory studies have demonstrated that prostate cancer cells that grow in the presence of physiologic androgen concentrations can paradoxically experience growth inhibition in the setting of supraphysiologic androgens. As the cells progress through the cell cycle, AR functions as a DNA licensing factor and must be degraded (60). In the setting of overexpressed androgen receptors—commonly seen with CRPC—testosterone binding to the overexpressed receptors can result in stabilization, inhibition of DNA relicensing, double-strand DNA breaks through recruitment of topoisomerase IIβ, and eventually growth arrest and apoptosis (61).

A pilot trial of supraphysiologic androgen therapy, administered at the FDA-approved dose of 400mg intramuscular monthly, was recently reported with promising initial results (62). PSA responses were seen in 7 of 14 patients with CRPC, and radiographic responses were seen in 5 of 10 evaluable patients. The initial trial co-administered etoposide; however, toxicity was excessive and further studies will omit chemotherapy. Larger trials of supraphysiologic androgen therapy in sequence with abiraterone and/or enzalutamide are underway. The first, called RESTORE (NCT02090114), will employ high-dose testosterone therapy to attempt to re-sensitize CRPC patients who have progressed on abiraterone and/or enzalutamide. The second, TRANSFORMER (NCT02286921), will randomize men with abiraterone-pretreated CRPC to either enzalutamide or high-dose testosterone therapy.

6 AR expression inhibitors

6.1 ENZ-4176

Another strategy to interfere with androgen signaling in CRPC is to attack the production of androgen receptors in the cell. In this approach, prostate cancer cells are treated with an antisense oligonucleatide to interfere with AR mRNA, so that AR protein is not translated. ENZ-4176 is a locked nucleic acid-based antisense oligonucleotide designed to bind and down-modulate AR mRNA. The agent was shown to be effective in cell-based experiments in reducing AR expression (63). Based upon that data, a phase I trial was performed in patients with CRPC (64). 22 patients were treated; however, no PSA or radiographic responses were seen, and the highest dosing level was associated with liver toxicity. Further trials with ENZ-4176 are not being pursued at this time.

7 BET/Bromodomain-targeted agents

The androgen-signaling pathway in CRPC may also be attacked by interfering with epigenetic chromatin binding of the activated AR to prevent transcription of its target gene products. BRD4, a protein that is a member of the bromodomain and extraterminal (BET) family, regulates expression of genes through histone binding. Recent work has shown that AR interacts with BRD4 in CRPC cell lines (65). In the presence of a BRD4 inhibitor, AR (both full-length and variants) was unable to bind androgen response elements (66). In addition, inhibition of BRD4 prevents transcription of other oncogenes, including c-Myc (67). Multiple BET bromodomain inhibitors have begun clinical trials in a variety of cancers, including two that include CRPC patients as discussed below.

7.1 GSK525762

GSK525762 (I-BET762) is the first BRD4 inhibitor to be tested in a trial that includes patients with CRPC. The BRD4–AR interaction in CRPC cell lines was disrupted by GSK525762 in preclinical models (65). In addition, CRPC cell lines treated with the drug also had growth inhibition through interference with Myc transcription (67). A phase I/II study that includes a cohort of patients with CRPC is currently underway (NCT01587703).

7.2 GS-5829

Gilead has announced clinical trials testing the BET/bromodomain small molecule inhibitor GS-5829. In the planned open-label trial (NCT02607228) that will be performed in patients with metastatic CRPC, GS-5829 will be tested as a single agent in the phase I portion of the trial. In the phase II portion, GS-5829 will be tested in combination with enzalutamide. The trial will prospectively evaluate cell-free DNA and CTCs for AR mutations and AR-variant expression to determine if GS-5829 retains clinical activity in AR-mutated and AR-V expressing phenotypes.

7.3 OTX015

Preclinical data for OTX015 (MK-8628), which is a drug targeting BRD4, has focused on models of hematologic malignancies (68, 69). Its clinical development is continuing in hematologic malignancies and CNS tumors. In addition, a phase IB study is open wherein patients with CRPC are specifically eligible. No clinical data for patients with CRPC is yet available.

8 AR degraders

8.1 AR PROTACs

One novel strategy for interfering with AR signaling is to use drugs that direct proteins for intracellular degradation (70). Several bi-functional molecules that combine a motif with affinity for a target of interest (such as AR) with an E3 ligase-recruiting element are under preclinical development for CRPC. These agents have been termed PROTACs (protein-targeting chimeras). ARV-330 is an example of an AR-directed PROTAC that has been shown to degrade AR in prostate cancer cell lines, resulting in growth inhibition (71). ARV-825 is a PROTAC directed at BRD4, which was effective against AR-V7–expressing tumors in prostate cancer models (72). Clinical trials using these agents are currently being planned.

9 Conclusion

The future is hopeful for patients with CPRC, as more options for treatment will become available as this research matures. These novel approaches to attack androgen axis signaling in CRPC have already led to FDA approval of two agents, and many other drugs are in late-stage clinical trials. One important area for future research is whether combinations of drugs with different mechanisms of action will provide the optimal therapy. Another area for future research will involve ongoing characterization of a patient’s tumor, in order to understand the evolving changes in AR, and to match those changes with the therapy with the appropriate mode of action. Such a precision medicine-based approach is increasingly being facilitated by evolving technologies allowing molecular characterization of ‘liquid biopsies’ in real time and tracked serially across the therapeutic continuum of CRPC (73–75).

Key Points.

• The success of abiraterone and enzalutamide serve as a proof-of-concept that further targeting of the androgen signaling pathway can benefit patients despite development of castration-resistant prostate cancer.

• Unique mechanisms of action of agents under investigation include: androgen synthesis inhibition with specificity for enzymes to avoid mineralocorticoid excess; antiandrogens active against activating AR mutations and AR splice variants; antiandrogens targeting the N-terminal domain of AR, compounds that degrade or downregulate AR; and bromodomain/BET inhibitors epigenetically silencing AR signaling.

• Future work should focus on selecting drugs based upon the specific AR resistance characteristics for an individual patient (precision oncology), as well as pairing of drugs with unique mechanisms of action to achieve optimal outcomes.