TAK-700 (orteronel) is an oral, selective, reversible, non-steroidal androgen synthesis inhibitor of the 17.20 lyase activity, one of two enzymatic reactions catalyzed by CYP17. Due to its low inhibition of 17-hydroxylase activity, administration does not require concomitant steroid replacement (Ryan and Tindall, 2011). This compound showed activity and good tolerability in a phase I/II clinical trial on chemotherapy naïve patients (Agus et al., 2011; Mortimer et al., 2011). Phase I patients received TAK-700 escalating doses (100–600 mg BID), while phase II ones were given TAK-700 300 mg BID, 600 mg once daily (QD), or two dose levels (400 or 600 mg BID) plus prednisone 5 mg BID. In the phase II portion, 97 patients received TAK-700. Most common adverse events (AEs) were fatigue (72%), nausea (44%), and constipation (31%), with fatigue (9%) and diarrhea (3%) as most common grade ≥3 AEs. TAK-700 resulted in reduced circulating testosterone and adrenal androgen dehydroepiandrosterone sulfate (DHEA-S) concentrations. Of 43 RECIST-evaluable patients, 6 had a partial response (PR), 23 had stable disease (SD), and 9 had progression disease (PD). PSA response rates (≥50% decrease) at 12 weeks were similar across treatment groups, with an overall response rate (ORR) of 53%. Researchers concluded that, in patients with mCRPC, oral TAK-700 (with/without prednisone) at doses of ≥300 mg BID was active and well tolerated. Based on these positive findings, two randomized, phase III, placebo-controlled multicenter studies are currently being investigated efficacy and safety of TAK-700 400 mg BID plus prednisone 5 mg BID versus placebo plus prednisone in chemotherapy naïve and docetaxel-pre-treated mCRPC patients.

Ligand-independent continued activation of ARs is one of the mechanisms that allow PC cells to survive and grow in the presence of castrate androgen levels. Receptor mutations (Taplin et al., 2003), alternative splicing with synthesis of AR splice variants (Sun et al., 2010), AR encoding gene amplification (Liu et al., 2009) as well as co-activators dysregulation have been described as potential escape mechanisms implicated in CRPC progression. First-generation AR antagonists, such as bicalutamide or flutamide, represent the standard of care for advanced PC since the eighties. However, they bind reversibly to ARs and may have androgen-agonist properties, as demonstrated in cells engineered to over-express higher AR amounts (Tran et al., 2009), limiting therapeutic activity. This has raised the need to develop more potent and efficacious molecules. In such a perspective, new antiandrogens with improved binding properties have been produced. One of these agents, MDV3100, is an oral AR antagonist small molecule that binds to ARs with higher affinity than bicalutamide, blocking AR nuclear translocation, co-activators recruitment, and DNA binding without agonist activity when AR is overexpressed (Tran et al., 2009). Unlike bicalutamide, MDV3100 do not induce expression of the AR target genes PSA and transmembrane serine protease 2 (TMPRSS2) in a pre-clinical model, indicating the absence of agonist activity in a castration-resistant setting (Tran et al., 2009). MDV3100 has demonstrated a promising clinical activity in CRPC in a phase I/II trial (Scher et al., 2010) evaluating drug escalating doses (from 30 to 600 mg/day) on 140 patients, 65 chemotherapy naïve and 75 previously treated with docetaxel. A recent update (Higano et al., 2011) after a long-term follow-up on time to PSA and radiographic progression, confirmed a durable anti-tumor activity of MDV3100. The median time to PSA progression, defined per-protocol as a ≥25% increase in PSA from baseline, was not met for naïve patients and was 8 months for post-chemotherapy patients. The median time to radiographic progression was 13 months for naïve and 6 months for post-chemotherapy group. MDV3100 was generally well tolerated, with fatigue as most frequently reported AE (Scher et al., 2010). Based on these interesting results, MDV3100 is currently being evaluated in two phase III studies, in pre- (PREVAIL) and post-docetaxel (AFFIRM) settings. AFFIRM is a randomized, placebo-controlled, double-blind, multi-national trial evaluating MDV3100 160 mg/day in mCRPC men previously treated with docetaxel-based chemotherapy. The primary endpoint is OS, secondary endpoints include progression-free survival (PFS), safety, and tolerability. On November 2011, the results of a planned interim analysis performed by the Independent Data Monitoring Committee (IDMC) showed that MDV3100 produced a 4.8-month advantage in median OS compared to placebo (18.4 months for MDV3100 versus 13.6 for placebo), with a 37% (HR = 0.631) reduction in the risk of death in the treated population. Consequently, IDMC recommended AFFIRM early stop, and men given placebo were offered MDV3100. A full analysis of the results from AFFIRM including safety data will be soon presented. ARN-509 is a novel small molecule AR antagonist with a mechanism of action similar to that of MDV3100, which showed powerful anti-cancer activity and induced durable remission in advanced CRPC mouse models. It seems to produce higher rates and longer duration of responses than MDV3100. An ongoing phase I/II clinical trial of continuous oral ARN-509 in patients with progressive CRPC with and without prior chemotherapy was started in July 2010 (Rathkopf et al., 2011a). Finally, AZD3514, a selective AR degrading and down-regulating agent, orally available, given on a daily continuous schedule, is now being tested in a phase I clinical trial, with an estimated completion date of 2013.

Some of the CYP17 inhibitors are of particular interest as they are also AR antagonists and cause receptor down-regulation. TOK-001 (VN/124-1) is the first compound to show superior efficacy compared with castration in PC xenograft models (Handratta et al., 2005). It also inhibits the proliferation of hormone-resistant PC cell lines (HP-LNCaP), which are no longer sensitive to bicalutamide and have an increased AR expression (Schayowitz et al., 2008). These impressive pre-clinical data led to the development of this compound in the clinical setting. The results of the phase I/II clinical trial ARMOR1, conducted in treatment-naïve CRPC patients progressing on ADT, are awaited after its completion in July 2012.

There is growing evidence indicating the presence of signaling mechanisms and cross-talk between growth factor receptor pathways and AR in androgen-dependent and hormone-resistant PC cell lines (Schayowitz et al., 2008), that lead to AR regulation by signal transduction pathways and vice versa (Traish and Morgentaler, 2009). Cross-talk between AR and growth factor pathways may represent a key factor during PC progression, conferring a survival, and invasion advantage to tumor cells, together with a resistance to hormonal therapy. This mutual relationship involves epidermal growth factor receptors (EGFR and HER-2), insulin-like growth factor receptor (IGFR), fibroblast growth factor receptor (FGFR), vascular endothelial growth factor (VEGF) receptor (VEGFR), transforming growth factor-β (TGFβ), phosphoinositide 3-kinase (PI3-K), Akt, and mammalian target of rapamycin (mTOR) pathways (Wen et al., 2000; Manin et al., 2002; Zhu and Kyprianou, 2008) and represents a potential target to overcome endocrine resistance. Drugs targeting EGFR and/or HER-2 did not produce significant results in CRPC (Boccardo et al., 2008; Nabhan et al., 2009; Slovin et al., 2009a; Whang et al., 2011), while mTOR inhibitors seemed to have some activity. In particular, in pre-clinical studies on cellular (Schayowitz et al., 2008; Wedel et al., 2011) and xenograft models (Morgan et al., 2008; Schayowitz et al., 2010) everolimus demonstrated positive results in combination with TOK-001 (Schayowitz et al., 2008, 2010), AEE788 (a dual VEGFR/EGFR inhibitor; Wedel et al., 2011), and docetaxel plus zoledronic acid (Morgan et al., 2008). A phase II study (Templeton et al., 2011) investigating the activity of everolimus 10 mg/daily as first-line treatment in patients with mCRPC has been recently presented. Among 37 enrolled patients, 12 (32%) remained progression-free at 12 weeks. Other phase II studies of everolimus, alone or in combination with bicalutamide, bevacizumab, or chemotherapy, as well as trials testing other mTOR inhibitors such as temsirolimus and ridaforolimus, are currently recruiting patients. IGF-1R inhibitors are also being evaluated in CRPC. Among these, cixutumumab (IMC-A12), a monoclonal antibody, is under study in a phase I/II trial in combination with temsirolimus in chemo-naïve mCRPC patients. The study has completed phase I accrual and early results have shown good tolerability (Rathkopf et al., 2011b).

Overexpression of proteins belonging to inhibitor of apoptosis (IAP) family, like Bcl-2, Bcl-X, survivin, and Mcl-1 (Raffo et al., 1995; Krajewska et al., 1996; Zhang et al., 2005), or of the stress-induced cytoprotective chaperone protein clusterin (Shiota et al., 2011) in PC cells is associated with disease progression, cell survival advantage, and resistance to ADT and chemotherapy-mediated apoptosis. Oblimersen, an anti-Bcl-2 antisense oligonucleotide (Sternberg et al., 2009), and AT-101, a small molecule inhibitor of the Bcl-2 family (Sonpavde et al., 2011), were studied as first-line treatment for CRPC in combination with docetaxel. However, clinical results were both negative in terms of PSA response and OS. Similarly, YM155, a small molecule suppressor of survivin, showed modest activity in taxane-pre-treated CRPC patients (Tolcher et al., 2012).

Custirsen (OGX-011) is an antisense oligonucleotide complementary to clusterin mRNA that inhibits its translation. In a randomized phase II trial (Chi et al., 2010) custirsen 640 mg intravenously weekly plus standard docetaxel as first-line treatment improved median OS in CRPC patients compared with docetaxel alone (23.8 versus 16.9 months), even if PSA and tumor response rates were similar. Another phase II trial tested custirsen in combination with mitoxantrone or docetaxel treatment as second-line therapy after progression during or within 6 months of initial docetaxel therapy (Saad et al., 2011). Pain relief was observed in 46 and 77% of patients, respectively. These rates were higher than expected, with interesting correlations between serum clusterin and survival. Two phase III studies are ongoing: SYNERGY, planned to enroll 800 patients, which will confirm whether adding custirsen to standard first-line docetaxel treatment slows tumor progression and enhances survival compared to chemotherapy alone, and SATURN, investigating custirsen in association with docetaxel as second-line retreatment.

As known, the hepatocyte growth factor/scatter factor (HGF/SF) and its receptor, the tyrosine kinase c-MET, promote tumor growth, invasion, and metastasis in several malignancies (Birchmeier et al., 2003). In PC, c-MET expression was demonstrated to be repressed by AR (Verras et al., 2007), with a consequent enhanced synthesis during ADT and in the hormone-independent state. Moreover, c-MET pathway activation is associated with progression to bone (Knudsen et al., 2002), and seems to induce a stem-like phenotype (Van Leenders et al., 2011) conferring a high invasive capacity, especially in the perimeter of prostate tumor, where this proto-oncogene is more expressed.

Cabozantinib (XL184) is an oral small molecule inhibitor of multiple kinase signaling pathways including c-MET and VEGFR2. An interim analysis of a phase II study, designed as a “randomized discontinuation” trial in patients with mCRPC, was recently presented (Hussain et al., 2011). After 12 weeks of treatment, patients with PR continued open-label cabozantinib, those with SD were blindly randomized to cabozantinib versus placebo, and those with PD discontinued treatment. Cabozantinib 100 mg/day led to improvement of bone parameters, especially bone scan and markers, as early as week 6. At this time, among bone evaluable patients, 86% complete or partial resolution of bone metastases and 12% stabilizations were reported. Prolonged PFS (21 versus 6 weeks) was registered in the cohort of patients undergone randomization. At week 12 disease control rate (PR + SD) was 71% with fatigue, hypertension, and hand-foot syndrome as major AEs. Based on this early clinical activity, two phase III trials are planned to test cabozantinib 60 mg/daily in patients with mCRPC after treatment with docetaxel and abiraterone: the 306 trial with mitoxantrone/prednisone as control arm and relief of bone pain as primary endpoint, and the 307 trial, comparing cabozantinib versus prednisone with OS as primary endpoint.

Src and Src-family kinases (SFKs) are a family of intracellular tyrosine kinases that mediate transduction of several molecular pathways implicated in PC growth, invasion, and progression (Tatarov et al., 2009). Src signaling is also involved in the development of bone metastasis, as it regulates different osteoclast functions including bone resorption (Miyazaki et al., 2004). Dasatinib is a SFK and Abl kinase inhibitor able to suppress PC cells-induced osteoclast differentiation and activity in pre-clinical models (Araujo et al., 2009; Vandyke et al., 2010). It was studied in a phase II trial (Yu et al., 2009) conducted in 47 chemotherapy naïve men with CRPC and biochemical progression. Patients were given dasatinib 100 or 70 mg BID. Both these schedules demonstrated biologic activity, especially on bone turnover markers, but remarkable AEs including diarrhea (62% of patients), pleural (51%), and pericardial (23%) effusions were recorded. The trial was amended because of toxicity and an expansion cohort of 48 patients was treated with dasatinib 100 mg once daily (Yu et al., 2011). At this dose the drug confirmed a good clinical activity and had a better tolerability profile with lesser grade 3/4 AEs (13% versus 32%), particularly 19% versus 51% pleural effusions, compared with the BID dosage. Dasatinib in combination with docetaxel has been studied in a recently published phase I/II trial (Araujo et al., 2012). Of 46 treated patients, 37 (80%) had any PSA decrease, including 26 (57%) who had a confirmed PSA response. Among RECIST-evaluable patients, 18 of 30 (60%) had PR with a 77% of overall disease control rate (PR + SD). Parallel bone scan improvements and urinary bone resorption markers decline were also observed. The combination was well tolerated, with a decreased frequency of pleural effusion (15%), probably due to the concomitant prednisone administration. READY, a phase III trial of docetaxel with or without dasatinib as first-line treatment of mCRPC is now ongoing. Another SFK inhibitor in clinical development for PC, saracatinib (AZD0530), showed limited clinical efficacy as monotherapy in a phase II trial (Lara et al., 2009).

The pathogenesis of osteoblastic bone metastasis, which frequently occur in mCRPC, is characterized by dysregulation of both bone resorption and formation (Guise et al., 2006). In particular, a role has been identified for the vasoactive peptide endothelin-1 (ET-1), produced by metastatic cancer cells in the microenvironment of new-formed bone, through the stimulation of the endothelin A receptor (ETaR) and its downstream pathways in osteoblastic cells (Yin et al., 2003). Atrasentan (ABT-627), an ETaR antagonist, reduced osteoblastic bone metastases, and bone tumor burden in in vitro and in vivo pre-clinical models (Yin et al., 2003), demonstrating an additive anti-tumor effect in combination with taxanes (Akhavan et al., 2006; Banerjee et al., 2007). Despite positive effects of atrasentan monotherapy in delaying median time to disease and PSA progression, as observed in a double-blinded, randomized, placebo-controlled phase II clinical trial (Carducci et al., 2003), data from two phase III studies carried out with this agent in either non-metastatic or metastatic disease failed to show a significant benefit in time to progression (Carducci et al., 2007; Nelson et al., 2008). Similarly, the phase III SWOG 0421 trial of atrasentan plus docetaxel as first-line therapy was closed early due to failure in reaching the primary endpoints (OS and PFS). Also zibotentan (ZD4054), another ETaR antagonist, presented discordant data among a phase II trial (James et al., 2010) and two of the subsequent phase III trials. The ENTHUSE clinical trial program consists of three phase III clinical studies designed to evaluate zibotentan monotherapy in men with metastatic (ENTHUSE M1 trial 14) and non-metastatic (ENTHUSE M0 trial 15) CRPC, as well as its combination with docetaxel as first-line treatment (ENTHUSE M1C trial 33). Both ENTHUSE studies 14 (Nelson et al., 2011) and 15 (not published) were stopped following the negative results to meet primary efficacy endpoints, while ENTHUSE study 33 will be continued and full results are expected.

Unlike strontium-89 and samarium-153, beta-emitting radiopharmaceuticals approved for palliation of bone metastasis-related pain (National Comprehensive Cancer Network (NCCN), 2011), radium-223 (alpharadin) targets bone metastasis with higher energy and shorter track length alpha-radiation. This allows hematopoietic bone marrow cells to be partly spared from damage due to radiation (Nilsson et al., 2007). A phase II trial reported minimum myelotoxicity and a significant effect on bone-alkaline phosphatase concentrations in patients treated with radium-223 versus placebo (Nilsson et al., 2007). The subsequent phase III ALSYMPCA trial was prematurely stopped in June 2011 after a preplanned interim efficacy analysis showing a significant 2.8-month OS benefit in the radium-223 arm over placebo arm (HR = 0.699). Based on these results approval procedures are ongoing.