Given the efficacy of androgen deprivation therapy in suppressing testosterone release and evidence of disease progression during castration, it was for a long time believed that castration-resistant prostate cancer represented a disease state in which the tumor had become independent of the androgen receptor signaling axis. Based on this belief, the disease state was initially referred to as “hormone-refractory prostate cancer” or “androgen-independent prostate cancer”.2 However, work in the past two and a half decades, including that related to the efficacy of abiraterone acetate, enzalutamide, apalutamide, and darolutamide in patients with castrate-resistant disease has demonstrated that the androgen receptor signaling axis remains both active and targetable in castration-resistant prostate cancer.
Among the first evidence of the ongoing importance of the androgen receptor signaling axis came from Visakorpi and colleagues in 1995 who demonstrated that there was androgen receptor gene amplification in a significant proportion of patients with castrate-resistant prostate cancer following androgen deprivation therapy.3 Subsequently, Chen, Sawyers, and colleagues demonstrated that reactivation of androgen receptor signaling (via androgen receptor mRNA) is both necessary and sufficient to induce a castrate-resistant prostate cancer phenotype in isogenic prostate cancer xenograft models.4
The androgen receptor signaling axis remains active in advanced castrate-resistant prostate cancer via a number of alterations in the androgen receptor pathway including direct modifications to the androgen receptor gene or indirectly through deregulation of co-factors and co-chaperones.5
Mechanism 1: ongoing androgenic stimulation of the AR despite castration
Despite the efficacy of androgen deprivation therapy in reducing circulating testosterone to castrate levels, there is consistent evidence that patients with castrate-resistant prostate cancer exhibit high levels of intratumoral androgens.5 There are a variety of mechanisms that underlie this phenomenon including epigenetic upregulation of steroidogenic enzymes driven by feedback from non-malignant prostate cancers. In some cases, these epigenetic changes can trigger de novo testosterone synthesis whereas other times this leverages adrenal androgen synthetic mechanisms.
This ongoing androgenic stimulation forms the basis of the rationale for treatment with agents that inhibit testosterone synthesis. In the pathway from cholesterol to testosterone and dihydrotestosterone, the cytochrome P450 family (namely, CYP17A1) is critical. Abiraterone acetate inhibits the 17,20-lyase activity of this enzyme thus preventing androgen synthesis.
Mechanism 2: AR overexpression
In a natural adaptive framework (independent of castration-resistance), there is an increased transcription of the androgen receptor in response to deprivation of its ligand (testosterone). Thus, ADT initially triggers the overexpression of the androgen receptor. This overexpression of the androgen receptor in patients with castrate-resistant prostate cancer results in hypersensitivity to even low levels of androgen.4
Androgen receptor overexpression is one of the most frequently identified abnormalities in patients with castrate-resistant prostate cancer.6 Further, it is associated with the development of resistance to first-generation anti-androgens, antagonist-to-agonist functional switching, and resistance to androgen receptor-targeted agents.2
In the majority of patients with castrate-resistant prostate cancer, androgen receptor of expression is mediated through X chromosome rearrangement with resultant focal copy number gain and resultant androgen receptor amplification.7 In patients who have received many lines of therapy, androgen receptor gene amplification is identified in up to half whereas such amplification is rarely found in treatment-naïve tumors.
In addition to androgen receptor gene amplification due to copy number alterations, androgen receptor activity may be increased due to transcriptional upregulation. In the wild state, the ligand-bound androgen receptor acts as negative feedback to its own expression through binding to the AR gene.8 Androgen receptor targeting therapies can release this negative feedback resulting in increased transcription of the androgen receptor mRNA.
On the basis of this rationale, drug development sought to identify agents that could more effectively act as competitive antagonists to the ligand-binding domain of the androgen receptor, compared to first-generation anti-androgens such as bicalutamide. This work led to the development of enzalutamide, darolutamide (originally called ODM-201), and apalutamide (originally called ARN-509). In a number of Phase III trials, these agents have demonstrated efficacy across the disease spectrum of advanced prostate cancer including castration-sensitive metastatic disease, non-metastatic castrate-resistance disease, metastatic castrate-resistant prostate cancer prior to chemotherapy, and metastatic castrate-resistant prostate cancer following chemotherapy. Interestingly, they have shown similar efficacy (when assessed using relative benefit compared to placebo) across the disease states.
Mechanism 3: AR gene mutation
Androgen receptor gene mutations may also contribute to the ongoing activity of the androgen receptor signaling axis in advanced prostate cancer. Most of these mutations affect the ligand-binding domain where the AR interacts with testosterone or the transactivation domain (the N-terminal domain).2 Alterations in the ligand-binding domain can contribute to androgen receptor signaling in spite of castration due to ligand promiscuity and receptor agonism from AR antagonists (so-called antagonist-agonist switching). In contrast, alterations in the transactivation domain result in alterations in the recruitment of co-activators and other transcriptional components, increased nuclear retention, and other changes that promote AR activation.
Mechanism 4: AR splice variants
The importance of the androgen receptor in prostate carcinogenesis places evolutionary pressure which can result in the emergence of alternative forms of the receptor. In nearly all cases, these variants are truncated proteins. Many androgen receptor variants lack all or part of the ligand-binding domain of the protein. As a result, these proteins have, at least theoretically, androgen-independent activity though this has not been demonstrated for all variants.2 The best characterized of these variants are AR-V7 and ARv567es. The presence of AR-V7 splice variants has been shown to correlate with poor responses to androgen-axis targeting agents such as abiraterone acetate and enzalutamide, but a continued response to taxane-based chemotherapy.9
Mechanism 5: de-regulation of AR co-chaperones
In a basal state in non-malignant prostate cancer cells, the androgen receptor exists as a cytosolic protein, in complex with heat shock proteins including HSP-27, HSP-70, and HSP-90 which serve to protect the androgen receptor from proteolytic degradation and maintain it in a stable conformation, available for binding to androgen when present.10 Following binding to androgens, the androgen receptor undergoes a conformational change that facilitates nuclear relocation.
In the castrate state, there is an innate stress response within prostate cells. This induces the activity of the heat shock proteins. This increased heat shock protein activity protects the cytosolic androgen receptor from degradation and also promotes its nuclear translocation.11 For example, over-expression of HSP-27 prevents cellular apoptosis and can induce castration-resistance.
This has led to the identification of heat shock proteins as potential druggable targets. While no such agents have yet come to market, many have been developed and tested. Perhaps the most well known is OGX-427 which is a second-generation antisense oligonucleotide directed against HSP-27. Despite evidence of pre-clinical activity, clinical trials of these agents have proven disappointing. Further, there is concern regarding the clinical utility of this approach due to the functional redundancy of heat shock proteins.
Mechanism 6: modification to AR co-activators
As alluded to above, the androgen receptor must translocate from the cytoplasm to the nucleus in order to biologically active. Once in the nucleus, its function requires mediation by a number of co-factors and co-activators. As a result, modification to these may influence the biologic activity of the androgen receptor signaling axis.
In patients with castrate resistance prostate cancer, a number of related genes, NCOA1, NCOA2, and NCOA3, have been investigated. These genes are members of the P160 steroid receptor co-activator family. The importance of NCOA1 or NCOA2 has been demonstrated in both gain of function and loss of function studies: overexpression of NCOA1 or NCOA2 increases the transactivation of the androgen receptor at physiologic levels of adrenal androgen that may be expected in a patient on ADT with castrate serum testosterone12 while depletion of NCOA2 can prevent the development of castrate-resistant prostate cancer in mice.13
Other co-factors, co-activators, and transcription factors may also be important in the ongoing role of the androgen receptor signaling axis in castrate-resistant prostate cancer, including the forkhead protein FOXA1 and GATA2. GATA2 functions as a transcription factor, recruiting NCOAs to the androgen receptor complex. High levels of GATA2 expression are associated with a poor prognosis in patients with castrate-resistant prostate cancer and, thus, this has been suggested as an actionable target.
Despite these mechanisms for the ongoing contribution of the androgen receptor signaling axis in advanced prostate cancer, tumors may evolve or adapt to become independent of its activity. In these cases, other hormonal pathways may become important including the estrogen receptor and glucocorticoid receptor. However, hormone-independent diseases such as neuroendocrine prostate cancer may also develop. These tumors exhibit a primarily neuronal phenotype. They both lack the typical behavior of prostate cancer (low PSA levels and predilection for visceral metastasis) and respond poorly to conventional prostate cancer-directed therapies.
Published Date: March 2020