Phase II trial of RAD001 and bicalutamide for castration-resistant prostate cancer

Regulation of Eukaryotic Translation Initiation Factor 4E as a Potential Anticancer Strategy

Eukaryotic translation initiation factors (eIFs) are highly expressed in cancer cells, especially eIF4E, the central regulatory node driving cancer cell growth and a potential target for anticancer drugs. eIF4E-targeting strategies primarily focus on inhibiting eIF4E synthesis, interfering with eIF4E/eIF4G interactions, and targeting eIF4E phosphorylation and peptide inhibitors. Although some small-molecule inhibitors are in clinical trials, no eIF4E inhibitors are available for clinical use. We provide an overview of the regulatory mechanisms of eIF4E and summarize the progress in developing and discovering eIF4E inhibitor strategies. We propose that interference with eIF4E/eIF4G interactions will provide a new perspective for the design of eIF4E inhibitors and may be a preferred strategy.

Effectiveness and durability of benefit of mTOR inhibitors in a real-world cohort of patients with metastatic prostate cancer and PI3K pathway alterations

BACKGROUND

Alterations in the PTEN/PI3K/AKT/mTOR pathway are prevalent in prostate cancer. In this retrospective study, we evaluated the clinical effectiveness of mTOR inhibitors (mTORi) in patients with metastatic prostate cancer (mPCA) and tissue assessed phosphatidyl-3-inositol kinase (PI3K) pathway alterations.

METHODS

This study used a nationwide (US-based) de-identified PCA clinico-genomic database, originating from approximately 280 US cancer clinics (~800 sites of care). We evaluated treatment data for patients with PCA from October 2014 to February 2020. In a cohort of 2301 PCA patients with 7208 evaluable treatment lines, we selected 17 mPCA patients (2 hormone-sensitive, 15 castrate-resistant) with tissue assessed PI3K pathway alterations by comprehensive genomic profiling who received mTORi treatment.

RESULTS

Patients had a median age of 72 years (IQR 68.0, 76.0) and were heavily pretreated with a median 3 lines of therapy prior to mTORi use (range 0-6). The PI3K pathway alterations included PTENdel (10 patients, 58.8%), AKT1mut (4 patients, 23.5%), PTENmut (2 patients, 11.8%), and dual PTENmut and PIK3CAmut (1 patient, 5.9%). Most (15 patients, 88.2%) were treated with everolimus monotherapy. Among 10 patients with on treatment PSA available, 2 patients had a PSA decrease ≥10% at week 12 and 5 patients overall had a subsequent PSA decrease. For those on mTORi, the median time to next treatment was 3.62 months (range 0, 8.52).

CONCLUSIONS

In this small cohort of mPCA patients with tissue assessed PI3K pathway alterations, mTORi therapy was not effective with few PSA responses and short duration of therapy.

PTEN-PI3K pathway alterations in advanced prostate cancer and clinical implications

BACKGROUND

Despite significant advances in molecular characterization and therapeutic targeting of advanced prostate cancer, it remains the second most common cause of cancer death in men in the United States. The PI3K (Phosphatidylinositol 3-kinase)/AKT (AKT serine/threonine kinase)/mTOR (mammalian target of rapamycin) signaling pathway is commonly altered in prostate cancer, most frequently through loss of the PTEN (Phosphatase and Tensin Homolog) tumor suppressor, and is critical for cancer cell proliferation, migration, and survival.

METHODS

This study summarizes signaling through the PTEN/PI3K pathway, alterations in pathway components commonly seen in advanced prostate cancer, and results of clinical trials of pathway inhibitors reported to date with a focus on more recently reported studies. It also reviews rationale for combination approaches currently under study, including with taxanes, immune checkpoint inhibitors and poly (ADP-ribose) polymerase inhibitors, and discusses future directions in biomarker testing and therapeutic targeting of this pathway.

RESULTS

Clinical trials studying pharmacologic inhibitors of PI3K, AKT or mTOR kinases have demonstrated modest activity of specific agents, with several trials of pathway inhibitors currently in progress. A key challenge is the importance of PI3K/AKT/mTOR signaling in noncancerous tissues, leading to predictable but often severe toxicities at therapeutic doses.

RESULTS

Further advances in selective pharmacologic inhibition of the PI3K/AKT/mTOR pathway in tumors, development of rational combinations, and appropriate biomarker selection to identify the appropriate tumor- and patient-specific vulnerabilities will be required to optimize clinical benefit from therapeutic targeting of this pathway.

Solamargine Inhibits Prostate Cancer Cell Growth and Enhances the Therapeutic Efficacy of Docetaxel via Akt Signaling

Prostate cancer (PCa) has become a leading cause of cancer-associated incidence and mortality in men worldwide. However, most primary PCas relapse to castration-resistant PCa (CRPC) after androgen deprivation treatment. The current treatment for CRPC is based on chemotherapeutic drugs such as docetaxel, while the development of chemoresistance and severe side effects limit the therapeutic benefit. Solamargine, a natural alkaloid isolated from a traditional Chinese herbal medicine known as Solanum nigrum, exhibits antitumor activity in various human cancers. In this study, we demonstrated that solamargine substantially inhibited CRPC cell growth in a dose-dependent manner through the suppression of phosphoinositide 3-kinase (PI3K)/Akt signaling. Moreover, solamargine exhibited significant antitumor effects in mouse xenograft models. Bioinformatics analysis of docetaxel-resistant PCa cells indicated that the PI3K/Akt pathway mediated the chemoresistance of CRPC. Furthermore, solamargine significantly enhanced the efficacy of docetaxel in PCa cells. These results reveal the therapeutic potential of solamargine against human PCa.

Role of PI3K-AKT-mTOR Pathway as a Pro-Survival Signaling and Resistance-Mediating Mechanism to Therapy of Prostate Cancer

Androgen deprivation therapy (ADT) and androgen receptor (AR)-targeted therapy are the gold standard options for treating prostate cancer (PCa). These are initially effective, as localized and the early stage of metastatic disease are androgen- and castration-sensitive. The tumor strongly relies on systemic/circulating androgens for activating AR signaling to stimulate growth and progression. However, after a certain point, the tumor will eventually develop a resistant stage, where ADT and AR antagonists are no longer effective. Mechanistically, it seems that the tumor becomes more aggressive through adaptive responses, relies more on alternative activated pathways, and is less dependent on AR signaling. This includes hyperactivation of PI3K-AKT-mTOR pathway, which is a central signal that regulates cell pro-survival/anti-apoptotic pathways, thus, compensating the blockade of AR signaling. The PI3K-AKT-mTOR pathway is well-documented for its crosstalk between genomic and non-genomic AR signaling, as well as other signaling cascades. Such a reciprocal feedback loop makes it more complicated to target individual factor/signaling for treating PCa. Here, we highlight the role of PI3K-AKT-mTOR signaling as a resistance mechanism for PCa therapy and illustrate the transition of prostate tumor from AR signaling-dependent to PI3K-AKT-mTOR pathway-dependent. Moreover, therapeutic strategies with inhibitors targeting the PI3K-AKT-mTOR signal used in clinic and ongoing clinical trials are discussed.

Inhibitors of the PI3K/Akt/mTOR Pathway in Prostate Cancer Chemoprevention and Intervention

The phosphatidylinositol 3-kinase (PI3K)/serine-threonine kinase (Akt)/mammalian target of the rapamycin (mTOR)-signaling pathway has been suggested to have connections with the malignant transformation, growth, proliferation, and metastasis of various cancers and solid tumors. Relevant connections between the PI3K/Akt/mTOR pathway, cell survival, and prostate cancer (PC) provide a great therapeutic target for PC prevention or treatment. Recent studies have focused on small-molecule mTOR inhibitors or their usage in coordination with other therapeutics for PC treatment that are currently undergoing clinical testing. In this study, the function of the PI3K/Akt/mTOR pathway, the consequence of its dysregulation, and the development of mTOR inhibitors, either as an individual substance or in combination with other agents, and their clinical implications are discussed. The rationale for targeting the PI3K/Akt/mTOR pathway, and specifically the application and potential utility of natural agents involved in PC treatment is described. In addition to the small-molecule mTOR inhibitors, there are evidence that several natural agents are able to target the PI3K/Akt/mTOR pathway in prostatic neoplasms. These natural mTOR inhibitors can interfere with the PI3K/Akt/mTOR pathway through multiple mechanisms; however, inhibition of Akt and suppression of mTOR 1 activity are two major therapeutic approaches. Combination therapy improves the efficacy of these inhibitors to either suppress the PC progression or circumvent the resistance by cancer cells.

Regulation of mRNA Translation by Hormone Receptors in Breast and Prostate Cancer

Simple Summary

The estrogen and androgen receptors (ER, AR) are key oncogenic drivers and therapeutic targets in breast and prostate cancer, respectively. These receptors bind to DNA and regulate gene expression but emerging evidence indicates that they also play important roles in controlling the process of mRNA translation, which dictates cellular protein production. Here, we review the mechanisms by which abnormal activities of ER and AR can dysregulate mRNA translation in breast and prostate cancer cells. Specifically, we explore how the intricate cellular signalling pathways that keep mRNA translation in check are perturbed by aberrant ER and AR signalling, which can lead to enhanced cancer cell growth. We also discuss the potential of targeting mRNA translation as a strategy to treat patients with breast and prostate cancer.

Abstract

Breast and prostate cancer are the second and third leading causes of death amongst all cancer types, respectively. Pathogenesis of these malignancies is characterised by dysregulation of sex hormone signalling pathways, mediated by the estrogen receptor-α (ER) in breast cancer and androgen receptor (AR) in prostate cancer. ER and AR are transcription factors whose aberrant function drives oncogenic transcriptional programs to promote cancer growth and progression. While ER/AR are known to stimulate cell growth and survival by modulating gene transcription, emerging findings indicate that their effects in neoplasia are also mediated by dysregulation of protein synthesis (i.e., mRNA translation). This suggests that ER/AR can coordinately perturb both transcriptional and translational programs, resulting in the establishment of proteomes that promote malignancy. In this review, we will discuss relatively understudied aspects of ER and AR activity in regulating protein synthesis as well as the potential of targeting mRNA translation in breast and prostate cancer.

Signaling Pathways That Control Apoptosis in Prostate Cancer

Prostate cancer is the second most common malignancy and the fifth leading cancer-caused death in men worldwide. Therapies that target the androgen receptor axis induce apoptosis in normal prostates and provide temporary relief for advanced disease, yet prostate cancer that acquired androgen independence (so called castration-resistant prostate cancer, CRPC) invariably progresses to lethal disease. There is accumulating evidence that androgen receptor signaling do not regulate apoptosis and proliferation in prostate epithelial cells in a cell-autonomous fashion. Instead, androgen receptor activation in stroma compartments induces expression of unknown paracrine factors that maintain homeostasis of the prostate epithelium. This paradigm calls for new studies to identify paracrine factors and signaling pathways that control the survival of normal epithelial cells and to determine which apoptosis regulatory molecules are targeted by these pathways. This review summarizes the recent progress in understanding the mechanism of apoptosis induced by androgen ablation in prostate epithelial cells with emphasis on the roles of BCL-2 family proteins and "druggable" signaling pathways that control these proteins. A summary of the clinical trials of inhibitors of anti-apoptotic signaling pathways is also provided. Evidently, better knowledge of the apoptosis regulation in prostate epithelial cells is needed to understand mechanisms of androgen-independence and implement life-extending therapies for CRPC.

Distinct roles for the RNA-binding protein Staufen1 in prostate cancer

BACKGROUND

Prostate cancer is one of the most common malignant cancers with the second highest global rate of mortality in men. During the early stages of disease progression, tumour growth is local and androgen-dependent. Despite treatment, a large percentage of patients develop androgen-independent prostate cancer, which often results in metastases, a leading cause of mortality in these patients. Our previous work on the RNA-binding protein Staufen1 demonstrated its novel role in cancer biology, and in particular rhabdomyosarcoma tumorigenesis. To build upon this work, we have focused on the role of Staufen1 in other forms of cancer and describe here the novel and differential roles of Staufen1 in prostate cancer.

METHODS

Using a cell-based approach, three independent prostate cancer cell lines with different characteristics were used to evaluate the expression of Staufen1 in human prostate cancer relative to control prostate cells. The functional impact of Staufen1 on several key oncogenic features of prostate cancer cells including proliferation, apoptosis, migration and invasion were systematically investigated.

RESULTS

We show that Staufen1 levels are increased in all human prostate cancer cells examined in comparison to normal prostate epithelial cells. Furthermore, Staufen1 differentially regulates growth, migration, and invasion in the various prostate cancer cells assessed. In LNCaP prostate cancer cells, Staufen1 regulates cell proliferation through mTOR activation. Conversely, Staufen1 regulates migration and invasion of the highly invasive, bone metastatic-derived, PC3 prostate cells via the activation of focal adhesion kinase.

CONCLUSIONS

Collectively, these results show that Staufen1 has a direct impact in prostate cancer development and further demonstrate that its functions vary amongst the prostate cancer cell types. Accordingly, Staufen1 represents a novel target for the development of much-needed therapeutic strategies for prostate cancer.

Interactions between androgen receptor signaling and other molecular pathways in prostate cancer progression: Current and future clinical implications

In last years several improvements have been made in the management of prostate cancer (PCa). Androgen receptor (AR) is considered the main driver in PCa growth and progression and most drugs are directed against AR pathway. Once PCa spreads outside the prostate, androgen deprivation therapy (ADT) represents the cornerstone of treatment in hormone-sensitive prostate cancer (HSPC). Unfortunately, the response is only transient and most patients eventually develop castration-resistant prostate cancer (CRPC). Most resistance mechanisms depend on maintenance of AR signalling in castration environment. Recent discoveries of multiple growth-promoting and survival pathways in PCa suggest the importance of alternative mechanisms involved in disease progression, such as DNA damage response pathway, PTEN/PI3K/AKT/mTOR pathway, cell cycle pathway, WNT pathway, TMPRSS2/ETS fusion, neuroendocrine pattern and immune system response. In this review, we discuss the interplay between AR signaling and other molecular pathways involved in PCa pathogenesis and their therapeutic implication in advanced disease.

The PTEN Conundrum: How to Target PTEN-Deficient Prostate Cancer

Loss of the tumor suppressor phosphatase and tensin homologue deleted on chromosome 10 (PTEN), which negatively regulates the PI3K-AKT-mTOR pathway, is strongly linked to advanced prostate cancer progression and poor clinical outcome. Accordingly, several therapeutic approaches are currently being explored to combat PTEN-deficient tumors. These include classical inhibition of the PI3K-AKT-mTOR signaling network, as well as new approaches that restore PTEN function, or target PTEN regulation of chromosome stability, DNA damage repair and the tumor microenvironment. While targeting PTEN-deficient prostate cancer remains a clinical challenge, new advances in the field of precision medicine indicate that PTEN loss provides a valuable biomarker to stratify prostate cancer patients for treatments, which may improve overall outcome. Here, we discuss the clinical implications of PTEN loss in the management of prostate cancer and review recent therapeutic advances in targeting PTEN-deficient prostate cancer. Deepening our understanding of how PTEN loss contributes to prostate cancer growth and therapeutic resistance will inform the design of future clinical studies and precision-medicine strategies that will ultimately improve patient care.

Cellular rewiring in lethal prostate cancer: the architect of drug resistance

Over the past 5 years, the advent of combination therapeutic strategies has substantially reshaped the clinical management of patients with advanced prostate cancer. However, most of these combination regimens were developed empirically and, despite offering survival benefits, are not enough to halt disease progression. Thus, the development of effective therapeutic strategies that target the mechanisms involved in the acquisition of drug resistance and improve clinical trial design are an unmet clinical need. In this context, we hypothesize that the tumour engineers a dynamic response through the process of cellular rewiring, in which it adapts to the therapy used and develops mechanisms of drug resistance via downstream signalling of key regulatory cascades such as the androgen receptor, PI3K-AKT or GATA2-dependent pathways, as well as initiation of biological processes to revert tumour cells to undifferentiated aggressive states via phenotype switching towards a neuroendocrine phenotype or acquisition of stem-like properties. These dynamic responses are specific for each patient and could be responsible for treatment failure despite multi-target approaches. Understanding the common stages of these cellular rewiring mechanisms to gain a new perspective on the molecular underpinnings of drug resistance might help formulate novel combination therapeutic regimens.

Randomized phase II trial of neoadjuvant everolimus in patients with high-risk localized prostate cancer

Background Despite definitive local therapy, patients with high-risk prostate cancer have a significant risk for local and distant failure. To date, no systemic therapy given prior to surgery has been shown to improve outcomes. The phosphatidilinositol 3-kinase/AKT/mTOR pathway is commonly dysregulated in men with prostate cancer. We sought to determine the clinical efficacy and safety of the mTOR/TORC1 inhibitor everolimus in men with high-risk prostate cancer undergoing radical prostatectomy. Methods This is a randomized phase II study of everolimus at two different doses (5 and 10 mg daily) given orally for 8 weeks before radical prostatectomy in men with high-risk prostate cancer. The primary endpoint was the pathologic response (histologic P0, margin status, extraprostatic extension) and surgical outcomes. Secondary endpoints included changes in serum PSA level and treatment effects on levels of expression of mTOR, p4EBP1, pS6 and pAKT. Results Seventeen patients were enrolled: nine at 10 mg dose and eight at 5 mg dose. No pathologic complete responses were observed and the majority of patients (88%) had an increase in their PSA values leading to this study being terminated early due to lack of clinical efficacy. Treatment-related adverse events were similar to those previously reported with the use of everolimus in other solid tumors and no additional surgical complications were observed. A significant decrease in the expression of p4EBP1 was noted in prostatectomy samples following treatment. Conclusions Neoadjuvant everolimus given at 5 mg or 10 mg daily for 8 weeks prior to radical prostatectomy did not impact pathologic responses and surgical outcomes of patients with high-risk prostate cancer. Trial registration NCT00526591 .

Renal Damaging Effect Elicited by Bicalutamide Therapy Uncovered Multiple Action Mechanisms As Evidenced by the Cell Model

Bicalutamide (Bic) is frequently used in androgen deprivation therapy (ADT) for treating prostate cancer. ADT-induced hypogonadism was reported to have the potential to lead to acute kidney injury (AKI). ADT was also shown to induce bladder fibrosis via induction of the transforming growth factor (TGF)-β level. We hypothesized that Bic can likely induce renal fibrosis. To understand this, a cell model was used to explore expressions of relevant profibrotic proteins. Results indicated that Bic initiated multiple apoptotic and fibrotic pathways, including androgen deprivation, downregulation of the androgen receptor → phosphatidylinositol-3-kinase → Akt pathway, upregulation of the extrinsic apoptotic pathway- tumor necrosis factor α →  nuclear factor κB → caspase-3, increased expressions of fibrosis-related proteins including platelet-derived growth factor β, fibronectin and collagen IV, and enhanced cell migration. The endoplasmic reticular stress pathway and smooth muscle actin were unaffected by Bic. Co-treatment with testosterone was shown to have an anti-apoptotic effect against Bic, suggesting a better outcome of Bic therapy if administered with an appropriate testosterone intervention. However, since Bic was found to inhibit the membrane transport and consumption rates of testosterone, a slightly larger dose of testosterone is recommended. In conclusion, these pathways can be considered to be pharmaceutically relevant targets for drug development in treating the adverse effects of Bic.

mTOR Signaling in Cancer and mTOR Inhibitors in Solid Tumor Targeting Therapy

The mammalian or mechanistic target of rapamycin (mTOR) pathway plays a crucial role in regulation of cell survival, metabolism, growth and protein synthesis in response to upstream signals in both normal physiological and pathological conditions, especially in cancer. Aberrant mTOR signaling resulting from genetic alterations from different levels of the signal cascade is commonly observed in various types of cancers. Upon hyperactivation, mTOR signaling promotes cell proliferation and metabolism that contribute to tumor initiation and progression. In addition, mTOR also negatively regulates autophagy via different ways. We discuss mTOR signaling and its key upstream and downstream factors, the specific genetic changes in the mTOR pathway and the inhibitors of mTOR applied as therapeutic strategies in eight solid tumors. Although monotherapy and combination therapy with mTOR inhibitors have been extensively applied in preclinical and clinical trials in various cancer types, innovative therapies with better efficacy and less drug resistance are still in great need, and new biomarkers and deep sequencing technologies will facilitate these mTOR targeting drugs benefit the cancer patients in personalized therapy.

Taxane resistance in castration-resistant prostate cancer: mechanisms and therapeutic strategies

Despite its good initial response and significant survival benefit in patients with castration-resistant prostate cancer (CRPC), taxane therapy inevitably encounters drug resistance in all patients. Deep understandings of taxane resistant mechanisms can significantly facilitate the development of new therapeutic strategies to overcome taxane resistance and improve CRPC patient survival. Multiple pathways of resistance have been identified as potentially crucial areas of intervention. First, taxane resistant tumor cells typically have mutated microtubule binding sites, varying tubulin isotype expression, and upregulation of efflux transporters. These mechanisms contribute to reducing binding affinity and availability of taxanes. Second, taxane resistant tumors have increased stem cell like characteristics, indicating higher potential for further mutation in response to therapy. Third, the androgen receptor pathway is instrumental in the proliferation of CRPC and multiple hypotheses leading to this pathway reactivation have been reported. The connection of this pathway to the AKT pathway has received significant attention due to the upregulation of phosphorylated AKT in CRPC. This review highlights recent advances in elucidating taxane resistant mechanisms and summarizes potential therapeutic strategies for improved treatment of CRPC.

Dual targeting of androgen receptor and mTORC1 by salinomycin in prostate cancer

Androgen receptor (AR) and PI3K/AKT/mTORC1 are major survival signals that drive prostate cancer to a lethal disease. Reciprocal activation of these oncogenic pathways from negative cross talks contributes to low/limited success of pathway-selective inhibitors in curbing prostate cancer progression. We report that the antibiotic salinomycin, a cancer stem cell blocker, is a dual-acting AR and mTORC1 inhibitor, inhibiting PTEN-deficient castration-sensitive and castration-resistant prostate cancer in culture and xenograft tumors. AR expression, its transcriptional activity, and androgen biosynthesis regulating enzymes CYP17A1, HSD3β1 were reduced by sub-micro molar salinomycin. Estrogen receptor-α expression was unchanged. Loss of phosphorylated AR at serine-81, which is an index for nuclear AR activity, preceded total AR reduction. Rapamycin enhanced the AR protein level without altering phosphoAR-Ser81 and CYP17A1. Inactivation of mTORC1, evident from reduced phosphorylation of mTOR and downstream effectors, as well as AMPK activation led to robust autophagy induction. Apoptosis increased modestly, albeit significantly, by sub-micro molar salinomycin. Enhanced stimulatory TSC2 phosphorylation at Ser-1387 by AMPK, and reduced inhibitory TSC2 phosphorylation at Ser-939/Thr-1462 catalyzed by AKT augmented TSC2/TSC1 activity, which led to mTORC1 inhibition. AMPK-mediated raptor phosphorylation further reduced mTOR's kinase function and mTORC1 activity. Our novel finding on dual inhibition of AR and mTORC1 suggests that salinomycin is potentially active as monotherapy against advanced prostate cancer.

RES-529: a PI3K/AKT/mTOR pathway inhibitor that dissociates the mTORC1 and mTORC2 complexes. Anticancer Drugs. 2016;27:475-487. [PMID: 26918392 PMCID: PMC4881730 DOI: 10.1097/cad.0000000000000354]

RES-529 (previously named Palomid 529, P529) is a phosphoinositide 3-kinase (PI3K)/AKT/mechanistic target of rapamycin (mTOR) pathway inhibitor that interferes with the pathway through both mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2) dissociation. This compound is currently being developed in oncology and ophthalmology. The oncology focus is for the treatment of glioblastoma, where it has received orphan designation by the US Food and Drug Administration, and prostate cancer. We present a review of the PI3K/AKT/mTOR pathway, its role in tumorigenesis, and the potential of RES-529 in cancer treatment. RES-529 inhibits mTORC1/mTORC2 activity in various cancer cell lines, as noted by decreased phosphorylation of substrates including ribosomal protein S6, 4E-BP1, and AKT, leading to cell growth inhibition and death, with activity generally in the range of 5–15 μmol/l. In animal tumor models where the PI3K/AKT/mTOR pathway is abnormally activated (i.e. glioblastoma, prostate cancer, and breast cancer), RES-529 reduces tumor growth by as much as 78%. RES-529 treatment is synergistic with radiation therapy, chemotherapy, and hormonal therapy in reducing tumor growth, potentially by preventing PI3K/AKT/mTOR pathway activation associated with these treatments. Furthermore, this compound has shown antiangiogenic activity in several animal models. mTORC1 and mTORC2 have redundant and distinct activities that contribute toward oncogenesis. Current inhibitors of this pathway have primarily targeted mTORC1, but have shown limited clinical efficacy. Inhibitors of mTORC1 and mTORC2 such as RES-529 may therefore have the potential to overcome the deficiencies found in targeting only mTORC1.

mTOR Inhibitors in Castration-Resistant Prostate Cancer: A Systematic Review

BACKGROUND

The progression of prostate cancer to castration-resistant prostate cancer (CRPC) is often a result of somatic alterations in the PI3K/Akt/mTOR (mammalian target of rapamycin) pathway, suggesting that therapies targeting this pathway might lead to improved survival and efficacy. Here, we systematically evaluate the results of clinical trials investigating mTOR inhibition in CRPC and utilize preclinical data to predict clinical outcomes.

METHODS

Trials included in the study were identified through PubMed and via review of conference abstracts cited by relevant review articles. The eligibility of trials was independent of sample size, clinical setting, or date.

RESULTS

A total of 14 studies were eligible for qualitative analysis. The clinical setting was variable among studies, and all utilized an allosteric mTOR inhibitor as either a monotherapy or in combination. Molecular criteria were evaluated in three trials. Among most studies, the prostate-specific antigen level declined during treatment, but often increased shortly thereafter. Partial responses to treatment were minimal, and no complete responses were reported. Two studies exploring therapy with an mTOR inhibitor in combination with bicalutamide resulted in minimal efficacy. Overall, allosteric mTOR inhibition was deemed to be inadequate for the treatment of CRPC.

CONCLUSION

Preclinical data suggest that a reciprocal feedback mechanism between PI3K and androgen receptor signaling is a potential mechanism behind the clinical inefficacy of mTOR inhibitors in CRPC, indicating combinatorial targeting of PI3K, mTORC1/2, and the androgen receptor might be more effective. Comprehensive analysis of preclinical data to assess clinical trial targets and efficacy may reduce the number of unproductive trials and identify potentially beneficial combinatorial therapies for resistant disease.

Targeting androgen-independent pathways: new chances for patients with prostate cancer?

Androgen deprivation therapy (ADT) is the mainstay treatment for advanced prostate cancer (PC). Most patients eventually progress to a condition known as castration-resistant prostate cancer (CRPC), characterized by lack of response to ADT. Although new androgen receptor signaling (ARS) inhibitors and chemotherapeutic agents have been introduced to overcome resistance to ADT, many patients progress because of primary or acquired resistance to these agents. This comprehensive review aims at exploring the mechanisms of resistance and progression of PC, with specific focus on alterations which lead to the activation of androgen receptor (AR)-independent pathways of survival. Our work integrates available clinical and preclinical data on agents which target these pathways, assessing their potential clinical implication in specific settings of patients. Given the rising interest of the scientific community in cancer immunotherapy strategies, further attention is dedicated to the role of immune evasion in PC.

The androgen receptor is a negative regulator of eIF4E phosphorylation at S209: implications for the use of mTOR inhibitors in advanced prostate cancer

The anti-androgen bicalutamide is widely used in the treatment of advanced prostate cancer (PCa) in many countries, but its effect on castration resistant PCa (CRPC) is limited. We previously showed that resistance to bicalutamide results from activation of mechanistic target of rapamycin (mTOR). Interestingly, clinical trials testing combinations of the mTOR inhibitor RAD001 with bicalutamide were effective in bicalutamide-naïve CRPC patients, but not in bicalutamide-pre-treated ones. Here we investigate causes for their difference in response. Evaluation of CRPC cell lines identified resistant vs sensitive in-vitro models, and revealed that increased eIF4E(S209) phosphorylation is associated with resistance to the combination. We confirmed using a human-derived tumor-xenograft mouse model that bicalutamide pre-treatment is associated with an increase in eIF4E(S209) phosphorylation. Thus, AR suppressed eIF4E phosphorylation, while the use of anti-androgens relieved this suppression, thereby triggering its increase. Additional investigation in human prostatectomy samples showed that increased eIF4E phosphorylation strongly correlated with the cell proliferation marker Ki67. SiRNA-mediated knock-down of eIF4E sensitized CRPC cells to RAD001+bicalutamide, while eIF4E overexpression induced resistance. Inhibition of eIF4E phosphorylation by treatment with CGP57380 (an inhibitor of MAPK interacting serine-threonine kinases Mnk1/2, the eIF4E upstream kinase) or inhibitors of ERK1/2, the upstream kinase regulating Mnk1/2, also sensitized CRPC cells to RAD001+bicalutamide. Examination of downstream targets of eIF4E-mediated translation, including survivin, demonstrated that eIF4E(S209) phosphorylation increased cap-independent translation whereas its inhibition restored cap-dependent translation which could be inhibited by mTOR inhibitors. Thus, our results demonstrate that while combinations of AR and mTOR inhibitors were effective in suppressing tumor growth by inhibiting both AR-induced transcription and mTOR-induced cap-dependent translation, pre-treatment with AR antagonists including bicalutamide increased eIF4E phosphorylation that induced resistance to combinations of AR and mTOR inhibitors by inducing cap-independent translation. We conclude that this resistance can be overcome by inhibiting eIF4E phosphorylation with Mnk1/2 or ERK1/2 inhibitors.

Safety and Efficacy of Docetaxel, Bevacizumab, and Everolimus for Castration-resistant Prostate Cancer (CRPC)

BACKGROUND

Previous data suggests that co-targeting mammalian target of rapamycin and angiogenic pathways may potentiate effects of cytotoxic chemotherapy. We studied combining mammalian target of rapamycin and vascular endothelial growth factor inhibition with docetaxel in castrate-resistant prostate cancer (CRPC).

METHODS

Eligible patients had progressive, metastatic, chemotherapy-naive CRPC. Docetaxel and bevacizumab were given intravenously day 1 with everolimus orally daily on a 21-day cycle across 3 dose levels (75:15:2.5, 75:15:5, and 65:15:5; docetaxel mg/m2, mg/kg bevacizumab, and mg everolimus, respectively). Maintenance therapy with bevacizumab/everolimus without docetaxel was allowed after ≥ 6 cycles.

RESULTS

Forty-three subjects were treated across all dose levels. Maximal tolerated doses for the combined therapies observed in the phase 1B portion of the trial were: docetaxel 75 mg/m2, bevacizumab 15 mg/kg, and everolimus 2.5 mg. Maximal prostate-specific antigen decline ≥ 30% and ≥ 50% was achieved in 33 (79%) and 31 (74%) of patients, respectively. Best response by modified Response Evaluation Criteria In Solid Tumors criteria in 25 subjects with measurable disease at baseline included complete or partial response in 20 (80%) patients. The median progression-free and overall survival were 8.9 months (95% confidence interval, 7.4-10.6 months) and 21.9 months (95% confidence interval, 18.4-30.3 months), respectively. Hematologic toxicities were the most common treatment-related grade ≥ 3 adverse events including: febrile neutropenia (12; 28%), lymphopenia (12; 28%), leukocytes (10; 23%), neutrophils (9; 21%), and hemoglobin (2; 5%). Nonhematologic grade ≥ 3 adverse events included: hypertension (8; 19%), fatigue (3; 7%), pneumonia (3; 7%), and mucositis (4; 5%). There was 1 treatment-related death owing to neutropenic fever and pneumonia in a patient treated at dose level 3 despite dose modifications and prophylactic growth factor support.

CONCLUSIONS

Docetaxel, bevacizumab, and everolimus can be safely administered in CRPC and demonstrate a significant level of anticancer activity, meeting the predetermined response criteria. However, any potential benefit of combined therapy must be balanced against increased risk for toxicities. Our results do not support the hypothesis that this combination of agents improves upon the results obtained with docetaxel monotherapy in an unselected population of chemotherapy-naive patients with CRPC.

Phase II trial of the PI3 kinase inhibitor buparlisib (BKM-120) with or without enzalutamide in men with metastatic castration resistant prostate cancer

BACKGROUND

Phosphatidylinositol-3-kinase (PI3K) and androgen receptor pathway activation is common in metastatic castration resistant prostate cancer (mCRPC). Buparlisib is an oral, pan-class I PI3 kinase inhibitor.

METHODS

This was a multisite single arm phase II trial of buparlisib 100 mg ± enzalutamide daily in men with mCRPC whose disease progressed on or who were not candidates for docetaxel. The primary end-point was the rate of radiographic/clinical progression-free survival (PFS) at 6 months.

RESULTS

Thirty men were accrued: 67% post-docetaxel; median prostate specific antigen (PSA) was 70 ng/dl, 83% had ≥4 prior therapies for mCRPC; 43% received concurrent enzalutamide. The final 6 month PFS rate was estimated to be 10% (95% confidence interval 2.5-23.6%). Median PFS was 1.9 months and was 3.5 months with concurrent enzalutamide. Median overall survival was 10.6 months. Concurrent enzalutamide led to a five-fold reduction in buparlisib concentrations. PSA declines were observed in 23%; no patients achieved a ≥50% decline, and no radiographic responses were observed. Severe adverse events occurred in four men including respiratory infection and multi-organ failure, urinary tract obstruction, confusion and one seizure in the setting of a new central nervous system (CNS) metastasis. Grade III adverse events were seen in 43% of patients; common toxicities included grade I-II weight loss, diarrhoea, nausea, fatigue, anorexia, rash, hyperglycemia and anxiety/mood disorders.

CONCLUSIONS

Buparlisib did not demonstrate significant activity in men with mCRPC, suggesting that PI3K inhibition is not sufficient to reverse resistant mCRPC progression. Future studies of PI3K pathway inhibitors with concurrent enzalutamide should develop optimal dosing and focus on selected patients more likely to benefit.

AR Signaling and the PI3K Pathway in Prostate Cancer

Prostate cancer is a leading cause of cancer-related death in men worldwide. Aberrant signaling in the androgen pathway is critical in the development and progression of prostate cancer. Despite ongoing reliance on androgen receptor (AR) signaling in castrate resistant disease, in addition to the development of potent androgen targeting drugs, patients invariably develop treatment resistance. Interactions between the AR and PI3K pathways may be a mechanism of treatment resistance and inhibitors of this pathway have been developed with variable success. Herein we outline the role of the PI3K pathway in prostate cancer and, in particular, its association with androgen receptor signaling in the pathogenesis and evolution of prostate cancer, as well as a review of the clinical utility of PI3K targeting.

A Phase I Study of Abiraterone Acetate Combined with BEZ235, a Dual PI3K/mTOR Inhibitor, in Metastatic Castration Resistant Prostate Cancer

LESSONS LEARNED

The combination of standard dose abiraterone acetate and BEZ235, a pan-class I PI3K and mTORC1/2 inhibitor, was poorly tolerated in men with progressive mCRPC.Although the clinical development of BEZ235 has been discontinued in prostate cancer, agents that more selectively target PI3K-AKT-mTOR signaling may have a more favorable therapeutic index and should continue to be explored.

BACKGROUND

Androgen receptor (AR) and phosphatidylinositol-3 kinase (PI3K) signaling are two commonly perturbed pathways in prostate cancer. Preclinical data have shown that the two pathways compensate for each other when one is inhibited, and combined inhibition of AR and PI3K signaling may be a viable strategy to prevent or overcome castration resistance.

METHODS

This phase I study evaluated the safety and tolerability of abiraterone acetate and prednisone combined with BEZ235, a dual PI3K and mTORC1/2 inhibitor, in men with progressive metastatic castration resistant prostate cancer (mCRPC) who have not received prior chemotherapy.

RESULTS

Six patients (n = 6) were treated at the starting dose level of abiraterone acetate 1,000 mg with prednisone 5 mg twice daily and BEZ235 200 mg twice daily in a 3 + 3 dose escalation design. The study was terminated early because three of the six patients (50%) experienced dose-limiting toxicities: grade 3 mucositis, grade 3 hypotension, and grade 4 dyspnea and pneumonitis. All six patients had previously progressed on abiraterone/prednisone. The median treatment duration was 27 days (range: 3-130 days). No prostate-specific antigen (PSA) decline or objective response were observed.

CONCLUSION

The combination of standard-dose abiraterone/prednisone with BEZ235 200 mg twice daily was poorly tolerated in patients with mCRPC. The on-target and off-target effects of dual PI3K and mTORC inhibition likely contributed to the unacceptable toxicity profile. The Oncologist 2017;22:503-e43.

Noninvasive Measurement of mTORC1 Signaling with 89Zr-Transferrin

Purpose: mTOR regulates many normal physiological processes and when hyperactive can drive numerous cancers and human diseases. However, it is very challenging to detect and quantify mTOR signaling noninvasively in clinically relevant animal models of disease or man. We hypothesized that a nuclear imaging tool measuring intracellular mTOR activity could address this unmet need.Experimental Design: Although the biochemical activity of mTOR is not directly amenable to nuclear imaging probe development, we show that the transferrin receptor can be used to indirectly measure intracellular changes in mTOR activity.Results: After verifying that the uptake of radiolabeled transferrin (the soluble ligand of the transferrin receptor) is stimulated by active mTORC1 in vitro, we showed that ⁸⁹Zr-labeled transferrin (Tf) can measure mTORC1 signaling dynamics in normal and cancerous mouse tissues with PET. Finally, we show that ⁸⁹Zr-Tf can detect the upregulation of mTORC1 by tumor cells to escape the antitumor effects of a standard-of-care antiandrogen, which is to our knowledge the first example of applying PET to interrogate the biology of treatment resistant cancer.Conclusions: In summary, we have developed the first quantitative assay to provide a comprehensive measurement of mTOR signaling dynamics in vivo, in specific normal tissues, and during tumor development in genetically engineered animal models using a nuclear imaging tool that is readily translatable to man. Clin Cancer Res; 23(12); 3045-52. ©2016 AACR.

Investigational serine/threonine kinase inhibitors against prostate cancer metastases

INTRODUCTION

Androgen deprivation therapy (ADT) is used as first therapeutic approach in prostate cancer (PCa) although castration resistant disease (CRPC) develops with high frequency. CRPC is the consequence of lack of apoptotic responses to ADT. Alternative targeting of the androgen axis with abiraterone and enzalutamide, as well as taxane-based chemotherapy were used in CRPC. Serine/threonine protein kinases (STKs) regulate different molecular pathways of normal and neoplastic cells and participate to development of CRPC as well as to the progression towards a bone metastatic disease (mCRPC). Areas covered: The present review provide data on STK expression and activity in the development of CRPC as well as summarize recent reports of different strategies to block STK activity for the control of PCa progression. Expert Opinion: Inhibitors for different STKs have been developed but clinical trials in PCa are comparatively rare and few exhibit satisfactory 'drug-like' properties. It is, however, necessary to intensify, when possible, the number of clinical trials with these drugs in order to insert new therapies or combinations with standard hormone- and chemo-therapies in the treatment guidelines of the mPCA.

Castration Resistance in Prostate Cancer Is Mediated by the Kinase NEK6

In prostate cancer, the development of castration resistance is pivotal in progression to aggressive disease. However, understanding of the pathways involved remains incomplete. In this study, we performed a high-throughput genetic screen to identify kinases that enable tumor formation by androgen-dependent prostate epithelial (LHSR-AR) cells under androgen-deprived conditions. In addition to the identification of known mediators of castration resistance, which served to validate the screen, we identified a mitotic-related serine/threonine kinase, NEK6, as a mediator of androgen-independent tumor growth. NEK6 was overexpressed in a subset of human prostate cancers. Silencing NEK6 in castration-resistant cancer cells was sufficient to restore sensitivity to castration in a mouse xenograft model system. Tumors in which castration resistance was conferred by NEK6 were predominantly squamous in histology with no evidence of AR signaling. Gene expression profiling suggested that NEK6 overexpression stimulated cytoskeletal, differentiation, and immune signaling pathways and maintained gene expression patterns normally decreased by castration. Phosphoproteome profiling revealed the transcription factor FOXJ2 as a novel NEK6 substrate, with FOXJ2 phosphorylation associated with increased expression of newly identified NEK6 transcriptional targets. Overall, our studies establish NEK6 signaling as a central mechanism mediating castration-resistant prostate cancer. Cancer Res; 77(3); 753-65. ©2016 AACR.

A phase 2 clinical trial of everolimus plus bicalutamide for castration-resistant prostate cancer

BACKGROUND

The mammalian target of rapamycin (mTOR) pathway is up-regulated in castration-resistant prostate cancer (CRPC). Nevertheless, inhibition of mTOR is ineffective in inducing apoptosis in prostate cancer cells, likely because of the compensatory up-regulation of the androgen receptor (AR) pathway.

METHODS

Patients who were eligible for this study had to have progressive CRPC with serum testosterone levels <50 ng/dL. No prior bicalutamide (except to prevent flare) or everolimus was allowed. Treatment included oral bicalutamide 50 mg and oral everolimus 10 mg, both once daily, with a cycle defined as 4 weeks. The primary endpoint was the prostate-specific antigen (PSA) response (≥30% reduction) from baseline. A sample size of 23 patients would have power of 0.8 and an α error of .05 (1-sided) if the combination had a PSA response rate of 50% versus a historic rate of 25% with bicalutamide alone.

RESULTS

Twenty-four patients were enrolled. The mean age was 71.1 years (range, 53.0-87.0 years), the mean PSA level at study entry was 43.4 ng/dL (range, 2.5-556.9 ng/dL), and the mean length of treatment was 8 cycles (range, 1.0-23.0 cycles). Of 24 patients, 18 had a PSA response (75%; 95% confidence interval [CI], 0.53-0.90), whereas 15 (62.5%; 95% CI, 0.41-0.81) had a PSA decrease ≥50%. The median overall survival was 28 months (95% CI, 14.1-42.7 months). Fourteen patients (54%; 95% CI, 0.37-0.78) developed grade 3 (13 patients) or grade 4 (1 patient with sepsis) adverse events that were attributable to treatment.

CONCLUSIONS

The combination of bicalutamide and everolimus has encouraging efficacy in men with bicalutamide-naive CRPC, thus warranting further investigation. A substantial number of patients experienced everolimus-related toxicity. Cancer 2016;122:1897-904. © 2016 American Cancer Society.

Cotargeting Androgen Receptor Splice Variants and mTOR Signaling Pathway for the Treatment of Castration-Resistant Prostate Cancer

PURPOSE

The PI3K/Akt/mTOR pathway is activated in most castration-resistant prostate cancers (CRPC). Transcriptionally active androgen receptor (AR) plays a role in the majority of CRPCs. Therefore, cotargeting full-length (FL) AR and PI3K/Akt/mTOR signaling has been proposed as a possible, more effective therapeutic approach for CRPC. However, truncated AR-splice variants (AR-V) that are constitutively active and dominant over FL-AR are associated with tumor progression and resistance mechanisms in CRPC. It is currently unknown how blocking the PI3K/Akt/mTOR pathway impacts prostate cancer driven by AR-Vs. Here, we evaluated the efficacy and mechanism of combination therapy to block mTOR activity together with EPI-002, an AR N-terminal domain (NTD) antagonist that blocks the transcriptional activities of FL-AR and AR-Vs in models of CRPC.

EXPERIMENTAL DESIGN

To determine the functional roles of FL-AR, AR-Vs, and PI3K/Akt/mTOR pathways, we employed EPI-002 or enzalutamide and BEZ235 (low dose) or everolimus in human prostate cancer cells that express FL-AR or FL-AR and AR-Vs (LNCaP95). Gene expression and efficacy were examined in vitro and in vivo

RESULTS

EPI-002 had antitumor activity in enzalutamide-resistant LNCaP95 cells that was associated with decreased expression of AR-V target genes (e.g., UBE2C). Inhibition of mTOR provided additional blockade of UBE2C expression. A combination of EPI-002 and BEZ235 decreased the growth of LNCaP95 cells in vitro and in vivo

CONCLUSIONS

Cotargeting mTOR and AR-NTD to block transcriptional activities of FL-AR and AR-Vs provided maximum antitumor efficacy in PTEN-null, enzalutamide-resistant CRPC. Clin Cancer Res; 22(11); 2744-54. ©2015 AACR.

Activity of Artemisia annua and artemisinin derivatives, in prostate carcinoma

BACKGROUND

Artemisia annua L, artemisinin and artesunate reveal profound activity not only against malaria, but also against cancer in vivo and clinical trials. Longitudinal observations on the efficacy of A. annua in patients are, however missing as of yet.

METHODS

Clinical diagnosis was performed by imaging techniques (MRT, scintigraphy, SPECT/CT) and blood examinations of standard parameters from clinical chemistry. Immunohistochemistry of formalin-fixed, paraffin-embedded tumor material was performed to determine the expression of several biomarkers (cycloxygenase-2 (COX2), epidermal growth factor receptor (EGFR), glutathione S-transferase P1 (GSTP1), Ki-67, MYC, oxidized low density lipoprotein (lectin-like) receptor 1 (LOX1), p53, P-glycoprotein, transferrin receptor (TFR, CD71), vascular endothelial growth factor (VEGF), von Willebrand factor (CD31)). The immunohistochemical expression has been compared with the microarray-based mRNA expression of these markers in two prostate carcinoma cell lines (PC-3, DU-145).

RESULTS

A patient with prostate carcinoma (pT3bN1M1, Gleason score 8 (4+4)) presented with a prostate specific antigen (PSA) level >800 µg/l. After short-term treatment with bacalitumide (50 mg/d for 14 days) and long-term oral treatment with A. annua capsules (continuously 5 × 50 mg/d), the PSA level dropped down to 0.98 µg/l. MRT, scintigraphy and SPECT/CT verified tumor remission. Seven months later, PSA and ostase levels increased, indicating tumor recurrence and skeletal metastases. Substituting A. annua capsules by artesunate injections (2 × 150 mg twice weekly i.v.) did not prohibit tumor recurrence. PSA and ostase levels rose to 1245 µg/l and 434 U/l, respectively, and MRT revealed progressive skeletal metastases, indicating that the tumor acquired resistance. The high expression of MYC, TFR, and VEGFC in the patient biopsy corresponded with high expression of these markers in the artemisinin-sensitive PC-3 cells compared to artemisinin-resistant DU-145 cells.

CONCLUSION

Long-term treatment with A. annua capsules combined with short-term bicalitumide treatment resulted in considerable regression of advanced metastasized prostate carcinoma. Controlled clinical trials are required to evaluate the clinical benefit of A. annua in prostate cancer.

Persistent androgen receptor addiction in castration-resistant prostate cancer

It is now understood that persistent activation of the androgen receptor (AR) signaling pathway often underlies the development of castration-resistant prostate cancer (CRPC). This realization led to renewed interest in targeting the AR and ultimately to the development of the potent next-generation AR-directed agents abiraterone and enzalutamide. While these drugs prolong survival in men with CRPC, they are unfortunately not curative. Perhaps not surprisingly, evidence points to persistent AR signaling as one of the key drivers by which resistances to these agents develops. In this context, activation of the AR signaling program can occur through a number of molecular adaptations, including alterations leading to persistent canonical AR signaling (e.g., AR amplification/overexpression, elucidations/concentration of intratumoral androgens), activation of the AR program via feedback pathways (e.g., AKT/mTOR/Pi3K, HER2/Neu), and activation of the AR program via mutation or substitution (e.g., AR ligand binding domain mutation; AR splice variants; glucocorticoid receptor signaling). This review will provide an overview of the more clinical relevant (i.e., druggable) pathways that have been implicated in the emergence of drug resistance in men with CRPC and highlight some of the ongoing efforts towards developing therapeutics to impair these mechanisms.

Phase II Trial of Carboplatin, Everolimus, and Prednisone in Metastatic Castration-resistant Prostate Cancer Pretreated With Docetaxel Chemotherapy: A Prostate Cancer Clinical Trial Consortium Study

OBJECTIVE

To conduct a phase II trial of the combination of carboplatin, prednisone, and everolimus in metastatic castrate-resistant prostate cancer (mCRPC) as mTOR inhibition can overcome resistance to chemotherapy in prostate cancer.

METHODS

Patients with progressive mCRPC pretreated with docetaxel-based regimen were eligible. Performance status of 0-1 and adequate bone marrow, renal, and liver function were required. Primary end point was time to progression. Treatment consisted of carboplatin (starting dose equal to area under the curve (AUC of 5) intravenously every 21 days along with oral everolimus 5 mg once daily and prednisone 5 mg twice daily.

RESULTS

Twenty-six patients were enrolled with median age of 69 years with 8 patients of African American origin. Grade 3 or 4 thrombocytopenia or neutropenia in 4 of 6 initial patients required dose adjustment of carboplatin to AUC of 4 for subsequent patients. There were no pharmacokinetic interactions between carboplatin and everolimus. The median time to progression was 2.5 months (90% confidence interval [CI], 1.8-4.3 months), and median overall survival was 12.5 months (90% CI, 7.7-18.7 months). Of 10 patients, 8 that demonstrated positive nuclear phosphorylated AKT (pAKT) staining on immunohistochemistry progressed within 9 weeks, whereas 2 patients with negative staining continued without progression for prolonged durations of 30 and 48 weeks. TSC1 gene mutations did not correlate with clinical outcome.

CONCLUSION

The addition of the mTOR inhibitor everolimus to carboplatin demonstrated minimal clinical efficacy in metastatic prostate cancer. pAKT testing warrants further evaluation as a predictive marker of response to everolimus therapy.

Evaluation of in vivo responses of sorafenib therapy in a preclinical mouse model of PTEN-deficient of prostate cancer

Background

Despite recent advances in the treatment for advanced prostate cancer, outcomes remain poor. This lack of efficacy has prompted the development of alternative treatment strategies. In the present study we investigate the effects of the multikinase inhibitor sorafenib in a genetically engineered mouse model of prostate cancer and explore the rational combination with the mTOR inhibitor everolimus.

Methods

Conditional prostate specific PTEN-deficient knockout mice were utilized to determine the pharmacodynamic and chemopreventive effects of sorafenib. This mouse model was also used to examine the therapeutic efficacy of sorafenib alone or in combination with everolimus. Preclinical efficacy was assessed by comparing the reduction of tumor burden, proliferation, angiogenesis and the induction of apoptosis. Molecular responses were assessed by immunohistochemical, TUNEL and western blot assays.

Results

Pharmacodynamic analysis revealed that a single dose of sorafenib decreased activation of the PI3K/AKT/mTOR signaling axis at doses of 30–60 mg/kg, but activated JAK/STAT3 signaling. Levels of cleaved casapase-3 increased in a dose dependent manner. Chemoprevention studies showed that chronic sorafenib administration was capable of inhibiting tumor progression through the reduction of cancer cell proliferation, angiogenesis and the induction of apoptosis. In intervention models of established castration-naïve and castration-resistant prostate cancer, treatment with sorafenib provided modest but statistically insignificant reduction in tumor burden. However, sorafenib significantly inhibited cancer cell proliferation and MVD but had minimal effects on the induction of apoptosis. Interestingly, the administration of sorafenib increased the expression levels of the androgen receptor, p-GSK3β and p-ERK1/2 in castration-resistant prostate cancers. In both intervention models, combination therapy demonstrated a clear tendency of enhanced antitumor effects over monotherapy. Notably, the treatment combination of sorafenib and everolimus overcame therapeutic escape from single agent therapy in castration-resistant prostate cancers.

Conclusions

In summary, we provide insights into the molecular responses of sorafenib therapy in a clinically relevant model of prostate cancer and present preclinical evidence for the development of targeted treatment strategies based on the use of multikinase inhibitors in combination with mTOR inhibitors for the treatment of advanced prostate cancer.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-015-0509-x) contains supplementary material, which is available to authorized users.

PI3K-AKT-mTOR signaling in prostate cancer progression and androgen deprivation therapy resistance

Prostate cancer (PCa) is the second most common malignancy among men in the world. Castration-resistant prostate cancer (CRPC) is the lethal form of the disease, which develops upon resistance to first line androgen deprivation therapy (ADT). Emerging evidence demonstrates a key role for the PI3K-AKT-mTOR signaling axis in the development and maintenance of CRPC. This pathway, which is deregulated in the majority of advanced PCas, serves as a critical nexus for the integration of growth signals with downstream cellular processes such as protein synthesis, proliferation, survival, metabolism and differentiation, thus providing mechanisms for cancer cells to overcome the stress associated with androgen deprivation. Furthermore, preclinical studies have elucidated a direct connection between the PI3K-AKT-mTOR and androgen receptor (AR) signaling axes, revealing a dynamic interplay between these pathways during the development of ADT resistance. Thus, there is a clear rationale for the continued clinical development of a number of novel inhibitors of the PI3K pathway, which offer the potential of blocking CRPC growth and survival. In this review, we will explore the relevance of the PI3K-AKT-mTOR pathway in PCa progression and castration resistance in order to inform the clinical development of specific pathway inhibitors in advanced PCa. In addition, we will highlight current deficiencies in our clinical knowledge, most notably the need for biomarkers that can accurately predict for response to PI3K pathway inhibitors.

A phase I study of everolimus and docetaxel in patients with castration-resistant prostate cancer

BACKGROUND

The PTEN tumor suppressor is frequently lost in CRPC, with activation of Akt-mTOR signaling, driving growth. We conducted a phase I trial of the mTOR inhibitor, everolimus, and docetaxel in CRPC.

PATIENTS AND METHODS

Eligible patients had progressive, metastatic, chemotherapy-naive CRPC. Patients received everolimus 10 mg daily for 2 weeks and underwent a restaging FDG-PET/computed tomography scan. Patient cohorts were subsequently treated at 3 dose levels of everolimus with docetaxel: 5 mg to 60 mg/m(2), 10 mg to 60 mg/m(2), and 10 mg to 70 mg/m(2). The primary end point was the safety and tolerability of combination therapy.

RESULTS

Accrual was 4 patients at level 1, 3 patients at level 2, and 8 patients at level 3. Common toxicities were hematologic and fatigue. Serum concentrations of everolimus when administered with docetaxel were 1.5 to 14.8 ng/mL in patients receiving 5 mg everolimus and 4.5 to 55.4 ng/mL in patients receiving 10 mg everolimus. Four patients had partial metabolic response (PMR) using FDG-PET, 12 had stable metabolic disease, and 2 had progressive metabolic disease after a 2-week treatment with everolimus alone. Five of 12 evaluable patients experienced a prostate-specific antigen (PSA) reduction ≥ 50% during treatment with everolimus together with docetaxel. All 4 patients with a PMR according to PET imaging experienced a PSA reduction in response to everolimus with docetaxel, and 3 of 4 had PSA declines ≥ 50%.

CONCLUSION

Everolimus 10 mg daily and docetaxel 60 mg/m(2) was safe in CRPC patients and these were the recommended doses in combination. FDG-PET response might serve as a biomarker for target inhibition by mTOR inhibitors.

Understanding the mechanisms of androgen deprivation resistance in prostate cancer at the molecular level

CONTEXT

Various molecular mechanisms play a role in the development of resistance to androgen deprivation therapy in castration-resistant prostate cancer (CRPC).

OBJECTIVE

To understand the mechanisms and biological pathways associated with the progression of prostate cancer (PCa) under systemic androgen depletion or administration of the novel antiandrogens abiraterone, enzalutamide, and ARN-509. This review also examines the introduction of novel combinational approaches for patients with CRPC.

EVIDENCE ACQUISITION

PubMed was the data source. Keywords for the search were castrate resistant prostate cancer, abiraterone, enzalutamide resistance mechanisms, resistance to androgen deprivation, AR mutations, amplifications, splice variants, and AR alterations. Papers published before 1990 were excluded from the review, and only English-language papers were included.

EVIDENCE SYNTHESIS

This review summarizes the current literature regarding the mechanisms implicated in the development of CRPC and the acquisition of resistance to novel antiandrogen axis agents. The review focuses on androgen biosynthesis in the tumor microenvironment, androgen receptor (AR) alterations and post-transcriptional modifications, the role of glucocorticoid receptor, and alternative oncogenic signaling that is derepressed on maximum AR inhibition and thus promotes cancer survival and progression.

CONCLUSIONS

The mechanisms implicated in the development of resistance to AR inhibition in PCa are multiple and complex, involving virtually all classes of genomic alteration and leading to a host of selective/adaptive responses. Combinational therapeutic approaches targeting both AR signaling and alternative oncogenic pathways may be reasonable for patients with CRPC.

PATIENT SUMMARY

We looked for mechanisms related to the progression of PCa in patients undergoing hormonal therapy and treatment with novel drugs targeting the AR. Based on recent data, combining maximal AR inhibition with novel agents targeting other tumor-compensatory, non-AR-related pathways may improve the survival and quality of life of patients with castration-resistant PCa.

Metformin reverses multidrug resistance in human hepatocellular carcinoma Bel‑7402/5‑fluorouracil cells

Metformin exhibits anti‑proliferative effects in tumor cells in vitro and in vivo. The present study investigated the ability of metformin to reverse multidrug resistance (MDR) in human hepatocellular carcinoma Bel‑7402/5‑fluorouracil (5‑Fu; Bel/Fu) cells. The synergistic anti‑proliferative effect of metformin combined with 5‑Fu was evaluated using a Cell Counting kit‑8 assay. The variation in apoptotic rates and cell cycle distribution were evaluated using a flow cytometric assay and variations in target gene and protein expression were monitored using reverse transcription‑polymerase chain reaction and western blot analysis. The results demonstrated that metformin had a synergistic anti‑proliferative effect with 5‑Fu in the Bel/Fu cells. The variations in the number of apoptotic cells and distribution of the cell cycle were consistent with the variability in cell viability. Metformin targeted the AMP-activated protein kinase (AMPK)/mammalian target of rapamycin (mTOR) pathway, suppressed the expression of hypoxia‑inducible factor‑1α (HIF‑1α) and transcriptionally downregulated the expression of multidrug resistance protein 1/P‑glycoprotein (P‑gp) and multidrug resistance‑associated protein 1 (MRP1). Collectively, these findings suggested that metformin may target the AMPK/mTOR/HIF‑1α/P‑gp and MRP1 pathways to reverse MDR in hepatocellular carcinoma.

High Efficacy of Combination Therapy Using PI3K/AKT Inhibitors with Androgen Deprivation in Prostate Cancer Preclinical Models

BACKGROUND

The phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/AKT pathway is frequently activated during prostate cancer (PCa) progression through loss or mutation of the phosphatase and tensin homolog (PTEN) gene. Following the androgen receptor (AR) pathway, it is the second major driver of PCa growth.

OBJECTIVE

To assess efficacy of novel PI3K/AKT-targeted therapies in PCa models, as a single agent and in combination with androgen deprivation.

DESIGN, SETTING, AND PARTICIPANTS

Twelve human PCa cell lines were tested in vitro for sensitivity to the AKT inhibitor AZD5363 and the PI3K beta/delta inhibitor AZD8186. The combination of AZD5363 and AZD8186 with castration was evaluated in vivo in PTEN-negative versus PTEN-positive patient-derived xenografts. Tumors and plasma were collected for biomarker analysis.

OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS

In vitro growth inhibition was determined by methylthiazolyldiphenyl-tetrazolium bromide assay. In vivo efficacy was monitored by caliper measurements of subcutaneous tumor volume. PI3K/AKT and AR pathway activity was analyzed by Western blot, enzyme-linked immunosorbent assay, and real-time polymerase chain reaction.

RESULTS AND LIMITATIONS

AZD5363 and AZD8186 inhibited in vitro growth of 10 of 12 and 7 of 12 PCa cell lines, respectively, with increased sensitivity under androgen depletion. In vivo, AZD5363 and AZD8186 as single agents significantly inhibited growth of PTEN-negative PC346C xenografts compared to placebo by 60% and 66%, respectively. Importantly, combination of either agent with castration resulted in long-lasting tumor regression, which persisted after treatment cessation. Expression of AR-target genes kallikrein-related peptidase 3 (KLK3, also known as PSA); transmembrane protease, serine 2 (TMPRSS2); and FK506 binding protein 5 (FKBP5) was upregulated after PI3K/AKT inhibition. Neither compound inhibited tumor growth in the PTEN-positive PC310 model.

CONCLUSIONS

Combination with hormonal therapy improved efficacy of PI3K/AKT-targeted agents in PTEN-negative PCa models. Upregulation of AR-target genes upon PI3K/AKT inhibition suggests a compensatory crosstalk between the PI3K-AR pathways. These data strongly advocate for further clinical evaluation.

PATIENT SUMMARY

Inactivation of the PTEN gene is a common event promoting prostate cancer (PCa) progression. This preclinical study illustrates the potent anticancer activity of novel PTEN-targeted drugs on PCa models, particularly in combination with hormonal therapy.

Growth factor and signaling pathways and their relevance to prostate cancer therapeutics

Treatments that target the androgen axis represent an effective strategy for patients with advanced prostate cancer, but the disease remains incurable and new therapeutic approaches are necessary. Significant advances have recently occurred in our understanding of the growth factor and signaling pathways that are active in prostate cancer. In conjunction with this, many new targeted therapies with sound preclinical rationale have entered clinical development and are being tested in men with castration-resistant prostate cancer. Some of the most relevant pathways currently being exploited for therapeutic gain are HGF/c-Met signaling, the PI3K/AKT/mTOR pathway, Hedgehog signaling, the endothelin axis, Src kinase signaling, the IGF pathway, and angiogenesis. Here, we summarize the biological basis for the use of selected targeted agents and the results from available clinical trials of these drugs in men with prostate cancer.

Expression of phosphorylated-mTOR during the development of prostate cancer

BACKGROUND

The PI3K pathway plays a significant role in the progression of prostate cancer (PCa) to an advanced stage. Mouse models suggest that the downstream effector molecule of the PI3K pathway, mTOR, is also important in the development of PCa, where it plays a pivotal role in forming precursor lesions such as high grade prostatic intraepithelial neoplasia (HGPIN). This study was conducted to determine the status of phosphorylated-mTOR (p-mTOR the activated state of mTOR) across the PCa progression model by looking at expression in normal prostate tissue, proliferative inflammatory atrophy (PIA), HGPIN, and PCa.

METHODS

Expression of p-mTOR was evaluated by immunohistochemistry on tissue microarrays constructed from 120 archival formalin-fixed paraffin embedded radical prostatectomy tissue specimens. Levels of expression were recorded as the percentage of positive epithelial cells multiplied by the intensity of staining scored as 0-3.

RESULTS

p-mTOR expression was found to increase across the progression model with mean staining in non-neoplastic samples of 40 compared to 98 in PIA, 107 in HGPIN, and 136 in cancer (P < 0.001), but without significant increase between HGPIN and PIA. Correlation of high p-mTOR expression with outcome in PCa showed a trend towards worse prognosis, but this was not statistically significant.

CONCLUSIONS

This study demonstrates that p-mTOR signaling has a potential role in both the initiation and progression of PCa. These data provide support for further research into the possible use of rapamycin analogues in the treatment of PCa, and raise the possibility that mTOR might be a potential target for chemoprevention.

Targeting the PI3K/Akt pathway in prostate cancer: challenges and opportunities (review)

The PI3K/Akt pathway is an actively pursued therapeutic target in oncology. In prostate cancer, the activation of this pathway appears to be characteristic of many aggressive prostate cancers. Further, activation of the PI3K/Akt pathway is more frequently observed as prostate cancer progresses toward a resistant, metastatic disease. Signalling from this pathway activates numerous survival, growth, metabolic and metastatic functions characteristic of aggressive cancer. Biomarkers of this pathway have correlated activation of this pathway to high grade disease and higher risk of disease progression. Therefore there is significant interest in developing effective strategies to target this pathway in prostate cancer. In this review, we discuss the pre-clinical and clinical data relevant to targeting of the PI3K/Akt pathway in prostate cancer. In particular, we review the rationale and relevance of co-targeting approaches against the PI3K/Akt pathway. It is anticipated that through an improved understanding of the biology of the PI3K/Akt pathway in prostate cancer, relevant biomarkers and rationale combination therapies will optimize targeting of this pathway to improve outcomes among patients with aggressive prostate cancer.

In vivo quantitative phosphoproteomic profiling identifies novel regulators of castration-resistant prostate cancer growth

Prostate cancer remains a leading cause of cancer-related mortality worldwide owing to our inability to treat effectively castration-resistant tumors. To understand the signaling mechanisms sustaining castration-resistant growth, we implemented a mass spectrometry-based quantitative proteomic approach and use it to compare protein phosphorylation in orthotopic xenograft tumors grown in either intact or castrated mice. This investigation identified changes in phosphorylation of signaling proteins such as MEK, LYN, PRAS40, YAP1 and PAK2, indicating the concomitant activation of several oncogenic pathways in castration-resistant tumors, a notion that was confirmed by tumor transcriptome analysis. Further analysis demonstrated that the activation of mTORC1, PAK2 and the increased levels of YAP1 in castration-resistant tumors can be explained by the loss of androgen inhibitory actions. The analysis of clinical samples demonstrated elevated levels of PAK2 and YAP1 in castration-resistant tumors, whereas knockdown experiments in androgen-independent cells demonstrated that both YAP1 and PAK2 regulate cell colony formation and cell invasion activity. PAK2 also influenced cell proliferation and mitotic timing. Interestingly, these phenotypic changes occur in the absence of obvious alterations in the activity of AKT, MAPK or mTORC1 pathways, suggesting that PAK2 and YAP1 may represent novel targets for the treatment of castration-resistant prostate cancer. Pharmacologic inhibitors of PAK2 (PF-3758309) and YAP1 (Verteporfin) were able to inhibit the growth of androgen-independent PC3 xenografts. This work demonstrates the power of applying high-resolution mass spectrometry in the proteomic profiling of tumors grown in vivo for the identification of novel and clinically relevant regulatory proteins.