Farnesyltransferase inhibitor effects on prostate tumor micro-environment and radiation survival

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.

Radiosensitization in prostate cancer: mechanisms and targets

Prostate cancer is the second most commonly diagnosed cancer in American men over the age of 45 years and is the third most common cause of cancer related deaths in American men. In 2012 it is estimated that 241,740 men will be diagnosed with prostate cancer and 28,170 men will succumb to prostate cancer. Currently, radiation therapy is one of the most common definitive treatment options for localized prostate cancer. However, significant number of patients undergoing radiation therapy will develop locally persistent/recurrent tumours. The varying response rates to radiation may be due to 1) tumor microenvironment, 2) tumor stage/grade, 3) modality used to deliver radiation, and 4) dose of radiation. Higher doses of radiation has not always proved to be effective and have been associated with increased morbidity. Compounds designed to enhance the killing effects of radiation, radiosensitizers, have been extensively investigated over the past decade. The development of radiosensitizing agents could improve survival, improve quality of life and reduce costs, thus benefiting both patients and healthcare systems. Herin, we shall review the role and mechanisms of various agents that can sensitize tumours, specifically prostate cancer.

Lonafarnib (SCH66336) improves the activity of temozolomide and radiation for orthotopic malignant gliomas

Malignant gliomas are highly lethal tumors resistant to current therapies. The standard treatment modality for these tumors, surgical resection followed by radiation therapy and concurrent temozolomide, has demonstrated activity, but development of resistance and disease progression is common. Although oncogenic Ras mutations are uncommon in gliomas, Ras has been found to be constitutively activated through the action of upstream signaling pathways, suggesting that farnesyltransferase inhibitors may show activity against these tumors. We now report the in vitro and orthotopic in vivo results of combination therapy using radiation, temozolomide and lonafarnib (SCH66336), an oral farnesyl transferase inhibitor, in a murine model of glioblastoma. We examined the viability, proliferation, farnesylation of H-Ras, and activation of downstream signaling of combination-treated U87 cells in vitro. Lonafarnib alone or in combination with radiation and temozolomide had limited tumor cell cytotoxicity in vitro although it did demonstrate significant inhibition in tumor cell proliferation. In vivo, lonafarnib alone had a modest ability to inhibit orthotopic U87 tumors, radiation and temozolomide demonstrated better inhibition, while significant anti-tumor activity was found with concurrent lonafarnib, radiation, and temozolomide, with the majority of animals demonstrating a decrease in tumor volume. The use of tumor neurospheres derived from freshly resected adult human glioblastoma tissue was relatively resistant to both temozolomide and radiation therapy. Lonafarnib had a significant inhibitory activity against these neurospheres and could potentate the activity of temozolomide and radiation. These data support the continued research of high grade glioma treatment combinations of farnesyl transferase inhibitors, temozolomide, and radiation therapy.

SmgGDS is up-regulated in prostate carcinoma and promotes tumour phenotypes in prostate cancer cells

SmgGDS is a guanine nucleotide exchange factor with the unique ability to activate multiple small GTPases, implicating it in cancer development and progression. Here, we investigated the role of SmgGDS in prostate cancer by studying the expression of SmgGDS in benign and malignant prostatic tissues. We also probed SmgGDS function in three prostate carcinoma cell lines using small interfering RNA (siRNA). Immunohistochemical analysis revealed that SmgGDS levels were elevated in prostatic intraepithelial neoplasia (PIN), prostate carcinoma, and metastatic prostate carcinoma. In addition, expression of SmgGDS positively correlated with that of cyclooxygenase-2 (COX-2), a protein believed to promote the development of prostate carcinoma. Reduction of SmgGDS expression in prostate carcinoma cells inhibited proliferation and migration, irrespective of androgen receptor status. These effects were accompanied by a reduction in COX-2 expression and in activity of NF-kappaB, a known regulator of COX-2. Taken together, these findings suggest that SmgGDS promotes the development and progression of prostate cancer, possibly associated with NF-kappaB-dependent up-regulation of COX-2.

Influence of PEG-conjugated hemoglobin on tumor oxygenation and response to chemotherapy

Hypoxic tumors are significantly more malignant, metastatic, radio- and chemoresistant. The use of artificial oxygen carriers represents a new approach to the problem of hypoxia. In the present study, female athymic BALB/c nude mice bearing the cervical carcinoma were untreated or treated with cisplatin to determine whether administration of artificial oxygen carrier (PEG-conjugated Hemoglobin, PEG-Hb) could improve the tumor oxygenation and enhance the anti-tumor efficacy of cisplatin. Pimonidazole staining was employed to detect tumor tissue oxygenation status. We found that the application of a higher dose (0.6 g/kg) PEG-Hb could significantly ameliorate the hypoxic condition in cervical carcinoma xenograft models. Co-administration of PEG-Hb (0.6 g/kg) with cisplatin produced significant tumor growth inhibition and pro-apoptotic and anti-proliferative effects as compared to cisplatin alone. These suggest the evaluated PEG-Hb in this experiment has positive effects on cisplatin or cisplatin-based chemotherapy, and further work to optimize its application is warranted.

A phase II trial of 17-allylamino-17-demethoxygeldanamycin in patients with hormone-refractory metastatic prostate cancer

PURPOSE

17-Allylamino-17-demethoxygeldanamycin (17-AAG) is a benzoquinone ansamycin antibiotic with antiproliferative activity in several mouse xenograft models, including prostate cancer models. A two-stage phase II study was conducted to assess the activity and toxicity profile of 17-AAG administered to patients with metastatic, hormone-refractory prostate cancer.

EXPERIMENTAL DESIGN

Patients with at least one prior systemic therapy and a rising prostate-specific antigen (PSA) were eligible. Patients received 17-AAG at a dose of 300 mg/m2 i.v. weekly for 3 of 4 weeks. The primary objective was to assess the PSA response. Secondary objectives were to determine overall survival, to assess toxicity, and to measure interleukin-6, interleukin-8, and maspin levels and quality of life.

RESULTS

Fifteen eligible patients were enrolled. The median age was 68 years and the median PSA was 261 ng/mL. Patients received 17-AAG for a median number of two cycles. Severe adverse events included grade 3 fatigue (four patients), grade 3 lymphopenia (two patients), and grade 3 back pain (two patients). The median PSA progression-free survival was 1.8 months (95% confidence interval, 1.3-3.4 months). The 6-month overall survival was 71% (95% confidence interval, 52-100%).

CONCLUSIONS

17-AAG did not show any activity with regard to PSA response. Due to insufficient PSA response, enrollment was stopped at the end of first stage per study design. The most significant severe toxicity was grade 3 fatigue. Further evaluation of 17-AAG at a dose of 300 mg/m2 i.v. weekly as a single agent in patients with metastatic, hormone-refractory prostate cancer who received at least one prior systemic therapy is not warranted.

Hypoxia signalling through mTOR and the unfolded protein response in cancer

Hypoxia occurs in the majority of tumours, promoting angiogenesis, metastasis and resistance to therapy. Responses to hypoxia are orchestrated in part through activation of the hypoxia-inducible factor family of transcription factors (HIFs). Recently, two additional O(2)-sensitive signalling pathways have also been implicated: signalling through the mammalian target of rapamycin (mTOR) kinase and signalling through activation of the unfolded protein response (UPR). Although they are activated independently, growing evidence suggests that HIF-, mTOR- and UPR-dependent responses to hypoxia act in an integrated way, influencing each other and common downstream pathways that affect gene expression, metabolism, cell survival, tumorigenesis and tumour growth.

Clinical target promiscuity: lessons from ras molecular trials

Mutated ras has been identified in approximately 30% of human tumors, and dysregulation of ras function and signal transduction pathways is a critical step in tumorigenesis. Herein, we review the early data that supports the concept that the intrinsic radiosensitivity of tumor cells can be altered by oncogenic ras expression and that this impacts the PI3K-dependent signaling cascade. This ras-induced radioresistance can be reversed using prenyl transferase inhibitors (PTIs.). We discuss the effects of PTIs as a radiosensitizer in both in vivo and in vitro studies and show that PTIs can lead to increased radiosensitization in vivo through a variety of potential mechanisms that enhance radiation-induced cell kill. We critically evaluate the use of ras biomarkers in predicting the clinical response to PTIs that may explain the mixed results seen thus far in clinical trials using PTIs as a clinical radiosensitizer. We conclude that Ras-mediated radioresistance is the result of multiple intercommunicating pathways functioning against a complex genetic background and a solitary biomarker may not be adequate to predict for PTI-mediated radiosensitization. Nonetheless, our knowledge of the ras-signaling pathway has led to development and testing of specific therapies directed against PI3K-AKT signaling pathways as a future approach towards clinical radiosensitization.

Ras inhibition results in growth arrest and death of androgen-dependent and androgen-independent prostate cancer cells

Prostate cancer is one of the most frequently diagnosed cancers in human males. Progression of these tumors is facilitated by autocrine/paracrine growth factors which activate critical signaling cascades that promote prostate cancer cell growth, survival and migration. Among these, Ras pathways have a major role. Here we examined the effect of the Ras inhibitor S-trans, trans-farnesylthiosalicylic acid (FTS), on growth and viability of androgen-dependent and androgen-independent prostate cancer cells. FTS downregulated Ras, inhibited signaling to Akt and reduced the levels of cell-cycle regulatory proteins including cyclin D1, p-RB, E2F-1 and cdc42 in LNCaP and PC3 cells. Consequently the anchorage-dependent and anchorage-independent growth of LNCaP and PC3 cells were inhibited. FTS also induced apoptotic cell death which was inhibited by the broad-spectrum caspases inhibitor, Boc-asp-FMK. Our study demonstrated that androgen-dependent and androgen-independent prostate cancer cells require active Ras for growth and survival. Ras inhibition by FTS results in growth arrest and cell death. FTS may be qualified as a potential agent for the treatment of prostate cancer.

Overcoming physiologic barriers to cancer treatment by molecularly targeting the tumor microenvironment

It is widely recognized that the vasculature of the tumor is inadequate to meet the demands of the growing mass. The malformed vasculature is at least in part responsible for regions of the tumor that are hypoxic, acidotic, and exposed to increased interstitial fluid pressure. These unique aspects of the tumor microenvironment have been shown to act as barriers to conventional chemotherapy or radiation-based therapies. It now seems that while the vasculature initiates these tumor-specific conditions, the cells within the tumor respond to these stresses and add to the unique solid tumor physiology. Gene expression changes have been reported in the tumor for vascular endothelial growth factor, carbonic anhydrase IX, and pyruvate dehydrogenase kinase 1. The activity of these gene products then influences the tumor physiology through alterations in vascular permeability and interstitial fluid pressure, extracellular acidosis, and mitochondrial oxygen consumption and hypoxia, respectively. Novel molecular strategies designed to interfere with the activities of these gene products are being devised as ways to overcome the physiologic barriers in the tumor to standard anticancer therapies.

Selective inhibition of Ras, phosphoinositide 3 kinase, and Akt isoforms increases the radiosensitivity of human carcinoma cell lines

Ras activation promotes the survival of tumor cells after DNA damage. To reverse this survival advantage, Ras signaling has been targeted for inhibition. Other contributors to Ras-mediated DNA damage survival have been identified using pharmacologic inhibition of signaling, but this approach is limited by the specificity of the inhibitors used and their toxicity. To better define components of Ras signaling that could be inhibited in a clinical setting, RNA interference was used to selectively block expression of specific isoforms of Ras, phosphoinositide 3 (PI3) kinase, and Akt. Inhibition of oncogenic Ras expression decreased both phospho-Akt and phospho-p42/44 mitogen-activated protein (MAP) kinase levels and reduced clonogenic survival. Because pharmacologic inhibition of PI3 kinases and Akt radiosensitized cell lines with active Ras signaling, whereas inhibition of the MAP/extracellular signal-regulated kinase (ERK) kinase/ERK pathway did not, we examined the contribution of PI3 kinases and Akts to radiation survival. Selective inhibition the PI3 kinase P110alpha + p85beta isoforms reduced Akt phosphorylation and radiation survival. Similarly, inhibition of Akt-1 reduced tumor cell radiation survival. Inhibition of Akt-2 or Akt-3 had less effect. Retroviral transduction and overexpression of mouse Akt-1 was shown to rescue cells from inhibition of endogenous human Akt-1 expression. This study shows that Ras signaling to the PI3 kinase-Akt pathway is an important contributor to survival, whether Ras activation results from mutation of ras or overexpression of epidermal growth factor receptor. This study further shows that selective inhibition of the PI3 kinase P110alpha + p85beta isoforms or Akt-1 could be a viable approach to sensitizing many tumor cells to cytotoxic therapies.