Hormone-Refractory Prostate Cancer: New Horizons

Abstract

The eighth session of the 13th International Prostate Cancer Update focused on the mechanisms of androgen-independent cancer growth and on new therapeutic approaches to the treatment of androgen-independent prostate cancer. Three possible mechanisms that might account for the development of hormone resistance are reviewed here. These are: changes in antigen receptor expression, changes in androgen receptor structure, and changes in androgen receptor function. Therapeutic approaches discussed include the endothelin receptor antagonist astrasentan; PS-341, a boronic acid dipeptide that is highly selective for proteosome inhibition; the microtubule-stabilizing agent epothilone B; and exisulind, a selective apoptotic antineoplastic drug. The cooperative cancer study groups will move these treatments from their preliminary phase 1 and 2 studies to comparison against the standard of care for hormone-refractory prostate cancer.

The eighth session of the 13th International Prostate Cancer Update focused on the mechanisms of androgen-independent cancer growth and on new therapeutic approaches to the treatment of androgen-independent prostate cancer. The presenters described several different clinical approaches based on the mechanisms leading to development of hormone-refractory disease. It is hoped that a more thorough understanding of these biologic processes will lead to therapeutic advances.

Androgen-Independent Growth and Androgen Receptor Function

The mechanisms of androgen-independent prostate cancer growth have yet to be fully characterized and are likely to be multifactorial. It is known that when a patient is castrated, duration of hormone response is usually 18–24 months. Castration imparts a clonal selection, whereby surviving prostate cancer cells will grow despite low or absent testosterone levels. Three possible mechanisms that might account for the development of hormone resistance are, changes in antigen receptor expression, changes in androgen receptor structure, and changes in androgen receptor function. The contribution of each mechanism has been difficult to define, in part owing to the lack of preclinical models.

Only one human prostate cancer cell line (LNCaP) expresses the antigen receptor, albeit in mutated form. Initial studies in the Dunning prostate cancer animal model, derived from a rat prostate cancer cell line, implied that androgen receptor loss was the primary mechanism of androgen-independent growth.¹ In contrast, primary and metastatic specimens from patients with prostate cancer were examined for antigen receptor expression. Nearly all specimens were found to conserve the androgen receptor, and thus loss of the androgen receptor function does not significantly contribute to the hormone-resistant phenotype. Of 12 primary tumors examined by Bartsch and colleagues, 6 expressed the antigen receptor in more than 50% of cells. Only 1 of the corresponding lymph node metastases did not express the antigen receptor. All 22 prostate cancer metastases examined expressed the antigen receptor, with 4 of 22 expressing the receptor weakly.²

Structural aberrations might also account for non-testosterone activation of the androgen receptor and loss of growth control. The androgen receptor comprises three distinct regions: a transactivation region, a deoxyribonucleic acid-binding region, and a ligand-binding region. Mutations have been observed in the other hormone-binding region of the antigen receptor; but it seems that androgen receptor mutations are late events in human prostate cancer, and the more likely mechanism for androgen-independent growth is the activation of the androgen receptor by non-testosterone-mediated signal transduction pathways.

Peptide growth factors, such as insulin-like growth factor, keratinocyte growth factor, epidermal growth factor, and interleukin 6 (IL-6) can activate the antigen receptor independent of androgen. An in vitro system can be used to evaluate the relative intensity of activation of the androgen receptor by different ligands. IL-6 has been found to be elevated in patients with hormone-refractory prostate cancer, with IL-6 receptors expressed in prostate cancer cell lines as well as most human prostate cancer specimens. With the use of in vitro techniques, IL-6 can activate the androgen receptor independently of testosterone. The epidermal growth factor receptor family includes EGF-r (Erb-1), Her-2/neu(erb-2), erb-3, and erb4. These tyrosine kinase membrane receptors can form complexes with each other and thus activate different signal transduction pathways. EGF-r is expressed in approximately 90% of hormone-refractory prostate cancer specimens and, although the rate of overexpression of Her-2/neu can be as low as 10%, interactions between these receptors might be critical for androgen-independent prostate cancer cell growth. In vitro, Her-2/neu had been found to phosphorylate the androgen receptor in the absence of androgen. Trials using 2C4, a monoclonal antibody that inhibits Her-2/neu as well as EGF-r function, are currently being designed, either singly or in combination with taxanes, in the treatment of hormone-refractory prostate cancer.³

Androgen-Independent Growth: Neuroendocrine Differentiation

Another form of androgen-independent growth is regulated by neuroendocrine peptides. Neuroendocrine prostate cancer cells stain negative for the androgen receptor. Neuroendocrine peptides, such as cholecystokinin, adrenocorticotropic hormone, gastrin-releasing peptide, and β endorphin, can be secreted by the prostate. The neuroendocrine cell can exist in a variety of different forms in human prostate cancer tissue and seems to be upregulated in patients with hormone-refractory disease. Neuroendocrine cells can coexist with adenocarcinoma; they can form a carcinoid or carcinoid-like tumor, as well as a small-cell cancer.⁴ An octreotide scan might select out those patients who might potentially respond to somatostatin.

Bone-Specific Targeted Therapy: Endothelin Receptor Antagonists

The endothelins are a class of peptides expressed in a variety of human tissues, which control vasoconstriction, mitogenesis, nociception, and bone matrix formation. Three ligands (ET-1, -2, and -3), consisting of 21 amino acids, can be found in endothelial cells, kidney and intestine, and brain, respectively. The endothelin receptor consists of 2 receptors, ET_A and ET_B. The endothelin receptor is expressed in a variety of human tumors, including prostate cancer. Binding of endothelin to its receptor results in cell proliferation, bone matrix synthesis, and resistance to apoptosis. Atrasentan, a specific ET-1_A inhibitor, decreases mitogenic activity, osteoblastic activity, rates of bone metastases, and angiogenesis, and blocks nociceptive effects. Atrasentan is orally bioavailable and is dosed once daily. Adverse effects include peripheral edema, rhinitis, headache, and dyspnea. Atrasentan has been evaluated in patients with hormone-refractory prostate cancer. M96-500 was a 12-week study that evaluated pain response as a primary endpoint. A total of 131 patients were entered in the study and received either placebo, atrasentan 10 mg, or atrasentan 2.5 mg. A significant improvement in visual analogue pain score was observed in the 10 mg atrasentan arm when compared with placebo. M96-594 randomized 288 patients to the same three arms as in M96-500; however, a different primary endpoint, time to disease progression, was used, with prostate-specific antigen (PSA) progression as a secondary endpoint. There was a significant difference in time to progression and survival for the evaluable patients who received atrasentan, compared with placebo. Unfortunately, these differences in survival and time to progression were not observed in the intent-to-treat analysis. An ongoing study is evaluating atrasentan compared with placebo in 811 hormone-refractory prostate cancer patients with metastatic disease and an asymptomatic rising PSA level.⁵

Proteosome Inhibition

Apoptosis can be activated by mechanisms other than the Bcl-2/Bax complex. The transcription factor nuclear factor κB (NF-κB) regulates the expression of genes related to tumor invasion, metastases, immune response, cell proliferation, and regulation of apoptotic death.⁶ The androgen-independent human prostate cancer cell lines PC3 and DU-145 are found to have significantly higher levels of NF-κB than does their hormone-sensitive counterpart, the LNCaP line.⁷ Further evidence to support the role of NF-κB in androgen-independent growth is the observation that androgen-independent tissues demonstrate higher NF-κB binding than do androgen-dependent tissues.

NF-κ is tightly regulated by protein kinases. NF-κB is inactive when bound and thus must be liberated from its binding protein, IκB through phosphorylation. Transcriptional activation occurs when free NF-κB translocates from the cytoplasm to the nucleus. A variety of antiapoptotic genes are regulated by NF-κB, including c-inhibitor of apoptosis and manganese superoxide dismutase.

Inhibition of the NF-κB pathway might increase the efficacy of taxane-based therapy. In human breast cancer cell lines, NF-κB inducible genes protect against paclitaxel-induced cell death.⁸ In prostate cancer, NF-κB is associated with increased Bcl-2 expression, and thus its inhibition might increase the ability of the prostate cancer cell to undergo apoptosis. In human prostate cancer cell lines, expression of a dominant negative IκB mutant protein significantly enhanced tumor necrosis factor α-mediated apoptosis; however, overexpression of Bcl-2 abrogated this effect.⁹ The constitutive expression of NF-κB has been demonstrated to be related to phosphorylation of IκB through activation of IκB kinase.^10,11

The ubiquitin-proteasome pathway is essential for the degradation of most short- and long-lived intracellular proteins in eukaryotic cells. Because the proteosome affects a broad range of proteins, including the degradation of IκB, inhibition of the 26S unit of the proteosome might result in arrest of cell growth, decreased angiogenesis, and sensitization of cells to chemotherapeutic agents. PS-341, a boronic acid dipeptide that is highly selective for proteosome inhibition, can inhibit degradation of IκB and thus effectively inhibit NF-κB release. Combinations of proteosome inhibitors with chemotherapeutic agents increase the number of cells undergoing apoptosis in vitro. In human colorectal carcinoma cell lines, pretreatment of cells with PS-341 followed by treatment with SN 38, the active metabolite of CPT-11, demonstrated a 64% to 75% decrease in cell counts. In comparison, treatment with either SN 38 or PS-341 alone resulted in 22% to 30% and 24% to 47% growth inhibition, respectively.¹² Because NF-κB expression seems to be associated with taxane resistance, proteosome inhibition might improve the efficacy of either paclitaxel or docetaxel. A phase 1/2 trial is currently evaluating the efficacy and toxicity of escalating doses of PS-341 administered 24 hours after weekly docetaxel. This trial is currently accruing patients at Vanderbilt University, The Cleveland Clinic, and New York Presbyterian.

New Antimicrotuble Agents: Epothilones

Newer agents focusing on microtubules are currently in phase 1 and 2 trials. Epothilone B, a semisynthetic analogue of natural epothilones derived from Sorangium cellulosum, is 2 to 20 times more cytotoxic than paclitaxel in vitro and demonstrates preclinical activity in paclitaxel-resistant cell lines. Preclinical data demonstrate that epothilone B has significant cytotoxicity against the DU-145 prostate cancer cell line in vitro.¹³ The microtubule-stabilizing agents epothilones A and B and their desoxy-derivatives induce mitotic arrest and apoptosis in human prostate cancer cells. The drug has completed phase 1 evaluation, with diarrhea as its major dose-limiting toxicity. Epothilones are being evaluated in patients with hormone-refractory prostate cancer. In a phase 1 study at Memorial Sloan-Kettering Cancer Center, the combination of epothilone B with estramustine phosphate resulted in PSA declines of more than 50% in patients with hormone-refractory disease. Eastern Cooperative Oncology Group (ECOG) study 1802 will evaluate the activity of epothilone B in patients who have failed either mitoxantrone- or taxane-based therapy.¹⁴ The Southwest Oncology Group (SWOG) is evaluating epothilone B in men who are androgen independent and have not received prior chemotherapy. To further define the role of estramustine in the treatment of men with hormone-refractory prostate cancer, a randomized phase 2 trial is being performed at Memorial Sloan-Kettering Cancer Center that will compare epothilone B alone with epothilone B plus estramustine. Epothilone D also has demonstrated promising activity in vitro and in human prostate cancer cell lines.

CP461 and Related Compounds

A new class of compounds, selective apoptotic antineoplastic drugs (SAANDs), activate a different apoptotic pathway than taxanes. This class of compounds inhibits the activation of cyclic guanosine monophosphate phosphodiesterase, which is responsible for the activation of protein kinase G.¹⁵ Protein kinase G in turn will decrease levels of B catenins and increase levels of Jun kinase. The changes in B catenin levels, as well as in Jun kinase, will in turn activate caspases, leading to apoptosis. The SAANDs will activate apoptosis in human prostate cancer cell lines but will not cause cell death in normal cell lines. This class of compounds is orally bioavailable. The first drug in this class to be evaluated in prostate cancer, exisulind, was found to decrease the rate of PSA rise in patients with a rising PSA level after prostatectomy.¹⁶ CP461, an analogue of exisulind, is 10 times more effective in inhibiting phosphodiesterase 5.¹⁷ A preliminary study in hormone-refractory prostate cancer has demonstrated activity, as evidenced by declines in PSA levels and responses in measurable soft tissue lesions. Phase 1 studies of exisulind plus docetaxel are currently being performed in patients with hormone-refractory prostate cancer.

Phase 3 Randomized Trials: Docetaxel Based-Therapy

The new therapies for the treatment of metastatic hormone-refractory prostate cancer outlined in this article must be evaluated in phase 3 trials. The cooperative groups—SWOG, ECOG, and Cancer and Leukemia Group B—will move these treatments from their preliminary phase 1 and 2 studies to comparison against the standard of care for hormone-refractory prostate cancer. The standard of care for hormone-refractory prostate cancer might change in the next year, with maturation of two phase 3 studies. SWOG is leading study 99-16, comparing a 5-day course of estramustine 280 mg po tid plus docetaxel 60 mg/m² with continuous prednisone and mitoxantrone mg/m² every 3 weeks. The trial was closed to accrual with 770 patients entered as of January 2003, and the final analysis is due in February 2004. The trial is powered to detect a 33% difference in survival between the two treatment arms. Other outcomes to be measured include quality of life, response in measurable disease, and improvement in bone pain.

Tax 327, an international study, accrued 1004 patients. The 3 arms of this trial study include the standard arm, mitoxantrone combined with prednisone, and two experimental arms. The primary endpoint is similar to that of SWOG 99-16; the trial is designed to detect a 33% difference in survival. Unfortunately, neither SWOG 99-16 nor TAX 327 will answer questions regarding the contribution of estramustine to taxane-based therapy.

Main Points.

• Three possible mechanisms that might account for the development of hormone resistance are 1) changes in antigen receptor expression; 2) changes in androgen receptor structure; and 3) changes in androgen receptor function.

• Atrasentan, a specific endothelin receptor antagonist, has been evaluated in patients with hormone-refractory prostate cancer; astrasentan decreases mitogenic activity, osteoblastic activity, rates of bone metastases, and angiogenesis, and blocks nociceptive effects.

• Inhibition of nuclear factor κB (NFκB) might increase the ability of the prostate cancer cell to undergo apoptosis. PS-341, a boronic acid dipeptide that is highly selective for proteosome inhibition, can effectively inhibit NF-κB release and is currently being evaluated in phase 1/2 trials.

• The microtubule-stabilizing agents epothilones A and B and their desoxy-derivatives induce mitotic arrest and apoptosis in human prostate cancer cells; epothilones are being evaluated in patients with hormone-refractory prostate cancer.

• Selective apoptotic antineoplastic drugs will activate apoptosis in human prostate cancer cell lines but will not cause cell death in normal cell lines; phase 1 studies of exisulind, the first drug in this class to be evaluated in prostate cancer, plus docetaxel are currently being performed in hormone-refractory prostate cancer patients.