Pharmacology
New pharmacological treatments for prostate cancer Bruce Turner and Lawrence Drudge-Coates
P
rostate cancer affects approximately 40 000 men annually in the UK (Cancer Research UK, 2009). Although improvements in the early diagnosis and treatment of prostate cancer have improved survival rates, some 10 100 men continue to die of prostate cancer each year (Cancer Research UK, 2010). Prostate cancer may be localized to the prostate, locally advanced, or advanced (metastatic). The most common site for metastatic deposits to arise is in the skeleton (Hatoum et al, 2008). Metastatic prostate cancer is incurable, and the aim of treatment is to delay time to progression and reduce the burden of symptoms from advanced disease (Turner and Drudge-Coates, 2012). When the disease metastasizes to bone and causes skeletal-related events, such as pathological fracture, spinal cord compression, and radiotherapy to bone (to control pain), hypercalcaemia may become a risk for the patient. Androgens (male sex hormones) are the key driver of growth for both the normal prostate and prostate cancer (Figure 1), and it is widely recognized that prostate cancer is sensitive to testosterone deprivation (Princivalle et al, 2007). The Leydig cells of the testes produce 90–95% of circulating testosterone. Another 5–10% of systemic testosterone is synthesized by the adrenal glands (Labrie, 2004). As such, treatment is initiated with androgen deprivation therapy (ADT), the aim of which is to reduce testosterone levels and cause a state of medical castration. ADT can be achieved using a range of medications (Table 1). Surgical castration with bilateral subcapsular orchidectomy is an alternative to medical castration for patients with advanced prostate cancer, but it is not as popular. Although men will initially benefit from this hormonal manipulation, ultimately the disease becomes hormone refractory (Berthold et al, 2008) Bruce Turner is a Uro-Oncology Nurse Practitioner, at Homerton University Hospital and Bart’s Health Trust, London; Lawrence Drudge-Coates is a Urological Oncology Clinical Nurse Specialist and Honorary Lecturer at Kings College Hospital, London Email:
[email protected]
Nurse Prescribing 2012 Vol 10 No 10
Abstract In this article, a summary of new drugs available for patients with prostate cancer, such as degarelix, abiraterone acetate, sipuleucel-T, bone-targeted therapies, and chemotherapeutic agents, and their pharmacologies is provided.
and continues to progress without serum testosterone being present. Disease progression after achieving castration levels of serum testosterone has been used as the definition of castration-resistant prostate cancer (CRPC) (Hou and Flaig, 2012). It is now appreciated that CRPC is driven by androgen production within the tumour and testosterone formation from weak adrenal precursors that are not reliant on serum testosterone (Cai and Balk, 2011).
Testosterone production
Luteinizing-hormone-releasing hormone (LHRH) agonists, such as leuprorelin, goserelin, and triptorelin, have become the initial treatment of choice for the majority of patients with advanced prostate cancer. These drugs are agonist analogues of hypothalamic LHRH (also known as gonadotropin-releasing hormone (GnRH)), which controls the secretion of LH and follicle-
Table 1. Medications used in androgen deprivation therapy Class
Existing drugs
Oestrogen
Diethylstilbestrol
LHRH agonists
Goserelin, leuprorelin, triptorelin, and histrelin
LHRH antagonists
Degarelix
Antiandrogens
Bicalutamide, flutamide, and cyproterone acetate
Non-specific cytochrome P450 enzyme inhibitors
Ketoconazole
Specific CYP17 inhibitors
Abiraterone acetate
Glucocorticoids
Prednisolone and dexamethasone
CYP17: cytochrome P450 17; LHRH: luteinizing-hormone-releasing hormone.
501
Pharmacology
stimulating hormone (FSH) from the anterior pituitary gland (Princivalle et al, 2007) (Figure 1). LHRH agonists eventually lead to desensitization of pituitary LHRH receptors, and, as such, eventually lead to an inhibition of testosterone production (Figure 2). However, a major drawback is that LHRH agonists initially stimulate testosterone production for approximately 2–3 weeks before the desensitization of LHRH receptors (Figure 2), and, during this time, patients may experience a ‘tumour flare’ phenomenon, which may cause their cancer to grow (Princivalle et al, 2007). For many years, there was a lack of proven therapeutic hormonal options for the treatment of prostate cancer after the development of castrationresistant disease (Hou and Flaig, 2012), with only docetaxel chemotherapy providing a clear survival benefit in this setting (Tannock et al, 2004). The introduction of several new hormonal agents has challenged the traditional management of CRPC. In this article, a summary of new drugs available for patients with prostate cancer, such as degarelix, abiraterone acetate, sipuleucel-T, bone-targeted therapies, and chemotherapeutic agents, and their pharmacologies is provided. Figure 1. Endocrine regulation of prostate growth. ACTH: adrenocorticotropic hormone; CRH: corticotropin-releasing hormone; FSH: follicle-stimulating hormone; LHRH: luteinizing-hormone-releasing hormone; T–DHT: testosterone– dihydrotestosterone. From: Drudge-Coates (2005)
Figure 2. Mechanism of action of luteinizing-hormone-releasing hormone analogues. In phase 1, testosterone production is stimulated for approximately 2–3 weeks and ‘tumour flare’ may occur. In phase 2, testosterone production is inhibited. LHRH: luteinizing-hormone-releasing hormone. From DrudgeCoates (2005)
502
Degarelix
Degarelix (Firmagon®; Ferring Pharmaceuticals Ltd) is a new class of LHRH antagonist that produces rapid androgen deprivation by immediate inhibition of LHRH receptors in the anterior pituitary gland and subsequent suppression of hormones secreted from the anterior pituitary, such as LH, FSH, and testosterone (Abraham, 2009). A randomized, controlled study to evaluate the efficacy of degarelix compared with LHRH agonists, involving 610 men, reported that medical castration (testosterone ≤0.5 ng/mL) was achieved by day three after commencing treatment in over 95% of patients; no patient in the LHRH agonist arm was castrate by day three (Klotz et al, 2008). Although the significance is yet to be elucidated, none of the patients receiving degarelix in the study had a testosterone surge, whereas 81% of patients on leuprorelin and 74% of patients on leuprorelin and antiandrogen cover for tumour flare were found to have testosterone microsurges (Klotz et al, 2008). In addition, prostate-specific antigen levels were lower at 2 and 4 weeks in the degarelix patients than those in the LHRH agonist group (p
