Prostate cancer is termed ‘castrate resistant’ when the disease progresses despite continuous androgen deprivation therapy. After this, further treatment is needed to maintain disease control. Androgen deprivation therapy is continued as patients who continue it survive longer than those who stop.6
Micro-levels of peritumoural androgen have been shown to persist despite castration levels of serum testosterone.7 Anti-androgens are therefore added to androgen deprivation therapy.
‘First-generation’ anti-androgens
The early anti-androgens included non-steroidal drugs (such as bicalutamide, nilutamide and flutamide) and the steroidal drug cyproterone acetate. Even though these drugs have less benefit compared to the newer drugs, they are widely used in castrate-resistant disease as first-line drugs to achieve combined androgen blockade. This is for several reasons – current prescribing rules state that new anti-androgens are only subsidised in Australia for patients who are unfit for, or have already progressed after, chemotherapy and many men wish to delay having chemotherapy. In addition, because of the longitudinal nature of prostate cancer treatment, these anti-androgens allow additional time on oral treatments. Many men will respond and these drugs can delay the time to progression and the need for newer drugs which can then be used later in the disease course. Bicalutamide is most widely used due to its once-daily dosing and better tolerability.8
Common adverse effects of first-line anti-androgens are similar to those of androgen deprivation therapy (hot flushes, sexual dysfunction). Nilutamide causes changes in light accommodation (reversible on cessation). Cyproterone acetate increases cardiovascular risk and is not widely used.
These drugs can also be used sequentially as second-line therapy after progression on another anti-androgen as there may be a brief response in a selected cohort of patients. For example, flutamide could be tried after a patient progresses despite taking bicalutamide.9 However, in the current era, the older anti-androgens are rarely used in second- or third-line settings.
New anti-androgens
The new anti-androgens target various steps of the androgen production pathway which is crucial in tumour growth. They are added to androgen deprivation therapy. These anti-androgens are used in men not fit for chemotherapy, and in those whose disease has progressed on previous chemotherapy.
In men who are fit for chemotherapy the drugs are usually reserved until after the disease progresses on first-line docetaxel chemotherapy. This is due to the current prescribing restrictions and also due to the ability to give the drugs later when these men may no longer be well enough for further chemotherapy.
Abiraterone acetate
The cytochrome P450 17 alpha-hydrolase and C17,20-lyase are enzymes involved in testosterone synthesis. They mediate conversion of pregnenolone-like steroids into androgens. Abiraterone is an oral inhibitor of these enzymes so it halts both extragonadal and testicular androgen synthesis.10
Abiraterone improves the survival of patients with metastatic castrate-resistant prostate cancer whether they are chemotherapy (docetaxel) naïve or have cancer that has progressed post chemotherapy.11,12 Co-administration of prednisone is important to minimise the abiraterone-induced reduction of serum cortisol and increase of mineralocorticoid. Patients need to be monitored for hypertension, hypokalaemia and peripheral oedema as well as the elevation of hepatic aminotransferases which may require a temporary suspension of treatment.
Enzalutamide
Enzalutamide is an androgen-receptor signalling inhibitor.13 It improves survival in metastatic castrate-resistant prostate cancer, and is significantly more effective than the older drug, bicalutamide.14,15 Enzalutamide has activity both before and after chemotherapy.15,16 Adverse effects include hypertension, fatigue, memory impairment, falls and, less commonly, seizures.
Chemotherapy
Anthracyclines and taxanes have been used to treat metastatic castrate-resistant prostate cancer.
Mitoxantrone
Mitoxantrone is an anthracycline compound. It was the first approved cytotoxic drug for the treatment of metastatic castrate-resistant prostate cancer and was widely used before docetaxel was available. Mitoxantrone significantly improved cancer symptoms and quality of life, but did not improve survival.17 A small number of patients may benefit from mitoxantrone and other older cytotoxic drugs such as cyclophosphamide and etoposide in third- and fourth-line settings.
Docetaxel
Docetaxel is a taxane that has been studied in a variety of cancers. It was the first cytotoxic drug to show a survival benefit for patients with prostate cancer. It became the standard of care for metastatic castrate-resistant disease after a phase III trial reported a median survival benefit of 2.4 months over mitoxantrone (18.9 vs 16.5 months). Although adverse events were more common with docetaxel, some patients had a better quality of life.18 Adverse effects include neutropenia, fatigue, diarrhoea, hair loss, nail changes and sensory neuropathy.
Cabazitaxel
Cabazitaxel is a newer taxane. In patients with cancer that has progressed following or during docetaxel chemotherapy it improved survival by 2.4 months compared with mitoxantrone (15.1 vs 12.7 months).19
Cabazitaxel is generally better tolerated than docetaxel with the common adverse effects of myelosuppression, diarrhoea, nausea and fatigue. A recent study evaluating a lower dose of cabazitaxel (20 mg/m2 every three weeks) reported non-inferiority in overall survival compared to the standard dose (25 mg/m2 every three weeks). There was less myelosuppression and infection which may benefit frail older patients.20