Abiraterone

Abiraterone is a drug used in castration-resistant prostate cancer (formerly hormone-resistant or hormone-refractory prostate cancer) (prostate cancer not responding to androgen deprivation or treatment with antiandrogens). It is formulated as the prodrug abiraterone acetate and sold under the trade name Zytiga. After an expedited six-month review, abiraterone was approved by the U.S. Food and Drug Administration (FDA) in April 2011. In Phase III trials, it extended median survival to 14.8 months versus 11.2 months placebo, and the trial was stopped because of the successful outcome.

In the UK, despite being licensed by the European Medicines Agency, the drug is not currently available for routine use on the NHS. In February 2012, the National Institute for Health and Clinical Excellence (NICE) issued preliminary guidance that the drug will not be made available on cost-effectiveness grounds, but this decision is open to consultation.

History
This drug was discovered by Gerry Potter in 1990 at the Cancer Research UK Centre for Cancer Therapeutics within the Institute of Cancer Research in London. Rights for commercialisation of the drug were assigned to BTG plc, a UK company that manages commercialisation activity in pharmaceuticals. BTG then licenced the product to Cougar Biotechnology which began development of the commercial product. In 2009, Cougar was acquired by Johnson & Johnson which developed and sells the commercial product, and is conducting ongoing clinical trials to expand its clinical uses.

Mechanism of action
Abiraterone inhibits 17 α-hydroxylase/C17,20 lyase (CYP17A1), an enzyme which is expressed in testicular, adrenal, and prostatic tumor tissues. CYP17 catalyzes two sequential reactions: (a) the conversion of pregnenolone and progesterone to their 17-α-hydroxy derivatives by its 17 α-hydroxylase activity, and (b) the subsequent formation of dehydroepiandrosterone (DHEA) and androstenedione, respectively, by its C17,20 lyase activity. DHEA and androstenedione are androgens and precursors of testosterone. Inhibition of CYP17 activity by abiraterone thus decreases circulating levels of testosterone.

Pharmacokinetics
After oral administration, abiraterone acetate, the prodrug form present in the commercial preparation, is converted into the active form, abiraterone; this conversion is likely to be esterase-mediated and not CYP-mediated. Administration with food increases absorption of the drug and thus has the potential to result in increased and highly variable exposures; the drug should be consumed in empty stomach. The drug is highly protein bound (>99%), and is metabolised in the liver by CYP3A4 and SULT2A1 to inactive metabolites. The drug is excreted by feces (~88%) and urine (~5%) with a terminal half life of 12 ± 5 hours.

Clinical studies
A phase III trial in subjects previously treated with docetaxel started in 2008. A placebo-controlled randomised phase III clinical trial in patients with castration-refractory prostate cancer who are chemotherapy-naive opened to accrual in April 2009.

In September 2010, an independent panel found that the interim results of the phase III clinical trial in previously treated docetaxel patients were so successful that it would have been unethical to keep half the trial participants on placebo, and all patients began receiving the drug. Overall survival was increased by 3.9 months according to this trial. It was approved by the FDA in April 2011.

The first clinical studies were run in 2004. A more recent study in patients who had not received chemotherapy reported in 2007 that abiraterone acetate induced decline in prostate specific antigen (PSA) in up to 70% of patients as well as radiological shrinkage of tumors, symptom improvement, normalisation of lactate dehydrogenase. However others cautioned in 2008 that it was too early to know whether abiraterone treatment will have long term benefit.

Results of two phase II trials indicate that abiraterone may reduce prostate specific antigen (PSA) levels, as well as shrink tumors. Many of the 21 men in the Phase II trial reported significant improvements in their quality of life and several were able to stop taking morphine, used to control the pain caused after the cancer spread into their bones. On average, progression-free survival (PFS) was prolonged by 161 days in patients which had been treated with chemotherapy, and by 236 days in chemotherapy naive patients. Phase II clinical trials of abiraterone's effectiveness in patients who have not yet received treatment with chemotherapy (33 patients) found a median time to PSA progression of 48 weeks. Another phase II trial in patients who had failed prior treatment with docetaxel (47 patients) showed a median time to PSA progression of 24 weeks.

A phase I/II clinical trial evaluating abiraterone acetate in advanced breast cancer patients is also underway.