Abiraterone acetate

Medical uses

Abiraterone acetate is indicated for use in combination with prednisone as a treatment for metastatic castration-resistant prostate cancer. It has received FDA (28 April 2011), EMA (23 September 2011), MHRA (5 September 2011) and TGA (1 March 2012) approval for this indication. In Australia it is covered by the Pharmaceutical Benefits Scheme when being used to treat castration-resistant prostate cancer and given in combination with prednisone/prednisolone (subject to the conditions that the patient is not currently receiving chemotherapy, is either resistant or intolerant of docetaxel, has a WHO performance status of <2, and his disease has not since become progressive since treatment with PBS-subsidised abiraterone acetate has commenced).

Adverse effects

Adverse effects by frequency:
Very common (>10% frequency):

Common (1-10% frequency):

Uncommon (0.1-1% frequency):

Rare (<0.1% frequency):

Contraindications

Contraindications include hypersensitivity to abiraterone acetate. Although documents state that it should not be taken by women who are or who may become pregnant, there is no medical reason that any woman should take it. Women who are pregnant should not even touch the pills unless they are wearing gloves. Other cautions include severe baseline hepatic impairment, mineralocorticoid excess, cardiovascular disease including heart failure and hypertension, uncorrected hypokalaemia, and adrenocorticoid insufficiency.

Interactions

Abiraterone acetate is a CYP3A4 substrate and hence should not be administered concurrently with strong CYP3A4 inhibitors such as ketoconazole, itraconazole, clarithromycin, atazanavir, nefazodone, saquinavir, telithromycin, ritonavir, indinavir, nelfinavir, voriconazole) or inducers such as phenytoin, carbamazepine, rifampin, rifabutin, rifapentine, phenobarbital. It also inhibits CYP1A2, CYP2C9, and CYP3A4 and likewise should not be taken concurrently with substrates of any of these enzymes that have a narrow therapeutic index.

Mechanism of action

Abiraterone, the active metabolite of abiraterone acetate, inhibits CYP17A1, which manifests as two enzymes, 17α-hydroxylase (IC₅₀ = 2.5 nM) and 17,20-lyase (IC₅₀ = 15 nM) (six-fold more selective for inhibition of 17α-hydroxylase over 17,20-lyase) that are expressed in testicular, adrenal, and prostatic tumor tissues. CYP17 catalyzes two sequential reactions: (a) the conversion of pregnenolone and progesterone to their 17α-hydroxy derivatives by its 17α-hydroxylase activity, and (b) the subsequent formation of dehydroepiandrosterone (DHEA) and androstenedione, respectively, by its 17,20-lyase activity. DHEA and androstenedione are androgens and precursors of testosterone. Inhibition of CYP17 activity by abiraterone thus decreases circulating levels of androgens such as DHEA, testosterone, and dihydrotestosterone (DHT).

Abiraterone also acts as a partial antagonist of the androgen receptor (AR), and as an inhibitor of the enzymes 3β-hydroxysteroid dehydrogenase (3β-HSD), CYP11B1 (steroid 11β-hydroxylase), and other CYP450s (e.g., CYP1A2, CYP2C9, and CYP3A4). In addition to abiraterone itself, part of the activity of the drug has been found to be due to a more potent active metabolite, Δ⁴-abiraterone (D4A), which is formed from abiraterone by 3β-HSD. D4A is an inhibitor of CYP17A1, 3β-hydroxysteroid dehydrogenase/Δ^5-4 isomerase, and 5α-reductase, and has also been found to act as a competitive antagonist of the AR reportedly comparable to the potent antagonist enzalutamide. However, the initial 5α-reduced metabolite of D4A, 3-keto-5α-abiraterone, is an agonist of the AR, and promotes prostate cancer progression. Its formation can be blocked by the coadministration of dutasteride, a potent and selective 5α-reductase inhibitor.

There has been interest in the use of abiraterone acetate for the treatment of breast cancer due to its ability to lower estrogen levels. However, abiraterone has recently been found to act as a direct agonist of the estrogen receptor, and induces proliferation of human breast cancer cells in vitro. If abiraterone acetate is used in the treatment of breast cancer, it should be combined with an estrogen receptor antagonist like fulvestrant. In spite of its antiandrogen and estrogenic properties, abiraterone acetate does not appear to produce gynecomastia as a side effect.

Abiraterone acetate, via its metabolite abiraterone, has the capacity to lower circulating testosterone levels to less than 1 ng/dL (i.e., undetectable), and these concentrations are much lower than those achieved by castration (20 ng/dL). The addition of abiraterone acetate to castration was found to reduce levels of DHT by 85%, DHEA by 97–98%, and androstenedione by 77–78% relative to castration alone.

In accordance with its antiandrogen action, abiraterone acetate decreases the weights of the prostate gland, seminal vesicles, and testes.

Pharmacokinetics

After oral administration, abiraterone acetate, the prodrug form 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 on an empty stomach at least one hour before or two hours after food. The drug is highly protein bound (>99%), and is metabolised in the liver by CYP3A4 and SULT2A1 to inactive metabolites. The drug is excreted in feces (~88%) and urine (~5%), and has a terminal half-life of 12 ± 5 hours.

History

In the early 1990s, Mike Jarman, Elaine Barrie, and Gerry Potter of the Cancer Research UK Centre for Cancer Therapeutics in the Institute of Cancer Research in London set out to develop drug treatments for prostate cancer. Starting from the drug ketoconazole, they developed abiraterone, filing a patent in 1993 and publishing the first paper describing it the following year. Rights for commercialisation of the drug were assigned to BTG, a UK-based specialist healthcare company. BTG then licensed 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.

Abiraterone acetate is licensed by the European Medicines Agency. Until May 2012 the National Institute for Health and Clinical Excellence (NICE) did not recommend use of the drug within the NHS on cost-effectiveness grounds. This position was reversed when the manufacturer submitted revised costs. The use is currently limited to men who have already received one docetaxel-containing chemotherapy regimen.

Prostate cancer

A phase III study in subjects previously treated with docetaxel started in 2008. In September 2010, an independent panel found that the interim results in patients previously treated with docetaxel were so much better compared to those treated with placebo that it was unethical to keep half the study participants on placebo, and all patients began receiving the drug. Overall survival was increased by 3.9 months in to this study (14.8 months versus 10.9 months for placebo).

A placebo-controlled double-blind randomized phase III study in patients with castration-refractory prostate cancer but who had not received chemotherapy opened to accrual in April 2009. 1,088 men received either abiraterone acetate (1000 mg daily) plus prednisone (5 mg twice daily), or placebo plus prednisone. The median radiographic progression-free survival was 16.5 months with abiraterone acetate–prednisone and 8.3 months with prednisone alone (hazard ratio (HR) = 0.53; 95% confidence interval (CI), 0.45 to 0.62; P<0.001). After a median follow-up period of 22.2 months, overall survival was better with abiraterone acetate plus prednisone (median not reached) compared to placebo plus prednisone (27.2 months); HR = 0.75; 95% CI, 0.61 to 0.93; P=0.01).

Breast cancer

Abiraterone acetate is currently under development for the treatment of breast cancer and is in phase II clinical trials for this indication.