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Abiraterone acetate for prostate cancer:
a new era of hormonal therapies

Abiraterone acetate for prostate cancer: a new era of hormonal therapies
Emmanuel S Antonarakis
Asian Journal of Andrology (2011) 13, 663–664; doi:10.1038/aja.2011.92; published online 11 July 2011
Johns Hopkins Sidney Kimmel Comprehensive Cancer / Center, Baltimore, MD 21231, USA
Correspondence: Dr ES Antonarakis (eantona1@jhmi.edu)

RESEARCH HIGHLIGHT

Therapies targeting the androgen recep­tor (AR) axis have constituted the Holy Grail in the management of advanced pro­state cancer for seven decades.1 These hor­monal therapies have traditionally taken two main forms: those that suppress gonadal androgen synthesis (e.g., the gonadotropin releasing hormone agonists/antagonists, such as leuprolide), and those that inhibit the AR directly (e.g., the anti-androgens, such as bicalutamide). However, although the vast majority of patients with prostate cancer ini­tially respond favorably to androgen-ablative therapies (manifested by tumor regressions and symptomatic improvements), all patients will eventually develop further disease pro­gression after a median of 18–24 months. This transformed disease state, known as cas­tration-resistant prostate cancer (CRPC), is invariably fatal.

Until recently, life-prolonging therapies for patients with metastatic CRPC (mCRPC) were limited, and consisted only of docetaxel (Taxotere) chemotherapy2 (U.S. Food and Drug Administration (FDA)-approved in 2004 as first-line chemotherapy for mCRPC). However, in 2010, two additional modalities were added to our armamentarium of FDA-approved treatment options for men with mCRPC: the autologous immunotherapy product sipuleucel-T (Provenge)3 (indicated for men with asymptomatic or minimally symptomatic disease) and the taxane chemo­therapy agent cabazitaxel (Jevtana)4 (indicated for men progressing after docetaxel). In that same year, the bone-targeting agent denosu­mab (Xgeva)5 wasalsoFDA-approvedfor the prevention of skeletal-related events (bone fractures, spinal cord compression, malignant hypercalcemia, the need for surgery, or the need for radiotherapy) in men with mCRPC, after demonstrating superiority over the bisphosphonate drug, zoledronic acid.

It is now known that androgen signaling remains active even in patients with CRPC, and that the AR is a critical mediator of the transition to the castration-resistant state. One mechanism by which CRPC maintains AR signaling is by overexpressing CYP17, a key enzyme in extra-gonadal (adrenal, pro­static and intratumoral) androgen biosyn­thesis. In the 26 May 2011 issue of the New England Journal of Medicine, de Bono and colleagues6 reported the results of a multicen­ter phase 3 trial evaluating the oral CYP17 inhibitor abiraterone acetate (1000 mg daily, administered together with prednisone 10 mg daily) compared against placebo and prednis­one in men with mCRPC with progression after prior docetaxel chemotherapy. The trial met its primary end point, demonstrating a 35% reduction in the relative risk of death among men receiving abiraterone compared to those receiving placebo, with a median sur­vival of 14.8 versus 10.9 months respectively (P,0.0001). In addition, when compared against placebo, abiraterone treatment resulted in more frequent reductions in pro­state-specific antigen (PSA) level by o50% (29% vs. 6%; P,0.0001), more frequent objective radiographic responses (14% vs. 3%; P,0.0001), longer time to PSA progres­sion (defined as a o25% PSA increase from nadir) (10.2 vs. 6.6 months; P,0.0001) and longer radiographic progression-free survival (5.6 vs. 3.6 months; P,0.0001). Additional data presented at the 2011 American Society of Clinical Oncology Annual Meeting showed that treatment with abiraterone prolonged the time to the first skeletal-related event (10.0 vs. 5.0 months; P50.0006) and also pro­vided pain relief and a delay in the time to pain progression (9.4 vs. 4.6 months; P50.0019).7 The results of this trial led to the FDA-approval of abiraterone acetate on 28 April 2011 for the treatment of men with mCRPC who had received prior docetaxel­containing chemotherapy, the second drug to be approved in the postdocetaxel setting.8

Adverse events related to abiraterone acet­ate were generally tolerable, and consisted mainly of a feedback syndrome of secondary mineralocorticoid excess resulting in fluid retention/peripheral edema (31%), hypoka­lemia (17%) and hypertension (10%). Notably, these toxicities had been more pre­valent in prior studies of abiraterone mono-therapy, suggesting that these effects were partially mitigated by the concurrent admin­istration of a corticosteroid (i.e., prednisone). Thirteen percent of abiraterone-treated men developed mild cardiac toxicities (mainly tachycardia, or atrial fibrillation). It is there­fore recommended that abiraterone be used with caution in patients with a history of cardiovascular disease, symptomatic heart failure, or a left ventricular ejection fraction of ,50%. In addition, elevated liver transa­minases were observed in 10% of abiraterone­treated patients; two-thirds of these cases were grade 1 events. Abiraterone is contrain­dicated in patients with severe liver failure, and should be administered at a reduced dose (250 mg daily) in men with moderate hepatic impairment. For patients who develop hepa­totoxicity during treatment, abiraterone should be held until recovery of the transami­nitis, and treatment may be reinitiated at a reduced dose once hepatic function has resolved. Therapy should be discontinued permanently in men who develop severe hepatotoxicity.

How does abiraterone compare to high-dose ketoconazole (600–1200 mg daily), a non­selective competitive inhibitor of CYP17? Although ketoconazole has not demonstrated a survival benefit in men with mCRPC,9 it is still commonly prescribed (although not FDA-approved) for this indication. The efficacy of ketoconazole in the docetaxel-refractory set­ting has only been examined in retrospective studies,10 where PSA responses (o50% PSA reductions) have been observed in approxi­mately 25% of patients. Although this figure is almost comparable to that seen with abirater­one in the current study, the toxicities of keto­conazole are more unfavorable and include fatigue, anorexia, emesis, cutaneous effects, peripheral neuropathy and significant transa­minitis. Importantly, a recent prospective trial aimed to discern the role of abiraterone in men with mCRPC who had previously received ketoconazole.11 In that study, PSA response rates were seen in 47% of ketoconazole­pre-treated patients, although those patients were all chemotherapy-naive. This suggests that abiraterone retains activity even in the ketoco­nazole-refractory population.

The FDA’s approval of abiraterone acetate will create new treatment opportunities but will also present new challenges. One dilemma will likely revolve around the optimal timing of abiraterone in men with mCRPC. Although the present study was conducted in docetaxel-pre-treated patients, there is no reason to believe that abiraterone would not be as (or more) effective in chemo­therapy-naive patients,12 and this hypothesis is currently being tested in an ongoing pla­cebo-controlled phase 3 trial in chemother­apy-untreated men. Although a common conceptual framework for understanding mCRPC relates to defining disease states before and after chemotherapy administra­tion, an alternative approach is to describe mCRPC in terms of metastatic burden and disease-related symptoms. To this end, designing trials based on amount of disease burden (or even expected survival as deter­mined by prognostic nomograms), rather than by chemotherapy status, may be a more rational way to examine novel prostate cancer therapies moving forward.

In conclusion, the observation that treat­ment with abiraterone can prolong survival in men with castration-resistant prostate cancer provides a proof of principle that even this form of the disease remains androgen-driven, and paves the way for a new era of AR-targeted therapies for prostate cancer. As sev­eral novel androgen-modulating therapies (e.g., orteronel, MDV-3100, TOK-001 and ARN-509) enter clinical trials,13 the new chal­lenge will be to determine how to optimally sequence or combine these agents with one another or with other established or experi­mental treatments for advanced prostate can­cer. In addition, the role of abiraterone and other AR-directed therapies in men with earl­ier disease states is currently undefined. Hopefully, the next decade will shed some light on some of these questions.

1 Attard G, Richards J, de Bono JS. New strategies in metastatic prostate cancer: targeting the androgen receptor signaling pathway. Clin Cancer Res 2011; 17: 1649–57.

2 Berthold DR, Pond GR, Soban F, de Wit R, Eisenberger M et al. Docetaxel plus prednisone or mitoxantrone plus prednisone for advanced prostate cancer: updated survival in the TAX 327 study. J Clin Oncol 2008; 26: 242–5.

3 Kantoff PW, Higano CS, Shore ND, Berger ER, Small EJ et al. Sipuleucel-T immunotherapy for castration-resistant prostate cancer. N Engl J Med 2010; 363: 411–22.

4 de Bono JS, Oudard S, Ozguroglu M, Hansen S, Machiels JP et al. Prednisone plus cabazitaxel or mitoxantrone for metastatic castration-resistant pro­state cancer progressing after docetaxel treatment: a randomised open-label trial. Lancet 2010; 376: 1147–54.

5 Fizazi K, Carducci M, Smith M, Damia˜o R, Brown J et al. Denosumab versus zoledronic acid for treatment of bone metastases in men with castration-resistant prostate cancer: a randomised, double-blind study. Lancet 2011; 377: 813–22.

6 de Bono JS, Logothetis CJ, Molina A, Fizazi K, North S et al. Abiraterone and increased survival in metastatic prostate cancer. NEngl JMed 2011; 364:1995– 2005.

7 Logothetis C, de Bono JS, Molina A, Basch EM, Fizazi K et al. Effect of abiraterone acetate on pain control and skeletal-related events in patients with metastatic castration-resistant prostate cancer post-docetaxel: results from the COU-AA-301 phase III study. J Clin Oncol 2011; 29 (Suppl): abstract 4520.

8 Antonarakis ES, Armstrong AJ. Evolving standards in the treatment of docetaxel-refractory castration-resistant prostate cancer. Prostate Cancer Prostatic Dis;e­pub ahead of print 17 May 2011; doi:10.1038/ pcan.2011.23.

9 Small EJ, Halabi S, Dawson NA, Stadler WM, Rini BI et al. Antiandrogen withdrawal alone or in combination with ketoconazole in androgen-independent prostate cancer patients: a phase III trial (CALGB 9583). JClin Oncol 2004; 22: 1025–33.

10 Nakabayashi M, Oh WK, Jacobus S, Regan MM, Taplin ME et al. Activity of ketoconazole after taxane-based chemotherapy in castration-resistant prostate cancer. BJU Int 2010; 105: 1392–96.

11 Ryan CJ, Smith MR, Fong L, Rosenberg JE, Kantoff P et al. Phase I clinical trial of the CYP17 inhibitor abiraterone acetate demonstrating clinical activity in patients with castration-resistant prostate cancer who received prior ketoconazole therapy. JClin Oncol 2010; 28: 1481–8.

12 Antonarakis ES, Eisenberger MA. Expanding treatment options for metastatic prostate cancer. N Engl J Med 2011; 364: 2055–8.

13 Antonarakis ES, Armstrong AJ. Emerging therapeutic approaches in the management of metastatic castration-resistant prostate cancer. Prostate Cancer Prostatic Dis; e-pub ahead of print 17 May 2011; oi:10.1038/pcan.2011.24.