Stephen Balk

PURPOSE: Abiraterone is a potent inhibitor of the steroidogenic enzyme CYP17A1 and suppresses tumor growth in patients with castration-resistant prostate cancer (CRPC). The effectiveness of abiraterone in reducing tumor androgens is not known, nor have mechanisms contributing to abiraterone resistance been established. EXPERIMENTAL DESIGN: We treated human CRPC xenografts with abiraterone and measured tumor growth, tissue androgens, androgen receptor (AR) levels, and steroidogenic gene expression versus controls. RESULTS: Abiraterone suppressed serum PSA levels and improved survival in two distinct CRPC xenografts: median survival of LuCaP35CR improved from 17 to 39 days (HR = 3.6, P = 0.0014) and LuCaP23CR from 14 to 24 days (HR = 2.5, P = 0.0048). Abiraterone strongly suppressed tumor androgens, with testosterone (T) decreasing from 0.49 ± 0.22 to 0.03 ± 0.01 pg/mg (P < 0.0001), and from 0.69 ± 0.36 to 0.03 ± 0.01 pg/mg (P = 0.002) in abiraterone-treated 23CR and 35CR, respectively, with comparable decreases in tissue DHT. Treatment was associated with increased expression of full-length AR (AR(FL)) and truncated AR variants (AR(FL) 2.3-fold, P = 0.008 and AR(del567es) 2.7-fold, P = 0.036 in 23 CR; AR(FL) 3.4-fold, P = 0.001 and AR(V7) 3.1-fold, P = 0.0003 in 35CR), and increased expression of the abiraterone target CYP17A1 (∼2.1-fold, P = 0.0001 and P = 0.028 in 23CR and 35CR, respectively) and transcript changes in other enzymes modulating steroid metabolism. CONCLUSIONS: These studies indicate that abiraterone reduces CRPC growth via suppression of intratumoral androgens and that resistance to abiraterone may occur through mechanisms that include upregulation of CYP17A1, and/or induction of AR and AR splice variants that confer ligand-independent AR transactivation.

The androgen-regulated enzyme fatty acid synthase (FAS), required for de novo lipogenesis, is overexpressed in several cancers including prostate carcinoma and has been associated with aggressive disease. FAS expression was assessed in 81 prostate carcinomas, both by immunohistochemistry in tissue microarrays and by Affymetrix Hu95Av2 oligonucleotide arrays. Both FAS mRNA and protein were significantly overexpressed in prostate carcinomas compared with the corresponding normal tissue. FAS mRNA and protein expression increased substantially from normal to prostatic intraepithelial neoplasia, to low grade, to high grade, and to androgen-independent bone metastases. A significant correlation between FAS mRNA and protein expression was found in two thirds of the cases. In 17% of the cases, FAS protein levels were high despite low mRNA levels, and these tumors exhibited a distinct molecular signature when compared with tumors that did not express FAS protein. Whereas the latter group of tumors expressed some proapoptotic genes, tumors with high FAS levels overexpressed, among other genes, its transcriptional regulator, steroid regulator binding protein, and apolipoprotein E. These data demonstrate (1) the consistent overexpression of FAS in prostate carcinoma compared with the adjacent normal tissue, (2) a strong association between FAS and prostate tumor initiation and progression, (3) the highest FAS expression occurring in androgen-independent bone metastases, (4) the transcriptional and posttranscriptional regulation of FAS in the majority and in a subset of prostate cancers, respectively, and (5) most importantly, the identification by FAS expression of prostate tumors with unique molecular signatures and potentially diverse biologic behavior.

Mutations in the androgen receptor (AR), that alter steroid hormone specificity have been identified in a series of androgen-independent prostate cancers. To address the functional properties of these mutant ARs that may have contributed to their selection in vivo, responses to a series of steroid hormones and antiandrogens were assessed. CV-1 cells were cotransfected with wild-type or mutant ARs and a luciferase reporter plasmid regulated by an androgen-responsive element. Dose-response curves were analyzed for 5alpha-dihydrotestosterone, the most active androgen in normal prostate, and androstenedione, a major androgen derived from the adrenals. Although the mutant ARs responded to both of these steroids, the responses were equivalent to or less than the wild-type AR. In contrast, responses to flutamide, a competitive antagonist of the wild-type AR, were markedly increased by three of the mutations. Similar responses were observed with a second antiandrogen, nilutamide. Bicalutamide, another antiandrogen related to flutamide, remained an antagonist for these mutant ARs. Finally, flutamide was observed to be a weak partial agonist of the wild-type AR in this system. These results indicate that flutamide used in conjunction with androgen ablation therapy for prostate cancer may select for tumor cells with flutamide-inducible ARs.