Identification of a Companion Diagnostic (CDx) that Utilizes Circulating Tumor Cells (CTCs) to Detect an Androgen Receptor Splice Variant (AR-V7) in Metastatic Castrate Resistant Prostate Cancer (mCRPC) D. T. Dransfield1 , D Jacoby1 , K. Mamlouk1 , K Ferrante1 1 Tokai Pharmaceuticals, Cambridge, MA
Background: Antonarakis et al1 recently demonstrated that mCRPC patients with AR-V7 had worse clinical outcomes than patients without the splice variant (SV). Galeterone, a small molecule, disrupts androgen receptor (AR) signaling via multi-targeted pathways through degradation of AR protein, competitive antagonism of the AR, and selective inhibition of CYP17 lyase. AR-V7, a truncated form of the AR lacking the ligand-binding domain, remains constitutively active as a transcription factor. The detection of AR-V7 status in CTCs from mCRPC patients is associated with resistance to enzalutamide and abiraterone. In preclinical models, galeterone demonstrated activity against multiple AR aberrations, including AR-V7, ARv567es, AR-T878A, and AR-F876L. Given the encouraging results from the Phase 2 ARMOR2 trial in patients with C terminal loss, 2 further clinical research and validation is warranted. ARMOR3-SV, a Phase 3 trial comparing galeterone to enzalutamide in M1 AR-V7 SV patients is expected to be initiated in first half of 2015. Patients will be screened for CTCs, and presence of the SV will be confirmed prior to enrollment. Therefore, availability of a validated, accurate, rapid, quantitative, and universally applicable CDx assay performed in a central CLIA-certified laboratory for determining AR-V7+ status is critical to initiating the trial.
Method: Patients will be screened for CTCs and the presence of the AR-V7 will be confirmed prior to enrollment in the study. Development of a validated assay performed in a central CLIA-certified laboratory is crucial to the identification of M1 AR-V7+ CRPC patients fit for enrollment in ARMOR3-SV. Three methods are being evaluated. 1) Johns Hopkins (JHU): Isolation and enrichment of CTCs using immunomagnetic capture via epithelial and tumor-associated antigens (AdnaTest ProstateCancerSelect kit (AdnaGen) mRNA purification followed by qRT-PCR using custom primers for CTC confirmation and AR and AR-V7 determination (AdnaTest ProstateCancerDetect kit (AdnaGen)/JHU assay) 2) Epic Sciences & Circulating Tumor Cell Analysis (used in Tokai's ARMOR2): Immunofluorescence using Epic Sciences' N/C-terminal CTC Assay (IHC) to measure existence of splice variants via the presence of a truncated Cterminal domain. 3) Selective AR-V7 option under development (both IHC and RNA-FISH assays) MD Anderson Cancer Center (MDACC): Detection of C-terminal loss using immunohistochemistry methods to produce a ratio of N and C-terminus AR staining of CTCs in bone marrow AR-V7 specific antibody.
Results: CDx vendor selection criteria and recommendations are expected to be based on premarket approval experience, regulatory support, and technical expertise in the methodologies employed to identify AR-V7+ patients. This will include two steps: 1. CTC isolation/characterization and 2. methods to quantify AR-V7. Conclusions: Choice of CDx assay to identify select mCRPC pts harboring a specific SV of the AR will be a key to enrollment strategy for ARMOR3-SV.
References: Antonarakis ES et al. NEJM. 2014 Sep 11;371(11):1028-38., Taplin ME et al. ESMO 2014.
Galeterone shows activity in a variant form of castration-resistant prostate cancer November 18, 2014 The European CanCer Organisation (ECCO) Results from a trial of the anti-cancer drug galeterone shows that it is successful in lowering prostate-specific antigen (PSA) levels in men with a form of prostate cancer that is resistant to treatment with hormone therapy (castration-resistant prostate cancer or CRPC). Results from a trial of the anti-cancer drug galeterone shows that it is successful in lowering prostate-specific antigen (PSA) levels in men with a form of prostate cancer that is resistant to treatment with hormone therapy (castration-resistant prostate cancer or CRPC). Associate professor Mary-Ellen Taplin, of the Dana-Farber Cancer Institute, Boston, USA, will tell the 26th EORTC-NCI-AACR Symposium on Molecular Targets and Cancer Therapeutics in Barcelona, Spain, that galeterone was well tolerated by patients in the ARMOR2 trial, and also lowered PSA levels in a subset of men with CRPC that was resistant to other drugs that target the cancer, such as enzalutamide and abiraterone. "Recent data have shown that a variant of the androgen receptor called AR-V7, found in tumour cells circulating in the blood of patients with metastatic CRPC, predicted resistance to treatment with enzalutamide and abiraterone," she will say. "Indeed, we believe AR-V7 and other, related variants are a mechanism of resistance in this disease and patients who have them may have a poorer prognosis." Researchers believed that galeterone could be effective against CRPC because it disrupts the androgen receptor signalling pathways that are involved in the cancer, and preclinical work has shown it is active against the AR-V7 variant. Several clinical centres in the USA and Canada recruited four groups of men with CRPC to a phase II study to receive 2550mg of galeterone orally once a day: 22 men had CRPC that had not metastasised (spread) and had received no previous treatment; 39 men had metastatic CRPC and no previous treatment with abiraterone or enzalutamide; 37 and nine men had metastatic CRPC and had failed treatment with abiraterone and enzalutamide respectively. As well as evaluating PSA responses to the drug, the researchers also analysed levels of circulating tumour cells, including identifying whether or not they contained the AR-V7 variant. The AR-V7 variant is formed when an androgen receptor loses the end part of the receptor, called the C-terminal end; this is deleted due to an error in RNA processing in tumour cells, leaving only the beginning part of the receptor, the N-terminal end. The researchers concluded that patients with circulating tumour cells with more N-terminals than C-terminals had the androgen receptor variants. "We found that galeterone resulted in meaningful PSA declines in patients with metastatic CRPC, and imaging showed that the disease was stable or had responded to the drug," Prof Taplin will say. "Galeterone was safe, without any unexpected toxicity. We also detected circulating tumour cells, which were found in higher numbers in patients who had received more prior therapies. "In a subset of seven patients who had circulating tumour cells with a higher ratio of N-terminal compared to C-terminal androgen receptors and so were likely to have the AR-V7 variant, six had favourable PSA responses to galeterone. This suggests that the presence of AR-V7 in circulating tumour cells does not preclude response to galeterone as has been shown to be the case for abiraterone and enzalutamide." Among the group of men who had non-metastatic and metastatic disease who had not received prior treatment with abiraterone and enzalutamide, data for 60 were available for analysis. PSA levels declined by 30% or more (PSA30) in 50 out of 60 (83%) patients, of whom 42 (70%) went on to have declines of 50% or more (PSA50). Among patients who were resistant to abiraterone, 37 were available for evaluation; 13 out of 37 (35%) had any PSA decline. Among patients who were resistant to enzalutamide, nine were evaluable; five out of nine (56%) had any PSA decline. The presence of circulating tumour cells were evaluated in 71 patients and were found to be higher in 64 (90%) of the patients who had more advanced cancer that had failed more previous treatments. Galeterone will now be tested in a phase III trial in which patients with metastatic CRPC with the AR-V7 variant will be randomised to receive either galeterone or enzalutamide. The researchers will be looking to correlate AR-V7 with response to galeterone and to see what effect the drug has on the length of time patients survive without their disease progressing. "This phase III trial will be noteworthy for being the first prostate cancer trial to assess a biomarker, namely AR-V7 in circulating tumour cells, as a predictor of response at the same time as testing the efficacy of the drug," Prof Taplin will conclude. Professor Josep Tabernero, a member of the scientific committee for the EORTC-NCI-AACR Symposium and head of the medical oncology department at Vall d'Hebron University Hospital and director of the Vall d'Hebron Institute of Oncology, Barcelona, Spain, commented: "These are encouraging results, which show that galeterone has significant clinical activity in men with castration-resistant prostate cancer that fails to respond to other drugs. Understanding the biological and genetic basis to drug resistance and cancer progression has enabled researchers to identify and develop a targeted drug that may prove to be beneficial in this type of cancer, without causing unmanageable side-effects. We look forward to the results from the phase III trial with interest."
Constitutively Active AR-V7 Plays an Essential Role in the Development and Progression of Castration-Resistant Prostate Cancer Scientific Reports 5, Article number: 7654 Received 28 September 2014 Accepted 01 December 2014 Published 07 January 2015 This study aimed to investigate the role of AR-V7 in development of castration-resistant prostate cancer (CRPC) and to determine whether the AR-V7 expression in CRPC tissues can predict cancer-specific survival. We enrolled 100 localized prostate cancer (PCa) (cohort 1), 104 newly diagnosed metastatic PCa (cohort 2), and 46 CRPC (cohort 3) patients treated at our institution. The expression of AR-V7 in PCa was assessed by immunohistochemistry. Cox regression models were used to evaluate the predictive role of all covariates for the development of CRPC in cohort 2 and for cancer-specific survival in cohort 3. Time to CRPC and cancer-specific survival curves were estimated using the Kaplan-Meier method. AR-V7 expression rate in cohort 3 was significantly elevated compared with other two cohorts (p < 0.001). Multivariate analysis revealed that AR-V7 was an independent predictive factor for CRPC development (HR = 2.627, p = 0.001) and for cancer specific survival (HR = 2.247, p = 0.033). Furthermore, the AR-V7 expression was associated with shorter survival in CRPC patients. Our results demonstrated protein AR-V7 levels in primary tumors can be used as a predictive marker for the development of CRPC and as a prognostic factor in CRPC patients. Therapy targeting AR-V7 may help prevent PCa progression and improve the prognosis of CRPC patients. Prostate cancer (PCa) is the most frequently diagnosed male malignancy in Western countries and represents the second leading cause of cancer-related death1. In China, the incidence of PCa, although lower than in developed countries, has increased remarkably over the past two decades partly attributed to increasing life expectancy, dietary changes and Westernized lifestyle. Furthermore, most newly diagnosed PCa patients already have metastatic disease because of a lack of PCa screening use prostate-specific antigen (PSA) and digital rectal examination in China. Androgen deprivation therapy remains the mainstay of advanced PCa management; however, almost all patients relapse and progress to castration-resistant prostate cancer (CRPC) within a median of 18–24 months, which is accompanied by poor outcome and high lethality. To date, the mechanisms underlying the transition to CRPC have not been fully clarified. Androgen and androgen receptors (ARs) play essential roles in the initiation and progression of PCa. Structurally, the human AR gene is composed of eight exons and encodes a multi-domain protein consisting of an N-terminal transactivation domain (NTD), a central DNA-binding domain (DBD), a hinge region, and a C-terminal ligand-binding domain (LBD). Recent results indicate that the LBD appears to be dispensable for AR transcriptional activity as its deletion leads to constitutive activation of AR transcription capability. Constitutively active, ligand-independent AR splice variants were proposed to be partly responsible for the development and growth of CRPC, irrespective of androgen level. Among more than 20 AR splice variants identified to date, AR-V7 is one of the most abundant and best characterized variants. The clinical relevance of AR-V7 has also been characterized in some respects. For instance, it was reported that increased transcription or protein AR-V7 levels has been detected in CRPC metastases and elevated expression of AR-V7 in PCa tissues is associated with biochemical recurrence and shorter survival. However, the predictive value of AR-V7 expression in primary PCa for the development of CRPC and its prognostic value for CRPC patients have not been well documented. In the current study, immunohistochemistry with an AR-V7 specific antibody, which is a feasible technique that is routinely used in clinical diagnosis, was employed to investigate the expression of AR-V7 in different stages of PCa (cohort 1, 100 localized PCa; cohort 2, 104 newly diagnosed metastatic PCa; cohort 3, 46 CRPC). We assessed the association between AR-V7 expression and patient characteristics and evaluated whether AR-V7 expression in primary tumors can predict the development of CRPC after adjusting for current prognostic factors, such as PSA nadir, time to PSA nadir, and PSA half-life (PSAHL) in cohort 2. Moreover, we determined whether AR-V7 expression in CRPC tissues can predict cancer-specific survival after transurethral resection of the prostate (TURP) in cohort 3 who underwent TURP due to dysuria at the CRPC stage.
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