Andrew J. Armstrong, MSc: There are 2 AR-V7 assays that are really coming close to market. The Epic Sciences assay is one of the first, as well as the Johns Hopkins QIAGEN assay. The Epic assay is a protein-based assay where the blood cells are smeared on a slide of glass, where all the cells are interrogated under a microscope and with a software program that looks at every single cell in that tube of blood. Multiple slides are made, and each cell is annotated with an immunofluorescent marker, such as for white blood cells and tumor cells. Those tumor cells are then counted, and the marker of interest, which is AR-V7 nuclear expression, has been quantified on a per-patient level. So, any detection of AR-V7 in the nucleus is considered a positive test.
Just like the Hopkins assay, which looks at PCR detection in the RNA, the prevalence of a positive test using the Epic assay goes up with resistance and with treatment sequences. A positive test using the Epic assay is in about 5% or 10% of patients in the frontline setting, and it goes up to about 10% to 20% or 30% as the disease progresses. So, the usefulness of the test really becomes important after abiraterone or enzalutamide particularly.
The second AR-V7 test, which was developed at Johns Hopkins by Dr. Luo’s lab, is an RNA-based expression. A very specific primer is developed to the cryptic exon of AR-V7, which is unique for that sequence, and is amplified and then detected in the circulating tumor cells of an enriched blood specimen. So, again, a liquid biopsy test—not visible in a cell but a kind of whole-cell extraction approach where you can measure the AR-V7 at the RNA level. A positive test is really defined using both positive and negative controls and then amplified RTP-CR cycle.
This is a very simple test, with rapid turnaround just like the Epic Sciences assay, within 1 to 2 days. And a positive test is, again, associated with rapid resistance and a low response rate to abiraterone and enzalutamide. So, both tests really do appear to have some predictive value. They certainly have poor prognostic value.
I want to point out that neither of these tests yet has been externally validated, so that’s actually our ongoing work is to compare these tests head-to-head in a prospective multicenter trial where there’s an external validation component to this study. We call this the Prostate Cancer Foundation November Challenge Grant. And this challenge study has enrolled 120 men. We completed enrollment in December of 2016. We expect to have results within about 2 or 3 months from today, and those results are anticipated to prospectively validate both the Epic Sciences AR-V7 test and the Hopkins QIAGEN AR-V7 PCR-based test. And all the patients in the study are either frontline or second-line therapy castrate-resistant patients who are treated at 1 of 5 different large academic centers around the United States and then followed prospectively for survival-based outcomes."
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pjoshea13
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Thanks for the information. At my last visit with my Oncologist I mentioned resistance and AR-V7 and she said " that's the usual culprit". From what I've been reading, about 30% of patients with CRPC are positive for AR-V7. At that point, I guess your only options are to go on chemo or a clinical trial for something new.
AR-V7 is this a gene variant that you are born with or one that develops with the disease? Different method, doing the test from tunor tissue; does Foundstion One do this test or Guardiant 360 or who does the tumor tissue test?
As far as I know, AR mutations are somatic and most common in CRPC. I'm not sure which genetic tests, if any are testing for them. I also think these can be tested from CTCs and need not be taken from a biopsy. That's my understanding, but someone can correct me if I'm wrong.
From what I have read, AR-V7 can be present at the start of ADT, but in very small numbers, since there is not yet a cancer survival advantage.
"AR-V1 and AR-V7 mRNA showed an average 20-fold higher expression in HRPC {i.e. CRPC} ... when compared with hormone-naive PCa ... Among the hormone-naive PCa, higher expression of AR-V7 predicted biochemical recurrence following surgical treatment ..." [4]
Regardless, If AR-V7 is going to be the primary cause of CRPC, there doesn't seem much point in testing for AR-V7 prematurely, in the absence of an add-on drug that targets AR-V7. Although the addition of a taxane may reduce the population:
"While 'conversions' to AR-V7-positive status were observed with both AR-directed therapies {i.e. abiraterone and enzalutamide} and taxane chemotherapies {i.e. docetaxel, cabazitaxel}, 'reversions' to AR-V7-negative status only occurred during taxane therapies." [1]
"Detection of AR-V7 in CTCs from men with metastatic CRPC is not associated with primary resistance to taxane chemotherapy. In AR-V7-positive men, taxanes appear to be more efficacious than enzalutamide or abiraterone therapy, whereas in AR-V7-negative men, taxanes and enzalutamide or abiraterone may have comparable efficacy. [2]
In CRPC related to AR-V7, the full-length AR is usually present:
"... overexpression of both the canonical full-length AR (AR-FL) and AR-Vs are frequently observed in CRPC." [3]
"In clinical CRPC specimens, individual AR-Vs are often co-expressed with AR-FL and are usually less abundant than the wild-type transcripts. For example, ... analysis of circulating tumor cells from men with CRPC has revealed the median mRNA ratio of AR-V7/AR-FL is 21% ..."
"AR-V7 protein was detected in 57% of men who developed disease progression within 4 months of starting enzalutamide, but was not detected in any patient who responded to enzalutamide for longer than 6 months. These finding were inline with an earlier study by the same investigators evaluating the combination of abiraterone and enzalutamide in 60 men with bone-metastatic CRPC. In that study, AR-V7 protein was detected in bone marrow biopsies from 66% of patients who developed progression within 4 months, but in none of the men who responded to therapy for more than 6 months."
It may seem obvious that AR splice variants that do not need DHT spell the end of ADT efficacy, but the authors write:
"The principal question that arises from the data presented is whether AR-Vs, particularly AR-V7, are drivers of malignant progression and treatment resistance in the clinic, or whether AR-Vs are passenger markers of aggressive disease. ....t it may be that AR-FL gains rather than AR-V expression is important given their relative abundance."
"AR-V7 protein was detected in 57% of men who developed disease progression within 4 months of starting enzalutamide, but was not detected in any patient who responded to enzalutamide for longer than 6 months."
That's a very significant finding. I've been reading about mechanisms of resistance in CRPC and it's thought that mutations in the AR are more responsible than mutations where neuroendocrine characteristics develop. It's still not completely understood and more research is needed.
The Androgen Receptors are constructed in the prostate cell, as any other proteins are. The AR gene is transcribed in to mRNA for the future AR protein, edited, and sent out into the cytoplasm to the ribosome to build (by translating the mRNA) the Androgen Receptor protein. "Splicing" takes place in the editing of the mRNA in the nucleus, after transcription. So a splice variant then seems to mean a variation of editing of the transcibed mRNA.
Does this mean that, in that particular cell, all the Androgen Receptors will have the same variant editing? Or is there a mix of the variations of Androgen Receptor in a given cell? If it were a error in the DNA, the answer would be easy: in any cell they are presumed to all be the same variant. But it does not seem to be related to a mutation in the DNA. So the answer is not clear.
It could be that just a few variants could alter the over-all lethality of the prostate cell, especially if one AR can start cell division.
The AR has, in addition to the ligand binding domain (the pocket for Testosterone), a DNA binding domain, where the AR hooks onto the DNA, upstream from the gene or genes to be expressed. The AR must have more than one function, since it controls many male pattern structures, so it must have multiple DNA hooks, to trigger the various appropriate genes. Hmmm.
Thanks, by the way, for the transcription of the video.
He says "So, any detection of AR-V7 in the nucleus is considered a positive test."
How often would one find any AR in the nucleus? And why look in only the nucleus for AR. Why is AR in the cytoplasm of less interest?
A general question, after AR has bound itself to the DNA and finished expressing some gene, does it reenter the cytoplasm, or what? The AR is classified as a transcription factor. What happens to transcription factors?
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Most likely positive for genetic mutation and AR-V7
AR-V7, Zytiga(abiraterone) and Xtandi (enzalutamide)
Are all AR inhibitors the same?
CCS 1477 3rd generation AR
