In yesterday’s post on novel targets in advanced prostate cancer, we looked at a potential new concept that is emerging and being evaluated in clinical trials.
Today, we continue that approach with a look at another novel – and quite different target – as well as ways of repurposing old drugs to help potentially overcome the drug resistance seen with newer therapies for this disease.
Warning: the concepts discussed here may well take you by surprise, although they are based on rational and logical evidence from recent scientific research.
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Dr Nima Sharifi (Cleveland Clinic) gave an intriguing talk at the ASCO Genitourinary symposium recently (#GU16), where he explored new ways of targeting and old target i.e. the androgen receptor with abiraterone.
He noted that 2015 was a landmark year for prostate cancer in terms of genetic characterisation of the disease with a key publication in Cell from several groups including Levi Garraway, Charles Sawyers and Arul Chinnaiyan’s labs via the Stand Up to Cancer research program.
It’s a well written and easy to read paper that I highly recommend reading (see Robinson et al., 2015 in the References below).
In addition, the TCGA work is summarised for those interested in Cell via the TCGA portal for prostate cancer (Link) based on integrated profiling of 333 primary prostate adenocarcinoma samples (the separate Cell paper is also included in the References below). For the geeky scientists here, you can also access and query the raw data they collated at the NCI (Link) and the full TCGA datasets for numerous cancers (most were updated in December 2015) can also be found here (Link).
Overall, the SU2C effort is particularly impressive on several fronts and the massive collaboration effort involved sought to determine the genetic factors at play in mCRPC:
“A multi-institutional integrative clinical sequencing analysis reveals that the majority of affected individuals with metastatic castration-resistant prostate cancer (mCRPC) harbor clinically actionable molecular alterations, highlighting the need for genetic counseling to inform precision medicine in affected individuals with advanced prostate cancer.”
The group identified the following key pathways involved in mCRPC, which may lead to more refined clinical work in the near future to optimise and improve the therapy options available for patients with this disease:
One thing that I really hope to see going forward is more focused and rational clinical trials in advanced prostate cancer, where different subsets such as those identified by the SU2C project are tackled with more logical combinations rather than treating everyone the same. This is a highly heterogeneous cancer, after all.
For example, we saw in yesterday’s post on DNA repair how trials are already starting or planned – some are allcomer trials (why oh why?!) and some are more specific including only patients with mismatch repair that the PARP inhibitors are supposed to target (a much better approach in our view).
It drives me crazy when, even in 2016, we see companies treating a targeted therapy in an untargeted fashion and expect to see positive results from such a diluted approach!
So what do we have in store today? At the recent ASCO Genitourinary symposium (#GU16), there were a number of intriguing talks that we will continue to highlight for the rest of this week.
After a look at what’s new in PARP inhibition, we now turn our focus and attention to the androgen receptor (AR) and abiraterone acetate (Zytiga), in particular. This drug was first approved by the FDA in 2010 for CRPC patients who had received prior chemotherapy (docetaxel) and then subsequently in the pre-chemotherapy setting. Both Zytiga and enzalutamide (Xtandi) compete with each other in both of these settings. One particular challenge is that the therapies work well as single agents but the effect lessens over time as resistance sets in.
A fresh look at abiraterone and steroidogenesis
In this particular talk, Dr Sharifi focused specifically on the androgen receptor (AR) where we have two drugs now approved – enzalutamide, which binds to the AR directly and abiraterone, which impacts steroidogenesis downstream.
The key question being addressed here was that we know abiraterone targets steroidogenesis, but does a bidirectional effect occur i.e. can steroidogenesis come back and regulate abiraterone? If so, is this also a potential mechanism of resistance and how can we improve the clinical situation for patients?
The majority of this work is also covered in recent publications by Li et al., (2015) and Sharifi (2015) see References.
What happens when we give abiraterone to mice and men?
First off, Sharifi described abiraterone and androgens as ‘kissing cousins’ with a shared metabolism, which I found quite amusing. Essentially, the abiraterone is converted to D4A, a metabolite. This occurs consistently in both species, although the concentrations are more variable in humans.
Is D4A inert and doesn’t do much or is it clinically relevant?
To answer this question, they looked at the essential steps in the androgen pathway in CRPC and looked at the impact on DHT synthesis. In two out of three cases, they found that D4A was a more potent inhibitor than abiraterone, while the other was equivalent.
Does D4A bind to the AR?
Both abiraterone and D4A have a common 3-Keto element in their structure, similarly to testosterone. Does this impact AR binding capacity?
In tests, they found that D4A binds to the AR receptor as least as well as enzalutamide and also blocks PSA expression in the same fashion. These data suggest that D4A is an antagonist, not an agonist.
Is their any evidence that this translates into anti-tumour efficacy?
In CRPC xenograft experiments, the researchers found that D4A has more potent anti-tumour activity than abiraterone or enzalutamide, based on the PFS in mice. This may not translate to clinical benefit in humans, but it should at least be worth exploring based on the findings of these models.
Sharifi also mentioned some unpublished work where they noticed that the D4A A/B rings also look like the testosterone structure. Interestingly, this is the substrate for 5α-reductase… so they asked the critical question: can the D4A metabolites be 5α-reduced and 5Ɓ-reduced? Note 5Ɓ-reductase serves to inactivate steroids via a different function.
The answer to both 5α-Abi and 5Ɓ-Abi was yes, they can be reduced.
In addition, Sharifi highlighted the chemical molecules and conformation – 5α-Abi retains a similar plane and conformation structrure as a steroid (i.e. a lookalike), whereas 5Ɓ-Abi is different and has a 90 degree bend at one end, which leads to steroid inactivation.
What happens when you expose prostate cancer cell lines (LNCaPr) to D4A?
They found that within 2 days, D4A was almost depleted because it was converted to the 5α-Abi metabolite.
What are the effects of 5α-Abi on enzymes required for DHT synthesis?
Essentially, they found that 5α-Abi < D4A, i.e. a weaker inhibitor.
What about downstream of metabolism?
Recall that 5α-Abi has the same A/B rings as DHT. They also found that 5α-Abi binds to AR, so does it cause agonist or antagonist effects? It turned out to be an agonist, which would explain the weaker inhibitory effects described above. In mouse xenograft models, this was alos confirmed by a much shorter PFS, which you would expect with an agonist.
What about the good and the bad effects?
By now, we can see that D4A can reduced to both good (5α-Abi) and bad (5Ɓ-Abi) metabolites. This leads to the next question: can we block the bad effects and enhance the good effects? Part of the answer to this lies in data from a phase 2 clinical trial that Dr Mary Ellen Taplin (DFCI) is running in mCRPC (Link).
Here, they are comparing the impact of adding a 5α-reductase inhibitor, dutasteride, to abiraterone plus prednisone versus abi plus steroid alone.
Remember that 5α-reductase inhibitors such as dutasteride and finasteride are old drugs originally approved by the FDA in the early 2000’s for symptomatic benign prostatic hyperplasia (BPH) in men with an enlarged prostate, so prostate cancer experts are well versed with these agents. The standard dose for dutasteride for BPH is 0.5mg/day, although this trial used a much higher dose, 3.5mg/day as an anti-cancer therapy.
The schematic for the trial is as follows – note that by exploring the doublet first then adding in the dutasteride, each patient acts as their own control, which may reduce intra-patient variability:
What did they find?
1) The Good: D4A concentrations nearly doubled when you add dutasteride
2) The Bad: 5α-Abi had a profound (90%) drop when dutasteride was added*
3) The Ugly: 5Ɓ-Abi had no real change in any of the 3 metabolites
*They saw a similar and highly consistent depletion with the other two 5α-Abi metabolites as well.
This preliminary research suggests that when a 5α-reductase inhibitor is added to abiraterone plus prednisone, it blocks the bad effects and enhances the good effects, exactly as we might expect or hope to see.
This is a very nice proof of principle in a small study that such a combination might be useful in mCRPC. What we do need to see now are outcomes data in a larger randomised controlled trial before attempting a more definitive conclusion.
Here are Dr Sharifi conclusions from their work, which I don’t disagree with:
Conclusions and Additional Commentary:
“Prostate cancer resistance to castration occurs because tumours acquire the metabolic capability of converting precursor steroids to 5α-dihydrotestosterone (DHT), promoting signalling by the androgen receptor and the development of castration-resistant prostate cancer.”
What this work suggests is that in the short term, there may be some clinical utility in combining a 5α-reductase inhibitor such as dutasteride with a CYP17A1 inhibitor such as abiraterone plus a steroid (prednisone), in order to reduce resistance to therapy and prolong outcomes for men with advanced prostate cancer.
In addition, the authors suggest a novel approach to next generation development of new AR agents targeting steroidogenesis:
“We propose that direct treatment with D4A would be more clinically effective than abiraterone treatment.”
If the clinical results from Dr Taplin’s triplet vs. doublet combination trial are encouraging then don’t be surprised to see a company with a prostate cancer franchise doing a deal with a small company developing anti-D4A inhibitors in the near future since the affinity of D4A for androgen receptors was significantly greater than that of abiraterone, similar to that of enzalutamide, and greater than that of bicalutamide.
As far as I know, the phase 2 study analysis was due to complete in December 2015, so an abstract may have been submitted to ASCO for the 2016 annual meeting (the deadline is this month). That’s probably the next likely milestone for a comprehensive readout and presentation of the full data.
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Cancer Genome Atlas Research Network. Electronic address: firstname.lastname@example.org, & Cancer Genome Atlas Research Network (2015). The Molecular Taxonomy of Primary Prostate Cancer. Cell, 163 (4), 1011–25 PMID: 26544944
Li Z, Bishop AC, Alyamani M, Garcia JA, Dreicer R, Bunch D, Liu J, Upadhyay SK, Auchus RJ, & Sharifi N (2015). Conversion of abiraterone to D4A drives anti-tumour activity in prostate cancer. Nature, 523 (7560), 347–51 PMID: 26030522
Sharifi N (2015). Prostate cancer: CYP17A1 inhibitor failure-lessons for future drug development. Nature reviews. Urology, 12 (5), 245–6 PMID: 25823374
Sharifi N (2015). Steroid sidestep: evading androgen ablation by abiraterone. Clinical cancer research : an official journal of the American Association for Cancer Research, 21 (6), 1240–2 PMID: 25432158