Results from a preclinical study, funded in part, by the National Cancer Institute, Department of Defense, and others and published in the June 18, 2021 edition of Clinical Cancer Research researchers from the University of Michigan Rogel Cancer Center describe the results of their examination of the underlying mechanisms that may help cancer cells evolve, adapt and overcome the anticancer treatments used against them.

In the article, the researchers describe how they uncovered a promising countermeasure of why some prostate tumors transition from being glandular to more nerve-like in response to drugs that block the androgen receptor.[1]

Extended Survival
Prostate cancer is the second-most commonly diagnosed cancer among men and the fifth most common cause of male cancer death worldwide. Adenocarcinoma of the prostate is dependent on androgen for tumor progression and depleting or blocking androgen action, by androgen receptor inhibitors such as enzalutamide, apalutamide, darolutamide, and abiraterone, has been a mainstay for decades. This approach has resulted in the expanded survival benefit for prostate cancer patients.

Androgen receptor inhibitors were designed to block the production or action of male hormones that fuel cancer, however, while prostate cancer is often initially sensitive to medical or surgical therapies that decrease levels of testosterone, over time, these drugs stop working. When that happens, a patient’s disease is considered incurable. Doctors refer to this as metastatic, castration-resistant prostate cancer.

Lineage plasticity
In the new study, a team of researchers led by Joshi Alumkal, M.D., who leads the prostate and genitourinary medical oncology section at the University of Michigan Rogel Cancer Center, uncovered new mechanisms underlying an important type of resistance called lineage plasticity.

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In prostate cancer, lineage plasticity is generally shown by loss of androgen receptor (AR) signaling.  When this happens castration-resistant prostate cancers undergo a deadly identity switch, shifting from resembling glandular cells to neuroendocrine cells, which can behave more like small cell lung cancer.

In contrast to de novo neuroendocrine prostate cancer (NEPC), treatment-emergent NEPC (t-NEPC), which is increasing in the era of novel AR inhibitors, tumors often express the AR. However, the functional role of AR in t-NEPC is still unknown. Furthermore, targetable factors that promote t-NEPC lineage plasticity are also unclear. As a result, there are only limited treatments for NEPC.

A promising treatment option
The findings of this study outline a promising path to overcoming of resistance using BET bromodomain inhibitors.

These compounds work against bromodomain and extra-terminal (BET) proteins, which are involved in regulating gene activation.

“We know that treatment-emergent neuroendocrine prostate cancer is becoming more frequent as we use new and more potent androgen receptor inhibitors,” Alumkal said.

“Our prior work examining patients progressing on these newer androgen receptor inhibitors demonstrated that neuroendocrine prostate cancer was found in 17% of cases. By comparison, we find it in less than 1% of patients who have not undergone any form of androgen receptor inhibition. This strongly suggests that interference with androgen receptor function contributes to the increased numbers of treatment-emergent neuroendocrine prostate cancers we now see clinically,” Alumkal explained.

Patients diagnosed with treatment-emergent neuroendocrine prostate cancer fare much worse than patients whose tumors remain adenocarcinomas — glandular tumors — surviving for only about one-third as long. Furthermore, there are very limited treatment options for patients with treatment-emergent neuroendocrine prostate cancer.

“We set out to understand how tumors shift their program to become neuroendocrine, the influence of androgen receptor inhibition on this process, and ways to block the switch to neuroendocrine prostate cancer,” noted Alumkal, who also co-leads of the translational and clinical research program at the U-M Rogel Cancer Center.

This line of investigation began when Alumkal was at the Knight Cancer Institute at Oregon Health & Science University and continued after his move to the U-M in 2019. The effort included a host of collaborators at other institutions, including scientists from the Dana-Farber Cancer Institute at Harvard University, the University of Washington, the Fred Hutchison Cancer Research Center, the Helen Diller Family Comprehensive Cancer Center at the University of California, San Francisco, and the Memorial Sloan Kettering Cancer Center.

A promising countermeasure
The major question the researchers wanted to answer was whether modern prostate cancer drugs, the new and more potent androgen receptor inhibitors, might be so effective in shutting down the androgen receptor in some tumors that these drugs could actually promote the tumors’ switch to become neuroendocrine tumors.

The researchers found that the answer appears to be yes!

Through a series of experiments using enzalutamide-resistant t-NEPC cell lines and their parental, enzalutamide-sensitive adenocarcinoma cell lines, cell laboratory models that were either sensitive to newer androgen receptor inhibitors or their counterpart cell models with acquired resistance, the research team found that androgen receptor inhibition was accentuating a neuroendocrine prostate cancer lineage plasticity program in the resistant cells, an effect not achievable in the sensitive cells.

“Ultimately, we found that a critical difference that may allow the resistant cells to react differently and become more neuroendocrine is due to the way their DNA is organized and packaged,” Alumkal said.

“In the treatment-resistant cells, the chromatin is organized in a way that’s more conducive to turning on that neuroendocrine program when the androgen receptor is blocked. It is as though the resistant cells have evolved to develop a rabbit hole through which they can change their identity and escape androgen receptor inhibition. We also found that high levels of a transcription factor called E2F1, which is involved in stemness and cell differentiation, is important for the resistant cells’ ability to switch identities,” he explained.

BET inhibitors
The results of this study uncovered the mechanisms that are giving rise to treatment-emergent neuroendocrine cancer and pointed toward BET inhibitors as a possible solution.

Although it is believed that E2F1 is not directly targetable, Alumkal and his team determined that E2F1 cooperates with BET bromodomain proteins to turn on a neuroendocrine prostate cancer lineage plasticity program. Blocking BET bromodomain proteins in cell models stopped the activation of this program that drives the development of neuroendocrine prostate tumors, the research team found.

“When we treated a variety of treatment-emergent neuroendocrine prostate cancer cell lines with BET inhibitors, we greatly reduced the viability of these tumors, including tumors derived from patients,” Alumkal further explained.

Building on previous research
The study builds on a previous clinical trial by Alumkal and colleagues, which found a BET bromodomain inhibitor compound developed by Zenith Epigenetics, ZEN-3694, looked most active in castration-resistant prostate tumors from patients who had the lowest androgen receptor activity. Additionally, patients in the trial whose tumors didn’t respond well to androgen receptor inhibitors prior to study enrollment appeared to have the most durable control with ZEN-3694, suggesting the most aggressive tumors may be particularly susceptible to BET bromodomain inhibition.

“We went back and determined that several patients from that ZEN-3694 clinical trial had treatment-emergent neuroendocrine prostate cancer,” Alumkal observed.

“When we looked at the subset of those patients who did the best, they had the highest expression of E2F1 and the BET bromodomain protein BRD4, and the lowest expression of the androgen receptor,” he added.

“When we examined the neuroendocrine prostate cancer lineage plasticity program we identified in our cellular models, we found these same genes were highly turned on in the tumors from patients with treatment-emergent neuroendocrine prostate cancer who responded best to treatment with ZEN-3694,” Alumkal said.

Ultimately, Alumkal explained, that the research points toward BET bromodomain inhibitors as potentially most beneficial for patients whose prostate cancer is no longer as dependent on the androgen receptor, and those with subsets of treatment-emergent neuroendocrine prostate cancer, particularly tumors in which transcription factor E2F1 may be playing an important role.

Building on the findings in the current study, a larger, international, randomized clinical trial is being planned to evaluate the effectiveness of ZEN-3694 with a particular focus on men whose tumors responded poorly to androgen receptor inhibitors, tumors that may be less dependent on the androgen receptor.

Development collaboration
Zenith Epigenetics, which develops novel epigenetic combination therapies for the treatment of cancers, announced that it has entered into an agreement with Astellas Pharma to evaluate ZEN-3694, Zenith’s leading BET inhibitor, in combination with Astellas and Pfizer’s androgen receptor inhibitor, XTANDI (enzalutamide), in patients with metastatic castration-resistant prostate cancer (mCRPC).

Clinical trials
ZEN-3694, Enzalutamide, and Pembrolizumab for the Treatment of Metastatic Castration-Resistant Prostate Cancer –  NCT04471974
A Study of ZEN003694 in Patients With Metastatic Castration-Resistant Prostate Cancer – NCT02705469
Continuation Protocol for ZEN003694 in Patients Experiencing Clinical Benefit While Enrolled in a ZEN003694 Protocol – NCT04145375
A Study of ZEN003694 in Combination With Enzalutamide in Patients With Metastatic Castration-Resistant Prostate Cancer – NCT02711956

Highlights of prescribing information
Enzalutamide (Xtandi®; Astellas Pharma/Pfizer) [Prescribing Information]
Apalutamide (Erleada®; Janssen Biotech)[Prescribing Information]
Darolutamide (Nubeqa®; Bayer)[Prescribing Information]
Abiraterone (Zytiga®; Janssen Pharmaceuticals) [Prescribing Information]

Reference
[1] Kim DH, Sun D, Storck WK, Welker Leng KR, Jenkins C, Coleman DJ, Sampson DA, Guan X, Kumaraswamy A, Rodansky ES, Urrutia JA, Schwartzman J, Zhang C, Beltran H, Labrecque MP, Morrissey C, Lucas JM, Coleman I, Nelson PS, Corey E, Handelman SK, Sexton JZ, Aggarwal R, Abida W, Feng FY, Small EJ, Spratt DE, Bankhead A, Rao A, Gesner EM, Attwell S, Lakhotia S, Campeau E, Yates JA, Xia Z, Alumkal JJ. BET Bromodomain Inhibition Blocks an AR-Repressed, E2F1-Activated Treatment-Emergent Neuroendocrine Prostate Cancer Lineage Plasticity Program. Clin Cancer Res. 2021 Jun 18:clincanres.4968.2020. doi: 10.1158/1078-0432.CCR-20-4968. Epub ahead of print. PMID: 34145028.

Featured image: Neuroendocrine prostate cancer cells. Photo courtesy: © 2021 Aaron Udager, Michigan Medicine. Used with permission.

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