Key Clinical Summary: Rationale for Targeting Epigenetic Pathways

This is a micro-learning module summary of a presentation by Dr. Michael Schweizer which you can find here. Before participating, please read our CME and disclosure information which can be found here.

This activity is supported by an independent medical educational grant from Pfizer. This online education program has been designed for healthcare professionals globally.

Introduction
This summary explores emerging strategies to target epigenetic pathways in metastatic castration‑resistant prostate cancer (mCRPC), outlining the biologic rationale for epigenetic modulation, the relationship between lineage plasticity and treatment resistance, and the therapeutic potential of EZH2 and PRC2 inhibition. It reviews differences between androgen receptor (AR)‑positive and neuroendocrine prostate cancer (NEPC), and early clinical trial results evaluating epigenetic agents in combination with AR‑directed therapy.

Epigenetic Regulation and Its Role in mCRPC Biology

In mCRPC, epigenetic mechanisms such as DNA methylation, histone acetylation, and histone methylation reshape transcriptional programs that contribute to treatment resistance without altering DNA sequence.
Key histone modifier classes include:
- Writers such as DNMTs and EZH2, which add methylation marks
- Erasers such as HDACs, which remove acetylation marks.
- Readers such as BET proteins, which bind acetylated histones and promote oncogene transcription.
- Movers such as ARID1A, which reposition nucleosomes to remodel chromatin.
Dysregulation of these pathways contributes to lineage plasticity, loss of luminal identity, and emergence of AR‑independent phenotypes.
Epigenetic alterations are increasingly recognized as central drivers of resistance to AR‑directed therapies and radioligand therapy.

Epigenetic Differences Between AR‑Positive and Neuroendocrine Prostate Cancer

Although AR‑positive prostate cancer (ARPC) and NEPC share many genomic alterations, they diverge markedly at the epigenetic level.
Genome‑wide methylation profiling demonstrates distinct clustering of ARPC and NEPC, reflecting divergent transcriptional programs.
In ARPC, H3K27me3 deposition by EZH2 silences neuroendocrine lineage transcription factors such as ASCL1, PROX1, and INSM1.
In NEPC, EZH2‑mediated repression targets luminal and epithelial genes including AR, HOXB13, REST, and GATA2.
These patterns support a model in which epigenetic remodeling facilitates lineage switching and contributes to aggressive, treatment‑refractory disease.

Mechanistic Rationale for Targeting EZH2

EZH2 is the catalytic component of the polycomb-repressive complex 2 (PRC2) and deposits repressive H3K27me3 marks.
Increased EZH2 expression and activity are observed as prostate cancer progresses from localized disease to mCRPC and NEPC.
Mechanisms through which EZH2 promotes resistance include:
- Suppression of luminal epithelial programs, enabling neuroendocrine transdifferentiation.
- Collaboration with N‑Myc, redirecting EZH2 to new genomic loci that reinforce lineage plasticity.
- Non‑canonical interactions with AR, enhancing AR‑driven transcription even in the absence of ligand.
Preclinical models show that EZH2 inhibition can block transdifferentiation, although single‑agent activity in NEPC is limited, supporting combination strategies with AR‑directed therapy.

Clinical Evidence for PRC2/EZH2 Inhibition in mCRPC

A phase 2 randomized study evaluated the EZH2 inhibitor mevrometostat plus enzalutamide versus enzalutamide alone in patients with mCRPC previously treated with abiraterone.
A total of 81 patients with progression per PCWG3 criteria were enrolled, with allowance for ≤1 prior chemotherapy regimen. All were receiving ongoing androgen-deprivation therapy.
Baseline characteristics were balanced, with nearly half of patients having prior taxane exposure and the majority presenting with bone metastases.
Efficacy outcomes (9.6 months’ median follow-up):
- Radiographic progression-free survival (PFS): 14.3 months with mevrometostat plus enzalutamide vs 6.2 months with enzalutamide alone (hazard ratio 0.51 [90% confidence interval 0.28–0.95]; a 49% reduction in risk of progression).
- Objective response rate: 26.7% with the combination vs 14.3% with enzalutamide alone.
- PSA50 response: 34.1% with the combination vs 15.4% with enzalutamide alone.
Safety profile:
- Most treatment‑emergent adverse events (TEAEs) were grade 1–2.
- Gastrointestinal (GI) toxicities such as diarrhea were among the most frequent with combination therapy.
- Dose reductions due to TEAEs were more common with the combination than with enzalutamide alone (36.6 vs 7.5%), but discontinuation rates were low (2.4 vs 5.0%).

Comparison With Other Epigenetic Approaches

A separate randomized study of enzalutamide with or without tazemetostat did not demonstrate a radiographic PFS benefit (p=0.3704).
The control arm in that study had an unexpectedly long PFS, complicating interpretation.
Differences in patient selection, drug potency, and pharmacokinetics may account for divergent results.
Beyond EZH2, additional epigenetic targets under investigation include:
- BRD4, a BET family protein involved in oncogene transcription.
- NSD2, a histone methyltransferase implicated in chromatin remodeling.
- p300/CBP, histone acetyltransferases that regulate enhancer activity.
  These emerging targets reflect the expanding therapeutic landscape aimed at reversing epigenetic drivers of resistance.

Clinical Implications for Treatment Planning

Epigenetic dysregulation is described as a key contributor to resistance in mCRPC, particularly through mechanisms that promote lineage plasticity and diminish reliance on AR‑driven luminal programs.
EZH2 represents a rational therapeutic target because of its dual role in suppressing luminal gene expression and facilitating neuroendocrine‑associated transcriptional programs, as well as its non‑canonical collaboration with AR signaling.
Patients with features associated with lineage plasticity, such as visceral metastases, low PSA relative to disease burden, or molecular alterations like TP53 mutation, may warrant consideration for clinical trials evaluating PRC2/EZH2 inhibitors.
Incorporating epigenetic therapies under investigation, including PRC2/EZH2 inhibitors, into treatment planning offers a strategy to address resistance mechanisms that emerge despite AR‑directed therapy and may help improve outcomes in advanced mCRPC.

Conclusions
Epigenetic dysregulation plays a central role in the evolution of treatment resistance in mCRPC, driving lineage plasticity and diminishing reliance on AR signaling. Targeting EZH2 and the PRC2 complex represents a promising therapeutic strategy, supported by mechanistic data and early clinical evidence demonstrating improved radiographic PFS and response rates when combined with AR‑directed therapy. As multiple epigenetic agents advance through clinical development, incorporating these approaches into treatment planning may help extend the durability of AR‑directed therapies and address the unmet needs of patients with aggressive, treatment‑refractory disease.

Content is accurate as of the date of release on 2 March 2026.