Home
This site is intended for healthcare professionals
Sign upLog in

Key Clinical Summary: Androgen Receptor Pathway Inhibitor Resistance

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 mechanisms of resistance to androgen receptor pathway inhibitors (ARPIs) in metastatic castration‑resistant prostate cancer (mCRPC), outlining the clinical context in which resistance emerges, the molecular pathways that drive therapeutic failure, and the growing recognition of epigenetic dysregulation and lineage plasticity as central contributors. It reviews the heterogeneity of mCRPC, the limited efficacy of sequential ARPI therapy, and the biologic underpinnings that shape disease progression and treatment decision‑making.

Hormone‑Sensitive to Castration‑Resistant Progression

  • mCRPC develops after an initial period of disease control with androgen deprivation therapy (ADT) combined with ARPIs such as abiraterone, enzalutamide, apalutamide, or darolutamide.
  • Despite strong initial responses, nearly all tumors eventually progress, reflecting adaptive and acquired resistance mechanisms.
  • Treatment options in the castration‑resistant setting include ARPIs, taxane chemotherapy, radiopharmaceuticals, poly ADP-ribose polymerase (PARP) inhibitors for homologous recombination repair (HRR)-mutated disease, and prostate-specific membrane antigen (PSMA)‑targeted radioligand therapy.
  • The clinical challenge lies in selecting therapies that address the dominant resistance mechanism while recognizing that mCRPC is molecularly diverse and evolves under therapeutic pressure.

Limited Efficacy of Sequential ARPI Therapy

  • Cross‑resistance between abiraterone and enzalutamide is well documented, with modest prostate-specific antigen (PSA) response rates when one agent is used after progression on the other.
  • Retrospective series consistently show:
    • Abiraterone after enzalutamide: PSA50 responses typically <10%.
    • Enzalutamide after abiraterone: PSA50 responses generally ~20–30%.
  • A prospective, phase 2 crossover trial in mCRPC demonstrated:
    • Significantly longer PFS2 for the sequence abiraterone → enzalutamide compared with enzalutamide → abiraterone.
    • No significant overall survival difference between sequences.
  • These findings highlight the need to understand underlying resistance biology rather than relying on sequential ARPI therapy.

Genomic Landscape of mCRPC

  • Large sequencing efforts demonstrate extensive heterogeneity in mCRPC, with recurrent alterations in:
    • Androgen-receptor (AR) pathway genes (amplifications, mutations, splice variants).
    • Phosphoinositide 3-kinase (PI3K) pathway components (PTEN loss).
    • DNA repair genes (BRCA1/2, ATM, other HRR genes).
    • Cell‑cycle regulators (RB1, TP53).
  • AR alterations are among the most common findings, present in more than half of mCRPC tumors, making AR amplification the most likely genomic event in patients progressing on long‑term ARPI therapy.

Categories of ARPI Resistance Mechanisms

  • AR‑activating alterations
    • AR gene amplification leading to hypersensitivity to low androgen levels.
    • Ligand‑binding domain (LBD) mutations that convert antagonists into agonists or broaden ligand specificity.
    • AR splice variants (e.g. AR‑V7) that signal independently of ligand binding.

Non–AR‑dependent oncogenic pathways
- Loss of tumor suppressors (e.g., PTEN) driving PI3K pathway activation.
- Cell‑cycle dysregulation through RB1 or TP53 alterations.
- Activation of alternative survival pathways that bypass AR signaling.

Lineage plasticity

  • Chronic AR suppression promotes tumor evolution toward AR‑null phenotypes. Key features include:
    • Loss of luminal markers and AR expression.
    • Acquisition of neuroendocrine or other alternative lineage programs.
    • Increased incidence of AR‑null tumors in the post‑abiraterone era, suggesting treatment‑driven selection.
  • Molecular drivers of lineage plasticity include:
    • RB1 and TP53 loss, which destabilize lineage fidelity and facilitate phenotypic switching.
    • Upregulation of transcription factors such as SOX2, MYCN, ASCL1, and REST.
    • Epigenetic dysregulation, particularly increased EZH2 activity, which reinforces repression of luminal/AR‑dependent programs.

Molecular Subtyping of mCRPC

  • Transcriptional and immunohistochemical profiling can classify mCRPC into biologically and clinically meaningful subtypes.
  • Subtyping based on AR and neuroendocrine marker expression helps distinguish:
    • AR‑high adenocarcinoma
    • AR‑low or AR‑null adenocarcinoma
    • Neuroendocrine prostate cancer
    • Double‑negative tumors lacking both AR and neuroendocrine markers
  • This framework supports more rational therapeutic strategies and may guide future clinical trial design.

Implications for Clinical Practice

  • Understanding resistance mechanisms is essential for interpreting progression on ARPIs and determining appropriate next therapeutic steps.
  • AR‑activating alterations such as amplification, LBD mutations, or splice variants remain among the most common findings in mCRPC and help explain why sequential ARPI therapy yields limited benefit.
  • Non–AR‑dependent pathways and lineage plasticity highlight the need to consider therapies beyond ARPIs, including options already used in the castration‑resistant setting such as taxane chemotherapy, PARP inhibitors for HRR‑mutated disease, radiopharmaceuticals, and PSMA‑targeted radioligand therapy.
  • Molecular evaluation through next‑generation sequencing and, when feasible, metastatic biopsy supports recognition of AR‑driven versus AR‑independent phenotypes and helps contextualize resistance patterns during treatment planning.

Conclusions
mCRPC is a biologically heterogeneous disease shaped by selective pressure from potent AR‑directed therapies. Resistance mechanisms fall into AR‑activating alterations, non-AR‑dependent oncogenic pathways, and lineage plasticity driven by transcriptional and epigenetic reprogramming. Increasing recognition of AR‑null and neuroendocrine phenotypes underscores the need for molecular characterization at progression. Integrating genomic and phenotypic data into clinical practice supports more informed therapeutic choices and highlights opportunities for developing targeted strategies that address the diverse mechanisms of ARPI resistance.

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