The Silent Surge in Our Bones
Prostate cancer claims over 350,000 lives yearly, but its deadliest turn occurs when it becomes castration-resistant (CRPC) and invades bones. Over 90% of advanced patients develop bone metastases (BMs) within 24 months of death, where tumors become resistant to standard therapies and immune attack 1 3 . Current treatments like bisphosphonates relieve pain but fail to extend survival. Why? The answer lies in a group of cellular turncoats: M2 macrophages. These immune cells, which normally promote tissue repair, get hijacked by tumors to fuel growth, suppress immunity, and resist therapy 8 .
In this article, we explore a groundbreaking strategy: drug repurposing. By combining transcriptomics (gene activity mapping) and network analysis (cellular relationship mapping), scientists identified existing drugs that can disarm M2 macrophages. This approach bypasses years of drug development, offering faster, cheaper solutions for a lethal problem.
Prostate Cancer Statistics
Annual global impact of prostate cancer and bone metastases.
Meet the M2 Macrophage: The Tumor's Accomplice
What Makes M2 Macrophages Dangerous?
Macrophages are immune sentinels that can adopt opposing roles:
- M1 "Warriors": Attack tumors and stimulate inflammation.
- M2 "Builders": Promote tissue repair and suppress inflammation.
In prostate bone metastases, tumors corrupt this system. They flood the microenvironment with signals (like IL-4 and CSF-1) that polarize macrophages toward the M2 state. These M2 cells then 8 :
Bone Destruction
Secrete bone-dissolving enzymes (e.g., MMPs, cathepsins), freeing tumor-growth factors.
Immunosuppression
Release immunosuppressive molecules (e.g., IL-10, TGF-β), paralyzing T cells.
Cancer Stem Cells
Act as "nurses" for cancer stem cells, enabling recurrence.
Key Insight: Single-cell RNA studies reveal M2 macrophages account for >50% of the tumor mass in bone metastases. Their gene signature predicts poor survival in patients 7 .
M2 Gene Signature
- CD163 overexpression
- IL-10 secretion
- TGF-β pathway activation
- CSF-1R signaling
The Pivotal Experiment: A Drug-Hunting Roadmap
Methodology: From Genes to Drug Candidates
A landmark 2022 study designed a 5-step "multiplex repurposing scheme" to find M2-targeting drugs 1 2 3 :
1. Transcriptomic Profiling
Compared gene activity in 51 bone metastasis samples (from datasets GSE32269 and GSE77930) vs. 44 primary prostate tumors. Identified 327 differentially expressed genes (DEGs), including ALPL, RUNX2, and COL family genes linked to bone remodeling.
2. Immune Deconvolution
Used algorithms (CIBERSORT, EPIC) to quantify immune cells in tumors. Confirmed M2 macrophages as the dominant immune subset in BMs (3–5× higher than in primary tumors).
3. Network Analysis
Mapped DEGs onto protein interaction networks (STRING database). Pinpointed 37 "hub genes" driving M2 polarization (e.g., EZH2, CD24).
4. Drug Screening
Screened 1,309 FDA-approved drugs using Connectivity Map and L1000 databases. Ranked drugs by their ability to reverse the M2 gene signature.
5. Redundancy Filtering
Clustered drugs by structural similarity (ChemBioServer 2.0). Selected top-scoring candidates per cluster.
Top Drug Candidates Identified
| Drug | Original Use | Anti-M2 Mechanism |
|---|---|---|
| Foretinib | Cancer (tyrosine kinase inhibitor) | Blocks CSF-1R, starving M2 macrophages |
| Norethindrone | Contraceptive | Modulates androgen receptor signaling |
| Menthol | Topical analgesic | Disrupts IL-10/STAT3 immunosuppression |
| Testosterone* | Hormone replacement | Paradoxically suppresses tumor growth in bone |
| Docetaxel | Chemotherapy | Re-sensitizes tumors to immune attack |
Results and Analysis
- Foretinib emerged as the top candidate 60% M2 reduction
- Menthol, a surprising entry, blocked IL-10 secretion T-cell reactivation
- Docetaxel validated the method Pipeline accuracy
45%
Reduction in bone lesions with Foretinib in preclinical models 1
Why This Matters: This workflow prioritized drugs that remodel the tumor microenvironment, not just kill cancer cells. By targeting M2 macrophages, they break the cycle of immunosuppression and bone destruction 3 .
Key Pathways Targeted by Repurposed Drugs
| Pathway | Drug Impact | Biological Effect |
|---|---|---|
| Polycomb Repressive Complex 2 (PRC2) | EZH2 inhibitors (e.g., tested in PDX models) | Silences genes that lock macrophages in M2 state |
| CSF-1R signaling | Foretinib | Depletes M2 macrophage recruitment |
| IL-10/STAT3 axis | Menthol | Reverses T-cell paralysis |
| Androgen receptor variants | Norethindrone | Blocks resistance to castration |
The Scientist's Toolkit: Essential Research Reagents
Drug repurposing relies on specialized tools to decode cellular networks. Here's what powers this research:
| Reagent/Resource | Function | Example Use in CRPC-BM Research |
|---|---|---|
| CIBERSORT | Immune cell deconvolution from RNA data | Quantified M2 macrophage infiltration in bone metastases 6 |
| Connectivity Map (CMap) | Matches gene signatures to drug effects | Identified menthol as an IL-10 inhibitor 2 |
| Patient-Derived Xenografts (PDX) | Human tumors grown in mice | Tested EZH2 inhibitors in CRPC bone models |
| Single-Cell RNA-Seq | Profiles gene activity per cell | Revealed M2-specific lncRNAs (e.g., SMIM25) 7 |
| STRING Database | Maps protein-protein interactions | Linked EZH2 to M2 polarization hubs 3 |
Research Workflow
From transcriptomic analysis to drug validation in preclinical models.
Data Sources
- GSE32269 - Bone metastasis transcriptomes
- GSE77930 - Primary tumor comparisons
- L1000 database - Drug signatures
- STRING - Protein interactions
Beyond Bone Metastases: A Template for the Future
This approach's power extends beyond prostate cancer. The same multiplex strategy—transcriptomics + network analysis + redundancy filtering—has identified drugs for breast cancer bone metastases and glioblastoma 5 8 . Key frontiers include:
Combination Therapies
Pairing foretinib with anti-PD-1 immunotherapy to boost T-cell attack .
Nanodelivery
Using nanoparticles to shuttle menthol or docetaxel directly to bone lesions (early trials in 2024) 5 .
Conclusion: A Faster Path to Hope
The integration of transcriptomics and network analysis isn't just techno-wizardry—it's a pragmatic shift in cancer therapeutics. By exposing the M2 macrophage as a linchpin in prostate cancer's bone invasion, and redeploying drugs like foretinib and menthol against it, scientists have turned the tumor's allies into vulnerabilities. As clinical trials advance (NCT04872153 testing foretinib in CRPC), this strategy promises to deliver solutions not in decades, but in years.
Further Exploration
For further exploration, see the open-access Prostate Cancer Transcriptome Atlas (Nature Communications, 2021) .