The 3D Collagen Network as a Determinant of Tumor Progression and Drug Delivery Efficiency in Breast Cancer

TL;DR

This paper reviews how the 3D collagen structure in breast cancer affects tumor growth, drug delivery, and treatment resistance.

Contribution

It synthesizes recent findings to highlight collagen's role as a biomarker and therapeutic target in breast cancer.

Findings

Collagen architecture promotes aggressive tumor behavior and immune evasion.

Collagen barriers hinder drug delivery and encourage metastasis.

Modulating collagen could improve treatment outcomes in fibrotic tumors.

Abstract

Background/Objectives: Breast cancer is a biologically complex malignancy whose high prevalence and therapeutic resistance represent a continuous challenge for global health. The Tumor Microenvironment (TME) is a crucial component in disease progression, and the Extracellular Matrix (ECM), particularly its 3D collagen architecture, is recognized for mediating interactions that influence invasion, metastasis, and pharmacological response. This review aims to critically synthesize recent evidence to elucidate the multifaceted role of collagen in the progression and modulation of therapeutic response in breast adenocarcinoma. Methods: A comprehensive literature review was conducted, analyzing studies addressing specific collagen subtypes, ECM stiffening (fibrosis), biomechanical signaling, and their impact on drug transport kinetics and immunomodulatory effects. Results: The results demonstrate that structural alterations of collagen not only orchestrate a pro-tumoral microenvironment, fostering aggressive phenotypes and immune evasion, but also create a physical barrier that compromises drug delivery efficiency and promotes metastatic dissemination. The synthesis of the data reinforces collagen as a potent prognostic biomarker and a promising therapeutic target for overcoming stroma-mediated resistance. Conclusions: Targeting the collagen-rich stroma and its 3D network is a critical frontier for therapeutic innovation. Developing adjuvant strategies to modulate the ECM has the potential to enhance clinical outcomes and optimize the distribution of antineoplastic agents, especially in patients with high degrees of tumor fibrosis.

Full text

1. Introduction

Breast cancer remains the most prevalent malignant neoplasm among women and one of the leading causes of cancer-related mortality worldwide, representing a persistent challenge for public health systems [1,2,3]. Although significant advances have been achieved in early diagnosis and therapeutic modalities, including surgery, chemotherapy, radiotherapy, targeted therapies, and immunotherapy, tumor heterogeneity and resistance mechanisms continue to compromise the effectiveness and durability of treatments [4,5].

Within this challenging clinical scenario, the tumor microenvironment (TME) has emerged as a determining factor in neoplastic progression and in the response to oncological therapies [6,7]. This microenvironment consists of a complex and dynamic network of stromal cells, such as cancer-associated fibroblasts (CAFs), immune and endothelial cells, blood and lymphatic vessels, in addition to the extracellular matrix (ECM), which acts as both a structural and functional support for tumor cells [8,9]. In the field of pharmaceutical sciences and tissue engineering, understanding this architecture is crucial for developing bio-inspired models and advanced drug delivery systems.

The ECM, in particular, plays a fundamental role in the regulation of cellular signaling, acting as a reservoir of cytokines and growth factors, while transmitting biochemical and biomechanical stimuli that directly affect cellular proliferation, differentiation, migration, and survival [10]. Among its components, collagen stands out as the most abundant fibrous protein in mammalian tissues, functioning as a natural 3D scaffold that exerts multiple functions in the tumor biology of breast cancer [11].

Alterations in the deposition, organization, and crosslinking of collagen fibers contribute to increased tissue stiffness and promote ECM alignment, favoring collective cell migration and tumor invasion [12,13,14]. These processes are not merely structural: they create a pro-tumoral microenvironment that intensifies uncontrolled proliferation, facilitates metastatic dissemination, and compromises therapeutic success [15,16]. Furthermore, the dense architecture of collagen presents a significant challenge for the diffusion of therapeutic agents, acting as a steric barrier that limits pharmacological efficacy.

Furthermore, the density and organizational pattern of collagen may constitute a physical barrier to drug penetration, also influencing signaling pathways that promote cellular resistance [17,18,19]. A deep understanding of the interface among collagen, tumor cells, and immune cells is essential to optimize therapeutic strategies, particularly in the design of next-generation delivery platforms capable of bypassing these fibrotic barriers [6].

Although previously published review studies have addressed general aspects of the extracellular matrix (ECM) in oncology or the role of collagen in specific tumor-related processes, a gap remains in the literature regarding a critical and integrated analysis of recent evidence correlating structural and functional modifications of collagen with tumor progression and therapeutic response in mammary adenocarcinoma. There is an emerging need to integrate findings on the different collagen subtypes involved with their direct impact on the pharmacokinetics and penetration of chemotherapeutic and immunotherapeutic agents [20,21]. Moreover, these biological insights provide the blueprint for 3D bioprinting technologies aimed at mimicking the tumoral stroma for drug screening.

Additionally, it is relevant to consider that cancer is a biologically dynamic and evolutionary condition, characterized by progressive genomic, epigenetic, and phenotypic changes that remodel the tumor microenvironment and extracellular matrix over time and under therapeutic pressure [22,23]. This plasticity clearly evidences that an exclusive focus on the tumor cell is insufficient to reverse the course of the disease. Variations in survival rates across different global regions, where access to early diagnosis and modern treatments remains disparate, underscore the urgent necessity of identifying universal prognostic factors and therapeutic targets that are intrinsic to tumor biology. In this context, focusing on collagen emerges as a critical area for therapeutic innovation [5].

Therefore, the present review aims to critically consolidate and analyze the most recent scientific evidence regarding the role of collagen as a modulator of tumor progression and therapeutic response in breast adenocarcinoma. By integrating data on collagen dynamics within the TME, this work seeks to identify prognostic and predictive biomarkers, as well as to validate potential therapeutic targets capable of overcoming resistance and optimizing the delivery and distribution of antineoplastic agents through the dense 3D tumor matrix.

2. Methods

2.1. Study Design and Research Question

This work constitutes a Comprehensive Narrative Review, meticulously designed to systematically and profoundly synthesize the existing evidence concerning the role of collagen in the progression and therapeutic response of breast adenocarcinoma. The central question guiding the data collection and analysis was: “What is the consolidated evidence regarding the role of collagen (dynamics, stiffness, alignment, or metabolism) as a prognostic factor, resistance mechanism, a barrier to drug delivery, and/or therapeutic target in Breast Adenocarcinoma?”

2.2. Search Strategy and Data Sources

The electronic search was finalized in November 2025 and encompassed recognized bibliographic databases, including PubMed/MEDLINE, Scopus, and Web of Science (WoS), to ensure maximum scientific and multidisciplinary coverage (bridging molecular biology, oncology, pharmacology, and bioengineering). Additionally, Gray Literature (websites of governmental organizations such as INCA and NCI) was consulted for up-to-date epidemiological data and reports.

2.2.1. Search Terms and Boolean Operators

To optimize the search sensitivity and comprehensively cover all investigation angles, controlled vocabulary terms (MeSH/DeCS) and free-text terms were employed. These were grouped into three conceptual axes and combined using the Boolean operators AND and OR: Collagen/ECM: “Collagen” OR “Extracellular Matrix” OR “Tumor Stroma” OR “Fibrosis” OR “Matrix Stiffness” OR “TACS” OR “LOX” OR “Lysyl Oxidase”; Neoplasm: “Breast Cancer” OR “Breast Neoplasm” OR “Breast Adenocarcinoma” OR “Tumor Microenvironment” OR “TME”; Outcome/Target: “Therapeutic Resistance” OR “Drug Resistance” OR “Prognosis” OR “Metastasis” OR “Target” OR “Biomarker” OR “Drug Delivery” OR “Pharmacological Penetration”.

2.2.2. Publication Period

The search period was strictly defined between January 2011 and November 2025. This temporal window was strategically selected to focus on the most recent literature (the past 15 years), which has seen the consolidation of knowledge regarding the biomechanical role of the matrix.

Classic or seminal articles crucial for establishing the fundamental concepts of tissue stiffness and collagen alignment, published prior to 2011, were included through a manual reference-based search (from the reference lists of the most recent articles) to provide adequate theoretical and historical context.

2.3. Eligibility and Study Selection Criteria

The eligibility criteria were rigorously applied in two distinct phases (title/abstract screening and full-text review) and are detailed as follows:

2.3.1. Inclusion Criteria

Study Type: Original studies (in vivo or in vitro), clinical trials, systematic reviews, and meta-analyses.

Topic/Intervention: Articles investigating, primarily or secondarily, the relationship between collagen (any subtype, synthesis, stiffness, alignment, TACS, metabolism, or enzymes like LOX) and breast adenocarcinoma (any histological/molecular subtype), and/or its correlation with therapeutic response (chemotherapy, radiotherapy, or immunotherapy) and studies focusing on the impact of collagen-rich matrices on the diffusion and delivery of antineoplastic agents.

Language: Articles published in the English language.

2.3.2. Exclusion Criteria

Publication Type: Editorials, letters to the editor, conference abstracts, and non-peer-reviewed preprints.

Topic Focus: Studies primarily focused on other ECM components (e.g., elastin, hyaluronic acid, glycosaminoglycans) without emphasis on collagen. Studies focused on other primary cancer sites (e.g., lung, prostate) without direct and explicit translational applicability to the mammary microenvironment.

Access: Full-text articles inaccessible after reasonable acquisition efforts.

3. 3D Collagen Network: From Structure to Therapeutic Challenge

The complexity of the 3D collagen network in breast adenocarcinoma requires sophisticated experimental approaches to evaluate its role in tumor progression and drug resistance. Understanding the biomechanical properties of the extracellular matrix (ECM) necessitates models that accurately replicate the desmoplastic reaction. In this context, several experimental platforms, ranging from 2D cultures to advanced 3D bioprinting and in vivo systems, have been developed to study how collagen fiber alignment and density impact therapeutic outcomes [24,25].

3.1. In Vitro Models: Recapitulating the 3D Collagen Architecture

While two-dimensional (2D) cell cultures have historically provided insights into cancer cell proliferation and migration, they are fundamentally limited by their inability to replicate the complex 3D collagen topography of the breast tumor microenvironment (TME). In 2D systems, the absence of a structured ECM leads to non-physiological cell morphology and altered gene expression, often resulting in overestimating drug efficacy [26,27].

To overcome these limitations, 3D organoids and scaffolds have emerged as superior platforms for studying the desmoplastic reaction. These models allow for the controlled manipulation of collagen density and stiffness, enabling researchers to investigate how fiber alignment promotes the Epithelial–Mesenchymal Transition (EMT) and physical resistance to drug penetration [28,29,30]. Furthermore, tumor-on-a-chip platforms integrate microfluidic flow to simulate interstitial fluid pressure (IFP), a critical factor in breast adenocarcinoma, where a dense collagen network hinders the convective transport of nanotherapeutics [31,32].

3.2. In Vivo Models: Biomechanical Remodeling and Host Interactions

In vivo systems remain the gold standard for evaluating the dynamic remodeling of the collagen matrix. Immunosuppressed mice (Xenografts) are instrumental in studying human-derived breast cancer cells; however, the murine host ECM may not fully recapitulate the cross-linking patterns found in human desmoplasia [33,34,35].

In contrast, Genetically Modified Models (GMMs), particularly those utilizing CRISPR/Cas9, allow for the precise deletion or overexpression of collagen-modifying enzymes, such as Lysyl Oxidase (LOX). These models are vital for demonstrating how collagen “stiffening” feedback loops drive tumor progression and immune evasion in a physiologically relevant context [36,37]. Spontaneous and syngeneic models (e.g., BALB/c) offer the advantage of an intact immune system, which is essential for investigating how the 3D collagen shield protects tumor cells from T-cell infiltration, a major hurdle in current immunotherapy [38,39,40].

3.3. The Ehrlich Adenocarcinoma (EAC) Model in Collagen Research

The Ehrlich Adenocarcinoma (EAC) model, specifically in its Solid Ehrlich Tumor (SET) form, serves as a robust preclinical platform for studying breast-like malignancies. Although EAC is highly aggressive and pleomorphic, its value in collagen research lies in its rapid induction of a stromal response upon subcutaneous inoculation [41,42,43].

While the EAC model lacks a highly metastatic phenotype, it provides a reproducible environment to test ECM-modulating agents and natural compounds aimed at reducing collagen density. By monitoring changes in the tumor’s physical consistency and stromal architecture, the EAC model facilitates the screening of therapies designed to “soften” the TME, thereby improving the delivery of conventional cytotoxic agents [44,45,46].

3.4. The 3D Collagen Network: Architectural Determinant of the Breast TME

In breast adenocarcinoma, the Extracellular Matrix (ECM) transcends its role as a structural scaffold, acting as a dynamic regulator of tumor evolution. While the ECM comprises proteoglycans (PGs), glycosaminoglycans (GAGs), and elastin, collagen is the primary driver of the physical barriers encountered during drug delivery [47,48].

3.5. Specific Role of Collagen in Tumor Progression and Immune Response

3.5.1. Structural Hierarchy as a Mechanical Determinant

The collagen superfamily, particularly the fibril-forming types I, II, and III, provides the essential structural scaffold of the mammary gland. Type I collagen, representing 90% of the total content, is organized into a triple-helix structure (tropocollagen) that assembles into microfibrils and fibers [49,50]. In the context of adenocarcinoma, this hierarchical organization is hijacked. The repetitive Gly-X-Y triplets and the dense interchain hydrogen bonding provide the mechanical stability that, when dysregulated by Cancer-Associated Fibroblasts (CAFs), results in the characteristic stiffness of the breast tumor microenvironment (TME) [51,52].

3.5.2. From Native to Hydrolyzed Collagen: Pharmaceutical Relevance

The transition from native to hydrolyzed collagen—via enzymatic or thermal denaturation—alters its physicochemical properties, increasing solubility and bioavailability [53,54]. While native collagen maintains the triple-helix integrity necessary for structural studies, hydrolyzed peptides and collagen-derived hydrogels have emerged as versatile biomaterials. In oncology, these hydrogels are increasingly used in 3D bioprinting and drug delivery systems, as they mimic the hydrated environment of the ECM while allowing for the controlled release of therapeutic agents and the scavenging of free radicals [55,56].

3.5.3. Collagen as a Functional Driver of Progression and Biomarker

As synthesized in Table 1, the remodeling of collagen expression is a hallmark of breast adenocarcinoma. Type I collagen (COL1A1) stands out as the primary modulator of the ECM, where its overdeposition is directly coupled with LOX-mediated crosslinking and matrix stiffening [57,58]. This “architectural plasticity” is not merely structural; it actively facilitates cellular trafficking. Imaging studies reveal that migratory tumor cells utilize linearized collagen fibers as “tracks” to escape the primary tumor, a process guided by Tumor-Associated Collagen Signatures (TACS), particularly the perpendicular alignment known as TACS-3 [59,60,61].

Beyond Type I, Table 2 highlights the role of less explored variants like collagens III, VI, and XII. For instance, COL12A1 secreted by CAFs can alter the organization of Type I fibers, creating a pro-metastatic niche [65]. These structural shifts are now recognized as robust prognostic biomarkers (Table 3), where high stromal Type IV expression and intratumoral collagen uniformity independently predict reduced recurrence-free survival [61,66].

3.5.4. Mechanotransduction and the “Immune Desert”

The interaction between tumor cells and the collagen matrix is mediated by specialized receptors, including integrins, DDR1, and DDR2 [74]. As summarized in Table 4, this biomechanical signaling activates the FAK/YAP and PI3K/AKT/mTOR pathways, leading to the formation of invadopodia—actin-rich protrusions that degrade the ECM and accelerate invasion [75,76].

A critical pharmaceutical challenge identified in recent studies is the role of collagen in immune reprogramming. The densely compacted and aligned collagen environment acts as a “physical shield” or immune desert, which:

• Excludes cytotoxic cells by limiting the infiltration of CD8+ T lymphocytes and NK cells [76].
• Polarizes macrophages by triggering the transition of Tumor-Associated Macrophages (TAMs) toward an immunosuppressive M2 phenotype [79,81].
• Hinders immunotherapy by creating a barrier that prevents contact between immune checkpoint inhibitors and their cellular targets.

3.5.5. Impact of Collagen Isoforms on Clinical Outcomes

The functional duality of collagen is evidenced in Table 5. While COL1A1 is universally associated with poor prognosis and hormonal therapy resistance [68,70], some isoforms like COL3A1 may exert a “tumor-restraining” effect depending on the Type I/Type III ratio [82]. Furthermore, hypoxia in the TME exacerbates this complexity by upregulating LOX and MMPs, which further stiffen the matrix and promote EMT [3,74].

Experimental strategies to modulate this “mechanobiology”—such as copper depletion with tetrathiomolybdate (TM) or LOX inhibition—are currently being explored to “soften” the TME and restore drug sensitivity [57,70].

3.6. Therapeutic Approaches and Translational Innovation

3.6.1. From Conventional Cytotoxicity to Matrix-Targeting Strategies

Conventional chemotherapy and radiotherapy, while effective, often fail in the context of highly desmoplastic breast tumors due to poor drug penetration. The dense collagenous stroma creates a high Interstitial Fluid Pressure (IFP), which counteracts the convective transport of small molecules and nanocarriers alike [42,88]. Modern oncology is shifting towards Targeted Therapies and Immunotherapy (e.g., anti-PD-1/PD-L1); however, their efficacy is strictly dependent on the ability to bypass the “collagen shield” that characterizes the immune desert [89,90].

3.6.2. Pharmacological Modulation of the Collagen Matrix

The mechanical properties of the TME, governed by Type I, III, and IV collagens, are now viewed as druggable targets.

Stiffness Normalization: Strategies aimed at inhibiting Lysyl Oxidase (LOX) or reducing fiber cross-linking are being explored to “soften” the TME, thereby lowering IFP and enhancing the delivery of co-administered chemotherapeutics [91].

The Dual Role of Collagen: While COL1A1 promotes invasion through TACS-3 alignment, recent evidence suggests that Type III Collagen (Col3) may act as a tumor-suppressive modulator. Recombinant human Col3 (rhCol3) hydrogels have shown potential in reducing tumor growth and metastatic burden, offering a biocompatible route for stromal “renormalization” [92].

3.6.3. Exosomes as Bioactive Shuttles in the Remodeled ECM

Exosomes (40–160 nm) act as the primary communication axis between tumor cells and the ECM. In a rigid environment, cancer-derived exosomes carry a specialized cargo, which includes MMP-14, TGF-β, and STAT3, that triggers the differentiation of resident fibroblasts into Cancer-Associated Fibroblasts (CAFs) [93,94]. From a biopharmaceutical standpoint, these vesicles represent a double-edged sword: they drive the pre-metastatic niche formation but also serve as inspiration for biomimetic nanocarriers. Understanding how the collagen matrix alters exosomal tropism is crucial for designing the next generation of targeted drug delivery systems [49,95].

4. Conclusions and Translational Implications

This review systematically consolidates the evidence positioning collagen as a central, bioactive regulator of breast adenocarcinoma. The transition from a 2D understanding of tumor biology to a 3D mechanobiological paradigm is essential for overcoming therapeutic resistance. We draw the following conclusions:

Biomechanical Barriers: Collagen architecture (TACS-3) and LOX-mediated stiffening are the primary physical obstacles to drug extravasation and immune cell infiltration.

Diagnostic Value: Collagen signatures should be integrated into clinical practice as prognostic biomarkers to stratify patients who may benefit from matrix-priming therapies.

Future Research: The pharmaceutical industry must prioritize the development of “Stromal-Permeable” nanotechnologies and inhibitors of collagen cross-linking.

By shifting the therapeutic focus from the tumor cell alone to the 3D collagenous architecture, we pave the way for more effective, personalized, and “stroma-aware” treatments for breast cancer.