Tumor Spheroids: Bridging the Gap Between 2D Cell Assays and Solid Tumors

For decades, two-dimensional (2D) cell-based assays have been a cornerstone of preclinical cancer drug discovery. Their simplicity, reproducibility, relatively low cost, and compatibility with high-throughput screening have made them invaluable for identifying and characterizing potential anticancer agents. But the very simplicity that makes 2D assays useful can also limit their ability to predict what happens in a solid tumor. Cells growing as a flat monolayer do not reproduce the three-dimensional architecture, cell–cell interactions, oxygen and nutrient gradients, extracellular matrix interactions, or other features of the tumor microenvironment that can influence drug penetration and therapeutic response. Consequently, activity observed in conventional 2D assays does not always translate into comparable activity in vivo. These limitations have driven increasing interest in three-dimensional (3D) models, including tumor spheroids, as a potential bridge between conventional in vitro assays and more complex in vivo models. [1][2[3][4]

A tumor spheroid is a three-dimensional (3D) spherical aggregate of cancer cells grown in vitro that reproduces selected structural and biological characteristics of a solid tumor. Unlike a conventional 2D monolayer, where cells are arranged on a flat surface and have relatively uniform access to the surrounding culture medium, spheroids create a more complex spatial environment. As spheroids increase in size, gradients of oxygen, nutrients, metabolites, and pH develop from the outer surface toward the center. These gradients can produce distinct populations of proliferating, quiescent, hypoxic, and, in sufficiently large spheroids, necrotic cells. [2][5][6][7][8]

Layered Zonation and the Tumor Microenvironment
One of the defining features of tumor spheroids is their spatial heterogeneity. Rather than behaving as a uniform collection of cells, a spheroid can exhibit different cellular and metabolic states depending on the cells’ location within the structure.

The precise size and organization of these regions vary with tumor cell type, growth characteristics, culture conditions, spheroid size, and the method used to generate the model. Therefore, the commonly described zones should be regarded as a general biological pattern rather than fixed anatomical boundaries. [5][6][7][8]

• Outer proliferative zone: Cells near the spheroid surface have relatively greater access to oxygen and nutrients from the surrounding culture medium. These cells tend to remain metabolically active and proliferate more rapidly. Their behavior can resemble that of tumor cells located closer to functional blood vessels in a solid tumor. [5][6][7]
• Middle quiescent and hypoxic zone: Cells farther from the surface experience progressively reduced access to oxygen and nutrients. As a result, proliferation may slow substantially, and cells can enter a quiescent state while remaining viable. Hypoxia and nutrient limitation can also alter cellular metabolism, signaling, gene expression, and sensitivity to anticancer therapy. [5][6][7][8]
• Inner necrotic zone: In sufficiently large spheroids, diffusion of oxygen and nutrients may become inadequate to support cellular viability in the center. At the same time, metabolic waste products can accumulate and local pH can decrease. These conditions may result in cell death and the development of a central necrotic region. [5][6][7]

Importantly, these regions are not necessarily separated by sharply defined boundaries. Instead, tumor spheroids generally contain continuous gradients of oxygen, nutrients, metabolites, and cellular activity. This spatial heterogeneity is one of the principal characteristics that makes spheroids useful as models of the solid tumor microenvironment. [5][6][7][8]

Why Tumor Spheroids Matter in Cancer Research
Tumor spheroids address several limitations inherent in conventional 2D cell culture. In a monolayer assay, cells are typically grown on a rigid, flat surface and exposed to nutrients, oxygen, and test compounds in a relatively uniform manner. Although this environment is highly useful for studying cellular signaling, molecular mechanisms, cytotoxicity, and other biological processes, it does not recapitulate many of the physical and biochemical constraints that cancer cells experience in a three-dimensional tumor. [1][3][4][5]

In contrast, the 3D organization of a spheroid introduces cell–cell interactions, spatial gradients, diffusion barriers, and differences in cellular proliferation and metabolism. These factors can influence both tumor-cell behavior and the response to therapy. [2][5][6][7][8]

Drug Penetration and Therapeutic Response
One of the most important applications of tumor spheroids is the study of drug penetration and therapeutic response. For a drug to affect cancer cells located deep within a solid tumor, it must reach the tumor, traverse its extracellular and cellular environments, and ultimately reach its intended target. In a densely packed 3D structure, transport can be affected by cellular density, cell–cell interactions, extracellular matrix components, molecular size, physicochemical properties, and other characteristics of the therapeutic agent. [6][8][9]

Tumor spheroids provide an experimentally accessible system for investigating these processes. Rather than measuring only whether a drug kills cells, researchers can evaluate how a therapeutic agent distributes through a 3D tumor-like structure and whether cells at different depths exhibit different responses. This can be particularly valuable when studying therapeutic approaches in which tumor penetration, cellular uptake, or target accessibility may influence activity. [6][9]

This application is especially relevant to targeted drug-delivery systems, including nanomedicines. Multicellular tumor spheroids have been used to investigate drug penetration, receptor targeting, cellular uptake, and therapeutic efficacy while providing a more physiologically relevant 3D environment than conventional monolayer cultures. [8][9]

Hypoxia, Cellular Heterogeneity, and Drug Resistance
Hypoxia is a characteristic feature of many solid tumors and can influence cancer-cell metabolism, proliferation, survival, signaling, and therapeutic response. Because oxygen diffusion becomes increasingly limited as spheroids grow, 3D models can reproduce gradients of oxygen availability that are difficult to generate in conventional 2D cultures. [7][10]

Cells exposed to hypoxia and nutrient deprivation may undergo substantial changes in phenotype and metabolism. Cells in the hypoxic and quiescent regions of a spheroid may also respond differently to treatment than rapidly proliferating cells at the periphery. These differences can contribute to heterogeneous responses to anticancer therapies. [7][10]

Importantly, drug resistance in spheroids is multifactorial. Reduced drug penetration, cellular quiescence, hypoxia, altered metabolism, changes in cell–cell and cell–matrix interactions, and alterations in intracellular signaling can all contribute to reduced treatment sensitivity. Consequently, observations from spheroid models should not be attributed to a single resistance mechanism without appropriate experimental evidence. [5][7][10]

The 3D environment can also alter gene and protein expression compared with 2D culture. These differences provide an opportunity to investigate how the physical organization of cancer cells influences phenotype and treatment response. [3][4][5][7]

A Practical Bridge Between 2D Culture and More Complex Models
Tumor spheroids occupy an important position within the continuum of experimental cancer models. Two-dimensional cultures offer simplicity and scalability, while animal models and patient-derived systems provide greater biological complexity but can be more expensive, time-consuming, and difficult to standardize. Three-dimensional spheroids offer an intermediate approach that adds biologically relevant spatial organization while retaining many of the experimental advantages of in vitro systems. [1][2][3][4]

Spheroids can be generated using a variety of techniques, including low-attachment culture, liquid-overlay methods, hanging-drop systems, hydrogels, microwell arrays, and microfluidic platforms. Depending on the experimental question, they can be formed from established cancer cell lines, primary tumor cells, or combinations of tumor cells with stromal or other cell populations. [3][5][11]

Their compatibility with multiwell formats and imaging-based readouts has also made spheroids attractive for drug screening. Advances in automated imaging and quantitative analysis have improved the ability to measure spheroid size, morphology, viability, growth, and treatment-associated changes. [1][5][11]

However, 3D does not automatically mean more predictive. Spheroid models have their own technical limitations. Variability in spheroid size and shape can affect drug response and complicate comparisons between experiments. Standardization of spheroid formation, size, morphology, culture conditions, dosing, and analytical methods is therefore important when spheroids are used for quantitative drug screening. [1][5][11]

From Simple Spheroids to More Complex Tumor Models
A conventional tumor spheroid composed primarily of cancer cells does not reproduce every component of a human tumor. Solid tumors are complex ecosystems containing cancer cells together with fibroblasts, endothelial cells, immune cells, extracellular matrix, blood vessels, and a range of soluble signaling factors.

For this reason, researchers have increasingly developed heterotypic or multicellular spheroid models that incorporate additional components of the tumor microenvironment. Co-culture approaches can incorporate stromal fibroblasts, endothelial cells, immune cells, or other relevant populations to investigate cell–cell interactions and their effects on tumor behavior and therapeutic response. [3][5][8]

These more complex systems may provide additional biological information, but they also introduce new challenges in reproducibility, characterization, and quantitative analysis. The appropriate model therefore depends on the biological question being asked. A relatively simple spheroid may be highly useful for studying drug penetration or cytotoxicity, whereas a more complex co-culture model may be better suited to investigating tumor–stroma interactions or immune responses. [3][5][8]

The Value—and the Limits—of the Spheroid Model
The major strength of tumor spheroids is their ability to reproduce several characteristics of solid tumors that are difficult to capture in conventional 2D culture. These include three-dimensional cellular organization, cell–cell interactions, spatial gradients of oxygen and nutrients, hypoxia, cellular quiescence, metabolic heterogeneity, necrosis, and physical barriers to drug penetration. [2][5][6][7][8][9][10][11]

For cancer drug development, these features allow researchers to ask questions that may be difficult to answer using a conventional monolayer assay alone. A spheroid can help investigators examine whether a therapeutic agent penetrates a 3D tumor-like structure, how cells in different microenvironmental states respond, and whether a treatment’s activity is maintained when cancer cells are organized in a more physiologically relevant architecture. [1][5][8][9]

This does not mean that spheroids replace 2D assays, animal models, or patient-derived systems. Rather, they can complement these approaches. The value of 3D models lies in adding biological complexity at an earlier stage of the drug-development process, potentially helping researchers identify differences in drug response that might otherwise remain hidden until later preclinical or clinical development. [1][2][3][4]

Ultimately, tumor spheroids represent an important bridge between conventional in vitro cell assays and more complex models of human cancer. Their ability to combine experimental accessibility with aspects of tumor architecture and microenvironment makes them increasingly useful for cancer biology, drug discovery, drug-delivery research, and the evaluation of emerging therapeutic strategies. [1][2][3][4][8]

References
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Feature image: Illustrative representation of a tumor spheroid and its heterogeneous 3D microenvironment. The image depicts the spatial organization of tumor cells within a three-dimensional spheroid, together with representative stromal, immune, extracellular matrix, and vascular components. The illustration is conceptual and is intended to highlight the biological complexity of 3D tumor models; it is not intended to represent the precise cellular composition or architecture of any specific human tumor. © 2026 CH/JCO Used with permission.

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