Multicellular 3D Tumor Spheroids Emerge as a Predictive Bridge Between 2D Assays and Solid Tumors in Preclinical Drug Testing
核心洞察
Multicellular 3D tumor spheroids (搜索) recreate key aspects of tumor architecture, including oxygen and nutrient gradients, hypoxia, quiescence, and necrosis, that conventional 2D monolayer assays cannot capture.
Incorporating stromal fibroblasts and immune cells into spheroid models enhances relevance for studying tumor-microenvironment interactions, drug penetration, efficacy, and resistance.
Spheroids serve as an accessible, scalable intermediate between 2D culture and more complex in vivo models, supporting New Approach Methodologies and more predictive oncology drug discovery.
Multicellular three-dimensional (3D) tumor spheroids (搜索) are gaining traction as a more physiologically relevant platform for preclinical cancer (搜索) drug testing, offering researchers a practical bridge between conventional two-dimensional (2D) cell assays and the biological complexity of solid tumors. While 2D monolayer cultures have long served as a cornerstone of early drug discovery due to their simplicity, reproducibility, low cost, and compatibility with high-throughput screening, their inability to reproduce the three-dimensional architecture, cell–cell interactions, oxygen and nutrient gradients, and extracellular matrix interactions of the tumor microenvironment limits their predictive value.
A tumor spheroid is a 3D spherical aggregate of cancer (搜索) cells grown in vitro that reproduces selected structural and biological characteristics of a solid tumor. Unlike a flat monolayer, where cells 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, producing distinct populations of proliferating, quiescent, hypoxic, and—in sufficiently large spheroids—necrotic cells.
Layered Zonation and the Tumor Microenvironment
One of the defining features of tumor spheroids (搜索) 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, so the commonly described zones should be regarded as a general biological pattern rather than fixed anatomical boundaries.
The outer proliferative zone contains cells near the spheroid surface that have relatively greater access to oxygen and nutrients, remaining metabolically active and proliferating more rapidly—behavior that can resemble tumor cells located closer to functional blood vessels in a solid tumor. The middle quiescent and hypoxic zone comprises cells farther from the surface that experience progressively reduced access to oxygen and nutrients, slowing proliferation and potentially entering a quiescent state while remaining viable. Hypoxia and nutrient limitation can also alter cellular metabolism, signaling, gene expression, and sensitivity to anticancer therapy. The inner necrotic zone develops in sufficiently large spheroids, where diffusion of oxygen and nutrients becomes inadequate to support cellular viability, metabolic waste products accumulate, and local pH decreases, resulting in cell death and a central necrotic region.
Importantly, these regions are not separated by sharply defined boundaries. Tumor spheroids (搜索) generally contain continuous gradients of oxygen, nutrients, metabolites, and cellular activity—a spatial heterogeneity that is one of the principal characteristics making spheroids useful as models of the solid tumor microenvironment.
Drug Penetration and Therapeutic Response
One of the most important applications of tumor spheroids (搜索) is the study of drug penetration and therapeutic response. For a drug to affect cancer (搜索) 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.
Tumor spheroids (搜索) 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 application is especially relevant to targeted drug-delivery systems, including nanomedicines, where multicellular tumor spheroids have been used to investigate drug penetration, receptor targeting, cellular uptake, and therapeutic efficacy within a more physiologically relevant 3D environment.
Hypoxia, Cellular Heterogeneity, and Drug Resistance
Hypoxia is a characteristic feature of many solid tumors and can influence cancer (搜索)-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. Cells exposed to hypoxia and nutrient deprivation may undergo substantial changes in phenotype and metabolism, and cells in the hypoxic and quiescent regions of a spheroid may respond differently to treatment than rapidly proliferating cells at the periphery.
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. The 3D environment can also alter gene and protein expression compared with 2D culture, providing an opportunity to investigate how the physical organization of cancer (搜索) cells influences phenotype and treatment response.
A Practical Bridge Between 2D Culture and More Complex Models
Tumor spheroids (搜索) occupy an important position within the continuum of experimental cancer (搜索) 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.
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 (搜索) cell lines, primary tumor cells, or combinations of tumor cells with stromal or other cell populations. Their compatibility with multiwell formats and imaging-based readouts has made spheroids attractive for drug screening, with advances in automated imaging and quantitative analysis improving the ability to measure spheroid size, morphology, viability, growth, and treatment-associated changes.
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.
From Simple Spheroids to More Complex Tumor Models
A conventional tumor spheroid composed primarily of cancer (搜索) 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.
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.
Advancing New Approach Methodologies
As interest in New Approach Methodologies (NAMs) continues to grow, multicellular spheroids have the potential to play an important role in developing more predictive and translational preclinical models. Advances in live-cell imaging and functional analysis are helping researchers monitor complex 3D models over time and generate richer, more informative data, supporting more predictive cancer (搜索) research and drug discovery.
The major strength of tumor spheroids (搜索) is their ability to reproduce several characteristics of solid tumors that are difficult to capture in conventional 2D culture, including 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. These features allow researchers to ask questions that may be difficult to answer using a conventional monolayer assay alone—examining 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 (搜索) cells are organized in a more physiologically relevant architecture.
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. Ultimately, tumor spheroids (搜索) represent an important bridge between conventional in vitro cell assays and more complex models of human cancer (搜索), combining experimental accessibility with aspects of tumor architecture and microenvironment that make them increasingly useful for cancer biology, drug discovery, drug-delivery research, and the evaluation of emerging therapeutic strategies.