A brief introduction to spheroids

Spheroids are aggregates of cells growing in a three-dimensional manner in suspension and developing complex interactions between cells or a three-dimensional matrix (like matrigel). Their primary interest is to reproduce an architecture similar to the original tissue or organ, which will generate different nutrients gradients, gas (oxygen), growth factors, etc. Spheroids are thought to mimic tumor behavior more effectively than normal 2D cultures because, like tumors, they contain both deeply embedded and surface-exposed cells, proliferating and non-proliferating cells, and a hypoxic center with a well-oxygenated outer layer of cells. They allow visualization of the physiological changes that differentiate tumor cells from healthy cells. Spheroids are also ideal for preclinical drug screening and identify potential cancer drugs. Specifically, they can reveal the potential of drugs to infiltrate the tumor, as well as their inhibitory effects on metastasis.

Compared to other 3D cell culture methods, spheroids have several advantages, including relevance to tumor biology, lower cost, reproducibility, ease of integration into high-throughput screening, and advanced imaging techniques. There are a number of distinct methods for promoting spheroid formation in culture, including hanging drop, liquid overlay, pellet culture, spinner culture, microfluidics, and methods incorporating biomaterial scaffolds like hydrogels or biofilms made of alginate, collagen, and hyaluronic acid. While the liquid overlay technique is advantageous for certain cell types, the formation of tumor spheroids is inhibited by agarose. For the spinner culture method, the sizes of the resultant spheroids are partially dependent on the size of the container, whereas for biomaterial scaffolds the set up can take a consistent time. Although the size of the spheroids can be relatively easy to control, the cell density within the spheroids is not. In addition, not all tumor cell lines can be used to generate spheroids.

The formation of magnetic spheroids with high sphericity allows the fine-tuning of the final spheroid diameter

Thanks to this technique, the authors investigated the cell penetration and distribution of liposomal-encapsulated doxorubicin (DOX) in magnetically molded spheroids of 1 mm initial size. They showed that the spheroid has a different cohesiveness than a hanging drop spheroid: whereas the magnetic one is well into its maturation state, the hanging drop spheroid has barely achieved a cohesive state by this maturation time, which translates to a more uniform drug penetration profile. The cells in the 3D spheroid configuration thus show a higher resistance to DOX compared to those in 2D culture. This indicates a limited penetration of the drug towards the spheroid core.

3D magnetically shaped spheroids and their multiple potential applications

The authors showed how the spheroid maturation impacts cell invasion and doxorubicin penetrability, highlighting the importance of this parameter in drug screening and therapeutic applications, and demonstrated that this method enables to measure the surface tension of spheroids, which is particularly relevant as an output parameter in the context of cancer cell invasion and metastasis.

In conclusion, as it can be magnetically patterned into specific shapes, the design of this tool has many advantages of size-tuning and low spheroid maturation times. Magnetic labelling can also be combined with agarose well patterning and bioprinting. It can be adapted to a lot of different types of cancer cells and environments, making it a good alternative or test-experiment before going into in vivo models. The reliability and robustness of preclinical research data could certainly be enhanced by the synergistic use of this kind of 3D models, thus reducing the use of animal testing. Advances in 3D cell culture techniques thus offer the potential to both better identify drug candidates and better understand and predict the occurrence of tumor resistance to anticancer drugs.