Ex vivo culture systems have been instrumental to preclinical oncology research because they enable researchers to study basic features of tumor cells and carry out large-scale screens of drugs or biologics. Ex vivo cultures do not fully recapitulate physiological conditions, although some advances have been made by developing three-dimensional (3D) culture methods.

Here we highlight the advantages and disadvantages of using ex vivo cultures for preclinical oncology research.

Cell culture: simple, inexpensive, but imperfect

Ex vivo cell cultures of tumors were first developed as adherent two-dimensional (2D) monolayers that were grown in culture flasks or flat dishes. This approach is useful for studying tumors' basic cell biology and carrying out drug screens and preliminary toxicology and pharmacokinetic studies^[1]. 2D cultures can be developed rapidly and cultured over longer periods of time using readily available tissue culture reagents. Unfortunately, 2D culture systems are limited in their applications because they do not accurately model the tumor microenvironment, and tumor cell morphology and function change under these culture conditions. Over the last several decades, 3D cultures have advanced significantly^[2]. Several different systems now exist for 3D cultures, including liquid suspension cultures passaged on non-adherent plates, cultures grown in semi-solid substrates like soft agar or Matrigel, and cells grown on scaffolds like collagen. 3D culture systems better mimic characteristics of the tumor microenvironment, including cell-cell interactions, hypoxia, and reduced sensitivity to drug treatment^[3].

3D cultures for drug discovery have become widely adapted over 2D models because they better predict in vivo efficacy and metabolic responses to drug treatments. A major advance in 3D tumor culture systems is the development of 3D spheroid and organoid cultures^[4]. Spheroids are clusters of cells derived from tumors or other complex tissues and they cluster together through cell-cell adhesion. Organoids are more complex cell clusters derived from stem cells and can self-assemble and regenerate to form a smaller version of the original tumor or organ tissue. Tumor spheroids are widely used for drug screening and cytotoxicity assays and can be formed from dissociated tumor tissue or circulating tumor cells^[5]. Tumor organoids are even more accurate reflections of tumors growing under physiological conditions and are now widely used to compare the efficacy of standard-of-care treatments versus targeted therapies in addition to their use in drug screens^[6]. Spheroids and organoids can undergo genomic editing as well, and this approach has been instrumental in identifying mutations that drive tumor growth or cause drug resistance^[7-8].

Ex vivo + in vivo = the most effective strategy

Animal models, especially patient-derived xenograft mouse models, are a critical bridge between preclinical studies and clinical trials. However, due to the lower costs and the compatibility with high throughput analysis, ex vivo tumor cultures are still the preferred method for ranking therapeutic agents, determining drug combination regimens, investigating mechanisms of action, and validating targets. Additionally, ex vivo tumor cultures are essential when studying certain hematological tumors, which are difficult to engraft and cannot be passaged in vivo because of their “liquid” nature.

Both ex vivo and in vivo studies continue to work together to advance oncology research, but each year brings new advances in ex vivo culture systems that enhance their overall impact in predicting treatment responses.