Figure 3.

Major differences between spheroid and monolayer culture that cause challenges for therapy development: (A) in contrast to monolayer culture, cells in a spheroid are tightly packed. This leads to the contact effect that induces radioresistance observed in 3D models as well as in patients. (B) The percentage of cycling cells in spheroids is size-dependent and varies between 40 to 70%, while in monolayer culture, more than 90% of cells are cycling. That makes the monolayer cells much more prone to medication and leads to misleading results, explaining the high number of failed clinical trials. (C) Oxygen, nutrient, and drug gradients are present in spheroids with concentration and penetration of all three decreasing towards the spheroid core, whereas in a monolayer culture, oxygen, nutrients, and drugs can reach each cell with equal efficiency. This leads again to false expectations on the true effectiveness of a drug candidate. In PDAC, the situation is even more severe, as fibrosis leads to a dense mesh-like network that hampers nutrients, drugs, as well as oxygen from entering the spheroid, leading to pronounced gradient formation. All three challenges are present and even more complex in co-culture spheroids.