What might explain why an inhibitor works in two-dimensional culture but not in organoids?

Why

Shi and colleagues mapped oxygen in patient-derived pancreatic spheroids and found a decay gradient that leaves cells deeper than about 20 micrometers from the surface hypoxic and quiescent. For spheroids larger than about 150 micrometers in diameter, a drug that acts on proliferating cells cannot reach an IC50 at any practical concentration. They also found that topoisomerase inhibitors behaved differently, penetrating past the oxygenated layer and still giving a dose response.

So the mechanism of the drug decides whether the effect survives the move to three dimensions. Other factors contribute. Extracellular matrix such as Matrigel binds some compounds and slows diffusion.

Cell-cell and cell-matrix contacts activate survival signaling absent in a monolayer. Organoids often grow more slowly, which changes any division-dependent readout. The assay itself differs, since a luminescent viability reagent must penetrate the structure as well.

None of this means the monolayer result was wrong. It means the three-dimensional model is testing a harder and more realistic question, and the failure is informative. Measuring drug concentration inside the structure, and the proliferating fraction by position, turns a disappointment into a mechanism.