...in mice
The biggest issue is that the mice we use for research typically have no or a highly depleted immune system. One of the biggest breakthroughs in the cancer field (IMO) is the development of humanized mice. These are mice that have had their immune system genes replaced with the human versions of these genes.
This is incredibly important for work like this where you’re studying cancer cell-immune cell interactions.
unfortunately, we need mice models for safety for now.
hopefully, computational models and personalized medicine will change this in the future.
1. Mice _love_ to get cancer naturally. If you have 100 mice, it's quite likely that around 20 of them will die within one year of cancer. This makes it difficult to extract useful signals. BTW, that's why if you see a study that a "chemical X results in cancer in mice", you should take that with a grain of salt.
2. Mice are small, so tumors are necessarily small too, with several orders of magnitude fewer cells than typical human tumors. So many drugs can just cure mice of cancer entirely, by killing cancerous cells too quickly to allow them to evolve resistance.
If you have 100 people, about 40 of them will be diagnosed with cancer over their lifetime (39/100 females and 41/100 males). Note "diagnosed with" is very different from "die" and "over lifetime" is not the same as "within one year", but the probability of people getting cancer naturally is high as well.
This all means that mice don't have a good natural cancer-suppression machinery, and this in turn makes them somewhat awkward to use to discover new treatments.