It's discussed elsewhere here, but to answer directly - as I understand it, cancer cells can only use energy from sugars. Cancer cells are also extremely energy hungry, since they spend so much energy growing. When a cell absorbs these particular sugars, it self-destructs in spectacular fashion.

It sounds like they're expecting cancer cells to absorb the vast majority of these sugars, leaving just a small amount for healthy cells. Which sounds to me like a kind of chemotherapy, but more effective and with weaker side effects.

(Not a biologist, most of this is over my head!)