This must be fairly slow-acting. I recall from having my late, beloved Lab treated for cancer that there is such a thing as too successful with large tumors. That much cell death all at once can have severe systemic effects upon the body, even leading to death of the patient.
A good friend has lung cancer; a single large tumor in one lung. He's having it treated with daily low-dose targeted radiation and low dose chemotherapy to shrink it. When it gets small enough, they'll hit it with an immune-system based treatment to destroy it completely. Can't use the immune therapy until it gets small.