Good question. I'm an author on both of the new CD47 studies, so I'll try to answer this for you:
1. CD47 appears to be higher on tumor cells than on normal cells. In general, the higher the expression of a marker on a diseased cell versus a healthy cell, the more of a therapeutic window/index [1] that is available for a treatment to specifically target diseased cells.
2. Inhibiting the anti-phagocytosis (i.e., "don't eat me") signal is only half of the equation. There are also pro-phagocytosis (i.e., "eat me") signal(s) that also are present [2] and that play a role in whether, and how well, macrophages, and other phagocytes, can phagocytose a cell. "Eat me" signal(s) also differ between normal and cancer cells, adding more nuance to the situation.
It does sound like a very promising avenue, but I imagine that this route could still be a problem for people already pre-disposed to autoimmune diseases. Still, most of those aren't as bad as the alternative here. Any drug based on this doesn't have to not have side effects, it just has to be better than normal chemotherapy.
You raise another good point here, that the allowable side effect profiles for cancer drugs are much less stringent than for drugs indicated for less life-threatening diseases.
1. CD47 appears to be higher on tumor cells than on normal cells. In general, the higher the expression of a marker on a diseased cell versus a healthy cell, the more of a therapeutic window/index [1] that is available for a treatment to specifically target diseased cells.
2. Inhibiting the anti-phagocytosis (i.e., "don't eat me") signal is only half of the equation. There are also pro-phagocytosis (i.e., "eat me") signal(s) that also are present [2] and that play a role in whether, and how well, macrophages, and other phagocytes, can phagocytose a cell. "Eat me" signal(s) also differ between normal and cancer cells, adding more nuance to the situation.
[1] Therapeutic index: http://en.wikipedia.org/wiki/Therapeutic_index
[2] A paper examining a putative "eat me" signal, as it relates to CD47: http://stm.sciencemag.org/content/2/63/63ra94.abstract