This is not exactly a bioinformatics question, I guess. You might be better of posting this at http://biology.stackexchange.com/ (This can get closed because not bioinformatics.)

Although I have no background in pharmacology, a protein should be accessible (preferentially cell surface molecule), your drug-protein interaction should be specific (avoiding side-effects by interaction of your drug with other proteins) and your protein should not have too many functions or vital in too many tissues (because, again, side-effects).

But obviously this would depend on the disease and protein in question. You would probably tolerate a few side effects in cancer therapy, but absolutely not when treating a common cold.