Rent seeking is not applicable for any company developing new medicines because that by definition is creating new wealth. I wouldn't disagree with that characterization for any company milking an out-of-patent treatment by trying to make it unfeasible for any other company to manufacture it. You are correct that does exist.

Cures are difficult to develop due to how variable human physiology is, but we still manage to do so. Vaccines are also a way more effective instrument for disease eradication; it's better to prevent anyone getting the disease in the first place.