Fig. 1.

Acquired and intrinsic drug resistance. Static, pre-existing subpopulations within a tumor can mediate intrinsic drug resistance. These subpopulations often harbor resistance mediating mutations, which quickly become the predominant population resulting in minor response and rapid progression. In contrast, drug sensitive tumors show a rapid regression in response to anti-cancer drugs followed by the development of a variety of acquired resistance mechanisms (resistance A, B or C)