Gleevec

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For most of the history of cancer treatment, patients and doctors had three options. Surgery aimed to completely excise local cancers from the body, while radiation and cytotoxic drugs were used to attack cells in the act of dividing. All three avenues are rather blunt in scope, carry frequently horrific side effects, and rarely resulted in true cures. Once the root causes of cancer became clear through genetic research in the 1980s, researchers first in America, then around the world, sought more precise weapons to bring to bear against the ghostly target of cancer. The first such drug to pass clinical trials is imatinib, marketed as Gleevec in the United States, and hit the market in 2001. You can read more about it here:
Gleevec

Much of the development work on Gleevec was performed at the old Sandoz lab in the rugged alpine meadows and mountains of Switzerland. This is the same place where Albert Hofmann discovered LSD a half century earlier, and just as LSD would revolutionize psychotherapy in the 1950s, so Gleevec revolutionized cancer therapy in the 2000s. Unlike Hofmann's psychedelic potion, though, Gleevec was found through a carefully planned research and test program, rather than through serendipity and happenstance. For decades oncologists groped for better drugs empirically, dosing patients with nearly anything that looked like it had at least a slightly better chance of killing cancer cells than a healthy human body. The difference, unfortunately, was often very slight, and researchers remained ignorant of the mechanisms that lay the foundation for cancer, tightly constraining their ideas for exploration.

When geneticists began to identify the hereditary instructions that govern cell division, it wasn't long until mutations were found that jammed the works and caused cells to divide endlessly. Working with researchers in Oregon, Italy, and the UK, Novartis eventually found a large molecule that fit like a key into the lock of a specific mutated cancer gene that causes chronic myelogenous leukemia. The side effects are mild, since it aims to inhibit cell division only of the effected, mutated cancer cells. Gleevec is a surgical strike where only carpet bombing had been tried before, and is certainly the shape of things to come in oncology.