Figure 2. The era of chemotherapy.

Since the discovery of nitrogen mustards and folic acid antagonist drugs in the 1940s, the history of chemotherapy has begun. In the mid-2000s, when PTKs were discovered and revealed to be involved in carcinogenesis, the approach toward chemotherapy evolved to target specific cancer-associated molecules like PTKs. However, due to chemoresistance, targeted therapy has still been challenged by combination chemotherapy in clinical trials.