What is the main premise of the lock-and-key model for drug activity?

Protein molecules are very complicated. Because of interactions between atoms within the molecules, they are often bent and twisted. This complicated structure gives each protein a specific shape. Because of the shape, the molecule can interact in very specific ways with other molecules. Think of a jigsaw puzzle. There is only one piece that will fit perfectly in each spot.

One model used to describe the behavior of molecules interacting because of their shapes is the lock and key model. Below is a generic representation of a lock and key site. The lock may represent a cancer-causing molecule and the key may represent a drug molecule that interacts with the cancer molecule to weaken it.