small-molecule drugs

Definition of small-molecule drug
National Cancer Institute

small-molecule drug

(… MAH-leh-kyool ...)
A drug that can enter cells easily because it has a low molecular weight. Once inside the cells, it can affect other molecules, such as proteins, and may cause cancer cells to die. This is different from drugs that have a large molecular weight, which keeps them from getting inside cells easily. Many targeted therapies are small-molecule drugs.

Source: NCI Dictionary of Cancer Terms