Small Molecule anti-cancer drugs can permeate into the inner cell domain and from there inhibit cell-growth signaling which can lead to cancer.
Tyrosine Kinase
An arch enemy. This is a protein with receptors that protrude outside the cell surface while the rest of the protein resides in the inner-cellular domain.
Growth Factors
The receptors can connect with a special growth factor for cancer -- the epidermal growth factor or "EGFR" -- and that can set off a tyrosine kinase reaction inside the cell. It is known as phosphorylation.
Phosphorylation... once this occurs cell growth signals are activated and the cell is on its way to becoming cancerous.
Inhibitors
The most important small molecule drugs in the fight against cancer are tyrosine kinase inhibitors. One example is Imatinib, which goes by the trade name Gleevec. It treats a few select cancers and was FDA-approved in 2002.