**4.2 Targeted molecular therapy**

Major advances in molecular biology, cellular biology and genomics have substantially improved our understanding of cancers. Now, these advances are being translated into therapy. The term targeted therapy ideally connotes the ability to identify a known therapeutic target that is important in the biology of cancer cells and use a specific agent that can treat the disease by modifying the expression or activity of the target in the growth and progression of the cancer. With this approach, only patients with a likelihood of benefit are treated, so the therapeutic index will hopefully be improved. Selecting a biologically active target is usually the first step in molecular imaging research and leads to the design of a molecular imaging agent. Potential targets include proteins, DNA, RNA, carbohydrates and lipids. Several properties of these biomolecules as imaging targets need to be considered that are summarized in the following (Meel et al., 2010):

