Examples are progesterone and testosterone, as well as thyroid hormones. They generally regulate transcription; or water soluble molecules that bind to receptors on the plasma membrane. They are either proteins like insulin and glucagons, or small, charged molecules like histamine and epinephrine.

Signal transduction pathways, which control the response of cells to various environmental signals, are mediated by the function of signaling proteins that interact with each other and activate one other with high specificity.

Proteins or peptides that participate in signal transduction processes in the cell. ( NCI Thesaurus)

The molecular components of these signal transduction pathways are always activated by a chemical signaling molecule. Signaling molecules can be grouped into three classes: cell-impermeant, cell-permeant, and cell-associated signaling molecules (Figure 8.3).

There are three major types: Ion channel linked receptors, G protein–coupled receptors, and enzyme-linked receptors.

Nanotechnology-based Drug Delivery in Cancer. Drug delivery in cancer is important for optimizing the effect of drugs and reducing toxic side effects. Several nanotechnologies, mostly based on nanoparticles, can facilitate drug delivery to tumors.

Once accumulated at the target site, hydrophobic biodegradable polymeric nanoparticles can act as a local drug depot depending on the make-up of the carrier, providing a source for a continuous supply of encapsulated therapeutic compound (s) at the disease site, e.g., solid tumors.

Nanoparticles are solid, colloidal particles consisting of macromolecular substances that vary in size from 10 nm to 1000 nm ( Kreuter, 1994a ). However, particles >200 nm are not heavily pursued and nanomedicine often refers to devices <200 nm (i.e., the width of microcapillaries).