Traditional chemotherapy drugs work by killing cells that grow and divide quickly, such as cancer cells. Targeted therapies, by contrast, take aim at the specific genetic changes and the proteins within cancer cells that drive their chaotic growth.
Targeted drugs are generally made of very small molecules that can block proteins involved in cancer cells’ growth and survival. Such proteins may arise from mutated genes, from genes that have been duplicated too many times, or genes that have ended up in the wrong position within a chromosome. The effect of these mistakes is that the cell spurns the normal controls on its growth—that divides incessantly, exceeds its normal lifespan, hoards nutrients from the bloodstream, guzzles energy, shirks its normal role within the body, disarms the immune system, and, in some cases, migrates to other organs and tissues—in short, a cancer cell.
A protein’s function within a cell depends on its shape. One protein’s shape may allow it to send signals involved in stimulating or slowing cell growth. Another’s may help it dispose of cellular waste. The small molecules within a targeted therapy are designed to wedge into the particular nooks and crevices within a cancer-related protein. This can cause the protein to change shape or alter the binding of necessary energy-containing molecules, rendering it less capable of wreaking havoc with cell growth.
Examples of targeted therapies for cancer include:
- Tamoxifen, often considered the first targeted therapy, which is used to treat patients with breast cancer fueled by estrogen or progesterone.
- Imatinib (Gleevec), which targets a “fusion” proein called BCR-ABL and is used to treat some patients with chronic myelogenous leukemia or gastrointestinal stromal tumors driven by the KIT or PDGFRA proteins.
- The targeted drugs gefitinib (Iressa) and erlotinib (Tarceva), which are used in patients with non-small cell lung cancer who carry a mutated EGFR gene, leading to an abnormal EGFR protein.
Because targeted therapies interfere with specific cogs in the machinery of cancer cells, such drugs may have fewer and less severe side effects than conventional chemotherapy agents, which, in addition to killing cancer cells, often kill fast-growing normal cells as well, potentially causing problems such as hair loss and digestive problems.
A shortcoming that targeted drugs and chemotherapy agents often have in common is drug resistance. Because the genome, or set of DNA, within cancer cells is so unstable, they may give rise to daughter cells that can avoid the effects of certain drugs. Many researchers believe that combinations of targeted therapies, tailored to the particular genetic features of a patient’s tumor cells, holds the best hope of cutting off cancer’s escape routes and defeating the disease.