targeted therapy

targeted therapy, form of cancer treatment that uses drugs designed to inhibit or otherwise attack a specific molecular anomaly that drives cancer cell growth, division, or metastasis. This approach differs from traditional chemotherapy in that it is selective, working against cells carrying the disease-related target while leaving healthy cells unaffected. Targeted therapy is a fundamental component of precision medicine, in which genetic, molecular, and other biological information is used to predict and prevent disease and to tailor treatments to individual patients.

Early developments

The idea of targeted therapy emerged in the late 19th and early 20th centuries. A crucial early discovery was in 1895, when Scottish surgeon George Thomas Beatson found that removing the ovaries could slow the advance of metastatic breast cancer, demonstrating a link between hormones and cancer growth. The removal of the ovaries as a form of treatment for breast cancer was arguably one of the earliest targeted treatment approaches to emerge.

Not long after, in the early 1900s, German physician Paul Ehrlich proposed a “magic bullet”—a drug that would specifically target a disease-causing agent without harming healthy tissue. This theory was initially applied to infectious diseases, as knowledge of cancer biology was still limited at the time. However, his development in 1909 of arsphenamine (Salvarsan), an arsenic-based compound effective against syphilis, provided early proof that a drug could act selectively on a biological target.

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The first targeted therapies

In the late 1970s, after decades of research in drug development and cancer biology, the first hormone therapy, tamoxifen, which works by specifically blocking the estrogen receptor in cancer cells, became available. Tamoxifen was a catalyst for the modern era of targeted therapy, which progressed in parallel with major advances in molecular biology and cell biology. In particular, the identification of specific genetic mutations and proteins that underlie the proliferation of cancer cells provided concrete targets for new drugs. An example is the HER2 gene, which in 1987 was found to be overexpressed in an aggressive form of breast cancer. This discovery fueled research into anti-HER2 antibodies, and by 1998 trastuzumab (Herceptin), the first HER2-targeted antibody, was approved by the U.S. Food and Drug Administration (FDA) for HER2-positive breast cancer.

The development of trastuzumab was a major milestone in oncology, as it required a diagnostic test to identify eligible patients, effectively launching the era of precision medicine. The drug also followed closely on the heels of rituximab (Rituxan)—the first monoclonal antibody used in cancer therapy—which in 1997 was approved by the FDA specifically for non-Hodgkin lymphoma. Another breakthrough came in 2001 with imatinib (Gleevec), a targeted therapy that received accelerated FDA approval for chronic myelogenous leukemia (CML). It was designed to target a fusion protein known as BCR-ABL, a constitutively active tyrosine kinase (meaning it constantly signals cancer cells to grow and divide) that is produced by the Philadelphia chromosome. Imatinib’s remarkable success made CML a manageable chronic condition and established a paradigm for developing small-molecule kinase inhibitors.

Core principles

• Molecular specificity: Treatment is engineered to bind to specific molecular “targets”—often proteins, such as receptors, enzymes, or signaling molecules, that are overexpressed, mutated, or activated in cancer cells.
• Selective inhibition: Treatment specifically blocks signals for growth and survival that are unique to the tumor, effectively cutting off the cancer’s life-support systems.
• Minimizing collateral damage: There are fewer and less-severe side effects compared with traditional chemotherapy because healthy cells, which lack the specific target or rely on different pathways, are largely unaffected.
• Individualized treatment: The presence of specific biomarkers in a patient’s tumor, assessed via molecular diagnostic testing, determines whether a patient is a suitable candidate for a particular drug.

Types and targeted pathways

The main types of targeted therapy are monoclonal antibodies, which work on the cell surface, and small-molecule drugs, which can enter the cell. Other types include antibody-drug conjugates (ADCs) and hormone therapies. These treatments are further categorized by the molecules they target and their mechanisms of action, such as blocking growth signals or inhibiting angiogenesis (the growth of new blood vessels).

The effects of targeted therapy are directly related to the molecular pathways that specific drugs disrupt. Examples of pathways targeted by these agents include the MAPK, PI3K/AKT/mTOR, EGFR, and VEGF pathways, which are involved in a wide range of cellular processes. The MAPK pathway, for example, activates a cell-signaling cascade that relays information from the cell surface to the nucleus, helping to regulate crucial functions—such as cell proliferation, differentiation, and survival—and to mediate drug resistance. The PI3K/AKT/mTOR pathway is a cell-signaling network that, when abnormally hyperactivated in cancer, promotes tumor growth, resistance to therapy, and the migration, invasion, and survival of cancer cells. EGFR inhibitors disrupt cell growth signals directly within or on the surface of cancer cells, whereas VEGF inhibitors cut off the tumor’s blood supply by interfering with angiogenesis.

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Other modalities have also emerged. For example, poly (ADP-ribose) polymerase (PARP) inhibitors block DNA repair enzymes to kill cancer cells that already have defective DNA repair mechanisms. Examples include the drugs olaparib, niraparib, rucaparib, and talazoparib, which are approved for specific types of ovarian, breast, pancreatic, and prostate cancer. These drugs are particularly effective in patients with certain genetic mutations, such as in BRCA.

ADCs are unique among targeted therapies in that they are hybrid agents, combining a monoclonal antibody with a cytotoxic drug. The antibody is designed to bind specifically to a protein on the surface of cancer cells. Upon binding, the ADC is transported to the interior of the cancer cell, where it releases the cytotoxic drug, thereby delivering the toxic effects directly to the tumor, with limited impact on healthy cells. Several ADCs are approved for breast, lung, ovarian, and hematologic malignancies. Examples include ado-trastuzumab emtansine (Kadcyla) and fam-trastuzumab deruxtecan (Enhertu) for HER2-positive breast cancer and sacituzumab govitecan (Trodelvy) for triple-negative breast cancer.

Advantages and limitations

The advent of targeted therapy marked a shift toward more advanced therapeutic strategies for cancer that offer enhanced precision, leading to greater effectiveness and fewer harmful side effects relative to conventional chemotherapy. Among its core benefits is the potential for better patient outcomes, including improved survival rates and quality of life. Furthermore, targeted therapy supports both precision medicine and personalized medicine by allowing the creation of combination therapies specifically adapted to a patient’s unique genetic makeup.

At the same time, targeted therapies often face limitations because of the development of drug resistance from gene mutations. They also are expensive and are not accessible to all patients, and they can cause harmful side effects. Side effects associated with targeted therapy are often different from those of conventional treatments and can vary widely, depending on the specific target and the individual patient. Examples of common side effects include skin rashes, diarrhea, fatigue, headaches, muscle aches, and high blood pressure. More severe conditions, such as cardiotoxicity, bleeding, and interstitial pneumonia (lung inflammation), can be fatal.

Other significant challenges for targeted therapy include the identification of reliable biomarkers that accurately predict how well a patient will respond to treatment. In addition, it is difficult to develop treatments for some drug targets because of their complex structure or essential cellular functions. Reaching such “undruggable” targets has necessitated innovative strategies, such as the generation of small molecules to actively degrade a target protein and the development of advanced drug delivery systems to ensure that high doses of the drug reach the desired target.