Targeted Therapy and Kinase Inhibitors

Targeted therapy uses drugs designed to inhibit a specific molecule that drives a cancer's growth, sparing most normal cells. Small-molecule kinase inhibitors and monoclonal antibodies are its principal forms; the breakthrough example, imatinib against the BCR-ABL kinase in chronic myeloid leukaemia, established that blocking a single oncogenic driver could control disease.

Definition

Targeted therapy is cancer treatment with agents that specifically inhibit a defined molecular alteration — most often an activated kinase or growth-factor receptor — on which a tumour depends, in contrast to the broad cytotoxicity of classical chemotherapy.

Scope

This topic covers the rationale of molecularly targeted treatment: oncogene dependence, the main classes of targeted agents (tyrosine kinase inhibitors and therapeutic antibodies), how predictive biomarkers select patients, and how resistance emerges. It is a conceptual reference and does not provide dosing or individualized treatment guidance.

Core questions

• What does it mean for a tumour to be 'addicted' to an oncogenic driver?
• How do small-molecule kinase inhibitors and therapeutic antibodies differ?
• How are predictive biomarkers used to select patients for targeted therapy?
• Why does acquired resistance so often develop, and how is it addressed?

Key concepts

• Oncogene dependence
• Tyrosine kinase inhibitors
• Therapeutic monoclonal antibodies
• Growth-factor receptors (e.g., HER2, EGFR)
• Predictive biomarkers and companion diagnostics
• ATP-competitive inhibition
• Acquired resistance and gatekeeper mutations
• On-target and off-target toxicities

Key theories

Oncogene addiction

Some tumours become so dependent on the continued activity of a single activated oncogene that inhibiting that gene's product alone can halt their growth or trigger cell death, providing the conceptual basis for single-target therapy.

Mechanisms

Many cancers depend on a constitutively active kinase produced by a fusion gene, mutation, or amplification. Small-molecule inhibitors typically occupy the kinase ATP-binding pocket and block downstream signalling, while monoclonal antibodies bind the extracellular domain of a receptor such as HER2 to interrupt signalling and recruit immune effector functions. Because efficacy depends on the presence of the target, predictive biomarkers and companion diagnostics select patients likely to respond. Tumours frequently escape through secondary mutations that hinder drug binding, activation of bypass pathways, or amplification of the target, which motivates next-generation inhibitors and combination strategies. Toxicities reflect both on-target effects in normal tissues that use the same pathway and off-target activity, including the cardiovascular effects seen with some agents.

Clinical relevance

Targeted agents are now integral to the management of many molecularly defined cancers and are commonly used alongside chemotherapy or immunotherapy. Familiarity with their logic supports critical reading of biomarker-driven trials and multidisciplinary care. This entry describes mechanisms and principles and is not a basis for selecting agents or doses for any individual patient.

Evidence & guidelines

Targeted therapy is governed by biomarker-defined indications in tumour-specific guidelines (e.g., NCCN, ESMO), supported by registration trials such as those that established imatinib in chronic myeloid leukaemia and trastuzumab in HER2-positive breast cancer. This reference summarizes the underlying principles rather than reproducing indication- or dose-level recommendations.

History

The field crystallized around the BCR-ABL fusion kinase of chronic myeloid leukaemia: imatinib, a rationally designed inhibitor, produced durable responses and proved that targeting a single oncogenic driver could be transformative. In parallel, the anti-HER2 antibody trastuzumab brought targeted treatment to a biomarker-defined subset of breast cancer. These successes launched a generation of kinase inhibitors and therapeutic antibodies and reframed oncology around molecular targets.

Debates

How best to overcome acquired resistance?

Tumours treated with kinase inhibitors commonly relapse through secondary mutations or bypass signalling; whether to deploy successive next-generation inhibitors, upfront combinations, or biomarker-guided switching remains an active question.

Key figures

• Brian J. Druker
• Charles L. Sawyers
• Dennis J. Slamon
• I. Bernard Weinstein
• Douglas Hanahan

Seminal works

• druker-2001
• hudis-2007
• hanahan-weinberg-2011

Frequently asked questions

How is targeted therapy different from chemotherapy?

Chemotherapy broadly damages dividing cells, whereas targeted therapy is designed to block a specific molecule that a particular tumour depends on, which can spare many normal cells and is usually guided by a biomarker test.

Why does targeted therapy sometimes stop working?

Tumours can acquire new mutations that prevent the drug from binding its target, or activate alternative signalling pathways, allowing resistant cells to grow despite continued treatment.