What Is Chemotherapy?

Chemotherapy is the treatment of disease, principally cancer, using chemical agents that inhibit cell division or induce cell death. The term encompasses a broad class of cytotoxic drugs designed to preferentially damage rapidly proliferating cells, which characterizes most tumors. First demonstrated as a therapeutic strategy in the 1940s with the clinical use of nitrogen mustard against lymphoma, chemotherapy has since expanded into hundreds of agents organized by mechanism, molecular target, and tumor type.

The field sits at the intersection of pharmacology, oncology, and biomedical engineering. Drug selection, dosing schedule, and delivery route are determined by the specific cancer type, its stage, the patient's organ function, and the drug's pharmacokinetics. Chemotherapy is often combined with radiation, surgery, or targeted biological agents in multimodal treatment plans.

Mechanisms of Action

Chemotherapeutic agents act through several distinct biochemical pathways. Alkylating agents, such as cyclophosphamide and cisplatin, form covalent bonds with DNA strands, blocking replication. Antimetabolites, including methotrexate and 5-fluorouracil, mimic naturally occurring nucleotides or folate cofactors and disrupt DNA synthesis by substituting into metabolic pathways. Topoisomerase inhibitors such as doxorubicin and irinotecan trap topoisomerase enzymes in a form that generates lethal strand breaks. Taxanes (paclitaxel, docetaxel) stabilize microtubule polymers and prevent chromosome segregation during mitosis. Because these mechanisms target cell division rather than cancer-specific molecular markers, normal tissues with high turnover rates, including bone marrow, gastrointestinal epithelium, and hair follicles, are also affected, producing the characteristic side effects associated with treatment.

Drug Delivery and Administration

Intravenous infusion is the predominant route for cytotoxic chemotherapy, allowing precise dose control and systemic distribution. Oral formulations are available for some agents and improve patient convenience, though bioavailability variability and adherence require careful monitoring. Nanomedicine approaches have modified drug delivery substantially: liposomal encapsulation of doxorubicin (Doxil) reduces peak plasma concentrations and cardiac toxicity compared with free drug, and polymeric nanoparticles enable controlled release profiles. Research on drug delivery systems for enhancing tumor penetration identifies that nanoparticles must traverse both the vascular and extracellular matrix barriers to reach cancer cells distant from tumor blood vessels, and describes charge-inversion and size-changing carrier designs that address this challenge. Regional delivery, such as hepatic arterial infusion for liver tumors or intrathecal injection for central nervous system disease, limits systemic exposure at the cost of added procedural complexity.

Patient Monitoring and Treatment Response

Monitoring patients during chemotherapy requires tracking hematologic toxicity, organ function, and tumor response simultaneously. Complete blood counts guide decisions to delay cycles or reduce doses when bone marrow suppression is severe. Imaging assessments using CT, MRI, or PET scans classify tumor response according to the Response Evaluation Criteria in Solid Tumors (RECIST) framework. Biomedical engineers have developed wearable and implantable monitoring devices that track physiological parameters continuously during outpatient treatment cycles. The NIH bioengineering strategies for targeted cancer therapy overview describes engineered scaffolds, nanoparticles, and cell-based delivery vehicles that aim to improve therapeutic index by concentrating drug effect at tumor sites. Circulating tumor DNA assays now allow molecular monitoring of treatment response and emerging resistance without tissue biopsy, and the Nature Biomedical Engineering study on patient-specific cancer immunotherapies illustrates how engineering approaches are extending the principles of targeted treatment beyond classical cytotoxic agents.

Applications

Chemotherapy has applications in the treatment of a wide range of conditions, including:

• Solid tumors including breast, lung, colorectal, and ovarian cancers
• Hematologic malignancies such as leukemia and lymphoma
• Preparation of patients for bone marrow and stem cell transplantation
• Combined chemoradiation for head, neck, and cervical cancers
• Metronomic low-dose regimens for anti-angiogenic effects