Targeted cancer therapy represents a paradigm shift in oncology, moving from non-specific cytotoxic agents to drugs designed to interfere with specific molecular pathways driving cancer growth and survival. These agents exploit vulnerabilities identified through understanding the genetic and molecular alterations that distinguish cancer cells from normal tissue.
What Is Targeted Cancer Therapy?
Targeted therapies include small molecule inhibitors that penetrate cells and interfere with intracellular signaling, and monoclonal antibodies that bind extracellular targets. Unlike cytotoxic chemotherapy, targeted agents often produce cytostatic rather than cytotoxic effects, requiring chronic administration and different response assessment criteria.
Drug Classes and Mechanisms
Tyrosine kinase inhibitors (TKIs) block tyrosine kinase enzymes that drive cell proliferation, survival, and angiogenesis. Imatinib targets BCR-ABL in chronic myeloid leukemia and KIT in gastrointestinal stromal tumors, transforming these previously fatal diseases into manageable chronic conditions. Erlotinib and gefitinib inhibit epidermal growth factor receptor (EGFR) in non-small cell lung cancer. Osimertinib is a third-generation EGFR TKI active against the T790M resistance mutation and with central nervous system penetration.
Monoclonal antibodies include trastuzumab, which targets HER2 in breast and gastric cancers, disrupting receptor signaling and inducing antibody-dependent cellular cytotoxicity. Rituximab targets CD20 on B cells for non-Hodgkin lymphoma. Cetuximab and panitumumab target EGFR in colorectal cancer and head and neck cancer. These antibodies may be used as monotherapy or conjugated to cytotoxic drugs as antibody-drug conjugates.
PARP inhibitors such as olaparib exploit synthetic lethality in tumors with defects in homologous recombination repair, particularly those with BRCA1 or BRCA2 mutations. By blocking single-strand break repair, PARP inhibitors cause accumulation of DNA damage leading to cell death.
BRAF inhibitors including vemurafenib and dabrafenib target the V600E mutation of BRAF found in melanoma, non-small cell lung cancer, and colorectal cancer. They are typically combined with MEK inhibitors to improve efficacy and reduce resistance and cutaneous toxicity.
mTOR inhibitors such as everolimus and temsirolimus inhibit the mammalian target of rapamycin, a key regulator of cell growth and metabolism. They are used in renal cell carcinoma, neuroendocrine tumors, and tuberous sclerosis complex-associated tumors.
Therapeutic Uses
Targeted therapy requires demonstration of the relevant molecular target in the tumor before initiation. EGFR mutations in lung adenocarcinoma predict response to erlotinib, gefitinib, and osimertinib. HER2 amplification in breast cancer mandates trastuzumab-based therapy. BRAF V600E mutant melanoma responds dramatically to BRAF-MEK inhibitor combinations. BCR-ABL positive chronic myeloid leukemia is treated with imatinib as first-line therapy. PARP inhibitors are approved for BRCA-mutated ovarian, breast, pancreatic, and prostate cancers.
Adverse Effects
TKIs commonly cause fatigue, diarrhea, skin rash, and hand-foot skin reactions with some agents. EGFR inhibitors produce acneiform rash and diarrhea correlating with treatment response. Imatinib causes fluid retention and muscle cramps. PARP inhibitors cause fatigue, nausea, and myelosuppression. BRAF inhibitors may cause photosensitivity, arthralgias, and secondary cutaneous squamous cell carcinomas.
Key Clinical Considerations
Biomarker testing is mandatory before initiating targeted therapy. Resistance inevitably develops through secondary mutations (T790M with EGFR TKIs), pathway reactivation, or histologic transformation. Liquid biopsy enables non-invasive resistance monitoring. Drug-drug interactions via CYP metabolism require careful management, particularly with many TKIs.
Conclusion
Targeted cancer therapy has transformed outcomes for molecularly defined patient subsets. The success of these agents depends on accurate biomarker identification, understanding resistance mechanisms, and strategic sequencing or combination with other treatment modalities.
