Cytotoxic Chemotherapy

From Single Agents to Working Combinations

The earliest cytotoxic drugs used in breast cancer were borrowed from other cancers. Nitrogen mustard, developed from wartime chemical weapons research, and later methotrexate, were tested singly and found to produce responses, though rarely durable ones. The leap forward came not from mechanistic reasoning but from trial and error — literally, from trials.

In the early 1970s, two combination regimens changed the frame entirely. The Cooper regimen, combining cyclophosphamide, methotrexate, fluorouracil, vincristine and prednisone, showed response rates in metastatic disease that single agents had not approached. At roughly the same time, the National Cancer Institute programme produced CMF — cyclophosphamide, methotrexate and fluorouracil — a three-drug combination that proved more tractable and became the workhorse of adjuvant therapy for more than a decade. Gianni Bonadonna and colleagues in Milan showed in the mid-1970s that CMF given after surgery reduced recurrence compared with surgery alone, which was the first credible demonstration that systemic chemotherapy could improve outcomes in early-stage disease.

Anthracyclines — doxorubicin and its relatives — entered the regimens next. Their activity against breast tumours was clear, but so was their toxicity, including dose-dependent cardiac damage. Regimens incorporating anthracyclines gradually displaced CMF in many settings; the Early Breast Cancer Trialists' Collaborative Group, through its Oxford-based meta-analyses of pooled trial data, demonstrated that anthracycline-containing regimens produced a modest but real survival advantage over CMF.

Taxanes — paclitaxel and docetaxel — arrived in the 1990s and added a further mechanism: disruption of the microtubule apparatus that cells need to divide. Large trials run by the NSABP and others showed that adding a taxane to an anthracycline-based backbone improved outcomes in node-positive disease. The progress was cumulative and stepwise, each generation of regimens tested against the last.

What chemotherapy does in every case is the same: it kills cells that divide rapidly, which tumour cells generally do. What it cannot do is distinguish malignant from normal rapidly-dividing cells, which is why bone marrow suppression, hair loss and mucositis are predictable consequences rather than rare ones. The understanding that chemotherapy's benefit varied by tumour biology — that hormone-receptor-positive tumours respond less dramatically than triple-negative ones — came later, driven by the same kind of trial evidence that built the regimens in the first place.