# Principles of Cancer Chemotherapy

> Principles of cancer chemotherapy for MBBS Pharmacology — cell cycle specificity, combination rules and key toxicities for exams.

- Canonical URL: https://prepelephant.com/topics/mbbs/pharmacology/principles-of-cancer-chemotherapy
- Exam / course: MBBS · Subject: Pharmacology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Principles of Cancer Chemotherapy", PrepElephant, https://prepelephant.com/topics/mbbs/pharmacology/principles-of-cancer-chemotherapy

## Direct answer

Cancer chemotherapy uses cytotoxic drugs that exploit tumour proliferation, killing the maximum number of malignant cells at acceptable cost to normal tissues. By the log-kill hypothesis of Skipper, each cycle destroys a fixed fraction of the tumour, not a fixed number of cells — hence multiple courses are essential. Chemotherapy may be curative, adjuvant, neoadjuvant or palliative depending on the tumour and stage.

## What you must remember

- Cell-cycle-specific agents act in S phase (antimetabolites — methotrexate, 5-fluorouracil, cytarabine) or M phase (vincristine, vinblastine, taxanes); cell-cycle non-specific agents — alkylating drugs, platinum compounds and anthracyclines — kill at any point and suit slow-growing tumours.
- Log-kill hypothesis: a given dose kills a constant proportion of cells, so clinical remission is not cure; residual cells are cleared by repeated cycles and host defences.
- Goals of treatment: curative regimes in acute lymphoblastic leukaemia, Hodgkin lymphoma, choriocarcinoma and testicular tumours; adjuvant therapy after surgery for micrometastases (breast, colorectal cancer); neoadjuvant therapy before surgery to downstage tumours and permit limb salvage (osteosarcoma); palliative therapy in advanced disease.
- Combination principles: drugs individually active against the tumour, different mechanisms to avoid cross-resistance, and non-overlapping toxicities; classic regimens include CHOP and ABVD.
- Dose-limiting toxicities worth memorising: cyclophosphamide — haemorrhagic cystitis from acrolein, prevented by MESNA and hydration; cisplatin — nephrotoxicity, ototoxicity and severe emesis; doxorubicin — cumulative cardiotoxicity mitigated by dexrazoxane; bleomycin — pulmonary fibrosis; vincristine — peripheral neuropathy and constipation (vinblastine is myelosuppressive); methotrexate — myelosuppression and mucositis, rescued by leucovorin.
- Shared toxicities: myelosuppression with a nadir around seven to fourteen days, mucositis, alopecia, infertility and secondary leukaemia from alkylators; tumour lysis syndrome after rapid lysis of bulky disease is prevented by hydration with allopurinol or rasburicase.
- Targeted agents complement cytotoxics — imatinib against BCR-ABL1 in CML and gastrointestinal stromal tumours, trastuzumab in HER2-positive breast cancer (itself cardiotoxic) and rituximab in CD20 lymphomas.

## Common confusion

Adjuvant and neoadjuvant are habitually interchanged. Adjuvant therapy follows definitive surgery, attacking micrometastatic disease invisible to the surgeon; neoadjuvant therapy precedes it, shrinking the primary to allow less mutilating operations and providing an in-vivo test of sensitivity. A second recurring mix-up is vincristine with vinblastine — vincristine is neurotoxic and sparing of marrow, vinblastine the reverse.

## Exam-focused takeaway

Theory answers should classify agents by cell-cycle specificity, state the log-kill hypothesis, list the goals of therapy and combination-regimen principles, then give the drug-toxicity map. Viva examiners ask why combinations outperform single agents and what MESNA or leucovorin rescue achieves. MCQs are dominated by drug-toxicity matching, acrolein and haemorrhagic cystitis, doxorubicin cardiotoxicity and tumour lysis syndrome biochemistry.

## Frequently asked questions

### What is the log-kill hypothesis?

Each dose of chemotherapy kills a constant fraction of tumour cells rather than a constant number, so repeated cycles are required to approach eradication.

### How does adjuvant differ from neoadjuvant chemotherapy?

Adjuvant therapy is given after surgery to eliminate residual micrometastases; neoadjuvant therapy is given before surgery to shrink the tumour and enable less radical operations.

### Why is MESNA given with cyclophosphamide?

MESNA detoxifies acrolein, the urotoxic metabolite responsible for haemorrhagic cystitis, alongside vigorous hydration.

### Which chemotherapeutic drug is cardiotoxic and how is this limited?

The anthracycline doxorubicin, causing cumulative dose-related cardiomyopathy; lifetime dose capping and dexrazoxane reduce the risk.

### What is tumour lysis syndrome?

Massive release of potassium, phosphate and uric acid when bulky tumours lyse, causing hyperkalaemia, hypocalcaemia and acute kidney injury — prevented by hydration and allopurinol or rasburicase.