Cancer Chemotherapy Principles

The conceptual backbone of every chemotherapy MCQ: how cytotoxic drugs kill dividing cells, why we combine them, why tumours become resistant, and the predictable toxicities that flow from non-selective attack on proliferating tissues. Master this once and the specific-drug questions become pattern recognition.

Why "principles" dominate the exam

Cytotoxic chemotherapy exploits a single therapeutic gap: cancer cells generally divide more often and repair damage less faithfully than most normal cells. Unlike antimicrobials, there is no unique target — the drugs hit any rapidly dividing cell. This poor selectivity explains both the narrow therapeutic index and the stereotyped toxicity profile (bone marrow, gut mucosa, hair follicle, gonad). The principles below — cell-cycle kinetics, log-kill, combination rationale, resistance, and general toxicity — are the lens through which all agent-specific facts make sense.

Cell-cycle kinetics & the growth fraction

Cells move through G1 → S (DNA synthesis) → G2 → M (mitosis), with resting cells parked in G0. The fraction of tumour cells actively cycling is the growth fraction, and it is the single most important determinant of chemosensitivity.

• High growth fraction (e.g. high-grade lymphomas, acute leukaemias, germ-cell tumours, choriocarcinoma) → highly chemosensitive, often curable.
• Low growth fraction (e.g. colon, non-small-cell lung, renal cell carcinoma) → relatively chemoresistant; cells sitting in G0 escape phase-specific agents.

Tumours follow Gompertzian growth: when small, growth fraction is high and doubling time short; as the tumour enlarges and outgrows its blood supply, growth fraction falls and many cells drop into G0. This is the kinetic basis for adjuvant chemotherapy — treat micrometastatic disease after surgery while tumour burden is low and growth fraction is highest.

High-yield: Growth fraction, not absolute tumour size, predicts chemosensitivity. Small residual tumours (adjuvant setting, post-debulking) have the highest growth fraction and respond best.

Cell-cycle phase specificity

Class	Phase specificity	Representative agents	Key kinetic feature
CCS – S phase	DNA synthesis	Antimetabolites (methotrexate, 5-FU, cytarabine, gemcitabine, 6-MP)	Schedule-dependent; benefit from prolonged/infusional exposure
CCS – M phase	Mitosis	Vinca alkaloids (vincristine, vinblastine), taxanes (paclitaxel)	Spindle poisons
CCS – G2 phase	Pre-mitotic	Bleomycin, etoposide (topo-II)	Bleomycin also G2-arrests
CCS – G1 phase	Post-mitotic	Corticosteroids, asparaginase	—
Cell-cycle non-specific (CCNS)	All phases incl. G0	Alkylating agents, cisplatin, anthracyclines, nitrosoureas, dactinomycin	Dose-dependent killing

Cell-cycle-specific (CCS) drugs kill only cycling cells; their efficacy plateaus with dose but improves with prolonged exposure / repeated cycles (schedule-dependent). Cell-cycle-non-specific (CCNS) drugs kill cycling and resting cells; their killing is dose-dependent (a steep dose–response), making them the backbone of high-dose regimens.

High-yield: Antimetabolites are S-phase specific and SCHEDULE-dependent (favour continuous infusion, e.g. infusional 5-FU). Alkylating agents are cell-cycle NON-specific and DOSE-dependent.

Mnemonic for M-phase (mitotic) poisons — "Very Toxic Poisons in Metaphase": Vinca, Taxanes, Podophyllotoxin... (etoposide actually arrests in G2/late S — see below), keep vinca/taxanes as the pure M-phase pair.

Log-kill hypothesis

Skipper's log-kill hypothesis states that a given dose of a drug kills a constant fraction of tumour cells, not a constant number — i.e. first-order kinetics. If one cycle kills 99.9% (3 logs) of a 10¹² cell burden, 10⁹ cells remain; the next identical cycle again removes 3 logs to leave 10⁶, and so on.

Consequences for exam:

1. Repeated cycles are essential — no single dose can sterilise a clinically detectable tumour (~10⁹ cells = 1 g).
2. Because each cycle kills a fraction, lower tumour burden is easier to cure — reinforcing adjuvant therapy.
3. The smallest detectable tumour already contains ~10⁹ cells; cure requires reducing below 1 cell despite immune clearance.

High-yield: Log-kill = constant FRACTION killed per cycle (first-order). It justifies multiple cycles and explains why micrometastatic (adjuvant) disease is most curable.

A refinement, the Norton–Simon hypothesis, notes that kill is proportional to growth rate at the moment of treatment; this underpins dose-dense scheduling (e.g. dose-dense AC in breast cancer, giving cycles every 2 weeks with G-CSF support rather than every 3 weeks).

Combination chemotherapy — the rationale

Single agents rarely cure; combinations are the rule. The principles for selecting a regimen are classic exam fodget:

Selection rules → 1. Each drug active as a single agent → 2. Different (non-overlapping) mechanisms of action → 3. Non-overlapping toxicities → 4. Different patterns of resistance → 5. Use at optimal dose and schedule → 6. Give at consistent intervals; keep the interval as short as marrow recovery allows.

The three goals of combining drugs:

• Maximal cell kill within tolerated toxicity.
• Broader coverage of a heterogeneous (resistant-subclone) tumour population.
• Prevent/slow emergence of drug resistance.

Non-overlapping toxicity is why classic regimens pair drugs that hit different organs — e.g. in MOPP (Mechlorethamine, Oncovin/vincristine, Procarbazine, Prednisone) and ABVD (Adriamycin/doxorubicin, Bleomycin, Vinblastine, Dacarbazine) for Hodgkin lymphoma. Vincristine's dose-limiting toxicity is neuropathy (marrow-sparing), so it slots into many marrow-toxic regimens.

High-yield: The cardinal rule of combination design is non-overlapping toxicities plus different mechanisms and resistance patterns, with each drug individually active.

Drug resistance

Resistance is the main reason chemotherapy fails. It may be intrinsic (present before exposure) or acquired (emerges under selection pressure, often clonal).

The Goldie–Coldman hypothesis links resistance to spontaneous mutation rate: the probability of harbouring a resistant clone rises with tumour size, so treat early, treat hard, and use non-cross-resistant combinations (rationale for alternating regimens like MOPP/ABVD).

Mechanisms of resistance (very high yield)

Mechanism	Example drug(s) affected	Note
↑ Drug efflux — P-glycoprotein (MDR1/ABCB1)	Doxorubicin, vinca alkaloids, taxanes, etoposide, dactinomycin	ATP-dependent efflux pump; classic multidrug resistance (MDR) — confers resistance to multiple structurally unrelated NATURAL-product drugs at once
↓ Drug uptake	Methotrexate (reduced folate carrier), melphalan
↓ Activation of prodrug	Cytarabine (↓ deoxycytidine kinase), 5-FU, 6-MP
↑ Inactivation	Cytarabine (↑ cytidine deaminase)
↑ Target / altered target	Methotrexate (↑ DHFR; altered DHFR)
↑ DNA repair	Alkylating agents, cisplatin (↑ nucleotide-excision repair, ↑ MGMT)	MGMT silencing predicts temozolomide response in glioblastoma
↑ Drug-detoxifying thiols	Alkylators, cisplatin (↑ glutathione/GST)
Defective apoptosis	Broad (p53 loss, ↑ BCL-2)

High-yield: P-glycoprotein (MDR1 gene product) is an ATP-dependent efflux pump causing multidrug resistance to large natural-product drugs — anthracyclines, vinca alkaloids, taxanes, epipodophyllotoxins, dactinomycin. It does NOT typically affect alkylators or antimetabolites.

High-yield: Methotrexate resistance = ↓ uptake via reduced folate carrier, ↑ DHFR (gene amplification), or altered DHFR with low affinity.

Classification of cytotoxic agents by mechanism

A quick organising map (specifics covered in agent-wise notes):

• Alkylating agents (CCNS): cyclophosphamide, ifosfamide, mechlorethamine, melphalan, busulfan, nitrosoureas (carmustine/lomustine — cross BBB), procarbazine, dacarbazine/temozolomide. Cross-link DNA at N7 of guanine.
• Platinum compounds (CCNS, alkylating-like): cisplatin, carboplatin, oxaliplatin — form intra-strand DNA cross-links.
• Antimetabolites (CCS, S-phase): folate analogue methotrexate; pyrimidine analogues 5-FU, capecitabine, cytarabine, gemcitabine; purine analogues 6-MP, 6-TG, fludarabine, cladribine.
• Microtubule agents (CCS, M-phase): vinca alkaloids (inhibit polymerisation), taxanes (stabilise/prevent depolymerisation).
• Topoisomerase inhibitors: topo-I — irinotecan, topotecan; topo-II — etoposide, teniposide, anthracyclines.
• Antitumour antibiotics: anthracyclines (doxorubicin, daunorubicin — free radicals, topo-II, intercalation), bleomycin (free radicals, G2 arrest), dactinomycin, mitomycin.
• Hormonal / targeted / biologics: tamoxifen, aromatase inhibitors, GnRH analogues, imatinib, rituximab, trastuzumab, immune checkpoint inhibitors (covered separately).

General toxicity of chemotherapy

Toxicity mirrors the normal tissues with the highest turnover. These appear in nearly every "which adverse effect" stem.

Tissues with high growth fraction → predictable toxicities

Toxicity	Basis	Timing / notes
Myelosuppression	Bone-marrow stem/progenitor turnover	Dose-limiting toxicity of MOST cytotoxics; neutrophil nadir ~7–14 days, recovery by 21–28 days
Mucositis / stomatitis & diarrhoea	GI epithelial turnover	Methotrexate, 5-FU, anthracyclines prominent
Alopecia	Hair-follicle matrix	Reversible; classic with anthracyclines, taxanes, cyclophosphamide
Gonadotoxicity / infertility	Germ-cell division	Alkylating agents worst (cyclophosphamide, procarbazine); azoospermia, premature ovarian failure
Nausea & vomiting	CTZ / 5-HT3, NK1 pathways	Cisplatin most emetogenic
Teratogenicity / mutagenicity	DNA damage in dividing fetal cells	Avoid in 1st trimester; methotrexate, aminopterin classic teratogens
Secondary malignancy	Mutagenic DNA damage	Alkylators → AML/MDS (often 5–7 yr, del 5/7); topo-II inhibitors → AML with t(11;23) MLL, earlier (~2–3 yr)
Hyperuricaemia / tumour lysis	Rapid cell breakdown	High-grade lymphoma, leukaemia

High-yield: Myelosuppression is the dose-limiting toxicity of most anticancer drugs. Notable marrow-SPARING exceptions: vincristine (DLT = peripheral neuropathy), bleomycin (DLT = pulmonary fibrosis), asparaginase, and steroids.

High-yield: Neutrophil nadir typically falls at day 7–14 after a cycle; cycles are timed (often q21d) to allow marrow recovery.

Selected organ-specific (off-target) toxicities — must-know pairings

• Doxorubicin/daunorubicin → dilated cardiomyopathy (cumulative, dose-related; prevented by dexrazoxane, an iron chelator).
• Bleomycin → pulmonary fibrosis (also busulfan).
• Cisplatin → nephrotoxicity (prevent with hydration + amifostine), ototoxicity, peripheral neuropathy.
• Cyclophosphamide/ifosfamide → haemorrhagic cystitis from acrolein (prevent with mesna + hydration).
• Vincristine → peripheral neuropathy; vinblastine → myelosuppression (the discriminator pair).
• Methotrexate → mucositis, hepatotoxicity, nephrotoxicity, pneumonitis (rescue with leucovorin/folinic acid).
• 5-FU → mucositis, diarrhoea, hand-foot syndrome, coronary vasospasm; toxicity ↑ in DPD deficiency.

Supportive care — rescuing the host

Because toxicity is the limiting factor, supportive agents let us push dose-intensity:

• G-CSF (filgrastim) / GM-CSF (sargramostim): accelerate neutrophil recovery, shorten the neutropenic period, reduce febrile-neutropenia risk, and enable dose-dense scheduling. Used as primary prophylaxis when febrile-neutropenia risk ≥20%, or therapeutically. Pegfilgrastim = long-acting, single per-cycle dose.
• Erythropoietin for chemo-induced anaemia (use cautiously — thrombosis, possible tumour progression).
• Leucovorin (folinic acid) — rescues normal cells after high-dose methotrexate by bypassing the DHFR block; note it potentiates 5-FU.
• Mesna — neutralises acrolein → prevents cyclophosphamide/ifosfamide haemorrhagic cystitis.
• Dexrazoxane — cardioprotection with anthracyclines.
• Amifostine — cytoprotective (cisplatin nephrotoxicity, radiation).
• Antiemetics — 5-HT3 antagonists (ondansetron) + NK1 antagonist (aprepitant) + dexamethasone for highly emetogenic regimens (cisplatin).
• Allopurinol/rasburicase + hydration — tumour lysis prophylaxis.

High-yield: Match the rescue to the drug — leucovorin↔methotrexate, mesna↔cyclophosphamide/ifosfamide, dexrazoxane↔doxorubicin, amifostine↔cisplatin, G-CSF↔neutropenia.

High-yield: G-CSF support is what makes dose-dense regimens (cycles every 2 weeks) feasible — a direct application of the Norton–Simon/log-kill logic.

Tumour lysis syndrome (a frequent corollary)

Massive cell death in highly chemosensitive tumours (Burkitt lymphoma, ALL, high-grade NHL) releases intracellular contents.

Biochemical tetrad: ↑K⁺, ↑PO₄³⁻, ↑uric acid, ↓Ca²⁺ (secondary to hyperphosphataemia) → risk of acute kidney injury and arrhythmia. Prevention/management: aggressive IV hydration, rasburicase (recombinant urate oxidase — first line in high risk) or allopurinol (lower risk), cardiac monitoring; avoid urinary alkalinisation if using rasburicase.

Approach to a "chemotherapy principle" MCQ

1. Identify the class/drug from the stem (toxicity clue or mechanism).
2. Decide CCS vs CCNS → predicts dose- vs schedule-dependence.
3. Map the toxicity to a high-turnover tissue (default = marrow) or recall the off-target organ pairing.
4. Choose the rescue/supportive agent if asked.
5. For "why combine / why resistance" stems → invoke non-overlapping toxicity, log-kill, Goldie–Coldman, P-glycoprotein.

Key differentials / discriminators

• CCS vs CCNS: antimetabolite (S-phase, schedule-dependent) vs alkylator (any phase, dose-dependent).
• Vincristine vs vinblastine: neuropathy vs myelosuppression ("crisp = neuro for vinCRIStine? — vinCRIStine = Constipation/CNS neuropathy; vinBLASTine = Bone marrow/Blasts down").
• Alkylator-induced AML (del 5/7, latency 5–7 yr) vs topo-II-induced AML (t(11;23) MLL, latency ~2 yr).
• MDR via P-glycoprotein (natural-product drugs) vs antimetabolite resistance (uptake/activation/target changes).

Recently asked / exam angle

• P-glycoprotein (MDR1) as the efflux pump of multidrug resistance, and which drug classes it affects (natural products: anthracyclines, vinca, taxanes, etoposide) — repeatedly tested.
• Log-kill hypothesis definition (constant fraction, not number) and its implication for cycles/adjuvant therapy.
• Cell-cycle specificity matching — e.g. "which is an S-phase specific agent?" (antimetabolites) or "which is cell-cycle non-specific?" (alkylating agents).
• Dose-limiting toxicity of specific drugs and the marrow-sparing exceptions (vincristine, bleomycin).
• Rescue/cytoprotective pairings — leucovorin–methotrexate, mesna–cyclophosphamide, dexrazoxane–doxorubicin.
• G-CSF indication and its role in dose-dense therapy.
• Secondary malignancy associations (alkylators → AML; topo-II → MLL-rearranged AML).
• Tumour lysis syndrome electrolyte pattern and rasburicase.
• Combination chemotherapy principle — "non-overlapping toxicity" as the answer to "principle behind combining agents."
• Gompertzian growth / growth fraction as the determinant of chemosensitivity (adjuvant rationale).

Rapid revision

1. Growth fraction (cycling fraction), not size, predicts chemosensitivity; small/micrometastatic tumours respond best — basis of adjuvant therapy.
2. CCS drugs (antimetabolites = S-phase; vinca/taxanes = M-phase) are schedule-dependent; CCNS drugs (alkylators, platinum, anthracyclines) are dose-dependent.
3. Log-kill = each cycle kills a constant fraction (first-order) → repeated cycles needed; cure requires very low residual burden.
4. Goldie–Coldman: resistant clones arise by mutation; treat early with non-cross-resistant combinations.
5. P-glycoprotein (MDR1) = ATP-dependent efflux pump → multidrug resistance to natural-product drugs (anthracyclines, vinca, taxanes, etoposide, dactinomycin).
6. Combination rules: each drug individually active, different mechanisms, non-overlapping toxicities, different resistance patterns.
7. Myelosuppression is the dose-limiting toxicity of most cytotoxics; nadir at day 7–14.
8. Marrow-sparing drugs: vincristine (neuropathy), bleomycin (pulmonary fibrosis), asparaginase, steroids.
9. Rescue pairings: leucovorin↔methotrexate, mesna↔cyclophosphamide/ifosfamide, dexrazoxane↔doxorubicin, amifostine↔cisplatin.
10. G-CSF shortens neutropenia and enables dose-dense scheduling (Norton–Simon logic).
11. Secondary AML: alkylators → del 5/7 (~5–7 yr); topo-II inhibitors → t(11;23) MLL (~2 yr).
12. Tumour lysis: ↑K⁺, ↑PO₄³⁻, ↑uric acid, ↓Ca²⁺ → AKI; prevent with hydration + rasburicase.