Oncology · Module 1 of 6

Principles of Cancer Pharmacology

Cell cycle kinetics, drug resistance, combination strategies, and supportive care

The log-kill hypothesis defines the fundamental arithmetic of chemotherapy: each drug course kills a constant fraction, not a fixed number, of tumor cells. Cure requires reducing the viable cell burden below one — a goal that demands repeat cycles and explains why very large initial tumor burdens are harder to eradicate even with active regimens.
Cell cycle specificity governs scheduling. Cycle-specific agents (antimetabolites in S phase, vinca alkaloids and taxanes in M phase) plateau at high doses and benefit from prolonged infusion to catch cells as they enter the sensitive phase; cycle-nonspecific alkylating agents and platinum compounds kill G0 cells and show a more linear dose-response, making dose escalation meaningful up to toxicity limits.
Drug resistance is multimechanistic: P-glycoprotein efflux (MDR1) actively pumps anthracyclines, taxanes, and vinca alkaloids out of cells; loss of activating enzymes (deoxycytidine kinase, hypoxanthine-guanine phosphoribosyltransferase) blocks prodrug activation; dihydrofolate reductase amplification overwhelms methotrexate; and p53 loss or BCL-2 overexpression prevents apoptotic cell death even after DNA damage is sustained.
Rational combination chemotherapy rests on two cardinal principles — non-overlapping dose-limiting toxicity and non-cross-resistant mechanisms — illustrated by CHOP (lymphoma) and BEP (testicular cancer), which achieve cure rates unavailable with any single constituent agent.
Supportive care pharmacology determines whether full-dose therapy is deliverable: G-CSF (filgrastim, pegfilgrastim) must not start within 24 hours of chemotherapy; antiemetics are matched to emetogenic tier (5-HT3 antagonist + NK-1 antagonist + dexamethasone for highly emetogenic regimens); and rasburicase is absolutely contraindicated in G6PD deficiency due to the hemolytic risk from hydrogen peroxide generated during uric acid oxidation.

ATP = adenosine triphosphate · BCL-2 = B-cell lymphoma 2 protein · BEP = bleomycin + etoposide + cisplatin · CHOP = cyclophosphamide + doxorubicin + vincristine + prednisone · DNA = deoxyribonucleic acid · G-CSF = granulocyte colony-stimulating factor · G6PD = glucose-6-phosphate dehydrogenase · HGPRT = hypoxanthine-guanine phosphoribosyltransferase · MDR1 = multidrug resistance 1 gene · NK-1 = neurokinin-1 · p53 = tumor protein 53 · 5-HT3 = serotonin type 3

Cell Cycle & Drug Specificity

Cycle-Specific Agents

S-Phase and M-Phase Drugs

Antimetabolites are S-phase specific: methotrexate, 5-fluorouracil, cytarabine
Vinca alkaloids are M-phase specific: vincristine, vinblastine
Taxanes are M-phase specific: paclitaxel, docetaxel
Dose-response plateaus once all cycling cells in the sensitive phase are killed — prolonged infusion maximizes exposure across the cell cycle
G0 quiescent cells escape cycle-specific agents — quiescence confers intrinsic resistance

Cycle-Nonspecific Agents

Alkylating Agents & Platinum Compounds

Kill cells regardless of cell cycle phase, including G0 quiescent cells
Dose-response relationship is more nearly linear — dose escalation is meaningful up to organ toxicity limits
Nitrogen mustards, platinum compounds (cisplatin, carboplatin), nitrosoureas, busulfan
Linear dose-response is the biological rationale for high-dose myeloablative conditioning regimens

Log-Kill Hypothesis — Fraction Kill, Not Fixed Number

Initial Burden

10¹⁰ cells (10 billion)

→

3-Log Kill (99.9%)

10⁷ cells remain

→

Repeat Cycle

10⁴ cells remain

→

Cure Requires

< 1 viable cell — multiple cycles are mathematically necessary

Drug Resistance Mechanisms

Efflux Pump

P-Glycoprotein (MDR1)

ATP-dependent efflux pump encoded by MDR1 gene
Pumps anthracyclines, taxanes, vinca alkaloids, and epipodophyllotoxins out of the cell
Overexpressed by gene amplification or selection of pre-existing high-expressing clones
Additional ABC transporters: MRP1 (anthracyclines, vinca, methotrexate) and BCRP (mitoxantrone, camptothecins)

Target & Metabolic

Enzyme Loss & Target Amplification

Loss of deoxycytidine kinase → cytarabine resistance (prodrug cannot be phosphorylated)
Loss of HGPRT → 6-mercaptopurine resistance
Dihydrofolate reductase amplification → methotrexate resistance (target overwhelms drug)
Topoisomerase II mutation → reduced etoposide and doxorubicin binding

Apoptosis Failure

BCL-2 Overexpression & p53 Loss

p53 loss (present in ~50% of cancers): no apoptotic signal after DNA damage → cells arrest but survive and repair
BCL-2 overexpression: sequesters pro-apoptotic proteins at mitochondrial outer membrane → prevents cytochrome c release
Cells sustain DNA damage but do not die — they repair and resume proliferation

Combination Chemotherapy Principles

Principle	Rationale	Example
Non-overlapping dose-limiting toxicity	Each drug can be used at or near its full single-agent dose; combined toxicity is additive only on different target organs	CHOP: cyclophosphamide and doxorubicin limit on marrow/cardiac; vincristine limits on peripheral nerve (minimal marrow); prednisone limits on metabolic (no marrow)
Non-cross-resistant mechanisms	The probability of simultaneous resistance to two independent drugs (each 1 in 10⁶) is approximately 1 in 10¹² — below tumor cell number even in microscopic disease	BEP: cisplatin DNA adducts + etoposide topoisomerase II inhibition + bleomycin free-radical strand breaks; cure rate >80% in metastatic testicular cancer
Dose density	Compresses cycle interval from 3 to 2 weeks — same per-cycle dose, higher dose intensity; prevents tumor regrowth between cycles during the exponential phase of residual disease	Dose-dense doxorubicin/cyclophosphamide/paclitaxel every 2 weeks with G-CSF support improved disease-free and overall survival vs standard 3-week schedule in node-positive breast cancer

Supportive Care — High-Yield Points

G-CSF Timing

Filgrastim & Pegfilgrastim Rules

G-CSF stimulates neutrophil precursor proliferation — shortens chemotherapy-induced neutropenia by 3–5 days and reduces febrile neutropenia incidence by ~50% in high-risk regimens
Filgrastim: non-glycosylated recombinant G-CSF; daily subcutaneous injection starting 24–72 h after last chemotherapy dose
Pegfilgrastim: PEG-conjugated filgrastim; single injection per cycle due to reduced renal clearance and prolonged half-life
Must not start within 24 h of chemotherapy — mobilizes proliferating precursors vulnerable to cycle-specific agents still present in plasma

Antiemetics

Emetogenicity-Based Selection

Acute nausea (<24 h): serotonin-mediated via 5-HT3 receptors on vagal afferents from enterochromaffin cells
Delayed nausea (24–120 h): substance P / NK-1 receptor-mediated in brainstem
Highly emetogenic (cisplatin, high-dose cyclophosphamide, dacarbazine): 5-HT3 antagonist + NK-1 antagonist + dexamethasone
Moderately emetogenic (carboplatin, doxorubicin, irinotecan): 5-HT3 antagonist + dexamethasone

Clinical Safety Rule

Tumor Lysis Syndrome — Prevention & Rasburicase Contraindication

Tumor lysis syndrome results from massive simultaneous cell death releasing intracellular contents: hyperuricemia (nucleic acid catabolism), hyperkalemia, hyperphosphatemia, and secondary hypocalcemia. Highest-risk tumors: Burkitt lymphoma, acute lymphoblastic leukemia with high white blood cell count, and acute myeloid leukemia with blast counts above 100,000 cells per microliter.

Standard prophylaxis: aggressive IV hydration plus allopurinol (xanthine oxidase inhibitor — blocks new uric acid synthesis; must start 24–48 h before chemotherapy). For high-risk patients, rasburicase (recombinant urate oxidase) is preferred because it rapidly degrades existing uric acid to allantoin, a soluble product — allopurinol only prevents new production and cannot lower pre-existing uric acid burden.

Rasburicase is absolutely contraindicated in G6PD deficiency. The hydrogen peroxide generated by the rasburicase reaction causes severe acute intravascular hemolysis in G6PD-deficient erythrocytes. Screen any patient with African, Mediterranean, or Southeast Asian ancestry before rasburicase administration; use allopurinol plus hydration as the default when G6PD status is unknown.

References

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