Nobody miscalculated. The error was conceptual: treating the dose as a property of the drug rather than of the drug in this patient.
Five days after starting capecitabine at the protocol dose, she is back in the emergency department. Violent diarrhea. Mouth ulcers. A neutrophil count near zero. She looks like someone who received many times the intended dose. She received exactly the dose the protocol specified. The error was not arithmetic. It was conceptual: treating the dose as a fixed property of the drug rather than of the drug in this patient.
What the tumor and normal tissues actually see depends on absorption, distribution, metabolism, and excretion — and each process varies between patients, sometimes by a factor of five or ten.
A prescribed milligram is intent. What the tumor and normal tissues actually see depends on how that milligram is absorbed, where it distributes, how quickly it is metabolized, and how efficiently it is excreted. Every one of those processes varies between patients, sometimes by a factor of five or ten. This chapter is about that variation, and about the resistance and toxicity that follow from it.
Pharmacokinetics gives us the acronym ADME. Absorption: most chemotherapy is intravenous, so variability is minimal — but oral agents like capecitabine must cross the gut wall, survive the liver's first pass, and do so consistently across patients with different food intake. Distribution: highly protein-bound drugs leave more free drug in a malnourished patient with low albumin. The blood-brain barrier is a sanctuary site, blocking drugs even when plasma concentrations are therapeutic. Metabolism: prodrugs like cyclophosphamide and irinotecan require hepatic activation — impaired activation changes the effective dose. Excretion: cisplatin clears through the kidney, doxorubicin through bile. Knowing which organ clears a drug is the minimum requirement for safe prescribing when organs aren't working normally.
Resistance can be intrinsic — present before the first dose — or acquired under treatment. The mechanisms are multiple and almost always coexist.
A cancer that responds to chemotherapy is not cured unless every cell is killed. The cells that survive treatment are, almost by definition, the least sensitive. Under selection pressure, those cells expand, and the resulting population is more resistant than the original. This is clonal evolution applied to treatment — the dominant reason cancers that initially respond eventually progress. Resistance can be intrinsic, present before the first dose, or acquired under treatment. The mechanisms are multiple and almost always coexist.
Six mechanisms recur. Reduced uptake: cells that downregulate a transporter never let the drug in — methotrexate depends on the reduced folate carrier. Increased efflux: P-glycoprotein pumps out taxanes, anthracyclines, and vinca alkaloids simultaneously — one mechanism, cross-resistance to unrelated drug classes. Drug inactivation: glutathione neutralizes alkylators; cytidine deaminase destroys analogs. Target alteration: thymidylate synthase upregulation blocks fluoropyrimidines; topoisomerase mutations block anthracyclines. Enhanced DNA repair: MGMT removes temozolomide adducts; ERCC1 fixes cisplatin crosslinks; BRCA reversion mutations restore homologous recombination after PARP inhibitor pressure. Apoptosis evasion: TP53 mutation and BCL-2 overexpression let cells survive DNA damage that should have triggered death.
For any emetogenic or myelosuppressive regimen, the supportive-care plan is written before the first dose — not assembled reactively as problems arise. For a highly emetogenic platinum regimen: five-H-T-3 antagonist, N-K-1 antagonist, dexamethasone, and olanzapine before and after each infusion, tapered over days for the delayed phase. Pre- and post-hydration for renal protection, with magnesium replacement because cisplatin causes renal wasting. G-CSF when febrile neutropenia risk exceeds twenty percent — not routinely, because the drug has its own side effects. A dated calendar: chemotherapy day one, antiemetic taper through day four, blood count check at days ten to twelve, emergency fever instructions for the nadir window, cycle two on day twenty-two after count recovery confirmed. Each element is a prediction based on known biology, acted on before the problem manifests.
Still open: which combinations of resistance mechanisms drive each specific tumor, and whether any of them create a new exploitable vulnerability.
The chapter's claim runs through every section. The dose is not a property of the drug — it is a property of the drug and the patient's biology together. Their enzyme activity: DPYD status before fluoropyrimidines, renal function before cisplatin, liver function before doxorubicin. Their tumor's resistance mechanisms: not a single pump to block, but a system of parallel defenses that requires asking which combination is driving resistance in this specific tumor and whether any creates a new vulnerability. And the supportive care plan that makes delivery of the dose survivable. The protocol number is the starting point. The plan is everything that happens after.
Cancer Research · Chapter 11 · Chemotherapy Pharmacology — Resistance and Toxicity Management
That's the frame. ADME governs what the patient's body does to the drug — and every step varies. Pharmacogenomics tells you when an inherited variant changes safe to lethal at a standard dose. Resistance is clonal evolution under selection pressure, running through parallel mechanisms that rarely yield to a single-target block. And supportive care is not an afterthought — it is the plan that makes the dose deliverable. Know the drug's pharmacology. Know this patient's biology. The two together are the prescription.