Oncology
Chemotherapy Mechanism
DNA damage and the mitotic crossfire
Chemotherapy uses cytotoxic drugs to kill cancer cells by targeting their most defining characteristic: rapid, uncontrolled cell division. These drugs disrupt the cell cycle through DNA damage or mitotic interference, hitting cancer cells harder than most normal tissues because they are "caught" in the act of dividing more often. However, this selectivity is imperfect, leading to the well-known side effects as normal rapidly-dividing cells in the bone marrow and gut are also caught in the crossfire.
- TargetRapidly dividing cells
- MechanismDNA damage / Spindle inhibition
- Neutrophil Nadir7–14 days post-treatment
- ExamplesCisplatin, Paclitaxel, Methotrexate
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How it works
Chemotherapy drugs are cell-cycle specific or non-specific. Some, like Methotrexate, mimic DNA building blocks to stop replication (S-phase). Others, like Paclitaxel, freeze the 'mitotic spindle' so the cell cannot pull itself apart into two (M-phase). Because cancer cells spend much more of their time in these active phases compared to normal cells, they accumulate lethal damage much faster. This is the 'Therapeutic Index'—the margin between killing the tumor and killing the host.
The Crossfire Problem
The body has several types of normal cells that also divide rapidly: those in the hair follicles, the lining of the digestive tract, and the bone marrow (where blood cells are made). When chemo hits these cells, it causes hair loss, nausea, and 'neutropenia'—a dangerous drop in white blood cells. This is why chemo is given in 'cycles,' allowing the body's normal fast-dividing cells time to recover before the next hit.
Common pitfalls
- Drug Resistance: Some cancer cells 'evolve' pumps to spit the drug back out before it can damage the DNA.
- Inadequate Nadir Monitoring: If a patient's white cell count drops too low, even a minor infection can become fatal sepsis.
- Tumor Lysis Syndrome: Killing too many cancer cells at once can flood the blood with toxins, damaging the kidneys.
| Feature | Traditional Chemotherapy | Targeted Therapy |
|---|---|---|
| Mechanism | Kills any cell that divides fast | Blocks specific mutations/proteins |
| Selectivity | Low (hits gut/hair/marrow) | High (hits specific cancer markers) |
| Side Effects | Systemic (Nausea, hair loss) | Specific (Rashes, liver issues) |
| Analogy | Sledgehammer | Precision scalpel |
Frequently asked questions
Why does chemotherapy cause hair loss?
Because hair follicle cells are among the fastest-dividing cells in the body, and chemo targets any cell that is actively dividing, regardless of whether it is cancer.
What is the 'Nadir'?
It is the point, usually 7–14 days after a chemo dose, when your blood counts (white cells, platelets) are at their lowest and you are most at risk of infection.
How is chemo delivered?
Most often through an IV (intravenous) line so it can travel through the whole body, but it can also be given as pills or injections.
Why is chemo given in cycles?
To give your healthy, fast-dividing cells (like your immune system) time to recover and regenerate between doses while keeping the pressure on the tumor.
Does chemo cure all cancers?
Some cancers (like certain leukemias) are highly curable with chemo alone. For others, it is used to shrink a tumor before surgery or to manage the disease and extend life.