Chemotherapy uses dozens of different chemicals, but they fall into a handful of major categories based on how they attack cancer cells. The main classes are alkylating agents, antimetabolites, plant alkaloids, topoisomerase inhibitors, and antitumor antibiotics. Each works through a distinct mechanism, but they all share the same basic goal: stopping cancer cells from copying their DNA or dividing into new cells.
Alkylating Agents
Alkylating agents are among the oldest and most widely used chemotherapy chemicals. They work by attaching chemical groups directly to a cell’s DNA, creating abnormal bonds between the two strands of the double helix. These bonds, called cross-links, prevent the DNA from separating when the cell tries to divide. The cell can’t copy its genetic material, so it dies.
This class includes several subgroups. Nitrogen mustards, the largest subgroup, include cyclophosphamide, melphalan, chlorambucil, and ifosfamide. Nitrosoureas like carmustine work slightly differently, targeting specific positions on DNA building blocks to create cross-links between strands. Other alkylating agents, such as procarbazine and dacarbazine, add small chemical groups to DNA that cause breaks in single strands.
Platinum-based drugs like cisplatin and carboplatin are often grouped with alkylating agents because they damage DNA in a similar way, forming bonds that block replication. Alkylating agents are generally most effective against slow-growing cancers and can act on cells at any stage of the growth cycle, including resting cells that aren’t actively dividing.
Antimetabolites
Antimetabolites are molecular imposters. They closely resemble the natural building blocks a cell needs to construct DNA and RNA. When a cancer cell absorbs the drug instead of the real molecule, its genetic code gets scrambled and it can no longer replicate.
There are three main types, each mimicking a different building block. Purine antagonists (like 6-mercaptopurine) block cells from making purines, one of the two chemical families that form the “letters” of the genetic code. Pyrimidine antagonists, including 5-fluorouracil (often called 5-FU), do the same for pyrimidines, the other family. Folic acid antagonists, most notably methotrexate, prevent cells from using folic acid, a vitamin essential for building both DNA and RNA. Unlike alkylating agents, antimetabolites are cell cycle specific. They primarily work during the S phase, when cells are actively synthesizing new DNA.
Plant-Derived Chemicals
Several powerful chemotherapy drugs come from compounds originally isolated from plants. These chemicals target microtubules, tiny protein structures that act like scaffolding inside a cell. When a cell divides, microtubules pull the two copies of chromosomes apart. Disrupting this scaffolding traps the cell mid-division and triggers its death.
Two major subgroups work in opposite ways. Vinca alkaloids, including vincristine and vinblastine, were isolated from the leaves of the Madagascar periwinkle plant. They bind to tubulin, the protein that microtubules are built from, and prevent the scaffolding from assembling at all. Without functional microtubules, the cell freezes in the middle of division.
Taxanes do the reverse. Paclitaxel, originally extracted from the bark of the Pacific yew tree, locks microtubules into an overly stable state. The scaffolding forms but can’t break down when it needs to, leaving the cell with a rigid, nonfunctional structure. Either way, the result is the same: the cell can’t complete division. These drugs act specifically during the M phase (mitosis), when cells are physically splitting in two.
Topoisomerase Inhibitors
To copy itself, DNA has to unwind from its tightly coiled state. Enzymes called topoisomerases handle this job by cutting one or both strands of DNA, letting it untwist, and then reattaching the cut ends. Topoisomerase inhibitors block the reattachment step, leaving the DNA permanently broken.
There are two types. Topoisomerase I inhibitors include irinotecan and topotecan, both derived from camptothecin, a compound first isolated from the bark of a Chinese tree (Camptotheca acuminata). These drugs bind to the enzyme-DNA complex and prevent the single-strand cut from being resealed.
Topoisomerase II inhibitors include etoposide and teniposide, semisynthetic chemicals derived from extracts of the American mayapple plant. They work on double-strand cuts instead, blocking the enzyme from reattaching both strands of DNA. The accumulation of unrepaired breaks stops the cell from replicating and eventually kills it.
Antitumor Antibiotics
Despite the name, these drugs don’t fight infections. They’re called antibiotics because they were originally extracted from Streptomyces bacteria found in soil. The most important subgroup is the anthracyclines, which include doxorubicin, daunorubicin, epirubicin, and idarubicin.
Anthracyclines work primarily by interfering with topoisomerase II, the same DNA-unwinding enzyme targeted by etoposide. They insert themselves between the base pairs of DNA and lock the enzyme in place, preventing the reattachment of cut DNA strands. This triggers a cascade that leads to cell death. Anthracyclines can act on cells at any stage of the cycle, including resting cells, making them cell cycle nonspecific agents. Doxorubicin is one of the most widely used chemotherapy drugs in the world, prescribed for cancers ranging from breast cancer to lymphoma.
Other Chemotherapy Chemicals
A few drugs don’t fit neatly into the categories above. Hydroxyurea works by blocking an enzyme cells need to produce the raw materials for DNA, specifically during the DNA-building phase of the cell cycle. Tretinoin, a derivative of vitamin A, takes an entirely different approach: rather than killing cancer cells directly, it forces immature leukemia cells to mature into normal cells that eventually die on their own schedule. Bleomycin, another soil-bacteria derivative, causes direct breaks in DNA strands.
Why These Chemicals Cause Side Effects
Every chemical listed above targets some aspect of cell division or DNA replication. Cancer cells divide rapidly, which makes them vulnerable, but they’re not the only fast-dividing cells in your body. Hair follicle cells, the cells lining your mouth and digestive tract, and bone marrow cells that produce blood all divide quickly too. That’s why hair loss, mouth sores, nausea, and low blood counts are the most common side effects across nearly all classes of chemotherapy.
The specific side effect profile varies by drug class. Anthracyclines carry a known risk of heart damage at higher cumulative doses. Platinum drugs can cause nerve damage and hearing changes. Vinca alkaloids are particularly associated with tingling or numbness in the hands and feet. The reason oncologists often combine drugs from different classes isn’t just to attack cancer from multiple angles. It also allows lower doses of each individual drug, which can reduce the severity of any single side effect while maintaining effectiveness against the tumor.
How These Chemicals Are Given
Chemotherapy drugs come in several forms depending on the specific chemical and the type of cancer being treated. Intravenous infusion is the most common delivery method, where the drug enters the bloodstream directly through a vein. Some drugs, like cyclophosphamide and methotrexate, are also available as oral pills or capsules. Others can be injected into muscle, delivered directly into the fluid surrounding the brain and spinal cord, or applied topically to the skin for certain skin cancers. The form a drug takes often determines how quickly it reaches the tumor and how it’s processed by the body.