Chemotherapy is the treatment of cancer with one or more cytotoxic antineoplastic drugs ("chemotherapeutic agents") as part of a standardized regimen. Chemotherapy may be given with a curative intent or it may aim to prolong life or to palliate symptoms. It is often used in conjunction with other cancer treatments, such as radiation therapy or surgery. Certain chemotherapeutic agents also have a role in the treatment of other conditions, including ankylosing spondylitis, multiple sclerosis, Crohn's disease, psoriasis, psoriatic arthritis, rheumatoid arthritis, and scleroderma.
Chemotherapeutic agents traditionally act by killing cells that divide rapidly, one of the main properties of most cancer cells. This means that chemotherapy also harms cells that divide rapidly under normal circumstances: cells in the bone marrow, digestive tract, and hair follicles. This results in the most common side-effects of chemotherapy: myelosuppression (decreased production of blood cells, hence also immunosuppression), mucositis (inflammation of the lining of the digestive tract), and alopecia (hair loss).
Some of the newer anticancer drugs (for example, various monoclonal antibodies) are not indiscriminately cytotoxic, but rather target proteins that are abnormally expressed in cancer cells and that are essential for their growth. Such treatments are often referred to as targeted therapy (as distinct from classic chemotherapy) and are often used alongside traditional chemotherapeutic agents in antineoplastic treatment regimens.
An older and broader usage of the word chemotherapy encompassed any chemical treatment of disease (for example, treatment of infections with antimicrobial agents). However, this usage has become archaic.
There are a number of strategies in the administration of chemotherapeutic drugs used today. Chemotherapy may be given with a curative intent or it may aim to prolong life or to palliate symptoms.
Combined modality chemotherapy is the use of drugs with other cancer treatments, such as radiation therapy or surgery. Most cancers are now treated in this way. Combination chemotherapy is a similar practice that involves treating a patient with a number of different drugs simultaneously. The drugs differ in their mechanism and side effects. The biggest advantage is minimising the chances of resistance developing to any one agent.
In neoadjuvant chemotherapy (preoperative treatment) initial chemotherapy is designed to shrink the primary tumor, thereby rendering local therapy (surgery or radiotherapy) less destructive or more effective.
Adjuvant chemotherapy (postoperative treatment) can be used when there is little evidence of cancer present, but there is risk of recurrence. This can help reduce chances of relapse. It is also useful in killing any cancerous cells that have spread to other parts of the body. This is often effective as the newly growing tumours are fast-dividing, and therefore very susceptible.
Palliative chemotherapy is given without curative intent, but simply to decrease tumor load and increase life expectancy. For these regimens, a better toxicity profile is generally expected.
All chemotherapy regimens require that the patient be capable of undergoing the treatment. Performance status is often used as a measure to determine whether a patient can receive chemotherapy, or whether dose reduction is required. Because only a fraction of the cells in a tumor die with each treatment (fractional kill), repeated doses must be administered to continue to reduce the size of the tumor. Current chemotherapy regimens apply drug treatment in cycles, with the frequency and duration of treatments limited by toxicity to the patient.
 
Types
The majority of chemotherapeutic drugs can be divided in to alkylating agents, antimetabolites, anthracyclines, plant alkaloids, topoisomerase inhibitors, and other antitumour agents.^[10] All of these drugs affect cell division or DNA synthesis and function in some way.
Some newer agents do not directly interfere with DNA. These include monoclonal antibodies and the new tyrosine kinase inhibitors e.g. imatinib mesylate (Gleevec or Glivec), which directly targets a molecular abnormality in certain types of cancer (chronic myelogenous leukemia, gastrointestinal stromal tumors). These are examples of targeted therapies.
In addition, some drugs that modulate tumor cell behaviour without directly attacking those cells may be used. Hormone treatments fall into this category.
Alkylating agents
Alkylating agents are so named because of their ability to alkylate many nucleophilic functional groups under conditions present in cells. Cisplatin and carboplatin, as well as oxaliplatin, are alkylating agents. They impair cell function by forming covalent bonds with the amino, carboxyl, sulfhydryl, and phosphate groups in biologically important molecules.^[10]
Other agents are mechlorethamine, cyclophosphamide, chlorambucil, ifosfamide.^[10] They work by chemically modifying a cell's DNA.
Anti-metabolites
Anti-metabolites masquerade as purines (azathioprine, mercaptopurine) or pyrimidines—which become the building-blocks of DNA. They prevent these substances from becoming incorporated in to DNA during the "S" phase (of the cell cycle), stopping normal development and division. They also affect RNA synthesis. Due to their efficiency, these drugs are the most widely used cytostatics.
Plant alkaloids and terpenoids
These alkaloids are derived from plants and block cell division by preventing microtubule function. Microtubules are vital for cell division, and, without them, cell division cannot occur. The main examples are vinca alkaloids and taxanes.
Vinca alkaloids
Vinca alkaloids bind to specific sites on tubulin, inhibiting the assembly of tubulin into microtubules (M phase of the cell cycle). They are derived from the Madagascar periwinkle, Catharanthus roseus (formerly known as Vinca rosea). The vinca alkaloids include:

• Vincristine
• Vinblastine
• Vinorelbine
• Vindesine

Podophyllotoxin
Podophyllotoxin is a plant-derived compound that is said to help with digestion as well as used to produce two other cytostatic drugs, etoposide and teniposide. They prevent the cell from entering the G1 phase (the start of DNA replication) and the replication of DNA (the S phase). The exact mechanism of its action is not yet known.
The substance has been primarily obtained from the American Mayapple (Podophyllum peltatum). Recently it has been discovered that a rare Himalayan Mayapple (Podophyllum hexandrum) contains it in a much greater quantity, but, as the plant is endangered, its supply is limited. Studies have been conducted to isolate the genes involved in the substance's production, so that it could be obtained recombinantly.
Taxanes
The prototype taxane is the natural product paclitaxel, originally known as Taxol and first derived from the bark of the Pacific Yew tree. Docetaxel is a semi-synthetic analogue of paclitaxel. Taxanes enhance stability of microtubules, preventing the separation of chromosomes during anaphase.
Topoisomerase inhibitors
Topoisomerases are essential enzymes that maintain the topology of DNA. Inhibition of type I or type II topoisomerases interferes with both transcription and replication of DNA by upsetting proper DNA supercoiling.

• Some type I topoisomerase inhibitors include camptothecins: irinotecan and topotecan.

• Examples of type II inhibitors include amsacrine, etoposide, etoposide phosphate, and teniposide. These are semisynthetic derivatives of epipodophyllotoxins, substances naturally occurring in the root of American Mayapple (Podophyllum peltatum).

Cytotoxic antibiotics
These include:

• actinomycin
• anthracyclines
  • doxorubicin
  • daunorubicin
  • valrubicin
  • idarubicin
  • epirubicin, which also inhibit topoisomerase II
• other cytotoxic antibiotics
• bleomycin. Bleomycin acts in unique way through oxidation of a DNA-bleomycin-Fe(II) complex and forming free radicals, which induce damage and chromosomal aberrations.
• plicamycin
• mitomycin

Newer and experimental approaches
Isolated infusion approaches
Isolated limb perfusion (often used in melanoma), or isolated infusion of chemotherapy into the liver or the lung have been used to treat some tumours. The main purpose of these approaches is to deliver a very high dose of chemotherapy to tumor sites without causing overwhelming systemic damage. These approaches can help control solitary or limited metastases, but they are by definition not systemic, and, therefore, do not treat distributed metastases or micrometastases.
Targeted delivery mechanisms
Specially targeted delivery vehicles aim to increase effective levels of chemotherapy for tumor cells while reducing effective levels for other cells. This should result in an increased tumor kill and/or reduced toxicity.
Specially targeted delivery vehicles have a differentially higher affinity for tumor cells by interacting with tumor-specific or tumor-associated antigens.
In addition to their targeting component, they also carry a payload - whether this is a traditional chemotherapeutic agent, or a radioisotope, or an immune-stimulating factor. Specially targeted delivery vehicles vary in their stability, selectivity, and choice of target, but, in essence, they all aim to increase the maximum effective dose that can be delivered to the tumor cells. Reduced systemic toxicity means that they can also be used in sicker patients, and that they can carry new chemotherapeutic agents that would have been far too toxic to deliver via traditional systemic approaches.
Nanoparticles
Nanoparticles have emerged as a useful vehicle for poorly soluble agents such as paclitaxel. 
Electrochemotherapy
Electrochemotherapy is the combined treatment in which injection of a chemotherapeutic drug is followed by application of high-voltage electric pulses locally to the tumor.