Alkylating Agents
The word “alkylating” derives from the drug’s mechanism of action. Alkylating agents are capable of forming from strong electrophiles usually via a carbonium ion or carbon radical. These electrophilic compounds naturally seek out nucleophilic atoms and functional groups to form their covalent bonds. Nucleophilic functional groups include phosphate, amino, sulfhydryl, carboxyl, and hydroxyl groups, among mothers. One atom that is particularly susceptible to alkylation is the nitrogen number 7 in the guanine nucleotide of DNA.
The action of alkylating agents as chemotherapeutic drugs comes from their ability to irreversibly bind to DNA. Once bound, the altered molecule disrupts the normal action and replication of the DNA strand. Alkylating agents throw a monkey-wrench into the cell’s DNA machinery. When this alkylation takes place at several places along the DNA molecule, the normal processes of the cell are interrupted. This leads to either programmed cell death (apoptosis) or at least an arrest in cellular replication. In either case, when applied to cancer cells, alkylating agents can limit tumor growth and lead to tumor destruction. This reaction is essentially a mutation that takes away the cancer cell’s ability to multiply.
In contrast to other cancer chemotherapeutic drugs, alkylating agents are distinguished by their ability to bind with DNA and disrupt cellular functions regardless of the stage in the cell cycle. Unfortunately this mechanism of action may lead to cytotoxic effects in non-cancerous cells. Alkylating compounds are sometimes referred to as mono- or bifunctional depending on the number on covalent bonds they can form within the DNA. Bifunctional agents lead to cross-linking of DNA which leads to more efficient cell death. Monofunctional agents cause single nucleotide disruption are more prone to mutagenesis and carcinogenesis—presumably because the drug forms a stable change in the DNA and is passed to subsequent generations of cells.
Cancer cells are sensitive to DNA damage. Alkylating agents work by reacting with the proteins that bond together to form the very delicate double helix structure of a DNA molecule, adding an alkyl group to some or all of them. This prevents the proteins from linking up as they should, causing breakage of the DNA strands and, eventually, the death of the cell.
While there are many different alkylating agents, they all work by this same chemical mechanism. Alkylating chemotherapy drugs have this effect on a cancer cell during every phase of its life cycle, although their biggest impact is in the S-phase. Oncologists use alkylating agents for a wide range of cancers. The biggest impact is on cancers that grow slowly, like solid tumors and leukemia, but they are also used to treat lung cancer, ovarian cancer, breast cancer, lymphomas, sarcomas, myelomas, and Hodgkin’s disease.
Alkylating agents were one of the earliest classes of drugs used to treat cancer, beginning in the 1940’s. Today there are 21 FDA-approved alkylating medicines for cancer treatment. Here is a pdf list of alkylating agents.
Classes of alkylating antineoplastic agents
Triazenes (Nonclassical)
Nitrogen Mustards
Mustards are a class of chemicals used as weapons in World War I. They are organic compounds of chlorine and sulfur and smell like mustard. Nitrogen mustards are organic compounds containing both nitrogen and chlorine atoms. Through serendipity, scientists found people exposed to them had lower rates of cancer.
The earliest experiments on human patients date to 1931, and by the 1960s nitrogen mustards were being used to treat leukemia on a fairly regular basis.
Cyclophosphamide is marketed as Cytoxan® or Neosar®, and is used to treat Hodgkin’s and non-Hodgkin’s lymphoma, Burkitt’s lymphoma, chronic lymphocytic leukemia, chronic myelocytic leukemia, acute myelocytic leukemia, acute lymphocytic leukemia, t-cell lymphoma, multiple myeloma, neuroblastoma, retinoblastoma, rhabdomyosarcoma, Ewing’s sarcoma; breast, testicular, endometrial, ovarian, and lung cancers. Because of the wide variety of cancers it treats, there is also a wide range of administering options. The most common methods are by intravenous injection or mouth in the form of tablets. Tablets should be taken with food and not tampered with. Less commonly, this drug is also injected directly into muscle, abdominal lining, or lung lining. Cyclophosphamide is actually not particularly active as a chemical; it is a prodrug that is turned into an active medicine once in the body.
Mechlorethamine, marketed under the trade name Mustargen®, is given by injection to treat Hodgkin’s disease and non-Hodgkin’s lymphoma, and as a palliative therapy for breast and lung cancers, and as a topical treatment for skin lesions of mycosis fungoides (cutaneous T-cell lymphoma).
Ifosfamide, sold under the trade name Ifex®, is used to treat both Hodgkin’s and non-Hodgkin’s lymphoma, as well as recurrent testicular cancer and germ cell tumors, sarcomas, lung cancer, bladder cancer, head and neck cancer, and cervical cancer. It is administered intravenously. Ifosfamide is actually an isomer of cyclophosphamide.
Melphalan is sold under the brand name Alkeran®, and is also referred to as L-PAM or phenylalanine mustard. It is used to treat multiple myeloma, ovarian cancer, neuroblastoma, rhabdomyosarcoma, and breast cancer. In the cell it akylates the N7 position of guanine residues. It comes as a 2 milligram pill to be taken daily on an empty stomach. More rarely, it is administered by injection. Melphalan flufenamide is similar. It is a peptide-drug conjugate sold by the name Pepaxto. Specifically it is an ethyl ester of a dipeptide and considered a prodrug of melphalan.
Chlorambucil is sold by the trade name Leukeran®, and is most widely used to treat chronic lymphocytic leukemia, malignant lymphomas including lymphosarcoma, giant follicular lymphoma, and Hodgkin’s disease. It has also been successfully used to treat non-Hodgkin’s lymphoma, breast, ovarian and testicular cancer, Waldenstrom’s macroglobulinemia, thrombocythemia, and choriocarcinoma. It comes in coated tablet form.
Nitrosoureas
The word nitrosurea refers to the molecule’s nitroso (R-NO) group and carmamide (or urea) group CO(NH2)2. These drugs are lipophilic and are able to cross the blood-brain barrier. That’s why there are sometimes used to treat brain cancer.
Streptozocin is sold under the trade name Zanosar®, and is used to treat islet cell pancreatic cancer. It is given intravenously and can cause tissue damage if allowed to escape from the vein. Therefore, it is important for the person giving the treatment to have specialized training.
Carmustine is also known as BiCNU® or BCNU, and is used for some kinds of brain tumors, glioblastoma, brainstem glioma, medulloblastoma, astrocytoma, ependymoma, and metastatic brain tumors. It is also used in treatment of multiple myeloma, Hodgkin’s disease, non-Hodgkin’s lymphoma, melanoma, lung cancer, and colon cancer. Usually, this drug is given intravenously, but there is a solid form that can be placed inside the empty space left from the removal of certain brain tumors.
Lomustine, also known as CCNU or CeeNU®, is used to treat primary and metastatic brain tumors, Hodgkin’s disease and non-Hodgkin’s lymphoma, and has also been used for melanoma, lung, and colon cancer. It is sold in 10 mg, 40 mg, and 100 mg capsules. It is typically taken once every 6 weeks, and it is important to take this drug with plenty of liquid and an empty stomach.
Alkyl Sulfonates
Busulfan, sold under trade names Busulfex® and Myleran®, is used to treat chronic myelogenous leukemia. It can be given in pill form or intravenously.
Triazines
Triazines are aromatic compounds with rings of carbon and nitrogen.
Dacarbazine is sold under the trade name DTIC-Dome® and is used to treat metastatic malignant melanoma, Hodgkin’s disease, soft tissue sarcomas, neuroblastoma, fibrosarcomas, rhabdomyosarcoma, islet cell carcinoma, and medullary thyroid carcinoma. It is given intravenously, but can be irritating to the vein as well as any surrounding tissue it contacts. Therefore, it is important for the person giving the treatment to have specialized training.
Temozolomide is sold under the trade name Temodar®, and is used to treat the specific types of brain tumors anaplastic astrocytoma and glioblastoma multiforme. This medication comes in 5 mg, 20 mg, 100 mg, and 250 mg pills.
Ethylenimines
These are derived from ethylenimine, which is a three-pointed ring of two carbon atoms and a nitrogen atom. Ethylenimine is also called aziridine.
Thiotepa, sold under the trade name Thioplex®, is an alkylating agent used to treat breast cancer, ovarian cancer, Hodgkin’s disease, and non-Hodgkin’s lymphoma. It is given by intravenous infusion.
Altretamine is sold under the trade name Hexalen®, and is also called hexamethylmelamine or HMM. It is used to treat ovarian cancer. It is given in pill form and should be taken after meals.
Others
Procarbazine (trade name Matulane®) is classified as an alkylating agent although it seems to work by a different mechanism from the others. It is one of the older chemo medicines and used in a combination regimen for lymphoma treatment. Trabectedin (Yondelis®) was discovered as part of an effort to find cytotoxic compounds in nature that might be beneficial in cancer treatment. Lurbinectedin (Zepzelca®) is the newest addition to this class, being approved for use in patients with lung cancer.
Platinums. Some drugs are included in the alkylating agent class even though they do not technically add an alkyl group to the DNA. The widely used platinum drugs act as catalysts to induce DNA cross-linking even though they may not donate an alkyl group.
Mechanism
Alkylating medicines are cell-cycle nonspecific agents. They attack the DNA at any point in the cell cycle. There are two types: SN1 and SN2. SN1 molecules react with chemicals in the body to produce a radical molecule that attacks the DNA. SN2 molecules directly react with the DNA.
These drugs work at the molecular level by binding to negatively charged sites on the DNA (oxygen, nitrogen, phosphorus and sulfur atoms). This is called alkylation because the result of the chemical reaction is a DNA molecule with an attached chain of carbon atoms. This extra strand impairs replication and division of strands of genetic matter. The long DNA strands can break, and the cross-linking needed for DNA replication can be inhibited.
The action of alkylating agents as chemotherapeutic drugs comes from their ability to irreversibly bind to DNA and, once bound, the altered molecule disrupts the normal action and replication of the DNA strand. The N7 atom of guanine is especially vulnerable. The agents promote intra- and inter-DNA strand linking at guanine bases on the strand. When alkylation takes place at several places along the DNA molecule, the normal processes of the cell are interrupted. This leads to either programmed cell death (apoptosis) or at least an arrest in cellular replication. (Alkylating agents do not discriminate and bond to RNA and other molecules in addition to DNA, but the DNA damage is the main source of anti-cancer activity.)
Neoplasms can develop resistance to alkylating agents. This resistance has been linked, at least in part, to the expression of an enzyme known as MGMT (O6-MethylguanineDNAmethyltransferase). MGMT is able to repair DNA errors caused by alkylating agents. For example, temozolomide causes a potentially cytotoxic lesion in oxygen 6 of guanine nucleotides in DNA. MGMT enzymatically removes this methyl group, repairs the DNA, and negates the effect of the temozolomide. In normal cells this error repair mechanism would be advantageous; a cellular mechanism to prevent DNA disruption in cells that are normal physiologically. However, cancers are also able to express this protein (and perhaps even overexpress it) thus rendering certain alkylating agents ineffective. Drugs that inhibit MGMT activity may be used as an adjunct to alkylating agents in order to overcome this resistance and improve the tumor-killing effect.
Alkylating agents can also cause secondary cancers. The most common one is Acute Myeloid Leukemia that can show up years after therapy stops.
Occasionally severe organ damage occurs with the administration of alkylating agents. Pulmonary fibrosis and veno-occlusive disease of the liver have been observed across all types of drugs within the class. The use of nitrosoureas has been associated with renal failure.
The central nervous system can be affected by alkylating agents as well. In addition to severe nausea and vomiting common to the class, certain agents (e.g. Ifosfamide) are quite neurotoxic, leading to acute confusion and delirium, seizures, paralysis, and coma.
Alopecia (hair loss) is known to occur with alkylating agents. Sex organs are not spared—women that are treated with alkylating agents may experience permanent amenorrhea (lack of menstruation) and in men, sperm production may cease. They should never be used on pregnant women because they elevate the risk of birth defects.
Bendamustine
Brand/Trade Names: Ribomustin, Treanda, and 4-[5-[Bis(2-chloroethyl)amino]-1-methylbenzimidazol-2-yl]butanoic acid.
Manufacturers: Teva Pharmaceuticals, Biophore India Pharmaceuticals, Dr Reddy’s Laboratories, Beijing Huikand Boyuan Chemical Techm, Euticals.
Formula: C16H21Cl2N3O2
Mechanism: Alkylating
Class: Mustard
Administration: Intravenous
Notes: Approved by the FDA in 2008. More. Used for treatment of B-cell non-Hodgkin lymphoma and chronic lymphocytic leukemia.
Mechlorethamine
Brand/Trade Names: Mustargen, Nitrogen Mustard, Mustine, Chlormethine
Formula: C5H11Cl2N
Mechanism: alkylating agent
Class: Mustard
Administration: Intravenous, Intracavity and Topical
Notes: Approved by the FDA in 2013. Approved for treatment of bronchogenic carcinoma. chronic lymphocytic leukemia, chronic myelogenous leukemia, Hodgkin’s disease, malignant pleural effusion, malignant pericardial effusion, malignant peritoneal effusion, mycosis fungoides, Non-Hodgkin’s lymphoma
Cyclophosphamide
Brand/Trade Names: Cytoxan, Neosar, Endoxan
Manufacturers: Transo-Pharm USA, Fermion Oy, Orion Corporation. Johnson Matthey, Jiangsu Hengrui Medicine Co
Formula: C7H15Cl2N2O2P
Mechanism: alkylating agent
Class: Nitrogen mustard
Administration: Oral, Intravenous
Notes: Approved by the FDA in 1959. Suppresses the immune system. Used for treatment of retinoblastoma, leukemias, Hodgkin’s disease, multiple myeloma, mycosis fungoides, neuroblastoma, Non-Hodgkin lymphoma, breast cancer, chronic granulocytic leukemia, and chronic lymphocytic leukemia.
Chlorambucil
Brand/Trade Names: Leukeran
Manufacturers: Biophore India Pharmaceuticals, Sionc Pharmaceuticals, SGMR Pharmaceuticals, Farmhispania S.A.
Formula: C14H19Cl2NO2
Mechanism: alkylating agent
Class: nitrogen mustard
Administration: Oral
Notes: Approved by the FDA in 1957. Used for treatment of chronic lymphocytic leukemia, Hodgkin’s disease. Non-Hodgkin’s lymphoma, ovarian cancer, and testicular cancer.
Melphalan
Brand/Trade Names: Alkeran
Manufacturers: Apothecon Pharmaceuticals, Biophore India Pharmaceuticals Pvt Ltd, ChemGenix Laboratories Pvt Ltd, ChemPacific Corp, ChemWerth Inc
Formula: C13H18Cl2N2O2
Mechanism: alkylating agent
Class: Mustard
Administration: Intravenous, Oral
Notes: Approved by the FDA in 1964. Used in treatment of multiple myeloma, ovarian, breast, and prostate cancer.
Melphalan flufenamide
Brand/Trade Names: Pepaxto
Manufacturer: Oncopeptides AB
Formula: C24H30Cl2FN3O3
Mechanism: alkylating agent
Class: Mustard
Administration: Intravenous
Notes: Dipeptide prodrug of melphalan. Approved by the FDA in 2021. Used in treatment of multiple myeloma.
Ifosfamide
Brand/Trade Names: Mitoxana, Ifex
Manufacturers: Aarti Industries Ltd, Aspen Biopharma Labs Pvt Ltd, Baxter Healthcare Corporation, Biotechnica Pharma Global, ChemWerth Inc
Formula: C7H15Cl2N2O2P
Mechanism: alkylating agent
Class: Nitrogen Mustard
Administration: Oral, Intravenous
Notes: Approved by the FDA in 1988. Used for treatment of sarcoma, lymphoma, leukemia, testicular, bladder, and lung cancer.
Busulfan
Brand/Trade Names: Myleran, Busulfex
Manufacturers: Excella, Chemwerth, Farambios, Mylan API US, Euticals
Formula: C6H14O6S2
Mechanism: alkylating agent
Class: alkyl sulfonate
Administration: Oral
Notes: Approved by the FDA in 1999. Used in treatment of chronic myelogenous leukemia.
Thiotepa
Brand/Trade Names: Thioplex
Manufacturers: Emcure Pharmaceuticals Heraeus Holding Hikma Pharmaceuticals IDT Australia Limited Jiangsu Hengrui Medicine Co.
Formula: C6H12N3PS
Mechanism: alkylating agent
Class: ethylenimine
Administration: Intravenous, Intrabladder, Intraperitoneal
Notes: Approved by the FDA in 1959. Used to treat bladder cancer, breast cancer, lymphoma, and ovarian cancer.
Altretamine
Brand/Trade Names: Hexalen
Manufacturers: AAIPharma Inc., Dr. Reddy’s Laboratories, F. Hoffmann-La Roche, Aptuit Inc
Formula: C9H18N6
Mechanism: alkylating agent
Class: ethylenimine
Administration: Oral
Notes: Approved by the FDA in 1990. Used in treatment of ovarian cancer.
Carmustine
Brand/Trade Names: BCNU, Gliadel, Carmubris
Manufacturers: Johnson Matthey Pharmaceutical, Pharm Eco Laboratories, AMPAC Fine Chemicals LLC, Agno Pharm
Formula: C5H9Cl2N3O2
Mechanism: alkylating agent
Class: nitrosourea
Administration: Oral
Notes: Approved by the FDA in 1996. Used for treatment of brain tumors, Hodgkin’s Disease, multiple myeloma, and Non-Hodgkin’s lymphoma.
Lomustine
Brand/Trade Names: Gleostine, CeeNU, CCNU
Manutacturer: NextSource Pharmaceuticals
Formula: C9H16ClN3O2
Mechanism: alkylating agent
Class: nitrosourea
Administration: Intravenous, Intrabladder, Intraperitoneal
Notes: Approved by the FDA in 1976. Approved for treatment of brain tumors and lymphoma.
Streptozocin
Brand/Trade Names: Zanosar
Manufacturer: Pfanstiehl
Formula: C8H15N3O7
Mechanism: alkylating agent
Class: nitrosourea
Administration: Intravenous
Notes: Approved by the FDA in 1982. Used to treat pancreatic cancer.
Dacarbazine
Brand/Trade Names: Imidazole Carboxamide, DTIC-Dome.
(Sometimes spelled decarbazine).
Manufacturer: Pfizer, Heraeus Holding, Apicore, Fujimoto Chemicals Co., Ltd. Kyowa Hakko Bio Co., Ltd.
Formula: C6H10N6O
Mechanism: alkylating agent
Class: trazine
Administration: Intravenous
Notes: Approved by the FDA in 1975. Used to treat melanoma, Hodgkin’s Disease, sarcomas, neuroblastoma, and thyroid cancer.
Temozolomide
Brand/Trade Names: Temodar, Temodal
Manufacturer: American Pacific Corporation, Aspire Lifesciences Pvt Ltd, Beijing Lunarsun Pharmaceutical Co., Berr Chemical Company, Biophore India Pharmaceuticals Pvt Ltd
Formula: C6H6N6O2
Mechanism: alkylating agent
Class: trazine
Administration: Intravenous, Oral
Notes: Approved by the FDA in 1999. Used in treatment of brain cancer.
Trabectedin
Brand/Trade Names: Yondelis
Manufacturer: BrightGene Bio-Medical Technology Co, Emcure Pharmaceuticals, Janssen Pharmaceutical, Jiangsu Hengrui Medicine Co., Lonza Group
Formula: C39H43N3O11S
Mechanism: alkylating agent
Class: other
Administration: Intravenous
Notes: Approved by the FDA in 2015. First discovered in extract from sea squid. Interesting article on that. Also called ET-743 or ecteinascidin 743. Approved for treatment of liposarcoma and leiomyosarcoma.
Lurbinectedin
Brand/Trade Names: Zepzelca
Manufacturer: Jazz Pharmaceuticals
Formula: C41H44N4O10S
Mechanism: alkylating agent
Class: other
Administration: Intravenous
Notes: Approved by the FDA in 2020. Used for treatment of small cell lung cancer.
Procarbazine
Brand/Trade Names: Matulane
Manufacturer: Apicore LLC, ChemPacific Corp, F. Hoffmann-La Roche, Mylan Inc
Formula: C12H19N3O
Mechanism: alkylating agent
Class: other
Administration: Oral
Notes: Approved by the FDA in 1969. Used for treatment of lymphoma, brain tumors, melanoma, lung cancer, and multiple myeloma.
Other Alkylating Agents
Other agents under investigation (but not approved by the FDA) include Treosulfan, Mannosulfan, Trofosfamide, Triaziquone, Carboquone, Nimustine, and Ranimustine.
Sources:
European Journal of Medicinal Chemistry
Canada Medical Association Journal