Alkylating Agents

Alkylating agents were one of the earliest classes of drugs used to treat cancer, beginning in the 1940’s. 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. This reaction is essentially a mutation that takes away the cancer cell’s ability to multiply.

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. The five major categories of alkylating agents are nitrogen mustards, nitrosoureas, alkyl sulfonates, triazines, and ethylenimines.  Here is a PDF list of alklating agents.

Nitrogen Mustards

Mustard is 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.

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.

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. It comes as a 2 milligram pill to be taken daily on an empty stomach. More rarely, it is administered by injection.

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.

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.

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.

Mechanism

Alkylating medicines are cell-cycle nonspecific agents.  They attack the DNA at any point in the cell cycle. 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. In either case, when applied to cancer cells, alkylating agents can limit tumor growth and cause tumor destruction.

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.

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.

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.

Mechlorethamine

Brand/Trade Names: Mustargen, Nitrogen Mustard, Mustine, Chlormethine

Formula: C5H11Cl2N

Mechanism: alkylating agent

Class: Mustard

Administration: Intravenous, Intracavity and Topical

Notes: Hodgkin disease, lymphosarcoma, certain leukemias, mycosis fungoides, and other cancers

Cyclophosphamide

Brand/Trade Names: Cytoxan, Neosar

Formula: C7H15Cl2N2O2P

Mechanism: alkylating agent

Class: Nitrogen mustard

Administration: Oral, Intraveneous

Notes: Supresses the immune system, used for Lymphoma, leukemia, and many carcinomas

Chlorambucil

Brand/Trade Names: Leukeran

Formula: C14H19Cl2NO2

Mechanism: alkylating agent

Class: nitrogen mustard

Administration: Oral

Notes:

Melphalan

Brand/Trade Names: Alkeran

Formula: C13H18Cl2N2O2

Mechanism: alkylating agent

Class: Mustard

Administration: Intravenous, Oral

Notes: Multiple myeloma, as well as ovarian, breast, and prostate cancer

Ifosfamide

Brand/Trade Names: Ifex

Formula: C7H15Cl2N2O2P

Mechanism: alkylating agent

Class: Nitrogen Mustard

Administration: Oral, Intravenous

Notes: Lymphoma, leukemia, testicular, and bladder cancers.

Busulfan

Brand/Trade Names: Myleran, Busulfex

Formula: C6H14O6S2

Mechanism: alkylating agent

Class: alkyl sulfonate

Administration: Oral

Notes: CML, other blood cancers

Thiotepa

Brand/Trade Names: Thioplex

Formula: C6H12N3PS

Mechanism: alkylating agent

Class: ethylenimine

Administration: Intravenous, Intrabladder, Intraperitoneal

Notes: Lymphoma and cancers of the breast, ovary, and bladder

Altretamine

Brand/Trade Names: Hexalen

Formula: C9H18N6

Mechanism: alkylating agent

Class: ethylenimine

Administration: Oral

Notes: Lymphoma, leukemia, testicular, and bladder cancers.

Carmustine

Brand/Trade Names: BCNU, Gliadel, Carmubris

Formula: C5H9Cl2N3O2

Mechanism: alkylating agent

Class: nitrosourea

Administration: Oral

Notes: Lymphoma, multiple myeloma, gliomas, and other types of cancer.

Lomustine

Brand/Trade Names: Gleostine, CeeNU, CCNU

Formula: C9H16ClN3O2

Mechanism: alkylating agent

Class: nitrosourea

Administration: Intravenous, Intrabladder, Intraperitoneal

Notes: Used for lymphoma and cancers of the breast, ovary, and bladder

Streptozocin

Brand/Trade Names: Zanosar

Formula: C8H15N3O7

Mechanism: alkylating agent

Class: nitrosourea

Administration: Intravenous

Notes:

Category: Triazenes (Nonclassical)

Dacarbazine

Brand/Trade Names: Imidazole Carboxamide, DTIC-Dome

Formula: C6H10N6O

Mechanism: alkylating agent

Class: trazine

Administration: Intravenous

Notes:

Temozolomide

Brand/Trade Names: Temodar

Formula: C6H6N6O2

Mechanism: alkylating agent

Class: trazine

Administration: Intravenous, Oral

Notes: Cancers of brain including anaplastic astrocytoma, glioblastoma multiforme (GBM), and gliomas

Trabectedin

Brand/Trade Names: Yondelis

Formula: C39H43N3O11S

Mechanism:

Class: alkylating agent

Administration: Intravenous

Notes: First discovered in extract from sea squid.  Interesting article on that.  Also called ET-743 or ecteinascidin 743.

Other Alkylating Agents

Other agents under investigation (but not approved by the FDA) include Treosulfan, Mannosulfan, Trofosfamide, Triaziquone, Carboquone, Nimustine, and Ranimustine.