Alkylating Agent

Any highly reactive drug that binds to certain chemical groups (amino, phosphate, sulfhydryl, and hydroxyl groups) commonly found in nucleic acids, as well as other macromolecules, causes changes in the RNA and DNA of cells is known as an alkylating agent. Alkylating agents were the first anticancer medications to be used, and despite their risks, they remain the cornerstone of anticancer therapy.

Examples of Alkylating Agents

A few examples of alkylating agents can be given as:

• cisplatin, nitrogen mustards (cyclophosphamide and chlorambucil),

• alkyl sulfonates (busulfan),

• nitrosoureas (lomustine, semustine, and carmustine),

• triazines (dacarbazine), and

• ethyleneimines (thiotepa).

Types of Molecular Changes

The types of molecular changes that are induced by the alkylating agents can be given as cross-linking between the DNA strands and the loss of a basic component (which is purine) from or the nucleic acid breaking. The result is, nucleic acid will not be replicated.

Either the altered DNA will be not able to carry out the cell functions, resulting in cell death (which is called cytotoxicity), or the altered DNA will change the characteristics of the cell, resulting in an altered cell (which is called mutagenic change). This change can result either in the ability or tendency to produce cancerous cells (which is called carcinogenicity). Normal cells can also be affected and become cancer cells.

Alkylating Agents Drugs

Alkylating agents were one first class of drugs to be used against cancer. There exist five traditional categories of alkylating agents, which are given as follows:

• Nitrogen mustards (for example, chlorambucil, bendamustine, ifosfamide, cyclophosphamide, melphalan, and mechlorethamine)

• Alkyl sulfonates (for example, busulfan)

• Nitrosoureas (for example, lomustine, carmustine, and streptozocin)

• Ethylenimines (for example, thiotepa and altretamine), and

• Triazines (for example, temozolomide and dacarbazine).

Cause of Alkylating Agents

Alkylating agents may cause critical vomiting and nausea and decreases the number of white blood cells and red blood cells as well. The decrease in the white blood cell count results in susceptibility to the infection. Alkylating agents have been found to be used in the treatment of leukaemia, lymphoma, melanoma, testicular cancer, breast cancer, and brain cancer. Often, they are the most used ones in combination with other anticancer drugs.

Some types of Alkylating Agents

Nonspecifically Acting Agents

A few of the alkylating agents are active under the conditions present in cells, and the similar mechanism that makes them toxic allows them to be used as anti-cancer drugs. They stop the tumour growth by cross-linking guanine nucleobases in the double-helix strands of DNA, directly attacking DNA. This process makes the strands unable to separate and uncoil. As this is quite necessary for DNA replication, the cells may no longer divide. These particular drugs act nonspecifically.

Agents Requiring Activation

A few of the substances that require conversion into the active substances in vivo (for example, cyclophosphamide).

Cyclophosphamide is the most potent immunosuppressive substance. In small doses, it is much efficient in the therapy of autoimmune hemolytic anaemia, systemic lupus erythematosus, granulomatosis with polyangiitis, including the other autoimmune diseases. High dosages will cause pancytopenia and hemorrhagic cystitis.

Dialkylating Agents

Dialkylating agents may react with two various 7-N-guanine residues, and if these are in varied DNA strands, the result can be cross-linkage of the DNA strands that prevents the DNA double helix from uncoiling. The effect is limpet binding of the drug molecule to the DNA if the two guanine residues are in the same chain. Busulfan is one of the examples of a di-alkylating agent: it is also the methanesulfonate diester of 1,4-butanediol. Methanesulfonate may be eliminated as a leaving group. Both the ends of the molecule may be attacked by the DNA bases by producing a butylene cross-link between the two different bases.

Monoalkylating Agents

Monoalkylating agents can only react with one 7-N of guanine.

Limpet Attachment

Monoalkylation and limpet attachment do not prevent the two DNA strands’ separation of the double helix but do prevent the vital DNA-processing enzymes from DNA accessing. The final result is given as the inhibition of cell growth or stimulation cell suicide, apoptosis.

Nitrogen Mustard

Nitrogen mustards are the cytotoxic organic compounds having the functional group - chloroethylamine (Cl(CH2)2NR2). Although originally it is produced as chemical warfare agents, they were the first chemotherapeutic agents for cancer treatment. Nitrogen mustards are said to be the nonspecific DNA alkylating agents.

Limitations

Alkylating antineoplastic agents have some limitations. Alkylating antineoplastic agent’s functionality has been found to be limited in the presence of the DNA-repair enzyme, which is O-6-methylguanine-DNA methyltransferase (MGMT). The cross-linking of double-stranded DNA by the alkylating agents can be inhibited by a mechanism of cellular DNA repair, MGMT.

If the MGMT promoter region gets methylated, the cells will no longer produce the MGMT, and they are thus more responsive to the alkylating agents. In gliomas, methylation of the MGMT promoter is a valuable indicator of tumour responsiveness to alkylating agents.