DNA (guanine N7 position)

Class

Small molecule alkylating agent — phenylalanine derivative of nitrogen mustard (melphalan); alkylates DNA and induces inter-strand cross-links, blocking DNA/RNA synthesis and causing cytotoxicity in dividing and non-dividing cells

Pathway

DNA damage / alkylation

This mechanism targets DNA itself rather than a specific protein. Nitrogen mustard alkylating agents form a reactive intermediate that attacks the N7 nitrogen of guanine bases in DNA, one of the most nucleophilic (electron-rich) sites in the genome. Because these agents carry two reactive arms (bifunctional alkylation), they can link guanines on opposite DNA strands, producing inter-strand cross-links. These cross-links physically prevent the DNA double helix from separating, which is required for both replication and transcription. Cells attempting to divide with cross-linked DNA either arrest or undergo programmed cell death when repair fails. The biological rationale for this approach is that rapidly dividing cells, including many cancer cells, are disproportionately vulnerable to DNA damage that blocks replication, though the damage itself is not selective for malignant tissue. This class of agent has broad relevance across hematologic malignancies and solid tumors, often used in high-dose regimens including conditioning before stem cell transplantation, reflecting the mechanism's potency and its lack of dependence on any single oncogenic pathway. Because alkylation affects any dividing cell, normal tissues with high turnover, such as bone marrow and gut lining, are also affected, which shapes how these agents are dosed and combined with supportive care in practice.