Which of the following is not categorized as an antimetabolite agent?

Cyclophosphamide is not categorized as an antimetabolite agent because it belongs to the class of alkylating agents. Alkylating agents work by directly damaging the DNA of cancer cells through the addition of alkyl groups, which leads to cross-linking of DNA strands and ultimately inhibits DNA replication and cell division. This mechanism of action differs from that of antimetabolites, which mimic the building blocks of DNA and RNA and interfere with nucleic acid synthesis.

In contrast, Mercaptopurine, Fluorouracil, and Thioguanine are classified as antimetabolites. These agents disrupt normal metabolic processes, primarily in rapidly dividing cells, by mimicking natural substrates involved in DNA and RNA synthesis, thereby inhibiting cell growth and replication. This distinct mechanism sets antimetabolites apart from alkylating agents like cyclophosphamide.