A polyfunctional, organophosphorus alkylating agent and a stable derivative of N,N',N''-triethylenephosphoramide (TEPA), with antineoplastic activity. Upon administration, thiotepa is converted into highly reactive ethylenimine groups, which covalently bind to nucleophilic groups in DNA and demonstrate a preference for the N7 position of guanine bases. This induces crosslinking of alkylated guanine bases in double-stranded DNA, interferes with both DNA replication and cell division, and results in both the induction of apoptosis and the inhibition of cell growth. or A drug used to treat certain types of bladder cancer, breast cancer, and ovarian cancer. It is also used to treat malignant effusion (a condition in which cancer causes an abnormal amount of fluid to collect in hollow spaces or between tissues of the body). It is also being studied in the treatment of other types of cancer. Thiotepa damages the cell’s DNA and may kill cancer cells. It is a type of alkylating agent.