Antimetabolites are drugs that interfere with one or more enzymes or their reactions that are necessary for DNA synthesis. They affect DNA synthesis by acting as a substitute to the actual metabolites that would be used in the normal metabolism (for example antifolates interfere with the use of folic acid).
Antimetabolites are drugs used in cancer chemotherapy. Cancer cells divide more rapidly compared to normal cells so antimetabolites affect cancer cell replication more than they affect normal cell replication.