An antimetabolite is a substance that blocks the function of a metabolite (a substance that is also produced normally as part of metabolism).
These molecules are usually structurally similar to the metabolite that they interact with, leading in inhibiting the activity of metabolites.
Antimetabolites can have toxic effects on cells, such as stopping cell growth and cell division, hence these substances are used as chemotherapy for cancer.
Classification of Antimetabolites-
Purine analogues
These analogues have thiol group at 6th position of the purine ring.