178 Heterocycles in Medicine
Some cytotoxic agents are used as immunosuppressive drugs rather than for treating cancer, acting by similar mecha-
nisms but with different balances of effects. These drugs would typically be used to prevent organ rejection following
transplantation or to treat severe infl ammatory diseases due to autoimmune reactions.
Other mechanisms include interference with the function of folic acid, an important growth factor. Methotrexate is a
well-established and widely-used anticancer and immunosuppressive agent that acts in this manner.
Irinotecan, a more selective synthetic analogue of camptothecin (a natural but toxic anticancer alkaloid), acts by inhib-
iting topoisomerase I, an enzyme involved in ordering the strands of DNA. Some compounds act by physical binding
with secondary structures of vital natural polymers – the complex indole alkaloid vincristine is a classic example – it
binds to tubulin, a protein essential to cell division.
Photochemotherapy
Photochemotherapy is used for the treatment of a number of diseases, predominantly, for obvious reasons, of the skin
(and some mucous membranes) and other areas of the body that are accessible to light, such as the eyes.