An item on the Nature Science Update website ("Magic bullet homes in", by Philip Ball, 4 March 2002) describes work by researchers at the University of Gottingen in Germany who have developed a custom-made drug molecule that turns lethal only when it reaches cancer cells. In healthy cells it is harmless, but it targets tumor cells selectively.
According to the NSU article, the Gottingen antitumour molecule is a ring of three carbon atoms that is highly strained and apt to burst open. Open, it is a reactive molecule that wreaks havoc among the nucleic acid molecules essential for normal cell function. The drug is delivered in the form of a 'prodrug' without the strained ring but with a sugar side-group. Once the sugar is clipped off, the molecule rearranges itself into a three-atom ring, and becomes lethally active. The Gottingen team uses an enzyme to cut away the sugar group. An antibody on the enzyme ensures that the enzyme targets the tumor cells, and activates the drug molecules there.
This method is vaguely similar to another molecular ësmart bombí that employs a single radioactive actinium atom contained inside a molecular cage and attached to a monoclonal antibody that homes in on cancer cells (see Nanodot post from 16 November 2001).