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Cancer-seeking antibodies guide light-sensitive drugs

By Andy Coughlon; 03 September 2007 (see article in New Scientist)
Magazine issue 2619

Attaching a light-sensitive cancer drug to an antibody fragment has boosted its potency while reducing its side effects.

Photodynamic therapy relies on light-sensitive drugs that soak into tumours and then destroy them when flashed with light. The problem is that existing drugs soak into healthy tissue as well, where they take weeks to clear. As a result, patients risk scorching normal tissue during treatment and if they go into the sun afterwards.

Now researchers led by Mahendra Deonarain of the company PhotoBiotics and Imperial College London have tested in mice a way of increasing the proportion of drug reaching the tumour by two to three times, thereby reducing the amount soaked up by normal tissue. The drug also cleared from tissue faster.

Their secret is to load the drug onto antibody fragments that seek out cancer cells, then inject the doped fragments instead of the drug on its own(Photochemical and Photobiological Sciences, D01:10.10391b708320c). Whole antibodies are useless as drug carriers because they can hold no more than three PDT molecules and take weeks to clear due to their bulk. The antibody fragments, which target a molecule unique to ovarian, breast and prostate cancer, carry eight PDT molecules and clear rapidly. "You end up with at least 10 times more drug in the tumour than in the blood, and 50 times more than in the muscle," says Deonarain.

The drug-laced fragments cured the mice of ovarian cancer (International Journal of cancer, in press). Next they will be tested against human prostate cancer in mice and rats.