We describe a novel version of antibody-directed
enzyme prodrug therapy (ADEPT), with the use of
amygdalin as
prodrug.
Amygdalin is a naturally occurring cyanogenic
glycoside, which can be cleaved by sweet almond
beta-glucosidase to yield free
cyanide. If
amygdalin could be activated specifically at the tumour site, then malignant cells would be killed without the systemic toxicity usually associated with
chemotherapy. To this end, we have conjugated
beta-glucosidase to a tumour-associated
monoclonal antibody (MAb) (HMFG1) and the conjugate has been tested in vitro for specificity and cytotoxicity subsequent to activation of
amygdalin.
Amygdalin was cytotoxic to HT1376
bladder cancer cells only at high concentrations, whereas the combination of
amygdalin with HMFG1-beta-glucosidase enhanced the cytotoxic effect of
amygdalin by 36-fold. When 2 concentrations of HMFG1-beta-glucosidase were compared, the toxic effect was dose dependent. The cytotoxicity of
amygdalin was also enhanced by the MAb-
enzyme conjugate even when the unbound conjugate was removed from the medium prior to exposure to
amygdalin and the cells were washed. In addition to the cytotoxic effect, we also demonstrated specificity, using a MAb-
enzyme conjugate that does not recognise the HT1376
bladder cancer cells. Finally, we studied the cytotoxic effect of the conjugate in co-culture of HMFG1-positive and-negative cell lines (HT 1376 and U87MG cells). We demonstrated that the rate of surviving cells corresponds well to the percentage of U87MG (HMFG1-negative) cells in the flask. Our findings indicate that ADEPT is more effective than non-directed enzyme activation of a
prodrug and can result in a non-toxic
cancer therapy.