Folate-targeted enzyme prodrug cancer therapy utilizing penicillin-V amidase and a doxorubicin prodrug

In antibody-targeted enzyme prodrug therapy, a monoclonal antibody (mAb) co valently linked to an enzyme is commonly exploited to concentrate the enzym e on the tumor cell surface prior to administration of a relatively nontoxi c prodrug. The tumor-localized enzyme then converts the prodrug into a cyto toxic agent, which in turn diffuses into the tumor causing localized cell d eath. In this paper, we have substituted folic acid for the mAb as a mean o f delivering an attached enzyme, penicillin-V amidase (PVA), to folate rece ptor (FR)-positive tumor cells. The enzyme PVA is capable of converting a d oxorubicin-N-p-hydroxyphenoxyacetamide prodrug (DPO) into its potent parent drug, doxorubicin. For PVA targeting, each PVA molecule was covalently lab eled with three molecules of folic acid via the formation of amide bonds. I n vitro binding assays showed that folate-PVA-I-125 conjugates bind specifi cally to KB cells (FR-positive tumor cells) but not to A549 cells (FR-negat ive tumor cells). Moreover, in a series of in vitro cytotoxicity tests, fol ate-PVA conjugates were found to kill folate receptor positive but not rece ptor negative cells, and when bound to FR-positive cells! folate-PVA conjug ates rendered the DPO prodrug as toxic as free doxorubicin (IC50, similar t o 0.6 mu M) Finally, preliminary in vivo plasma clearance studies in normal mice revealed that i.v. administered folate-PVA-125I and PVA-I-125 are bot h cleared from the blood within a 24 h time period, removing concern that n onspecifically trapped folate-PVA might activate prodrug in nontargeted tis sues. In view of the fact that only a small number of folate-PVA molecules are required to mediate killing of target cells in vitro, these data argue that folate-targeted enzyme prodrug therapy should be considered for tumor eradication in vivo.