Jy. Lu et al., Folate-targeted enzyme prodrug cancer therapy utilizing penicillin-V amidase and a doxorubicin prodrug, J DRUG TAR, 7(1), 1999, pp. 43-53
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.