Fibroblast-Activation
Protein-α (FAP) is a membrane-bound
serine protease that is expressed on the surface of reactive stromal fibroblasts present within the majority of human epithelial
tumors but is not expressed by normal tissues. FAP is a postprolyl
peptidase that differs from other dipeptidyl prolyl
peptidases such as diprolylpeptidase 4 in that it also has
gelatinase and
collagenase endopeptidase activity. Therefore, FAP represents a potential pan-
tumor target whose enzymatic activity can be exploited for the intratumoral activation of
prodrugs and protoxins. To evaluate FAP as a
tumor-specific target, putative FAP-selective
peptide protoxins were constructed through modification of the prodomain of
melittin, a 26
amino acid amphipathic cytolytic
peptide that is the main toxic component in the
venom of the common European honeybee Apis milefera.
Melittin is synthesized as
promelittin, containing a 22
amino acid NH(2)-terminal prodomain rich in the
amino acids proline and
alanine. In this study,
peptides containing truncated
melittin prodomain sequences were tested on erythrocytes to determine the optimal prodomain length for inhibiting cytolytic activity. Once optimized, modified
promelittin peptides were generated in which previously identified FAP substrate sequences were introduced into the prodomain.
Peptide protoxins were identified that were efficiently activated by FAP and selectively toxic to FAP-expressing cell lines with an IC(50) value in the low micromolar range that is similar to
melittin. Intratumoral injection of an FAP-activated protoxin produced significant lysis and growth inhibition of human breast and
prostate cancer xenografts with minimal toxicity to the host animal.