Challenges associated with the targeted delivery of gelonin to claudin-expressing cancer cells with the use of activatable cell penetrating peptides to enhance potency

BACKGROUND: Treatment of tumors with macromolecular toxins directed to cytoplasmic targets requires selective endocytosis followed by release of intact toxin from the endosomal/lysosomal compartment. The latter step remains a particular challenge. Claudins 3 and 4 are tight junction proteins that are over-expressed in many types of tumors. This study utilized the C-terminal 30 amino acid fragment of C. perfringens enterotoxin (CPE), which binds to claudins 3 and 4, to deliver a toxin in the form of recombinant gelonin (rGel) to the cytoplasm of the human ovarian carcinoma cell line 2008. RESULTS: CPE was fused to rGel at its N-terminal end via a flexible G(4)S linker. This CPE-G(4)S-rGel molecule was internalized into vesicles from which location it produced little cytotoxicity. To enhance release from the endosomal/lysosomal compartment a poly-arginine sequence (R(9)) was introduced between the CPE and the rGel. CPE-R(9)-rGel was 10-fold more cytotoxic but selectivity for claudin-expressing cells was lost. The addition of a poly-glutamic acid sequence (E(9)) through a G(4)S linker to R(9)-rGel (E(9)-G(4)S-R(9)-rGel) largely neutralized the non-selective cell membrane penetrating activity of the R(9 )motif. However, introduction of CPE to the E(9)-G(4)S-R(9)-rGel fusion protein (CPE-E(9)-G(4)S-R(9)-rGel) further reduced its cytotoxic effect. Treatment with the endosomolytic reagent chloroquine increased the cytotoxicity of CPE-E(9)-G(4)S-R(9)-rGel. Several types of linkers susceptible to cleavage by furin and endosomal cathepsin B were tested for their ability to enhance R(9)-rGel release but none of these modifications further enhanced the cytotoxicity of CPE-E(9)-G(4)S-R(9)-rGel. CONCLUSION: We conclude that while a claudin-3 and -4 ligand serves to deliver rGel into 2008 cells the delivered molecules were entrapped in intracellular vesicles. Incorporation of R(9 )non-specifically increased rGel cytotoxicity and this effect could be masked by inclusion of an E9 sequence. However, the putative protease cleavable sequences tested were inadequate for release of R(9)-rGel from CPE-E(9)-G(4)S-R(9)-rGel.