Development of New Classes of Peptide Therapeutics

Signaling cascades that propagate biological information in the cell consist of complex networks of protein-protein interactions which play a central role in a large number of cellular processes like cellular homeostasis, proliferation, migration, angiogenesis, immune responses, apoptosis and cell death. De-regulation of these interactions lead to different diseases, most prominent of which is cancer. Protein–protein interactions, being part of a large number of signaling networks, are potential targets for drug development and peptides are better inhibitors of protein-protein interactions than small molecules. The major aim of this work has been to develop peptide inhibitors of a particular protein-protein interaction and test their therapeutic potential, especially in malignant melanoma cells. S100B, a calcium-regulated protein, plays a crucial role in the proliferation of malignant melanoma cells through protein–protein interactions. The particular protein-protein interaction that has been chosen in this study is the S100B-p53 interaction. S100B is a member of the S100 protein family of Ca2+ binding proteins of the EF-hand type, comprising more than 20 members and is expressed in many cancers like malignant melanoma, glioblastoma etc. Recently S100B has come into focus as it binds directly to the tumor suppressor p53, reduces p53 protein levels and inhibits wild-type p53 functions in malignant melanoma. TRTK-12, a peptide derived from the actin-binding protein Cap Z and the C-terminal region of p53 (residues 367-388) have overlapping binding sites on S100B. Also each monomeric subunit of S100B binds to one peptide. Both the peptides are unstructured in solution but become helical upon binding to S100B. Hence, dimeric helically constrained peptides derived from TRTK-12 and the p53 C-terminal end have been designed, synthesized and purified, which induce cell death in malignant melanoma cells where S100B is overexpressed and downregulates wild type p53 by binding to the latter.The peptide derived from TRTK-12, Arg-Br-aib-TRTK, induces rapid apoptosis in SK-Mel- 5 melanoma cells. This peptide binds to its target S100B in vivo. Apoptosis induced by Arg-Br-aib-TRTK peptide occurs due to the mitochondrial translocation of cytosolic p53. This apoptotic cell death is independent of p53-dependent gene expression as well as caspase activation. No ROS generation is involved in the cell death induced by Arg-Braib- TRTK peptide. This peptide is a better therapeutic molecule than either pentamidine or Nutlin-3 in melanoma cells. Thus, this peptide has the potential to become a candidate drug for malignant melanoma. Monomeric and dimeric helically constrained peptides have been derived from the Cterminal negative regulatory domain of p53 and their efficacy has been studied in malignant melanoma cells. The branched dimeric peptide p53CT-B-Br-6DR binds tightly to S100B in vitro and induces cell death in melanoma cells. It is more potent than the corresponding helically constrained monomeric peptide p53CT-B-6DR. However, the mutant branched peptide p53CT-B-Br-6DR-AAA also induces cell death to some extent. This may be attributed to p53 C-terminus binding to other protein targets apart from S100B. The other part of the work deals with peptides which specifically target to cancerousb tumors and/or blood vessels associated with the tumors. These peptides are called tumor homing peptides and their major application is targeted delivery of a drug conjugated to such peptides. They have also been used for delivery of oligonucleotides, imaging agents, inorganic nanoparticles, liposomes and viruses apart from drugs. A tumor homing peptide HPM and its designed mutant HPV, have been studied for their tumor targeting efficacy in a syngenic mouse melanoma model in vivo by animal biodistribution studies. Both the HPM and HPV peptides are deemed unsuitable for targeting Arg-Br-aib-TRTK peptide to this particular syngenic mouse melanoma model