GnRH-based Drug Targeting : Cell Adhesion and Migration Modulator Effects of GnRH Derivatives in Tumor Cells

Abstract

The decapeptide gonadotropin-releasing hormone (GnRH) is responsible for the regulation of reproductive function in the pituitary. Several natural isoforms (GnRH-I, GnRH-II, GnRH-III) and synthetic GnRH analogues, both agonists and antagonist, have been proven to exert antiproliferative effects mediated by GnRH receptors (GnRH-Rs) that are overexpressed in cancer cells compared to their expression in normal peripheral tissues. The application of cytotoxic drugs covalently linked to a GnRH peptide represents a promising approach to deliver the drug selectively to tumors overexpressing GnRH-Rs and consequently to improve the efficacy of tumor therapy. The targeted chemotherapy favors synthetic GnRH analogues such as GnRH-III and its derivatives (dimers and fragments), which directly inhibit the tumor growth without exerting significant endocrine side effects. The activity of cell physiological functions such as cell adhesion and chemotaxis (migration) basically determine the prognosis (dissemination) and the targeted treatment of cancer. The possible use of anticancer and antimetastatic GnRH-III peptides as drug delivery systems was investigated. The GnRH-III derivatives were applied as targeting moieties, while their drug (e.g., daunorubicin–Dau, doxorubicin–Dox) containing conjugates were tested by using adequate, cutting edge techniques of impedimetry acting on cell adhesion, chemotaxis/migration and proliferation in diverse model cells. Many types of cancer cells exhibit impaired adhesion and enhanced motility which allow the metastatic spreading. This prompted us to investigate the effects of GnRH-III derivatives and conjugates in tumor model cells such as Mono Mac 6 human leukemia cell line. The adhesion inducer effect of GnRH-III peptides with targeting potency is a rather general character in the leukemia model cell, while the chemotactic effects of the monomers containing a branch on Lys8 of GnRH-III proved to be dependent on the length of the branch. In case of conjugates the application of the doxorubicin as a drug (GnRH-III(Dox-ester)) or dimerization of the conjugates ([GnRH-III(Dau-oxime-C)]2) could significantly increase the adhesion inducer and chemorepellent character, activities which are favorable for drug targeting conjugates to inhibit the invasion of tumor cells. The majority of the conjugates (e.g., GnRH-III(Doxester)) could inhibit the growth of different types of tumor cells (leukemia–Mono Mac 6, human breast cancer–MCF-7, human colon cancer–HT-29). In addition to the notable antitumor activity of the GnRH-III based conjugates, the combined adhesion inducer and chemorepellent effects of both targeting units and their conjugates suggested their possible inhibitory action on invasion of a primary tumor and potential use in targeted tumor therapy.