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A series of new somatostatin analogs have been developed and tested for antitumor activity. Some analogs strongly inhibited tyrosine kinase activity of human colon tumor cells and this activity correlated well with their antiproliferative effect, but did not correlate with GH release inhibition. The best analogs strongly inhibited the metastasis formation in the Lewis lung metastasis model in mice. On the basis of these in vitro and in vivo data we were able to select one analog with strong tyrosine kinase inhibitory and antitumor activity, without inhibiting growth hormone release.Copyright 1993, 1999 Academic Press

Following the observation that the activity of gonadotropin-releasing hormone III (GnRH-III) in the suppression of growth of MDA-MB-231 and MCF-7 breast cancer cells surpasses that of GnRH and other analogs thereof, analogs of GnRH-III were synthesized to investigate the structural basis for the improved antitumor activity. Compounds synthesized include analogs with changes in the central sequence in which GnRH-III differs from GnRH and in the C- and N-terminal regions. The results indicate that a salt bridge between Asp⁶ and Lys⁸ stabilizes the active conformation of GnRH-III and show the importance of the Trp⁷. Replacement of the C-terminal Gly-NH2 with d-Ala-NH2 was not well tolerated, but replacement with ethylamide was. Replacement of pGlu¹ with Ac-d-Trp appears to have a significantly deleterious effect on a unique conformation of GnRH-III which is responsible for its binding to the receptors on cancer cell lines and the resultant antitumor activity.

Since permanently high levels of GnRH analogues are necessary to exert direct and/or indirect antitumor effect on mammary tumors, much emphasis was put on the development of retarded-release devices (e.g. microcapsules) for GnRH derivatives. Alternatively, these compounds can be covalently coupled to high-molecular mass carrier molecules for the design of bioconjugates acting as (a) prodrugs producing prolonged release or (b) macromolecular therapeutics. In order to evaluate the feasibility of this approach, a prototype construct has been prepared with a potent GnRH antagonist Ac(d-Trp^1,3,d-Cpa²,d-Lys⁶,d-Ala¹⁰)-GnRH (MI-1544). As a carrier, a representative of a new generation of synthetic, biodegradable branched poly[Lys(Xi-dl-Alam)] (XAK) type polypeptides with poly(l-lysine) backbone has been used in which X is an acetylated derivative of glutamic acid (AcEAK). This polyanionic polypeptide with free γ-carboxyl groups was conjugated to MI-1544, which has only a single amino group at position 6. In this paper, we describe (i) the synthesis and structure (primary structure, conformation) properties of the MI-1544−AcEAK conjugate with a 33% degree of substitution, (ii) the effect of the covalent attachment of MI-1544 to AcEAK on its blood clearance and tissue distribution, and (iii) the hormone-related indirect (ovulation inhibitory) or direct (antiproliferative) antitumor activity of the conjugate studied by in vitro assays. Data obtained with ¹¹¹In- and ¹²⁵I-labeled conjugates have demonstrated that in fact the body/blood survival of MI-1544 was prolonged by 1.5−3 times. The direct in vitro antitumor effect of MI-1544 was maintained or even enhanced in the MI-1544−AcEAK conjugate. Furthermore, we have shown that this conjugate was able to antagonize the effect of GnRH in vitro or to act as free MI-1544 both in short- and long-term inhibition of ovulation even after single subcutaneous injection. These data suggest that it is feasible to use a biodegradable polymeric polypeptide for development of a macromolecular therapeutic with GnRH antagonists.

The new agonistic analogues (I), in which Gly6 of the title hormone is replaced by lipophilic D‐amino acids, are synthesized by solution‐phase peptide synthetic methods and tested for their LH‐releasing activity and their ovulation suppressing ability.