Gonadotropin-releasing hormone analogue conjugates with strong selective antitumor activity

Gonadotropin-releasing hormone analogue conjugates with strong selective antitumor activity

^*National Institute of Oncology, Budapest, H-1525, P.O. Box 21, Hungary; ^‡Creighton University, School of Medicine, Department of Biomedical Sciences, Omaha, NE 68178; ^§Joint Research Organization of the Hungarian Academy of Sciences and Semmelweis University Medical School, Department of Medical Chemistry, Molecular Biology and Pathobiochemistry, Budapest, H-1444, P.O. Box 260, Hungary; ^¶Central Research Institute for Chemistry of the Hungarian Academy of Sciences, Budapest, H-1025, P.O. Box 17, Hungary; and ^‖1st Institute of Pathology and Experimental Cancer Research, Semmelweis University Medical School, Budapest, H-1085, Hungary

Communicated by Tibor Farkas, Hungarian Academy of Sciences, Szeged, Hungary (received for review May 13, 1998)

Abstract

Conjugation of gonadotropin-releasing hormone (GnRH) analogues GnRH-III, MI-1544, and MI-1892 through lysyl side chains and a tetrapeptide spacer, Gly-Phe-Leu-Gly (X) to a copolymer, poly(N-vinylpyrrolidone-co-maleic acid) (P) caused increased antiproliferative activity toward MCF-7 and MDA-MB-231 breast, PC3 and LNCaP prostate, and Ishikawa endometrial cancer cell lines in culture and against tumor development by xenografts of the breast cancer cells in immunodeficient mice. MCF-7 cells treated with P-X-1544 and P-X-1892 displayed characteristic signs of apoptosis, including vacuoles in the cytoplasm, rounding up, apoptotic bodies, bleb formation, and DNA fragmentation. Conjugates, but not free peptides, inhibited cdc25 phosphatase and caused accumulation of Ishikawa and PC3 cells in the G₂/M phase of the cell cycle after 24 h at lower doses and in the G₁ and G₂ phases after 48 h. Since P-X-peptides appear to be internalized, the increased cytotoxicity of the conjugates is attributed to protection of peptides from proteolysis, enhanced interaction of the peptides with the GnRH receptors, and/or internalization of P-X-peptide receptor complexes so that P can exert toxic effects inside, possibly by inhibiting enzymes involved in the cell cycle. The additional specificity of P-X-peptides compared with free peptides for direct antiproliferative effects on the cancer cells but not for interactions in the pituitary indicates the therapeutic potential of the conjugates.

Footnotes

↵ † To whom reprint requests should be addressed at: National Institute of Oncology, Ráth György str. 7–9, Budapest, H-1122, Hungary. e-mail: palyi{at}oncol.hu.
ABBREVIATIONS:

GnRH,

gonadotropin-releasing hormone;

P,

poly(N-vinylpyrrolidone-co-maleic acid) copolymer;

X,

Gly-Phe-Leu-Gly;

SRB,

sulforhodamine B