Antitumour effect of a gonadotropin-releasing-hormone antagonist (MI-1544) and its conjugate on human breast cancer cells and their xenografts

The extrapituitary effects of GnRH antagonists and their potential clinical implications: a narrated review

Potential roles of gonadotropin-releasing hormone (GnRH) antagonists on GnRH/GnRH receptor systems and their effects on the extrapituitary tissues are largely elusive. In this narrated review, we summarized the systemic effects of GnRH antagonists on ovary, endometrium, embryo implantation, placental development, fetal teratogenicity, reproductive tissue cancer cells, and heart while briefly reviewing the GnRH and GnRH receptor system. GnRH antagonists may have direct effects on ovarian granulosa cells. Data are conflicting regarding their effects on endometrial receptivity. The GnRH antagonists may potentially have detrimental effect on early placentation by decreasing the invasive ability of cytotrophoblasts if the exposure to them occurs during early pregnancy. The GnRH antagonists were not found to increase the rates of congenital malformations. Comparative clinical data are required to explore their systemic effects on various extrapituitary tissues such as on cardiac function in the long term as well as their potential use in other human cancers that express GnRH receptors.

Drug targeting by macromolecules without recognition unit?

his review will summarize available information on the ability of macromolecular conjugates containing no specific recognition motifs to deliver anthracyclines (daunomycin, adriamycin) or methotrexate to target cells such as tumour cells or macrophages. Conjugates with natural (proteins, DNA, carbohydrates) and synthetic macromolecules (linear and branched chain poly-alpha-amino acids, non-biodegradable DIVEMA, HPMA etc.) will be reviewed. Experimental data from several laboratories indicate that these conjugates are taken up by cells mainly by fluid-phase or adsorptive endocytosis. It is believed that these processes do not involve 'specific receptors'. Two examples of methotrexate and daunomycin conjugates will be discussed to show the effect of the chemical structure of branched chain polypeptides on the uptake and antitumour or antiparasitic (Leishmania donovani infection) efficacy of conjugates.

Synthesis and conformational studies of poly(L-lysine) based branched polypeptides with Ser and Glu/Leu in the side chains

In a new group of polypeptides, the branches were composed of DL-Ala oligopeptide, L-serine and L-leucine or L-glutamic acid residues. The synthesis of eight different side-chain combinations is described. In the first group, Ser was attached directly to the epsilon-amino groups of polylysine, and Leu or Glu was situated at the side chain end (poly[Lys(X(i)-DL-Ala(m)-Ser(j))]). Alternatively, Leu or Glu was positioned next to the polylysine backbone (poly[Lys(Ser(j)-DL-Ala(m)-X(i))], where X=L-Leu or L-Glu and m approximately 3-6, i</=1 and j</=1). The second group of polymers was synthesised by grafting oligo(DL-alanine) chains to the epsilon-amino groups of polylysine, followed by coupling of Ser and Leu or Glu consecutively to the chain ends, however, in a different order, resulting in the polymers (poly[Lys(X(i)-Ser(j)-DL-Ala(m))] and poly[Lys(Ser(j)-X(i)-DL-Ala(m))], where X=L-Leu or L-Glu and m approximately 3-6, i</=1 and j</=1). The effect of amino-acid composition and sequence of side chains in branched polypeptides on solution conformation was studied by CD spectroscopy. CD spectra recorded in aqueous solutions of various pH (2-11) and ionic strengths (0.02-2.0 M NaCl) suggest that leucine- and serine-containing polypeptides have more ordered (alpha-helical) structure than the polymers with glutamic acid and serine residues in the same position. The influence of serine residues on ordered structure (helical or beta-sheet) formation depends on its position in the side chain as well as on the nature of amino acid X (Leu or Glu). The incorporation of Ser into the branches resulted in polypeptides possessing prolonged shelf stability and high water-solubility. No toxic effect of this new class of polymers was observed on mouse spleen cells, even after 4 h of incubation.

Carrier design: new generation of polycationic branched polypeptides containing OH groups with prolonged blood survival and diminished in vitro cytotoxicity

For the construction of macromolecule-drug conjugates, it is important to provide rational basis to the selection of proper carrier. With respect to the importance of the side-chain structure and charge of the branched polypeptides in biological properties, we have prepared a new class of branched polypeptides with single or multiple hydroxyl groups and studied their solution conformation, in vitro cytotoxicity, biodistribution, and immunoreactivity. For comparative studies, polypeptides were designed to contain serine at various positions of the side chains, varying also the number. Ser was attached to the end of oligo(DL-Ala) side chains grafted to polylysine resulting polypeptides with the general formula poly[Lys(Ser(i)-DL-Ala(m))], (SAK). Ser was also coupled directly to the polylysine backbone poly[Lys(Ser(i))] (S(i)K) and then elongated by polymerization of N-carboxy-DL-Ala anhydride resulting poly[Lys(DL-Ala(m)-Ser(i))] (ASK). An additional polymer was also prepared, but instead of the oligo(DL-Ala) branches, oligo(DL-Ser) side chains were introduced (poly[Lys(DL-Ser(m))], SK). The presence of hydroxyl groups resulted in compounds with improved of water solubility. CD spectra of polypeptides showed significant differences correlating with the position and numbers of Ser residues in the side chains. Under physiological conditions, polycationic polypeptides assumed ordered secondary structure (S(i)K and LSK) or partially unordered conformation (SK, SAK, and ASK). Data of selected polymers demonstrate that these polycationic compounds are essentially nontoxic in vitro on normal rat liver or mouse spleen cells and have no cytostatic effect on mouse colorectal carcinoma C26 cells. The blood clearance and biodistribution of these derivatives were greatly dependent on the position and number of Ser residues in the branches and possess a rather extended blood survival in mice. Polypeptides were taken up predominantly by the liver and kidney (S(i)K, LSK, and ASK) or kidney and lung (SK and SAK). The best survival in the blood was found with SAK, representing the first polycationic branched polypeptide, which show extended blood clearance. The relative position of Ser residue had also a marked influence on the immunogenicity of polypeptides. The characteristics of the antibody response to polypeptide containing Ser at the end of the branches (SAK) or adjacent to the polylysine backbone (ASK) was also dependent on the genetic background of the mouse strains. We also found that these compounds have no effect on to the SRBC-specific humoral immune response, indicating the lack of nonspecific immunostimulatory potential. In conclusion, these studies suggest that synthetic branched polypeptides with Ser can be considered as candidates for constructing suitable conjugates for drug/epitope delivery. It is not only due to the presence of hydroxyl group to be used for oxime chemistry but also to their beneficial biological features.

Gonadotropin-releasing hormone analogue conjugates with strong selective antitumor activity

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.

Direct anticancer activity of gonadotropin-releasing hormone-III

In previous studies GnRH-III, a variant of the hypothalamic neurohormone GnRH, was isolated from the brain of the sea lamprey and structurally characterized. GnRH-III is a hypothalamic neurohormone in both female and male sea lampreys. In the present work biological activities of GnRH-III in mammalian systems were examined. In superfused rat pituitary cells, GnRH-III at 1 nM to 100 nM neither induced LH-secretion nor inhibited the LH-secretion elicited by native GnRH and elicited LH release only at 1 microM. At high dose (500 microg/day) in vivo, GnRH-III behaved as a GnRH agonist, though, it was 1000-fold less active than ovurelin. The in vitro and in vivo results were in good agreement in showing that GnRH-III is only a weak agonist of the endocrine activity of GnRH. GnRH-III specifically bound to receptors on cancer cells and recognized not only the high-, but also the low-affinity binding sites. GnRH-III significantly suppressed growth of human cancer cells which have GnRH receptors. The inhibitory effect of GnRH-III on growth of cancer cells was specific and direct since the peptide did not have endocrine activity in the concentration range found to be effective in anticancer assays. GnRH-III inhibited equally the growth of ER-positive and -negative breast and TeR-positive and negative prostate cells.

Synthesis, conformation, biodistribution, and hormone-related in vitro antitumor activity of a gonadotropin-releasing hormone antagonist-branched polypeptide conjugate

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-Trp1,3, 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 gamma-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.

Analysis of macromolecular branched chain polypeptides by capillary electrophoresis and micellar electrokinetic chromatography

Amphoteric poly(Lys-[Glu1.0-DL-Ala4.1]), (EAK) and anionic poly(Lys-Ac-Glu0.98-DL-Ala3.98]), (AcEAK) branched chain polypeptides were analyzed by capillary electrophoresis (CE) and micellar elektrokinetic chromatography (MEKC) in the following buffers. A1: 0.25 N triethyl ammonium phosphate (TEAP) buffer (pH 2.25); A2: 100 mM sodium dodecyl sulfate (SDS) in buffer A1; B1: Na-borate buffer (pH 7.7); B2: 100 mM SDS in buffer B1; C1: Na-borate buffer (pH 11.0); C2: 100 mM SDS in buffer C1. Both EAK and AcEAK could be separated by a CE mechanism at pH 2.25 and by an MEKC mechanism at pH 11.0. Optimum results were achieved with CE in buffer A1 and with MEKC in buffer C2.