Synthesis and cytotoxic activity of N-(2-chloroethyl)-N-nitroureas and N-(2-chloroethyl)-N-nitrocarbamates.

As analogs of the widely used anti-tumor agents, N-(2-chloroethyl)-N-nitrosoureas, N-(2-chloroethyl)-N-nitroureas and N-(2-chloroethyl)-N-nitrocarbamates were synthesized by nitration following the reaction of the appropriate amines or alcohols with 2-chloroethyl isocyanate. All tested compounds exert cytotoxic effect with IC50 values of 10(-4) to 10(-6) M and most of them show somewhat higher cytotoxicity in nitrogen than in air.

The antiproliferative action of a melphalan hexapeptide with collagenase-cleavable site.

PURPOSE: The objective of the present study was to examine the relevance of collagenase in the antitumor action of a melphalan peptide (MHP) with a collagenase-cleavable sequence. The question was addressed as to whether collagenase may act as an activator or a target in the antiproliferative mechanism of MHP. METHODS: Melphalan was inserted into peptides representing the sequence Pro-Gln-Gly-Ile-Ala.Gly of the collagenase-cleavable site in collagens. Changes in growth and collagenase IV activities of HT-1080, HT-29, HT-168, and MCF-7 cell cultures were investigated. RESULTS: The present investigations provide data indicating that Pro-Gln-Gly-Ile-Mel-Gly (melphalan hexapeptide, MHP) is a substrate for both bacterial and 72-kDa type IV collagenases and that in this way it can generate Ile-Mel-Gly (melphalan tripeptide, MTP) of higher cytotoxic potency. Indeed, the formation of MTP was detected in the conditioned medium of HT-1080, a collagenase IV-producing human fibrosarcoma. In a comparison of equimolar concentrations of melphalan and its two peptide derivatives (MHP and MTP), superior antiproliferative action of MTP was seen in HT-29, HT-1080, and HT-168 tumor cell cultures. However, the relatively modest cytostatic actions of MHP were increased when bacterial collagenase was added to the cell cultures. After melphalan treatment, reduced levels of both 92 and 72-kDa type IV collagenases were seen in the HT-1080 cell cultures. However, the reduction of collagenase activity and the cell counts did not run parallel in the MTP- or MHP-treated cultures; indeed, collagenase activity related to cell numbers showed an elevated level. CONCLUSIONS: As the conversion of MHP to the more toxic MTP was detected in the presence of collagenases, it is possible that collagenase-directed activation of prodrugs may be a promising approach for the development of more selective cytostatic drugs against malignant tumors with high collagenase activities.

Effect of Ca2+ on the secondary structure of linear and cyclic collagen sequence analogs.

To investigate the role of secondary structure in the substrate specificity of human 72 kDa type IV collagenase, we synthesised linear and cyclic collagen sequence analogs. As Ca2+ plays a crucial role in the enzyme activity, the CD and FTIR spectra of the peptides were also measured in the presence of Ca2+. Most of the linear, but none of the cyclic peptides form stable 1:1 Ca2+ complexes. The cyclic hexapeptides adopt significantly different backbone conformations comprising not only beta-turns but also the less frequent gamma-turns. Consequently, in the cyclopeptides the scissile Gly-Ile(Leu) bond is embedded into a different conformational environment, but in spite of that none of them is a substrate or an inhibitor of the enzyme. The best substrate Ac-Pro-Leu-Gly-Leu-Ala-Gly-D-Lys-OH binds Ca2+, but does not form a stable 1:1 Ca2+ complex, which suggests that instead of a folded structure an extended flexible conformation is preferred by the enzyme.

Substrate- and inhibitor-specificity of a non-endothelial enzyme which forms [Met5]-enkephalin from [Met5]-enkephalin-Arg6,Phe7 in isolated rabbit ear artery: pharmacological characterization.

The captopril-inhibited enzyme which forms [Met5]-enkephalin from [Met5]-enkephalin-Arg6,Phe7 in isolated rabbit ear artery was characterized further by using various natural substrate candidates/analogues ([Met5]-enkephalin-Arg6,Phe7 and its amide, [Met5]-enkephalin, angiotensin I and bradykinin), peptidase inhibitors such as captopril, enalaprilate and thiorphan and by endothelial removal. 10(-5) and 10(-4) M but not 10(-6) M captopril reduced the effectiveness of [Met5]-enkephalin-Arg6,Phe7 and potentiated the effect of bradykinin but did not affect markedly the action of the other peptides. Of the inhibitors, enalaprilate was less effective than captopril, and thiorphan had no effect. The [Met5]-enkephalin-Arg6,Phe7-->[Met5]-enkephalin conversion was not affected by endothelial removal. The substrate and inhibitor spectrum of this non-endothelial enzyme activity bears no relationship in other, hitherto characterized dipeptidylcarboxypeptidases/endopeptidases known to be involved in the metabolism of the tested peptides.

Receptor-mediated cytotoxicity of alpha-MSH fragments containing melphalan in a human melanoma cell line.

Four alpha-MSH drug conjugates have been synthesized, 2 C-terminal (Pep 3 and 4) and 2 central fragments (Pep 1 and 2), the latter being the 4-10 sequence known to be the main alpha-MSH-receptor-recognition site. Melphalan was introduced into each sequence at different locations. Their ability to recognize alpha-MSH receptors as well as their cytotoxic effects were compared in 3 cell lines: melanoma, carcinoma and fibroblast cells. Pep 1 and 2 were able to specifically bind to MSH receptors on melanoma cells by displacing labelled alpha-MSH from its binding sites at concentrations similar to the 4-10 heptapeptide sequence known to contain the main receptor-recognition site. They subsequently penetrate the cell, most probably by a receptor internalization mechanism, since about half of their effect could be inhibited by competition at the receptor level. Significant and selective cytotoxic effects to melanoma cells could be observed after only 2 hr exposure to the drug conjugates. Interestingly, these 2 conjugates, differing only in melphalan position, showed completely different cytotoxicity in terms of IC50 values, Pep 1 being 24 times more toxic to all cells; but the 2 were equally specific to melanoma cells. However, they both were less toxic to all cells than melphalan itself. Furthermore, Pep 1 and 2 were able to block the receptor and, unlike Pep 3 and 4, their cytotoxic effect could be significantly inhibited by an alpha-MSH agonist. Pep 3 and 4 were 5 to 10 times less toxic than melphalan to melanoma and carcinoma cells and 50 times less to fibroblast cells, and did not show any cell-type selectivity. They were less toxic than Pep 1 to melanoma and carcinoma cells by a factor of 2, but equally toxic to fibroblasts. In contrast, they were more toxic than Pep 2 to fibroblasts, melanoma and carcinoma by a factor of 3, 10 and 24 respectively. Our data strongly suggest a receptor-mediated cytotoxicity mechanism occurring with alpha-MSH central fragments in human melanoma cells due to the presence of alpha-MSH-specific receptors. This mechanism appeared to be both peptide- and cell-type-specific.

Novel, delta-opioid receptor-selective peptide antagonists: demonstration of a possible opioid interaction in the absence of a protonated nitrogen and attempts to locate the protonated nitrogen site.

We have recently (Rónai et al., 1992) introduced a family of novel delta-opioid receptor-selective peptide antagonists, based on the Tyr-Pro-Gly-Phe-Leu-Thr structure, where the nitrogen accepts substituents that make protonation possible (e.g. diallyl) as well as substituents (e.g. t-Boc) where protonation cannot occur. In this paper, we present the details of a design strategy where the structurally closely related biologically active and inactive compounds are suggestive of conformational requirements of action. Furthermore, since even those derivatives of the antagonist peptides where the N-terminus was free were either devoid of opioid agonist activity or were extremely weak agonists, it is suggested that these antagonists do not interact with the conventional "opioid nitrogen site". To find this "conventional" site, a number of N-substituted (phenylglycyl-, alpha-Boc-lysyl-, alpha-Phe-beta-alanyl-) derivatives of Tyr-Pro-Gly-Phe-Leu-Thr hexapeptide were synthesized and their biological activities were determined in the mouse vas deferens bioassay.

Synthesis and characterization of some collagen sequence analogs.

Some analogs of natural collagen sequences (773-779) were synthesized. The peptides were hydrolyzed at the Gly-Ile bond not only by crude collagenase isolated from normal rat liver, but also by the bacterial Clostridium histolyticum collagenase. The reason for this unusual cleavage site in the latter case may lie in the unordered secondary structure of the substrates measured by CD spectroscopy.

A novel opioid structure which accepts protonated as well as non-protonated nitrogen: a family of pure, delta receptor selective antagonists.

Conventional opioids including opioid peptides require an "opioid" nitrogen which exists in protonated state while interacting with the receptor. In the present paper we demonstrate that the Tyr-Pro-Gly-Phe-Leu-Thr hexapeptide sequence accepts N-terminal substituents such as N-t-Boc, N-phenylacetyl and N-diphenylacetyl where the N cannot become protonated, as well as "traditional" substitutions such as N,N-diallyl, where protonation is likely under physiological conditions. The opioid peptides bearing these substituents are pure antagonists of medium affinity (Ke values in the mouse vas deferens bioassay against [Met5]-enkephalin are in the 3 x 10(-7)-4 x 10(-6) M range) with a high delta receptor preference (50-350-fold delta over mu selectivity ratios).

Investigations on the antitumor effect and mutagenicity of alpha-MSH fragments containing melphalan.

alpha-MSH fragments containing melphalan were tested in vivo on L1210 leukemia and on human amelanotic melanoma xenograft in mice and in vitro on human amelanotic melanoma cell lines. The compounds exhibit significant antitumor activity, but no selectivity in targeting of melanoma can be achieved. There is a difference between melphalan and the melphalyl-peptide in their action on protein synthesis. The peptide derivatives also are less mutagenic than melphalan, according to the SCE assay, furnishing further evidence for the positive effect of natural carrier molecules.

Synthesis of alpha-MSH fragments containing phenylalanine mustard for receptor studies.

For chemical affinity labeling of the melanotropin receptor several alpha-MSH fragments containing phenylalanine mustard were synthesized in solution. Tested in the frog skin bioassay the derivatives roughly preserved the biological activity of the corresponding natural sequences. Alkylating peptides with prolonged biological activity containing phenylalanine mustard in place of arginine, phenylalanine or methionine are inhibitors of alpha melanotropin, suggesting an irreversible binding to reactive nucleophiles on the part of the MSH receptor, where the Met-Glu-His-Phe-Arg sequence is attached.