Novel Crizotinib-GnRH Conjugates Revealed the Significance of Lysosomal Trapping in GnRH-Based Drug Delivery Systems

Several promising anti-cancer drug–GnRH (gonadotropin-releasing hormone) conjugates have been developed in the last two decades, although none of them have been approved for clinical use yet. Crizotinib is an effective multi-target kinase inhibitor, approved against anaplastic lymphoma kinase (ALK)- or ROS proto-oncogene 1 (ROS-1)-positive non-small cell lung carcinoma (NSCLC); however, its application is accompanied by serious side effects. In order to deliver crizotinib selectively into the tumor cells, we synthesized novel crizotinib analogues and conjugated them to a [d-Lys⁶]–GnRH-I targeting peptide. Our most prominent crizotinib–GnRH conjugates, the amide-bond-containing [d-Lys⁶(crizotinib*)]–GnRH-I and the ester-bond-containing [d-Lys⁶(MJ55*)]–GnRH-I, were able to bind to GnRH-receptor (GnRHR) and exert a potent c-Met kinase inhibitory effect. The efficacy of compounds was tested on the MET-amplified and GnRHR-expressing EBC-1 NSCLC cells. In vitro pharmacological profiling led to the conclusion that that crizotinib–GnRH conjugates are transported directly into lysosomes, where the membrane permeability of crizotinib is diminished. As a consequence of GnRHR-mediated endocytosis, GnRH-conjugated crizotinib bypasses its molecular targets—the ATP-binding site of RTKs— and is sequestered in the lysosomes. These results explained the lower efficacy of crizotinib–GnRH conjugates in EBC-1 cells, and led to the conclusion that drug escape from the lysosomes is a major challenge in the development of clinically relevant anti-cancer drug–GnRH conjugates.

Protein expression profile of HT-29 human colon cancer cells after treatment with a cytotoxic daunorubicin-GnRH-III derivative bioconjugate

Targeted delivery of chemotherapeutic agents is a new approach for the treatment of cancer, which provides increased selectivity and decreased systemic toxicity. We have recently developed a promising drug delivery system, in which the anticancer drug daunorubicin (Dau) was attached via oxime bond to a gonadotropin-releasing hormone-III (GnRH-III) derivative used as a targeting moiety (Glp-His-Trp-Lys(Ac)-His-Asp-Trp-Lys(Dau = Aoa)-Pro-Gly-NH₂; Glp = pyroglutamic acid, Ac = acetyl; Aoa = aminooxyacetyl). This bioconjugate exerted in vitro cytostatic/cytotoxic effect on human breast, prostate and colon cancer cells, as well as significant in vivo tumor growth inhibitory effect on colon carcinoma bearing mice. In our previous studies, H-Lys(Dau = Aoa)-OH was identified as the smallest metabolite produced in the presence of rat liver lysosomal homogenate, which was able to bind to DNA in vitro. To get a deeper insight into the mechanism of action of the bioconjugate, changes in the protein expression profile of HT-29 human colon cancer cells after treatment with the bioconjugate or free daunorubicin were investigated by mass spectrometry-based proteomics. Our results indicate that several metabolism-related proteins, molecular chaperons and proteins involved in signaling are differently expressed after targeted chemotherapeutic treatment, leading to the conclusion that the bioconjugate exerts its cytotoxic action by interfering with multiple intracellular processes.

Rationally designed cyclic analogues of luteinizing hormone-releasing hormone: enhanced enzymatic stability and biological properties

This article describes the rational design, synthesis and pharmacological properties of amide-linked cyclic analogues of Luteinizing Hormone-Releasing Hormone (LHRH) with substitutions at positions 1 (Pro), 6 (D-Leu/D-Trp), 9 (Aze) and 10 (BABA/Acp). These LHRH analogues fulfil the conformational requirements that are known in the literature (bend in the 5-8 segment) to be essential for receptor recognition and activation. Although, they are characterised by an overall low binding affinity to the LHRH-I receptor, the cyclic analogues that were studied and especially the cyclo(1-10)[Pro(1), D-Leu(6), BABA(10)] LHRH, exhibit a profoundly enhanced in vitro and in vivo stability and improved pharmacokinetics in comparison with their linear counterpart and leuprolide. Upon receptor binding, cyclo(1-10)[Pro(1), D-Leu(6), BABA(10)] LHRH causes testosterone release in C57/B16 mice (in vivo efficacy) that is comparable to that of leuprolide. Testosterone release is an acutely dose dependent effect that is blocked by the LHRH-I receptor antagonist, cetrorelix. The pharmacokinetic advantages and efficacy of cyclo(1-10)[Pro(1), D-Leu(6), BABA(10)] LHRH render this analogue a promising platform for future rational drug design studies towards the development of non-peptide LHRH mimetics.

[Synthesis of GnRH analogs and their application in targeted gene delivery systems]

A set of GnRH analogues containing nuclear localization signal (NLS) of SV-40 virus large T-antigen have been synthesized using solid phase peptide synthesis and chemical ligation technique. Selective chemical ligation was achieved as a result of hydrazone formation in the course of interaction between NLS hydrazide and GnRH analog modified by pyruvic acid. The efficiency of synthesized peptide carriers was demonstrated in experiments with human cancer cells transfected by reporter luciferase and beta-galactosidase genes or suicide HSV-1 thymidine kinase gene. It was shown that selectivity of action on cancer cells can be achieved as a result of peptide/DNA complex penetration through the cell membrane by GnRH receptor-mediated endocytosis pathway.

[GnRH analogues containing SV-40 virus T-antigen nuclear localization sequence]

To improve the efficiency of anticancer drugs due to their delivery to intracellular targets a set of GnRH analogues containing nuclear localization signal (NLS) of SV-40 virus large T-antigen have been synthesized. NLS was attached to the parent molecule via ε-amino group of D-Lysine in position 1 or 6 of peptide sequence using orthogonal protection strategy. The biological activity studies revealed that incorporation of NLS moiety significantly increases cytotoxic activity of palmitoyl-containing GnRH analogues in vitro. The influence of tested peptides on tumor cells does not accompanied by the destruction of cell membrane, as confirmed in experiments with normal fibroblasts, used as a control.

Structure-activity study on the LH- and FSH-releasing and anticancer effects of gonadotropin-releasing hormone (GnRH)-III analogs

GnRH-III was reported to have selective FSH-releasing activity in rats and significant anticancer potency on human breast cancer cells. To improve either of these effects, 14 analogs were synthesized and investigated for FSH/LH stimulation and breast cancer inhibition. Analogs with single amino acid changes in positions 5-7 or 10 showed small or no difference in the FSH- or LH-releasing activity compared with GnRH-III but their anticancer potency decreased significantly. Modification of the terminal amino acids, side chain cyclization at the 6-8 regions, or combined amino acid changes at positions 4, 6 and/or 8 resulted in the decrease of both effects. Gonadotropin-releasing activity of Arg(8)-GnRH-III was improved 3-11-fold. A copolymer conjugate of GnRH-III showed 2-3-fold anticancer activity while losing endocrine potency.

CONCLUSION

The activation of GnRH-receptors on pituitary and breast cancer cells requires a specific structure and/or conformation that makes possible to improve the anticancer selectivity of GnRH analogs.

Design, synthesis, and molecular modeling of a novel amide-linked cyclic GnRH analogue cyclo(4-9)[Lys4,D-Trp6,Glu9]GnRH: stimulation of gonadotropin gene expression

This report describes the rational design, synthesis, and pharmacological properties of an amide-linked cyclic analogue of gonadotropin-releasing hormone (GnRH) namely Cyclo(4-9)[Lys(4),d-Trp(6),Glu(9)]GnRH. The conformationally restricted analogue is characterized by reduced flexibility of the peptide strand due to the introduction of a beta-turn mimetic through 4,9 residue amide cyclization. The cyclic analogue was found to stimulate gonadotropin gene expression in the goldfish pituitary with similar potency compared to two native forms of GnRH. Simulation studies based on ROE connectivities in linear GnRH and potency of cyclic analogue supports the His(2), Trp(3), Tyr(5) clustering considered important for triggering receptor activation.

Synthesis, in vivo and in vitro biological activity of novel azaline B analogs

Several azaline B analogs (2-10) were synthesized and evaluated for their ability to antagonize GnRH in vitro and for duration of action in inhibiting luteinizing hormone secretion in a castrated male rat assay in vivo. Analogs, 8 (IC(50) = 1.85 nM), and 9 (IC(50) = 1.78 nM), are equipotent with azaline B (1, IC(50) = 1.36 nM) in vitro. Whereas 9 is short acting, 8 is as long acting as azaline B. Other analogs have IC(50) greater than 2.0 nM and are all short acting.

Novel gonadotropin-releasing hormone antagonists with substitutions at position 5

Gonadotropin-releasing hormone (GnRH) antagonists with high potency and improved duration of action are needed for potential clinical applications. We synthesized four new antagonists (2-5) of GnRH homologues to Azaline B (1), with a common core sequence of [Aph(X)5, D-Aph(Cbm)6]Azaline B. In these analogs, (X) contains hydrophobic aromatic moieties (like homoveratoyl in 2, homovanillyl in 3, 2,5-dimethoxyphenylacetyl in 4, and 3,5-dimethoxyphenylacetyl in 5) designed to improve the duration of action over that of Azaline B. These analogs were tested in vitro for their ability to antagonize the GnRH receptor and in vivo for duration of action in a castrated male rat assay. Analogs 2, 4, and 5 were potent in vitro, but were found to be short acting in vivo. However, analog 3 [Aph(Hvn)5,D-Aph(Cbm)6]Azaline B is a potent human GnRH receptor antagonist in vitro (IC50 1.47 nM) and exhibits a longer duration of action than azaline B.

?-Sulfonamido gonadotropin-releasing hormone analogs: synthesis and evaluation of several parent hormone properties

With the aim of producing long-acting analogs of gonadotropin releasing hormone (GnRH), four analogs, containing -X(6) (aa)psi(CH(2)SO(2)NH)-Leu(7) building unit (X(aa)=Gly, Ala, Val or Phe), and a reduced-size analog [Des-Tyr(5)]-GnRH which includes the unit Phe(5)psi(CH(2)SO(2)NH)-Leu(6), and [beta-Ala(6)]-GnRH were synthesized. The peptides were evaluated for their capacity to induce LH-release from rat pituitary cells and to withstand proteolysis by pituitary-derived enzymes, compared with the parent peptide GnRH. Albeit stable toward enzymatic degradation, the sulfonamido containing peptides were only marginally bioactive. [beta-Ala(6)]-GnRH, however, induced LH-release and bound to pituitary receptors nearly as efficiently as GnRH. This analog was also highly stable toward proteolysis suggesting that it may serve as a long-acting GnRH-analog.

Azalide 3,6-ketals: antibacterial activity and structure-activity relationships of aryl and hetero aryl substituted analogues

Aryl and hetero aryl substituted 3,6-ketals of 15-membered azalide analogues were synthesized and were found to have potent in vitro antibacterial activity against veterinary pathogens, including Staphylococcus aureus and Pasteurella multocida.

A bicyclic cispentacin derivative as a novel reverse turn inducer in a GnRH mimetic

A cispentacin-derived bicyclic beta-amino acid (Bic) has been synthesized and incorporated into the 6-position of GnRH. The new GnRH analogue has been characterized with respect to its structure in solution and its activity and affinity toward the human GnRH receptor.

Three forms of gonadotropin-releasing hormone, including a novel form, in a basal salmonid, Coregonus clupeaformis

Multiple forms of GnRH within individual brains may have different functions. However, some vertebrates such as salmonids continue to reproduce even though they have lost or do not express 1 of the 3 forms of GnRH found in most other teleosts. We examined a basal salmonid, lake whitefish, to determine the mechanism by which a reduction in the number of GnRH forms occurs. We identified for the first time 3 distinct GnRHs in a salmonid. One form is novel and is designated whitefish GnRH. The primary structure is pGlu-His-Trp-Ser-Tyr-Gly-Met-Asn-Pro-Gly-NH(2). HPLC and RIA were used for purification followed by Edman degradation for sequence determination. Mass spectroscopy was used to confirm the sequence and amidation of the peptide. The other 2 forms, salmon GnRH and chicken GnRH-II, are identical to the 2 forms found in salmon, which evolved later than whitefish. Synthetic whitefish GnRH is biologically active, as it increased mRNA expression of growth hormone and the alpha-subunit for LH and thyroid-stimulating hormone in dispersed fish pituitary cells. Our data support the hypothesis that the ancestral salmonid had a third GnRH form when the genome doubled (tetraploidization), but the third form was lost later in some salmonids due to chromosomal rearrangements. We suggest that the salmon GnRH form compensated for the loss of the third form.

Guinea pig GnRH: localization and physiological activity reveal that it, not mammalian GnRH, is the major neuroendocrine form in guinea pigs

The isolation of GnRH cDNA from guinea pig hypothalamus predicted a novel form of GnRH with two unique amino acid substitutions relative to all known forms of this essential decapeptide. The predicted substitution at amino acid 2 in guinea pig (gp) GnRH was particularly intriguing because of the proposed importance of position 2 for binding and activation of the GnRH receptor. In the present study, gpGnRH was synthesized, and a specific antibody was generated and used to assess translation of the gpGnRH transcript. The localization of intensely labeled gpGnRH-positive cell bodies and processes in tissue sections through the preoptic area and hypothalamus argue that gpGnRH is the major neuroendocrine form of GnRH in guinea pigs. Guinea pig GnRH stimulated LH release in guinea pigs and increased LH output from guinea pig pituitary fragments, thus demonstrating biological activity in this species. In contrast, gpGnRH demonstrated little ability to stimulate LH release in rats, a species known to possess the highly conserved mammalian GnRH receptor. These findings suggest that: (1) the amino acid substitutions in gpGnRH impede binding to and/or activation of the mammalian GnRH receptor, and (2) the unique amino acid substitutions in gpGnRH are accompanied by changes in the guinea pig GnRH receptor.

Design, synthesis, and evaluation of a long-acting, potent analogue of gonadotropin-releasing hormone

The design, synthesis, and biological evaluation of a gonadotropin-releasing hormone (GnRH) agonist, [D-Lys(6)(1,3,8-trihydroxy-6-carboxyanthraquinone)]GnRH ([D-Lys(6)(Emo)]GnRH), is described. Synthesis of this analogue was carried out in a homogeneous solution as well as on a polymer support. [D-Lys(6)(Emo)]GnRH was found to bind to rat pituitary GnRH receptors (IC(50) = 0.25 nM), to induce luteinizing hormone (LH) release (ED(50) = 27 pM), and to be devoid of any toxicity. This analogue also proved to be a very potent agonist in vivo and exhibited a prolonged bioactivity. Six hours after its administration to rats, LH levels were substantially higher than those of rats treated with a 10-fold higher dose of the parent peptide. Moreover, chronic treatment of adult male rats with [D-Lys(6)(Emo)]GnRH (0.1 nmol/rat) for one week resulted in a further decrease of the weight of the testes and prostate as compared to those of rats that were treated with a higher dose of [D-Lys(6)]GnRH (1 nmol/rat). The prolonged activity of [D-Lys(6)(Emo)]GnRH may be attributed to its emodic acid moiety, which enhances the binding affinity of the analogue to human serum albumin. Indeed, we found that emodic acid binds to human serum albumin almost completely at the examined range of concentrations.

Primary structure of a novel gonadotropin-releasing hormone in the brain of a teleost, Pejerrey

The neuropeptide GnRH is the major regulator of reproduction in vertebrates acting as a first signal from the hypothalamus to pituitary gonadotropes. Three GnRH molecular variants were detected in the brain of a fish, pejerrey (Odontesthes bonariensis), using chromatographic and immunological methods. The present study shows that one form is identical to chicken GnRH-II (sequence analysis and mass spectrometry) and the second one is immunologically and chromatographically similar to salmon GnRH. The third form was proven to be a novel form of GnRH by isolating the peptide from the brain and determining its primary structure by chemical sequencing and mass spectrometry. The sequence of the novel pejerrey GnRH is pGlu-His-Trp-Ser-Phe-Gly-Leu-Ser-Pro-Gly-NH(2), which is different from the known forms of the vertebrate and protochordate GnRH family. The new form of GnRH is biologically active in releasing gonadotropin and GH from pituitary cells in an in vitro assay.

A new photoreactive antagonist cross-links to the N-terminal domain of the gonadotropin-releasing hormone receptor

A new photoreactive gonadotropin-releasing hormone (GnRH) antagonist [Ac-(4-azidobenzoyl)-D-Lys1, D-4-Cl-Phe2, D-Trp3, D-Arg6, D-Ala10]GnRH (PAnt-1) was synthesized and shown to bind covalently to mouse and human GnRH receptors after ultraviolet irradiation. PAnt-1 exhibited high binding affinity (Ki = 3.1 +/- 0.8 nM), and high crosslinking efficiency as shown by loss of 78% of binding sites following crosslinking at saturating concentration. Crosslinking resulted in irreversible receptor blockade as shown by inhibition of GnRH-stimulated inositol phosphate production. PAnt-1 has a photoreactive group at residue 1 of the peptide, a region believed to be critical in determining antagonist versus agonist properties of GnRH analogues. The attachment site of PAnt- to the receptor was localized between residues 11 and 19 of the extracellular N-terminal domain of the receptor by peptide mapping studies using natural sequence differences between human, mouse and sheep GnRH receptors, as well as a panel of GnRH receptor constructs with a series of engineered protease cleavage sites. A disulphide bridge between Cys14 and Cys200 was cleaved during crosslinking, suggesting that Cys14 is the crosslinked residue. These results suggest that peptide GnRH antagonists bind to the receptor with the N-terminal end of the peptide positioned in a site comprising the constrained regions of the N-terminal domain and second extracellular loop in the vicinity of the Cys14-Cys200 disulphide bridge.

Reaction medium engineering in enzymatic peptide fragment condensation: synthesis of eledoisin and LH-RH

The influence of different reaction systems on alpha-chymotrypsin-catalyzed synthesis of eledoisin and LH-RH peptides from (7 + 4) and (5 + 5) fragments was investigated. The peptide yield was determined in the following systems: buffered aqueous media, frozen solutions, organic media, and cosolvent mixtures. The experimental set up was tailored to allow the screening of an array of conditions with minimum consumption of peptide fragments (2.1 and 2.5 mM). The best yields (22% yield for eledoisin and 68% yield for LH-RH) were obtained in buffered aqueous solutions. It was found that the choice of buffer had a strong influence on the peptide yield; boric-borate and ammonium acetate buffers at pH 9, gave the best results. In buffered aqueous systems, both syntheses were scaled up by using a 10-fold increase in fragment concentration (21 and 25 mM). Under these conditions the yields rose to 57% and 80% of eledoisin and LH-RH, respectively. Moreover, during the synthesis of eledoisin and in the presence of boric-borate buffer pH 9, the peptide precipitated from the reaction medium preventing a secondary hydrolysis and facilitating the in situ product purification.

Synthesis of GnRH analogs having direct antitumor and low LH-releasing activity

New chicken I GnRH agonists and antagonists have been synthesized and tested for their biological activities. The common feature of these analogs was that the molecules had a beta-L-aspartyl residue inserted in position 6. The agonist bound to the pituitary still had low endocrinological activity. On the other hand, it exhibited direct antitumor effect in in vitro assays. The endocrinological activity of the antagonist was low; however, it showed potent, direct antitumor activity. These observations might lead to the development of new GnRH analogs with selective antitumor effect.

Synthesis of gonadotropin-releasing hormone III analogs. Structure-antitumor activity relationships

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.

Introduction of the maleimide function onto resin-bound peptides: a simple, high-yield process useful for discriminating among several lysines

Incorporation of epsilon-Adpoc-lysine as a residue in solid phase peptide synthesis allows selective deprotection of this residue on the resin-bound peptide relative to other acid labile groups such as Boc. Premature resin cleavage is avoided. A maleimide group, a useful thiol-capture reagent, was readily introduced by reacting the liberated amino function with an acylating agent containing the maleimide functionality. Acidic cleavage from the resin, with an appropriate scavenging system, afforded peptides that are derivatized with a maleimide functionality on a specific lysine. This is advantageous for producing peptide-carrier conjugates of defined specificity, useful as immunogens, by maleimide-thiol coupling. The derivatization and resin removal chemistries appear to proceed in excellent yield with respect to the maleimide group. The structures were confirmed by tandem mass spectrometry.

Cytotoxic analogs of luteinizing hormone-releasing hormone containing doxorubicin or 2-pyrrolinodoxorubicin, a derivative 500-1000 times more potent

Doxorubicin (DOX) and its daunosamine-modified derivative, 2-pyrrolino-DOX, which is 500-1000 times more active than DOX, were incorporated into agonistic and antagonistic analogs of luteinizing hormone-releasing hormone (LH-RH). The conjugation of DOX with LH-RH analogs was performed by using N-(9-fluorenylmethoxycarbonyl)-DOX-14-O-hemiglutarate, a dicarboxylic acid ester derivative of DOX. Coupling this derivative covalently to the epsilon-amino group of the D-Lys side chain of agonist [D-Lys6]LH-RH or antagonistic analog AC-D-Nal(2)-D-Phe(4Cl)-D-Pal(3)-Ser-Tyr-D-Lys-Leu-Arg-Pro-D-Ala-NH 2 [where Nal(2) = 3-(2-naphthyl)alanine, Pal(3) = 3-(3-pyridyl)alanine, and Phe(4CI) = 4-chlorophenylalanine] was followed by the removal of the 9-fluorenylmethoxycarbonyl protective group to yield cytotoxic derivatives of LH-RH analogs containing DOX. From these DOX containing LH-RH hybrids, intensely potent analogs with daunosamine-modified derivatives of DOX can be readily formed. Thus, cytotoxic LH-RH agonist containing DOX (AN-152) can be converted in a 66% yield by a reaction with a 30-fold excess of 4-iodobutyraldehyde in N,N-dimethylformamide into a derivative having 2-pyrrolino-DOX (AN-207). Hybrid molecules AN-152 and AN-207 fully preserve the cytotoxic activity of their radicals, DOX or 2-pyrrolino-DOX, respectively, in vitro, and also retain the high binding affinity of the peptide hormone portion of the conjugates to rat pituitary receptors for LH-RH. These highly potent cytotoxic analogs of LH-RH were designed as targeted anti-cancer agents for the treatment of various tumors that possess receptors for the carrier peptide. Initial in vivo studies show that the hybrid molecules are much less toxic than the respective cytotoxic radicals incorporated and significantly more active in inhibiting tumor growth.

Betidamino acids: versatile and constrained scaffolds for drug discovery

Betidamino acids (a contraction of "beta" position and "amide") are N'-monoacylated (optionally, N'-monoacylated and N-mono- or N,N'-dialkylated) aminoglycine derivatives in which each N'acyl/alkyl group may mimic naturally occurring amino acid side chains or introduce novel functionalities. Betidamino acids are most conveniently generated on solid supports used for the synthesis of peptides by selective acylation of one of the two amino functions of orthogonally protected aminoglycine(s) to generate the side chain either prior to or after the elongation of the main chain. We have used unresolved Nalpha-tert-butyloxycarbonyl-N'alpha-fluorenylmethoxycarbonyl++ + aminoglycine, and Nalpha-(Nalpha-methyl)-tert-butyloxycarbonyl-N'alpha-fluo renylmethoxycarbonyl aminoglycine as the templates for the introduction of betidamino acids in Acyline [Ac-D2Nal-D4Cpa-D3Pal-Ser-4Aph(Ac)-D4Aph(A c)-Leu-Ilys-Pro-DAla-NH2, where 2Nal is 2-naphthylalanine, 4Cpa is 4-chlorophenylalanine, 3Pal is 3-pyridylalanine, Aph is 4-aminophenylalanine, and Ilys is Nepsilon-isopropyllysine], a potent gonadotropin-releasing hormone antagonist, in order to test biocompatibility of these derivatives. Diasteremneric peptides could be separated in most cases by reverse-phase HPLC. Biological results indicated small differences in relative potencies (<5-fold) between the D and L nonalkylated betidamino acid-containing Acyline derivatives. Importantly, most betide diastereomers were equipotent with Acyline. In an attempt to correlate structure and observed potency, Ramachandran-type plots were calculated for a series of betidamino acids and their methylated homologs. According to these calculations, betidamino acids have access to a more limited and distinct number of conformational states (including those associated with alpha-helices, beta-sheets, or turn structures), with deeper minima than those observed for natural amino acids.

Synthesis of LHRH antagonists suitable for oral administration via the vitamin B12 uptake system

Conjugates have been synthesized between vitamin B12 and two lysyl derivatives of the LHRH antagonist, ANTIDE. Lys6-ANTIDE and Lys8-ANTIDE were both found to have similar activities to the native analogue in the in vitro pituitary cell assay. The in vitro bioactivity of the VB12-ANTIDE conjugates was preserved following linkage using a number of spacers; however, the in vivo bioactivity was lost. In order to produce conjugates which had similar in vivo bioactivity to the native analogue, it was necessary to link the VB12 to the ANTIDE analogues using thiol cleavable spacers. The resultant conjugates had similar activity to ANTIDE both in vitro and in vivo and were also found to be much more water soluble than ANTIDE. These VB12-ANTIDE conjugates show potential utility as water soluble ANTIDE analogues for parenteral use and are protease resistant LHRH antagonist analogues suitable for uptake from the intestine via the VB12-transport system following oral administration.

In vitro and in vivo activities of reduced-size antagonists of luteinizing hormone-releasing hormone

A novel series of octapeptide LHRH antagonists was designed on the basis of the structure of the (2-9) fragment of a LHRH agonist. By adopting a systematic SAR study, we were able to improve first the in vitro activity and then the in vivo LH suppression, raising them up to the range of the decapeptide antagonists NalGlu (51) and A-75998 (50), resulting in A-76154 (49). The octapeptide antagonist A-76154 is the most potent reduced-size LHRH antagonist reported. It suppresses LH in the castrated rat by over 80% for a period of 4 h following sc bolus administration of 30 micrograms/kg.

Synthesis and bioassay of LHRH-antagonists with N-Ac-D-O-phenyltyrosine and N-Ac-D-3-(2-dibenzofuranyl)alanine in position 1

N-Ac-D-O-phenyltyrosine was synthesized via the corresponding azlactone. Resolution of the DL methyl esters was achieved by Subtilisin Carlsberg. Treatment with palladium(II) acetate in trifluoroacetic acid converted N-Ac-D-O-phenyltyrosine into N-Ac-D-3-(2-dibenzofuranyl)alanine. These two amino acids were incorporated instead of N-Ac-D-2-Nal into position 1 of the LHRH-antagonist (N-Ac-D-2-Nal1, D-p-ClPhe2, D-3-Pal3, c-PzACAla5, D-PicLys6, ILys8,D-Ala10)-LHRH. The more rigid N-Ac-D-3-(2-dibenzofuranyl)alanine was structurally more effective than N-Ac-D-O-phenyltyrosine; the AOAs for the corresponding analogs were 82 and 38%, respectively, at 0.5 micrograms. Replacement of c-PzACAla in position 5 by O-phenyltyrosine significantly decreased potency.

Temporary serine protection in solid phase synthesis of LH-RH analogs

Methodology for the large-scale, solid-phase synthesis of nafarelin, I, an LH-RH agonist and RS-26306, III, and LH-RH antagonist, is described. N alpha-Boc protected amino acids were used in the synthesis. The only side-chain-protected amino acids required were BocHis(Tos)-OH and BocSer(t-Bu)-OH. The use of temporary protection on serine eliminates the formation of bis-serine derivatives (II and IV), which presents a major limitation to the use of minimal protection schemes for large-scale synthesis. Using this approach, the side-chain protecting groups are cleaved during the synthesis, and HF deprotection in a separate step is not required.

Conformational analysis and proteolytic processing of synthetic pre-pro-GnRH/GAP protein

Homogeneous pre-pro-GnRH/GAP protein was recently synthesized in 100 mg quantities by solid-phase methods and surprisingly, the synthetic pre-pro-protein, which normally does not escape the endoplasmic reticulum, was found to inhibit the release of prolactin from cultured pituitary cells. This is the first demonstration of significant biological activity associated with a precursor protein and provides the rationale for its further study. We now report the results of our initial examination of the conformational properties of pre-pro-GnRH/GAP protein as a prelude to solving its solution phase conformation by homonuclear 1H-NMR protocols. Thermal and pH titration fluorescence and circular dichroism spectroscopies reveal that the protein is resistant to thermal-induced conformational changes but is particularly sensitive to pH-induced conformational changes; while Asp/Glu and Arg residues may contribute to structural stability, His and Lys residues predominate. Pre-pro-GnRH/GAP is about 30% helix in the range of 2-40 degrees C; however, even at 90 degrees C, the peptide retains nearly 50% of its helix character. There is no evidence for a cooperative transition; for this reason, differential scanning calorimetry failed to yield a defined transition thermogram. Pre-pro-GnRH/GAP apparently does not pass through a transition state as a function of temperature but appears to flex and retain a high percentage of helix structure, resulting in subtle changes in secondary structure. There is no discernible isodichroic point. On either side of the neutral pH range, however, there are dramatic changes in structure that result in nonreversible denaturation of the protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of N-methyl substitution of the peptide bonds in luteinizing hormone-releasing hormone agonists

Each peptide bond in leuprolide (1), deslorelin (13), and nafarelin (24) was separately substituted with N-methyl. The synthesized compounds were tested for in vitro receptor binding, LH release, and stability against chymotrypsin and intestinal degradation. The NMe-Ser4 (30), NMe-Leu7 (33), and Sar10 (35) analogues of nafarelin had pD2 values 2-, 20-, 9-fold higher than their respective parent. All the other N-methyl agonists were less active. For the first time, conversion of LHRH agonists to antagonists was observed as a result of N-methyl substitution in the peptide backbone. [NMe-Phe2,DLeu6,Pro9NHEt]LHRH (4), [NMe-1Nal3,DLeu6,Pro9NHEt]LHRH (6), [NMe-His2,DTrp6,Pro9NHEt]LHRH (14), [NMe-Phe2,DNal6]LHRH (27), and [D2Nal6,NMe-Arg8]LHRH (34) exhibited antagonist responses. Substitutions of NMe-1Nal3, NMe-Ser4, or NMe-Tyr5 in leuprolide rendered the 3-4 peptide bond in these compounds completely stable to chymotrypsin. Examination of the three-dimensional structure of leuprolide when bound to the active site of chymotrypsin, reveals the NH's of residues 3 and 5 are involved in hydrogen bond interactions with the enzyme. N-Methylation at these positions is not only disrupting the hydrogen bond interactions, but is also sterically preventing the substrate from fitting in the enzyme's active site. All the compounds in the leuprolide series were also tested against intestinal degradation using an in vitro rat jejunum sac assay. In this model the pattern of stabilization was similar, but not identical, to that against chymotrypsin. The pharmacokinetics of all the analogues in the leuprolide series and of several others in the deslorelin and nafarelin series were determined. The clearance values of all the three NMe-Tyr5 analogues, 8, 20, and 31 were lower than their respective parents. These slower clearances suggest lower rates of metabolism.

Short-chain analogs of luteinizing hormone-releasing hormone containing cytotoxic moieties

Five hexapeptide and heptapeptide analogs of luteinizing hormone-releasing hormone (LH-RH) were synthesized for use as carriers for cytotoxic compounds. These short analogs were expected to enhance target selectivity of the antineoplastic agents linked to them. Native LH-RH-(3-9) and LH-RH-(4-9) containing D-lysine and D-ornithine at position 6 were amidated with ethylamine and acylated on the N terminus. The receptor-binding affinity of one hexapeptide carrier AJ-41 (Ac-Ser-Tyr-D-Lys-Leu-Arg-Pro-NH-Et) to human breast cancer cell membranes was similar to that of [D-Trp6]LH-RH. Alkylating nitrogen mustards (melphalan, Ac-melphalan), anthraquinone derivatives including anticancer antibiotic doxorubicin, antimetabolite (methotrexate), and cisplatin-like platinum complex were linked to these peptides through their omega-amino group at position 6. The hybrid molecules showed no LH-RH agonistic activity in vitro and in vivo but had nontypical antagonistic effects on pituitary cells in vitro at the doses tested. These analogs showed a wide range of receptor-binding affinities to rat pituitaries and cell membranes of human breast cancer and rat Dunning prostate cancer. Several of these conjugates exerted some cytotoxic effects on MCF-7 breast cancer cell line.

Total solid-phase synthesis and prolactin-inhibiting activity of the gonadotropin-releasing hormone precursor protein and the gonadotropin-releasing hormone associated peptide

The human gonadotropin-releasing hormone precursor protein, pHGnRH (Met-23-Ile69) (preproGnRH), and three of its fragment peptides, pHGnRH (Asp14-Ile69) (gonadotropin-releasing hormone associated peptide--GAP), pHGnRH (Phe38-Ile69), and pHGnRH (Ser47-Ile69), were assembled in a stepwise solid-phase cosynthesis employing Boc/Bzl tactics and an optimized acylation schedule which included recoupling steps with hexafluoro-2-propanol to help overcome the aggregation of the pendant peptide chains of the peptidoresin during difficult couplings. Reversed-phase high-performance liquid chromatography (HPLC) purification yielded products which were characterized by analytical reversed-phase HPLC, ion-exchange chromatography, capillary zone electrophoresis, SDS-polyacrylamide gel electrophoresis, and ion-spray mass spectrometry to reveal a high degree of homogeneity. Biological characterization demonstrated that only GAP stimulated luteinizing hormone and follicle-stimulating hormone release from primary cultures of rat anterior pituitary cells, while GAP, pHGnRH (Phe38-Ile69), and preproGnRH all inhibited prolactin release, with the latter being the most potent at concentrations comparable to bromocryptine. However, only GAP and pHGnRH (Phe38-Ile69) were able to displace a labeled gonadotropin-releasing hormone agonist from binding to rat pituitary membrane preparations. This first demonstration of significant biological activity with a precursor protein also suggests that the gonadotropin-releasing and prolactin release-inhibiting functions of GAP are not mediated through the same pituitary receptors.

The use of reproductive hormones in canine reproduction

Naturally produced and synthetically prepared reproductive hormones are frequently employed as pharmaceuticals in canine reproduction. Specific indications for the use of these preparations are few; too often hormonal therapies are used incorrectly and at times deleteriously. Appropriate uses of reproductive hormones are based on known physiological mechanisms in order to improve reproductive efficiency or to treat specific disorders.

Disulfide bond as peptide-resin linkage in Boc-Bzl SPPS, for potential biochemical applications

Use of disulfide bonds for labile linkage in solid-phase peptide synthesis was investigated using polyacrylic polymers (Expansin). Three bifunctional disulfide handles were synthesized for the introduction of disulfide linkage to the synthesis support. This work showed that only N-Boc aminoethyl 2-propionic acid and N-Boc aminoethyl 2-isobutyric acid were fully compatible with Boc/Bzl peptide synthesis and trifluoromethane sulfonic acid side-chain protection. Qualitative and quantitative synthesis results were comparable to those obtained by conventional peptide synthesis using polyacrylic resins. The resulting peptidyl-resins, which swelled in water or aqueous buffers, may be suitable for various biochemical applications, including use as peptide-resin conjugates for antibody production. Thiolysis by aqueous dithiothreitol released mercapto amide peptides suitable for various uses in solution (e.g., direct coupling with activated protein carrier, specific labeling in the mercapto amide group). Partial thiolysis of the disulfide linkage allowed easy post-synthetic adjustment of the peptide loading of peptidyl-resins.

Novel gonadotropin-releasing hormone antagonists: peptides incorporating modified N omega-cyanoguanidino moieties

In order to minimize the deleterious effects of histamine release resulting from the administration to rats and humans of some potent gonadotropin-releasing hormone (GnRH) antagonists, various arginine residues were replaced with the less basic N omega-cyano-N omega-alkyl- or -arylhomoarginine, -arginine, or -p-aminophenylalanine and N omega-triazolyllysine, -ornithine or -p-aminophenylalanine residues in active analogues. These novel analogues were synthesized on a solid-phase support via a two-step modification of the N omega-NH2 of lysine, ornithine, or p-aminophenylalanine residues in otherwise protected resin bound peptides. Most analogues were tested in the rat antiovulatory assay (AOA) and three in vitro assays; a pituitary cell culture assay, a binding assay to pituitary cell membranes, and a histamine release assay. Introduction of the cyanoguanidino and N omega-triazolyl moieties into GnRH analogues yielded several water-soluble antagonists which showed a desirable therapeutic ratio (low histamine release activity to high in vivo potency). Among them, "Azaline" (10, [Ac-DNal1,DCpa2,DPal3,Lys5(atz), DLys6(atz),ILys8,DAla10]GnRH), inhibited ovulation in the rat by 90% at 2 micrograms/rat with an ED50 in the in vitro histamine release assay comparable to that of GnRH itself.

Gonadotropin releasing hormone antagonists: novel structures incorporating N omega-cyano modified guanidine moieties

A series of GnRH antagonists with substitutions at positions 1, 2, 3, 5 and 6 that included the recently reported homoArg-N omega-cyano-N omega'-alkyl- or Lysine-N epsilon-5'-(3-amino-1H-1,2,4-triazole) [Lys(atz)] amino acid derivatives was synthesized, characterized and tested for antiovulatory and anaphylactoid activities and binding affinity. Overall, these analogs were found to be considerably more soluble at neutral pH than their homologs Nal-Glu or Antide. The decapeptides with these substitutions in positions 5 and/or 6 retained high in vivo potency while those with similar substitutions at positions 1, 2 and 3 were significantly less potent than Nal-Glu or Antide. Of the 16 new analogs reported here, Azaline (Ac-DNal1, DCpa2, DPal3, Lys5(atz),DLys6(atz), ILys8,DA1a10]-GnRH) showed the most promising physico-chemical and biological properties [Lys(atz) = N epsilon-5'-(3-amino-1H-1,2,4-triazole) lysine]. Azaline is readily soluble in dilute buffers at pH 7.0, completely inhibits ovulation at 2.0 to 3.0 micrograms per rat, is equipotent to GnRH in releasing histamine in the rat and has a weaker anaphylactoid response in the rat than other analogs such as Nal-Glu or even Antide.

Synthesis and bioactivities of new LHRH antagonists containing a novel unnatural amino acids at position five

Synthesis and bioactivities of new antagonists of luteinizing hormone releasing hormone (LHRH) with novel unnatural amino acids at position five are reported. Most of them showed some antiovulatory activity at 0.5 microgram/rat and two of them inhibited ovulation completely at 1 microgram/rat using saline as vehicle.

Prolonged inhibition of luteinizing hormone and testosterone levels in male rats with the luteinizing hormone-releasing hormone antagonist SB-75

Inhibitory effects of the potent antagonist of luteinizing hormone-releasing hormone N-Ac-[3-(2-naphthyl)-D-alanine1,4-chloro-D-phenylalanine2,3- (3-pyridyl)-D- alanine3,D-citrulline6,D-alanine10]luteinizing hormone-releasing hormone (SB-75) free of edematogenic effects were investigated in male rats. In a study to determine the effect on luteinizing hormone levels in castrated male rats, SB-75 was injected s.c. in doses of 0.625, 1.25, 2.5, 5.0, and 10 micrograms. Blood samples were taken at different intervals for 48 hr. All doses of SB-75 significantly decreased luteinizing hormone levels for greater than 6 hr (P less than 0.01); this inhibition lasted for greater than 24 hr (P less than 0.01) with a dose of 5.0 micrograms and greater than 48 hr with 10 micrograms (P less than 0.05). Serum testosterone levels were also measured in intact male rats injected with SB-75 in doses of 25, 50, and 100 micrograms. All doses produced a dramatic fall in testosterone to castration levels 6 hr after injection (P less than 0.01); this inhibition of serum testosterone was maintained for greater than 72 hr, but only the 100-micrograms dose could keep testosterone in the castration range for greater than 24 hr (P less than 0.01). In another study using a specific RIA, we obtained the pharmacokinetic release pattern of SB-75 from two sustained delivery formulations of SB-75 pamoate microgranules and examined their effect on serum testosterone. After a single i.m. injection of 20 mg of one batch of microgranules, a large peak corresponding to SB-75 at 45.8 ng/ml was observed, corresponding to the "burst" effect. Levels of the analog decreased to 19.6 ng/ml on day 2, gradually reached a concentration of 4.7 ng/ml on day 7, and kept declining thereafter. Testosterone levels were reduced on day 1 (P less than 0.01) and were maintained at low values for greater than 7 days (P less than 0.05). In rats injected with 10 mg of SB-75 pamoate microgranules of the second batch, SB-75 serum levels rose to 33 ng/ml 3 hr after administration and then fell gradually to approximately 3.4 ng/ml on day 16, but a second small peak was seen on day 28. Subsequently, the analog levels decreased slowly to 2.9 ng/ml on day 42. At this time, testosterone serum levels were still significantly lower than in controls. These overall results demonstrate the efficacy of SB-75 in the suppression of the pituitary-gonadal axis. This modern luteinizing hormone-releasing hormone antagonist can possibly be used for treating sex hormone-sensitive cancers and other disorders.

Repetitive BOP coupling (REBOP) in solid phase peptide synthesis. Luliberin synthesis as model

The coupling reagent (benzotriazol-1-yloxy)tris-(dimethylamino)phosphonium (BOP) hexafluorophosphate was tested in the synthesis of luliberin (LH-RH) with inexpensive classically protected Boc-amino acids, in slight excess, and benzhydryl amino resin, without any other additive. The good solubility of this reagent and its by-products is of particular interest for automated peptide synthesis. [D-His2]LH-RH was also synthesized and compared with LH-RH by proton nuclear magnetic resonance spectroscopy. As shown by the biological tests and the high performance liquid chromatography study, unprotected pyroGlu and Boc-His can be used without any significant racemization but Boc-His(Boc) was found to be preferable since it gave no detectable racemization and no by-products. The difficult isolation of the minority D-derivative from the crude preparation of LH-RH was resolved by a recycling procedure in reversed phase HPLC.

Active reduced-size hexapeptide analogues of luteinizing hormone-releasing hormone

A series of reduced-size hexapeptide analogues of LH-RH were synthesized that contain the residues corresponding to amino acid positions 4-9 and are linked to various carboxylic acids in place of residue 3. These compounds were tested in vitro in the rat pituitary receptor binding and LH release assays. A wide range of binding affinities was obtained up to and exceeding that of LH-RH. Both agonists and antagonists were obtained. From the SAR studies, it appears that a very precise size, length, and shape of the substituent at position 3 is required to achieve agonist activity, whereas the structural requirements for antagonist activity appear to be much less stringent. Depending on the nature of the substituent at positions 6 and 4, the biological response switches from antagonist to agonist or vice versa. The results suggest that conformational changes at position 6 or 4 feed back to the substituent at position 3, which induces the change from agonist to antagonist. The most potent compounds in the series were tested in vivo and found to be active.

Highly potent analogues of luteinizing hormone-releasing hormone containing D-phenylalanine nitrogen mustard in position 6

The nitrogen mustard derivatives of 4-phenylbutyric acid and L-phenylalanine, called chlorambucil (Chl) and melphalan (Mel), respectively, have been incorporated into several peptide hormones, including luteinizing hormone-releasing hormone (LH-RH). The alkylating analogues of LH-RH were prepared by linking Chl, as an N-acyl moiety, to the complete amino acid sequence of agonistic and antagonistic analogues. These compounds, in particular the antagonistic analogues, showed much lower potency than their congeners carrying other acyl groups. To obtain highly potent alkylating analogues of LH-RH, the D enantiomer of Mel was incorporated into position 6 of the native hormone and some of its antagonistic analogues. Of the peptides prepared, [D-Mel6]LH-RH (SB-05) and [Ac-D-Nal(2)1,D-Phe(pCl)2,D-Pal(3)3,Arg5,D-Mel6,D-Ala10++ +]LH-RH [SB-86, where Nal(2) is 3-(2-naphthyl)alanine and Pal(3) is 3-(3-pyridyl)alanine] possessed the expected high agonistic and antagonistic activities, respectively, and also showed high affinities for the membrane receptors of rat pituitary cells, human breast cancer cells, human prostate cancer cells, and rat Dunning R-3327 prostate tumor cells. These two analogues exerted cytotoxic effects on human and rat mammary cancer cells in vitro. Thus these two D-Mel6 analogues seem to be particularly suitable for the study of how alkylating analogues of LH-RH could interfere with intracellular events in certain cancer cells.

Highly potent metallopeptide analogues of luteinizing hormone-releasing hormone

Metal complexes related to the cytotoxic complexes cisplatin [cis-diamminedichloroplatinum(II)] and transbis(salicylaldoximato)copper(II) were incorporated into suitably modified luteinizing hormone-releasing hormone (LH-RH) analogues containing D-lysine at position 6. Some of the metallopeptides thus obtained proved to be highly active LH-RH agonists or antagonists. For instance, SB-40, a PtCl2-containing metallopeptide in which platinum is coordinated to an N epsilon-(DL-2,3-diaminopropionyl)-D-lysine residue [D-Lys(DL-A2pr] at position 6, showed 50 times higher LH-releasing potency than the native hormone. SB-95, [Ac-D-Nal(2)1,D-Phe(pCl)2, D-Pal(3)2, Arg5,D-Lys[DL-A2pr(Sal2Cu)]6,D-Ala10]LH-RH, where Nal(2) is 3-(2-naphthyl)alanine, Pal(3) is 3-(3-pyridyl)alanine, and copper(II) is coordinated to the salicylideneimino moieties resulting from condensation of salicylaldehyde with D-Lys(DL-A2pr)6, caused 100% inhibition of ovulation at a dose of 3 micrograms in rats. Most metallopeptide analogues of LH-RH showed high affinities for the membrane receptors of rat pituitary and human breast cancer cells. Some of these metallopeptides had cytotoxic activity against human breast cancer and prostate cancer cell lines in vitro (this will be the subject of a separate paper on cytotoxicity evaluation). Such cytostatic metallopeptides could be envisioned as targeted chemotherapeutic agents in cancers that contain receptors for LH-RH-like peptides.

Photoaffinity labelling of gonadotropin releasing hormone binding sites in human epithelial ovarian carcinomata

A photoaffinity labelled derivative of [D-Lys6]-GnRH was prepared with a bifunctional photolabile reagent (4-azidobenzoyl)-N-hydroxysuccinimide. In rat pituitary membranes, this analog retained high binding affinity (Ka = 0.12 x 10(9) M-1) consistent with a single class of receptors. The analog was iodinated and used for the identification of GnRH binding sites in human epithelial ovarian carcinomata. By sodium dodecyl sulfate electrophoresis in 10% polyacrylamide gel the presence of two labelled components could be demonstrated: a high molecular weight component of 63,200 and a smaller component of 46,000. Competition experiments with unlabelled ligand suggest that it is the high molecular weight component which specifically binds GnRH.

[Gonadotropins, LHRH and analogs]

Beside the gonadotrophines, that are used for therapeutical purposes since a rather long time, LHRH appeared as well as similar substances, whose actual clinical uses are mentioned.

New antagonists of LHRH. II. Inhibition and potentiation of LHRH by closely related analogues

Modifications of the previously described LHRH antagonists, [Ac-D-Nal(2)1, D-Phe(4Cl)2, D-Trp3, D-Cit6, D-Ala10]LHRH and the corresponding D-Hci6 analogue, have been made to alter the hydrophobicity of the N-terminal acetyl-tripeptide portion. Substitution of D-Trp3 with the less hydrophobic D-Pal(3) had only marginal effects on the antagonistic activities and receptor binding potencies of the D-Cit/D-Hci6 analogues, but it appeared to further improve the toxicity lowering effect of D-Cit/D-Hci6 substitution. Antagonists containing D-Pal(3)3 and D-Cit/D-Hci6 residues, i.e. [Ac-D-Nal(2)1, D-Phe(4Cl)2, D-Pal(3)3, D-Cit6, D-Ala10]LHRH (SB-75) and [Ac-D-Nal(2)1, D-Phe(4Cl)2, D-Pal(3)3, D-Hci6, D-Ala10]LHRH (SB-88), were completely free of the toxic effects, such as cyanosis and respiratory depression leading to death, which have been observed in rats with the D-Trp3, D-Arg6 antagonist and related antagonists. Replacement of the N-acetyl group with the hydrophilic carbamoyl group caused a slight decrease in antagonistic activities, particularly in vitro. Introduction of urethane type acyl group such as methoxycarbonyl (Moc) or t-butoxycarbonyl (Boc) led to analogues that showed LHRH-potentiating effect. The increase in potency induced by these analogues, e.g. [Moc-D-Nal(2)1, D-Phe(4Cl)2, D-Trp3, D-Cit6, D-Ala10]LHRH and [Boc-D-Phe1, D-Phe(4Cl)2, D-Pal(3)3, D-Cit6, D-Ala10]LHRH, was 170-260% and persisted for more than 2 h when studied in a superfused rat pituitary system.

Human follicular gonadotropin releasing peptide analogs. Evaluation of biological (in vitro) and immunological activity

Human follicular gonadotropin releasing peptide (hF-GRP) has been shown to stimulate pituitary LH and FSH secretion in vitro. Six hF-GRP analogs have been synthesized and evaluated for gonadotropin releasing activity in a rat anterior pituitary primary cell culture system. A tyrosine analog of hF-GRP, [Tyr4]-hF-GRP, retained comparable biological activity in releasing gonadotropins. However, acetylation of hF-GRP in Ac-hF-GRP greatly reduced the in vitro activity. The shorter segments of hF-GRP, hF-GRP-(5-14), and hF-GRP-(10-14), were tested for LH and FSH releasing activity, and it was found that the decapeptide retained moderate activity while the activity of the pentapeptide was markedly lower than hF-GRP. The baboon alpha 1 antitrypsin-(27-40) peptide, b-alpha 1 AT-(27-40), is relatively less potent in releasing LH than hF-GRP. Interestingly, the baboon peptide is more potent (2.5-fold) in releasing FSH under identical conditions. The effect of hF-GRP in releasing LH and FSH was not affected by the presence of LHRH antagonists in cell culture systems. When these peptides were tested for immunological activity in a hF-GRP radioimmunoassay, it was found that hF-GRP and [Tyr4]-hF-GRP have comparable activities. The C-terminal decapeptide of hF-GRP is more active (1.5-fold) in the RIA, and the C-terminal pentapeptide had only one third of the immunoreactivity. The b-alpha 1-AT-(27-40) failed to cross-react in the RIA even at a concentration of 20 micrograms per tube.

A synthetic luteinizing hormone releasing hormone vaccine. I. Conjugation and specificity trials in BALB/c mice

The immunobiology of luteinizing hormone releasing hormone (LHRH) was explored, to provide a conceptual and practical basis for the use of LHRH in immunocastration. Cysteine substituted analogues of LHRH were synthesized including Cys1-LHRH (C1-LHRH), Cys6-LHRH (C6-LHRH) and Cys10-LHRH (C10-LHRH). These were reacted to carrier molecules using the heterobifunctional cross-linking reagent m-maleimidobenzoylsulfosuccinimide ester (SMBS), producing peptide-carrier conjugates of known peptide content and conjugation orientation. This reaction regime was found to be rapid, efficient and allowed for easy control of peptide to carrier ratios. Conjugates were used in active immunization trials in BALB/c mice to characterize the murine immune response against LHRH. BALB/c mice were shown to have the capacity to recognise all three cysteine substituted LHRH analogues and to produce antibodies cross-reactive with native LHRH. The specificity of LHRH antisera generated was found to be dependent on the site of conjugation of the peptide to carrier molecule. C1-LHRH generated carboxy terminal directed antibodies, C10-LHRH generated amino terminal directed antibodies, while C6-LHRH could generate amino terminal directed or carboxy terminal directed antibodies, or both within a given animal. No intrinsically immunodominant epitopes were seen within the LHRH molecule.

Photosubstitution of cymantrenylalanine as a tool in peptide chemistry. Synthesis and biological activity of new GnRH analogs

Beyond its aromatic character and important hydrophobicity, cymantrenylalanine, a metallocenic amino-acid, can be easily photosubstituted with phosphine and phosphite ligands to readily yield new analogs with different hydrophobicity and steric hindrance. The incorporation of phosphine and phosphite ligands is described. As an illustration of the offered possibilities, the synthesis and the biological activity of two new GnRH analogs modified in position 6 are reported.

Photoaffinity labeling of pituitary GnRH receptors: significance of the position of photolabel on the ligand

Photoreactive derivatives of GnRH and its analogues were prepared by incorporation of the 2-nitro-4(5)-azidophenylsulfenyl [2,4(5)-NAPS] group into amino acid residues at positions 1, 3, 6, or 8 of the decapeptide sequence. The modification of Trp3 by the 2,4-NAPS group led to a complete loss of the luteinizing hormone (LH) releasing as well as LH-release-inhibiting activity of the peptide. The [D-Lys(2,4-NAPS)]6 analogue was a very potent agonist that, after covalent attachment by photoaffinity labeling, caused prolonged LH secretion at a submaximal rate. [Orn(2,4-NAPS)]8-GnRH, a full agonist with a relative potency of 7% of GnRH, after photoaffinity labeling caused prolonged maximal LH release from cultured pituitary cells. In contrast, [Orn(2,5-NAPS)]8-GnRH, although being equipotent with the 2,4-NAPS isomer in terms of LH releasing ability, was unable to cause prolonged LH release after photoaffinity labeling. Thus, [Orn(2,4-NAPS)]8-GnRH is a very effective photolabeling ligand of the functionally significant pituitary GnRH receptor. Based on this compound, a pituitary peptidase resistant derivative, D-Phe6,[Orn(2,4-NAPS)]8-GnRH-(1-9)-ethylamide, was synthesized. This derivative showed high-affinity binding to pituitary membranes with a Kd comparable to those of other GnRH analogues. A radioiodinated form of this peptide was used for pituitary GnRH-receptor labeling. This derivative labeled 59- and 57-kDa proteins in rat and 58- and 56-kDa proteins in bovine pituitary membrane preparations, respectively. This peptide also labeled pituitary GnRH receptors in the solubilized state and therefore appears to be a suitable ligand for the isolation and further characterization of the receptor.

Design of potent cyclic gonadotropin releasing hormone antagonists

In order to improve the biological potency of cyclic gonadotropin releasing hormone (GnRH) antagonists, we have synthesized analogues, the conformations of which were restrained through internal side chain/side chain amide bridges linking aspartic acid or glutamic acid and L-2,3-diaminopropionic acid or L-ornithine. A disulfide bridge linking L-cysteine residues was also introduced. Residues belonging to the bridge spanned from position 4 to positions 9 or 10. Two series of analogues were synthesized and are characterized by residues at positions 1 [Ac-D-3-(2'-naphthyl)alanine], 2 [D-(4-chlorophenyl)alanine or D-(4-fluorophenyl)alanine], 3 [D-3-(3'-pyridyl)alanine or D-tryptophan], 5 (arginine or tyrosine), and 6 [D-3-(3'-pyridyl)alanine or D-arginine], respectively. These substitutions were selected in an effort to optimize high biopotency for inhibition of luteinizing hormone secretion, minimization of histamine release activity, and high (relative) hydrophilicity. The most potent analogues in the antiovulatory assay were cyclo(4-10) [Ac-DNal1,DCpa2,DPal3,(Asp4 or Glu4),Arg5,DPal]6,Dpr10]GnRH (compounds 5 and 7), which were fully active at ca. 12.5 micrograms/rat in the first series, and cyclo(4-10)[Ac-DNal1,DFpa2,DTrp3,Asp4,DArg6++ +,Dpr10]GnRH (compound 12), which was fully active at 2.5 micrograms/rat in the second.