Synthesis and biological evaluation of compact, conformationally constrained bifunctional opioid agonist - neurokinin-1 antagonist peptidomimetics.

A reported mixed opioid agonist - neurokinin 1 receptor (NK1R) antagonist 4 (Dmt-D-Arg-Aba-Gly-(3',5'-(CF3)2)NMe-benzyl) was modified to identify important features in both pharmacophores. The new dual ligands were tested in vitro and subsequently two compounds (lead structure 4 and one of the new analogues 22, Dmt-D-Arg-Aba-ß-Ala-NMe-Bn) were selected for in vivo behavioural assays, which were conducted in acute (tail-flick) and neuropathic pain models (cold plate and von Frey) in rats. Compared to the parent opioid compound 33 (without NK1R pharmacophore), hybrid 22 was more active in the neuropathic pain models. Attenuation of neuropathic pain emerged from NK1R antagonism as demonstrated by the pure NK1R antagonist 6. Surprisingly, despite a lower in vitro activity at NK1R in comparison with 4, compound 22 was more active in the neuropathic pain models. Although potent analgesic effects were observed for 4 and 22, upon chronic administration, both manifested a tolerance profile similar to that of morphine and cross tolerance with morphine in a neuropathic pain model in rat.

Identification of Dmt-D-Lys-Phe-Phe-OH as a highly antinociceptive tetrapeptide metabolite of the opioid-neurotensin hybrid peptide PK20.

BACKGROUND: Recently, we presented a novel compound (PK20, Dmt-D-Lys-Phe-Phe-Lys-Lys-Pro-Phe-Tle-Leu-OH) that targets single entity opioid and neurotensin pharmacophores. This endomorphin-2-like opioid peptide was introduced as a highly active analgesic because it elicited a strong dose- and time-dependent antinociceptive response when administered centrally and peripherally. Its pain-relieving activity was observed as rapidly as 5 min after drug injection. Such promising results led us to perform further studies, such as determining the resistance to enzymatic degradation, which resulted in obtaining a very stable opioid pharmacore PK20 metabolite. METHODS: The synthesis of PK20 and its N-terminal tetrapeptide fragment has been accomplished using solid phase peptide chemistry. The biological stability of peptides has been measured in human serum and analyzed by HPLC/MS. Peptides were pharmacologically characterized in in vitro MOP and DOP receptor binding as well as [(35)S]GTPγS receptor binding assays. Antinociceptive properties of compounds were measured by in vivo assays in C57Bl6 mice after intravenous or intrathecal applications. RESULTS: Dmt-D-Lys-Phe-Phe-OH (PK20M), an N-terminal tetrapeptide metabolite of the opioid-neurotensin hybrid peptide PK20, is characterized by a long duration of action, as demonstrated by a preserved, long-lasting analgesic effect even 2 h post-injection (average % MPE = 69.33). In rat brain membranes, PK20M efficiently displaced both the MOP and DOP receptor selective radioprobes [(3)H]DAMGO and [(3)H]DIDI (pKi of 9.52 and 7.86, respectively) and potently stimulated [(35)S]GTPγS binding, proving full agonism at both receptor types. In the [(35)S]GTPγS assay, which measured the agonist-mediated G protein activation, PK20M together with PK20 and Met-enkephalin were potent stimulators of the regulatory G proteins. The relative affinities of PK20M for the µ and δ receptor subtypes revealed µ-receptor selectivity. CONCLUSION: The novel MOP receptor selective metabolite has been shown to possess opioid subtype receptor selectivity, high potency, and effective analgesic activities as measured in various bioassays.

[3H]IVDE77, a novel radioligand with high affinity and selectivity for the insulin-regulated aminopeptidase.

The hexapeptide angiotensin IV (Ang IV) induces diverse biological effects such as memory enhancement and protection against ischemic stroke. Studies on the mechanism of Ang IV however are hampered by its instability and its lack of selectivity. The high-affinity binding site for Ang IV is the insulin-regulated aminopeptidase (IRAP, EC 3.4.11.3), but Ang IV also acts as a weak agonist for the Ang II-receptor (AT1), implying the need for stable and highly selective Ang IV-analogues. Here we present the screening of novel Ang IV-analogues, selected on basis of high affinity for IRAP, high selectivity (compared to aminopeptidase N and the AT1 receptor) and resistance against proteases. The selected compound IVDE77 possesses a number of advantages compared to Ang IV: (i) it has a 40 times higher affinity for IRAP (Ki 1.71 nM), (ii) it does not activate the AT1 receptor, (iii) it is easily radiolabeled with tritium and (iv) it is resistant to proteolysis, even in human plasma. In addition, pre-treatment of intact CHO-K1 cells with IVDE77 led to a virtually complete inhibition of subsequent intracellular accumulation of [(3)H]IVDE77-IRAP complexes. IVDE77 thus represents the first Ang IV-analogue able to abolish IRAP-availability completely at the cell surface in vitro. In summary, IVDE77 is a useful tool for the detection of IRAP under physiological conditions, and may contribute to elucidating the mechanism of Ang IV to ascertain which functions are IRAP-dependent.

In vivo antinociception of potent mu opioid agonist tetrapeptide analogues and comparison with a compact opioid agonist-neurokinin 1 receptor antagonist chimera.

BACKGROUND: An important limiting factor in the development of centrally acting pharmaceuticals is the blood-brain barrier (BBB). Transport of therapeutic peptides through this highly protective physiological barrier remains a challenge for peptide drug delivery into the central nervous system (CNS). Because the most common strategy to treat moderate to severe pain consists of the activation of opioid receptors in the brain, the development of active opioid peptide analogues as potential analgesics requires compounds with a high resistance to enzymatic degradation and an ability to cross the BBB. RESULTS: Herein we report that tetrapeptide analogues of the type H-Dmt1-Xxx2-Yyy3-Gly4-NH2 are transported into the brain after intravenous and subcutaneous administration and are able to activate the µ- and δ opioid receptors more efficiently and over longer periods of time than morphine. Using the hot water tail flick test as the animal model for antinociception, a comparison in potency is presented between a side chain conformationally constrained analogue containing the benzazepine ring (BVD03, Yyy3: Aba), and a "ring opened" analogue (BVD02, Yyy3: Phe). The results show that in addition to the increased lipophilicity through amide bond N-methylation, the conformational constraint introduced at the level of the Phe3 side chain causes a prolonged antinociception. Further replacement of NMe-D-Ala2 by D-Arg2 in the tetrapeptide sequence led to an improved potency as demonstrated by a higher and maintained antinociception for AN81 (Xxx2: D-Arg) vs. BVD03 (Xxx2: NMe-D-Ala). A daily injection of the studied opioid ligands over a time period of 5 days did however result in a substantial decrease in antinociception on the fifth day of the experiment. The compact opioid agonist-NK1 antagonist hybrid SBCHM01 could not circumvent opioid induced tolerance. CONCLUSIONS: We demonstrated that the introduction of a conformational constraint has an important impact on opioid receptor activation and subsequent antinociception in vivo. Further amino acid substitution allowed to identify AN81 as an opioid ligand able to access the CNS and induce antinociception at very low doses (0.1 mg/kg) over a time period up to 7 hours. However, tolerance became apparent after repetitive i.v. administration of the investigated tetrapeptides. This side effect was also observed with the dual opioid agonist-NK1 receptor antagonist SBCHM01.

Conformational constraints in angiotensin IV to probe the role of Tyr², Pro5 and Phe6.

The aromatic amino acids Tyr and Phe in angiotensin IV (Ang IV) were conformationally constrained by the use of ß-Me substituted analogs, or cyclic constrained analogs. None of these modifications was allowed for Tyr¹, while only e-ß-MePhe6 substitution resulted in an AngIV analog with high IRAP potency and selectivity versus AP-N or the AT1 receptor. This indicates an important role of the orientation of the Phe6 for inducing selectivity. Pro5 replacement with 2-aminocyclopentanecarboxylic acid maintained IRAP potency and abolished AT1 affinity. These results confirm the importance of conformational constrained amino acids to generate selectivity in bioactive peptides.

PK20, a new opioid-neurotensin hybrid peptide that exhibits central and peripheral antinociceptive effects.

BACKGROUND: The clinical treatment of various types of pain relies upon the use of opioid analgesics. However most of them produce, in addition to the analgesic effect, several side effects such as the development of dependence and addiction as well as sedation, dysphoria, and constipation. One solution to these problems are chimeric compounds in which the opioid pharmacophore is hybridized with another type of compound to incease antinociceptive effects. Neurotensin-induced antinociception is not mediated through the opioid system. Therefore, hybridizing neurotensin with opioid elements may result in a potent synergistic antinociceptor. RESULTS: Using the known structure-activity relationships of neurotensin we have synthesized a new chimeric opioid-neurotensin compound PK20 which is characterized by a very strong antinociceptive potency. The observation that the opioid antagonist naltrexone did not completely reverse the antinociceptive effect, indicates the partial involvement of the nonopioid component in PK20 in the produced analgesia. CONCLUSIONS: The opioid-neurotensin hybrid analogue PK20, in which opioid and neurotensin pharmacophores overlap partially, expresses high antinociceptive tail-flick effects after central as well as peripheral applications.

A new structural motif for mu-opioid antagonists.

On the basis of the structural features of the Dmt-Tic pharmacophore, a new motif leading to a fairly potent mu-opioid antagonist is described. This motif contains the 4-amino-1,2,4,5-tetrahydro-2-benzazepine-3-one skeleton as a substitute for the Tic residue, which provides the conformational constraint compatible with the mu-opioid receptor. The stereoselective synthesis of four stereoisomers is performed starting from homochiral 2',6'-dimethyltyrosine (Dmt) and o-aminomethylphenylalanine.

Solid-supported solution-phase synthesis of 4-amino-1,2,4,5-tetrahydro-2-benzazepine-3-ones.

Starting from Boc-o-aminomethylphenylalanine, a solution-phase parallel synthesis of 2,4-substituted 4-amino-1,2,4,5-tetrahydro-2-benzazepine-3-ones is described. This heterocycle has two nitrogen functions, which are differentiated and can be selectively substituted. The sources of diversity are aldehydes for the R(1) position and carboxylic acids, sulfonyl chlorides, or isocyanates for the R(2) position. High-throughput synthesis and purification of this multistep synthetic sequence was accomplished using polymer-bound reagents and scavengers and liquid-liquid extraction protocols, and a small library of compounds was prepared. Polymer-bound cyanoborohydride was found to work well for the reductive amination. Scavenging of excess of amine was performed by polymer-bound benzaldehyde, and cyclization was performed in the presence of polymer-bound coupling reagent 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC). After Boc-deprotection, the second nitrogen can be acylated using carboxylic acids, sulfonylated or converted to a urea. The acylation is again performed by polymer-bound EDC. Excellent yields and purities were obtained.