Partial retro-inverso, retro, and inverso modifications of hydrazide linked bifunctional peptides for opioid and cholecystokinin (CCK) receptors

The Effects of Side-Chain Configurations of a Retro-Inverso-Type Inhibitor on the Human T-Cell Leukemia Virus (HTLV)-1 Protease

In this study, the effects of side-chain configurations of D-Ile residues of a retro–inverso (RI)-type inhibitor on the human T-cell leukemia virus type 1 (HTLV-1) protease containing a hydroxyethylamine dipeptide isostere were clarified. Prior to evaluation using the RI-type inhibitor, the effects of side-chain configurations of Ile residues of the substrate peptide on the HTLV-1 protease were examined to estimate the influence of side-chain configurations on enzyme activity. Based on the estimation of the influence of side-chain configurations on protease affinity, the RI-type inhibitors containing a D-allo-Ile residue in the corresponding substrate sequence, instead of a D-Ile residue, were synthesized via 9-fluorenylmethoxycarbonyl-based solid-phase peptide synthesis. Refolded recombinant HTLV-1 protease (1-116, L40I) was used for the simple and short evaluation of the inhibitory activities of the synthesized RI-type inhibitors. The results clearly indicated that mimicking the whole topology, comprising both the main- and side-chain structures of the parent inhibitor, is effective for the design of potent RI-modified protease inhibitors.

Fentanyl Structure as a Scaffold for Opioid/Non-Opioid Multitarget Analgesics

One of the strategies in the search for safe and effective analgesic drugs is the design of multitarget analgesics. Such compounds are intended to have high affinity and activity at more than one molecular target involved in pain modulation. In the present contribution we summarize the attempts in which fentanyl or its substructures were used as a μ-opioid receptor pharmacophoric fragment and a scaffold to which fragments related to non-opioid receptors were attached. The non-opioid ‘second’ targets included proteins as diverse as imidazoline I₂ binding sites, CB₁ cannabinoid receptor, NK₁ tachykinin receptor, D₂ dopamine receptor, cyclooxygenases, fatty acid amide hydrolase and monoacylglycerol lipase and σ₁ receptor. Reviewing the individual attempts, we outline the chemistry, the obtained pharmacological properties and structure-activity relationships. Finally, we discuss the possible directions for future work.

In Vivo, In Vitro and In Silico Studies of the Hybrid Compound AA3266, an Opioid Agonist/NK1R Antagonist with Selective Cytotoxicity

AA3266 is a hybrid compound consisting of opioid receptor agonist and neurokinin-1 receptor (NK1R) antagonist pharmacophores. It was designed with the desire to have an analgesic molecule with improved properties and auxiliary anticancer activity. Previously, the compound was found to exhibit high affinity for μ- and δ-opioid receptors, while moderate binding to NK1R. In the presented contribution, we report on a deeper investigation of this hybrid. In vivo, we have established that AA3266 has potent antinociceptive activity in acute pain model, comparable to that of morphine. Desirably, with prolonged administration, our hybrid induces less tolerance than morphine does. AA3266, contrary to morphine, does not cause development of constipation, which is one of the main undesirable effects of opioid use. In vitro, we have confirmed relatively strong cytotoxic activity on a few selected cancer cell lines, similar to or greater than that of a reference NK1R antagonist, aprepitant. Importantly, our compound affects normal cells to smaller extent what makes our compound more selective against cancer cells. In silico methods, including molecular docking, molecular dynamics simulations and fragment molecular orbital calculations, have been used to investigate the interactions of AA3266 with MOR and NK1R. Insights from these will guide structural optimization of opioid/antitachykinin hybrid compounds.

Gram-Scale Preparation of C-Terminal-Modified Enkephalin Analogues by Typical Liquid-Phase Peptide Synthesis

This article describes the gram-scale liquid-phase peptide synthesis of C-terminal-modified enkephalin analogues that possess high analgesic efficacy in animals, high potency for mu and delta opioid receptors, and high metabolic stability and potential blood-brain barrier permeability. Despite the long cycle time and tedious purification steps, liquid-phase synthesis is still a preferred method for large-scale peptide synthesis due to its cost effectiveness (i.e., amount of amino acids and reagents required), easy detection, and isolation of impurities compared with solid-phase synthesis. A robust liquid-phase synthesis protocol is described, involving BOP-assisted coupling and Boc deprotection, which has been well established in the laboratory and is a useful synthetic protocol for cost-effective production of peptide drugs. © 2019 by John Wiley & Sons, Inc.

Novel Molecular Strategies and Targets for Opioid Drug Discovery for the Treatment of Chronic Pain

Opioid drugs like morphine and fentanyl are the gold standard for treating moderate to severe acute and chronic pain. However, opioid drug use can be limited by serious side effects, including constipation, tolerance, respiratory suppression, and addiction. For more than 100 years, we have tried to develop opioids that decrease or eliminate these liabilities, with little success. Recent advances in understanding opioid receptor signal transduction have suggested new possibilities to activate the opioid receptors to cause analgesia, while reducing or eliminating unwanted side effects. These new approaches include designing functionally selective ligands, which activate desired signaling cascades while avoiding signaling cascades that are thought to provoke side effects. It may also be possible to directly modulate downstream signaling through the use of selective activators and inhibitors. Separate from downstream signal transduction, it has also been found that when the opioid system is stimulated, various negative feedback systems are upregulated to compensate, which can drive side effects. This has led to the development of multi-functional molecules that simultaneously activate the opioid receptor while blocking various negative feedback receptor systems including cholecystokinin and neurokinin-1. Other novel approaches include targeting heterodimers of the opioid and other receptor systems which may drive side effects, and making endogenous opioid peptides druggable, which may also reduce opioid mediated side effects. Taken together, these advances in our molecular understanding provide a path forward to break the barrier in producing an opioid with reduced or eliminated side effects, especially addiction, which may provide relief for millions of patients.

Investigational peptide and peptidomimetic μ and δ opioid receptor agonists in the relief of pain

INTRODUCTION

Current methods for treating prolonged and neuropathic pain are inadequate and lead to toxicities that greatly diminish quality of life. Therefore, new approaches to the treatment of pain states are needed to address these problems.

AREAS COVERED

The review primarily reviews approaches that have been taken in the peer-reviewed literature of multivalent ligands that interact with both μ and δ opioid receptors as agonists, and in some cases, also with pharmacophores for antagonist ligands that interact with other receptors as antagonists to block pain.

EXPERT OPINION

Although there are a number of drugs currently on the market for the treatment of pain; none of them are 100% successful. In the authors' opinion, it is clear that new directions and modalities are needed to better address the treatment of prolonged and neuropathic pain; one drug or class clearly is not the answer for all pain therapy. Undoubtedly, there are many different phenotypes of prolonged and neuropathic pain and this should be one avenue to further develop appropriate therapies.

Stability, toxicity, and biological activity of host defense peptide BMAP28 and its inversed and retro-inversed isomers

Host defense peptides (HDPs) contribute to immune defense through direct antimicrobial activity as well as modulation of host immune responses. While the antimicrobial activity of HDPs has been successfully exploited as topical antibiotics, their use as systemic immunomodulatory antimicrobials has been limited by their toxicity and biological instability. Peptide modification strategies to address these characteristics, while maintaining biological activity, are likely essential to capture the full therapeutic potential of HDPs. Here we investigate the stability, toxicity, and biological activity of the L, inversed (D), and retro-inversed (RI) isomers of BMAP28. The D and RI isomers both form symmetrically related structures to L BMAP28 and resist proteolytic degradation. With respect to toxicity, the considerable hemolytic activity of L BMAP28 is approximately halved with the D isomer and eliminated with RI BMAP28. Furthermore, while D BMAP28 maintains the same cytotoxicity profile against epithelial cells and monocytes as the natural peptide, RI BMAP28 is markedly less toxic against these cell types. As prophylactic antimicrobials, all three isomers significantly reduced bacterial loads [99.99% bacterial clearance by each peptide at the highest dose (20 mg kg(-1) )], when administered 18 h prior to challenge in a mouse model of peritoneal infection. This protection appears to be mediated through neutrophil recruitment and activation of macrophages for bacterial clearance. Collectively, the increased stability and retained biological activity of D and RI BMAP28 make these isomers attractive as antimicrobial therapeutics. In particular, the protection conferred by RI BMAP28, combined with its reduced toxicities, make it a strong candidate for further consideration.

Twin and triplet drugs in opioid research

Twin and triplet drugs are defined as compounds that contain respectively two and three pharmacophore components exerting pharmacological effects in a molecule. The twin drug bearing the same pharmacophores is a "symmetrical twin drug", whereas that possessing different pharmacophores is a "nonsymmetrical twin drug." In general, the symmetrical twin drug is expected to produce more potent and/or selective pharmacological effects, whereas the nonsymmetrical twin drug is anticipated to show both pharmacological activities stemming from the individual pharmacophores (dual action). On the other hand, nonsymmetrical triplet drugs, which have two of the same pharmacophores and one different moiety, are expected to elicit both increased pharmacological action and dual action. The two identical portions could bind the same receptor sites simultaneously while the third portion could bind a different receptor site or enzyme. This review will mainly focus on the twin and triplet drugs with an evaluation of their in vivo pharmacological effects, and will also include a description of their pharmacology and synthesis.

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.

Design and synthesis of trivalent ligands targeting opioid, cholecystokinin, and melanocortin receptors for the treatment of pain

It has been known that co-administration of morphine with either cholecystokinin (CCK) receptor or melanocortin (MC) receptor antagonists enhance morphine's analgesic efficacy by reducing serious side effects such as tolerance and addiction. Considering these synergistic effects, we have designed trivalent ligands in which all three different pharmacophores for opioid, CCK, and MC receptors are combined in such a way as to conserve their own topographical pharmacophore structures. These ligands, excluding the cyclic compound, were synthesized by solid phase synthesis using Rink-amide resin under microwave assistance in very high yields. These trivalent ligands bind to their respective receptors well demonstrating that the topographical pharmacophore structures for the three receptors were retained for receptor binding. Ligand 10 was a lead compound to show the best biological activities at all three receptors.

Modulation of cell surface expression of nonactivated cholecystokinin receptors using bivalent ligand-induced internalization

CCK(2) receptor antagonists potentiate pain relief by MOP receptor agonists. In an attempt to enhance this effect, we prepared bivalent ligands incorporating CCK(2) receptor antagonist and MOP receptor agonist pharmacophores. (9) Ligands with 16- to 22-atom spacers could simultaneously bind both receptors but provided no advantage in activity over individual ligands. We now examine the effect of these ligands on receptor internalization as a mechanism of receptor regulation. We prepared CHO cell lines expressing nonfluorescent halves (YN and YC) of yellow fluorescent protein attached to each receptor. Spatial approximation of constructs was needed to yield fluorescence. Monovalent MOP agonist 1 signaled normally and internalized the MOP receptor. Monovalent CCK(2) antagonist 2 did not stimulate receptor internalization. In the dual receptor-bearing cells, bivalent ligands 3a-c capable of simultaneously binding both receptors resulted in cell surface fluorescence and internalization of the fluorescent complex in a time- and temperature-dependent manner. Bivalent ligand 4 with spacer too short to occupy both receptors simultaneously yielded no signal. Receptor tethering with appropriate bivalent ligands can down-regulate signaling by moving a nonactivated receptor into the endocytic pathway.

Retro-inversion enhances the adjuvant and CpG co-adjuvant activity of host defence peptide Bac2A

Host defence peptides (HDPs) have a variety of potential therapeutic applications, including as vaccine adjuvants, energizing efforts for modification strategies to address their toxicity and instability. Here we compare l, d and RI-Bac2A as vaccine adjuvants. d and RI-Bac2A are equally resistant to proteolytic degradation with no increases in toxicity, however, only RI-Bac2A maintains adjuvant activity of the natural peptide through conserved induction of a Th2 immune response. As HDPs potentiate the adjuvant activity of CpG ODNs, the isomers were also evaluated as co-adjuvants. l-Bac2A has no significant co-adjuvant activity while CpG/RI-Bac2A induces antibody titres significantly higher than CpG (P<0.01), CpG/l-Bac2A (P<0.01) or CpG/d-Bac2A (P<0.01). None of the isomers influence ODN duration or distribution but l and RI-Bac2A promote ODN uptake into B cells and antigen presenting cells. The enhanced adjuvant and co-adjuvant of RI-Bac2A is hypothesized to result from an undefined combination of increased stability and retained biological activity supporting application of retro-inversion to this, and potentially other HDPs.

An APP inhibitory domain containing the Flemish mutation residue modulates gamma-secretase activity for Abeta production

Gamma-secretase is an aspartyl protease that cleaves multiple substrates within their transmembrane domains. Gamma-secretase processes the amyloid precursor protein (APP) to generate gamma-amyloid (Agamma) peptides associated with Alzheimer's disease. Here, we show that APP possesses a substrate inhibitory domain (ASID) that negatively modulates gamma-secretase activity for Agamma production by binding to an allosteric site within the gamma-secretase complex. Alteration of this ASID by deletion or mutation, as is seen with the Flemish mutation (A21G), reduces its inhibitory potency and promotes Agamma production. Notably, peptides derived from ASID show selective inhibition of gamma-secretase activity for Agamma production over Notch1 processing. Therefore, this mode of regulation represents an unprecedented mechanism for modulating gamma-secretase, providing insight into the molecular basis of Alzheimer's disease pathogenesis and a potential strategy for the development of therapeutics.

Synthesis and investigations of double-pharmacophore ligands for treatment of chronic and neuropathic pain

Acids 9a-f as possible bivalent ligands designed as a structural combination of opioid mu-agonist (Fentanyl) and NSAID (Indomethacin) activities and produced compounds which were tested as analgesics. The obtained series of compounds exhibits low affinity and activity both at opioid receptors and as cyclooxygenase (COX) inhibitors. One explanation of the weak opioid activity could be stereochemical peculiarities of these bivalent compounds which differ significantly from the fentanyl skeleton. The absence of significant COX inhibitory properties could be explained by the required substitution of an acyl fragment in the indomethacin structure for 4-piperidyl.

Bi- or multifunctional opioid peptide drugs

Strategies for the design of bi- or multifunctional drugs are reviewed. A distinction is made between bifunctional drugs interacting in a monovalent fashion with two targets and ligands containing two distinct pharmacophores binding in a bivalent mode to the two binding sites in a receptor heterodimer. Arguments are presented to indicate that some of the so-called "bivalent" ligands reported in the literature are unlikely to simultaneously interact with two binding sites. Aspects related to the development of bi- or multifunctional drugs are illustrated with examples from the field of opioid analgesics. The drug-like properties of the tetrapeptide Dmt(1)[DALDA] with triple action as a micro opioid agonist, norepinephrine uptake inhibitor and releaser of endogenous opioid peptides to produce potent spinal analgesia are reviewed. Rationales for the development of opioid peptides with mixed agonist/antagonist profiles as analgesics with reduced side effects are presented. Progress in the development of mixed micro opioid agonist/delta opioid antagonists with low propensity to produce tolerance and physical dependence is reviewed. Efforts to develop bifunctional peptides containing a micro opioid agonist and a cholecystokinin antagonist or an NK1 receptor antagonist as analgesics expected to produce less tolerance and dependence are also reviewed. A strategy to improve the drug-like properties of bifunctional opioid peptide analgesics is presented.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.