Multifunctional Enkephalin Analogs with a New Biological Profile: MOR/DOR Agonism and KOR Antagonism.

In our previous studies, we developed a series of mixed MOR/DOR agonists that are enkephalin-like tetrapeptide analogs with an N-phenyl-N-piperidin-4-ylpropionamide (Ppp) moiety at the C-terminus. Further SAR study on the analogs, initiated by the findings from off-target screening, resulted in the discovery of LYS744 (6, Dmt-DNle-Gly-Phe(p-Cl)-Ppp), a multifunctional ligand with MOR/DOR agonist and KOR antagonist activity (GTPγS assay: IC50 = 52 nM, Imax = 122% cf. IC50 = 59 nM, Imax = 100% for naloxone) with nanomolar range of binding affinity (Ki = 1.3 nM cf. Ki = 2.4 nM for salvinorin A). Based on its unique biological profile, 6 is considered to possess high therapeutic potential for the treatment of chronic pain by modulating pathological KOR activation while retaining analgesic efficacy attributed to its MOR/DOR agonist activity.

Structure-Activity Relationships of [des-Arg7]Dynorphin A Analogues at the κ Opioid Receptor.

Dynorphin A (Dyn A) is an endogenous ligand for the opioid receptors with preference for the κ opioid receptor (KOR), and its structure-activity relationship (SAR) has been extensively studied at the KOR to develop selective potent agonists and antagonists. Numerous SAR studies have revealed that the Arg7 residue is essential for KOR activity. In contrast, our systematic SAR studies on [des-Arg7]Dyn A analogues found that Arg7 is not a key residue and even deletion of the residue does not affect biological activities at the KOR. In addition, it was also found that [des-Arg7]Dyn A(1-9)-NH2 is a minimum pharmacophore and its modification at the N-terminus leads to selective KOR antagonists. A lead ligand, 14, with high affinity and antagonist activity showed improved metabolic stability and could block antinociceptive effects of a KOR selective agonist, FE200665, in vivo, indicating high potential to treat KOR mediated disorders such as stress-induced relapse.

Cyclic non-opioid dynorphin A analogues for the bradykinin receptors.

Nerve injury and inflammation cause up-regulation of an endogenous opioid ligand, dynorphin A (Dyn A), in the spinal cord resulting in hyperalgesia via the interaction with bradykinin receptors (BRs). This is a non-opioid neuroexcitatory effect that cannot be blocked by opioid antagonists. Our systematic structure-activity relationships study on Dyn A identified lead ligands 1 and 4, along with the key structural feature (i.e. amphipathicity) for the BRs. However, the ligands showed very low metabolic stability in plasma (t1/2 <1h) and therefore, in order to improve their metabolic stabilities with retained biological activities, various modifications were performed. Cyclization of ligand 4 afforded a cyclic Dyn A analogue 5 that retained the same range of binding affinity as the linear ligand with improved metabolic stability (t1/2 >5h) and therefore possesses the potential as a pharmacophoric scaffold to be utilized for drug development.

Discovery of Stable Non-opioid Dynorphin A Analogues Interacting at the Bradykinin Receptors for the Treatment of Neuropathic Pain.

Dynorphin A (Dyn A) is a unique endogenous ligand that possesses well-known neuroinhibitory effects via opioid receptors with a preference for the kappa receptor but also neuroexcitatory effects, which cause hyperalgesia. We have shown that the neuroexcitatory effects are mediated through bradykinin (BK) receptors and that intrathecal (i.th.) administration of our lead ligand 1, [des-Arg7]-Dyn A-(4-11), which shows good binding affinity (IC50 = 150 nM) at the BK receptors, blocks Dyn A-induced hyperalgesia in naïve animals and reverses thermal and tactile hypersensitivities in a dose-dependent manner in nerve-injured animals. However, 1 has a serious drawback as a potential drug candidate for the treatment of neuropathic pain because of its susceptibility to enzymatic degradation. In an effort to increase its stability, we modified ligand 1 using non-natural amino acids and found that analogues substituted at or near the N-terminus with a d-isomer retain binding at the receptor and provide a large increase in stability. In particular when Leu5 was modified, with either the d-isomer or N-methylation, there was a large increase in stability (t1/2 = 0.7-160 h in rat plasma) observed. From these studies, we have developed a very stable Dyn A analogue 16, [d-Leu5,des-Arg7]-Dyn A-(4-11), that binds to BK receptors (IC50 = 130 nM) in the same range as ligand 1 and shows good antihyperalgesic effects in both naïve rats and L5/L6 spinal nerve ligation rats.

Various modifications of the amphipathic dynorphin A pharmacophore for rat brain bradykinin receptors.

As a unique endogenous opioid ligand, dynorphin A shows paradoxical neuroexcitatory effects at bradykinin receptors, and the effects are known to be amplified by the upregulation of dynorphin A under chronic pain and inflammatory conditions. In our earlier structure-activity relationship studies, the amphipathic dynorphin A fragment, [Des-Arg(7) ]-Dyn A-(4-11), was identified as a pharmacophore for the bradykinin receptors along with key structural features. Here, further modifications of the pharmacophore showed that the position of a Pro residue is also an important feature because of its role in making (or disrupting) a ß-turn or 310 helix structure which is crucial for receptor recognition.

Blockade of non-opioid excitatory effects of spinal dynorphin A at bradykinin receptors.

Dynorphin A (Dyn A) is an endogenous opioid ligand that possesses neuroinhibitory (antinociceptive) effects via µ, δ, and κ opioid receptors. However, under chronic pain conditions, up-regulated spinal Dyn A can also interact with bradykinin receptors (BRs) to promote hyperalgesia through a neuroexcitatory(pronociceptive) effect. These excitatory effects cannot be blocked by an opioid antagonist, and thus are non-opioid in nature. On the basis of the structural dissimilarity between Dyn A and endogenous BR ligands, bradykinin(BK) and kallidin (KD), Dyn A's interaction with BRs could not be predicted, and provided an opportunity to identify a novel potential neuroexcitatory target. Systematic structure-activity relationship (SAR) studies discovered a minimum pharmacophore of Dyn A, [des-Arg7]-Dyn A-(4-11) LYS1044 for antagonist activity at the BRs, along with insights into the key structural features for BRs recognition, i.e., amphipathicity. The des-Tyr fragment of dynorphin does not bind to opioid receptors. Intrathecal administration of des-Tyr dynorphin produces hyperalgesia reminiscent of behaviors seen in peripheral n europathic pain models and at higher doses, neurotoxicity. Our lead ligand LYS1044 negatively modulated Dyn A-(2-13)-induced neuroexcitatory effects in naïve animals and blocked mechanical hypersensitivity and thermal hyperalgesia in a dose-dependent manner in animals with experimental neuropathic pain. Based on these results, ligand LYS1044 might prevent abnormal pain states by blocking the neuroexcitatory effects of increased levels of Dyn A that are seen in experimental models of neuropathic pain and that likely promote excitation mediated by BRs in the spinal cord.

Modification of amphipathic non-opioid dynorphin A analogues for rat brain bradykinin receptors.

It has been shown that under chronic pain or nerve injury conditions, up-regulated dynorphin A (Dyn A) interacts with bradykinin receptors (BRs) to cause hyperalgesia in the spinal cord. Thus BRs antagonist can modulate hyperalgesia by blocking Dyn A's interaction with the BRs in the central nervous system. In our earlier structure-activity relationship (SAR) study, [des-Arg(7)]-Dyn A-(4-11) 13 was discovered as a minimum pharmacophore for rat brain BRs with its antagonist activity (anti-hyperalgesic effect) in in vivo tests using naïve or injured animals. We have pursued further modification on the [des-Arg(7)]-Dyn A analogues and identified a key insight into the pharmacophore of the rat brain BRs: amphipathicity.

Structure-activity relationships of non-opioid [des-Arg(7)]-dynorphin A analogues for bradykinin receptors.

In our earlier studies, bradykinin receptors (BRs) were identified as a potential target for the neuroexcitatory effects of dynorphin A (Dyn A) in the central nervous system (CNS), and [des-Arg(7)]-Dyn A-(4-11) (6) was discovered as a lead ligand to modulate Dyn A-(2-13) induced neuroexcitatory effects in the CNS as an antagonist. In an effort to gain insights into key structural features of the Dyn A for the BRs, we pursued further structure-activity relationships (SAR) study on the [des-Arg(7)]-Dyn A analogs and confirmed that all of the [des-Arg(7)]-Dyn A analogues showed good binding affinities at the BRs.

Analysis of the HNO and NO donating properties of alicyclic amine diazeniumdiolates.

Nitroxyl (HNO) donors have been shown to elicit a variety of pharmacological responses, ranging from tumoricidal effects to treatment of heart failure. Isopropylamine-based diazeniumdiolates have been shown to produce HNO on decomposition under physiological conditions. Herein, we report the synthesis and HNO release profiles of primary alicyclic amine-based diazeniumdiolates. These compounds extend the range of known diazeniumdiolate-based HNO donors. Acetoxymethyl ester-protected diazeniumdiolates were also synthesized to improve purification and cellular uptake. The acetoxymethyl derivative of cyclopentylamine diazeniumdiolate not only showed higher cytotoxicity toward cancer cells as compared to the parent anion but was also effective in combination with tamoxifen for targeting estrogen receptor α-negative breast cancer cells.

Discovery of amphipathic dynorphin A analogues to inhibit the neuroexcitatory effects of dynorphin A through bradykinin receptors in the spinal cord.

We hypothesized that under chronic pain conditions, up-regulated dynorphin A (Dyn A) interacts with bradykinin receptors (BRs) in the spinal cord to promote hyperalgesia through an excitatory effect, which is opposite to the well-known inhibitory effect of opioid receptors. Considering the structural dissimilarity between Dyn A and endogenous BR ligands, bradykinin (BK) and kallidin (KD), this interaction could not be predicted, but it allowed us to discover a potential neuroexcitatory target. Well-known BR ligands, BK, [des-Arg(10), Leu(9)]-kallidin (DALKD), and HOE140 showed different binding profiles at rat brain BRs than that previously reported. These results suggest that neuronal BRs in the rat central nervous system (CNS) may be pharmacologically distinct from those previously defined in non-neuronal tissues. Systematic structure-activity relationship (SAR) study at the rat brain BRs was performed, and as a result, a new key structural feature of Dyn A for BR recognition was identified: amphipathicity. NMR studies of two lead ligands, Dyn A-(4-11) 7 and [des-Arg(7)]-Dyn A-(4-11) 14, which showed the same high binding affinity, confirmed that the Arg residue in position 7, which is known to be crucial for Dyn A's biological activity, is not necessary, and that a type I ß-turn structure at the C-terminal part of both ligands plays an important role in retaining good binding affinities at the BRs. Our lead ligand 14 blocked Dyn A-(2-13) 10-induced hyperalgesic effects and motor impairment in in vivo assays using naïve rats. In a model of peripheral neuropathy, intrathecal (i.th.) administration of ligand 14 reversed thermal hyperalgesia and mechanical hypersensitivity in a dose-dependent manner in nerve-injured rats. Thus, ligand 14 may inhibit abnormal pain states by blocking the neuroexcitatory effects of enhanced levels of Dyn A, which are likely to be mediated by BRs in the spinal cord.