Muhammad Faheem Asim

The µ Opioid Receptor and Ligands Acting at the µ Opioid Receptor, as Therapeutics and Potential Therapeutics

Although the µ opioid receptor (MOR) was pharmacologically and biochemically identified in binding studies forty years ago, its structure, function, and true complexity only have emerged after its cloning in 1993. Continuous efforts from many laboratories have greatly advanced our understanding of MORs, ranging from their anatomic distribution to cellular and molecular mechanisms, and from cell lines to in vivo systems. The MOR is recognized as the main target for effective pain relief, but its involvement in many other physiological functions has also been recognized. This review provides a synopsis on the history of research on MORs and ligands acting at the MOR with the focus on their clinical and potential use as therapeutic drugs, or as valuable research tools. Since the elucidation of the chemical structure of morphine and the characterization of endogenous opioid peptides, research has stimulated the development of new generations of MOR ligands with distinct pharmacological profiles (agonist, antagonist, mixed agonist/antagonist and partial agonist) or site of action (central/peripheral). Discovery of therapeutically useful morphine-like drugs and innovative drugs with new scaffolds, with several outstanding representatives, is discussed. Extensive efforts on modifications of endogenous peptides to attain stable and MOR selective analogs are overviewed with stimulating results for the development of peptide-based pharmaceuticals. With pharmacophore modeling as an important tool in drug discovery, application of modern computational methodologies for the development of morphinans as new MOR ligands is described. Moreover, the crystal structure of the MOR available today will enable the application of structure-based approaches to design better drugs for the management of pain, addiction and other human diseases, where MORs play a key role.

In vitro and in vivo pharmacological profile of the 5-benzyl analogue of 14-methoxymetopon, a novel μ opioid analgesic with reduced propensity to alter motor function

Opioids are the most effective analgesics for pain management, and efficient pain control is a therapeutic priority. Herein, we describe the synthesis and pharmacological activities of the 5-benzyl analogue of the μ opioid analgesic 14-methoxymetopon (14-MM). The result of the replacement of the 5-methyl in 14-MM with a benzyl group on in vitro opioid receptor binding and functional profiles, and in vivo behavioural properties, i.e. nociception and motor activity, was investigated. In rodent brain membranes, the 5-benzyl derivative showed high affinity at the μ opioid receptor and decreased interaction with δ and κ receptors, hence displaying a similar binding profile as 14-MM. It displayed potent agonist activity in vitro and in vivo. In in vitro guanosine-5′-O-(3-[35S]thio)-triphosphate ([35S]GTPγS) binding assay, it activated G-proteins in rat brain membranes through a μ opioid receptor-mediated mechanism having significantly enhanced potency compared to DAMGO (d-Ala2,Me-Phe4,Gly-ol5]enkephalin), and to the μ opioid agonist morphinans 14-MM, 14-O-methyloxymorphone (14-OMO) and morphine. In vivo, the 5-benzyl analogue of 14-MM elicited dose-dependent and naloxone-sensitive antinociceptive effects in hot-plate and tail-flick tests in mice after subcutaneous (s.c.) administration. Its analgesic potency was comparable to 14-MM, and was 50-fold higher than that of morphine. Contrary to morphine, 14-MM and 14-OMO, no motor dysfunction was produced by the new opioid in the mouse rotarod test at any of the tested doses. In summary, the 5-benzyl analogue of 14-MM emerged as a novel potent μ opioid antinociceptive agent with reduced propensity to cause unwanted motor impairment.

Novel approach to demonstrate high efficacy of μ opioids in the rat vas deferens: A simple model of predictive value

14-O-Methyloxymorphone and 14-methoxymetopon were reported as highly selective and potent micro opioid receptor agonists. The aim of this study was to demonstrate the opioid activity of these compounds in vitro and in vivo in comparison to oxymorphone, morphine and DAMGO. The micro opioid receptor efficacy, full or partial agonist nature of opioids was analyzed in the rat vas deferens (RVD) bioassay. Compared to oxymorphone, 14-O-methyloxymorphone and 14-methoxymetopon showed greater affinities to the rodent brain micro opioid receptors in receptor binding assays. In isolated organs 14-O-methyloxymorphone and 14-methoxymetopon were 3-10-fold more potent than the micro agonist opioid peptide, DAMGO. All tested compounds reached at least 70% maximum inhibition in mouse vas deferens (MVD) except morphine and oxymorphone. In the RVD, morphine could not exceed 50% inhibition of the twitches while 14-O-methyloxymorphone and 14-methoxymetopon showed inhibitory effects more than 70%. Oxymorphone reached only 4% maximal agonist effect and antagonized the inhibitory effect of DAMGO. The investigated morphinans produced dose-dependent antinociceptive activities in mice and rats. Both, 14-O-methyloxymorphone and 14-methoxymetopon are highly efficacious micro opioid receptor agonists in the RVD exhibiting full micro agonist properties. The RVD tissue contains mu receptors indicated by the comparable K(e) values of the micro antagonist naltrexone against DAMGO in the MVD. RVD may be a good alternative to assess the mu receptor efficacy of opioid agonists providing a more physiological environment for the ligand-receptor interaction than other efficacy measuring methods such as the [(35)S]GTPgammaS binding assay.

Current κ opioid receptor ligands and discovery of a new molecular scaffold as a κ opioid receptor antagonist using pharmacophore-based virtual screening.

The κ opioid receptor (KOR) plays a significant role in many physiological functions, including pain relief, stress, depression, drug abuse, anxiety and psychotic behaviors. KORs are widely distributed in the central and peripheral nervous systems, and are specifically activated by endogenous opioids derived from prodynorphin. They are members of the G protein-coupled receptor superfamily, and the crystal structure of the human KOR was recently elucidated. KORs were initially studied for their involvement in mediation of pain as stimulation of KOR produces analgesia and minimizes abuse liability and other side effects. Nowadays, the KOR is rapidly emerging as an important target for the treatment of a variety of other human disorders. Specifically, the KOR system has become increasingly implicated as a modulator of stress-related and addictive behaviors. Several selective KOR partial agonists and antagonists have been developed as potential antidepressants, anxiolytic and anti-addiction medications. Although many KOR ligands have not demonstrated desirable pharmacological properties, some have been shown to be viable drug candidates. Herein, we describe chemical and pharmacological developments on KOR ligands, advantages and challenges, and potential therapeutic applications of ligands for KORs. In the second part, recent advances in the KOR drug design utilizing computational approaches are presented, with focus on the discovery of a new naturally derived scaffold, sewarine, as a novel class of selective KOR ligands with antagonist properties, using a pharmacophore-based virtual screening strategy.

Synthesis and pharmacological activities of 6-glycine substituted 14-phenylpropoxymorphinans, a novel class of opioids with high opioid receptor affinities and antinociceptive potencies.

The synthesis and the effect of a combination of 6-glycine and 14-phenylpropoxy substitutions in N-methyl- and N-cycloproplymethylmorphinans on biological activities are described. Binding studies revealed that all new 14-phenylpropoxymorphinans (11−18) displayed high affinity to opioid receptors. Replacement of the 14-methoxy group with a phenylpropoxy group led to an enhancement in affinity to all three opioid receptor types, with most pronounced increases in δ and κ activities, hence resulting in a loss of μ receptor selectivity. All compounds (11−18) showed potent and long-lasting antinociceptive effects in the tail-flick test in rats after subcutaneous administration. For the N-methyl derivatives 13 and 14, analgesic potencies were in the range of their 14-methoxy analogues 9 and 10, respectively. Even derivatives 15−18 with an N-cyclopropylmethyl substituent acted as potent antinociceptive agents, being several fold more potent than morphine. Subcutaneous administration of compounds 13 and 14 produced significant and prolonged antinociceptive effects mediated through peripheral opioid mechanisms in carrageenan-induced inflammatory hyperalgesia in rats.

Identification of novel ligands interacting with kappa opioid receptors

Background The opioid (KOP) receptor belongs to the family of seven transmembrane G protein-coupled receptors (GPCRs) and it plays a significant role in a broad range of physiological functions. Stimulation of the KOP receptor results in analgesic actions, and KOP agonists appear to have some advantages over the μ opioid (MOP) receptor agonists. Inhibiting KOP receptors is proposed to be useful for treating addiction and stress-related conditions, such as depression and anxiety. The pharmacology of currently available KOP antagonists shows a delay in onset of action and an extremely long duration of action in vivo, which might limit their therapeutic application. The search for new ligands with potent biological activities, particularly as potential novel therapeutic agents, utilizing computational and synthetical approaches, is a key goal of life science research and drug development. Herein, we present the in silico, in vitro and in vivo profiles of new molecular scaffolds as novel KOP receptor ligands.

A ligand-based 3D pharmacophore model for the μ opioid receptor: application to the morphinan class of opioids

Background Opioid receptors belong to the rhodopsin subclass within the superfamily of G protein-coupled receptors (GPCR), which are characterized by the presence of seven transmembrane (7TM) helices. They interact with morphine and related opioid alkaloids as well as with endogenous opioid peptides. There are three main types of opioid receptors (μ, δ, ), which are differently implicated in opioid function. Ligands specifically targeting each opioid receptor type are of high interest both as research tools and potential therapeutic agents. Activation of the μ opioid receptor produces many other effects, besides its main involvement in pain control, including immunomodulation, respiratory depression, constipation, tolerance and physical dependence. With the lack of an experimental 3D structure of the μ opioid receptor, discovery of 3D pharmacophores for the receptor that can explain the activity of a series of ligands represents an important approach in drug discovery.

Biological, pharmacological and immunological activities of novel 6-amino-acid-substituted 14-alkoxy-N-methylmorphinans

Background Of the three opioid receptors, μ (MOP), δ (DOP) and (KOP), the MOP type is the most involved in the action of opioids in the gut. Experimental studies on models of intestinal inflammation and inflammatory bowel disease (IBD) support an anti-inflammatory role of peripheral MOP receptors in the gut, besides their involvement in pain control (analgesia) and gastrointestinal motility (anti-diarrheal effects). Research focuses increasingly on exploring the therapeutic potential of peripheral MOP receptors aiming for identification of peripheral ligands as improved treatment for debilitating conditions associated with bowel functions. One strategy to increase peripheral selectivity includes chemical modifications that enhance hydrophilicity [1,2]. Our work in the field of peripherally acting opioids has led to a series of opioids with zwitterionic moieties (i.e. amino acid residues) attached to the C-6 position of 14-O-methyloxymorphone, which may represent novel therapeutic molecules for IBD. These 14-alkoxymorphinans were pharmacologically and immunologically characterized.

Efficacy of systemic HS-198, an analogue of oxymorphone, on cancer pain-related behaviour in mice

Background Cancer pain is a significant clinical problem being one of the first symptoms of disease with 75–90% of the patients experiencing chronic pain syndromes in advanced stages [1]. The management of cancer pain is mainly based on the use of opioid drugs; however their clinical use is limited by high incidence of adverse effects. There is a continued search for highly efficacious opioid analgesics with reduced complications and improved patient compliance. An analogue of the clinically used oxymorphone, 5-methyl-substituted 14-O-methyloxymorphone (HS198), is a selective μ opioid agonist and a potent antinociceptive agent in animal models of nociceptive and inflammatory pain, while exhibiting a favourable dissociation between analgesia and the occurrence of side effects [2]. We report data on efficacy of this opioid agonist after subcutaneous administration (s.c.) in a murine model of cancer pain. The opioid receptor-mechanistic basis of the antinociceptive action was also investigated.

483 ANALGESIC EFFECTS OF 14-METHOXYMETOPON IN A MURINE MODEL OF CANCER PAIN

The pain appeared after longer exercises (cycling), and then got worse after palpation. He had no pain in the rest. One year prior the admission MRI showed small circumscribed spherical lesion (venous varix?). Before admission was diagnosed the small deep tumor (neurofibroma?) on the sonography and NMR. It was not palpable mass. We took aim the surgical excision using ultrasonography. We removed a demarcate hard tumor 1cm in diameter coming from deep intermuscular septum. The histological diagnosis was monophasic variant of synovial sarcoma. Therefore was indicated the excision of the muscular tissue around previous tumor place. No malignant cells were found in the excised parts. One year after treatment is patient without problems. The MRI of thigh is without signs of tumor. Conclusions: The occurrence of pain at the tumour place prior to development of the swelling is not common. Awareness of this unusual presentation and appropriate investigation may enable detection of synovial sarcoma at a prognostic favourable early stage.