Novel Drug Development
New keys unlock the hidden potential of the melanocortin system A system of receptors, the structure of which has been conserved through millennia of evolution, are being targeted by Prof Victor J Hruby and his team at the University of Arizona, opening the door to new treatments for a panoply of diseases.
What has been the most significant advance in peptide synthesis during your career? • Optimised solid phase peptide synthesis including the use of microwave methods for some aspects • The ability to form many different kinds of constrained and cyclic and macrocyclic peptide structures with designed conformations and topographical structures • Asymmetric synthesis in chiral chi space What is the greatest challenge in getting novel compounds in to clinical trials? People! Money! Clinical trials are expensive, and most people in most drug companies fear novel compounds, and do not want to think about or do something novel. Who has the potential to benefit most from your research? Ultimately sick people! Currently, medical doctors, biologists, and medicinal chemists
who wish to explore new modalities for the treatment of disease. What is there still to discover about GPCRs? We know very little about GPCRs, so there is still much to discover! Especially how their structures change in response to ligands that interact with them, and how these structural changes lead to changes in biological activities. Also, how downstream signalling is differentially affected by the different ligand-receptor structures. What other applications or treatments is this technique likely to open up? We now have highly receptor-selective orthosteric and allosteric agonists and antagonists for the MCiR, MC3R, NC4R and MC5R. This will provide new tools to discover new biology associated with the MCRs. This, in turn, can lead to new leads to new approaches for the treatment of many of our most common degenerative diseases including mental illness.
This work can lead to new approaches for the treatment of many of our most common degenerative diseases, including mental illness
Novel Drug Development
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he melanocortin system of receptors is a group of five G-protein coupled receptors (GPCRs), a common receptor type which spans the cell membrane to sense molecules outside the cell. When a specific molecule is bound on the extracellular portion, the protein undergoes a conformational (shape) change which releases molecules inside the cell to stimulate intracellular signalling pathways. These receptors are generally very specific for their activating molecules, known as ligands, and this is particularly true for the melanocortin family. Melanotropins bind to a particular site on the protein, created by a unique sequence of amino acids; each of the naturally occurring receptor sub-types is the result of evolutionary selection of minor variations in these binding sites. The melanocortin family of receptors has sites which include highly conserved sequences (the same layout of amino acids in each of the receptors). This makes it more difficult to find or create ligands which will be specific for just one receptor sub-type.
is then utilised for the final purification and analysis steps. All this before their biological action can even begin to be investigated; competition binding assays (seeing how it competes with a known ligand labelled with a radioactive isotope) and functional assays in cultured cells which express the receptor on their surface, are employed for this purpose. This work, though laborious and complex, has the potential to influence many treatments being used to moderate human health and behaviour. Often, people associate the prefix melano- with pigmentation disorders such as the skin cancer melanoma and albinism’s opposite number, melanism. Endogenous peptide hormones, made within the body to act on the melanocortin system, do indeed regulate pigmentation processes, so novel ligands have the potential to be utilised to adjust these. Some of these hormones also act at other receptors, such as those associated with pain, so mimetics of these have the potential to relieve that pain as novel analgesics without the problems of addiction associated with substances such as opiates.
THE MASTER LOCKSMITH With nearly forty years of experience in the field, Prof Hruby has dedicated his life’s work to developing novel ligands for GPCRs, and designing and synthesising biologically active peptides and their mimetics (molecules that mimic the behaviour of these proteins). Through analysis of the conformation, dynamics and biological activity of these transmitters, particularly those that relate to human behaviour, he has uncovered potential treatments or drug targets for the control of pain, obesity and sexual behaviour among others.
THE BRAIN IS KEY The field has now opened up even further with the relatively recent discovery that other aspects of human behaviour disorders, such as the regulation of appetite, have their roots in this system. In recent studies of sub-types three and four of the melanocortin receptor, removal of these receptors in mice had a complex effect on obesity and related diseases, such as diabetes, which is not completely understood. Other studies have shown that mouse models, with a genetic variation which makes them prone to obesity, endogenously express a protein which activates these receptors (an agonist). Conversely, chemicals which compete with the agonist for the binding site but without generating a response (antagonists) inhibit this overeating activity. Novel compounds, with high specificity for a particular receptor sub-type and antagonist activity, are
There are significant challenges to designing and synthesising peptides with very specific biological activity. Modern computer modelling techniques and computational chemistry have made it possible to test the theory of novel molecular constructions and their interactions. However, there then follows complex and painstaking work to realise the compound and test its structural biology.
Detail
Prof Hruby’s team have pushed ever deeper into the realms of human behaviour disorders with their research into the role of melanocortin receptors in erectile dysfunction. Following successful research in mice, clinical trials of melanotropic peptides have shown significant positive effects on both erectile function and the more complex issue of sexual appetite.
RESEARCH OBJECTIVES Professor Hruby’s research focuses on the melanocortin 1, 2, 3, 4 and 5 receptors and designed ligands that will act on these receptors. The receptors are involved in many diseases including pigmentary disorders, melanoma cancer, stress, immune response, sexual dysfunction and behaviour, feeding disorders including obesity, neurodegeneration and many others. The ligands therefore have an extremely wide field of potential applications.
The regulation of appetite and sexual activity is clearly linked to this receptor subpopulation of types three and four, which is predominantly located in the brain and spinal cord. This brings particular challenges around the design of peptides and mimetics which are able to cross the notorious blood–brain barrier and reach the desired target. As the different sub-types clearly interact with each other in mediating physiological responses, it is necessary to develop compounds which are able to cross this barrier and act at one type, as well as those which are unable to and will instead act at another. Highly specific agonists and antagonists with these properties have been developed by the team and are in the process of being rigorously tested for efficacy. A LOCK ON EVERY DOOR With the melanocortin receptors also associated with the regulation of cardiovascular function, through blood pressure and heart rate, and kidney function in terms of salt secretion and high blood pressure related to salt sensitivity, there are clearly many applications for melanotropins yet to be uncovered. Due to his extensive experience of creating and evaluating novel peptides and their mimetics, Prof Hruby and his team are perfectly placed to continue pushing at these previously locked doors. The keys they are creating promise to unlock new treatments, ones which will treat the diseases threatening to become ever more prevalent in the ageing populations of developed economies.
Their work promises to unlock new treatments, ones which will treat the diseases threatening to become ever more prevalent in the ageing populations of developed economies
CRACKING THE SAFE Prof Hruby’s team had to make use of previously proven methods of protein synthesis to create the peptides they had designed. They then used High Pressure Liquid Chromatography to purify them. A second chromatography method (Ion Exchange Chromatography) or gel filtration
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therefore of great interest in the treatment of certain eating disorders.
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FUNDING National Institutes of Health (NIH) COLLABORATORS • Professor Mac E Hadley (deceased) • Professor Robert Dorr • Professor Roger D Cone • Professor Norm Levine • Professor Hunter Wessells • Research Professor Minying Cai • Dr Tomi Sawyer And many others worldwide BIO Dr Victor J Hruby received his PhD from Cornell University in 1965 and did his postdoctoral research with Nobel Laureate Vincent du Vigneaud. He has been a professor at the University of Arizona in the Department of Chemistry and Biochemistry since 1968. His major research interests are in peptide hormones and neurotransmitters and their receptors in relation to human behaviour and degenerative diseases. He has over 1200 publications. CONTACT Prof Victor J Hruby Regents Professor Emeritus Department of Chemistry and Biochemistry 1306 East University Boulevard PO Box 210041 University of Arizona, Tucson, AZ 85721 USA E: hruby@email.arizona.edu T: +1 520 621-6332 W: http://uacc.arizona.edu/profile/ victor-hruby
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