Current Pharmaceutical Design - Volume 14, Issue 1, 2008

In this issue of Current Pharmaceutical Design the superfamily of TRP (Transient Receptor Potential) cation channels is the subject of four detailed reviews. A variety of more than 30 cation channels, members of the TRP family, permeable to Ca2+ and other cations are involved in several pathological and physiological conditions. Based on sequence homology, this superfamily of cation channels has been divided in six main subfamilies: canonical (TRPC), vanilloid (TRPV), melastatin (TRPM), polycystin (TRPP), mucolipin (TRPML) and the ankyrin (TRPA). The search for the molecular targets for naturally occurring substances, especially of plant origin, and studies of the mechanism of the transduction of physical stimuli such as temperature, mechanical pressure and light, have allowed the characterization and classification of many TRP channels. In fact, attempts to understand the molecular mechanism of action of the pro-nociceptive effects of vanillyl moietycontaining compounds such as capsaicin, the pungent component of hot chilli peppers (from Capsicum sp.), or its potent analogue resiniferatoxin (RTX, from Euphorbia sp.), led to the cloning of the “vanilloid” receptor (TRPV1) already 10 years ago, and of other heatsensitive TRPV channels soon thereafter. More recently, TRP channels sensitive to low temperatures, such as TRPM1 and TRPA1, have also been identified. While celebrating the first decennium of TRPV1 research, one should bear in mind that in addition to its expression in primary sensory afferents and its well documented biological role in pain perception, a significant number of studies have been published indicating that functionally active populations of TRPV1 receptors are expressed also in the central nervous system (CNS), thus suggesting that they are involved in many more functions than just nociception. The four reviews in this issue of the journal highlight and discuss some important aspects of TRP channel-related issues. Appendino and colleagues [1] provide an overview of the role that natural products have played in the identification of TRP channels and of their function as possible candidates for drug discovery. Vennekens and coworkers [2] summarize the fundamental properties of all members of the TRPV subfamily (TRPV1- TRPV6), in the light of their multi-faceted cellular functions, expression, molecular structure, regulation and pharmacology. The subsequent review by Gunthorpe and Szallasi [3] focusses on the function and potential therapeutic exploitation of TRPV1 channels located in the peripheral nervous system. The authors discuss the role of TRPV1 not only in pain but also in cancer, obesity and diabetes, among others, and the clinical development of TRPV1 agonists and antagonists. The issue of TRPV1 as a potential target in non-neuronal tissues is also reviewed. Finally, Starowicz, Cristino and Di Marzo [4] discuss current ‘hot’ data on the functional significance of TRPV1 channels in the brain, where these receptors are unlikely to be activated by irritant or noxious stimulus like high temperature or low pH, hence implying the existence of “endovanilloids”. Starowicz and colleagues focus not only on the role of potential endovanilloids in central aspects of pain control, but also on the regulation of body temperature, cardiovascular and respiratory functions, emesis, anxiety and locomotion. The common intention to all authors is to provide the reader with the most up-to-date and state-of-the-art aspects of TRP, and particularly TRPV, function under both physiological and pathological conditions, and emphasize the potential of TRP targeting for therapeutic purposes. References [1] Appendino G, Minassi A, Pagani A, Ech-Chahad A. The Role of Natural Products in the Ligand Deorphanization of TRP Channels. Curr Pharm Des 2008; 14(1): 2-17. [2] Vennekens R, Owsianik G, Nilius B. Vanilloid Transient Receptor Potential Cation Channels: An Overview. Curr Pharm Des 2008; 14(1): 18-31. [3] Gunthorpe MJ, Szallasi A. Peripheral TRPV1 Receptors As Targets for Drug Development: New Molecules and Mechanisms. Curr Pharm Des 2008; 14(1): 32-41. [4] Starowicz K, Cristino L, Di Marzo V. TRPV1 Receptors in the Central Nervous System: Potential for Previously Unforeseen Therapeutic Applications. Curr Pharm Des 2008; 14(1): 42-54.

The ligand deorphanization of TRP channels has a tremendous potential for biomedical and nutritional research, and this review highlights the role that natural products have played in the identification of ligands for these targets and their establishment as viable candidates for drug discovery. Specific ligands have so far been discovered only for some thermoTRPs, like TRPV1, TRPV3, TRPV4, TRPM8 and TRPA1, and the lack of selective pharmacology has been a major drawback for unraveling the biological role of TRPs. While genetic approaches (transgenic animal models) have partially compensate for the lack of ligands, the universal expression of TRPs in living systems and the success achieved with TRPV1 suggest that a systematic investigation of the natural products pool might alleviate this shortage, fostering adoption by small molecules within this class of still largely orphan biological targets.

The mammalian branch of the Transient Receptor Potential (TRP) superfamily of cation channels consists of 28 members. They can be subdivided in six main subfamilies: the TRPC (‘Canonical’), TRPV (‘Vanilloid’), TRPM (‘Melastatin’), TRPP (‘Polycystin’), TRPML (‘Mucolipin’) and the TRPA (‘Ankyrin’) group. The TRPV subfamily comprises channels that are critically involved in nociception and thermo-sensing (TRPV1, TRPV2, TRPV3, TRPV4) as well as highly Ca2+ selective channels involved in Ca2+ absorption/ reabsorption in mammals (TRPV5, TRPV6). In this review we summarize fundamental physiological properties of all TRPV members in the light of various cellular functions of these channels and their significance in the systemic context of the mammalian organism.

Based on the painful effects of exposure to capsaicin, TRPV1 (transient receptor potential vanilloid subfamily member 1) lo-calization is most readily associated with peripheral sensory neurons, however, TRPV1 is now known to be expressed, albeit at lower levels, in the spinal cord, brain and a wide-range of non-neuronal cells. The latter includes epithelial cells (e.g. keratinocytes, urothe-lium, gastric epithelial cells, enterocytes, and pneumocytes) through vascular endothelium and cells of the immune system (e.g. T-cells and mast cells) to smooth muscle, fibroblasts and hepatocytes. Despite extensive research, the physiological function of TRPV1 in the brain and in non-neuronal tissues remains elusive. The preliminary results are exciting, but many are unconfirmed and/or contradictory. As yet, studies with TRPV1 knock-out mice have proven unhelpful in clarifying such biological roles. Now that a range of potent and selective TRPV1 antagonists are available in this rapidly expanding research field, further understanding of the biological roles of TRPV1 throughout the body is within reach. In this article, we will summarize the known roles of peripheral TRPV1 receptors in physi-ology and disease and review the current perspectives for the therapeutic potential of TRPV1 agonists and antagonists in the treatment of a wide range of conditions such as pain, cancer, migraine, chronic cough, asthma, rectal hypersensitivity, inflammatory bowel disease, obesity, overactive bladder and diabetes. New applications of targeting central TRPV1 receptors are reviewed in the accompanying arti-cle by Starowicz et al. (in this issue).

Increasing evidence exists to support the presence of functional transient receptor potential vanilloid type 1 (TRPV1) channels in the brain, where these receptors are unlikely to be activated by high temperature and low pH. Here we review this evidence as well as the literature data pointing to the potential role of endovanilloid-activated brain TRPV1 channels not only in the supraspinal control of pain, body temperature, cardiovascular and respiratory functions and emesis, but also in anxiety and locomotion. This literature provides the first bases for the possible future development of new therapeutic approaches that, by specifically targeting brain TRPV1 receptors, might be used for the treatment of pain as well as affective and motor disorders.

Many examples of specific binding between small molecules are known that are associated with modified physiological and pharmacological activities. Conversely, the antagonism or synergism of small molecules is often correlated with specific binding between the molecules. It follows that small molecule binding can be used as a relatively quick, easy, and specific screen for functionally useful drug actions and interactions. These actions and interactions may manifest themselves as functional antagonisms; binding may correlate with enhancement or synergism; the formation of some complexes may yield clues about how drugs may be targeted to specific cell types in vivo and provide leads for the development of antidotes for drug overdoses or poisoning; the binding of one molecule to another may mimic receptor binding; and complexation may provide novel ways of protecting and delivering drugs. Relevant examples from each type of application are reviewed involving peptide-peptide interactions; peptide-aromatic compound interactions; aromatic-aromatic compound interactions; vitamin-aromatic compound interactions; and polycyclic compound interactions. We argue that screening for molecular complementarity of small molecules turns ligands such as neurotransmitters and their metabolites, hormones, and drugs themselves, into direct targets of drug development that can augment screening new compounds for activity against receptors and second messenger systems. We believe that the small molecule complementarity approach is novel, fruitful and under-utilized.

Novel drug delivery systems are one of the widely used delivery systems. In the present scenario, amongst them, “Drug Loaded Erythrocytes” is one of the growing and potential systems for delivery of drugs and enzymes. Erythrocytes are biocompatible, biodegradable, posses long circulation half-life and can be loaded with variety of biologically active substances. Carrier erythrocytes are prepared by collecting blood sample from the organism of interest and separating erythrocytes from plasma. By using various physical and chemical methods the cells are broken and the drug is entrapped into the erythrocytes, finally they are resealed and the resultant carriers are then called “resealed erythrocytes”. Surface modification with glutaraldehyde, antibodies, carbohydrates like sialic acid and biotinylation of loaded erythrocytes (biotinylated erythrocytes) is possible to improve their target specificity and to increase their circulation half-life. Upon reinjection the drug loaded erythrocytes serve as slow circulation depots, targets the drug to the reticuloendothelial system (RES), prevents degradation of loaded drug from inactivation by endogenous chemicals, attain steady state concentration of drug and decrease the side-effects of loaded drug. Nowadays, Nanoerythrosomes based drug delivery systems have excellent potential for clinical application.

About 50 peptides, and a similar number of peptide receptors, are known to be present in the gut and this amount is likely to rise significantly over the next few years. While there has been a massive research effort to define their functions and their anatomical distribution in the central nervous system (CNS), the understanding of their roles in the gut is far more limited. Classically, the physiological functions include the control of motility, fluids, electrolytes, and digestive enzymes secretion, or vascular and visceral pain function, and more recently, the role-played in cell proliferation and survival, and in immune-inflammatory responses. The term inflammatory bowel disease (IBD) that encompasses Crohn's disease and ulcerative colitis, is clearly an inflammatory disease where several mediators such as cytokines, chemokines, prostanoids, nitric oxide or free radicals, produced by infiltrating cells, play a critical role in intestine tissue alteration. Some peptides, initially known for their neuroregulative properties, have been suggested to act as endogenous immune factors, with predominant antiinflammatory effects. Based on these actions, these molecules are proposed as potential agents for the treatment of IBD and selective peptide analogs are being developed as novel therapeutic strategies for IBD patients. Patients with IBD have an increased risk for developing colorectal cancer (CRC). Up to the present time, no known genetic basis has been identified to explain CRC predisposition in these IBD. Instead, it is assumed that chronic inflammation is what causes cancer. This is supported by the fact that colon cancer risk increases with longer duration of colitis, greater anatomic extent of colitis, the concomitant presence of other inflammatory manifestations, and the fact that certain drugs used to treat inflammation, may prevent the development of CRC. However, though different regulative peptides play a beneficial role in experimental IBD, an increasing number of articles about cancer pathology are starting to implicate different peptides in tumor initiation and progression. The complexities of cancer could be described in terms of a small number of underlying principles and the malignant growth is dependent upon a multi-step process including different basic essential alterations. The activities of many peptides that are overexpressed in cancer cells help them to develop several of the molecular and physiological features that are now considered the basis of malignant growth. These collective findings implicate regulative peptides, receptors, or peptide-levels modulators, as important biological targets for developing intervention strategies against intestinal immunological disorders and cancers.