Drug Design Reviews - Online (Discontinued) - Volume 1, Issue 3, 2004

It is now well established that many tissues express binding sites that are defined pharmacologically as “imidazoline” receptors. These are still undergoing full molecular characterisation but candidate clones have been identified for both the I 1 and I2 subtypes. The physiological roles of these receptors remain to be the subject of continuing investigation although there is strong evidence that I1 sites play a role in the central regulation of blood pressure while I2 sites may control feeding and other aspects of behaviour. In molecular terms, the third imidazoline receptor subtype (I3) is the least well characterised although progress in understanding its functional pharmacology is continuing. This receptor plays a role in the regulation of insulin secretion and mediates glucose-dependent increases in secretion in response to synthetic ligands such as efaroxan and antazoline. These agents were originally developed as selective α2-adrenoceptor antagonists but it is now clear that they also act as I3 agonists. Molecular characterisation of the functional activity of I3 agonists has been complicated by the fact that most of the ligands also interact with other cellular proteins, leading to potential difficulties in the interpretation of experimental data. In particular, it was established more than a decade ago that many I3-agonists induce the closure of ATP-sensitive potassium (KATP) channels and it is now clear that an imidazoline binding site is located within the pore-forming subunit (Kir6.2) of the KATP channel. However, increasing evidence implies that closure of this channel is not primarily responsible for the ability of imidazoline drugs to enhance insulin secretion. In support of this, imidazoline compounds have recently been described that do not close KATP channels but which potentiate insulin secretion very effectively. These probably interact with intracellular targets within pancreatic β- cells and the results suggest that the I3 receptor may be distinct from the imidazoline binding site associated with Kir6.2. Rather, the I3-site appears to control a distal step in the exocytotic pathway. The present review examines recent progress in identifying the I 3 receptor and describes the pharmacology of I3-agonists that regulate insulin secretion. The possibility that these agonists might then serve as prototypes for the development of novel ligands having therapeutic potential is also considered.

Lysolipids can be divided into lysoglycerolipids and lysosphingolipids. These lipids are pleoitropic and perform many biological functions including cell arrest, cell growth, angiogenesis, tumour cell invasion and migration of lymphocytes, among many others. Receptors for these lipids belong to the heptahelical family of G protein-coupled receptors (GPCRs), making them prime targets for drug intervention. Receptors for lysophosphatidic acid (LPA) are known as LPA1, LPA2 and LPA3, and those for sphingosine 1-phosphate (S1P) are named S1P1, S1P 2, S1P3, S1P 4 and S1P5. Lysophosphatidylcholine (LPC or lysolecithin) binds at least three receptors known as G2A, GPR4 and PAFR, whereas sphingosylphosphorylcholine (SPC or lysosphingomyelin) binds OGR1, GPR4 and G2A. Psychsoine (Dgalactosyl- β1-1' sphingosine) which accumulates in the brain and other tissues of patients with globoid cell leukodystrophy (GLD), and D-glucosyl-β1-1' sphingosine, which accumulates in patients with Gaucher disease bind a receptor known as TDAG8. The accumulation of lysolipids results in various diseases, such as atherosclerosis, cancer, Krabbe and Gaucher diseases, to cite few examples. However, recent results showed that some of these lipids are important for maintaining the homeostasis of the immune system and for eradicating infections. The expression of receptors for these lipids has been reported in several cell types. However, such expression in lymphocytes has not been clearly established. This review will examine the up-to-date knowledge regarding the expression of receptors for lysolipids in natural killer (NK) and T lymphocytes. Because these cells are the major cell types in the innate and adaptive immunity respectively, it is hoped that the information provided might help in developing drugs to treat various diseases.

Chronic myelogenous leukemia (CML) is characterized by the expression of Bcr-Abl, a constitutively active form of the Abl tyrosine kinase. Gleevec, a small molecule Abl inhibitor developed by Novartis, is effective in treating early-stage CML patients, however those with advanced disease often acquire resistance to the drug. In many cases, this resistance is associated with mutations in the Abl kinase domain that no longer allow Gleevec to bind to the protein. In contrast to Gleevec, which does not inhibit members of the Src family of kinases (SFKs), several compounds initially identified as Src inhibitors also inhibit Abl. Crystallography showed that while Gleevec binds only to the inactive conformation of the Abl catalytic domain, PD 173955, a dual Src / Abl inhibitor, binds to both the active and inactive conformations. This finding is consistent with the activity of several Src / Abl inhibitors against cell lines harboring certain Gleevec resistant mutated forms of Bcr-Abl. In addition, since SFKs are implicated in Bcr-Abl signaling, these dual Src / Abl inhibitors may prove to be highly effective agents for the treatment of CML, both as first line agents and for those patients who develop Gleevec resistance. This review summarizes the considerable efforts focused on the design of dual inhibitors of Src and Abl kinases.

There has been a long interest and history in using porphyrins for applications in Photodynamic Therapy (PDT) for cancer treatment. These applications arise from their photosensitizing and phototoxic properties. PDT combines a photosensitizer and light to produce highly toxic singlet oxygen and therefore destroy any unwanted cells or tissues. The therapeutic properties of porphyrins and their interactions with different cell organelles and components are dependent on their chemical structures. Research on porphyrins with sugar moieties has been of great interest in the last decade. Glycosylated porphyrins can have greater water solubility than most naturally occurring and synthetic, meso substituted, porphyrins. This property can not only increase the efficacy of drug delivery but also assist the drug elimination from the organism after treatment. The proper lipophilicity of neutral saccharide conjugated porphyrins enable them to permeate better in both lipophilic and hydrophilic biological structures. Furthermore, they can have specific interactions with proteins on cell membranes and thus exhibit specific targeting of cancer cells. Therefore, porphyrins with carbohydrate moieties are very promising candidates for PDT in cancer treatment, and for other therapeutics.

The alignment of molecules in 3D-QSAR studies is an important step that affects considerably the outcome and the quality of CoMFA and CoMSIA models and subsequently the drug design and synthesis. Several applications will be reviewed with regard to the flexibility of the structures involved and the alignment procedures applied. Emphasis will be given to the contribution of NMR spectroscopy coupled with theoretical calculations to provide bioactive conformers in environments, which mimic the biological site of action. The aim of this review article is to inform medicinal chemists involved in the design and synthesis of novel drugs about the existing weaponry, which can be of aid in their research work.

MicroRNAs (miRNAs) are phylogenetically widespread small RNAs in animals and plants; the origin and function of these molecules in drug design is unknown. Our recent findings of small regulatory RNAs in noncoding regions of the human genome indicate a new strategy for modern drug designs. A novel gene modulation system was found within mammalian introns, regulating intracellular gene transcripts homologous to certain 5'- proximal regions of a native intron. These regions are normally located between the 5'-splice site and the next branch-point domain. Using Pol-II-directed expression and splicing excision of an artificial intron with various 5'- proximal synthetic inserts, we have successfully identified the generation of intron-derived microRNA-like molecules (Id-miRNA) from these regions as a tool for analysis of gene function and development of gene-specific therapeutics. The Id-miRNAs in hairpin conformations deliver maximal RNAi-related gene silencing effects on target genes in several mammalian cells. Based on the diversity of known miRNA structures and the complexity of genomic non-coding regions, the Id-miRNA generation mechanism may lead to one of the major gene modulation systems for developmental regulation, intracellular immunity, heterochromatin inactivation and genomic evolution in eukaryotes.

Traditional approach of natural products chemistry-collection, bioassay, isolation, structure elucidation is still effective in discovering new bioactive substances. Chemical ecological aspects of living organisms give useful consideration in managing reasonable collections and bioassays. Since diversity of species gives structural diversity of secondary metabolites, collection of unexplored organisms is one of effective approaches in finding new compounds. Development of new bioassay is also important for drug discovery. This review presents a bioassay method, which detects deformations of mycelia germinated from conidia of Pyricularia oryzae, and application to culture broths of marinederived fungi and Chinese traditional medicines. The bioassay was developed using an antimitotic fungal metabolite rhizoxin, the causative toxin of rice seedling blight, isolated from Rhizopus chinensis. Antimitotic and antifungal agents revealed morphological deformations on hyphal growth from conidia such as curling, swelling, hyper-divergency, beads shape, and so on. This bioassay method is cheap, quick, and easy to handle, and can be applied to high throughput screening and quantitative estimations. So far 30 new compounds have been isolated from five fungal culture broths and seven Chinese medicinal plants. Some of new and rediscovered compounds are expected to become drug candidates as a new class of antitumor agents.