Current Pharmaceutical Design - Volume 14, Issue 24, 2008

The production of new molecular entities endowed with salutary medicinal properties is a formidable challenge; synthetic molecules that can bind with high sequence specificity to a chosen target in a protein or gene sequence are of major interest in medicinal and biotechnological contexts. The general awareness of the importance of peptides in physiology and pathophysiology has markedly increased over the last few years. With progresses in the analysis of whole genomes, the knowledge base in gene sequence and expression data useful for protein and peptide analysis has drastically increased. The medical need for relevant biomarkers is enormous. This is particularly true not only for many types of cancers, but also for other diseases, e.g. type 2 diabetes or cardiac diseases, which also lack adequate diagnostic markers with high specificity and sensitivity. Imaging technologies for early detection of diseases, proteomic and peptidomic multiplex techniques have markedly evolved in recent years. Peptides can indeed be regarded as ideal agents (as “magic bullets”) for diagnostic and therapeutic applications, because of their fast clearance, rapid tissue penetration, and low antigenicity, and also of their easy production, allowing innumerable biological applications. They can easily be engineered to improve their biological activities as well as their stability and their efficient delivery to specific targets. The present issue of Current Pharmaceutical Design, for which I have the honour to be Executive Guest Editor, addresses topical issues to some of these potential utilisations of peptide motifs for a variety of genetic and acquired diseases. During the past years, proofs of the fact that peptide receptors can be successfully used for in vivo targeting of human cancers, have been provided. The molecular basis for targeting grounds on the in vitro observation is that peptide receptors can be expressed in large quantities in certain tumours. The clinical impact is at the diagnostic level: in vivo receptor scintigraphy uses radiolabelled peptides for the localisation of tumours and of their metastases. Peptidic tumour targeting agents can be sub-divided into the following segments: peptide, spacer, bifunctional chelator, and radioisotope. Denise Zwanziger and Annette G. Beck-Sickinger [1] summarise the biological and chemical properties of peptide hormones and their prerequisites for use as tumour targeting agents for both diagnostic and therapeutic purposes, alone or in combination with other peptide hormones or as the carriers for cytotoxic agents. Radiolabelled peptides have emerged as an important class of radiopharmaceuticals for imaging and therapy of inflammatory diseases and malignancies. These radiopharmaceuticals, which bind with high affinity and specificity to their receptors present in these structures, have an excellent diagnostic potential for the imaging of patients with chronic inflammatory diseases or tumours. The challenge is to label bioactive peptides without affecting their receptor binding properties. Size, plasma protein binding, lipophilicity and sensitivity to proteolysis are to be considered, with biodistribution, metabolism and excretion characteristics. G. Malviya, A. Signore, B. Laganà and R.A. Dierckx [2] describe the characteristics of peptides, cytokines and monoclonal antibodies with a particular emphasis to their role for therapy decision making and follow up in different inflammatory diseases. Cell penetrating peptides (CPPs) have the promising ability to cross the plasma membranes of mammalian cells in an apparently energy- and receptor-independent fashion. Most of the currently recognised CPPs are of cationic nature and derived from viral, insect or mammalian proteins endowed with membrane translocation properties. The exact mechanism underlying this translocation remains poorly understood, but this ability is being exploited to deliver a broad range of problematic therapeutic cargos, such as proteins, DNA oligomers, antibodies, peptide-nucleic acids, imaging agents, magnetic nanoparticles and liposomes in a variety of situations and biological systems. Veerle Kersemans, Ken Kersemans and Bart Cornelissen [3] present an overview of the use of CPPs for molecular imaging and discussed the difficulties and pitfalls of their utilisation. Peptide microarray technologies, based on the high-density immobilisation of surface-bound peptides on the solid planar supports, allowing them to sense protein activity (like substrates) or to act as small molecule ligands (for potential therapeutic leads) in profiling, detection or diagnostic applications. Peptides can be rapidly synthesised as large, defined library sets, which can be installed with orthogonal or directed chemical tags for convenient immobilisation on arrays. These approaches, allowing to miniaturise, parallelise and automate high throughput screening, have emerged as one of the most prominent and revolutionary technologies currently available for multiplexed detection.

Radiometal labeled peptide hormones are promising tools for a new generation of radiopharmaceuticals, because their receptors frequently are overexpressed in many human tumors. Furthermore, peptide hormones are characterized by different advantages for clinical application, such as high tumor-to-background ratios as well as rapid blood clearance. Peptidic tumor targeting agents can be subdivided into the following segments: peptide, spacer, bifunctional chelator and radioisotope. Here the biological and chemical properties of peptide hormones are summarized as well as their prerequisites for their use as tumor targeting agents. Additionally, promising bifunctional chelators and radioisotopes for radiometal labeling are reviewed. Some few special peptide hormones that have been pre-clinically or clinically investigated are furthermore presented, such as somatostatin, bombesin (BBS) / gastrin releasing peptide (GRP), vasoactive intestinal peptide (VIP) and neuropeptide Y (NPY). In vitro and in vivo investigations of the binding affinity, selectivity, metabolic stability, bioavailability and biodistribution of radiolabeled peptide hormones could lead to potential peptide-based tumor targeting agents for tumor diagnosis and therapy.

Radiolabelled peptides and monoclonal antibodies are an emerging class of radiopharmaceuticals for imaging inflammation with clinical implications for several chronic inflammatory disorders for diagnosis, therapy decision making and follow up. In the last decades, a number of novel monoclonal antibodies and peptides have been introduced for the treatment of different inflammatory disorders and also labelled with a variety of radionuclides depending upon the specific applications, diagnostic or therapeutic, by using direct or indirect methods. These radiopharmaceuticals bind to their targets with high affinity and specificity and therefore have an excellent diagnostic potential for the imaging of patients with chronic inflammatory diseases. In this review article we describe the characteristics of peptides, cytokines and monoclonal antibodies with a particular emphasis on their role in therapy decision making and follow up in different inflammatory diseases.

Cell penetrating peptides (CPPs) are a relatively new class of peptides that have the promising capability to cross cell membranes. While details remain to be resolved, various non-receptor-mediated endocytic pathways likely contribute most to the cell penetrating properties of these peptides. CPPs have been used to deliver many different cargos - ranging from radionuclides and other peptides to antibodies and nanoparticles - into cells. Besides many different drug delivery applications, CPPs have also seen a limited use in molecular imaging. Molecular imaging of intracellular and intranuclear targets, by techniques such as SPECT, PET, optical imaging, and MRI, relies heavily on the delivery of contrast agents to the cytoplasm and/or nuclei of the target tissue. Therefore, the number of applications in molecular imaging of intracellular targets has remained relatively low, because of the effective barrier presented by the cell membrane. One of the key strategies to overcome this challenge is the introduction of membrane-transducing peptides in the design of new contrast agents. This review presents an overview of the literature on CPPs, focusing on their use for molecular imaging. Applications using proteins and peptides, DNA/RNA, and CPP-loaded cells as the imaging agents will be looked at. Moreover, the difficulties and pitfalls regarding the use of CPPs in molecular imaging will be discussed.

Peptide microarrays have become increasingly accessible in recent years and as a result, more widely applied. Beyond its initial utility in substrate profiling, researchers are adopting peptide microarrays for the comparative screening of many different classes of enzymes, proteins/ proteomes and even living cells. Understanding the basis of peptide interactions at these diverse levels provides an unprecedented window into dissecting the complex cellular circuitries and molecular architectures of living systems. The peptides on the arrays may serve to sense protein activity (like substrates) or act as small molecule ligands (for potential therapeutic leads) in profiling, detection or diagnostic applications. This review will chart the progress made in peptide microarrays, with a focus on the recent advances that could impact how the field will be shaped in the coming years. These developments, along with the diminishing costs of library synthesis and growing commercial support, recognize that peptide microarrays will no longer remain just a vital research tool, but also a platform that could now be harnessed for wider drug discovery and point-of-care applications.

The peptide nucleic acids (PNAs) constitute a remarkable new class of synthetic nucleic acids analogs, based on peptide-like backbone. This structure gives to PNAs the capacity to hybridize with high affinity and specificity to complementary RNA and DNA sequences, and a great resistance to nucleases and proteinases. Originally conceived as ligands for the study of double stranded DNA, the unique physico-chemical properties of PNAs have led to the development of a large variety of research and diagnostic assays in the field of genetics, including genome mapping and mutation detection. Over the last few years, the use of PNAs has also proven its powerful usefulness in cytogenetics for the rapid in situ identification of human chromosomes and the detection of aneuploidies. Recent studies have reported the successful use of chromosome-specific PNA probes on human lymphocytes, amniocytes, spermatozoa as well as on isolated oocytes and blastomeres. Multicolor peptide nucleic acid-fluorescence in situ hybridization (PNA-FISH) protocols have been described for the identification of several human chromosomes, indicating that PNAs could become a powerful tool for in situ chromosomal investigation.

The early identification of susceptibility to adverse cardiovascular outcomes and risk stratification amongst asymptomatic individuals, as well as amongst those with overt disease continues to be one of the major priorities of clinically-orientated research in the field of atherothrombosis. Available data from epidemiological studies indicate that traditional risk factors do not fully explain the predisposition to cardiovascular disease, its dynamics in different population groups and treatment responses. The pressing need for the development and clinical implementation of new markers of atherothrombotic disease has fuelled rapidly expanding research into cardiac biomarkers. This review outlines the main principles of biomarker qualification that have entered clinical practice, as well as an overview of the development of targeted biomarkers across the cardiovascular “continuum”. We discuss in detail the evidence from epidemiological and clinical studies advocating the potential clinical use of the most promising candidate plasma biomarkers (more specifically, C-reactive protein, coagulation and inflammatory mediators and natriuretic peptides). Such an application of biomarkers to aid clinical risk assessment would be important in our efforts to improve risk stratification of subjects at risk of cardiovascular events.

Over two decades of research on venom peptides derived from cone snails (“conopeptides or conotoxins”) has led to several compounds that have reached human clinical trials, most of them for the treatment of pain. Remarkably, none of the conopeptides in clinical development mediate analgesia through the opioid receptors, underlying the diverse and novel neuropharmacology evolved by Conus snails. These predatory animals produce an estimated ∼100,000 distinct conotoxins, a vast majority yet to be discovered and characterized. The conopeptides studied to-date in animal models, have exhibited antinociceptive, antiepileptic, neuroprotective or cardioprotective activities. Screening results also suggest applications of conotoxins in cancer, neuromuscular and psychiatric disorders. Additional potentially important applications of conotoxin research are the discovery and validation of new therapeutic targets, also defining novel binding sites on already validated molecular targets. As the structural and functional diversity of conotoxins is being investigated, the Conus venoms continue to surprise with the plethora of neuropharmacological compounds and potential new therapeutics. This review summarizes recent efforts in the discovery of conopeptides, and their preclinical and clinical development.

Many peptides are potent and highly selective blockers or modulators of calcium channel function, and as such are valuable pharmacological tools and potentially valuable leads for the development of human therapeutics. Cone shells and spiders are rich sources of such peptides, although they are also found in scorpions and insects. In this article we compare the amino acid sequences of toxins active against calcium channels and describe their three-dimensional structures and structure-function relationships. Certain structural motifs, in particular the inhibitor cystine knot, prove to be quite common amongst this class of toxins. Aspects of the pharmacology and physiology of these toxins in mammalian systems are also discussed, with an emphasis on their application in the treatment of chronic pain. We then consider the prospects for peptide-based therapeutics targeting calcium channels for this and other indications, including the development of non-peptide (peptidomimetic) compounds based on a detailed understanding of toxin structure-function relationships.

Throughout millions of years of evolution, nature has supplied various organisms with a massive arsenal of venoms to defend themselves against predators or to hunt prey. These venoms are rich cocktails of diverse bioactive compounds with divergent functions, extremely effective in immobilizing or killing the recipient. In fact, venom peptides from various animals have been shown to specifically act on ion channels and other cellular receptors, and impair their normal functioning. Because of their key role in the initiation and propagation of electrical signals in excitable tissue, it is not very surprising that several isoforms of voltage-activated sodium channels are specifically targeted by many of these venom peptides. Therefore, these peptide toxins provide tremendous opportunities to design drugs with a higher efficacy and fewer undesirable side effects. This review puts venom peptides from spiders, scorpions and cone snails that target voltage-activated sodium channels in the spotlight, and addresses their potential therapeutical applications.

Animal venoms are rich natural sources of bioactive compounds, including peptide toxins acting on the various types of ion channels, i.e. K+, Na+, Cl- and Ca2+. Among K+ channel-acting toxins, those selective for voltage-gated K+ (Kv) channels are widely represented and have been isolated from the venoms of numerous animal species, such as scorpions, sea anemones, snakes, marine cone snails and spiders. The toxins characterized hitherto contain between 22 and 60 amino acid residues, and are cross-linked by two to four disulfide bridges. Depending on their types of fold, toxins can be classified in eight structural categories, which showed a combination of β-strands, helices, or a mixture of both. The main architectural motifs thereof are referred to as α/β scaffold and inhibitor cystine knot (ICK). A detailed analysis of toxin structures and pharmacological selectivities indicates that toxins exhibiting a similar type of fold can exert their action on several subtypes of Kv channels, whereas a particular Kv channel can be targeted by toxins that possess unrelated folds. Therefore, it appears that the ability of structurally divergent toxins to interact with a particular Kv channel relies onto a similar spatial distribution of amino acid residues that are key to the toxin-channel interaction (rather than the type of toxin fold). The diversity of Kv channel blockers and their therapeutic value in the potential treatment of a number of specific human diseases, especially autoimmune disorders, inflammatory neuropathies and cancer, are reviewed.