Current Pharmaceutical Design - Volume 11, Issue 25, 2005

Drug discovery and development is time consuming and a costly procedure. The challenges for pharmaceutical companies range from the evaluation of potential new drug candidates, the determination of drug pharmacokinetics/pharmacodynamics, the measurement of receptor occupancy as a determinant of drug efficacy, and the pharmacological characterisation of mechanisms of action. Radiolabelled molecules have a unique ability to monitor biochemical reactions, ligand-receptor or enzyme interactions and many other biological pathways at subnanomolar concentrations. The recent advances in computer emission tomography, image reconstruction and animal models of disease has led to the development of extremely sensitive and specific tools for imaging biochemical processes in vivo therefore representing a new means of providing information for drug development and evaluation. This issue of Current Pharmaceutical Design highlights the capabilities of PET and SPECT in preclinical and clinical research and acknowledges the efforts from each contributor in putting this issue together. John Gatley and co-workers [1] at the Brookhaven National Laboratory outlines the use of PET and SPECT in the study of human psychopharmacology. These in vivo imaging studies have been useful in conjunction with neuropsychological testing of subjects, to allow correlation of imaging data with uniquely human aspects of the effects of drugs, such as euphoria and craving. Frederic Dolle [2] at the Service Hospitaleir Frederic-Joliot outlines the recent applications of these nucleophilic heteroaromatic substitutions in the pyridine series and highlights its potential in the design and preparation, of often drug-based, fluorine-18-labelled radiotracers and radiopharmaceuticals of high specific radioactivity for PET imaging. Denis Guilloteau and Sylvie Chalon [3] at the University of Tours outline the in vivo exploration of monoamine transporters. This will enhance our knowledge of physiopathological mechanisms of brain disorders; allow early diagnosis of cerebral dysfunctions and early use of new therapies, selection of homogenous classes of subjects for therapeutic assays, objectiveness of drug-molecular target interaction, and follow-up of disease progression and treatment. Martin Pomper and co-workers [4] at the Johns Hopkins University outlines strategies relating to animal imaging and how these studies have the potential to play a major role in drug development. Most of molecular imaging research is undertaken in small animals, which provide a conduit between in vitro studies and human clinical imaging. The ability to study those animals noninvasively and quantitatively with new, high-resolution imaging devices provides the most relevant milieu in which to find and examine new therapies. References [1] Gatley SJ, Volkow ND, Wang G-J, Fowler JS, Logan J, Ding Y-S, Gerasimov M. PET Imaging in Clinical Drug Abuse Research. Curr Pharm Design 2005; (25): 3203-3219. [2] Dolle F. Fluorine-18-Labelled Fluoropyridines: Advances in Radiopharmaceutical Design. Curr Pharm Design 2005; (25): 3221-3235. [3] Guilloteau D, Chalon S. PET and SPECT Exploration of Central Monoaminergic Transporters for the Development of New Drugs and Treatments in Brain Disorders. Curr Pharm Design 2005; (25): 3237-3245. [4] Pomper MG, Lee JS. Small Animal Imaging in Drug Development. Curr Pharm Design 2005; (25): 3247-3272.

Over the last two decades, SPECT (single photon emission computed tomography) and especially PET (positron emission tomography) have proven increasingly effective imaging modalities in the study of human psychopharmacology. Abusing populations can be studied at multiple times after abstinence begins, to give information about neurochemical and physiological adaptations of the brain during recovery from addiction. Individual human subjects can be studied using multiple positron labeled radiotracers, so as to probe more than one facet of brain function. PET and SPECT have been used to help our understanding of many aspects of the pharmacokinetics and pharmacodynamics of abused drugs, and have made valuable contributions in terms of drug mechanisms, drug interactions (e.g. cocaine and alcohol) and drug toxicities. They have also been employed to study the acute effects of drugs on populations of active drug abusers and of normal controls, and to evaluate the neurochemical consequences of candidate therapies for drug abuse. A particularly productive strategy has been the use of PET in conjunction with neuropsychological testing of subjects, to allow correlation of imaging data with uniquely human aspects of the effects of drugs, such as euphoria and craving.

Positron Emission Tomography is a high-resolution, sensitive, functional imaging technique, which can efficiently give access to the distribution, pharmacokinetics and -dynamics of a drug in vivo and which can therefore advantageously play a key-role in both drug discovery and development. This molecular imaging technique requires the preparation of a positron-emitting radiolabelled probe or radiotracer and for this purpose, fluorine-18 is becoming, more and more often, the radionuclide of choice (adequate physical and nuclear characteristics and potential wide use and - distribution of fluorine-18-labelled radiopharmaceuticals). Considering chemical structures showing a fluoropyridinyl moiety, nucleophilic heteroaromatic substitution at the ortho-position with no-carrier-added [18F]fluoride appears today as the most efficient method for the radiosynthesis of radiotracers and radiopharmaceuticals of high specific radioactivity when compared to homoaromatic-, but also aliphatic, nucleophilic radiofluorination. Like for the aliphatic nucleophilic radiofluorinations, only a good leaving group is required (a halogen, or better a nitro- or a trimethylammonium group). There is no need for an additional strong electron-withdrawing substituent for activation of the aromatic ring such as in the homoaromatic nucleophilic radiofluorinations, except if one considers meta-fluorination. Nucleophilic heteroaromatic substitution and consequent fluorine-18 incorporation are generally performed in DMSO with the no-carrier-added, activated K[18F]F-K222 complex using conventional heating at a moderately high temperature (120-150°C) or microwave irradiation (100 Watt) for a short period of time (1-2 minutes) and often lead to high radiochemical yields. This review summarizes some of the recent applications of these nucleophilic heteroaromatic substitutions in the pyridine series and highlights its potential in the design (not seldom by hydrogen, hydroxyl or halogen replacement by fluorine) and preparation, of often drug-based, fluorine-18-labelled radiotracers and radiopharmaceuticals of high specific radioactivity for PET imaging.

Membrane and vesicular monoaminergic transporters, responsible for the homeostasis of neurotransmitter pools at nerve endings, are very involved in the physiology and diseases of central nervous system. Recent progresses of cerebral molecular imaging using SPECT and PET methods allow the extend of in vivo exploration of these transporters. For this aim, an increasing number of radiopharmaceuticals labelled with [123I], [99mTc], [11C] or [18F] have been developed such as cocaine derivatives for the DAT, compounds from the diphenyl sulfide family for the SERT, and dihydrotetrabenazine derivatives for the VMAT2. These functional imaging methods can be very useful in several neurological and psychiatric disorders which involve the monoaminergic neurotransmission systems such as Parkinson's disease, ADHD, depression and autism. For example, the DAT is a specific index of the density of dopaminergic endings which progressively degenerate in Parkinson's disease. In vivo exploration of this transporter can therefore be a relevant way (i) to realize an early detection of the loss of dopaminergic neurons, (ii) to assess the progression of the disease, (iii) to validate and improve the efficacy of new therapeutic strategies such as neuroprotection and neuroreparation. In all, the extend of in vivo exploration of monoamine transporters will allow great progress for (1) knowledge of physiopathological mechanisms of brain disorders, (2) early diagnosis of cerebral dysfunctions, allowing early use of new therapies, (3) selection of homogenous classes of subjects for therapeutic assays, (4) objectiveness of drug-molecular target interaction, (5) follow-up of disease evolution and treatment.

Better mechanistic understanding of disease through mapping of the human and mouse genomes enables rethinking of human infirmity. In the case of cancer, e.g., we may begin to associate disease states with their underlying genetic defects rather than with the organ system involved. That will enable more selective, nontoxic therapies in patients who are genetically predisposed to respond to them. Because one of the major goals of molecular imaging research is to interrogate gene expression noninvasively, it can impact greatly on that process. Most of molecular imaging research is undertaken in small animals, which provide a conduit between in vitro studies and human clinical imaging. We are fortunate to be able to manipulate small animals genetically, and to have increasingly better models of human disease. The ability to study those animals noninvasively and quantitatively with new, high-resolution imaging devices provides the most relevant milieu in which to find and examine new therapies.

Hypertension is one of the most powerful cardiovascular risk factors. By lowering blood pressure, antihypertensive drug treatment reduces the incidence of stroke and coronary heart disease. In general, the currently recommended antihypertensive agents (α-blockers, angiotensin converting-enzyme inhibitors, angiotensin type-1 receptor blockers, β-blockers, calciumchannel blockers, and diuretics) provide similar cardiovascular protection [1]. In randomized controlled trials, the achieved systolic blood pressure largely accounts for cardiovascular outcome, emphasizing the need of tight blood pressure control. The failure in showing the superiority of new antihypertensive drugs (α-blockers, angiotensin converting-enzyme inhibitors, angiotensin type-1 receptor blockers, or calcium-channel blockers) to conventional therapy (diuretics or β-blockers) should not discourage the pharmaceutical industry to invest in developing novel blood pressure lowering agents, because the control rate of hypertension remains low in all communities, particularly for the elevated systolic blood pressure [2]. Indeed, in clinical trials as well as regular clinical practice, hypertensive patients often require 2 to 3 or even more antihypertensive drugs to normalize their systolic and diastolic blood pressure (below 140/90 mm Hg). The emerging basic research provides the potential to develop more potent antihypertensive drugs with a better safety profile. In this issue of Current Pharmaceutical Design, 5 experts and their colleagues give a broad overview on the current status and future perspectives of the development of antihypertensive drugs on the basis of their own work and/or by intense scrutiny of the literature. Magy and colleagues [3] provide a comprehensive review of the recent experimental evidence on the renin-angiotensin system and demonstrate a new concept that angiotensin II and its derivatives are involved in protective mechanisms against cerebral ischemia probably via the non-angiotensin II type 1 receptors. Dr. Quaschning [4] summarizes the vasopeptidase inhibition approach in the treatment of hypertension and heart failure. Dr. Ferrandi and colleagues [5] describe a new antihypertensive pathway and a newly-designed compound, PST 2238, that selectively antagonizes the pressor effect and the alteration of renal Na+-K+ pump and efficiently lowers blood pressure. Dr. Augustyniak and colleagues [6] present a series of experimental work on the role of nitric oxide and nitric oxide synthase in the development of hypertension, and highlight that the nitric oxide pathway is elucidating novel antihypertensive drug targets, in particular for refractory hypertension. Drs Blacher and Safar [7] discuss the role of arterial stiffness measurements in risk assessment as well as in risk reduction strategies by monitoring arterial stiffness under different pharmacological regimens with a blood pressure lowering action. References [1] Staessen JA, Wang JG, Thijs L. Cardiovascular prevention and blood pressure reduction: a quantitative overview updated until 1 March 2003. J Hypertens 2003; 21: 1055-76. [2] Gu D, Reynolds K, Wu X, Chen J, Duan X, Muntner P, Huang G, Reynolds RF, Su S, Whelton PK, He J, InterASIA Collaborative Group. The International Collaborative Study of Cardiovascular Disease in ASIA. Prevalence, awareness, treatment, and control of hypertension in china. Hypertension 2002; 40: 920-7. [3] Magy L, Vincent F, Messerli FH, Wang JG, Achard JM, Fournier A. The renin-angiotensin systems : evolving pharmacological perspectives for cerebroprotection. Curr Pharm Design 2005; 11(25): 3275-3291. [4] Quaschning T. Vasopeptidase inhibition for blood pressure control: emerging experience. Curr Pharm Design 2005; 11(25): 3293-3299. [5] Ferrandi M, Barassi P, Molinari I, Torielli L, Tripodi G, Minotti E, Bianchi G, Ferrari P. Ouabain antagonists as antihypertensive agents. Curr Pharm Design 2005; 11(25): 3301-3305. [6] Augustyniak RA, Thomas GD, Victor RG, Zhang W. Nitric oxide pathway as new drug targets for refractory hypertension. Curr Pharm Design 2005; 11(25): 3307-3315. [7] Blacher J, Safar ME. Large artery stiffness and antihypertensive agents. Curr Pharm Design 2005; 11(25): 3317-3326.

During the last 20 years, the renin-angiotensin system (RAS) has become an increasingly important focus of basic and clinical cardiovascular research. One main conceptual step forward was made with the discovery of a tissue RAS and the understanging of its critical pathophysiological role in atherogenesis and plaque destabilisation [1]. Major effort to find new strategies for blocking the RAS has produced new classes of drugs which were expected to be clinically important in the management of hypertension and heart failure. As landmark clinical studies have demonstrated that inhibition of the RAS significantly reduces morbidity and mortality from coronary heart disease, myocardial infarction and heart failure, the concept has rapidly emerged that blocking the RAS was the strategy of choice for preventing cardiovascular diseases [2]. More recently, basic research has however continuously extended our understanding of the complexity of the systemic and tissue RASs, that can no longer be viewed as one-way streets in which one single effector, angiotensin II acts solely through its major (AT1) receptor. Meanwhile, clinical trials have challenged the concept that blocking the RAS is the most effective preventive strategy for all patients and all target organs [3]. Consistent with the recent understanding that the RAS encompasses a number of distinct effectors acting through different receptors to promote opposite effects, a growing body of basic and clinical evidence suggests that blunting the RAS is a double-edge sword, with beneficial effects counterbalanced by deleterious ones, resulting in a net effect that critically depends on the experimental conditions, or the clinical characteristics of the study population. Of particular clinical relevance, a number of clinical trials point to the somewhat provocative conclusion that beyond their blood pressure lowering effect antihypertensive drugs that decrease angiotensin II formation are less stroke protective than the ones that increase angiotensin levels [4]. This review focuses on the recent experimental evidence demonstrating that angiotensin II and its derivatives acting through the non-AT1 receptors are involved in protective mechanisms against cerebral ischaemia and discusses in the light of the recent large cardiovascular prevention trials the clinical relevance of this new concept. The perspective of a renewal of therapeutical strategies to optimise the prevention of target organ damage and perhaps even some of the diseases of ageing, such as loss of cognitive function is emphasised.

Vasopeptidase inhibition is a novel treatment approach in cardiovascular disease such as hypertension and heart failure. Since the inhibition of the angiotensin-converting enzyme (ACE) turned out to represent a very successful principle in the treatment of hypertension in numerous large scale clinical studies, their results encouraged attempts to inhibit other key enzymes in the regulation of vascular tone as well - such as the neutral endopeptidase (NEP). Similar to ACE, NEP is an endothelial cell surface metalloproteinase, which is involved in the degradation of several regulatory peptides including the natriuretic peptides and thus augments vasodilatation and natriuresis through increased levels of atrial natriuretic peptide (ANP). By simultaneous inhibition of the RAS and potentiation of the natriuretic peptide system, combined NEP/ACE inhibitors - the so called vasopeptidase inhibitors - reduce vasoconstriction and enhance vasodilatation, therefore, decreasing peripheral vascular resistance and blood pressure. Based on these considerations, numerous preclinicial studies with vasopeptidase inhibitors were performed and revealed promising results in experimental hypertension. Correspondingly, large scale clinical studies in patients with hypertension are on the way. Their preliminary results indicate that combined inhibition of ACE and NEP by vasopeptidase inhibitors represents an effective strategy in the treatment of hypertension and other cardiovascular disease such as heart failure. However, clinical data also suggest that the incidence of angioedema may increase on vasopeptidase inhibition. Therefore, careful evaluation of the safety of this promising therapeutic principle in large scale clinical studies is mandatory before vasopeptidase inhibition may be considered a novel option in the treatment of cardiovascular disease.

The evidence that high levels of endogenous ouabain (EO), a closely related isomer of ouabain, are implicated in human hypertension and cardiac hypertrophy and failure stimulated the pharmacological research for developing novel anti-hypertensive agents active as ouabain antagonists. The pathogenetic mechanisms through which increased EO levels affect cardiovascular system involve the modulation of Na-K ATPase, the key enzyme responsible for renal tubular sodium reabsorption and the activation of signalling transduction pathways implicated in growth-related gene transcription. By studying both genetic and experimental rat models of hypertension and comparing them with humans, our group has demonstrated that elevated levels of circulating EO and the genetic polymorphism of the cytoskeletal protein adducin associate with hypertension and high renal Na-K pump activity. Ouabain itself induces hypertension and up-regulates renal Na-K pump when chronically infused at low doses into rats (OS). In renal cultured cells, either incubated for several days with nanomolar concentrations of ouabain or transfected with the hypertensive adducin genetic variant, the Na-K pump results enhanced. Moreover, both EO and adducin polymorphism affect cardiac complications associated to hypertension, the former through the activation of a signalling transduction pathway. As a consequence, a compound able to interact with the cellular and molecular alterations, sustained by EO or mutated adducin, may represent the suitable treatment for those patients in whom these mechanisms are at work. A new antihypertensive compound, PST 2238, that selectively antagonises the pressor effect and the alteration of renal Na-K pump, sustained both by ouabain and adducin polymorphism, is described. A selective ability of PST 2238 to antagonise the ouabain-induced organ hypertrophy is also documented. The specificity of PST 2238 mechanism of action is supported by the absence of interactions with receptors or hormones involved in blood pressure regulation and by the lack of diuretic activity and diuretic-associated side effects. It is concluded that this compound could be useful for the treatment of those forms of essential hypertension in which renal Na handling alterations and cardiac complications are associated with either increased EO levels and/or adducin polymorphism.

Nitric oxide (NO) is thought to reduce blood pressure by evoking vasodilation either directly by causing relaxation of vascular smooth muscle or indirectly by acting in the rostral brainstem to reduce central sympathetic outflow, which decreases the release of norepinephrine from sympathetic nerve terminals. An increasingly large body of literature suggests that alterations in the NO system may play an important role in the development or maintenance of clinical hypertension. As proof of concept, pharmacological inhibition of nitric oxide synthase (NOS) in humans and animals causes moderate to severe hypertension. Certain forms of secondary hypertension are accompanied by the accumulation of endogenous NOS inhibitors, which may contribute to the development of hypertension. Furthermore, targeted disruption of the endothelial isoform of NOS in mice causes moderate hypertension, implying that hypertension may also develop from reductions in NOS expression. These gene knockout studies in animals have initiated the search for single nucleotide polymorphisms in human NOS genes, which could potentially lead to decreases in NOS protein expression. Conversely, increases in NOS expression or NO production have been linked with several commonly used cardiovascular therapies, including exercise training and the use of both statins and angiotensin-converting enzyme inhibitors. Finally, increases in the production of oxidants such as superoxide anion can lead to the inactivation of NO, thereby reducing NO bioavailability. Thus, alterations in the expression or activity of NOS or in the availability of NO have the potential to play a causal role in clinical hypertension. The purpose of this article is to show how emerging basic research on the NO pathway is elucidating novel antihypertensive drug targets that are on the cusp of clinical application.

Purpose of Review: Since in hypertensive populations, concentration on peripheral blood pressure only does not achieve 100% of blood pressure-attributable risk reduction, taking into consideration other hemodynamic parameters than peripheral blood pressure could perhaps improve cardiovascular prevention. The main purpose of this review is to analyse the scientific data in favour of considering arterial stiffness parameters as interesting intermediate cardiovascular endpoints in order to optimise risk assessment and risk reduction strategies. Summary: Aortic pulse wave velocity (PWV), a marker of aortic stiffness, has been shown to be a strong independent predictor of cardiovascular morbid events, cardiovascular and all-cause mortality in numerous studies in different populations. Furthermore, it has been shown in a therapeutic trial that the lack of aortic PWV attenuation despite significant drug-induced reduction in mean blood pressure was a significant predictor of cardiovascular death in subjects with end-stage renal disease. In essential hypertension, the Reason Study has shown that, despite a similar decrease in peripheral diastolic blood pressure, different effects on central hemodynamic parameters were observed between blockade of the renin-angiotensin system and atenolol. Novel therapeutic approaches available to reduce the increase of pulse pressure and arterial stiffness with age involve converting enzyme inhibitors in association with diuretic compounds; nitrate derivatives; agents acting on collagen cross-linking; and finally spironolactone and vasopeptidase inhibitors. Conclusion: These results support the hypothesis that measurement of aortic PWV could then help, not only in risk assessment strategies but also in risk reduction strategies by monitoring arterial stiffness under different pharmacological regimens.