Current Medicinal Chemistry - Cardiovascular & Hematological Agents - Volume 2, Issue 3, 2004

The activation of platelets and the resultant aggregation have been shown to play important role in the pathogenesis of cardiovascular, cerebrovascular and peripheral vascular diseases and in acute coronary syndromes. Hence platelet adhesion and aggregation have been identified as promising targets for the development of anti-thrombotic drugs. Glycoprotein (GP) IIb / IIIa antagonism exerts a strong anti-platelet effect, because this interference inhibits the final common pathway of platelet aggregation and is not dependent on a single activation pathway. Three GPIIb / IIIa antagonists have been approved by the US Food and Drug administration. They include abciximab (the chimeric monoclonal antibody 7E3 Fab fragment), eptifibatide (the cyclic heptapeptide based on the KGD amino acid sequence) and tirofiban (a nonpeptide tyrosine derivative). In addition, nonpeptide oral GPIIb / IIIa antagonists are also in various stages of clinical development. This paper reviews the molecular biology of the GPIIb / IIIa receptor, history of development of GPIIb / IIIa antagonists, some issues about GPIIb / IIIb antagonists including their affinity, reversibility and receptor specificity, adverse effects including bleeding and thrombocytopenia, clinical trials and costs. Future direction in the development of GPIIb / IIIa antagonists is also discussed.

Obesity is associated with increased incidence of cardiovascular mortality. However, the mechanisms that link increased fat mass with hypercholesterolemia, hypertension, endothelial dysfunction and coronary heart disease have not been fully elucidated. Unravelling the diverse neuroendocrine systems, which regulate energy balance and body fat has been a long-standing challenge in biology, with obesity as an increasingly important public health focus. Until recently, the adipocyte has been considered only a passive tissue for the storage of excess energy in the form of fat. However, there is now compelling evidence that adipocytes act as endocrine, secretory cells. It has been shown that several hormones, growth factors and cytokines are actually expressed in white adipose tissue. In a dynamic view of the adipocyte a wide range of signals emanates from white adipose tissue such as tumour necrosis factor-α (TNF-α), interleukin-6 (IL-6), and their respective soluble receptors. White adipose tissue also secretes important regulators of lipoprotein metabolism like lipoprotein lipase (LPL), apolipoprotein E (apoE) and cholesteryl ester transfer protein (CETP). The increasing number of products secreted by adipocytes also includes leptin, estrogen, angiotensinogen, plasminogen activator inhibitor-1 (PAI-1), tissue factor and transforming growth factor-β (TGF-β). Nitric oxide synthase (NOS) has been also reported to be expressed in white adipose tissue. Acylation stimulating protein (ASP), adipophilin, adipoQ, adipsin, monobutyrin, agouti protein and factors related to pro-inflammatory and immune processes have also been shown to be released by white adipocytes. Since blood vessels express receptors for most of the adipocyte-derived factors, adipose tissue seems to play a key role in cardiovascular physiology through the existence of a network of local and systemic signals. The current knowledge in this field will be reviewed in the broader perspective of cardiovascular physiology and pathophysiology.

Phospholipase A2 (PLA2) catalyzes the hydrolysis of sn-2 fatty acids from membrane phospholipids resulting in the production of several biologically active phospholipid metabolites such as lysophospholipids, arachidonic acid, eicosanoids and platelet-activating factor. The majority of myocardial PLA2 activity is membrane-associated and does not require Ca2+ for activity (iPLA2). Myocardial iPLA2 demonstrates unique characteristics when compared to other PLA2 isoforms described previously, including a selectivity for plasmalogen phospholipids and resistance to inhibition by methyl arachidonyl fluorophosphonate. Activation of myocardial iPLA2 results in the production of lysoplasmenylcholine and arachidonic acid, both of which can change the electrophysiologic properties of the myocardium. Arachidonic acid can modulate ion channel activity via protein kinase C activation and has been demonstrated to decrease gap junctional conductance. Lysoplasmenylcholine directly produces action potential derangements and alters calcium cycling in cardiac myocytes. Thus, inhibition of iPLA2 activity to block production of phospholipid metabolites that mediate pathologic changes in the myocardium would be of considerable benefit. However, there are situations where inhibition of PLA2 activity would be detrimental to the myocardium, in particular if iPLA2 acts as a phospholipid repair enzyme following oxidative damage. Although little is known regarding the function of cPLA2 or sPLA2 in the myocardium, it is possible that they may be important for signal transduction or may modulate the activity of iPLA2.

In this paper, we review our current understanding of the medicinal chemistry of the major peptide systems, which influence body fluid homeostasis. Electrolytes play pivotal roles in intra- and intercellular communication, acidbase equilibrium and, when bound to several macromolecules, they regulate a myriad of enzymatic proteins, receptors and transcription factors. Cell turgor influences the plasma membrane, which activates mechanically-gated ion channels or mechanoreceptors, and the expression of a number of genes which underlie long-term metabolic responses to hormones, substrates and reactive oxygen intermediates. The altered kinetics and enzymatic cleavage of peptides during waterelectrolyte imbalance can contribute to cardiac and renal damage associated with elevated blood pressure. Identification of the enzymes which are responsible for cleavage, together with emerging information about the mechanisms of action and structures of regulatory and effector peptides, has laid a foundation for the discovery of novel drugs, some of which are in use or are now undergoing evaluation in experimental trials. The development of models of hydrosaline challenge with relative efficiency to induce selective water-electrolyte imbalance has permitted the identification of kallikrein-kinin, renin-angiotensin-aldosterone, vasopressin-oxytocin, thyrotropin-releasing hormone and luteinizing hormone-releasing hormone as susceptible substrates. At present, the angiotensin-I converting enzyme inhibitors are well-known efficacious, orally active, blood pressure-lowering agents which have been used in hypertensive patients. In addition to several new analogues of this class of drug, some selective dual inhibitors of angiotensin-I converting enzyme and neutral endopeptidase and inhibitors of aminopeptidases are now also being rationally assayed and their beneficial effects on hypertension and hydromineral balance indicate that this type of drug may have powerful therapeutic effects for disorders of body fluid homeostasis.

A novel human plasma protein was found in the eluate from the dextran sulfate column, which was used for the treatment of the patients with hypercholesteremia to reduce plasma low density lipoprotein. The results of sequence analysis revealed that this protein was a homologue of heavy chains of inter-alpha-trypsin inhibitor (ITI) family, and it was termed IHRP (ITI family heavy chain related protein). IHRP was identified as an acute-phase protein in animals, and slightly increased concentrations in human plasmas were observed in the patients with inflammatory disorders. IHRP bound to actin and inhibited its polymerization, and IHRP suppressed the phagocytosis and chemotaxis of polymorphonuclear cells. These results suggest that IHRP may function as an anti-inflammatory protein. Plasma hyaluronan binding protein (PHBP) is a novel serine protease which was also found in human plasma. It is consisted of three epidermal growth factor domains, one kringle domain and one serine protease domain from its amino terminus. The amino acid sequence of PHBP is homologous to that of hepatocyte growth factor activator. Purified 75-kDa single chain pro-form of PHBP was auto-activated (auto-cleaved) to 50-kDa heavy chain and 25-kDa light chain, both of them are bridged by a disulfied bond. PHBP digested α-chain and β-chain of fibrinogen to prevent coagulation and cleaved single chain urokinase type plasminogen activator (scuPA) to the active hetero dimer form (tcuPA). The auto-activation of PHBP was accelerated in the presence of dextran sulfate or phosphatidylethanolamine as well as factor XII of the coagulation system. C1 inhibitor of the complement system was identified as the main inhibitor of PHBP in human plasma. Partial hepatectomy and administration of carbon tetrachloride or galactosamine caused the conversion of pro- PHBP to the active form in mouse but administrations of turpentine and mercury chloride did not, suggesting the hepatic injury specific activation of PHBP. These results indicate that PHBP participates not only in the fibrinolytic system but also in the degradation cascade of extracellular matrix (ECM), i. e., PHBP activates scuPA to tcuPA, tcuPA activates matrix metalloproteases (MMPs) and activated MMPs degrade ECM for the tissue remodeling after hepatic injury.

Cardiovascular disease continues to be a major health problem. Tremendous efforts have been invested in clinical and laboratory research in the hopes of decreasing the risk of patients with cardiovascular disease undergoing cardiac and non-cardiac surgeries. A powerful endogenous mechanism of cardioprotection, termed ischemic preconditioning, was reported in a laboratory setting whereby the myocardium can be preconditioned by a brief ischemic episode and protected against a subsequent prolonged ischemic attack. Since this initial observation, several pharmacological agents have been demonstrated to mimic ischemic preconditioning. These include opioids, potassium channel openers such as pinacidil and diazoxide, and adenosine agonists. Recently, volatile anesthetics were found to be powerful cardiac preconditioning agents. Infarct size reduction as the result of anesthetic-induced preconditioning paralleled that of ischemic preconditioning. The use of volatile anesthetics as preconditioning agents in high-risk patients undergoing cardiac and non-cardiac surgeries can potentially result in the reduction of morbidity and disability in this class of patients. However, to fully realize the potential use of this novel property of volatile anesthetics, the underlying mechanism of anesthetic-induced preconditioning would need to be elucidated. This review highlights the major recent findings on the cardioprotective effects of volatile anesthetics.

Prostaglandin I2 (PGI2, prostacyclin), an eicosanoid of the cyclooxygenase pathway, causes relaxation of vascular smooth muscle in most blood vessels and inhibits platelet aggregation. PGI2 and its stable analogues activate a specific cell-surface receptor (IP receptor, IPR), which is coupled to adenylyl cyclase through Gs-protein. Elevation of 3': 5'-cyclic monophosphate (cyclic AMP, cAMP) levels has been considered to be a key cellular event to trigger blood vessel relaxation by IP agonists; however, its exclusive role has been recently challenged. Downstream effectors of the IP agonist metabolic cascade are plasma membrane K+ channels that upon activation would cause smooth muscle cell hyperpolarization and relaxation. The K+ channel candidates include ATP-sensitive K+ (KATP) channel and large conductance, Ca2+-activated K+ (MaxiK, BK) channel. The contribution of each K+ channel subtype would be governed by their relative expression and / or particular co-localization with different proteins of the IPR signaling cascade in each vascular bed. Scrutiny of the cellular mechanisms underlying IPR-activated vascular relaxation of a large conduit artery revealed that relaxation by an IP agonist, beraprost, is elicited through cAMP-independent pathway as well as by a cAMPdependent route. Both mechanisms include activation of MaxiK channels. The cAMP-independent vasorelaxant mechanism is partly attributed to a direct activation of MaxiK channel by Gs-protein. In this review article, we discuss cAMP-dependent and -independent mechanisms by which IPR stimulation activates MaxiK channel. Our recent work demonstrates a functional tight coupling between IPR and MaxiK channel through a cAMP-independent, Gs-protein mediated mechanism(s) in vascular smooth muscle.

Thrombin converts fibrinogen to fibrin and is the most powerful activator of platelets thus playing a crucial role in arterial and venous thrombosis. The limitations of heparin, largely used in the therapy of arterial and venous thromboembolism, has prompted the development of new antithrombotic drugs, able to directly inhibit thrombin. They comprise hirudin, bivalirudin and argatroban, which are antithrombins for parenteral use, and the orally active ximelagatran which, once absorbed, is converted to the active compound melagatran. Hirudin is a polypeptide able to irreversibly block both the active site and the fibrin(ogen) binding site of thrombin; bivalirudin, a synthetic hirudin derivative, has the same binding sites of hirudin to thrombin but has a shorter pharmacological action and is safer for clinical use. Several clinical trials which tested these drugs in acute coronary syndromes, coronary angioplasty and venous thromboembolism, demonstrate that hirudin and bivalirudin are superior to heparin in significantly reducing cardiac major events. The advantage of hirudin and bivalirudin over heparin was also confirmed in adjuncts to thrombolytic therapy as well as in percutaneous angioplasty relating to thrombotic events but not to restenosis. Hirudin was also significantly better than both unfractionated heparin and low molecular weight heparin (LMWH) in the prophylaxis of venous thromboembolism in patients undergoing elective arthroplasty. Major bleeding associated to hirudin was not different from that observed with heparin. Preliminary data also indicate that melagatran / ximelagatran may be used in the prophylaxis of venous thromboembolism and in the prevention of arterial embolism in patients with non-valvular atrial fibrillation.