Current Drug Targets - Cardiovascular & Hematological Disorders - Volume 1, Issue 1, 2001

Advances in biochemistry, protein chemistry and molecular biology over the last twenty-five years have spurred the increased use and development of proteins as injectable therapeutic agents. Introduction of proteins into the circulation exposes them to numerous different cells, enzymes and routes of extravasation that contribute to their clearance and their catabolism. Overly rapid clearance, particularly of small proteins, can limit therapeutic efficacy. Many strategies have been devised to retard the clearance of therapeutic or potentially therapeutic proteins, but relatively few proteins with clearance-retarding modifications are in clinical use. Proteins have been chemically modified towards this end by covalent attachment of polyethylene glycol or dextran chains or by protein-protein cross-linking. Genetic modification has also been employed to fuse proteins of interest to long-lived plasma proteins like albumin or immunoglobulins, or portions of these proteins. While all modifications may reduce the biological activity of the protein of interest or elicit antibody formation in recipient animals or patients, there now exists sufficient experience in this area that an optimal clearance-extending strategy can often be designed and successfully executed. With the explosive growth of genomic and proteomic information, an exponentially increasing number of engineered proteins are likely to be developed, with a probable need for clearance-related modification.

Intracellular Ca2+ transients have been shown to control several transition points within the eukaryotic cell cycle. We focus here on the G1-to-S phase transition triggered by an increase in the intracellular Ca2+ concentration {[Ca2+]i } in rodent vascular smooth muscle cells {VSMC} and its potential targeting for the treatment of vaso-occlusive processes such as atherosclerosis, hypertension and post-angioplasty restenosis.The transcription factor c-Myb generates a G1 / S transition-specific Ca2+ transient via its regulation of a high affinity Ca2+ efflux pump, the plasma membrane Ca2+ ATPase-1 {PMCA1}. Two c-Myb binding sites in the PMCA1 promoter mediate the cell cycle-associated repression of PMCA1. As c-Myb levels increase in late G1 phase of proliferating VSMC, transcription from the PMCA1 promoter is reduced, expression of the PMCA1 gene falls, and the resultant reduced rate of Ca2+ efflux underlies a G1 / S-associated increase in [Ca2+]i . Blocking either the upregulation of c-Myb levels, or the down regulation in expression of the efflux pump, leads to significant reductions in S phase entry and proliferation of VSMC. A search for functional c-Myb sites within the promoters of other Ca2+ transporters has been undertaken in order to extend the molecular framework of the G1 / S-specific Ca2+ signal mediated by the c-Myb transcription factor.Animal studies with c-myb antisense oligodeoxynucleotides and an anti-c-myb ribozyme as well as in vitro results with dominant negative c-Myb mutants and a doxycycline-inducible c-Myb neutralizing antibody point to the potential of c-Myb-targeted gene therapy for treating pathologic VSMC proliferation and highlight the need for clinical trials in this field.

Thrombin-specific inhibitors directly diminish thrombin-induced coagulation and cellular activities without the side effects of heparin. Hirudin is the most potent natural thrombin-specific inhibitor. Recombinant hirudins (such as desirudin) have been shown to be effective in the treatment of heparin-induced thrombocytopenia (HIT) and in the prevention of thrombotic complications after hip or knee surgery. The application of recombinant hirudin has been limited mainly by hemorrhagic complications. Synthetic thrombin-specific inhibitors, including oligopeptides, tripeptides and non-peptide low molecular weight (LMW) thrombin inhibitors, have been designed according to their interactions with the active sites of thrombin. Bivalirudin (an anti-thrombin oligopeptide) has been approved for preventing thrombosis in unstable angina patients following angioplasty in adjunct to aspirin. Argotroban (a tripeptide thrombin inhibitor) has been used for the treatment of HIT, peripheral and cerebral thrombotic diseases. The benefit of using thrombin-specific inhibitors alone in acute myocardial infarction or unstable angina remains uncertain. A number of LMW thrombin-specific inhibitors have been developed. Some of them can be administrated orally, and cause less increase in bleeding time than other thrombin inhibitors. The efficacy, safety, stability and oral bioavailability of the thrombin inhibitors may be considerably improved through structural optimization. Most of the LMW thrombin inhibitors are currently being tested in animal models or at early stages of clinical trials. In this review, we will present an overview of recent advances in thrombin-specific inhibitors.

The statins are lipid-lowering agents that act by inhibition of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase. This enzyme is responsible for the conversion of HMG-CoA to mevalonate. Products of mevalonate metabolism are critical for several cellular processes of eukaryotic cells, and inhibition of the mevalonate pathway by statins has pleiotropic effects. It has been reported that statins inhibit the migration and proliferation of vascular smooth cells (VSMCs), reduce interleuk in - 6 expression in VSMCs, improve endothelium - dependent vasomotion, and inhibit the expression of plasminogen activator inhibitor-1 and matrix metalloproteinases in endothelial cells. These effects of statins are independent of plasma cholesterol level, and are completely blocked by exogenous mevalonate and some isoprenoids. These findings suggest that statins exert direct antiatherosclerotic effects on the vascular wall beyond their effects on plasma lipids.