Effects of terbogrel on platelet function and prostaglandin endoperoxide transfer

Human cytochrome P450 enzymes 5-51 as targets of drugs and natural and environmental compounds: mechanisms, induction, and inhibition - toxic effects and benefits

Cytochrome P450 (P450, CYP) enzymes have long been of interest due to their roles in the metabolism of drugs, pesticides, pro-carcinogens, and other xenobiotic chemicals. They have also been of interest due to their very critical roles in the biosynthesis and metabolism of steroids, vitamins, and certain eicosanoids. This review covers the 22 (of the total of 57) human P450s in Families 5-51 and their substrate selectivity. Furthermore, included is information and references regarding inducibility, inhibition, and (in some cases) stimulation by chemicals. We update and discuss important aspects of each of these 22 P450s and questions that remain open.

Thromboxane A2: physiology/pathophysiology, cellular signal transduction and pharmacology

Thromboxane A(2) (TXA(2)), an unstable arachidonic acid metabolite, elicits diverse physiological/pathophysiological actions, including platelet aggregation and smooth muscle contraction. TXA(2) has been shown to be involved in allergies, modulation of acquired immunity, atherogenesis, neovascularization, and metastasis of cancer cells. The TXA(2) receptor (TP) communicates mainly with G(q) and G(13), resulting in phospholipase C activation and RhoGEF activation, respectively. In addition, TP couples with G(11), G(12), G(13), G(14), G(15), G(16), G(i), G(s) and G(h). TP is widely distributed in the body, and is expressed at high levels in thymus and spleen. The second extracellular loop of TP is an important ligand-binding site, and Asp(193) is a key amino acid. There are two alternatively spliced isoforms of TP, TPalpha and TPbeta, which differ only in their C-terminals. TPalpha and TPbeta communicate with different G proteins, and undergo hetero-dimerization, resulting in changes in intracellular traffic and receptor protein conformations. TP cross-talks with receptor tyrosine kinases, such as EGF receptor, to induce cell proliferation and differentiation. TP is glycosylated in the N-terminal region for recruitment to plasma membranes. Furthermore, TP conformation is changed by coupling to G proteins, showing several states of agonist binding. Finally, several drugs modify TP-mediated events; these include cyclooxygenase inhibitors, TXA(2) synthase inhibitors and TP antagonists. Some flavonoids of natural origin also have TP receptor antagonistic activity. Recent advances in TP research have clarified TXA(2)-mediated events in detail, and further study will supply more beneficial information about TXA(2) pathophysiology.

Antiplatelet Agents: Current Drugs and Future Trends

Antiplatelet drugs in clinical use are discussed in terms of their mechanisms of action and the relevancy of that to the physiology of platelets and the pathophysiology of arterial thrombosis. Current clinical usage is outlined in detail for each drug. Experimental antiplatelet drugs also are discussed.

Advances in antiplatelet therapy

Cardiovascular diseases (CVDs) are the leading cause of morbidity and mortality in the western world. These disorders share a common pathophysiology -- atherosclerosis, which affects various arterial beds, leading to protean manifestations (coronary artery disease [CAD], stroke, peripheral arterial disease [PAD]). The platelet plays a pivotal role in the perpetuation and clinical expression of these disorders. The platelet, once believed to have a role confined to modulation of thrombosis and haemostasis, also plays an active role in vascular inflammation. Antiplatelet agents have become first-line therapy for CVD, and their unequivocal benefits are demonstrated in various basic and experimental models and supported by overwhelming evidence from clinical trials. Search is underway for more effective and safer antiplatelet therapy. Novel therapies are emerging to target the redundant pathways of platelet adhesion, activation and aggregation. Efforts are also ongoing to enhance implementation of existent therapy, target therapy selectively to high-risk patients and to those likely to respond (pharmacogenomics), and study the incremental benefits and safety of various antiplatelet combinations and their interaction with other medications in patients with CVD treated with polypharmacy.

The thromboxane receptor antagonist PBT-3, a hepoxilin stable analog, selectively antagonizes the TPalpha isoform in transfected COS-7 cells

The hepoxilin analog PBT-3 [10(S)-hydroxy-11,12-cyclopropyleicosa-5Z,8Z,14Z-trienoic acid methyl ester] was previously shown to inhibit the aggregation of human platelets and to antagonize the binding of the thromboxane receptor agonist I-BOP [[1S-[1alpha,2alpha (Z),3beta(1E,3S*),4alpha]]-7-[3-[3-hydroxy-4-(4-iodophenoxy)-1-butenyl]-7-oxabicyclo[2.2.1]hept-2-yl]-5-heptenoic acid] in human platelets (Pace-Asciak et al., 2002). We show herein that PBT-3 inhibits, to different degrees, binding of the TP receptor antagonist [3H]SQ 29,548 [[1S-[1alpha,2alpha (Z),3alpha,4alpha]]-7-[3-[[2-[(phenylamino)carbonyl]hydrazino]methyl]-7-oxabicyclo[2.2. 1]hept-2-yl]-5-heptenoic acid], to the TP receptor isoforms in TPalpha- and TPbeta-transfected COS-7 cells. These isoforms possess a different tail length, the alpha being shorter than the beta isoform. In contrast, SQ 29,548 shows no selection for the two TP isoforms. The IC50 value for PBT-3 = 2.0 +/- 0.3 x 10-7 M was observed for TPalpha, whereas this was one-sixth less active on the TPbeta isoform (IC50 = 1.2 +/- 0.2 x 10-6 M), suggesting selectivity for the TPalpha isoform. To investigate whether the tail contributes to the difference in competition binding by PBT-3, we investigated the tailless TP isoform expressed in transfected COS-7 cells. Its IC50 was similar to that of the TPalpha isoform. In additional studies, we investigated the effect of PBT-3 on the collagen and I-BOP evoked intracellular calcium release and on the collagen and I-BOP evoked phosphorylation of pleckstrin. PBT-3 blocked both pathways further demonstrating its TP receptor antagonist activity. These results demonstrate that the action of PBT-3 in inhibiting platelet aggregation is mediated via inhibition of the TPalpha isoform of the thromboxane receptor and that the tail may play an important role in recognition of this TP receptor antagonist.

Functional and biochemical evaluation of platelet aspirin resistance after coronary artery bypass surgery

BACKGROUND

Aspirin inhibits platelet activation and reduces atherothrombotic complications in patients at risk of myocardial infarction and stroke. However, a sufficient inhibition of platelet function by aspirin is not always achieved. The causes of this aspirin resistance are unknown.

METHODS AND RESULTS

Patients undergoing coronary artery bypass grafting (CABG) have a high incidence of aspirin resistance. To evaluate functional and biochemical responses to aspirin, platelet-rich plasma was obtained before and at days 1, 5, and 10 after CABG. Thromboxane formation, aggregation, and alpha-granule secretion were effectively inhibited by 30 or 100 micromol/L aspirin in vitro before CABG, but this inhibition was prevented or attenuated after CABG. Whereas the inhibition of thromboxane formation and aggregation by aspirin in vitro partly recovered at day 10 after CABG, oral aspirin (100 mg/d) remained ineffective. The inducible isoform of cyclooxygenase in platelets, COX-2, has been suggested to confer aspirin resistance. In fact, immunoreactive COX-2 was increased 16-fold in platelets at day 5 after CABG, but the COX-2 selective inhibitor celecoxib did not alter aspirin-resistant thromboxane formation. By contrast, the combined inhibitor of thromboxane synthase and thromboxane receptor antagonist terbogrel equally prevented thromboxane formation of platelets obtained before (control) and after CABG.

CONCLUSIONS

Platelet aspirin resistance involves an impairment of both in vivo and in vitro inhibition of platelet functions and is probably due to a disturbed inhibition of platelet COX-1 by aspirin.

Effects of the thromboxane synthetase inhibitor and receptor antagonist terbogrel in patients with primary pulmonary hypertension

BACKGROUND

Circulating mediators, including thromboxane A2, the vasoconstrictor, platelet aggregant, and smooth muscle mitogen, may contribute to the progression of vascular narrowing in primary pulmonary hypertension (PPH).

METHODS

To further understand the contribution of thromboxane and to provide novel therapy for PPH, we administered the potent orally active thromboxane synthetase inhibitor and thromboxane receptor antagonist terbogrel for 12 weeks to patients with New York Heart Association functional classification II and III PPH. The study had a multicenter randomized placebo-controlled design. The primary endpoint was a change in the distance walked during 6 minutes. The pharmacologic effects of terbogrel on thromboxane and prostacyclin metabolism also were studied.

RESULTS

Although the planned enrollment was 135 patients, the study was halted after only 71 patients had been randomized because of the unforeseen side effect of leg pain, which occurred almost exclusively in patients with terbogrel treatment. Only 52 patients completed the 12-week study, and only 22 patients (31%) were fully compliant with the study medication. The leg pain confounded the primary endpoint of walking distance. On an intention-to-treat analysis, no improvements in 6-minute walk distance or in hemodynamics in patients with terbogrel treatment were seen. However, terbogrel was effective from a pharmacologic standpoint, reducing thromboxane metabolites by as much as 98% (P <.0001), with a modest but statistically insignificant (39%) rise in prostacyclin metabolites.

CONCLUSION

Inhibition of thromboxane with an orally active agent is feasible in PPH, but the incidence of severe leg pain with terbogrel precludes its use in this disorder. Similar therapeutic efforts, with other thromboxane inhibitors, should be considered.

Angiotensin II receptor-independent antiinflammatory and antiaggregatory properties of losartan: role of the active metabolite EXP3179

Angiotensin II (Ang II) type 1 receptor (AT(1)) antagonists such as losartan (LOS) are widely used for the treatment of hypertension and elicit antiinflammatory and antiaggregatory in vitro and in patients, although the underlying mechanism are unclear. Following computer-based molecule similarity, we proposed that on cytochrome-P450 degradation, the LOS metabolite EXP3179 is generated, which shows molecule homology to indomethacin, a cyclooxygenase inhibitor with antiinflammatory and antiaggregatory properties. Subsequently, serum-levels of EXP3179 were determined for 8 hours in patients receiving a single oral dose of 100 mg LOS. High-performance liquid chromatography followed by liquid chromatography-mass spectrometry (GC-MS) [corrected] from serum samples revealed a maximum of 10(-7) mol/L for EXP3179 peaking between 3 to 4 hours. The increase in serum-EXP3179 levels was associated with a significant reduction in platelet aggregation in vivo (-35+/-4%, P<0.001 versus control). EXP3179 generation was investigated in a chemical reaction mimicking the liver cytochrome-P450-dependent LOS-degradation and human endothelial cells were exposed to Ang II or lipopolysaccharides (LPS) in the presence of EXP3179 (10(-7) mol/L). LPS- and Ang II-induced COX-2 transcription was abolished by EXP3179. Moreover, EXP3179 significantly reduced Ang II- and LPS-induced formation of prostaglandin F2alpha as determined by GC-MS [corrected]. Thus, antiinflammatory properties of LOS are mediated via its EXP3179 metabolite by abolishing COX-2 mRNA upregulation and COX-dependent TXA2 and PGF2alpha generation. Serum levels of EXP3179 are detectable in patients in concentrations that exhibit antiinflammatory and antiaggregatory properties in vitro.

Antiplatelet and anticoagulant effects of "HN-11 500," a selective thromboxane receptor antagonist

The antiplatelet and anticoagulant effect of a thromboxane receptor (TX receptor) antagonist developed by Nycomed (Linz) has been studied in a placebo-controlled double-blind phase I study. Sixteen healthy male volunteers received different single oral doses of "HN-11 500" (C(14)H(15)NO(5)S(2); 1, 10, 100, 200, and 400 mg). Eight volunteers received placebo. The washout period between each dosage applied was at least 12 days. Platelet aggregation induced by the thromboxane mimetic "U 46 619" (C(21)H(34)0(4)) and platelet adhesion to siliconized glass were significantly and dose-dependently inhibited. The effect lasted between 3 and 4 h (10 mg) and 8 h (400 mg), respectively, and correlated well with the pharmacokinetic data. Platelet aggregation seems to be more sensitive to monitor the effects of HN-11 500 on platelet function than platelet adhesion. Plasma levels of 300 ng/ml HN-11 500 probably leads to >90% inhibition of platelet aggregation. The template bleeding time slightly increased but did not exceed the normal range. Furthermore, there was a wide variation of results. There were no significant changes in platelet counts, platelet-induced thrombin generation time (PITT), and blood coagulation parameters. All doses of HN-11 500 were well tolerated. HN-11 500 is a potent TX receptor antagonist (TXRA), which inhibits either platelet aggregation or platelet adhesion, which has not yet been described. In clinical routine, TXRAs have to demonstrate the effectiveness in large clinical trials for different clinical indications and to compete with single or combined administrations of cyclooxygenase (COX) inhibitors, thienovridines, thromboxane synthase inhibitors, and GIIb/IIIa inhibitors.

Constitutive cyclooxygenase-2 expression in healthy human and rabbit gastric mucosa

Selective cyclooxygenase (COX)-2 inhibitors are expected to cause fewer gastric side effects because of sparing of COX-1-dependent prostaglandin (PG) synthesis in the gastric mucosa. However, the possible contribution of COX-2 to overall gastric PG biosynthesis is not known. This study demonstrates constitutive expression of COX-2 mRNA and protein in apparently healthy human and rabbit gastric mucosa. This basal expression of COX-2 protein in human gastric mucosa was increased by lipopolysaccharide and phorbol ester, indicating its up-regulation in response to appropriate stimuli. The functional significance of COX-2-dependent PG formation was studied in terms of PGE2 generation in the rabbit mucosa and its inhibition by the COX-2-selective inhibitor flosulide. There was concentration-dependent (IC50 = 107 +/- 55 nM) and ultimately complete inhibition of PGE2 generation by flosulide. In addition, gastric mucosa generated 15-hydroxyeicosatetraenoic acid upon treatment with acetylsalicylic acid. The data suggest an important role for COX-2-dependent PG production in apparently healthy gastric mucosa and raise the issue of whether selective COX-2 inhibitors might also interfere with physiological PG formation and actions in the stomach.