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Thromboxane A2 (TXA2) is a key mediator of platelet aggregation and smooth muscle contraction. Its action is mediated by its G protein-coupled receptor of which two isoforms, termed TPalpha and TPbeta, occur in humans. TXA2 has been implicated in pathologies such as cardiovascular diseases, pulmonary embolism, atherosclerosis, and asthma. This study describes the pharmacological characterization of BM-613 [N-n-pentyl-N'-[2-(4'-methylphenylamino)-5-nitrobenzenesulfonyl]urea], a new combined TXA2 receptor antagonist and TXA2 synthase inhibitor. It exhibits a strong affinity for human platelet TP receptors (IC50 = 1.4 nM), TPalpha and TPbeta expressed in COS-7 cells (IC(50) = 2.1 and 3.1 nM, respectively), and TPs expressed in human coronary artery smooth muscle cells (IC50 = 29 microM). BM-613 shows a weak ability to prevent contraction of isolated rat aorta (ED50 = 1.52 microM) and guinea pig trachea (ED50 = 2.5 microM) induced by TXA2 agonist U-46619 (9.11-dideoxy-9.11-methanoepoxy-prostaglandin F2). Besides, BM-613 antagonizes TPalpha (IC50 = 0.11 microM) and TPbeta (IC50 = 0.17 microM) calcium mobilization induced by U-46619 and inhibits human platelet aggregation induced by U-46619 (ED50 = 0.278 microM), arachidonic acid (ED50 = 0.375 microM), and the second wave of ADP. BM-613 also dose dependently prevents TXA2 production by human platelets (IC50 = 0.15 microM). In a rat model of ferric chloride-induced thrombosis, BM-613 significantly reduces weight of formed thrombus by 79, 49, and 28% at 5, 2, and 1 mg/kg i.v., respectively. In conclusion, BM-613 is a dual and potent TP receptor antagonist and TXA2 synthase inhibitor characterized by a strong antiplatelet and antithrombotic potency. These results suggest that BM-613 could be a potential therapeutic drug for thrombotic disorders.

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.

We report herein a novel class of thromboxane receptor (TP receptor) antagonists modeled on unstable natural lipids that we identified several years ago, the hepoxilins. These antagonists have been rendered chemically and biologically more stable than the natural compounds through structural modification by chemical synthesis. We demonstrate that the analogs inhibit the aggregation of human platelets in vitro evoked by the thromboxane receptor agonists, I-BOP ([1S-[1alpha,2alpha(Z),3beta(1E,3S*),4alpha]]-7-[3-[3-hydroxy-4-(4-iodophenoxy)-1-butenyl]-7-oxabi-cyclo[2.2.1]hept-2-yl]5-heptenoic acid) and U46619 (9,11-dideoxy-9alpha,11alpha-methanoepoxy-prosta-5Z,13E-dien-1-oic acid). The most potent of the analogs described, PBT-3 [10(S)-hydroxy-11,12-cyclopropyl-eicosa-5Z,8Z,14Z-trienoic acid methyl ester], has an IC(50) versus aggregation by I-BOP = 0.6 x 10(-7) M and versus U46619 = 7 x 10(-7) M, representing one of the most potent anti-aggregating substances so far described. PBT-3 also inhibits thromboxane formation and aggregation evoked by collagen with an IC(50) = 8 x 10(-7) M. Other PBT (hepoxilin cyclopropane) analogs so far tested were 5- to 10-fold less active, and the native hepoxilins were about 500-fold less active. Neither PBT-3 nor the other analogs inhibited 12-lipoxygenase, phospholipase A(2), or cyclooxygenase 1 or 2, and weakly stimulated adenyl cyclase (threshold stimulation at 10(-7) M and little selectivity for each of the PBT compounds). TP antagonism by PBT-3 was further demonstrated in receptor binding studies through use of (125)I-BOP, where the IC(50) for PBT-3 was 8 x 10(-9) M, approximately 16-fold less than for I-BOP itself. These findings identify a new mode of action of PBT-3 and other related analogs as primarily TP antagonists. These studies identify a new family of compounds useful in further development as novel therapeutics for thromboxane-mediated diseases.

Bleomycin has been suggested to incite plasma extravasation and influx of inflammatory cells leading to pulmonary fibrosis. We hypothesized that stable analogs of the 12-lipoxygenase product, hepoxilin, may attenuate these effects. We initially investigated the effects of the four hepoxilin analogs (PBT-1 to -4) coadministered intradermally with bleomycin and found that PBT-1 and -2 significantly opposed the vascular permeability effects of bleomycin in rat skin. We subsequently tested the hepoxilin analogs for their actions in opposing the intratracheal bleomycin-evoked acute inflammatory phase of lung fibrosis in the mouse, characterized by a marked accumulation of macrophages and an increase in the rate of collagen synthesis and deposition. We found that the bleomycin-evoked effects on macrophage influx were inhibited by all the hepoxilin analogs (PBT-1, -3, and -4 > PBT-2) administered i.p. for 8 days. Increased total lung collagen was completely abrogated by PBT-1 and -2, whereas PBT-3 and -4 had little effect. A dose-response study with PBT-1 indicated that the effective dose for inhibition of bleomycin-induced inflammatory and histological changes was below 10 microg/day. These studies demonstrate an in vivo action of stable analogs of hepoxilin and support an effect on inflammation and vascular permeability from these novel compounds, especially for PBT-1.

We studied the release of 6-keto-prostaglandin (PG) E1-like material into urinary and venous effluents of the isolated perfused kidney of the rabbit. After high performance liquid chromatographic (HPLC) separation, material coeluting with authentic 6-keto-PGE1 was measured by radioimmunoassay, platelet antiaggregation bioassay and characterized further by gas chromatography/mass spectrometry-single ion monitoring (GC/MS-SIM). Injection of either arachidonic acid or ATP into the renal artery stimulated the release of prostacyclin [measured as immunoreactive (i)-6-keto-PGF1 alpha] and i-6-keto-PGF1. HPLC fractions containing i-6-keto-PGE1, and coeluting with 6-keto-PGE1 standard, exhibited potent inhibition of platelet aggregation. The presence of authentic 6-keto-PGE1 was verified by the GC/MS-SIM spectra in HPLC zones from which radioimmunoassayable and bioassayable 6-keto-PGE1-like material was recovered.