XV454, a novel nonpeptide small-molecule platelet GIIb/IIIa antagonist with comparable platelet alpha(IIb)beta3-binding kinetics to c7E3

Assessment of anti-metastatic effects of anticoagulant and antiplatelet agents using animal models of experimental lung metastasis

It is well established that the blood coagulation system is activated in cancer. In addition, there is considerable evidence to suggest that clotting activation plays an important role in the biology of malignant tumors, including the process of blood-borne metastasis. For many years our laboratory has used experimental models of lung metastasis to study the events that follow the introduction of procoagulant-bearing tumor cells into circulating blood. This chapter focuses on the basic methods involved in assessing the anti-metastatic effects of anticoagulants and anti-platelet agents using rodent models of experimental metastasis. In addition, it summarizes our experience with these models, which collectively suggests that intravascular coagulation and platelet activation are a necessary prelude to lung tumor formation and that interruption of coagulation pathways or platelet aggregation may be an effective anti-metastatic strategy.

A perspective on the toxicological mechanisms possibly contributing to the failure of oral glycoprotein IIb/IIIa antagonists in the clinic

In clinical trials in patients with acute or unstable coronary syndromes and/or undergoing percutaneous coronary intervention, oral glycoprotein (GP) IIb/IIIa antagonists did not show therapeutic benefit over aspirin during long-term administration. Moreover, high-dose oral administration of these agents was associated with greater fatality risk compared with that of lower doses. This article postulates that continuous exposure of the GP IIb/IIIa receptor (integrin alpha(IIb)beta(3)) to these agents may result in some form of resistance or activation of other biological systems. These toxicological mechanisms may help explain some factors that could potentially contribute to the failure of these agents in clinical trials. Several hypotheses are presented: (i) modulation of platelet response because of long-term exposure to GP IIb/IIIa antagonists; (ii) role of related integrins and associated proteins to compensate for the loss of platelet activity because of dysfunctional GP IIb/IIIa receptors occupied by inhibitors; (iii) effects of the GP IIb/IIIa antagonists on other cellular systems such as the caspase and procaspase enzymes in apoptosis and possibly the ryanodine receptor involved in sarcoplasmic reticulum calcium release. These toxicological mechanisms could potentially limit the utility of these oral agents in long-term administration.

Oral platelet glycoprotein IIb/IIIa receptor inhibitors--part II

Although the hypothesis of benefit from prolonged oral IIb/IIIa inhibition was appealing, the large Phase III trials have uniformly shown there was no improvement in outcome. In addition, there was an increased mortality seen in patients treated with the oral IIb/IIIa inhibitor. This latter finding is not adequately explained, but is likely a multifactorial problem of this strategy of platelet inhibition. The trials found that, even with no improvement in efficacy, there was increased bleeding, meaning that for chronic therapy with IIb/IIIa inhibition there does not appear to be a therapeutic window. Accordingly, chronic oral IIb/IIIa inhibition appears to have been well tested but has not worked. Fortunately, there are several other oral antiplatelet agents available that have shown beneficial results, including clopidogrel. In addition, other newer classes of antiplatelet agents are in earlier stages of development. Thus, agents targeted more "upstream" in platelet activation pathways may offer a more tolerable and efficacious approach to long-term antiplatelet therapy.

Anti-platelet effects of GPIIb/IIIa and P-selectin antagonism, platelet activation, and binding to neutrophils

Platelet activation with GPIIb/IIIa binding to fibrinogen, aggregation and interaction with leukocytes constitute the principal mediator of thrombosis. Although the clinical benefits of GPIIb/IIIa antagonists have been documented, the relationship between their anti-platelet properties, platelet activation and binding to leukocytes is still debated. We investigated the effects of abciximab, tirofiban, roxifiban, and an anti-P-selectin blocking monoclonal antibody (Mab) on isolated human platelet aggregation using optical aggregometer, and on platelet P-selectin and GPIIb/IIIa expression, and platelet-neutrophil binding using flow cytometry. Thrombin at 0.025 U/ml induced maximal platelet aggregation (76.3 +/- 2.6%), P-selectin expression (88.5 +/- 4%), GPIIb/IIIa activation (PAC-1 binding, 86.2 +/- 8.9%) and platelet-neutrophil binding (58.0 +/- 6.4%). The GPIIb/IIIa antagonists inhibited in a concentration-dependent manner platelet aggregation (IC50 of 100 nM for abciximab and tirofiban and 50 nM for roxifiban) and PAC-1 binding, without any effect on P-selectin. None of these agents affected significantly platelet-neutrophil binding, whereas an anti-P-selectin Mab abolished this binding and amplified the effect of abciximab on platelet aggregation. These results indicate that the effects of these GPIIb/IIIa antagonists on platelet aggregation are not related to inhibition of platelet activation, as P-selectin levels and platelet-neutrophil binding remained unaffected, and highlight the participation of P-selectin with GPIIb/IIIa in platelet aggregation.

Platelet GPIIb/IIIa binding characteristics of small molecule RGD mimetic: distinct binding profile for Roxifiban

A number of non-peptide orally active RGD mimetic prodrug such as Orbofiban, Sibrafiban, SR121566, Roxifiban and others entered into the clinical evaluation stage. Some of these agents were terminated and some are still in clinical trials. The present study examined the platelet GPIIb/IIIa binding profiles for the active form of Roxifiban, Sibrafiban, SR121566 and Orbofiban using 3H-Roxifiban active form (XV459), 3H-DMP728, 125I-Echistatin, and 125I-Fibrinogen. Either DMP728, Orbofiban, Sibrafiban, SR121566 or Roxifiban active form as well as other RGD mimetic bind to the same binding site(s) on human platelets as evident from the competitive inhibition of binding of each other to human platelet. Additionally, Roxifiban active form competed with FITC labeled GPIIb/IIIa antagonist cyclic RGD peptidomimetic (XL086) as demonstrated using confocal microscopy technique. Roxifiban active form (XV459) demonstrated the highest potency in inhibiting 3H-XV459, 3H-DMP728, 125I-Echistatin, and 125I-Fibrinogen binding to human platelets as compared to the others. Structure activity relationship within the isoxazoline Roxifiban series showed that substituent at the alpha-carbon next to the carboxy terminal represents an exosite for the affinity binding to human platelets leading to slow platelet dissociation rate. These data indicated a distinct binding profile for Roxifiban (high affinity to both activated and resting platelets associated with a relatively slow K(off)) as compared to others. These differences might determine the pharmacodynamics and pharmackokinetics of the different GPIIb/IIIa antagonists.

Comparative antiplatelet efficacy of a novel, nonpeptide GPIIb/IIIa antagonist (XV454) and abciximab (c7E3) in flow models of thrombosis

Glycoprotein (GP) IIb/IIIa is pivotal in homotypic platelet aggregation and may also be involved in the heterotypic adhesion of leukocytes and tumor cells to platelets. This study was primarily undertaken to compare the antiplatelet efficacy of a novel, nonpeptide GPIIb/IIIa antagonist, XV454, to that of abciximab in 2 flow models of platelet thrombus formation: (1) direct shear-induced platelet aggregation imposed by a cone-and-plate rheometer and (2) platelet adhesion onto von Willebrand factor (vWF)/collagen I followed by aggregation in a perfusion system. XV454 inhibited platelet aggregation in a concentration-dependent manner in both experimental models. Maximal inhibition of aggregation was achieved by XV454 at approximately 70% receptor occupancy, which is lower than the >/=85% previously reported for abciximab. At similar levels of receptor blockade (approximately 45%), XV454 appeared to be relatively more effective than abciximab in suppressing platelet aggregation. Neither XV454 nor abciximab inhibited platelet adhesion to collagen. Pretreatment of surface-adherent platelets with either XV454 or abciximab inhibited the attachment of monocytic THP-1 cells under flow. In contrast, the rapidly reversible GPIIb/IIIa inhibitor orbofiban failed to suppress these heterotypic interactions. These findings demonstrate that XV454 is a potent GPIIb/IIIa antagonist with a long receptor-bound lifetime like abciximab and may be beneficial for the treatment/prevention of thrombotic complications.

Immobilized platelets support human colon carcinoma cell tethering, rolling, and firm adhesion under dynamic flow conditions

Accumulating evidence suggests that successful metastatic spread may depend on the ability of tumor cells to undergo extensive interactions with platelets. However, the mechanisms mediating tumor cell adhesion to platelets under conditions of flow remain largely unknown. Therefore, this study was designed to analyze the ability of 3 human colon carcinoma cell lines (LS174T, COLO205, and HCT-8) to bind to surface-anchored platelets under flow and to identify the receptors involved in these processes. Immobilized platelets support LS174T cell adhesion at wall shear stresses up to 1.4 dyn/cm2. Our data suggest that platelets primarily recruit LS174T cells through a 2-step, sequential process of adhesive interactions that shares common features but is distinct from that elaborated for neutrophils. Platelet P-selectin mediates LS174T cell tethering and rolling in a PSGL-1- and CD24-independent manner. Moreover, platelet αIIbβ3-integrins appear to be capable of directly capturing LS174T cells from the fluid stream, and also convert instantaneously transient tethers initiated by P-selectin into stable adhesion. This step is at least partially mediated by von Willebrand factor, but not fibrinogen or fibronectin, that bridges platelet αIIbβ3 with a yet unidentified receptor on the LS174T cell surface via an RGD-dependent mechanism. The sequential engagement of platelet P-selectin and αIIbβ3 is also requisite for the optimal adhesion of COLO205. Furthermore, HCT-8 cells, which fail to interact with P-selectin, tether minimally to surface-anchored platelets under flow, despite their extensive adhesive interactions under static conditions. This cascade of events depicts an efficacious process for colon carcinoma arrest at sites of vascular injury.

Immobilized platelets support human colon carcinoma cell tethering, rolling, and firm adhesion under dynamic flow conditions

Accumulating evidence suggests that successful metastatic spread may depend on the ability of tumor cells to undergo extensive interactions with platelets. However, the mechanisms mediating tumor cell adhesion to platelets under conditions of flow remain largely unknown. Therefore, this study was designed to analyze the ability of 3 human colon carcinoma cell lines (LS174T, COLO205, and HCT-8) to bind to surface-anchored platelets under flow and to identify the receptors involved in these processes. Immobilized platelets support LS174T cell adhesion at wall shear stresses up to 1.4 dyn/cm2. Our data suggest that platelets primarily recruit LS174T cells through a 2-step, sequential process of adhesive interactions that shares common features but is distinct from that elaborated for neutrophils. Platelet P-selectin mediates LS174T cell tethering and rolling in a PSGL-1- and CD24-independent manner. Moreover, platelet αIIbβ3-integrins appear to be capable of directly capturing LS174T cells from the fluid stream, and also convert instantaneously transient tethers initiated by P-selectin into stable adhesion. This step is at least partially mediated by von Willebrand factor, but not fibrinogen or fibronectin, that bridges platelet αIIbβ3 with a yet unidentified receptor on the LS174T cell surface via an RGD-dependent mechanism. The sequential engagement of platelet P-selectin and αIIbβ3 is also requisite for the optimal adhesion of COLO205. Furthermore, HCT-8 cells, which fail to interact with P-selectin, tether minimally to surface-anchored platelets under flow, despite their extensive adhesive interactions under static conditions. This cascade of events depicts an efficacious process for colon carcinoma arrest at sites of vascular injury.