Potential therapeutic target to inhibit thrombogenesis without affecting haemostasis

Tilly P, Charles AL, Ludwig S et al.
Cardiovasc Res. 2014 Jan 2. [Epub ahead of print]

Background

Anti-platelet agents partly prevent atherothrombosis, but they may cause bleeding, specifically more potent drugs or drug combinations. Thus, means to intensify the anti-platelet effect without altering haemostasis are needed. This is challenging because the molecular mechanisms at play in haemostasis and thrombosis are highly intertwined. One strategy is to target pro-thrombogenic factors that are produced in much higher amounts by atherosclerotic plaques than by healthy vessel wall.
The inflammatory environment of atherosclerotic plaques produces several factors like prostaglandin E2 (PGE2), which in turn activates signalling cascades. PGE2 increases platelet response through activation of EP-receptors on platelets: on its own, PGE2 does not induce platelet aggregation [1-4]. PGE2-receptor EP3 has the opposite effect of receptor EP2 and EP4, but normally the EP3-effect predominates. PGE2 increases the platelet response via EP3. Specific inactivation of EP3 synergizes with the stimulating effect of PGE2-activated EP2 and EP4 to inhibit the platelet response [5]. Bleeding was not increased in EP3-deficient mice [4]. It is thus hypothesised that targeting the PGE2/EP3 pathway might prevent atherothrombosis without altering haemostasis.
This study investigated whether atherothrombosis can be controlled without inducing bleeding in mice, by using the selective EP3-blocker DG-041, which is known to inhibit PGE2-induced potentiation of aggregation of rat and human platelets [6-8].

Main results

• The EP3-blocker DG-041 strongly inhibited platelet aggregation in mice, as well as atherothrombosis, as provoked by scratching atherosclerotic plaque surface.
• In response to tail injury, there is no local PGE2 production in DG-041 treated mice. Blocking EP3-function did not affect liver or brain haemostasis either.
• EP3 activation normally increases platelet sensitivity. PGE2 could partially restore sensitivity of clopidogrel-treated platelets (which blocks the P2Y12 receptor to prevent thrombosis).
• After induction of thrombosis, clopidogrel reduced thrombosis, but increased bleeding time. Also adding DG-041 further decreased thrombotic scores, while bleeding time did not significantly change. Blocking EP3 can thus potentiate platelet inhibition without aggravating clopidogrel-induced bleeding.
• To explore whether the dissociation of thrombosis from haemostasis observed in mice is clinically relevant, studies in human material were performed. Human atherosclerotic plaques also produce PGE2, and it also facilitated platelet aggregation in whole blood of healthy volunteers.
• Platelets obtained from patients with peripheral artery disease on oral anti-platelet treatment, were re-sensitised by EP3 activation. This suggests that locally produced PGE2 can increase the sensitivity of platelets to their activators, even when P2Y12 receptor is therapeutically blocked.
• When healthy volunteers took PG-041, ex vivo platelet aggregation in response to collagen and PGE2 was inhibited. Bleeding times were unchanged. Thus, targeting the PGE2/EP3 pathway with DG-041 reduced platelet aggregation in healthy volunteers, without affecting cutaneous haemostasis.

Conclusion

PGE2 is produced by human atherosclerotic plaques and potentiates platelet function in human blood. Blocking EP3 in mice or healthy volunteers inhibited platelet function while haemostatic function was preserved. Thus, haemostasis and thrombosis are differently affected by the PGE2/EP3-pathway. Targeting the PGE2/EP3-pathway allows for local restriction of the anti-thrombotic effect to the plaque, due to the local production of PGE2 at the plaque. Targeting the EP3 receptor may therefore increase the efficiency of conventional anti-platelet drugs without increasing the bleeding risk.

Editorial comment [9]

The persisting challenge in the development and use of antithrombotic drugs is to find a better benefit-risk balance, as a consequence of the inherent risk of bleeding. Tilly et al publish findings that support their hypothesis that this class of novel antiplatelet agents might be different from those currently available. Some reservations to the viability of this potential strategy are, however, appropriate, one of which is the ubiquitous nature and diverse actions of EP3, in a variety of tissues. Compounds like DG-041 might thus also have effects in other organs.
De Catarina concludes the editorial with ”Therefore, while we should welcome the testing of a new hypothesis for inhibiting thrombosis with a better efficacy–safety profile than with current drugs, we warn about premature enthusiasm and emphasize the need for further studies in this exciting field.’

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