Many blood-sucking ectoparasites synthesize substances to thwart the defence mechanisms of the hosts on which they feed. Ticks, in particular, produce salivary substances capable of modulating the host immune responses and to maintain blood in a sufficiently fluid state to effectively acquire and digest their blood meal. Hemostasis involves a network of factors organized in different pathways that can be activated independently (see figure, below). Ticks are, therefore, confronted with a complex biochemical system and must simultaneously block several different pathways to obtain effective antihemostasis. Ticks produce a variety of compounds with antihemostatic activity, compounds which have been naturally selected over the long course of tick–host co-evolution.
Ir-CPI is a novel serine protease inhibitor isolated from the saliva of the Ixodes ricinus tick. This protein is capable of effectively inhibiting the intrinsic coagulation pathway. Indeed, in vitro experiments have showed that Ir-CPI considerably prolongs the activated partial thromboplastin time (aPTT), which measures the intrinsic pathway of coagulation, without modifying the prothrombin time (PT), which measures the extrinsic pathway, the thrombin time, which measures the conversion of fibrinogen to fibrin and its subsequent polymerization, or the Russel Viper venom (RVV) time, which measures the activation of FX into FXa and the subsequent coagulation steps.
Ir-CPI specifically binds to factors of the contact phase (namely, FXII, FXI, and kallikrein) but not to factors belonging to the common and TF pathways. In addition, Ir-CPI binds with a very high affinity to the activated form of the target enzymes and does not bind to the zymogenic form (see figure below). This indicates that Ir-CPI does not block the activation of the target factor but, rather its action on the next factor in the clotting cascade.
By binding to FXIa and FXIIa, Ir-CPI prevents the activation of FXII and FXI into FXIIa and FXIIa, respectively.
In addition, Ir-CPI has antithrombotic properties in vivo. It inhibits in a dose-dependent manner the formation of both venous and arterial thrombi in rodents without disturbing the clotting balance and primary hemostasis This indicates that Ir-CPI does not inhibit platelet adhesion/activation and aggregation processes. Additional data show that Ir-CPI acts mainly on clot propagation. A profound defect in the formation but also stabilization of platelet-rich thrombi was observed in Ir-CPI–treated mice.
These data demonstrate that Ir-CPI, an exogenous molecule targeting specific contact phase factors (FXII and FXI), can protect against the formation of a thrombus without altering coagulation and bleeding times.
The results demonstrate a highly attractive profile justifying the drug’s evaluation in further pre-clinical models and suggest utility in a number of clinical indications including arterial thrombosis,, cerebral ischemia, deep vein thrombosis and pulmonary embolism. Ir-CPI may have particular benefit in situations where the contact pathway of coagulation is activated. Thrombosis is a particular concern where biomaterials, such as catheters, extracorporeal circuits, stents and other medical devices, potentially activate contact phase coagulation factors.
Bioxodes believes that prevention of catheter occlusion, catheter-induced thrombosis and extra-corporeal pump occlusion are important potential indications for a contact pathway inhibitor such as Ir-CPI alongside systemic anticoagulation in such procedures as percutaneous coronary intervention (PCI) and cardiac artery bypass graft (CABG). Other potential indications include medical and/or upstream treatment of acute myocardial infarction, acute pulmonary embolism and acute stroke.
Early Programmes: Ixac and Ir-LBP
In addition to the blockbuster markets being addressed by Ir-CPI, Bioxodes’ early stage pipeline is likewise addressing very significant pharmaceutical markets in the areas of respiratory, renal and cardiovascular medicine. Bioxodes will apply the experience gained in development of Ir-CPI to these earlier programs.
Bioxodes has two additional projects at very early stage of development. Like Ir-CPI, IxAC and Ir-LBP are proteins, which have been identified in the saliva of the Ixodes tick.
IxAC, via its specific properdin-binding and its C3-convertase properties, inhibits the alternative complement pathway and thereby is expected to inhibit renal ischemia. Ischemia reperfusion injury in the kidney is a major cause of acute renal failure and acute kidney injury in both native and transplanted kidneys. Such problems follow transient falls in blood flow to the kidney resulting in shock, sepsis and need for transplantation. Reperfusion is essential for the survival of the kidney but, as in stroke and myocardial infarction, causes additional injury.
The second project is based on the Ir-LBP protein which is a scavenger of leukotriene B4 (LTB4) with an affinity of the order of 1nM. This protein inhibits neutrophil chemotaxis and delays LTB4-induced apoptosis in vitro and inhibits the host inflammatory response in vivo by decreasing the number and activation of neutrophils.