Anticoagulant therapy during cardiopulmonary bypass

Generation of an anticoagulant aptamer that targets factor V/Va and disrupts the FVa-membrane interaction in normal and COVID-19 patient samples

Coagulation cofactors profoundly regulate hemostasis and are appealing targets for anticoagulants. However, targeting such proteins has been challenging because they lack an active site. To address this, we isolate an RNA aptamer termed T18.3 that binds to both factor V (FV) and FVa with nanomolar affinity and demonstrates clinically relevant anticoagulant activity in both plasma and whole blood. The aptamer also shows synergy with low molecular weight heparin and delivers potent anticoagulation in plasma collected from patients with coronavirus disease 2019 (COVID-19). Moreover, the aptamer's anticoagulant activity can be rapidly and efficiently reversed using protamine sulfate, which potentially allows fine-tuning of aptamer's activity post-administration. We further show that the aptamer achieves its anticoagulant activity by abrogating FV/FVa interactions with phospholipid membranes. Our success in generating an anticoagulant aptamer targeting FV/Va demonstrates the feasibility of using cofactor-binding aptamers as therapeutic protein inhibitors and reveals an unconventional working mechanism of an aptamer by interrupting protein-membrane interactions.

Cardiovascular and Respiratory Toxicity of Protamine Sulfate in Zebrafish and Rodent Models

Protamine sulfate (PS) is the only available option to reverse the anticoagulant activity of unfractionated heparin (UFH), however it can cause cardiovascular and respiratory complications. We explored the toxicity of PS and its complexes with UFH in zebrafish, rats, and mice. The involvement of nitric oxide (NO) in the above effects was investigated. Concentration-dependent lethality, morphological defects, and decrease in heart rate (HR) were observed in zebrafish larvae. PS affected HR, blood pressure, respiratory rate, peak exhaled CO₂, and blood oxygen saturation in rats. We observed hypotension, increase of HR, perfusion of paw vessels, and enhanced respiratory disturbances with increases doses of PS. We found no effects of PS on human hERG channels or signs of heart damage in mice. The hypotension in rats and bradycardia in zebrafish were partially attenuated by the inhibitor of endothelial NO synthase. The disturbances in cardiovascular and respiratory parameters were reduced or delayed when PS was administered together with UFH. The cardiorespiratory toxicity of PS seems to be charge-dependent and involves enhanced release of NO. PS administered at appropriate doses and ratios with UFH should not cause permanent damage of heart tissue, although careful monitoring of cardiorespiratory parameters is necessary.

Combination of aptamer and drug for reversible anticoagulation in cardiopulmonary bypass

Unfractionated heparin (UFH), the standard anticoagulant for cardiopulmonary bypass (CPB) surgery, carries a risk of post-operative bleeding and is potentially harmful in patients with heparin-induced thrombocytopenia-associated antibodies. To improve the activity of an alternative anticoagulant, the RNA aptamer 11F7t, we solved X-ray crystal structures of the aptamer bound to factor Xa (FXa). The finding that 11F7t did not bind the catalytic site suggested that it could complement small-molecule FXa inhibitors. We demonstrate that combinations of 11F7t and catalytic-site FXa inhibitors enhance anticoagulation in purified reaction mixtures and plasma. Aptamer-drug combinations prevented clot formation as effectively as UFH in human blood circulated in an extracorporeal oxygenator circuit that mimicked CPB, while avoiding side effects of UFH. An antidote could promptly neutralize the anticoagulant effects of both FXa inhibitors. Our results suggest that drugs and aptamers with shared targets can be combined to exert more specific and potent effects than either agent alone.

Protamine (heparin)-induced thrombocytopenia: a review of the serological and clinical features associated with anti-protamine/heparin antibodies

Protamine is widely used in medicine as a rapidly-acting antidote to heparin, particularly for reversing heparin anticoagulation after cardiac surgery. Protamine is also used as a stabilizing additive to certain preparations of insulin. Recent reports demonstrate that protamine and heparin form multimolecular complexes that result in high rates of immunization in post-cardiac surgery patients, particularly of immunoglobulin G (IgG) class antibodies; a subset of these anti-protamine/heparin IgG antibodies activates platelets through their FcγIIA (IgG) receptors. Although the clinical consequences of anti-protamine/heparin antibodies that are newly generated after cardiac surgery are unknown, there is evidence that platelet-activating anti-protamine/heparin antibodies already present at the time of cardiac surgery might occasionally explain more severe thrombocytopenia with delayed platelet count recovery, as well as thromboembolic complications, in the post-cardiac surgery setting. Triggers for such antibodies remain poorly-defined, but could include preoperative administration of heparin to diabetic patients receiving protamine-insulin as well as recent previous cardiac surgery. Anti-protamine/heparin antibodies have several features in common with anti-platelet factor 4 (PF4) PF4/heparin antibodies implicated in heparin-induced thrombocytopenia (HIT), including immunization by heparin-containing multimolecular complexes, predominant IgG class, pathological platelet-activating properties, relatively rapid IgG formation without IgM precedence, and antibody transience. Despite these similarities, the risk of anti-protamine/heparin antibody-mediated complications seems to affect the early post-cardiac surgery period, whereas HIT usually occurs at least 5 days following cardiac surgery. Clinicians need to become aware of this recently recognized immunohematological disorder, and research is needed to identify triggers of immunization, improve detection of pathological antibodies and identify patients at risk of this complication.

Platelet-activating protamine-heparin-antibodies lead to higher protamine demand in patients undergoing cardiac surgery

OBJECTIVES

Platelet-activating antibodies against protamine-heparin-complexes were described in patients undergoing cardiac surgery, but their clinical consequences remain unclear. This prospective single-center observational study aimed to describe the prevalence and clinical consequences of protamine-heparin-complex antibodies in patients undergoing cardiac surgery with cardiopulmonary bypass.

METHODS

A total of 200 patients undergoing cardiac surgery with cardiopulmonary bypass were included. Blood samples were collected preoperatively and 1 hour, 24 hours, and 7 days after weaning from cardiopulmonary bypass. All sera were tested for the presence of protamine-heparin-complex antibodies using a modified heparin-induced platelet-activation assay. Specific Fcγ receptor IIa-dependent platelet activation was confirmed by repeated testing in the presence of the Fcγ receptor IIa-blocking antibody IV.3.

RESULTS

Samples from 185 patients were obtained, of whom 24 patients (13%) were positive for protamine-heparin-complex antibodies preoperatively. In all positive samples, functional reactivity was reversible in the presence of IV.3. Although patients with a preoperative presence of protamine-heparin-complex antibodies were significantly older compared with patients negative for protamine-heparin-complex antibodies (73 ± 9.8 years vs 68 ± 10 years, P = .037), no other potential risk factors were identified at 1 day before operation. Patients with protamine-heparin-complex antibodies required significantly more protamine to neutralize heparin (47.66 mg vs 41.67 mg, P = .027). Protamine-heparin-complex antibodies have no significant influence on perioperative platelet numbers, bleeding complications, transfusion requirement, thromboembolic events, major cardiovascular and cerebrovascular events, inflammation parameters, or kidney function.

CONCLUSIONS

Protamine-heparin-complex antibodies occur frequently in patients undergoing cardiac surgery on cardiopulmonary bypass, resulting in specific platelet activation in vitro. Protamine-heparin-complex antibodies are associated with increased protamine requirement after cardiopulmonary bypass and possibly slower recovery of platelet numbers.

Probing the coagulation pathway with aptamers identifies combinations that synergistically inhibit blood clot formation

Coordinated enzymatic reactions regulate blood clot generation. To explore the contributions of various coagulation enzymes in this process, we utilized a panel of aptamers against factors VIIa, IXa, Xa, and prothrombin. Each aptamer dose-dependently inhibited clot formation, yet none was able to completely impede this process in highly procoagulant settings. However, several combinations of two aptamers synergistically impaired clot formation. One extremely potent aptamer combination was able to maintain human blood fluidity even during extracorporeal circulation, a highly procoagulant setting encountered during cardiopulmonary bypass surgery. Moreover, this aptamer cocktail could be rapidly reversed with antidotes to restore normal hemostasis, indicating that even highly potent aptamer combinations can be rapidly controlled. These studies highlight the potential utility of using sets of aptamers to probe the functions of proteins in molecular pathways for research and therapeutic ends.

Bovine and porcine heparins: different drugs with similar effects on human haemodialysis

Background

Heparins from porcine and bovine intestinal mucosa differ in their structure and also in their effects on coagulation, thrombosis and bleeding. However, they are used as undistinguishable drugs.

Methods

We compared bovine and porcine intestinal heparin administered to patients undergoing a particular protocol of haemodialysis. We compared plasma concentrations of these two drugs and also evaluated how they affect patients and the dialyzer used.

Results

Compared with porcine heparin, bovine heparin achieved only 76% of the maximum plasma concentration as IU mL^-1. This observation is consistent with the activities observed in the respective pharmaceutical preparations. When the plasma concentrations were expressed on weight basis, bovine heparin achieved a maximum concentration 1.5 fold higher than porcine heparin. The reduced anticoagulant activity and higher concentration, on weight basis, achieved in the plasma of patients under dialysis using bovine instead of porcine heparin did not affect significantly the patients or the dialyzer used. The heparin dose is still in a range, which confers security and safety to the patients.

Discussion

Despite no apparent difference between bovine and porcine intestinal heparins in the haemodialysis practice, these two types of heparins should be used as distinct drugs due to their differences in structure and biological effects.

Conclusions

The reduced anticoagulant activity achieved in the plasma of patients under dialysis using bovine instead of porcine heparin did not affect significantly the patients or the dialyzer.

The effect of surface contact activation and temperature on plasma coagulation with an RNA aptamer directed against factor IXa

The anticoagulant properties of a novel RNA aptamer that binds FIXa depend collectively on the intensity of surface contact activation of human blood plasma, aptamer concentration, and its binding affinity for FIXa. Accordingly, anticoagulation efficiency of plasma containing any particular aptamer concentration is low when coagulation is strongly activated by hydrophilic surfaces compared to the anticoagulation efficiency in plasma that is weakly activated by hydrophobic surfaces. Anticoagulation efficiency is lower at hypothermic temperatures possibly because aptamer-FIXa binding decreases with decreasing temperatures. Experimental results demonstrating these trends are qualitatively interpreted in the context of a previously established model of anticoagulation efficiency of thrombin-binding DNA aptamers that exhibit anticoagulation properties similar to the FIXa aptamer. In principle, FIXa aptamer anticoagulants should be more efficient and therefore more clinically useful than thrombin-binding aptamers because aptamer binding to FIXa competes only with FX that is at much lower blood concentration than fibrinogen (FI) that competes with thrombin-binding aptamers. Our findings may have translatable relevance in the application of aptamer anticoagulants for clinical conditions in which blood is in direct contact with non-biological surfaces such as those encountered in cardiopulmonary bypass circuits.

A high affinity, antidote-controllable prothrombin and thrombin-binding RNA aptamer inhibits thrombin generation and thrombin activity

BACKGROUND

The conversion of prothrombin to thrombin is one of two non-duplicated enzymatic reactions during coagulation. Thrombin has long been considered an optimal anticoagulant target because it plays a crucial role in fibrin clot formation by catalyzing the cleavage of fibrinogen, upstream coagulation cofactors and platelet receptors. Although a number of anti-thrombin therapeutics exist, it is challenging to use them clinically due to their propensity to induce bleeding. Previously, we isolated a modified RNA aptamer (R9D-14) that binds prothrombin with high affinity and is a potent anticoagulant in vitro.

OBJECTIVES

We sought to explore the structure of R9D-14 and elucidate its anticoagulant mechanism(s). In addition to designing an optimized aptamer (RNA(R9D-14T)), we also explored whether complementary antidote oligonucleotides can rapidly modulate the optimized aptamer's anticoagulant activity.

METHODS AND RESULTS

RNA(R9D-14T) binds prothrombin and thrombin pro/exosite I with high affinity and inhibits both thrombin generation and thrombin exosite I-mediated activity (i.e. fibrin clot formation, feedback activity and platelet activation). RNA(R9D-14T) significantly prolongs the aPTT, PT and TCT clotting assays, and is a more potent inhibitor than the thrombin exosite I DNA aptamer ARC-183. Moreover, a complementary oligonucleotide antidote can rapidly (< 2 min) and durably (>2 h) reverse RNA(R9D-14T) anticoagulation in vitro.

CONCLUSIONS

Powerful anticoagulation, in conjunction with antidote reversibility, suggests that RNA(R9D-14T) may be ideal for clinical anticoagulation in settings that require rapid and robust anticoagulation, such as cardiopulmonary bypass, deep vein thrombosis, stroke or percutaneous coronary intervention.

Potent anticoagulant aptamer directed against factor IXa blocks macromolecular substrate interaction

An aptamer targeting factor IXa has been evaluated in animal models and several clinical studies as a potential antithombotic therapy. We elucidate the molecular mechanism by which this aptamer acts as an anticoagulant. The aptamer binds tightly to factor IXa and prolongs the clotting time of human plasma. The aptamer completely blocks factor IXa activation of factor X regardless of the presence of factor VIIIa. However, the aptamer does not completely block small synthetic substrate cleavage, although it does slow the rate of cleavage. These data are consistent with the aptamer binding to the catalytic domain of factor IXa in such a way as to block an extended substrate-binding site. Therefore, unlike small molecule inhibitors, aptamers appear to be able to bind surfaces surrounding an active site and thereby sterically interfere with enzyme activity. Thus, aptamers may be useful agents to probe and block substrate-binding sites outside of the active site of an enzyme.

Cardiac transplantation and/or mechanical circulatory support device placement using heparin anti-coagulation in the presence of acute heparin-induced thrombocytopenia

BACKGROUND

Patients with congestive heart failure, decreased left ventricular function, and debilitation are frequently maintained on anti-coagulants, including heparin. As such, these patients are at high risk for developing heparin-induced thrombocytopenia (HIT). Some of these HIT-positive individuals will ultimately undergo urgent heart transplantation or placement of a mechanical circulatory support device (MCSD). Such procedures require cardiopulmonary bypass (CPB) and full anti-coagulation. The safety of re-exposure to heparin during CPB despite the presence of recent-onset thrombocytopenia and Hep/PF4 antibodies has not been studied in the transplant or MCSD populations.

METHODS

Over a 24-month period, 75 heart transplants and 55 MCSD implants were performed. Fourteen patients with acute HIT (thrombocytopenia and positive Hep/PF4 antibody by enzyme-linked immunosorbent assay [ELISA]) and 3 patients with a history of remote HIT underwent cardiac transplantation (n = 9) and/or MCSD placement (n = 8) using heparin as the anti-coagulant during CPB. The median time from diagnosis to CPB was 4.5 days in patients with acute HIT.

RESULTS

Thirty-day survival was 100% in transplant patients and 75% in MCSD patients. Post-transplant ELISA testing was found to be negative in 4 patients with acute HIT and 3 with remote HIT.

CONCLUSIONS

This series demonstrates that short-term re-exposure to heparin during urgent CPB for heart transplantation or MCSD placement is safe despite the presence of thrombocytopenia and Hep/PF4 antibodies. Moreover, the rapid clearance of Hep/PF4 antibodies in our transplant patients suggests a potential therapeutic role for immunosuppressive therapy in the management of HIT in the acute setting.