Protamine inhibits plasma carboxypeptidase N, the inactivator of anaphylatoxins and kinins

Hereditary angioedema with normal C1 inhibitor associated with carboxypeptidase N deficiency

Background

Hereditary angioedema (HAE) is a potentially life-threatening disorder characterized by recurrent episodes of subcutaneous or submucosal swelling. HAE with normal C1 inhibitor (HAE-nC1-INH) is an underdiagnosed condition. Although the association with genetic variants has been identified for some families, the genetic causes in many patients with HAE-nC1-INH remain unknown. The role of genes associated with bradykinin catabolism is not fully understood.

Objective

We sought to investigate the biological parameters and the genes related to kallikrein-kinin system in families with a clinical phenotype of HAE-nC1-INH and presenting with a carboxypeptidase N (CPN) deficiency.

Methods

This study includes 4 families presenting with HAE-nC1-INH and CPN deficiency. Patients' clinical records were examined, biological parameters of kallikrein-kinin system were measured, and genetics was analyzed by next-generation sequencing and Sanger sequencing. Predictive algorithms (Human Splicing Finder, Sorting Intolerant From Tolerant, Polymorphism Phenotyping v2, MutationTaster, and ClinPred) were used to classify variants as affecting splicing, as benign to deleterious, or as disease-causing.

Results

Patients presented with angioedema and urticaria, mainly on face/lips, but also with abdominal pain or laryngeal symptoms. Affected patients displayed low CPN activity-30% to 50% of median value in plasma. We identified 3 variants of the CPN1 gene encoding the catalytic 55-kDa subunit of CPN: c.533G>A, c.582A>G, and c.734C>T. CPN deficiency associated with genetic variants segregated with HAE-nC1-INH symptoms in affected family members.

Conclusions

CPN1 gene variants are associated with CPN deficiency and HAE-nC1-INH symptoms in 4 unrelated families. Genetic CPN deficiency may contribute to bradykinin and anaphylatoxin accumulation, with synergistic effects in angioedema and urticarial symptoms.

What's fishy about protamine? Clinical use, adverse reactions, and potential alternatives

Protamine, a highly basic protein isolated from salmon sperm, is the only clinically available agent to reverse the anticoagulation of unfractionated heparin. Following intravenous administration, protamine binds to heparin in a nonspecific electrostatic interaction to reverse its anticoagulant effects. In clinical use, protamine is routinely administered to reverse high-dose heparin anticoagulation in cardiovascular procedures, including cardiac surgery with cardiopulmonary bypass. Despite the lack of supportive evidence regarding protamine's effectiveness to reverse low-molecular-weight heparin, it is recommended in guidelines with low-quality evidence. Different dosing strategies have been reported for reversing heparin in cardiac surgical patients based on empiric dosing, pharmacokinetics, or point-of-care measurements of heparin levels. Protamine administration is associated with a spectrum of adverse reactions that range from vasodilation to life-threatening cardiopulmonary dysfunction and shock. The life-threatening responses appear to be hypersensitivity reactions due to immunoglobulin E and/or immunoglobulin G antibodies. However, protamine and heparin-protamine complexes can activate complement inflammatory pathways and inhibit other coagulation factors. Although alternative agents for reversing heparin are not currently available for clinical use, additional research continues evaluating novel therapeutic approaches.

Bioactive Sphingolipids, Complement Cascade, and Free Hemoglobin Levels in Stable Coronary Artery Disease and Acute Myocardial Infarction

BACKGROUND

Acute myocardial infarction (AMI) and coronary artery bypass graft (CABG) surgery are associated with a pathogen-free inflammatory response (sterile inflammation). Complement cascade (CC) and bioactive sphingolipids (BS) are postulated to be involved in this process.

AIM

The aim of this study was to evaluate plasma levels of CC cleavage fragments (C3a, C5a, and C5b9), sphingosine (SP), sphingosine-1-phosphate (S1P), and free hemoglobin (fHb) in AMI patients treated with primary percutaneous coronary intervention (pPCI) and stable coronary artery disease (SCAD) undergoing CABG.

PATIENTS AND METHODS

The study enrolled 37 subjects (27 male) including 22 AMI patients, 7 CABG patients, and 8 healthy individuals as the control group (CTRL). In the AMI group, blood samples were collected at 5 time points (admission to hospital, 6, 12, 24, and 48 hours post pPCI) and 4 time points in the CABG group (6, 12, 24, and 48 hours post operation). SP and S1P concentrations were measured by high-performance liquid chromatography (HPLC). Analysis of C3a, C5a, and C5b9 levels was carried out using high-sensitivity ELISA and free hemoglobin by spectrophotometry.

RESULTS

The plasma levels of CC cleavage fragments (C3a and C5b9) were significantly higher, while those of SP and S1P were lower in patients undergoing CABG surgery in comparison to the AMI group. In both groups, levels of CC factors showed no significant changes within 48 hours of follow-up. Conversely, SP and S1P levels gradually decreased throughout 48 hours in the AMI group but remained stable after CABG. Moreover, the fHb concentration was significantly higher after 24 and 48 hours post pPCI compared to the corresponding postoperative time points. Additionally, the fHb concentrations increased between 12 and 48 hours after PCI in patients with AMI.

CONCLUSIONS

Inflammatory response after AMI and CABG differed regarding the release of sphingolipids, free hemoglobin, and complement cascade cleavage fragments.

Fine-tune regulation of carboxypeptidase N1 controls vascular patterning during zebrafish development

Vascular development is regulated by complicated signals and molecules in vertebrates. In this study, we characterized a novel function of carboxypeptidase N1 (Cpn1) in the vasculature. We show that cpn1 mRNA is expressed in developing vessels. The knockdown of cpn1 by morpholino injection impairs the growth of intersegmental vessels (ISV) and caudal vein plexus (CVP), suggesting the role of cpn1 in vascular development. We showed that vascular defects are not caused by cell death but are due to the impairment of migration and proliferation. Consistent with vascular growth defects, loss of cpn1 affects the expression of the vascular markers flt4, mrc1, flk, stabilin, and ephrinb2. Furthermore, the overexpression of cpn1 impaired the growth of ISV and CVP, but the remodeling expression of vascular markers was different from the knockdown of cpn1, indicating the differential regulation mechanisms in cpn1-overexpressing embryos. We examine the interaction between cpn1 and multiple signals and observed that cpn1 is regulated by Notch/VEGF signals for ISV growth and likely regulates BMP signals for CVP patterning. In conclusion, we demonstrate that cpn1 has a critical role in the vascular development of zebrafish. We also reveal a fine-tune regulation of cpn1 that controls vascular patterning mediated by multiple signals.

Hereditary and acquired C1-inhibitor-dependent angioedema: from pathophysiology to treatment

Uncontrolled generation of bradykinin (BK) due to insufficient levels of protease inhibitors controlling contact phase (CP) activation, increased activity of CP proteins, and/or inadequate degradation of BK into inactive peptides increases vascular permeability via BK-receptor 2 (BKR2) and results in subcutaneous and submucosal edema formation. Hereditary and acquired angioedema due to C1-inhibitor deficiency (C1-INH-HAE and -AAE) are diseases characterized by serious and potentially fatal attacks of subcutaneous and submucosal edemas of upper airways, facial structures, abdomen, and extremities, due to inadequate control of BK generation. A decreased activity of C1-inhibitor is the hallmark of C1-INH-HAE (types 1 and 2) due to a mutation in the C1-inhibitor gene, whereas the deficiency in C1-inhibitor in C1-INH-AAE is the result of autoimmune phenomena. In HAE with normal C1-inhibitor, a significant percentage of patients have an increased activity of factor XIIa due to a FXII mutation (FXII-HAE). Treatment of C1-inhibitor-dependent angioedema focuses on restoring control of BK generation by inhibition of CP proteases by correcting the balance between CP inhibitors and BK breakdown or by inhibition of BK-mediated effects at the BKR2 on endothelial cells. This review will address the pathophysiology, clinical picture, diagnosis and available treatment in C1-inhibitor-dependent angioedema focusing on BK-release and its regulation. Key Messages Inadequate control of bradykinin formation results in the formation of characteristic subcutaneous and submucosal edemas of the skin, upper airways, facial structures, abdomen and extremities as seen in hereditary and acquired C1-inhibitor-dependent angioedema. Diagnosis of hereditary and acquired C1-inhibitor-dependent angioedema may be troublesome as illustrated by the fact that there is a significant delay in diagnosis; a certain grade of suspicion is therefore crucial for quick diagnosis. Submucosal edema formation in hereditary and acquired C1-inhibitor-dependent angioedema is potentially life threatening and can occur at any age. To date effective therapies for acute and prophylactic treatment are available.

Complement activation and cardiac surgery: a novel target for improving outcomes

Complement activation and the resulting inflammatory response is an important potential mechanism for multisystem organ injury in cardiac surgery. Novel therapeutic strategies using complement inhibitors may hold promise for improving outcomes for cardiac surgical patients by attenuating complement activation or its biologically active effector molecules. Recent clinical trials evaluating complement inhibitors have provided important data to further delineate the impact of complement activation and its inhibition on clinical outcomes. In this review we examine the role of complement activation and its inhibition as a therapeutic approach in cardiac surgery.

Plasmin alters the activity and quaternary structure of human plasma carboxypeptidase N

Human CPN (carboxypeptidase N) is a tetrameric plasma enzyme containing two glycosylated 83 kDa non-catalytic/regulatory subunits that carry and protect two active catalytic subunits. Because CPN can regulate the level of plasminogen binding to cell surface proteins, we investigated how plasmin cleaves CPN and the consequences. The products of hydrolysis were analysed by activity assays, Western blotting, gel filtration and sequencing. When incubated with intact CPN tetramer, plasmin rapidly cleaved the 83 kDa subunit at the Arg457-Ser458 bond near the C-terminus to produce fragments of 72 and 13 kDa, thereby releasing an active 142 kDa heterodimer, and also cleaved the active subunit, decreasing its size from 55 kDa to 48 kDa. Further evidence for the heterodimeric form of CPN was obtained by re-complexing the non-catalytic 72 kDa fragment with recombinant catalytic subunit or by immunoprecipitation of the catalytic subunit after plasmin treatment of CPN using an antibody specific for the 83 kDa subunit. Upon longer incubation, plasmin cleaved the catalytic subunit at Arg218-Arg219 to generate fragments of 27 kDa and 21 kDa, held together by non-covalent bonds, that were more active than the native enzyme. These data show that plasmin can alter CPN structure and activity, and that the C-terminal 13 kDa fragment of the CPN 83 kDa subunit is a docking peptide that is necessary to maintain the stable active tetrameric form of human CPN in plasma.

Structure and function of human plasma carboxypeptidase N, the anaphylatoxin inactivator

Human carboxypeptidase N (CPN) was discovered in the early 1960s as a plasma enzyme that inactivates bradykinin and was identified 8 years later as the major "anaphylatoxin inactivator" of blood. CPN plays an important role in protecting the body from excessive buildup of potentially deleterious peptides that normally act as local autocrine or paracrine hormones. This review summarizes the structure, enzymatic properties and function of this important human enzyme, including insights gained by the recent elucidation of the crystal structure of the CPN catalytic subunit and structural modeling of the non-catalytic regulatory 83 kDa subunit. We also discuss its physiological role in cleaving substrates such as kinins, anaphylatoxins, creatine kinase, plasminogen receptors, hemoglobin and stromal cell-derived factor-1alpha (SDF-1alpha).

Electrochemiluminescence assay for basic carboxypeptidases: inhibition of basic carboxypeptidases and activation of thrombin-activatable fibrinolysis inhibitor

Carboxypeptidases catalyze the removal of the C-terminal amino acid residues in peptides and proteins and exert important biological functions. Assays for carboxypeptidase activity that rely on change of absorbance generally suffer from low sensitivity and are difficult to adapt to high-throughput screening. We have developed a sensitive, robust assay for basic carboxypeptidase activity that makes use of electrochemiluminescent (ECL) detection of reaction product. In this assay, a peptide substrate contains the epitope for antibody (G2-10) binding which is masked by a C-terminal arginine. Carboxypeptidase activity exposes the epitope, allowing the binding of ruthenylated G2-10 which is then detected using ECL. High sensitivity allowed detection limits of 1-2 pM enzyme for carboxypeptidase B and activated thrombin-activatable fibrinolysis inhibitor (TAFIa). The inhibition of several basic carboxypeptidases by commercially available inhibitors was studied. This antibody-based method can be extended to other sensitive detection techniques such as amplified luminescent proximity homogeneous assay. The high sensitivity of the assay allowed the determination of the activatable levels of TAFI in human and other animal plasma in the presence of epsilon -aminocaproic acid, an active-site inhibitor that stabilizes TAFIa. A method to isolate in situ activated TAFIa from human serum in the presence of epsilon -aminocaproic acid was also developed.

Inactivation of C3a and C5a octapeptides by carboxypeptidase R and carboxypeptidase N

Pro-carboxypeptidase R (proCPR), also known as thrombin-activatable fibrinolysis inhibitor (TAFI), precursor of carboxypeptidase U and plasma carboxypeptidase B is present in plasma and following activation by thrombin/thrombomodulin and/or plasmin can remove arginine from the carboxyterminal of C3a and C5a. We have shown that this enzyme can remove terminal arginine from the C5a octapeptide much more efficiently than the classical anaphylatoxin inactivator, carboxypeptidase N (CPN). Since we have previously demonstrated that proCPR is significantly upregulated in the inflammatory state, this enzyme would appear to significantly contribute to the inactivation of C5a, the most potent of the complement derived anaphylatoxins.

Rational structure-based design of a novel carboxypeptidase R inhibitor

A novel carboxypeptidase R (CPR) inhibitor, related to potato carboxypeptidase inhibitor (PCI), was designed using rational structure-based strategies, incorporating two principle facts: CPR has a strong affinity for basic amino acids, and the two lysine and arginine residues of PCI are orientated in the same direction and held in close spatial proximity by three disulfide bonds. Initially, a disulfide-bonded fragment of PCI was synthesized showing weak competitive inhibitory activity against CPR. Subsequently, a smaller linear 9-mer peptide, designated CPI-2KR, was designed/synthesized and found to be a more efficient competitive inhibitor of CPR, without affecting the activity of the other plasma carboxypeptidase, carboxypeptidase N. In vitro studies showed that, together with tissue plasminogen activator, CPI-2KR synergistically accelerated fibrinolysis, representing a lead compound for the design of smaller organic molecules for use in thrombolytic therapy.

The effect of differing rates and injection sites on the amount of protamine delivered before detection of hemodynamic alterations in dogs

OBJECTIVES

To determine the effect of the route and rate of protamine administration on the amount of protamine that could be delivered before a hemodynamic reaction occurred in dogs.

STUDY DESIGN

Prospective randomized experimental study.

ANIMALS

Twenty adult mixed-breed dogs weighing 25.1+/-2.5 kg.

METHODS

Before vascular surgery, the dogs were heparinized to reach an activated clotting time (ACT) of 300 seconds. After completion of the vascular surgery, protamine was administered intravenously until a hemodynamic reaction was recorded. The 4 groups of dogs were given protamine at 5 mg/min (slow) or 10 mg/min (fast) via the cephalic or the jugular veins. Systemic and pulmonary arterial pressures, central venous pressure (CVP), and pulmonary arterial occlusion pressure (PAOP) were recorded before and after protamine administration. The dose of protamine was recorded when a reaction occurred, which was defined as mean arterial pressure (MAP) <60 mm Hg or mean pulmonary arterial pressure (MPAP) >20 mm Hg or more than double the baseline value.

RESULTS

Significant decreases in systolic arterial pressure (SAP), MAP, and diastolic arterial pressure (DAP) and significant increases in systolic (SPAP), mean (MPAP), and diastolic (DPAP) pulmonary arterial pressures were recorded after protamine administration. The cephalic slow group had significantly fewer protamine reactions than other groups (chi-square = 8.57, P = .03, df = 3). Significantly more protamine could be delivered from the cephalic vein (52.5+/-14.5 mg) compared with the jugular vein (37.6+/-16 mg) before a reaction occurred (P = .048).

CONCLUSION

The rate of administration did not have an effect on the amount of protamine delivered. Adverse reactions were minimized when protamine was administered via the cephalic vein at a slow rate.

CLINICAL RELEVANCE

We would recommend delivering protamine after cardiopulmonary bypass or vascular surgery through a peripheral venous route.

Management of anticoagulation and its reversal during paediatric cardiopulmonary bypass: a review of current UK practice

Protocols for management of heparin and protamine administration in patients undergoing open-heart surgery have been developed from experience gained mainly in adult practice. However, it has been demonstrated that there are marked differences between paediatric and adult patients in their response to systemic anticoagulation and its reversal. The aim of this study was to obtain an overview of current practice of management of anticoagulation and its reversal from paediatric cardiac surgical units of Great Britain and Ireland. All centres performing paediatric cardiac surgery agreed to participate in the survey (n = 16). Telephone interviews were carried out with the chief or a senior perfusionist from all participating institutions, which were based on a structured questionnaire compiled specifically for the purpose. The answers were anonymised. At present, in the UK and Ireland, unfractionated heparin is the anticoagulant of choice in all units, with a slight prevalence of porcine mucosal (9/16, 56.5%) versus bovine lung preparation (7/16, 44.0%). The policy for administration of heparin to the patient is uniform, with a dose of 300 IU/kg. However, there is great variability in the amount of heparin added to the prime and to the volume infused during cardiopulmonary bypass (CPB). Monitoring of anticoagulation is achieved by activated coagulation time alone in all but one centre, with lower limits varying between 400 and 750 s. Use of aprotinin is widely accepted, but clinical indications are highly variable. No centre adopts heparin-bonded or heparin-coated circuitry for CPB. Calculation of initial and additional protamine doses followed a variety of criteria, resulting in a very wide distribution of doses. The data obtained highlighted the lack of uniformity among paediatric cardiac surgical units of Great Britain and Ireland with regard to most of the issues related to the management of anticoagulation and its reversal. The striking heterogeneity of our cross-sectional observations clearly underlines the need for prospective, multicentre studies on a national basis to relate different clinical practices to outcome measures.

Low-dose protamine based on heparin-protamine titration method reduces platelet dysfunction after cardiopulmonary bypass

OBJECTIVE

The heparin-protamine titration method that uses the Hepcon hemostasis management system (Medtronic HemoTec Inc, Englewood, Colo) reduced blood loss in cardiac surgery in previous reports, but the mechanism is not fully understood. This study tests the hypothesis that reduced protamine administration preserves platelet function in human cardiac surgery.

METHODS

Platelet count, alpha-granule secretion, and aggregation to thrombin before and after cardiopulmonary bypass in human beings were evaluated. In the control group (n = 14), a fixed dose of protamine (3 mg/kg) was administered. In the titration group (n = 20), protamine doses were based on the heparin concentration measured by the Hepcon system.

RESULTS

Heparin concentrations before protamine administration were higher in the titration group (P =.0012), but protamine doses of patients in the titration group were markedly lower than those of the control group (P <.0001). During protamine infusion at a rate of 0.3 mg. kg(-1). min(-1), the percentage of granule membrane protein-140-positive platelets significantly increased in the control group compared with the titration group (18.8% +/- 8.6% vs 13.0% +/- 5.3%, P =.0188). After protamine administration, aggregation of washed platelets to thrombin recovered almost to the preoperative level in the titration group; however, it remained lower in the control group (20% +/- 20% vs 55% +/- 18%, P =.0009).

CONCLUSION

Low-dose administration of protamine, based on a heparin-protamine titration method, restores not only the blood coagulation but also the platelet responses to thrombin and attenuates platelet alpha-granule secretion during heparin neutralization. Overdose of protamine activates platelets and may predispose patients to excessive bleeding after cardiac surgery.

Low molecular weight protamine: a potential nontoxic heparin antagonist

Protamine sulfate is the universal clinical antagonist to heparin and is used routinely after cardiovascular surgery to neutralize the anticoagulant function of heparin. Its clinical use, however, is associated with adverse effects including idiosyncratic fatal reactions. An examination of the mechanism of heparin neutralization and protamine toxicity suggests that the reversal of heparin anticoagulation may only require a small arginine-rich fragment of protamine to electrostatically dissociate antithrombin III from its binding to a specific pentasaccharide sequence in heparin. A review of literature indicates that chain-shortened peptide fragments derived from their parent proteins are normally accompanied with significantly reduced antigenicity and immunogenicity, which are two primary contributing factors to protamine-induced life-threatening toxic effects via an immunoglobulin-mediated pathway. Based on these observations, we propose our general hypothesis: if a chain-shortened low molecular weight protamine fragment containing the heparin-neutralizing domain could be derived directly from a native protamine, it could be a potent and nontoxic heparin antagonist. In this article, we present our experimental results to support the above hypothesis. LMWP fragments containing an intact arginine sequence and an average molecular weight of approximately 1.1 kDa were prepared successfully by enzymatic digestion of native protamine with thermolysin. In vitro studies demonstrated that such LMWP fragments completely neutralized the anticoagulant functions of heparin, based on the anti-Xa chromogenic assay and aPTT clotting time assay. Our in vivo results indicated that while administration of protamine to mice led to obvious production of antiprotamine antibodies, injection of LMWP did not elicit any detectable immunogenic responses. In addition, the LMWP fragments showed a significantly reduced antigenicity or, in other words, cross-reactivity towards the mice antiprotamine antibodies produced by the administration of protamine.

Metabolism of Bradykinin by Peptidases in Health and Disease

This chapter provides an overview of the metabolism of bradykinin (BK) by peptidases in health and disease. The enzymatic breakdown of kinins affects the duration of their biological actions as the plasma half-life of intravenously injected BK is in the range of seconds. Kinins are cleaved in vitro and in vivo by enzymes that belong to families, such as zinc-metallopeptidases, astacin-like metallopeptidases, and catheptic enzymes. Vane noted the importance of the pulmonary circulation in the metabolism of vasoactive substances, such as BK as well as angiotensin 1 and 5- hydroxytryptamine. It is clear after decades of research that angiotensin 1-converting enzyme (ACE) on the vascular endothelial cell surface is the most important inactivator of blood-borne BK. BK may act primarily in an autocrine and paracrine fashion, establishing the importance of local regulation of its activity by enzymes on cell surfaces. Thus, the assortment of other enzymes that can inactivate BK is important in a variety of physiological and pathological situations. Most physiological systems have redundant pathways of metabolism so that the abolishment of one pathway is compensated for by the presence of others. This is demonstrated by the pharmacological inhibition of ACE in hypertension.

Mast cell chymase in complex with heparin proteoglycan is regulated by protamine

Protamines are polycationic proteins that are widely used for neutralisation of the anticoagulant action of heparin. However, several reports have shown adverse, mast cell-dependent reactions to protamine. The exact mechanism by which protamine causes these adverse effects is not clear. In the present study, the possibility that protamine may influence mast cell chymase function was investigated. Mast cell chymase is in vivo recovered in a macromolecular complex with heparin proteoglycan, and this interaction is essential for expression of optimal enzymatic activity. Protamine was shown to strongly reduce the activity of mast cell chymase by a mechanism that involved displacement of the chymase from heparin proteoglycan.

Effects of heparin on the inhibitory action of protamine on endothelium-mediated vasorelaxation

The precise mechanism(s) of inhibitory action of protamine on endothelium-mediated vasorelaxation has not been fully elucidated. In addition, no information is available regarding the effects of a heparin-protamine complex on the endothelium-mediated relaxation. Employing isometric tension recording methods, we studied the effects of heparin, an anionic substance, on the protamine-induced inhibition of acetylcholine (ACh)-induced vasorelaxation in isolated rabbit small mesenteric artery. Protamine (> or = 50 micrograms/ml) inhibited ACh (0.03-10 microM)-induced relaxation under a norepinephrine (10 microM)-stimulated condition (P < 0.05). The ACh relaxation, even 20 min after washout of protamine (150 micrograms/ml), was still significantly inhibited as compared to the control (before protamine) ACh relaxation, and further, it was not significantly different from the ACh relaxation maximally inhibited in the presence of protamine. Preapplication of heparin (700 U/ml) almost abolished the protamine inhibition (50 & 150 micrograms/ml) of the ACh relaxation. However, heparin (700 U/ml), applied on washout of protamine (150 micrograms/ml), had no effect on the prolonged protamine inhibition. In conclusion, a heparin-protamine complex had no direct effect on the endothelium-mediated relaxation, and the inhibitory action of protamine on the endothelium-mediated relaxation might be due to its polycationic property. The prolongation of protamine inhibition and the lack of effects of heparin on the prolonged protamine inhibition may suggest a toxic effect of protamine on the endothelium.

Effective and less toxic reversal of low-molecular weight heparin anticoagulation by a designer variant of protamine

PURPOSE

This investigation assessed protamine reversal of heparin anticoagulation by formation of a protamine-heparin alpha-helix by use of a new designer-variant protamine [+18BE] that was made from an existing protamine variant [+18B] whose non-alpha-helix-forming amino acid proline (P) was replaced by an alpha-helix-forming glutamic acid (E). The rate of administration of the new [+18BE] variant protamine on efficacy and toxicity in comparison to that of [+21] standard protamine and [+18B] was also studied.

METHODS

Acetyl-EAA(K2A2K2A)4K2-Amide [+18BE] was administered intravenously in a 1:1 dose to low-molecular-weight heparin (LMWH)-anticoagulated (intravenous 150 IU antifactor Xa/kg) dogs over 10 seconds or 3 minutes (n = 7, each group). Reversal efficacy was documented by measuring activated clotting time, thrombin clotting time, antifactor Xa, and antifactor IIa. Toxicity was defined by measuring systemic blood pressure, heart rate, cardiac output, pulmonary artery pressure, and oxygen consumption. Measurements were made at baseline, after administration of LMWH, before its reversal, and for 30 minutes thereafter. Results were compared with those after LMWH reversal with [+21] standard protamine and the [+18B] variant. A total toxicity score (TTS) was calculated for each compound from maximal declines in blood pressure, heart rate, cardiac output, and oxygen consumption.

RESULTS

LMWH anticoagulation reversal was significantly (p < 0.01) less toxic over 10 seconds and 3 minutes with the [+18BE] designer variant (TTS -2.3, -2.2) compared with the [+21] standard protamine (TTS -6.4, -7.2). Percent LMWH reversal at 3 minutes revealed [+18BE] to have antifactor Xa activity as high as 91%, compared with 68% for protamine [+21], when given over 3 minutes (p < 0.05).

CONCLUSIONS

This investigation documents that a new designer variant of protamine [+18BE] has superior efficacy compared with [+21] standard protamine for reversal of LMWH anticoagulation and that this occurs with a highly favorable toxicity profile.

Heparin-coated cardiopulmonary bypass circuits

The indications for heparin-coated extracorporeal circuits cannot be defined or limited at present. Clinical investigation remains at an early stage of development. In situations where the risk of systemic anticoagulation is high, this technology would seem to hold great promise. Examples include extracorporeal lung assist and resuscitation from accidental hypothermia. Some have also suggested the use of heparin-coated circuits for percutaneous bypass in cardiopulmonary resuscitation. A significant advantage might also accrue in noncardiac surgical procedures requiring cardiopulmonary bypass, such as complex cerebral aneurysm or arteriovenous malformation resections, resections of the tracheal carina, or bilateral lung transplantations. Its role in routine cardiac surgical procedures remains uncertain, but the work of von Segesser et al suggests a need for continued investigation in that setting using reduced levels of systemic anticoagulation. That endeavor will be greatly assisted by the recent development of heparin-coated cardiotomy reservoirs. Although heparin-coated circuits have been safely used for extracorporeal lung assist with little or no systemic anticoagulation, prospective studies are clearly needed to determine if this approach is advantageous, and it would seem appropriate to develop heparin coating for silicone-based membrane oxygenators.

Heparin and protamine use in peripheral vascular surgery: a comparison between surgeons of the Society for Vascular Surgery and the European Society for Vascular Surgery

It was the intent of this study to document, in general, the patterns and complications of heparin and protamine usage during carotid endarterectomy, aortic and femoral-popliteal-tibial reconstructions for occlusive disease, elective and emergent abdominal aortic aneurysmectomy, thromboembolectomy, and dialysis arteriovenous (AV) fistula placement by surgeons from North America and Europe. All vascular surgeons from the Society for Vascular Surgery (SVS) and the European Society for Vascular Surgery (ESVS) were surveyed by a voluntary, self-reported questionnaire. Six hundred and forty-six completed questionnaires (284 from SVS and 362 from ESVS), representing a 62% response rate, were returned for evaluation. Systemic and regional administration of heparin was common during vascular procedures performed by both SVS and ESVS surgeons. Use of protamine to reverse heparin anticoagulation varied among SVS and ESVS surgeons, respectively, during: carotid endarterectomy (54% vs. 26%, p < 0.01), elective aortic reconstruction for occlusive disease (58% vs. 23%, p < 0.001), elective aortic reconstruction for abdominal aortic aneurysm (63% vs. 27%, p < 0.001), and femoral-popliteal-tibial reconstruction (44% vs. 15%, p < 0.001). Adverse reactions to protamine among the 25,219 and 12,902 cases reported from SVS and ESVS surgeons, respectively, included: hypotension (1209 and 495 cases), pulmonary artery hypertension (65 and eight cases), anaphylaxis (52 and 10 cases), and death (seven and two cases). These adverse responses accounted for 5.3% and 4.0% of the SVS and ESVS cases, respectively. Although this study is subject to the known limitations of a retrospective survey, it is clear that heparin use is common. Protamine reversal of heparin anticoagulation is more common in North America.(ABSTRACT TRUNCATED AT 250 WORDS)

Calculating the protamine-heparin reversal ratio: a pilot study investigating a new method

There is no consensus as to the dosage of protamine required to reverse a given dose of heparin. The amounts advised vary widely. The hypothesis was investigated that doses of protamine smaller than those usually recommended could be used following cardiac surgery to successfully reverse heparin activity as measured by the activated coagulation time (ACT). A group of 18 patients scheduled for cardiopulmonary bypass (CPB) were investigated with their informed consent. A baseline ACT was measured before anticoagulation with heparin. At the end of CPB, an initial neutralizing dose (IND) of protamine (2 mg/kg) was administered. The ACT was measured after 5 minutes and a further dose of protamine (2 mg/kg) was then administered to make up the full dose. The heparin activity (HA) before and after the IND of protamine reversal was calculated according to the method described by Bull. The IND of protamine (2 mg/kg) was expressed as a ratio of the change in HA (the latter also expressed as mg/kg). The average +/- standard deviation (SD) preoperative ACT was 155 +/- 21 seconds with a range of 130 to 199 seconds. Following heparin administration the ACT increased to 701 +/- 152 seconds. After the IND of protamine, the average ACT of 160 +/- 31 (range, 121 to 250) was not statistically (NS) significantly different from the starting value. A further dose of 2 mg/kg of protamine ("full-dose") decreased (NS) the ACT only minimally to an average of 151 +/- 18 (range, 128 to 206) seconds.(ABSTRACT TRUNCATED AT 250 WORDS)

Heparin content of washed red blood cells from the cardiopulmonary bypass circuit

Reinfusion of red blood cells (RBC) from the extracorporeal circuit following cardiopulmonary bypass (CPB) reduces patient exposure to homologous blood. Because infusing unneutralized heparin might exacerbate postoperative bleeding, this study examines the heparin content of the washed packed RBC produced by a commonly used autotransfusion device. This RBC product was derived from the residual whole blood in the oxygenator circuit after CPB. A wash volume of 750 mL of normal saline produced heparin concentrations below 0.04 USP U/mL. A 500 mL wash volume yielded heparin concentrations ranging from 0.08 to 0.22 USP U/mL, and could be used if time did not permit an additional wash. RBCs produced by the usual complete wash cycle do not contain clinically significant amounts of heparin; thus, they would not require a supplemental protamine dose.

Carboxypeptidase N concentration during cardiopulmonary bypass in humans

Carboxypeptidase N (CPN) is an inactivator of anaphylatoxins and kinins, peptides implicated in the pathogenesis of complications in extracorporeal circulation. To investigate whether the level of CPN is altered during cardiopulmonary bypass (CPB) we studied 15 patients undergoing cardiac surgery utilizing CPB. The concentration of CPN decreased to about 48% of the initial value upon initiation of CPB and remained low throughout the procedure. A similar decrease was observed in the level of alkaline phosphatase, an enzyme that was measured to assess the degree of haemodilution. When the data were normalized for dilution, no difference in the concentration of CPN was observed during CPB. Moreover, no changes in the concentration of CPN were observed when protamine was given to neutralize heparin and none of the 15 patients experienced any side-effects of protamine administration. We conclude that the decrease in CPN during CPB was due primarily to dilution and not to changes in CPN synthesis or catabolism. Protamine administration is not associated with significant changes in the level of CPN in patients who have an asymptomatic reversal of heparin anticoagulation.

Heparin rebound: a comparative study of protamine chloride and protamine sulfate in patients undergoing coronary artery bypass surgery

Heparin rebound has been suggested to occur when protamine sulfate, but not protamine chloride, is used to neutralize heparin. This study was undertaken to compare these two protamine salts in 32 patients undergoing coronary artery bypass surgery. Initial heparin and subsequent protamine doses were determined by constructing a heparin-activated coagulation time response curve. Heparin was neutralized either with protamine sulfate or protamine chloride. The total protamine/heparin dose ratio was 0.71 +/- 0.05 for protamine sulfate and 0.77 +/- 0.07 (mg/100 U) for protamine chloride. The initial neutralization effect, the subsequent behavior of the plasma heparin level, and the various coagulation parameters did not differ significantly between the groups. Two hours after neutralization, a small and temporary increase of plasma heparin level was observed in both groups. The postoperative blood losses were comparable in both groups. Thus, protamine chloride was not a clinically superior antidote to heparin than protamine sulfate. The observed heparin rebound levels were low and clinically insignificant in terms of blood loss, but they were associated with slight changes in coagulation monitoring.

Association of protamine IgE and IgG antibodies with life-threatening reactions to intravenous protamine

Life-threatening reactions to intravenous protamine, administered to reverse heparin anticoagulation, have been reported with increasing frequency as a consequence of the escalating use of cardiac catheterization and coronary bypass surgery. Retrospective studies have shown that such reactions are more common in diabetic patients receiving daily subcutaneous injections of protamine-insulin preparations. To determine whether anti-protamine IgE or IgG antibodies might explain the increased risk for protamine reactions among patients with protamine-insulin-dependent diabetes, we conducted a case-control study of 27 patients (diabetic and nondiabetic) who had acute reactions to intravenous protamine and 43 diabetic patients who tolerated protamine without a reaction during diagnostic or surgical procedures. Cases and controls were grouped according to previous exposure to protamine-insulin preparations. In diabetic patients who had received protamine-insulin injections, the presence of serum antiprotamine IgE antibody was a significant risk factor for acute protamine reactions (relative risk, 95; P = 1.0 X 10(-5), as was antiprotamine IgG (relative risk, 38; P = 1.2 X 10(-5). No patients without previous exposure to protamine-insulin injections had serum protamine IgE antibodies. In this group, anti-protamine IgG antibody was a risk factor for protamine reactions (relative risk, 25; P = 0.0062). We conclude that in protamine-insulin-dependent diabetics, the increased risk of serious reactions when intravenous protamine was given appeared to be caused largely by antibody-mediated mechanisms. In nondiabetic subjects, the presence of protamine IgG was significantly associated with an increased risk of acute protamine reactions, although many nondiabetic subjects who had reactions had no IgG antibodies.