Protein S Deficiency

Practice Essentials

Protein S is a vitamin K–dependent anticoagulant protein that was first discovered in Seattle, Washington in 1979 and arbitrarily named after that city. The major function of protein S is as a cofactor to facilitate the action of activated protein C (APC) on its substrates, activated factor V (FVa) and activated factor VIII (FVIIIa). Protein S deficiencies are associated with thrombosis.

Protein S deficiency may be hereditary or acquired; the latter is usually due to hepatic disease or a vitamin K deficiency. Protein S deficiency usually manifests clinically as venous thromboembolism (VTE).

Any association of protein S deficiency with arterial thrombosis appears coincidental or weak at best. Arterial thrombosis is not evident with other hereditary anticoagulant abnormalities (eg, protein C or antithrombin III deficiency, factor V Leiden gene mutation).

Protein S and C levels are lower in sickle cell anemia and they decrease further significantly during crisis.^[1]

Hereditary protein S deficiency is an autosomal dominant trait. Thrombosis is observed in both heterozygous and homozygous genetic deficiencies of protein S.

Protein S deficiency is diagnosed using laboratory tests for the protein S antigen and by using other tests for functional protein S activity (based on clotting assays); see Workup. Management takes place in the event of acute VTE. Prophylaxis may be used in selected patients with asymptomatic carrier states without a thrombotic event. (See Treatment and Medication.)

For patient education information, see Deep Vein Thrombosis.

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Pathophysiology

To understand how thrombosis occurs in protein S deficiency, its physiological function should be briefly reviewed. Protein S is part of a system of anticoagulant proteins that regulate normal coagulation mechanisms in the body.^[2] Under most normal circumstances, the anticoagulant proteins prevail and blood remains in a liquid nonthrombotic state. Whenever procoagulant forces are locally activated to form a physiologic or pathologic clot, protein S participates as part of one mechanism of controlling clot formation.^{[3, 4, 5]}

Protein S functions predominantly as a nonenzymatic cofactor for the action of another anticoagulant protein, activated protein C (APC). This activity occurs via a coordinated system of proteins, termed the protein C system. The image below shows a simplified outline of the function of protein S in the protein C system.

View Image

A simplified outline of the protein C system.

During the process of clotting, multimolecular complexes are formed on membrane surfaces. These membranes are usually negatively charged phospholipids and/or activated platelets. These multimolecular complexes are referred to as the tenase and prothrombinase complexes for their key activities of activation of factor X and prothrombin, respectively. Anchoring these two complexes are the activated form of factor VIII (FVIIa) used for the tenase complex and the activated form of factor V (FVa) for the prothrombinase complex. These two large proteins are homologous in structure and are cofactors, not enzymes, in the clotting process.

In one of many examples of nature's efficiency, the same enzyme that clots blood, thrombin, is converted from clotting to an anticoagulant mechanism on the surface of the endothelium and it then activates protein C to its active enzymatic form, APC. APC requires protein S as a cofactor in its enzymatic action on its 2 substrates, FVa and FVIIIa. Thus, this process is designed to dampen and shut off clotting by switching off the key cofactor proteins FVa and FVIIIa. Protein S and APC are sufficient to inactivate FVa. However, for the inactivation of FVIIIa, APC and protein S require the help of the nonactivated clotting protein, factor V. This is another example of dual use of a protein in this same process.

Factor Va as noted above is cleaved by APC to an inactive form. However, in assisting protein S to inactivate FVIIIa, it is the inactive FV that is cleaved by APC. An important consequence of this dual procoagulant and anticoagulant property of factor V, is that the mutant factor V Leiden, which resists APC cleavage, cannot be switched off but also cannot function here at this step as an anticoagulant protein (see factor V Leiden gene mutation in Race). In addition to its cofactor role in the protein C system, protein S functions independently of protein C to directly inhibit the factor X clotting factor–activating complex and the prothrombin-activating complex.^{[6, 7]}

Furthermore, protein S interacts with the complement system and may play a role in phagocytosis of apoptotic cells. Protein S binds to phosphatidyl serine on apoptotic cells and stimulates macrophage phagocytosis of early apoptotic cells.^[8] The physiological impact of protein S deficiencies on these nonanticoagulant roles of protein S is not yet known.

Protein S is a single-chain glycoprotein, and it is dependent on vitamin K action for posttranslational modification of the protein to a normal functional state. Vitamin K–dependent proteins are synthesized with a unique recognition propeptide piece. The propeptide sequence serves as a recognition site for the vitamin K–dependent gamma-carboxylase enzyme that modifies the nearby glutamic acid residues to gamma-carboxyglutamic acid (Gla) residues. Gla residues are responsible for calcium-dependent binding to membrane surfaces. Structural studies indicate that protein S contains 10-12 Gla residues, a loop region sensitive to thrombin (ie, thrombin-sensitive region [TSR]), 4 epidermal growth factor (EGF)–like modules, and a carboxy-terminal portion that is homologous to a sex hormone-binding globulin (SHBG)–like region.

In blood plasma, protein S exists in both a bound and a free state. A portion of protein S is noncovalently bound with high affinity to the complement regulatory protein C4b-binding protein (C4BP). The C4BP molecule consists of 7 alpha chains that bind to the complement protein, C4b, and one beta chain. The beta chain of the C4bBP molecules contains the binding sites for protein S. The role of protein S in complement regulation by C4BP is not completely understood.

In healthy individuals, approximately 30-40% of total protein S is in the free state. Only free protein S is capable of acting as a cofactor in the protein C system. This distinction between free and total protein S levels is important and gives rise to the current terminology regarding the deficiency states. Type I protein S deficiency is a reduction in the level of free and total protein S. Type III deficiency is a reduction in the level of free protein S only. Type II deficiency is a reduction in the cofactor activity of protein S, with normal antigenic levels.

Age affects total protein S but not free protein S levels. Generally, the total protein S level increases in persons older than 50 years. This rise is in association with total increases in the complement binding protein, C4BP. Free protein S levels do not increase with age. These factors may explain the observation that families with the same recognized genetic defect in protein S can have both type I and type III deficiencies. When families with the same genetic type I defect are surveyed, older individuals even with deficiency in protein S have an increase in total protein S and now appear to have type III deficiency.

APC and protein S require negatively charged phospholipids (PL) and Ca²⁺ for normal anticoagulant activity. Studies of the structure and function relationships of protein S demonstrate that the APC interaction sites are located in the Gla, TSR, and first EGF-like modules of protein S. The binding site for C4BP is located in the SHBG-like region, which is also important for full anticoagulant activity.

Heeb et al reported that protein S has APC-independent anticoagulant activity, termed PS-direct, that directly inhibits thrombin generated by the factor Xa/factor Va prothrombinase complex, a process made possible by the presence of zinc (Zn²⁺) content in protein S. The investigators found Zn²⁺ content positively correlated with PS-direct in prothrombinase and clotting assays, but the APC-cofactor activity of protein S was independent of Zn²⁺ content. In addition, protein S that contained Zn²⁺ bound factor Xa more efficiently than protein S without Zn²⁺, and, independent of Zn²⁺ content, protein S also efficiently bound tissue factor pathway inhibitor.^[9]

Heeb et al suggested that conformation differences at or near the interface of 2 laminin G-like domains near the protein S C terminus may indicate that Zn²⁺ is necessary for PS-direct and efficient factor Xa binding and could have a role in stabilizing protein S conformation.^[9]

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Epidemiology

Frequency

United States

Congenital protein S deficiency is an autosomal dominant disease, and the heterozygous state occurs in approximately 2% of unselected patients with venous thromboembolism (VTE).

Protein S deficiency is rare in the healthy population without abnormalities. Frequency is approximately one out of 700 based on extrapolations from a study of over 9000 blood donors who were tested for protein C deficiency. When looking at a selected group of patients with recurrent thrombosis or family history of thrombosis, the frequency of protein S deficiency increases to 3-6%.^[10]

Very rarely, protein S deficiency occurs as a homozygous state, and these individuals have a characteristic thrombotic disorder, purpura fulminans. Purpura fulminans is characterized by small-vessel thrombosis with cutaneous and subcutaneous necrosis, and it appears early in life, usually during the neonatal period or within the first year of life.^[11]

International

Data for European studies indicated the same frequencies for protein S deficiency as in the United States. Studies have indicated that the prevalence of protein S deficiency is particularly high in the Japanese population. In several reported series of patients with VTE in the United States, protein S deficiency was seen in 1-7% of patients. The deficiency is rare in population surveys of Caucasians, at approximately 0.03%.

Japanese patients with VTE have reported a frequency of approximately 12.7% protein S deficiency and similarly elevated population frequencies of approximately 0.48%-0.63%.^[12] In a Japanese study that evaluated patients with VTE for congenital thrombophilia, Ikejiri et al diagnosed congenital protein S deficiency in eight of 130 patients (6.2%).^[13]

Mortality/Morbidity

VTE develops in 60-80% of patients who are heterozygous for protein S deficiency. The remaining patients are asymptomatic, and some heterozygous individuals never develop VTE.

Though it is controversial, no clear association exists between protein S deficiency and arterial thrombosis. Many case reports and small case series describe protein S deficiency as one factor found in patients with arterial thrombosis, most commonly stroke. However, prospective and cohort studies have not shown convincing increased risk for arterial thrombosis.

Protein S deficiency is also associated with fetal loss in women, in the absence of VTE. Some authors suggest that as many as 40% of women with obstetric complications other than VTE may carry some form of thrombophilia. Protein S deficiency is one of these factors along with several other more common genetic thrombophilic states.

Cases of warfarin-induced skin necrosis have been reported in patients with protein S deficiency.^[14]

Mortality is from pulmonary embolism. In several studies, the 3-month mortality rate of pulmonary embolism ranged from 10.0-17.5%. In a study of Medicare recipients with pulmonary embolism, men had a 13.7% mortality rate compared to 12.8% in women; the mortality rate was 16.1% in blacks, compared to 12.9% in whites.

Race

Race-related variations exist in thrombophilic disorders, as one may expect from genetic-based population traits. In general, a significant difference exists in the frequency of thrombophilic disorders in whites compared with thrombophilic disorders in Japanese (Asian) and black African persons. Current research indicates that protein S deficiency is 5-10 times higher in Japanese populations compared with Caucasians. Protein C deficiency is estimated to be 3 times higher in Japanese populations.

The factor V Leiden mutation is common in white populations and is now known to be the result of a founder effect estimated to be 30,000 years old. This mutation is rare and almost never found in Japanese or Asian populations. In general, black Africans and African Americans with VTE have a lower detection of any of the currently recognized thrombophilic disorders, especially factor V Leiden.

Sex

No difference exists in the male-to-female rate of occurrence.

Age

Protein S deficiency is a hereditary disorder, but the age of onset of thrombosis varies by heterozygous versus homozygous state. Most VTE events in heterozygous protein S deficiency occur in persons younger than 40-45 years. The rare homozygous patients have neonatal purpura fulminans, as described above; onset occurs in early infancy. As noted above in the discussion on genetics, age does affect total protein S antigen levels, but not free protein S levels. Older patients deficient in protein S have low free S levels, even if their total protein S level rises into the normal range.

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History

Signs and symptom s of protein S deficiency are those associated with deep venous thrombosis (DVT), thrombophlebitis, or pulmonary embolus. Some women may have fetal loss as their only manifestation of a thrombophilic disorder (eg, protein S deficiency).

With venous thrombosis of the lower limbs, lower limb swelling and discomfort are the usual symptoms. Occasionally, redness or discoloration also is present, with or without associated cellulitis.

A family history of thrombosis is an important finding, which suggests inherited thrombophilia. Thrombosis at an early age (eg, usually < 55 y) or recurrent thrombosis is also frequently indicative of an inherited thrombophilic state (eg, protein S deficiency).

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Physical

Direct the examination to identify signs of venous thrombosis or pulmonary embolism. The results of the physical examination are nonspecific and often misleadingly indicate the diagnosis of DVT. Unusual sites of thrombosis (eg, mesenteric vein,^[15] cerebral sinuses) are rare (< 5%) but, when observed, characteristically suggest one of the inherited thrombophilias (eg, protein S deficiency)

Deep vein thrombosis

See the list below:

• The most common manifestation is venous thrombosis of the lower extremities, whichaccounts for approximately 90% of all events
• Superficial veins that are obviously thrombosed usually appear distended, firm, and noncompressible (cords), with or without associated redness or pain
• Superficial thrombophlebitis can be observed in some cases, with or without DVT
• Suspect DVT if identifying signs of venous obstruction and local inflammation are present on examination
• The classic presentation of DVT is a triad of calf pain, edema, and pain on dorsiflexion of the foot (ie, Homan sign). However, less than a third of DVT cases exhibit these three findings; physicians observe unilateral leg or calf swelling with mild or moderate pain more often, which suggests DVT; rarely, calf discomfort without swelling is the only sign of DVT
• Differential diagnoses for DVT include muscle strains and tears, passive swelling of a paralyzed or immobilized limb, Baker cyst, cellulitis, knee trauma or derangement, lymphatic obstruction, and postphlebitic syndrome
• In postphlebitic syndrome, chronic swelling and pain are present in the limb, and the occurrence of a new venous thrombosis is often impossible to assess without Doppler or venography.

Pulmonary embolism

See the list below:

• Some patients may have associated or isolated pulmonary embolism and may experience dyspnea, chest pain, syncope, or cardiac palpitations; dyspnea is the most frequent symptom of pulmonary embolism, and tachypnea is the most frequent sign
• Some patients with massive pulmonary embolism can present with syncope or cyanosis
• Classic pleuritic chest pain, cough, or hemoptysis suggests an embolism with pleural involvement
• Acute right-sided heart failure occurs rarely and is associated with massive embolus
• Findings of right ventricular dysfunction include bulging neck veins, a left parasternal lift, and an accentuated pulmonic component of the second heart sound; these findings are not specific for pulmonary embolism

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Causes

Protein S deficiency may be hereditary or acquired.

Hereditary protein S deficiency

Researchers have identified 2 genes for human protein S; both are linked closely on chromosome 3p11.1-3q11.2.^[16]  One gene is the active gene, PROS-b (ie, PROS1), and the other, PROS-a, is an evolutionarily duplicated nonfunctional gene, which is classified as a pseudogene because it contains multiple coding errors (eg, frameshifts, stop codons).^[17]  The expressed (alpha) PROS1 gene is more than 80 kb long and contains 15 exons and 14 introns. The protein S pseudogene (beta) has 97% homology to the PROS-a gene.^[18]  Molecular studies into the genetic causes of protein S deficiency are complicated by the presence of the pseudogene, PROS-b, and phenotypic variation.

A number of mutations in PROS1 have been identified as causes of protein S deficiency and thrombophilia. Most are point mutations, such as transversion mutations that generate a premature stop codon and thus result in a truncated protein S molecule.^{[19, 20, 21]}  In addition, deletions of large portions of the PROS1 gene have been reported.^[22]  Researchers located the first such deletion in the central portion of the gene.^[23]  The second deletion described (5.3 kb) was a deletion of coding exon XIII, which resulted in a truncated protein product.^[24]

Acquired protein S deficiency

Acquired conditions associated with decreased protein S levels include the following^[25] :

• Oral contraceptive use
• Warfarin anticoagulant use
• Vitamin K deficiency
• Chronic liver disease
• Certain viral infections (eg, HIV, varicella)
• Systemic lupus erythematosus
• Myeloproliferative disorders

In addition, protein S levels decrease in pregnancy and can fall into the abnormal-low laboratory range. These low levels of protein S in pregnancy do not cause thrombosis by themselves.

Another seldom recognized cause for acquired protein S deficiency is sickle cell anemia. However, this condition alone does not produce a thrombophilic state.

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Laboratory Studies

Protein S deficiency is diagnosed using laboratory tests for the protein S antigen and by using other tests for functional protein S activity (based on clotting assays), as follows^[5] :

• Protein S antigen: Laboratories can test protein S antigen as total antigen (ie, protein S bound to C4BP plus free protein) or free protein S antigen. The free form of protein S has functional activity, and researchers have developed assays specifically for the free protein S antigen. Both free and total protein S are measured by ELISA methods in the laboratory.
• Functional protein S: Assays for functional protein S are indirect and are based on prolongation of blood clotting by the generation of APC and its function in the assay. These functional tests are difficult to perform. In addition, the tests introduce several other factors that can alter the interpretation of test results. Most importantly, a falsely low protein S functional assay value can be observed in patients with factor V Leiden genetic defect, which is another common cause of hereditary thrombophilia that interferes with protein C function. Some new commercial methods for determining protein S deficiency can measure activity in factor V Leiden patients accurately after dilution of test plasma.^{[28, 29]}

Several clinical conditions affect the blood levels of protein S on both antigenic and functional assays. As one would expect, vitamin K deficiency, liver disease, or antagonism with warfarin reduces protein S levels. In the setting of acute thrombosis, protein S levels fall, sometimes into the deficient range. Pregnancy also results in lower blood levels of protein S, especially as measured by functional assays. As noted previously in the section on genetics of protein S, total protein S levels actually rise with age. Free protein S levels are not affected by age.

Based on the measurement of free and total protein S antigen and functional protein S activity, scientists classify protein S deficiency into the following three phenotypes, using the classification proposed at the 1991 meeting of the Scientific Subcommittee of the International Society on Thrombosis and Haemostasis in Munich, Germany:

• Type I deficiency is characterized by a decrease in the total protein S antigen and free protein S antigen together (quantitative deficiency)
• Type II deficiency is characterized by normal total and free antigen levels but reduced protein S activity (functional deficiency)
• Type III deficiency is characterized by low free protein S levels, whereas the total plasma concentration of protein S is normal

Although reports document a few type II deficiencies, they are rare. The most common types are I and III. The distinction between type I and type III has no clinical implications. In both type I and type III deficiencies, free protein S levels are reduced.

Physicians should request free protein S antigen testing for any patient suspected of having deficiencies of protein S because this test detects most cases (ie, type I or III), and the use of a total protein S assay is not routinely needed. Consider use of the functional assay for protein S deficiency if the other test results are normal and a reliable assay can be performed after excluding other interfering defects.

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Medical Care

Management of protein S deficiency takes place in the event of acute venous thromboembolism (VTE). Prophylaxis may be used in selected patients with asymptomatic carrier states without a thrombotic event. Following an acute thrombosis, administer heparin therapy and then transition to warfarin oral anticoagulation.

Heparin

Heparin is administered as follows:

• Initial heparin treatment may be with intravenous unfractionated heparin or subcutaneous low molecular weight heparin (LMWH)
• Heparin should be given for a minimum of 5 days
• Manage heparin with standard protocols; see Deep Venous Thrombosis or Pulmonary Embolism for additional details

Warfarin

Warfarin administration can start on day 1 or 2 of heparin therapy. After two consecutive therapeutic International Normalized Ratio (INR) clotting tests and a minimum of 5 days of heparin therapy, the patient can continue on warfarin alone. In most patients, specialists recommend 6-9 months of initial treatment with warfarin.

The question of whether to continue lifelong warfarin in patients with identified protein S deficiency after their first thrombotic event is controversial. If the first thrombotic event was life threatening or occurred in multiple or unusual sites (eg, cerebral veins, mesenteric veins), most experts recommend lifelong therapy initially. If precipitated by a strong event (eg, trauma, surgery) and the thrombosis did not meet the criteria of life threatening or multiple or unusual sites, some experts argue that these patients may have a lower risk of recurrence and deserve a trial without warfarin after 9 months.

Direct Factor Xa Inhibitors

These agents bind to factor Xa and prevent it from cleaving prothrombin to thrombin. Although there are no direct trials to support use of these drugs in protein S deficiency, these drugs are widely being used in multiple hypercoagulable states, including protein S deficiency. Currently the oral agents that are available are rivaroxaban, apixaban, and edoxaban.^[30]

Direct Thrombin Inhibitors

Dabigatran is another option for treatment of hypercoagulable states, however there are no direct trials to support its use in protein S deficiency. Specific reversal agents for non–vitamin K antagonist oral anticoagulants are lacking, but idarucizumab, an antibody fragment, is available for reversing the anticoagulant effects of dabigatran.

Prophylaxis

In patients who are asymptomatic carriers of protein S deficiency, the goal of therapy is prevention of the first thrombosis. In such patients, avoid drugs that predispose to thrombosis, including oral contraceptives. In these patients, if surgery or orthopedic injury occurs, prophylaxis with heparin is mandatory.

In pregnancy, experts recommend prophylaxis with heparin; however, the timing is controversial. Most experts would treat from the second trimester through 4-6 weeks postpartum.

The patient's bleeding risks must be assessed on an individual basis for any of these prophylactic recommendations. No single prescription fits all cases.

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Diet and Activity

Dietary issues relate to patients with protein S deficiency who are on oral anticoagulation with warfarin. Avoid foods that are rich in vitamin K.

Restrictions apply to activity shortly after acute venous thrombosis (ie, DVT, pulmonary embolism). See Deep Venous Thrombosis or Pulmonary Embolism for additional details concerning such restrictions. While on anticoagulation therapy, patients should avoid vigorous contact activities.

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Medication Summary

Heparin is used for patients with acute thrombotic events or for the prevention of thrombosis. Heparin treatment currently is available in two forms: unfractionated (standard) heparin or low molecular weight heparin (LMWH).

Unfractionated heparin for treatment of thrombosis is administered properly by a weight-based dosing protocol, with a target heparin therapeutic range as monitored by the activated partial thromboplastin time (aPTT) test and for a minimum of 5 days. A heparin dosing protocol includes the specified weight-dosing regimen, the target therapeutic aPTT range, the time for measuring aPTT tests after bolus or adjustment in dose (4-6 h), and a standard means of adjusting the unfractionated heparin infusion based on the aPTT test result (eg, subtherapeutic, therapeutic, supratherapeutic).

A commonly used weight-adjusted unfractionated heparin regimen is termed 80/18, and consists of an 80 U/kg IV bolus followed by 18 U/kg continuous IV infusion. The target therapeutic heparin range is ideally individualized to the institution's laboratory aPTT test instrument and reagent.

To obtain an institutional heparin therapeutic range, employ a method such as that described by Brill-Edwards or any other similar comparison of in vitro and ex vivo heparin levels with aPTT test results in multiple individuals. In the absence of an established institutional therapeutic range, an aPTT ratio of 1.5-2.0 is commonly used; however, aPTT reagents and patient responses to unfractionated heparin vary, and the ratio can be 1.8-3.0 for some reagents.

The pharmacodynamics of LMWHs are different from the parent unfractionated heparin. LMWHs are administered subcutaneously. The aPTT test is not affected significantly by LMWH and is not used to monitor LMWH therapy. Several different LMWHs are available in the United States, but they have different pharmacodynamic properties and are not considered interchangeable. Weight-based dosing regimens for each LMWH and for treatment or prophylaxis indications are available from each manufacturer.

LMWHs are approved for treatment of deep venous thrombosis (DVT) with or without pulmonary embolism in the inpatient hospital setting. LMWHs are approved for treatment of DVT without pulmonary embolism in the outpatient setting.

Warfarin is used for long-term oral anticoagulant management of patients with protein S deficiency after first or subsequent thrombosis.

Direct factor Xa inhibitor

These agents bind to factor Xa and prevent it from cleaving prothrombin to thrombin. Currently the oral agents that are available are:

Rivaroxaban

Description: Its anticoagulant action develops within 2-4 hours of ingestion and lasts for approximately 24 hours. It should be used if the creatinine clearance is < 15mL/min.

Dosage: In cases of DVT, PE: Given by oral route, to start with 15 mg twice daily with food for 21 days followed by 20 mg once daily with food

Pharmacokinetics:  Largely metabolized and also excreted unchanged in urine.

Advantage: Rivaroxaban does not require monitoring of PT and aPTT. However, presence can be tested by Prothrombin time (PT) or antifactor Xa activity.

Apixaban

Description: Used in patients with atrial fibrillation to prevent venous thromboembolism and stroke.

Dosage: For treatment of DVT and PE, the dosage is 10 mg twice daily for 7 days followed by 5 mg twice daily

Pharmacokinetics: Metabolized in the liver via CYP3A4/5

Advantage: PT, aPTT and INR are prolonged but, routinely no monitoring is required. Anti-Xa assay can be used to rely upon if testing is necessary.

Edoxaban

Description: Orally active, with a half life of 6-11 hours.

Pharmacokinetics: undergoes hydrolysis, conjugation and oxidation by CYP3A4

Advantage: Inhibits free Xa, prothrombinase activity and thrombin-induced platelet aggregation without requiring routine monitoring. Dosed once daily.

Dabigatran

Description:  It is a direct thrombin inhibitor with a plasma half- life is 12-14 hours and duration of action is 24 hours.

Dosage: In cases of VTE and PE, administered in a dose of 150mg twice daily after 5-10 days of parenteral anticoagulation

Pharmacokinetics:

Dabigatran etexilate is a prodrug and gets rapidly hydrolyzed to dabigatran (active form) in the liver by plasma and hepatic esterases. It undergoes hepatic glucuronidation to active acylglucuronide isomers.

Advantage:  Does not require routine monitoring. However, the measurement of activated partial thromboplastin time (aPTT), ecarin clotting test (ECT) if available, or thrombin time may be useful to determine presence of dabigatran and level of coagulopathy. Also, reversal agent has

ANTICOAGULANTS IN DEVELOPMENT

Tissue factor pathway inhibitors –  TFPI act in two ways. One, by directly inhibiting factor Xa and second, by forming a complex together with factor Xa inhibits TF/FVIIa. Currently the recombinant form is being tested. (1).

Factor VIII inhibitor – with an approximate three week half-life, TB-402 is a human IgG4 monoclonal antibody with a partial inhibitor activity. TB-402 has been found to be effective for a 10 days equivalent of LMW Heparin in a randomized phase II trial in patients who have undergone total knee replacement.

Factor IXa inhibitor – REG1 has two components, RB006 which binds and inhibits factor IXa and RB007, which neutralizes its anti-IXa activity. Trials are still underway to determine its effect when combined with anit-platelet therapy in patients with coronary artery disease.

Factor XI inhibitor – FXI-ASO is a factor XI antisense oligonucleotide that potentially reduces factor XI.

Factor XIIa inhibitor – rHA-Infestin-4 is a factor XIIa inhibitor that is being investigated further in the management of acute ischemic cardiovascular and cerebrovascular events.

Polyphosphate inhibitors – Platelets when activated release polyphosphate that impact coagulation via the intrinsic pathway. And the compounds which inhibit the polyphosphate are being investigated.

Thrombomodulin –  ART-123 is the recombinant part of the extracellular domain of thrombomodulin.  It has also been approved in Japan for the treatment of DIC. Also, with a plasma half-life of 2-3 days, it can be administered subcutaneously every five to six days.

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Heparin

• Dosing, Interactions, etc.

Clinical Context:  Usually administered as CIV infusion for the treatment of acute thrombosis. For prevention of thrombosis, unfractionated heparin is administered SC.

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Enoxaparin (Lovenox)

• Dosing, Interactions, etc.

Clinical Context:  Enhances inhibition of factor Xa and thrombin by increasing antithrombin III activity. In addition, preferentially increases inhibition of factor Xa.

Average duration of treatment is 7-14 d.

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Dalteparin (Fragmin)

• Dosing, Interactions, etc.

Clinical Context:  Enhances inhibition of factor Xa and thrombin by increasing antithrombin III activity. In addition, preferentially increases inhibition of factor Xa.

Average duration of treatment is 7-14 d.

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Fondaparinux sodium (Arixtra)

• Dosing, Interactions, etc.

Clinical Context:  Only synthetic compound in this class of LMW heparins. This compound is a novel pentasaccharide capable of inhibiting factor Xa via the action of antithrombin (AT) but devoid of anti-factor IIa (thrombin) activity. Interestingly, this compound does not appear to cross-react with HIT antibodies.

Approved for use in hip fracture surgery, knee replacement surgery, and hip replacement surgery. Only FDA-approved anticoagulant drug for hip fracture surgery. Also used and approved for extended prophylactic dosing for 21 d following hip fracture surgery.

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Tinzaparin (Innohep)

• Dosing, Interactions, etc.

Clinical Context:  Enhances inhibition of factor Xa and thrombin by increasing antithrombin III activity. In addition, preferentially increases inhibition of factor Xa.

Average duration of treatment is 7-14 d.

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Warfarin (Coumadin)

• Dosing, Interactions, etc.

Clinical Context:  Oral anticoagulant that antagonizes action of vitamin K in normal synthesis of clotting factors II, VII, IX, and X. Safe and effective for long-term oral management of thrombotic disorders. See articles on Deep Venous Thrombosis or Pulmonary Embolism (discussed in Treatment section) for additional details on dosing and monitoring of warfarin. Therapy is initiated without a loading dose at a dose range of 5-10 mg qd for 70-kg adult. Monitor PT/INR daily during initiation of therapy to measure anticoagulation effect. After initial 5-10 d and stabilization of warfarin dose, measure PT/INR 2-3 times qwk for 2-4 wk, then monthly thereafter.

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Class Summary

Unfractionated IV heparin and fractionated low molecular weight SC heparins are the 2 choices for initial anticoagulation therapy. Warfarin therapy may be initiated after 1-3 days of effective heparinization.

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Deterrence/Prevention

In patients with heterozygous protein S deficiency and no history of thrombosis, physicians may administer prophylactic heparin during situations that present high risk for thrombosis. Such situations include surgery, orthopedic trauma (especially with a cast), pregnancy, and prolonged bed rest. Heparin may be administered subcutaneously in standard protocols for venous thromboembolism (VTE) prevention.

The risk of VTE during pregnancy and for the first 6 weeks postpartum varies among the hereditary thrombophilic states. Protein S and protein C deficiencies significantly elevate the risks for thrombosis when compared with the modest increase in thrombosis seen with factor V Leiden mutation. Protein S deficiency was also associated with a seven-fold increase in fetal loss. Many experts recommend that women with protein S deficiency and a history of fetal loss, and severe or recurrent eclampsia, receive low-dose aspirin and prophylactic-dose low molecular weight heparin (LMWH) therapy during pregnancy, with the LMWH prophylaxis extending for 6 weeks postpartum.

For women with heterozygous protein S deficiency and no prior VTE history of fetal loss, treatment choices vary. Some recommend VTE prophylaxis only during the 6 weeks postpartum (the highest risk period for VTE) unless the pregnancy is complicated. Others recommend prophylaxis for the entire pregnancy and 6 weeks postpartum. Recommendations for other scenarios include the following:

• For women with no prior history of VTE and protein S deficiency plus any other thrombophilic defect, active prophylaxis with LMWH should be given during pregnancy and for 6 weeks postpartum.
• For women with a prior VTE history and confirmed protein S deficiency, experts recommend prophylactic or intermediate dosing of LMWH during pregnancy and for 6 weeks postpartum.
• For women with a prior history of VTE who are already receiving oral anticoagulants at the time of pregnancy, full anticoagulant dosing of LMWH is recommended with transition back to oral anticoagulant postpartum.
• Patients with recurrent thrombosis should remain on lifelong warfarin.

Interruption of anticoagulation

In patients with a history of thrombosis who are taking warfarin, no standard exists for "bridging" (ie, on and off use of warfarin for surgery or other procedures that require cessation of warfarin). Some institutions cover with subcutaneous heparin while holding warfarin for 3-4 days. In other situations, this temporary interruption of warfarin is not covered by heparin. Each clinician should weigh the thrombosis risk with the bleeding risk in the individual patient because no data in controlled trials are available to answer this difficult question.

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