WO1991001382A1

WO1991001382A1 - A method for determining the functional activity of free protein s or protein c in a plasma sample

Info

Publication number: WO1991001382A1
Application number: PCT/SE1990/000478
Authority: WO
Inventors: Steffen ROSÉN
Original assignee: Kabivitrum Ab
Priority date: 1989-07-14
Filing date: 1990-07-04
Publication date: 1991-02-07
Also published as: DE69022425T2; EP0486515B1; ES2079481T3; SE8902532D0; IE902431A1; JPH04506603A; DE69022425D1; EP0486515A1; SE464135B; SE8902532L; US5439802A; NO920125L; JP2892828B2; DK0486515T3; ATE127853T1; CA2063564A1; AU6053390A; NO920125D0

Abstract

There is described a method for assaying the functional activity of free Protein S or Protein C in a plasma sample, by adding to the plasma sample a coagulation enzyme, FIXa, which enzyme promotes the formation of coagulation enzyme FXa optionally together with further coagulation factors, whereafter the sample is incubated and the quantity of thrombin formed from prothrombin, this process being promoted by FXa, is measured in a known manner. The level of functionally active Protein S or Protein C, which is inversely correlated to the amount of thrombin formed, is then determined on the basis of the quantity of thrombin measured, in a known manner with the aid of a standard.

Description

A method for determining the functional activity of free Protein S or Protein C in a plasma sample.

The present invention relates to the assay of com¬ ponents in the blood coagulation and fibrinolysis sys¬ tems, and particularly to the assaying or determination of the functional activity of certain proteins included in these systems, namely the plasma proteins designated Protein C and Protein S.

One purpose of the blood coagulation or clotting pro¬ cess is to effectively stop bleeding. This process involves a complicated so-called enzymatic cascade of enzyme-activating reactions initiated by contact activation, e.g. through an injured blood vessel, of a proenzyme. Factor XII (the word Factor is abbreviated hereinafter to F, as is normal practice) to an active enzyme FXII (the suffix "a" stands for active and this labelling method is used generally in the following text) . FXII catalyzes a subsequent activation reaction of proenzyme to enzyme and the blood coagulum or blood clot is finally formed through a series (cascade) of enzyme activations, by the conversion of soluble fibrinogen to insoluble fibbrin. The many activation stages, i.e. the cascade of reactions, contribute to the rapid formation of the blood clot, so as to stop bleeding rapidly.

The reverse process, i.e. the lysis of formed blood clots, so-called fibrinolysis also comprises a similar enzyme reaction cascade, wherein plasmin is formed in the ultimate stage of the cascade. The plasmin formed functions to degrade the clot, i.e. the fibrin, quickly into smaller, soluble fragments, by proteolysis.

These systems incorporate a large number of factors, as will be described in more detail herebelow.

Enhanced or reduced quantities of one or more of these factors in the blood, due to acquired or inherited disorders in the coagulation and/or fibrinolysis sys¬ tems will often lead to pathalogical conditions, which may be fatal, and in the case of the individual may, for instance, mean a predisposition to the formation of arterial and/or venous thrombosis (or blood clots) . Reduced quantities of functionally active antithro bin and plasminogen and enhanced quantities of FVII, fibri- nogen and plasminogen activator inhibitors PAI-1 are examples of disorders which can lead to thrombosis (or blood clots) .

Recently, two novel proteins, designated Protein C (PC) and Protein S (PS) have been identified in plasma.

Similar to the well-known antithrombin, these proteins, which are vitamin K dependent, have an anticoagulating activity, said proteins coacting such that Protein S (molecular weight 80 kD) will stimulate the activity of activated Protein C and therewith both counteract clot formation and promote degradation of clots that have already formed. Reduced quantities of these proteins, or of one of said proteins, will also lead to disorders in the coagulation and fibrinolysis systems. The signi- ficance of Protein S has been confirmed by studies carried out on the protein. On the basis of these stud¬ ies, it is believed that in the case of patients afflicted with deep venous thrombosis and younger than 50 years of age, the clinically manifested thrombosis in 5-8% of cases is due to an inherited deficiency of Protein S, whereas an inherited deficiency of antith¬ rombin is believed to be responsible for only about 3% of all cases.

Considerable benefit would therefore be gained if it were possible to assay, or determine the content of, both or one of the proteins as a matter of routine, since this will enable the predisposition of an indivi¬ dual to the formation of such thrombosis to be establi- shed more easily and to enable prophylaxis and/or therapeutical treatment to be administered in good time.

These plasma proteins, PC and PS, influence the forma- tion of FX and thrombin, by coacting to cleave the cofactors FV and FVIII , necessary for effecting for- mation of thrombin. In order to facilitate an under¬ standing of the function of Protein S and Protein C (in an activated form) , a brief description is given below of the essential parts of the coagulation system, of which the stages from and including activation of FXII are shown in Schedule 1:

Schedule 1

I_^>Jactivating j^ Λ inhibiting

As will be seen from this schedule (1), the inactive protein prothrombin (proenzyme) is converted to active thrombin (enzyme) , through the influence of activated FX, i.e. FX . Ca , a phospholipid designated TF3 (thrombocyte factor 3) which is exposed on the surface of activated thrombocytes, and a helper-protein FV in its activated form FV . The thrombin cleaves the fibri- a nogen enzymatically, such as to obtain fibrin, which forms the structural framework of the clot, by being cross-linked with the aid of the enzyme FXIII . These processes are illustrated in Schedule 2 below, which thus forms part of Schedule 1.

Schedule 2

Fibrinogen. - Fibrin (soluble) FXIII ->^FXIIIa Ftibrin clot

In turn, FX has been formed by inactive FX through the ^a 2+ action of FIXa and with the aid of TF-_5, Ca and a further helper protein FVIII in the active form FVIII . These processes are illustrated in Schedule 3 below, which also forms part of Schedule 1.

Schedule 3

As will be seen from these schedules, thrombin is self- regulating with respect to the quantity formed, by contributing to the activation of FVa (Schedule 2) and, together with FX^a., also to the activation of FVIIIa (Schedule 3) , these active factors being necessary for effective thrombin formation, and also by contributing to the activation of Protein C which, in active form, together with Protein S decreases thrombin formation by cleaving the factors FVIIIa and FVa, as will be ex- plained here below. The enzyme FIX is formed by inactive FIX through the action of FXIa and Ca . FXIa has been obtained by FXI through the action of FXIIa,. FXIIa, is formed by FXII as a result of vessel injury or the like, which conse¬ quently is the initiating factor in the clotting sys^¬ tem. This is illustrated in Schedule 4 below, which also forms part of Schedule 1.

Schedule 4

Trauma

In addition to the processes disclosed in Schedule 2 and Schedules 3 + 4, the formation of thrombin is pro^¬ moted by activation of FIX and FX to FIX_a and FX_a res^¬ pectively, via the extrinsic system with the assistance of a tissue factor, tissue thromboplastin, which supplies helper-protein and phospholipid, and by the

2+ action of the enzyme FIIa in the presence of Ca .

The activated forms of the coagulation and fibrinolysis factors do not normally occur in the blood, but are only formed when needed. The blood contains a number of proteins, so-called enzyme inhibitors, which guard against erroneous activation. One well known inhibitor is antithrombin, which binds thrombin to a complex lacking proteolytic activity. Protein C (PC) and Protein S (PS) , to which the present invention relates, also act as inhibitors or anticoagulants, the effect of these proteins being based on the activation of Protein C to active Protein

C, APC, which cleaves FVa with the assistance of Prote- in S. This reduces the conversion of prothrombin to thrombin (Schedule 5) dramatically, and thus the for¬ mation of clots.

Protein

In addition, APC exerts, together with PS, a cleaving effect on FVIII , which also assists in the anticoagu- lating effect, by reduced formation of FX . This pro¬ cess is illustrated in Schedule 6 below.

Schedule 6

'thrombin)

As will be seen from these schedules, the activation of Protein C is controlled by some kind of feedback mecha¬ nism which incorporates thrombin for the purpose of restricting clot formation. When clots are formed in a blood vessel, thrombin is passed from the clot to the vessel wall, where thrombin is bound to thrombomodulin (TM) , which is a protein on the endothelial cell sur- no longer have a coagulating effect, i.e. it is no longer able to cleave the fibrinogen, and instead the complex TM/thrombin quickly activates Protein C to APC which, in the form of a complex with Protein S, inhi- bits coagulation by cleaving FVa and FVIIIa. This lat- ter complex thus has an important anti-clotting effect.

As mentioned earlier, APC also promotes clot lysis. This takes place indirectly, by protection of the tis- sue plasminogen activator (t-PA) against the effect of inhibitors. In this way, a high conversion rate of plasminogen to plasmin is maintained, through the effect of t-PA. Plasmin dissolves the fibrin clot, by cleaving the clot into smaller, soluble fragments.

Too low contents of Protein C and/or Protein S, will render an individual predisposed to thrombotic disea¬ ses, as a result of insufficient moderation of thrombin formation, which results in excessive forma- tion of fibrin with the accompanying risk of thrombo¬ sis. Consequently, there is a need of a simple and safe process by means of which the levels of Protein C and/- or Protein S in a sample can be measured in a routine fashion, so as to enable prophylax and/or therapeutic treatment of thrombosis to be improved. It is the func¬ tional activity of Protein C and/or Protein S which shall be determined in particular, since about 10% of all individuals suffering from a Protein C deficiency exhibit normal immunological levels of Protein C, des- pite the fact that the functional activity is greatly reduced or totally lacking. With regard to Protein S, the concentration of which in plasma is about 22 μg/ml, the greater part thereof, about 60%, is bound to a protein within the complementary system, the C4b-bind- ing protein, and is functionally inactive. The remaind- er, about 10 μg/ml, which is present in a free state in plasma, is responsible for the biological activity, i.e. is bound to activated Protein C to form an active complex. Since individuals suffering from a Protein S deficiency can exhibit normal levels of bound Protein S, despite lacking or exhibiting very low levels of free Protein S, it is the functional activity which shall be measured also in the case of Protein S deficiency.

Hitherto, however, no satisfactory process has been developed for assaying the functional activity of pri¬ marily Protein S.

The functional activity of Protein S has, admittedly, been the subject of earlier assaying methods, although not as matter of routine, and the known methods are not sufficiently reliable with respect to distinguishing between various levels of biological activity. Those methods known at present are based on recording the time at which a fibrin clot occurs. These methods are designated APTT-based or FXa-based coagulation methods, the abbreviation "APTT" standing for "Activated Partial Thromboplastin Time". In this regard, plasma is activated in a first stage with a reagent containing phospholipid and a contact activator, such as kaolin or ellaginic acid, thereby initiating the calcium-depen¬ dent coagulation reactions (see Schedule 4) , i.e. FXII and FXI . Ca ++-i.ons are added i.n a second stage, where^a- with FIX, FX and prothrombin are activated to varying degrees. A known quantity of one or more coagulation factors is also added, for the purpose of determining the Protein S activity.

The coagulation sequence initiated in stage l thus proceeds to completion, so as to form a blood clot. The time (in seconds) taken for the clot to form (APTT) is recorded and correlated to the functional activity of Protein S, with the aid of standard samples. Generally, an increasing content of free Protein S in plasma will require a longer time for a blood clot to form, since larger quantities of the coagulation inhibiting APC/ Protein S-complex are formed.

The activating coagulation factors added to the system in these known methods are activated Protein C (APC) and FX [see P. COMP and C. ESMON, "Recurrent Venous a ^L '

Thromboembolism in Patients with a Partial Deficiency of Protein S", New Eng. J. Med. 311, 1525-1528 (1984), and B. DAHLBACK, ' "Inhibition of Protein Ca Cofactor

Function of Human and Bovine Protein S by C4b-Binding Protein", (1986)]. According to A. d'ANGELO, S. VIGANO- d'ANGELO, C. ESMON, P. COMP, "Acquired Deficiencies of Protein S. Protein S Activity During Oral Anti- coagulation, in Liver Disease and in Disseminated In- travascular Coagulation". J. Clin Invest 81, 1445-1454 (1988) first free Protein S in plasma is extracted with the aid of a specific monoclonal antibody, whereafter APC and FX are added to the extracted protein. In K. SUZUKI and J. NISHIOKA, "Plasma Protein S Activity

Measured Using Protac, a Snake Venom Derived Activator of Protein C", Thromb Res. 49, 241-251 (1988), there is used instead of APC a substance which activates PC in plasma to APC, namely Protac® C (from Pentapharm, Switzerland) , the active component of which is a selec¬ tive snake venom enzyme from Agkistrodon Contortrix contortrix.

These known methods, however, have several drawbacks. For instance, the low resolution, i.e. the discrimin- ation of different levels of free Protein S, is limi¬ ted. Typically, the coagulation time is extended by only 10 s within the range of 70-200% free Protein S, the coagulation time for normal plasma, i.e. 100% free Protein S, lying within a range of 40-80 s, depending on the method used. Furthermore, these methods are difficult to carry out on a routine basis, e.g. in coagulation laboratories, and require thorough stan¬ dardization and are deficient with respect to accuracy.

Accordingly, it is an object of the present invention to provide a method for assaying the levels of func¬ tionally active Protein S or Protein C in blood plasma, which can be carried out simply and which will essen- tially circumvent the drawbacks associated with known methods.

In accordance with the invention, this object is achieved with a method comprising the addition of the coagulation enzyme FIXa, this enzyme promoting the formation of the coagulation enzyme FXa, optionally together with further coagulation reagent, (s) , to a plasma sample whose content of functionally active

Protein S or Protein C shall be determined, incubating the sample and measuring, in a known manner, the amount of thrombin derived from prothrombin, this process being catalyzed by FXa, and measuring the content of functionally active Protein S or Protein C, which is inversely correlated to the amount of thrombin formed, on the basis of the measured quantity of thrombin in a known manner with the aid of a standard.

Thus, the quantity of thrombin formed constitutes a measure of the quantity of functionally active Protein S or Protein C present. Methods for assaying the throm- bin content in plasma are well known, and the invention is not limited to any particular measuring method. All methods known at present and also those methods which may be devised in the future are usable. Examples of suitable measuring methods include substrate cleaving based, preferably photometric, measuring processes and coagulation-based measuring processes.

In accordance with the invention, it is found that the photometric substrate-cleaving method is well-suited for determining the functional activity of Protein S and Protein C on the basis of the quantity of thrombin formed, and this method provides considerably greater resolution than that achieved with the known coagulation-based methods, particularly with respect to Protein S. According to one preferred embodiment of the invention, photometric measurement of the quantity of thrombin formed is therefore applied, wherein a selec¬ tive thrombin substrate, preferably a chromogenic thrombin substrate, is used. Such substrates are nor¬ mally based on amino acids or, preferably, on short peptides which are provided with a group (marker) which can be readily and selectively cleaved by thrombin and can be readily measured, e.g. photometrically, when a chromogenic marker, such as p-NA (p-nitroaniline) is used. These substrates are available commercially, for instance, from Kabi Diagnostica, Mδlndal, Sweden and/or can be easily prepared by the person skilled in this art.

It is also surprisingly found, in accordance with the invention, that by suitable selection of the selective thrombin substrate, the inventive method can be carried out optionally in either two stages or in one single stage. It is thus surprising and beneficial that the method can be carried out in one single stage when a suitable selective substrate is chosen. The substrates which make a single-stage process possible are, for instance, (S-2846) (from KabiVitrum AB, Sweden) and H-D-CHG-Ala-Arg-pNA (Nycomed Th-1) (from Nycomed AS, Norway) . The known thrombin substrate H-D-Phe-Pip-Arg- pNA (S-2238) (from KabiVitrum AB, Sweden) , on the other hand, is not suited for a single-step process, due to insufficient resolution among other things.

The inventive method also enables the quantity of thrombin formed to be established with the aid of co¬ agulation methods in a known manner. The coagulation sequence is therewith permitted to proceed to comple- tion and the time taken for a clot to form on the plas¬ ma sample is measured and compared with corresponding times for plasma with known functional activity of Protein S or Protein C. The process can be carried out in either one stage or in two stages.

The present method is based on an addition of FIX for promoting activation of FX to FXa, instead of adding

FXa, directly, as is often the case when practicing prior art methods. The FIX used in accordance with the invention is normally of mammal origin, e.g. from bo¬ vviinnee.. FFIIXXaa from humans or pigs can also be used with advantage.

Addition of FIXa in accordance with the invention pro- vides the advantage of enabling activation of FX to FX to be controlled and, indirectly, also of enabling the inactivating effect of APC/PS on formed FVa, and FVIIIa, to be optimized, which in turn greatly influences the formation of thrombin. The strong influence of FIX on the process, for instance in comparison with the addi- tion of FXa at corresponding plasma contents, could not be foreseen. Neither has the addition of FIXa for the purpose intended with the invention been earlier described.

The inventive method can be further improved by adding certain other coagulation factors, in addition to FIX . Thus, according to one preferred embodiment, the method is carried out while adding activated Protein C, i.e. APC. The APC used is normally of mammal origin, such as from bovine (bovine APC) or pigs (porcine APC) or pre¬ ferably from humans (human APC) . Usable APC can be prepared from plasma or by recombinant techniques, and is normally highly purified. The earlier mentioned commercial preparation Protac® C including a snake venom enzyme can be used instead of APC. This enzyme activates Protein C to APC and thus promotes indirectly formation of the APC/PS complex, the effect of which has been discussed earlier.

According to a further suitable embodiment of the in¬ vention, FVa is also added to the plasma sample. The addition of FVa is used to control the rate at which thrombin is formed, and the conversion of prothrombin to thrombin can be effected more quickly and more effi¬ ciently by the suitable addition of FVa. There is pre- ferably used an FVa of mammal origin, such as human origin, bovine origin or porcine origin, and preferably bboovviinnee oorriiggiinn,, FFVVa. prepared from plasma or via recom- binant techniques.

Phospholipid is another coagulation reagent which can be appropriately added to the plasma sample when carry¬ ing out the present invention. This addition is able to accelerate the activation of FX to FX and prothrombin to thrombin. The choice of the phospholipid source is not critical. For instance, both commercial APTT- reagents, with or without contact activator, such as kaolin or ellaginic acid, and synthetic phospholipids or phospholipid mixtures have been found usable.

In certain cases, the addition of prothrombin, FX and/ or FVIII has been found advantageous. With respect to all coagulation proteins used in accordance with the present method, it will be understood that these co¬ agulation proteins are not limited with respect to species origin and that they are embraced by the inven¬ tion irrespective of whether they are extracted from plasma or prepared via recombinant techniques from native or molecular biologically modified variants thereof. With regard to species origin, however, mammal origin is normally preferred, and then particularly human, bovine or porcine origin.

The proportions in which the majority of components are present when applying the present method are not par¬ ticularly critical. However, in order to facilitate an understanding of the invention, suitable concentration ranges for a number of components have been set forth in the following Table 1.

Table 1

Component/ conditions Final Content in Test

Plasma 0.02-10 percent by volume, particularly 0.1-2 percent by volume* up to 50 percent by volume**

FIXc 1-10~¹⁵-1-10~⁶ mol/1***

APC 5-10^_11-l-10~⁷ mol/1, prefer¬ ably l-10~¹⁰-l-lθ^"8 mol/1

FV 1-10~¹²-1-10~⁹ mol/1, prefer¬ ably 2-lθ^""10-l-lθ^"11 mol/1

Phospho1ipids 1-10^-6 - 3-10^-4 mol/1, prefer¬ ably 3-10~⁶-l-10~⁴ mol/1

Ca²⁺ 10~⁴-5-10~² mol/1, preferably 10~³-10~² mol/1****

Thrombin- substrate, e.g. chromogenic 2-10~⁶-2-10~³ mol/1, prefer¬ ably 10~⁴-10~³ mmol/1 pH 6.5-9.5, preferably 7-8.5

Ion strength (I) 0-0.6, preferably 0-0.25

Protac® C 5-10~³-1.5 U/ml

Prothrombin l-10~⁹-2-10~⁶ mol/1

FX l-10~¹³-5-lθ^"8 mol/1

FVIII l-lθ^"4-5-10~¹ IU/ml

*Thrombin substrate method, e.g. chromogen method. **Coagulation method. ***Higher contents in test without an addition of FVIII and/or FV_a. ****Calcium may be added in the form of CaCA or some other suitable salt.

In order to maintain the pH in the range preferred, the component-solutions are prepared in buffer solutions, using conventional buffer types, such as Tris-buffer.

The inventive method involves incubating the sample at a temperature of 18-45°C, preferably 35-40°C and more preferably at 37°C, for a short period of time, e.g. 0.5-15 minutes, suitably 1-10 minutes and particularly 0.5-6 minutes, whereafter the functional activity of Protein S or C is measured, all in accordance with a so-called single-stage method. In the case of systems having pronounced short reaction times, such as pre- ferably coagulation-based methods, it is preferred to heat the sample in an introductory stage, optionally together with a coagulation factor, to the same tem¬ peratures as those recited above, over a short period of time, normally 2-3 minutes, before adding FIXa and optionally also other coagulation factors to the sam¬ ple. In certain cases, it may be advantageous to apply a two-stage method incorporating two incubation per¬ iods, for instance when Protac® C is used to activate Protein C in the plasma sample.

It may also be beneficial to use in the present method polyclonal or monoclonal antibodies against a coagula¬ tion factor, with the intention of eliminating the influence of this factor. For instance, the biological activity of FVIII can be blocked by adding anti-FVHI:C antibodies.

The aforesaid applies generally for assaying the func¬ tional activity of Protein S. When the present method is applied for assaying the functional activity of Protein C, it may be necessary to make some modifica¬ tion. For instance, Protein S, preferably of human origin, is added to the plasma sample either before or after activation of Protein C in the plasma sample, this activation advantageously being effected with

Protac® C, as disclosed above. FIXa and optionally other coagulation factors is, or are, then added in a second stage.

The invention will be explained in more detail with the aid of the following examples, which are intended sole¬ ly to illustrate the invention without limiting the scope thereof. These examples include references to the accompanying drawings, of which Figure 1 illustrates the assaying of Protein S with a chromogenic single- stage method in the presence of FIXa or FXa,; Figure 2 illustrates the effect of anti-FVIII:C antibodies and bovine FX when assaying Protein S; Figure 3 illustrates the effect of various substrates in a single-stage method for assaying Protein S; Figure 4 illustrates a single-stage coagulation-based method for assaying

Protein S on the basis of a FIX addition; and Figure 5 illustrates a FIXa-based chromogenic method for assay- ing Protein C.

Example l

(a) A plasma sample was diluted 1:15 in Tris-buffer I.

100 μl thereof were mixed with 100 μl of human APC dissolved in Tris-buffer I, 200 μl of a chromogenic substrate H-D-Ala-Pro-Arg-pNA (S2846) from KabiVitrum

Diagnostica, Sweden) and 200 μl in total of human FIXa dissolved in Tris-buffer II, CaCl Δ, bovine FVa dis- solved in Tris-buffer I and a phospholipid mixture comprising 40% cholesterol, 40% phosphatidylcholine and 20% phosphatidyl serine.

The Tris-buffer I comprised 0.05 mol/1 Tris-HCl having a pH of 7.4 and an ion strength (I) of 0.15, and 0.2% bovine serum albumin (BSA) .

The Tris-buffer II comprised 0.05 mol/1 Tris-HCl, pH 8.0, I = 0.15 and 0.2% BSA. The concentrations of the starting solutions were such as to obtain the final concentrations given for the reactants in Table 1.

600 μl (total volume)

The mixture was incubated for 4 minutes at 37°C, where¬ after 20%-acetic acid was added, with the intention of interrupting the reaction processes. The absorbance of the sample was then determined at 405 nm (A. A with standard equipment, such as a photometer, e.g.

Hitachi 100-20. A 4,_n05_c was also determined for standard samples prepared by mixing normal plasma, the Protein S content of which was set at 100%, and Protein S- deficient plasma, the Protein S content of which was set at 0%. A4.0_~5_ was also measured for such plasma as that containing 100% PS and 0% PS respectively.

The values obtained for A _ are plotted against %PS in Figure 1 (-B-). It will be seen from these values that very good resolution was achieved, which is re¬ flected in the wide difference between absorbance at 100% and at 0%, this difference being designated A ._Q5 (0-100%), namely roughly 0.85. It will also be seen from Figure 1 that the Protein S content is in- versely correlated to the quantity of thrombin formed, i.e. that increasing quantities of PS result in a con¬ siderable reduction in the formation of thrombin.

(b) In Example 1 (a) , the assay was carried out with the addition of APC to the plasma sample. By way of comparison, an analogous assay was carried out without adding APC, wherein a substantially smaller, not sig¬ nificant difference £, A._n(_ (0-100%) was obtained, i.e. the test had a low resolution degree. This is evident from the plotted values (- H ~) in Figure 1. The mea¬ sured absorbance remained substantially unchanged at a high level when carrying out the assay according to Example 1 (a) subsequent to initial incubation of the sample with antibodies against Protein S (designated PS-Ab) , whereby the biological activity of PS, and therewith also of the APC/PS-complex, was eliminated. The values obtained are also plotted (-x-) in Figure 1.

(c) By way of comparison with known techniques, an assay was also carried out analogously with 1 (a) above, with the addition of human APC and without the addition of human APC, but with the substitution of

FIXa, with bovine FXa (final concentration 0.4. pmol/1 and 5-min. incubation) . The A._n -values obtained are also plotted in Figure 1 (- A - and - A - respective- ly) . It will be evident herefrom that no significant values of A 4._n05_c. (^v100-0%)' were obtained and that the thrombin formation was constantly relatively low.

Although the incubation time was shortened somewhat when the FXa-addition was increased to a final propor- tion of 40 pmol/1, the resolution was impaired still further. A slightly better result was achieved when the amount of APC addition was increased, although not even a three-fold increase in the APC-addition gave a iv ^A ₄₀₅ (0-100%)-value higher than 30% of the value obtained in accordance with Example 1 (a) .

Example 1 illustrates a one-stage chromogenic method for assaying the functional activity of PS in plasma.

The surprisingly high effect achieved when adding FIX, to the sample is clearly evident from the results ob¬ tained (set forth in Figure 1) . It is admittedly known that APC will degrade the cofactors FV and FVIII via proteolytic cleaving (W. KISIEL, W. CANFIELD, L.

ERICSSON, E. DAVIE, Anticoagulant Properties of Bovine Plasma Protein C Following Activation by Thrombin. Biochemistry 16, 5824-5831 (1977); and R. MARLAR, A. KLEISS, J. GRIFFIN, "Mechanism of Action of Human Activated Protein C, a Thrombin-Dependent Anticoagulant

Enzyme". Blood 59, 1067-1072 (1982)), which process, however, is not effective before FV and FVIII have been activated respectively to FVa and FVIIIa by thrombin and/or FXa,. Consequently, it is quite surprising that the addition of a preformed FX will result in considerably lower APC/PS activity than the addition of

FIXa, (according to the invention) , which assists in the formation of FX during incubation, since activation of FVIII via the influence of FX /thrombin can take place in both instances. The following Example illustrates the significance of the cofactor FVIII to thrombin formation by blocking the activity of the cofactor.

Example 2

One volume of normal plasma (100% PS) was preincubated at 37°C for 15 minutes with one volume of 0.2 μg/ml of monoclonal anti-FVIII:C antibodies 2A3 (from Kabi AB, Sweden) . These antibodies bind specifically to FVIII, which is thereby inactivated. Analogously with Example 1 (a) , but with a plasma content of 1.7% and with an FIX -addition, A.-.,- was determined for plasma samples having 0, 50 and 100% PS respectively after incubation for 4 minutes at 37°C, and with an addition (final content 0.02 U/ml) of bovine FX or without such an addition. The values obtained are plotted in Figure 2.

It will be evident herefrom that in the absence of FVIIIa, APC/PS will still produce an effect when adding

FX (- 0 -) , namely through the influence of APC/PS on the FV supplied.

In comparison with Example l (c) , comprising direct addition of FXa, (Figure 1; - A -) , it would again appear advantageous to form FXa during the incubation period, from FX that has been added instead of adding

FXa directly to the system. (Figure 2; - V -) .

Example 3

Analogously with Example 1 (a) , _4Q_ was determined for normal plasma (100% PS) and Protein S deficient plasma (0% PS) . Instead of using APC, however, Protac® C (final content 0.17 U/ml) was used, which activates (final content 0.17 U/ml) was used, which activates

Protein C to APC upon incubation for 2 minutes at 37°C, and while using 12 pmol/1 of FIXa and applying an in- cubation time of 4.5 minutes, The results obtained are set forth in Table 2.

Table 2

"405 + Protac® C - Protac® C

Protein S-deficient plasma (0% PS) 0.87 Normal plasma (100% PS) 0.18 1.6

It will seen from these values that Protein S exhibits high activity in the presence of APC. It is not neces¬ sary to supply APC initially, since APC can be formed in vitro through the action of Protac® C.

Example 4

A for normal plasma and Protein S-deficient plasma was assayed analogously with Example 1 (a) , although with a final proportion of 18 μmol/1 of the synthetic phospholipid mixture. Analogous assays were also made with the aid of an APTT-reagent, Cephotest® (from Nycomed AS, Norway) , which also includes a contact activator, namely ellaginic acid, instead of the phos¬ pholipid mixture, which in this case comprised an ex¬ tract from bovine brain. The values obtained are set forth in Table 3. Table 3

COMPARISON BETWEEN CEPHOTEST® AND SYNTHETIC PHOSPHOLIPID (PL) MIXTURE

405

% Protein S Cephotest® PL-mixture

0 0.48 0.44

100 0.15 0.18

These A _Q5 values exhibit good agreement, which indi¬ cates that different phospholipid sources can be used, including mutually different compositions of synthetic phospholipids and also mutually different contact activators.

Example 5

Analogously with Example 1 (a) , plasma containing dif¬ ferent percentages of PS were incubated in a single- stage method with the use of three different substrates and additions of bovine FIX . a

The final sample had a plasma content of 1.1 percent by volume, a substrate content of 0.3 mmol/1 and contained 0.1 nmol/1 FIXa,, 75 pmol/1 FVa and 2 nmol/1 APC. The substrates tested consisted of S-2846 and S-2238 (both from KabiVitrum Diagnostica, Sweden) and Nycomed Th-1, the incubation times being 4 min. , 5 min. and 4 min. respectively. The results are set forth in Figure 3. It will be seen from these results that S-2846 (— £Ξ2 —) is a far better substrate than S-2238 (-- -) • It will also be seen from Figure 3 that a good effect was also obtained with the substrate Nycomed Th-1, (- O -) (from Nycomed AS, Oslo, Norway) .

For comparison purposes, the aforesaid method was also carried out as a two-stage method with the use of sub- strates S-2846 and S-2238, i.e. the plasma sample to which APC was added was incubated prior to the sub¬ strate addition. In the two-stage method, the ^ A _Q5 (0-100%) -values obtained were 0.8 and 0.6 respectively, i.e. no pronounced difference in effectiveness was observed. The following substrates were also tested in a single-stage method and found to be useful, ^ _{ά ()}- (0-100%) = 0.2-0.5) .

Chromozym Th: Tosyl-Gly-Pro-Arg-pNA (from Pen- tapharm AB, Basel, Switzerland) .

Spectrozym Th: H-D-CHT-Ala-Arg-pNA (from American

Diagnostica, Greenwich, U.S.A.) .

CBS 34.47: H-D-CHG-But-Arg-pNA (from Diagnos¬ tica Stago, Asnieres, France) . Thrombin substrate from Behring: H-D-CHA-But-Arg-pNA (from Behring- werke AG, Marburg, Federal Republic of Germany) .

wherein

CHT = cyclohexyltyrosine

CHG = cyclohexylglycine

But = α-amino butyric acid

CHA = cyclohexylalanine pNA = p-nitroanilide Tosyl = p-toluene sulphonyl

Exam le 6

100 μl of human APC were added to 100 μl of a plasma sample diluted to 1:1 in a 0.9%-NaCl solution and the sample was incubated for 2 minutes at 37°C. 100 μl totally of bovine FIX,a, bovine FVa, Cephotest® and

CaCl_ were then added. The contents of the starting solutions used were such as to obtain the following final concentrations:

Plasma samples with 0, 50 and 100% PS, i.e. Protein S- deficient plasma (0%), normal plasma (100%) and a 1:1 mixture (50%) thereof were used in the assay, which was carried out as a single-stage coagulation-based method. c<re

The coagulation times of these samplesrregistered and plotted (Figure 4; -■ -) against % PS for the sample.

Analogously with the aforegoing, the coagulation time was also recorded for plasma samples which had not been activated with APC (see Figure 4; - Δ -) .

As will be seen from Figure 4, an almost 60 second extension of the coagulation time was achieved for 100% PS in comparison with 0% PS, this effect being a considerable improvement on the effect achieved with earlier known methods which lie closest to the afore- described method with respect to plasma contents (P. Comp et al, loc cit; B. Dahlback, loc cit) . No difference (i.e. no extension) in coagulation time between 0 and 100% PS in the above method is obtained in the absence of an APC-addition (- A -) .

Example 7

A plasma sample to which human Protein S was added was diluted 1:15 with Tris-buffer I. 100 μl of the sample were mixed with 100 μl of Protac® C at a concentration such that the content when activating Protein C was 0.17 U/ml, whereafter the sample was incubated at 37°C for 5 minutes for activation of PC.

200 μl of thrombin substrate S-2846 were then added together with a total of 200 μl of bovine FIX in Tris- buffer II, bovine FV in Tris-buffer I, Cephotest® and CaCl , the concentrations used being such as to obtain the final concentrations given below. The test solution obtained was incubated for 6 minutes at 37°C, whereafter the reaction was interrupted by adding 200 μl of 20%-acetic acid.

This process was carried out with plasma samples having the PC contents and final contents of the included com¬ ponents given below:

0% PC = Protein C-deficient plasma. 50% PC = A mixture of equal parts of normal plasma and Protein C-deficient plasma. 100% PC = Normal plasma.

Component Content

Added human Protein S 0.11 μg/ml

Cephotest® 4.2 percent by volume ca²⁺ 4.2 mmol/1

A4.,0,5,- was measured for these plasma and the values ob- tained are plotted against the PC contents (%) in Figure 5. It will be seen that good resolution was ob¬ tained, i.e. a high value of Λ A. (0-100%) of 0.65.

The Figure also shows that the quantity of thrombin formed is clearly and inversely correlated to the pro¬ portion of Protein C in plasma. This is also supported by the fact that, as expected, no APC-activity what¬ soever was obtained when plasma samples were prein- cubated with polyclonal anti-PC-antibodies for 5 min¬ utes prior to the activating with Protac® C.

Example 8

The method has been tested clinically, where the method according to Example 1 (a) was carried out with plasma from an individual having normal Protein S content and with plasma from a thrombosis patient. For control purposes, the free Protein S content was also assayed by a conventional RIA-method ("Radio Immuno Assay") .

The result achieved when practicing the invention was 115% PS and 53% PS respectively, which is in good agreement with the RIA values of 100% PS and 41% PS respectively.

Claims

1. A method for determining the functional activity of free Protein S or Protein C in a plasma sample, c h a r a c t e r i z e d by adding to the plasma sample coagulation enzyme FIXa,, which enzyme promotes the formation of coagulation enzyme FXa. optionally together with further coagulation reagent(s) , incubat¬ ing the sample, measuring in a known manner the quan- tity of thrombin formed from prothrombin, this process being promoted by FXa; and determining the proportion of functionally active Protein S or Protein C, which is inversely correlated to the quantity of thrombin formed, on the basis of the measured quantity of throm- bin, in a known manner with the aid of a standard.

2. A method according to Claim 1, c h a r a c ¬ t e r i z e d by adding to the sample as said further coagulation reagent(s) when determining Protein S, active Protein C (APC) or Protac® C prior to adding

FIXa, and when determining Protein C adding Protac® C and human Protein S to the plasma sample, and by pre¬ ferably incubating the plasma sample subsequent to adding Protac® C but prior to adding FIXa.

3. A method according to Claim l or 2, c h a r ¬ a c t e r i z e d by carrying out the method in one or in two stages, and by measuring the quantity of throm¬ bin formed by means of a method based on substrate cleaving or a method based on coagulation.

4. A method according to Claim 1, 2 or 3, c h a r ¬ a c t e r i z e d by using a chromogenic thrombin sub¬ strate when applying the method based on substrate cleaving, and measuring the quantity of thrombin obtained photometrically.

5. A method according to Claim 4, c h a r a c ¬ t e r i z e d by using a chromogenic thrombin sub- strate which will enable the method to be carried out in one stage, said substrate preferably comprising H-D- Ala-Pro-Arg-pNa, H-D-CHG-Ala-Arg-pNA, tosyl-Gly-Pro- Arg-pNA, HD-CHT-Ala-Arg-pNA or H-D-CHG-But-Arg-pNA.

6. A method according to Claim 3, c h a r ¬ a c t e r i z e d by using in the substrate cleaving method a plasma sample having a final plasma content of 0.02-10 percent by volume, and preferably 0.1-2 percent by volume.

7. A method according to any one of the preceding C Cllaaiimmss,, cc hh ea r a c t e r i z e d in that FIXa is of bovine origin.

8. A method according to any one of the preceding

Claims, c h a r a c t e r i z e d by adding a phos¬ pholipid mixture and optionally one or more of the following coagulation factors: FV, FVa, FVIII, FVIIIa,,

FX and prothrombin.

9. A method according to any one of the preceding

Claims, c h a r a c t e r i z e d in that FIXa and other coagulation proteins are of mammal origin, such as bovine, human or porcine origin, or may be formed by recombinant technique from native or genetically modi¬ fied variants of these factors.

10. A method according to any one of the preceding Claims, c h a r a c t e r i z e d by preincubating the plasma sample with polyclonal or monoclonal antibodies against FVIII, with the intention of inac¬ tivating FVIII.