Blood coagulation factor XI concentrate having high specific activity, suitable for therapeutic use, and process for preparing same
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a process for preparing a concentrate of human plasma Factor XI having very high specific activity and intended for therapeutic use. 
     2. Description of Related Art 
     Factor XI or the precursor of plasma thromboplastin, is a glycoprotein that forms part of the contact phase, in the hemostasis mechanism, through its Factor IX activating effect and, on the other hand, of the fibrinolytic system through its plasminogen activating effect. 
     Factor XI deficiency is hereditary and is transmitted as a recessive autosomal character. This is a rare deficiency but one that is widespread in certain populations of the Middle East. 
     As with other factors in which a deficiency is rare (Factors V, XIII, X), therapeutic products purified from human plasma are nonexistent or rare, and the only replacement treatment is carried out using total plasma or the supernatant fraction of the cryoprecipitated plasma, but this entails the simultaneous injection of useless quantities of other plasma proteins, hence a risk of various major secondary reactions after multiple injections. 
     The purification of Factor XI, on an experimental scale, was difficult to achieve, and then, only with the use of powerful inhibitors, which suggests that this molecule is highly labile. Purification was carried out in a series of 4 or 5 steps of ion exchange chromatography and affinity chromatography, either starting from plasma (Bouma and Griffin, 1977, J. Biol. Chem. 252, 6432-6437) or from platelets (Schiffman and Yeh, 1990, Thromb. Res. 60, 87-97). A highly purified bovine Factor XI was also purified from 20 liters of plasma by precipitation and chromatographic steps in about 9 days (Koide et al., 1975, in: Methods in Enzymology--Blood Clotting Enzymes, pp. 65-73). Only one preparation having a quality compatible with therapeutic use has been described (Winkelman et al., 1988, Internat. Congress ISBT-BBTS London), and was obtained by adsorption on heparin-sepharose after recovery of Factors VIII and IX, but its specific activity does not exceed 5 U/mg of protein, and contains high doses of residual AT III. Applicants are aware of no other process of preparing Factor XI which allows for production on an industrial level, and which provides larger quantities of Factor XI of suitable quality for therapeutic use on a large scale. 
     SUMMARY OF THE INVENTION 
     Thus, the Applicants have sought to develop a new purification process suitable for very large volumes of plasma and making it possible to obtain, in a small number of steps that are easy to carry out on an industrial level, a Factor XI concentrate of suitable quality for therapeutic use. 
     The present invention thus relates to a human Factor XI concentrate, the preparation of which comprises only two steps: the first is a filtration-adsorption step which retains Factor XI quite selectively; after it has been desorbed, it is subjected to the second step, which involves chromatography on a cation exchange resin. 
     The Factor XI preparation may be further subjected to a conventional solvent-detergent viral inactivation treatment prior to the chromatography step, which serves to eliminate the residual products of this decontamination step completely. 
     The present method allows the production of therapeutic Factor XI from about 1100 liters of human plasma in a relatively short time (about 28 hours), that include the 8 hours - viral inactivation step. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The purification process according to the present invention is applied to cryoprecipitated plasma supernatant which is obtained by conventional methods well known in the art and can be adapted to volumes of 1000 to 1200 liters. 
     The first purification step consists of filtering the cryoprecipitated plasma supernatant on a battery of 3 cartridges of filters, the porosity of which is between 0.5 and 2μ and which are essentially negatively charged (Zeta plus™ CF filters supplied by Cuno, Process Filtration Products, a subsidiary of Commercial Intertech Corp., Meriden, CT, 06450, hereinafter designated &#34;Cuno filters&#34;) which are described in U.S. Pat. Nos. 4,783,262 and 4,859,340. These filters are composed of perlites and purified cellulose having a small quantity of positively charged resin to retain particulate material, for example, having about 75% negative charges and 25% positive charges, and are completely free of asbestos and glass microfibers. Other commercially available filter systems can also be used. 
     The filters with adsorbed Factor XI are rinsed with a citrate/phosphate buffer solution comprising sodium citrate, disodium phosphate, potassium phosphate, sodium chloride and disodium EDTA and adjusted to a pH of between 5.5 and 6.5 and, preferably, to a pH of 6. This filtration step eliminates a large proportion of the plasmatic proteins. 
     The Factor XI, which has remained adsorbed on the layers of filters, owing to their negative charge, is desorbed therefrom by an increase in the ionic strength of the buffer solution by adjusting the final sodium chloride concentration. To this last buffer solution is also added a small quantity of antithrombin III (AT III) 0.1-0.2 U/ml, to protect the Factor XI from the action of the proteases. 
     The desorbed, dialyzed and concentrated fraction is then injected onto a chromatography column using a cation exchange resin, more particularly sepharose sulphate gel, equilibrated with a buffer solution composed of sodium citrate, sodium chloride, lysine and arginine, at a pH of 5.5 to 6.5, and preferably a pH of 6. 
     This column allows the remaining contaminant proteins and the viral inactivation agents to pass through. 
     It has been found by the present invention that sepharose sulphate gel has, unexpectedly, a very high Factor XI retention capacity (from 300 to 450 U/ml of gel), which makes it possible to avoid a subsequent ultrafiltration step which would lead to a loss of yield. In addition, the adsorption to the sepharose sulphate permits elution by practically physiological buffer solution, whereas the other resins (described earlier), which are sulphopropyl group based, necessitate more drastic conditions of elution. 
     After the column has been rinsed with the citrate/phosphate buffer comprising sodium citrate, disodium phosphate, potassium phosphate, sodium chloride, lysine and arginine, adjusted to a pH of between 6.1 and 6.9, and preferably to a pH of 6.5, the Factor XI is desorbed by increasing the pH of the buffer to between 7 and 8, and preferably to 7.5, and increasing the sodium chloride concentration to between 0.15 and 0.20 M, and preferably to 0.17. 
     As soon as it has been eluted, the Factor XI is stabilized by the addition of 0.5 to 3 U/ml high purity AT III and 0.5-4 U/ml heparin. Then the solution is sterilized by filtration, dispensed in vials and freeze dried by conventional methods. 
     The Factor XI obtained by the process according to the present invention has a specific activity at least equal to 100 U/mg of proteins. 
     The high purity of Factor XI obtained is demonstrated by electrophoresis on SDS-polyacrylamide gel, by biochemical analyses, and its harmlessness by biological tests on animals. 
     The Factor XI concentrate obtained using the process according to the present invention is thus particularly suitable for therapeutic use, in particular as a replacement therapy in cases of congenital or acquired deficiency in Factor XI. 
     The following example illustrates a form of embodiment of the present invention without, however, limiting the scope thereof. 
     EXAMPLE 
     Starting Material 
     Each batch of Factor XI is prepared from a volume of approximately 1000 liters of conventionally prepared human plasma cryoprecipitate supernatant. Cryoprecipitate supernatant is prepared by freezing fresh plasma at -60° C., gently thawing the frozen plasma at about 4° C., and centrifuging to separate the cryoprecipitate and the supernatant. 
     First Purification Step 
     The cryoprecipitate supernatant is passed over Zeta Plus Charged Filteraid cartridges arranged in batteries of three. Either the 0.5-1μ (type 50 S) or 1-2μ (type 30 S) filters can be used. 
     After removal of the filtrate containing a majority of the proteins of the cryoprecipitate supernatant, the cartridges are washed with a citrate/phosphate buffer solution comprising 5 mM of sodium citrate, 5 mM of disodium phosphate, 5 mM of potassium phosphate, 0.065 M of sodium chloride and 0.5 mM of disodium EDTA, and adjusted to a pH of 6 with citric acid. 
     A relatively selective adsorption of Factor XI on the layers of filters is observed. 
     The Factor XI is desorbed from the filters by increasing the ionic strength of the washing buffer by adjusting the NaCl concentration to 1 M. 0.2 U/ml of AT III are also added thereto to protect the Factor XI against the action of residual plasma proteases; the EDTA of the buffer solution also contributes to this protective action. 
     The Factor XI solution thus recovered is concentrated 30-fold and dialyzed to remove the EDTA, with the help of a Millipore™ ultrafiltration system formed of 10 cassettes with 10 K membranes. 
     The dialysis buffer solution is composed of 5 mM of sodium citrate, 0.14 M of sodium chloride, 5.5 mM of L-lysine and 20 mM of L-arginine, and adjusted to a pH of 6. 
     The dialyzed solution is passed over a DSLK2NLP (PALL™) 0.45μ filter to clarify the solution and remove any bacterial contaminants. 
     Viral Inactivation Treatment 
     The solution containing the Factor XI is subjected to a solvent-detergent treatment known for its efficiency in destroying lipid enveloped type viruses (Horowitz et al., 1985, Transfusion 25, 516-522) and which includes incubation for 8 hours at 25° C. in the presence of 0.3% of tri-n-butyl-phosphate (TnBP) and 1% of Tween 80. 
     Second Purification Step 
     A chromatography column is used with a cation exchange resin, more particularly &#34;sulfate-sepharose fast flow&#34;™ gel (supplied by Pharmacia, Uppsala, Sweden). 
     The column is equilibrated with the dialysis buffer solution described above. 
     The column is equilibrated with the dialysis buffer solution described above. 
     After loading the column with the protein solution, the column is rinsed with 10 to 15 volumes of rinsing buffer solution to remove the slightly adsorbed proteins and the viral inactivating agents. This rinsing buffer comprises 10 mM of sodium citrate, 5 mM of disodium phosphate, 5 mM of potassium phosphate, 0.12 M of sodium chloride, 27 mM of lysine and 11.5 mM of arginine, at a pH of 6.5. 
     The linear flow rate of the equilibration, washing and elution buffers is 50 cm/h. 
     The Factor XI is eluted from the column by increasing the pH of the buffer to 7.5 and increasing the NaCl concentration to 0.17 M. 
     Upon elution of Factor XI, 2 U/ml of AT III (i.e. approximately 1.5 to 2.5% of the amount of FXI) and 2 U/ml of heparin (i.e. 1.5 to 2.5% of the amount of FXI) are added thereto to stabilize it. (Both products are of high purity and of a quality suitable for injection in humans.) 
     The Factor XI solution thus prepared is sterilized by filtration on a DSLKINFZP (PALL™) 0.22μ filter, dispensed (10 ml/vial) and freeze dried. 
     Biochemical and Biologial Analyses of the Factor XI Concentrate 
     Six successive batches were analyzed. 
     The specific activity of Factor XI ranges from 130 to 150 U/mg of proteins (i.e. Factor XI+AT III). Before the addition of AT III, the specific activity is 210 U/mg. 
     The Factor XI, which had been adjusted to approximately 100-120 U/ml before freeze drying has a coagulating activity of 90 to 110 U/ml. 
     The following Tables 1 and 2 set forth the characteristics of the purified product. 
     
                       TABLE 1                                                     
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Factor XI recovery and purity of the purification fraction                
(mean values of 6 batches)                                                
                     SPECIFIC                                             
                     ACTIVITY     Purification                            
FRACTIONS YIELD (%)  (U/MG)       Factor (fold)                           
______________________________________                                    
Filteraid 37         12           ≅800                          
eluate                                                                    
Solvent-  98         12                                                   
detergent-                                                                
treated                                                                   
fraction                                                                  
S-Sepharose                                                               
          62         210          ≅14000                        
eluate                                                                    
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                       TABLE 2                                                     
______________________________________                                    
Characteristics of the freeze-dried Factor XI                             
(mean values of 6 batches)                                                
______________________________________                                    
Protein content (g/l)                                                     
                   0.7-1.1                                                
Factor XI (U/ml)    90-110                                                
Specific activity (U/mg)                                                  
                   130-150                                                
(after addition of AT III)                                                
Cl-inhibitor*      0.34                                                   
Fibronectin*       0.46                                                   
alpha 2-macroglobulin*                                                    
                   0.13                                                   
IgG*               0.17                                                   
Albumin            &lt;0.0006                                                
Fibrinogen         &lt;0.002                                                 
NAPPT (1/10 dilution)                                                     
                   &gt;600&#34;                                                  
NAPPT (1/100 dilution)                                                    
                   221&#34;                                                   
NAPPT (1/10)**     166&#34;                                                   
NAPPT 1/100)**     216&#34;                                                   
FCT (20° C.)                                                       
                   &gt;24 h                                                  
FCT (37° C.)                                                       
                    &gt;6 h                                                  
______________________________________                                    
 *Expressed in mg/1000 U F XI                                             
 **Dilutions tested after heparin neutralization                          
 
    
     As shown in Table 1, the adsorption step on the Filteraid layers allowed the separation of Factor XI from bulk of plasma protein resulting in 12 U Factor XI/mg protein. This specific activity represents about 800-fold purification factor from starting plasma. Chromatography on S-Sepharose of the partially purified Factor XI increased the specific activity to &gt;200 U/mg corresponding to a purification factor of &gt;14000 times from plasma. The washing step before elution of Factor XI, achieved by increasing the buffer pH, contributed to the improved purity by removing part of the contaminating proteins, mainly fibronectin. Under the conditions used, the gel capacity for Factor XI was about 300 U/ml and for proteins was approximately 36 mg/ml. 
     These results are to be compared with the therapeutic preparation of Winkelman et al. (Internat. Congr. ISBT-BBTS-London 1988, Abstract No. P.M.4.15) which has a specific activity of about 5 U/mg protein and contains equivalent doses of AT III. 
     The small quantity of protein contaminants (with the exception of the deliberately added AT III) is confirmed by immunonephelometry. 
     Electrophoresis on SDS-polyacrylamide gel shows a single major band at 160 KDa and a small band at 62 KDa which corresponds to the AT III. After reduction of the proteins with β-mercapto-ethanol, no band is detectable in the 50-30 KDa region, which shows that the Factor XI molecules have not been activated in the course of the purification process (Bouma and Griffin, 1986,Blood Coagulation, Ed. Hemker - pp 103-128). 
     The conventional methods are used to check carefully that there is no residual contamination by coagulation factors and constituents of the kinin system. 
     After reconstitution of the freeze dried product, conventional tolerance tests on animals are conducted: 
     thrombogenicity test on rabbits, 
     hypotension test on rats, 
     toxicity test on mice. 
     The test on rabbits shows that the product is not thrombogenic since the effective dose 50 (ED 50) is greater than 1000 U FXI/kg, while this same value ranges from 40 to 60 U/kg in the case of a PPSB concentrate, which is thus far more thrombogenic, and can effectively entail thromboses and disseminated intravascular coagulation when injected in high doses in man. 
     The concentration does not induce hypotension phenomena when intravenously injected into rats in doses of 50 U FXI/kg. This animal model is very sensitive to the presence of plasma components with vasoactive properties and demonstrates the absence of these components in the concentrate of Factor XI obtained using the process described. 
     When injected intravenously in mice in a high dose (2500 U FXI/kg), the Factor XI concentrate of the present invention does not induce any lethality or behavioral disturbance over a period of 7 days.