STABLE REAGENT FOR FERRIC ION COMPLEX INDICATOR SYSTEMS

Abstract: The invention is a stable reagent used in assay systems that form a colored ferric ion complex, such as Prussian Blue. Such reagents are useful for the detection or measurement of an analyte from a fluid sample. Surprisingly, it has been found that the inclusion of certain ferric ion chelating agents, such as 3-sulfobenzoic acid, will inhibit formation of the blank reaction in the reagent.

Description: STABLE REAGENT FOR FERRIC ION COMPLEX INDICATOR SYSTEMS Field of the Invention The invention relates to a stable reagent useful for forming a ferric ion chelation complex, as an indicator, in the detection or measurement of an analyte from a fluid sample. The invention also relates to a method of making a stable reagent, incorporating the reagent into an analytical element, and a method of determining the presence of an analyte in a liquid sample. Background of the Invention Many clinical chemistry assays are performed using reagents that form colored indicators. In such assays, the intensity of the color of the indicator is correlated to the concentration of analyte in the fluid sample being measured. Exemplary of such assays is the analysis of glucose from a blood sample by utilizing a reagent that forms Prussian Blue (or Turnbull's Blue) as a colored indicator. The reagent may include the enzyme glucose oxidase (GOD) , and potassium ferricyanide and ferric sulfate for the formation of Prussian Blue. The reaction that generates Prussian Blue may be depicted as follows (Scheme I) : glucose + ferricyanide > gluconic acid GOD + ferrocyanide ferrocyanide + ferric sulfate > Prussian Blue (color indicator) Accordingly, the more Prussian Blue that is formed by this reaction, then the more glucose is present in the blood sample being analyzed. Such assays can be calibrated to However, a particular problem with reagents that are used to form the Prussian Blue indicator, and other reagents that are used to form indicators that involve ferric ion complexes, is stability. Such reagents, when in liquid form, are unstable to light and heat, and, when in lyophilized form or included in a film, are additionally unstable to humidity. In the case of a reagent useful for forming the Prussian Blue indicator in a glucose assay, the instability is manifested by premature formation of Prussian Blue ("the blank reaction") in the reagent prior to addition of a blood sample containing glucose. Summary of the Invention The invention is a stable reagent used in assay systems that form a colored ferric ion complex, such as Prussian Blue. Such reagents are useful for the detection or measurement of an analyte from a fluid sample. However, in liquid form these reagents are unstable to heat and light. In lyophilized form or in a film, these reagents are further unstable to humidity. This instability is often manifested in the form of a blank reaction. For example, in a reagent used for forming the colored complex Prussian Blue, Prussian Blue is prematurely formed in the reagent. Surprisingly, it has been found that the inclusion of certain ferric ion chelating agents, such as 3-sulfobenzoic acid, will inhibit formation of the blank reaction in the reagent. Description of the Invention The present invention is a stable reagent, capable of forming a ferric ion colored complex in the presence of an analyte from a fluid sample, and useful for the detection or measurement of the analyte from the fluid sample. At a minimum, the reagent includes a first compound that will react in a reaction involving the analyte to form a second compound that complexes with ferric ion to form a colored complex; a source of ferric ions; and a ferric ion chelator of sufficient type and in sufficient amount to substantially inhibit formation of the colored complex prior to addition of the analyte to the reagent and to substantially not inhibit formation of the colored complex after addition of the analyte to the reagent. Ferric ion chelators, such as citric acid, aspartic acid, ethylenediamine tetraacetic acid (EDTA) , and succinic acid, which have too high an affinity for ferric ion are not of the type of ferric ion chelators included in this invention. When the colored complex is Prussian Blue, the first compound may be a ferricyanide, such as potassium ferricyanide, the second compound may be a ferrocyanide, the source of ferric ions may be ferric sulfate, and the ferric ion chelator may be 3-sulfobenzoic acid, 3-hydroxybutyric acid, 4-hydroxybutyric acid, 5-aminovaleric acid, butyric acid, or propionic acid, or salts thereof. (see Scheme I in Background of the Invention section.) The ferric ion chelator may also be 2-sulfobenzoic acid, or tartaric acid, or a salt thereof. Importantly, inclusion of the ferric ion chelator, of the type specified herein, stabilizes a liquid reagent against heat and light and stabilizes a dry reagent (lyophilized or in a film) against heat, light and humidity. A molar ratio of about 3:1 ferric ion chelator: ferric ion in the reagent is sufficient to stabilize the reagent. The reagent will be increasingly stabilized with molar ratios of ferric ion chelator to ferric ion above about 3:1. The increased stabilization will be beneficial until the ratio of ferric ion chelator to ferric ion becomes high enough to deleteriously affect the reaction kinetics of the assay, that is, the colored complex (e.g., Prussian Blue) forms too slowly in the assay, and, as a result, assay time becomes too long and precision and accuracy of measurement are adversely affected. A catalyst is preferably included in the reagent. The catalyst should be of sufficient type and in sufficient amount to catalyze the reaction involving the analyte and the first compound. When the analyte is glucose, the catalyst may be the enzyme glucose oxidase. A buffer is also preferably included, and when 2- sulfobenzoic acid is used a buffer should be included, in the reagent. The buffer should be of sufficient type and in sufficient amount to provide a desired pH for the reagent and the reaction involving the analyte and the first compound. Further, the buffer should not deleteriously bind to ferric ion. For example, phosphate buffer precipitates ferric ion and should not be used in the reagent. In diagnostic assays, biological buffers, such as "Good" buffers (available from, e.g., Sigma Chemical Company) are often used buffers. Pyruvic acid, 2-amino butyric acid, gluconic acid, and 2-hydroxyisobutyric acid are useful buffers. The particular buffer used will depend upon the particular assay system being employed. For a reagent that is capable of forming the Prussian Blue indicator in a diagnostic assay, 4-amino butyric acid is a useful buffer. The buffer may even be a weak chelator of ferric ion as long as the buffer does not substantially inhibit formation of the colored complex (the indicator) after the addition of the analyte to the reagent. If the buffer does weakly chelate ferric ion, then a lesser amount of ferric ion chelator (that is, less than a 3:1 molar ratio of ferric ion chelator to ferric ion) may be needed to stabilize the reagent. The reagent may be formulated in liquid (aqueous) form, in lyophilized form, or incorporated into a film or a diagnostic kit. When incorporated into a film, a film former, for example, an aqueous vinyl propionate/vinyl acetate copolymer dispersion sold under the mark PROPIOFAN ® 70 D (obtained from BASF) or a polyvinylacetate ethylene copolymer, such as film formers sold under the mark ELVACE (available from Reichhold Chemicals) or AIRFLEX 300 (available from Air Products) , is needed. Other additives may be helpful in producing a film that incorporates the reagent of the present invention. For example, a viscosity controlling agent, such as hydroxy-methyl cellulose, a surfactant, such as polyoxyethylene-sorbitan monolaurate, an anti-foaming agent, such as acetone, and a film opener, such as diatomaceous earth or the film openers disclosed in Voαel et al.. U.S. Patent No. 4,312,834, issued January 26, 1982, the disclosure of which is hereby incorporated by reference, may be helpful additives in formulating a film. Addition of a pigment, such as titanium dioxide, may be helpful in reflectance films; and addition of an oxidizing agent, such as potassium dichromate, may further increase the stability of a film that includes the reagent of the present invention by providing an oxidizing environment. The present invention is generally applicable to any reagent that includes a source of ferric ions and that forms, as an indicator, a colored ferric ion chelation complex in the detection or measurement of an analyte from a fluid sample. Including a ferric ion chelator that substantially inhibits formation of the colored ferric ion chelation complex prior to addition of the analyte to the reagent and that does not substantially inhibit formation of the colored ferric ion chelation complex after addition of the analyte to the reagent will protect the reagent from the destabilizing effects of heat and light, if the reagent is in liquid form, and additionally from humidity, if the reagent is in dry form (lyophilized or incorporated in a film) . The present invention is specifically applicable to the compositions, methods, and analytical elements described in Freitag, U.S. Patent No. 4,929,545, issued May 29, 1990, the disclosure of which is hereby incorporated by reference. In any of the examples found in the U.S. Patent No. 4,929,545, a reagent that is stable to heat and light, if the reagent is in liquid form, and additionally stable to humidity, if the reagent is incorporated into a film on a test strip, may be prepared by adding to the reagent an amount of ferric ion chelator sufficient to make the molar ratio of ferric ion chelator to ferric ion about 3:1. Increasing this molar ratio will further stabilize the reagent until the ratio becomes high enough to deleteriously affect the reaction kinetics of an analyte assay that utilizes the reagent. The molar ratio of ferric ion chelator to ferric ion may become so high that assay time is too long and assay accuracy and precision become poor. Another example of a reagent, or coating mass as it is referred to in Example 3 of U.S. Patent No. 4,929,545, is as follows: Step No. 1 Thoroughly mix 1376 grams(g) water, 58g sodium hydroxide (pellets) , 52g 4-amino butyric acid, 331g 3- sulfobenzoic acid, 52 g ferric sulfate, and 234g potassium ferricyanide, thereby forming a penultimate aqueous ionic mixture. The penultimate aqueous ionic mixture is filtered to remove large particles and impurities. 0.17g potassium dichromate, 288g diatomaceous earth (available from Eagle- Picher Minerals, Inc. under the mark CELABRITE) , and a sonicated suspension of 414g titanium dioxide in 907g water are mixed into the resulting filtrate, thereby forming the ultimate aqueous ionic mixture. Step No. 2 Thoroughly mix 1067g water, 1739g PROPIOFAN ® 70 D film former that has been demonomerized, 485g of 4% aqueous hydroxymethyl cellulose (sold under the mark NATROSOL) , 75g acetone, and 4.7g polyoxyethylenesorbitan monolaurate surfactant (sold under the mark TWEEN 20) , thereby forming a polymer mixture. This mixture is filtered to remove large particles and impurities. Step No. 3 Thoroughly mix 368g water and 64g glucose oxidase (G02A grade having 185 kilounits glucose oxidase/g, available from Biozyme Laboratories International, Ltd.), thereby forming an enzyme mixture. This enzyme mixture is filtered. Step No. 4 With mixing, the ultimate aqueous ionic mixture is slowly added to the filtrate of the polymer mixture. Once addition is complete, the filtrate of the enzyme mixture is added to form the penultimate coating mass, which is filtered to form the ultimate coating mass. The ultimate coating mass may be coated onto a clear, polyester foil, as described in U.S. Patent No. 4,929,545, Example 3 (col. 6), and dried to form a film coated test strip. This test strip may be used in detecting or measuring glucose in a fluid sample as described in Example 3 (col. 6) of U.S. Patent No. 4,929,545. The present invention has been disclosed in the above teachings and drawings with sufficient clarity and conciseness to enable one skilled in the art to make and use the invention, to know the best mode for carrying out the invention, and to distinguish it from other inventions and from what is old. Many variations and obvious adaptations of the invention will readily come to mind, and these are intended to be contained within the scope of the invention as claimed below.

Claims: We Claim : 1. A reagent useful for the detection or measurement of an analyte from a fluid sample, comprising: (a) a first compound that will react in a reaction involving the analyte to form a second compound that complexes with ferric ion to form a colored complex; (b) a source of ferric ions; and (c) a ferric ion chelator of sufficient type and in sufficient amount to substantially inhibit formation of the colored complex prior to addition of the analyte to the reagent and to substantially not inhibit formation of the colored complex after addition of the analyte to the reagent. 2. The reagent of claim 1, further comprising: (d) a catalyst of sufficient type and in sufficient amount to catalyze the reaction involving the analyte and first compound. 3. The reagent of claim 1, wherein the molar ratio of ferric ion chelator to ferric ion is at least about 3:1. 4. The reagent of claim 2 , further comprising: (e) a buffer of sufficient type and in sufficient amount to provide a desired pH for the reagent and the reaction involving the analyte and first compound and which will not deleteriously bind ferric ion. 5. The reagent of claim 4, wherein the catalyst is an enzyme. 6. The reagent of claim 5, wherein the molar ratio of ferric ion chelator to ferric ion is at least about 3:1. 7. The reagent of claim 5, wherein the reagent is an aqueous reagent. 8. The reagent of claim 5, further comprising: (f) at least one film-forming agent in sufficient amount to form a film. 9. The reagent of claim 7, wherein the first compound is ferricyanide, the second compound is ferrocyanide, the colored complex is Prussian Blue, and the ferric ion chelator is 3-sulfobenzoic acid, 2-sulfobenzoic acid, tartaric acid, 3-hydroxybutyric acid, 4-hydroxybutyric acid, 5-aminovaleric acid, butyric acid, or propionic acid, or a salt thereof. 10. The reagent of claim 7 , wherein the first compound is ferricyanide, the second compound is ferrocyanide, the colored complex is Prussian Blue, and the ferric ion chelator is 3-sulfobenzoic acid or a salt thereof. 11. The reagent of claim 8, wherein the first compound is ferricyanide, the second compound is ferrocyanide, the colored complex is Prussian Blue, and the ferric ion chelator is 3-sulfobenzoic acid, 2-sulfobenzoic acid, tartaric acid, 3-hydroxybutyric acid, or 4-hydroxybutyric acid, or a salt thereof. 12. The reagent of claim 8, wherein the first compound is ferricyanide, the second compound is ferrocyanide, the colored complex is Prussian Blue, and the ferric ion chelator is 3-sulfobenzoic acid or a salt thereof. 13. The reagent of claim 9, wherein the molar ratio of ferric ion chelator to ferric ion is at least about 3:1. 14. The reagent of claim 11, wherein the molar ratio of ferric ion chelator to ferric ion is at least about 3:1. 15. A method of stabilizing a reagent that includes a source of ferric ions, which complex with a compound to form a colored complex used for the detection or measurement of an analyte from a fluid sample, comprising: adding to the reagent a ferric ion chelator of sufficient type and in sufficient amount to substantially inhibit formation of the colored complex prior to addition of the analyte to the reagent and to substantially not inhibit formation of the colored complex after addition of the analyte to the reagent. 16. A method of stabilizing a reagent that includes components for the formation of a Prussian Blue indicator in the detection or measurement of an analyte from a fluid sample, comprising: adding 3-sulfobenzoic acid, 2-sulfobenzoic acid, tartaric acid, 3-hydroxybutyric acid, 4-hydroxybutyric acid, 5-aminovaleric acid, butyric acid, or propionic acid, or a salt thereof in sufficient amount to substantially inhibit formation of Prussian Blue prior to addition of the analyte to the reagent and to substantially not inhibit formation of Prussian Blue after addition of the analyte to the reagent. 17. A composition useful in determining an analyte in a sample, comprising: (a) an enzyme which specifically reacts with said analyte; (b) a soluble ferricyanide compound which is reducible in the presence of an electron to produce a ferrocyanide compound; (c) a soluble ferric compound which reacts with a ferrocyanide compound to form a reaction product therebetween; (d) a buffer which does not prevent formation of said reaction product; and (e) a ferric ion chelator that substantially inhibits formation of said reaction product prior to addition of said sample to said composition and that does not substantially inhibit formation of said reaction product after addition of said sample to said composition, wherein said composition has a pH from about 3.0 to about 6.0. 18. The composition of claim 17 in lyophilized form. 19. A method for determining the presence of an analyte in a liquid sample, comprising: contacting the liquid sample to the composition of claim 19, wherein said composition contains an enzyme which reacts with said analyte and determining formation of said reaction product, wherein formation of said reaction product is indicative of the presence of said analyte in said sample. 20. An analytical element useful in determining an analyte in a sample, comprising: (a) a support carrier; and (b) a reagent layer applied to said support carrier, said reagent layer comprising (i) an inert film, (ii) an enzyme which specifically reacts with said analyte, (iii) a soluble ferricyanide compound which is reduced in the presence of an electron to a ferrocyanide compound, (iv) a soluble ferric compound which reacts with said ferrocyanide compound to form a reaction product , (v) a buffer which does not prevent formation of said reaction product, and (vi) a ferric ion chelator that substantially inhibits formation of said reaction product prior to addition of said sample to said reagent layer and that does not substantially inhibit formation of said reaction product after addition of said sample to said reagent layer, wherein said reagent layer has a pH from about 3.0 to about 6.0.

Inventors: CARR WILLIAM R, DEAN KENNETH J, HAN CHI-NENG ARTHUR, JERNIGAN WALTER W, CARR, WILLIAM, R., DEAN, KENNETH, J., HAN, CHI-NENG, ARTHUR, JERNIGAN, WALTER, W.
Applicants: BOEHRINGER MANNHEIM CORP, BOEHRINGER MANNHEIM CORPORATION
Application year: 1993
Main classification: C12Q1
IPC codes: C12Q1, G01N31, G01N33