COMPOSITION AND ANALYTICAL ELEMENT HAVING STABILIZED PEROXIDASE

Abstract: An analytical element has a peroxidase-labeled ligand analog distributed within a water-soluble binder composition comprising at least 50 percent, by weight, of poly(vinyl alcohol). As a result, the stability of peroxidase is improved prior to use. Such elements can be used to determine any of a number of immunologically reactive analytes, such as digoxin.

Description: The present invention relates to clinical chemistry. In particular, it relates to binder compositions, to analytical elements containing stabilized peroxidase-labeled ligand analogs and to their use in analytical methods to assay liquids, for example, biological fluids. It is well known to perform a quantitative or qualitative analysis of an aqueous liquid by contacting that liquid with an analytical element containing a combination of reagents capable of yielding a detectable product in proportion to the concentration of the predetermined analyte in the liquid. As used herein, this combination of reagents is termed an interactive composition which is capable of chemical reactivity, catalytic activity, or any other form of chemical or physical interaction that can result in the ultimate production of a change in the element that is detectable with suitable procedures and equipment. One type of particularly useful analytical elements utilizes enzymatic reactions wherein the analyte, upon contact with reagents in the element, reacts with oxygen in the presence of a suitable enzyme to produce peroxide in proportion to the concentration of the analyte. A detectable product is then produced by the reaction of the peroxide in proportion to the concentration of the analyte in the tested liquid. Such useful elements are known in the art. Unfortunately, because of the intrinsic instabilities of certain reagents and the need to dry these down in contact with other materials, the reagents contained therein may deteriorate during storage and thus deleteriously affect the accuracy and reliability of the assay. For example, exposure to air and moisture may adversely affect peroxidase which is often included in an element to catalyze the oxidation of interactive compositions by a peroxide. Peroxidases are used for diverse purposes, including diagnostic determinations of analytes such as glucose, uric acid, cholesterol, and so forth. In such determinations, excess peroxidase can be added to an element to overcome the effect of enzyme deterioration during storage. However, enzyme immunoassays using a peroxidase-labeled ligand analog have become important for determining an immunologically reactive ligand such as a drug, antigen or other immunologically reactive compound. In such assays, excess peroxidase cannot be added and the adverse effect of its instability is more prominent because of the relatively low concentration of ligand to be determined. U.S. Patent 4,283,491 describes the stabilization of peroxidase in elements with a vinyl copolymer prepared from specific ethylenically unsaturated polymerizable monomers. These elements further contain reagents needed for the determination of particular analytes such as glucose and uric acid. However, immunoassays are not described. The copolymer shown therein is included in the element carrier materials in an amount of from 20 to 50 weight percent. The remainder of the carrier materials can be one or more of a variety of binder materials, for example, gelatin, hydrophilic celluloses, poly(vinyl alcohol), polysaccharides, and others. It has been found, however, that peroxidase is not sufficiently stabilized by the materials described in U.S. Patent 4,283,491 when it is used as a label in a ligand analog. Moreover, the problem of peroxidase instability is more acute in the determination of a low level ligand in an immunoassay than in an assay of analytes such as glucose or uric acid which are generally found in test liquids in higher concentrations. Therefore, there is a need for a means to stabilize peroxidase-labeled ligand analogs in dry immunoassays. The problems noted above are overcome with a water-soluble composition comprising a peroxidase-labeled ligand analog, the composition characterized as further comprising a water-soluble binder composition comprising as sole stabilizer for the ligand analog, at least 50 percent, by weight, of poly(vinyl alcohol). This invention also provides an analytical element for the determination of an immunologically reactive ligand comprising an absorbent carrier material, the element characterized as containing the water-soluble composition described above. Further, a method for the determination of an immunologically reactive ligand comprises the steps of: A. in the presence of a receptor for the ligand, contacting a sample of a liquid suspected of containing the ligand with the analytical element described above, the contacting carried out in such a manner as to form a complex of receptor and ligand analog, and B. determining the amount of the ligand as a result of the presence of complexed or uncomplexed ligand analog. The present invention provides a means for stabilizing peroxidase-labeled ligand analogs in analytical elements. The peroxidase is stabilized sufficiently such that it can be retained in elements in low concentrations. Therefore, analytes, for example, immunologically reactive ligands present in a test fluid in low concentrations can be rapidly and accurately determined. These advantages are achieved by putting the ligand analog in a water-soluble binder composition which includes at least 50 percent, by weight, of poly(vinyl alcohol). This composition is the sole stabilizer used in this invention. The present invention relates to the determination (qualitative or quantitative measurement) of an immunologically reactive ligand in aqueous liquids. In particular, the invention can be used to assay biological fluids of either animals or humans. Such fluids include, but are not limited to, whole blood, plasma, sera, lymph, bile, urine, spinal fluid, sputum, perspiration and the like as well as stool secretions. It is also possible to assay fluid preparations of human or animal tissue such as skeletal muscle, heart, kidney, lungs, brains, bone marrow or skin. In the assay of this invention, the ligand to be determined and the corresponding labeled ligand analog compete for a fixed amount of common reactant. This reactant which specifically recognizes the ligand and ligand analog and reacts to form complexes with them is referred to herein as the receptor. The method of this invention is practiced with a dry analytical element. The simplest element can be composed of an absorbent carrier material, for example, a thin sheet of a self-supporting absorbent or bibulous material, such as filter paper or strips, which contains the binder composition described below and any other desired reagents. Alternatively, the reagents needed for an assay can be added to the element at the time of the assay. The element can be divided into two or more discrete zones with different reagents incorporated into individual zones of the carrier material. Such elements are known in the art as test strips, diagnostic elements, dip sticks or diagnostic agents. Useful absorbent carrier materials are water insoluble and maintain their structural integrity when exposed to water or biological fluids such as whole blood or serum. Useful elements can be prepared from paper, porous particulate structures, porous polymeric films, cellulose, glass fibers or woven and nonwoven fabrics (synthetic and nonsynthetic). Useful materials and procedures for making such elements are well known in the art as exemplified in U.S. Patents 3,092,465, 3,802,842, 3,915,647, 3,917,453, 3,936,357, 4,248, 829, 4,255,384, 4,270,920 and 4,312,834. Preferably, the absorbent carrier material of the dry analytical element of this invention is a porous spreading zone. This zone can be self-supporting (that is, composed of a material rigid enough to maintain its integrity), but preferably it is carried on a separate support. Such a support can be any suitable dimensionally stable, and preferably, nonporous and transparent material which transmits electromagnetic radiation of a wavelength between 200 and 900 nm. A support of choice for a particular element should be compatible with the intended mode of detection (fluorescence, transmission or reflectance spectroscopy). Useful supports can be prepared from paper, metal foils, films of polystyrene, polyesters, polycarbonates or cellulose esters. The porous spreading zone of the element can be prepared from any suitable fibrous or non-fibrous material or mixtures of either or both, as described in U. S. Patents 4,292,272, 3,992,158, 4,258,001 and 4,430,436 and Japanese Patent Publication 57(1982)-101760. It is desirable that the spreading zone be isotropically porous, meaning that the porosity is the same in each direction in the zone as caused by interconnected spaces or pores between particles, fibers or polymeric strands. The elements can have two or more discrete zones, either in the same layer or superimposed. At least one of which is preferably a porous spreading zone. The other zones can be reagent zones, registration zones, additional spreading zones, radiation-blocking or filter zones, subbing zones, barrier zones, and so forth, as those zones are known in the art. The zones are generally in fluid contact with each other, meaning that fluids, reagents and reaction products (for example, color dyes) can pass or be transported between regions of adjacent zones. In other words, when the element is contacted with fluid, reagents become mixed and can readily move within the element. Preferably, each zone is a separately coated layer, although two or more zones can be separate regions in a single layer of the element. The peroxidase-labeled ligand analog can be present in any zone or layer as long as it is uniformly distributed in the water-soluble binder composition described below. In a preferred embodiment, the water-soluble binder composition forms a distinct zone or layer in the element. More preferably, this zone or layer is adjacent to the porous spreading layer, although the two layers can be separated by a subbing or intermediate layer if desired. The practice of this invention allows one to determine the amount of unknown ligand in a liquid sample. The ligand can be any immunologically reactive compound including, for example, antigens, haptens, antibodies, toxins, hormones, therapeutic drugs, natural or synthetic steroids, proteins and other species which will complex specifically with a corresponding receptor. This invention is particularly useful for the determination of therapeutic drugs, for example, digoxin. The ligand analog useful in this invention is formed using any suitable technique known to one skilled in the art. Generally, they are prepared by covalently binding the peroxidase label to the ligand molecule which may be modified in any suitable way to achieve the binding. The peroxidase-labeled ligand analog is uniformly distributed in a water-soluble binder composition comprising poly(vinyl alcohol) in an amount of at least 50% and up to 100%, by weight, of the binder composition (that is, total binder weight). Preferably, at least 80%, by weight, of the composition is poly(vinyl alcohol). The remainder of the binder composition can be one or more synthetic or natural binder materials which are present in amounts that do not adversely affect the water-solubility of the composition. Examples of useful binder materials which can be used in combination with poly(vinyl alcohol) include gelatin, polyacrylic acid, poly(acrylamide- co -N-vinyl-2-pyrrolidone) (50:50 weight ratio) and similar copolymers, poly(N-vinyl-2-pyrrolidone), polyacrylamide, water absorbent starch-containing polymers such as those described in U.S. Patent 3,935,099 and similar materials. The coverage of the water-soluble binder composition in the element is at least 0.1, and preferably from 0.5 to 5, g/m². The method of this invention is carried out in the presence of a receptor for the ligand to be determined. For example, if the ligand is an antigen, the receptor is the corresponding antibody. The receptors are generally commercially available, or they can be readily prepared using known techniques and starting materials. Generally, the appropriate receptors, for example, polyclonal antibodies, are produced by inoculating a suitable animal with the ligand to produce antibodies according to an appropriate protocol, and removing the generated antibodies from the animal. Monoclonal antibodies can be obtained using standard hybridoma technology. The receptor can be added to the element prior to or substantially simultaneously with the test sample. Alternatively and preferably, the receptor is immobilized within the element prior to the assay, for example during manufacture. For example, it can be immobilized within the absorbent carrier material. More particularly, it can be immobilized within the porous spreading zone on a carrier material, such as glass or polymeric beads or other particles, resins or fibers. One useful carrier material is a microorganism, such as Staphylococcus aureus . Alternatively, the porous spreading zone components, for example, beads, can serve as the carrier material for the receptor. The method of this invention is carried out in such a manner that either complexed or uncomplexed ligand analog is measured. In a preferred embodiment, the method is carried out so that the complex formed between the receptor and labeled ligand is determined. This complex can be determined in any of a number of ways. For example, the complex can be determined by a competitive radiometric immunoassay. Alternatively and preferably, the complex is determined using an interactive composition which provides a spectrophotometric signal in the presence of a substrate for peroxidase. This composition comprises one or more reagents which can react to produce hydrogen peroxide which in turn can react with a dye precursor in the presence of peroxidase to produce a detectable dye. The interactive composition can be added to the element at the time of the assay, with or separate from the test sample. Preferably, it is incorporated in the element during manufacture. When so incorporated, the individual components of the composition can be located in one or more zones of the element. Alternatively, some of the reagents of the interactive composition can be incorporated in the element while others are added at the time of the assay. During the assay, all of the reagents are mixed and interact in the desired manner. The amounts of each component of the interactive composition to be used in the assay can be readily determined by one skilled in the art. Useful dye precursors which can be converted into detectable dyes in the presence of hydrogen peroxide and peroxidase include various leuco dyes such as imidazole derivatives described, for example, in U.S. Patent 4,089,747, E.P. Application 122,641 and Japanese Patent Publications 58(1983)-045,557 58(1983)-068009 and 59(1984)-193353, and triarylmethanes described, for example, in E.P. Publication 162,685. In a preferred embodiment, the interactive composition comprises α-glycerol phosphate oxidase and a triarylimidazole leuco dye. This composition can be used for the determination of digoxin. The element can have a number of other useful but optional components in one or more zones, including surfactants, buffers, hardeners, antioxidants, solvents, and others known in the art. The amounts of these materials are also within the skill of a worker in the art. In a preferred embodiment, the element contains a phenol or aniline electron transfer agent which increases the reaction rate of peroxidase. Useful phenol and aniline electron transfer agents include 4-hydroxyacetanilide. The amounts of peroxidase-labeled ligand analog and receptor used in the practice of this invention are readily determined by one skilled in the art. Generally, the ligand analog is present in the element in an amount of at least 10⁻⁶, and preferably from 10⁻⁵ to 10⁻², g/m². The receptor (whether incorporated in the element or added during the assay) is generally used in an amount which provides at least 10⁻⁷, and preferably from 10⁻⁶ to 10⁻², g/m². These amounts refers to the receptor alone and not to the receptor immobilized on a carrier. The receptor is generally immobilized on a carrier in the ratio of at least 1 part of receptor to 10⁶ parts of carrier, by weight. In a preferred embodiment, a multilayer analytical element comprises a nonporous support having thereon, in order, a registration layer, a water-soluble layer containing a peroxidase-labeled ligand analog for an immunologically reactive ligand uniformly distributed in the water-soluble binder composition described above, and a porous spreading layer, the element further comprising an interactive composition which is capable of interacting with the ligand analog to provide a spectrophotometric signal in the presence of a substrate for peroxidase. A variety of different elements, depending on the method of assay, can be prepared in accordance with the present invention. Elements can be configured in a variety of forms, including elongated tapes of any desired width, sheets, slides or chips. The method of this invention can be manual or automated. In general, ligand determination is made by taking the element from a supply roll, chip packet or other source and physically contacting it with a sample (for example, 1-200 µl) of the liquid to be tested so that the sample, ligand analog, receptor and reagents within the element interact. Such contact can be accomplished in any suitable manner, for example, dipping or immersing the element into the sample or, preferably, by spotting the element by hand or machine with a drop of the sample with a suitable dispensing means. After sample application, the element is exposed to any conditioning, such as incubation, heating or the like, that may be desirable to quicken or otherwise facilitate obtaining any test result. Once the receptor has complexed with ligand and ligand analog, any suitable separation technique can be used to vertically or horizontally separate bound (or complexed) ligand analog from unbound (or uncomplexed) ligand analog. In one embodiment, contact of the sample can be accomplished in such a manner that complexation of receptor and ligand and substantial horizontal separation of uncomplexed and complexed ligand occur during sample introduction. This contact can be carried out by hand or with a machine using a pipette or other suitable dispensing means to dispense the test sample. The sample of liquid can be applied to the element in a number of ways to effect horizontal separation. For example, a relatively large liquid sample (for example, up to 100 µl) can be applied slowly (for example, over at least 5 seconds) in a continuous manner using a suitable dispensing means. Alternatively, the sample can be applied in small portions, for example, as a series of two or more droplets (for example, 0.1 to 1 µl) over a period of time (for example, over at least 5 seconds). In another embodiment, horizontal or vertical separation can be accomplished by slowly adding a wash fluid after the liquid sample has been applied to the element. This wash causes uncomplexed materials to move away from the complexed materials. The amount of ligand in the test sample is then determined by passing the element through suitable apparatus for detecting the receptor-ligand analog complex directly or a detectable species formed as a result of the reaction of peroxidase and substrate (for example, change in reflection or transmission density or fluorescence). Alternatively, the uncomplexed ligand analog can be determined in a suitable manner. In the embodiments noted above involving horizontal separation, the complexed ligand analog is measured in a finite area in the center of the contacted area. The amount of the ligand in the test sample is inversely proportional to the amount of ligand analog measured in that finite area. Generally, ligand analog measurement is carried out from 5 to 500 seconds after the test sample has been applied to the element. As used in the context of this disclosure and the claims, I.U. represents the International Unit for enzyme activity defined as one I.U. being the amount of enzyme activity required to catalyze the conversion of 1 micromole of substrate per minute under standard pH and temperature conditions for the enzyme. Examples 1-3: Comparative Example These examples demonstrate the improved stability of peroxidase observed with the elements of the present invention as compared to an element outside the scope of this invention. Three elements of this invention were prepared having a format like that shown in Example 4 below except that the antibodies on S. aureus were omitted in Examples 1 and 2, and the water-soluble layer of Examples 2 and 3 contained a binder composition comprising poly(vinyl alcohol) (0.55 g/m²) and gelatin (0.55 g/m²) with 50 percent, by weight, of the binder composition being poly(vinyl alcohol). Control elements were similarly prepared except that the binder composition of the water-soluble layer was composed solely of unhardened gelatin (Control A) or poly(acrylamide- co -N-vinyl-2-pyrrolidone) (50:50 weight ratio) (Control B). The stability of the peroxidase in the ligand analog in each element was evaluated in the following manner. A 10 µl sample of fluid containing 100 mmole of α-glycerol phosphate was applied to elements which had been incubated, some at 0°C in the freezer and others at 22°C/50% relative humidity. Then the elements were incubated at 37°C for 5 minutes and the rate of dye formation was determined in the most linear region of a plot of reflection density vs. time (usually after 1-3 minutes of incubation). The percent peroxidase activity retained was calculated by dividing the rate of dye formation observed for the element kept at 22°C/50% relative humidity by that observed for the freezer element. The results of these tests are shown in Table I below. It is apparent from these data that the elements of the present invention have significantly improved peroxidase stability (that is, retained peroxidase activity). Example 1 shows the most improvement since high peroxidase activity is retained for up to 3 weeks. However, Examples 2 and 3 exhibited improved keeping over the Control elements (at least 60% retained activity after 1 week keeping in an open container). Both Control elements had lost a significant amount of peroxidase activity even after the one week keeping period. Example 4: Determination of Digoxin This example illustrates the practice of this invention for the determination of digoxin. An element of the present invention was prepared having the following format and components: Digoxin was determined using this element in the following manner. A series of test samples containing various amounts of the ligand, digoxin, were prepared in a buffered solution (pH 7). A 10 µl sample of each test sample was applied to the element prior to incubation for about 5 minutes at 37°C. At this time, a 10µl sample of a wash fluid containing 100 mmole of α-glycerol phosphate was applied to the element over the area of the spreading layer contacted with the test sample to wash uncomplexed ligand analog horizontally away from complexed ligand analog, and to initiate the enzymatic reactions which produce a detectable dye. Complexed ligand analog was then determined by monitoring reflection densities at 670 nm in the center of the spotted area using a standard reflectometer. The rate of change in dye density was calculated from measurements taken between 60 and 120 seconds after application of the second fluid. The Clapper-Williams transform ( J. Optical Soc. Am. , 43 , 595, 1953) was used to determine transmission density values from reflectance density values. The concentration of digoxin in the test fluid was observed to be inversely related to the rate of dye formation.

Claims: A water-soluble composition comprising a peroxidase-labeled ligand analog, the composition characterized as further comprising a water-soluble binder composition comprising as sole stabilizer for the ligand analog, at least 50 percent, by weight, of poly(vinyl alcohol). The composition as claimed in claim 1 wherein poly(vinyl alcohol) is present in the binder composition in an amount of at least 80 percent, by weight. The composition as claimed in either of claims 1 or 2 wherein the binder composition comprises at least one additional binder material which is gelatin, polyacrylic acid, an acrylamide-N-vinyl-2-pyrrolidone copolymer, polyacrylamide, poly(N-vinyl-2-pyrrolidone) or a water-absorbent starch-containing polymer. The composition as claimed in any of claims 1 to 3 wherein the ligand analog is peroxidase-labeled digoxin. An analytical element for the determination of an immunologically reactive ligand comprising an absorbent carrier material, the element characterized as containing the water-soluble composition as claimed in any of claims 1 to 4. The element as claimed in claim 5 further comprising an interactive composition which is capable of reacting with the ligand analog to provide a spectrophotometric signal in the presence of a substrate for peroxidase. The element as claimed in claim 6 wherein the interactive composition comprises α-glycerol phosphate oxidase and a triarylimidazole leuco dye. The element as claimed in any of claims 5 to 7 further comprising an immobilized receptor for the immunologically reactive ligand. A method for the determination of an immunologically reactive ligand comprising the steps of: A. in the presence of a receptor for the ligand, contacting a sample of a liquid suspected of containing the ligand with the analytical element as claimed in any of claims 5 to 8, the contacting carried out in such a manner as to form a complex of receptor and ligand analog, and B. determining the amount of the ligand as a result of the presence of complexed or uncomplexed ligand analog. The method as claimed in claim 9 for the determination of digoxin in a biological fluid wherein the ligand analog is peroxidase-labeled digoxin.

Inventors: EIKENBERRY, JON NATHAN
Applicants: EASTMAN KODAK COMPANY (A NEW JERSEY CORPORATION)
Application year: 1987
Main classification: C12Q1
IPC codes: C12Q1, G01N33