ANALYTICAL ELEMENT AND METHOD FOR THEOPHYLLINE DETERMINATION USING BUFFER IN SPREADING ZONE

Abstract: Theophylline can be determined with an analytical element and method which utilize the inhibition, by theophylline, of alkaline phosphatase activity on an appropriate substrate. The assay is carried out at a pH of 9 or less. The element comprises a porous spreading zone and at least one other zone in fluid contact with the spreading zone. The isoenzyme of alkaline phosphatase and a suitable substrate for the isoenzyme are located in different zones of the element. The element also contains a buffer for maintaining the pH at 9 or less, substantially all of which is located in the porous spreading zone.

Description: The present invention relates to clinical chemistry and to the assay of biological fluids for theophylline. More specifically, it relates to a dry analytical element and a method for the determination of theophylline in human biological fluids. Theophylline is a drug frequently administered for treatment of asthma and pulmonary diseases. For the drug to be used successfully without serious undesirable effects, it must be frequently and carefully monitored in a patient because it has a relatively narrow therapeutic range of use, that is, 1-2 mg/dl. Numerous techniques have been used to determine the amount of theophylline in human serum. Most of these techniques have serious drawbacks. For example, known spectrophotometric methods require large sample volumes, extensive pretreatment and suffer from interferences by similarly structured xanthines, such as caffeine and theobromine. Known gas chromatographic methods are more specific, but require derivitization and are time consuming. Nonisotropic immunoassy techniques are most frequently used because they provide rapid results and are simple to use. Although satisfactory sensitivity has been generally obtained with immunoassay techniques, it has been found recently that they may produce highly elevated results depending upon a patient's renal condition and the specificity of the antibody used in the assay. Moreover, immunoassays require the use of generally costly reagents which have limited stability. High performance liquid chromatography techniques are also known. These techniques vary in specificity depending upon whether pretreatment of the test sample is carried out. Organic extraction steps are necessary to improve the accuracy and specificity of the assay. Many chromatography methods are susceptible to interference from a number of substances including some common antibiotics. Other disadvantages include the need for expensive intrumentation and a specialized technical staff to perform the assays. It is known that theophylline can be determined by measuring its inhibitor effect on alkaline phosphatase activity. However, when assaying human biological fluids in this manner, it is known that endogenous alkaline phosphatase can affect the assay and render inaccurate results on the high side. Endogenous alkaline phosphatase must then be detroyed or removed in some manner prior to the assay to avoid this problem. A significant advange in the art is described in EP-A-0 188 372. The element described and claimed therein, however, has an isoenzyme for alkaline phosphatase and a buffer for maintaining the pH at 9 or less during the assay located in the registration layer. That layer is located adjacent to the support. There is no buffer described in the spreading layer of the element. While this element provides a simple and rapid assay for theophlline in which endogenous alkaline phosphatase exhibits a reduced effect, there is a need for further improvement in that assay. The element described above has a limited rate range. Further, the element described above has reduced keeping properties which means that it is susceptible to environmental conditions. The problems noted above are overcome with a dry analytical element for the determination of theophylline comprising, in fluid contact, a porous spreading zone and at least one addition zone. the element containing an isoenzyme of alkaline phosphatase which is capable of acting on a substrate for the isoenzyme at a pH or 9 or less, and a substrate for the isoenzyme, provided the phosphatase and substrate are in different zones of the element, the element characterized in that it further contains a buffer which is capable of maintaining the pH at 9 or less during the determination, substantially all of which is in the porous spreading zone. This invention also provides a method for the determination of theophylline comprising the steps of: A. at a pH of 9 or less, contacting a sample of a biological fluid suspected of containing theophylline with the dry analytical element described above, and B. determining a detectable change resulting from the contact. The present invention provides a simple and rapid assay for theophylline having all of the advantages of the element and assay described and claimed in EP-A-0 188 372. In addition, the element of the present invention has significantly improved keeping properties. Therefore, it is less responsive to the environment and keeping conditions are less restrictive. The assay also has a significantly increased rate range. It was further observed that swelling of certain layers in the element of this invention was significantly reduced. These improvements were achieved unexpectedly by locating substantially all of the buffer used to keep the assay pH at 9 or less in the spreading zone of the element. Relatively little buffer is located in other zones of the element. The present invention relates to the determination of theophylline in biological fluids, and especially human biological fluids. As used herein, determination refers to either qualitative or quantitative measurements of the amount of theophylline in a test sample. In particular, this invention can be used to determine theophylline in human biological fluids which contain endogenous alkaline phosphatase (that is, naturally occurring enzyme) in any of its enzymatic forms (for example, liver, intestinal, placental or bone). For example, this invention can be advantageously used to assay sera, whole blood, plasam, spinal fluid, sputum, bile, saliva, and other biological fluids. It is also possible to use the invention to assay fluid preparations of tissue such as human skeletal muscle, kidney, placenta, heart, intestine, lung or other tissue. The preferred biological fluids used in the practice of this invention are human sera and whole blood. Theophylline is determinable in the practice of this invention by inhibiting the activity of alkaline phosphatase, an enzyme which can act on a number of substrates to produce a detectable reaction products. For example, the following representative equation illustrates the production of a detectable dye by the action of alkaline phosphatase using a typical substrate, p -nitrophenyl phosphate: The dye can then be colorimetrically detected with suitable spectrophotometric detection equipment. The amount of theophylline present in the test sample contacted with the substrate and enzyme is inversely proportional to the amount of dye measured. The present invention is practiced at a pH of 9 or less, and preferably at a pH of from 7 to 9. As noted in EP-A-0 188 372, noted above, endogenous alkaline phosphatase in human fluids has less activity at a pH of 9 or less. However, isoenzymes of alkaline phosphatase which are not inactivated in an environment of pH 9 or less can be used in the assay to indicate the presence of theophylline. Any isoenzyme from any suitable source which has that desired property, that is, activity measurable at a pH of 9 or less, is useful in the practice of this invention. Particularly useful isoenzymes are those obtained from bovine sources, for example tissues and organs (such as liver) of cattle or calves. Isoenzymes from various other sources (for example mcroorganisms, avian and nonhuman mammalian sources) are also useful. It is well within the skill of a worker in a clinical chemstry to find isoenzymes which will be useful in the practice of this invention. One or more of a variety of alkaline phosphatase substrates can be used in the practice of this invention. The substrate must be such that upon enzymatic reaction with the isoenzyme, a directly detectable change occurs. For example, the substrate is converted into one or more detectable reaction products, such as a chromogen, fluorogen, radioisotopically labeled species, and other suitable detectable products. The detectable change measured during the assay can be the appearance or disappearance of such a detectable product, or the change of one detectable product into another. Alternatively, the detectable change can be brought about through a series of reactions which are initiated by the action of the isoenzyme on the substrate. For example, the alkaline phosphatase isoenzyme can act on the substrate to release another enzyme or reagent which then is used in one or more reactions to produce a detectable product. The detectable product may be directly measurable, or require some physical separation or handling for measurement. In a preferred embodiment of this invention, the assay provides a chromogen or fluorogen as a detectable product of the enzymatic reaction. Generally, the substrates which are useful in such reactions have a phosphate group which is cleaved from the substrate molecule during the enzymatic reaction. Such substrates include organic mono- or dieseters of phosphoric acid or salts thereof. Examples of particularly useful substrates include p -nitrophenyl phosphate, phenolphthalein monophosphate, phenophthalein diphosphate, thymolphthalein monophosphate, indoxyl phosphate, phenyl phosphate, α-naphthol phosphate, β-naphthol phosphate, α-glycerol phosphate, o -methylfluorescein phosphate, o -carboxyphenyl phosphate, alkali metal salts thereof and others known in the art, for example, in U.S. Patent 3,425,912 and European Patent Publication 61,731. Preferred substrates are p -nitrophenyl phosphate and 4-(4-nitro-2-methylsulfonyl phenylazo)naphthol-1-phosphate. The isoenzyme and substrate must be kept separated in different zones of the element until contacted with the liquid test sample. The amount of isoenzyme and substrate in the element can be varied widely. Generally, the element contains at least 10 I.U./m² of the isoenzyme. In a preferred embodiment, the isoenzme is present in a coverage of at least 100 I.U./m², and more preferably, from 125 to 200 I.U./m². In the context of this disclosure, I.U. represents the International Unit for isoenzyme activity defined as one I.U. being the amount of isoenzyme activity required to catalyze the conversion of 1 micromole of substrate per minute under standard pH and temperature conditions for the isoenzyme. The substrate for the enzyme is generally present in an amount of from 1 to 5, and preferably from 2 to 4, g/m². Other addenda are incorporated into the element in amounts which are within the skill of an ordinary worker in clinical chemistry. The assay is carried out at a pH of 9 or less, and preferably from 7 to 9. Any suitable buffer or mixture of buffers can be used in the practice of this invention as long as it is capable of maintaining the pH during the assay at 9 or less. Particularly useful buffers are nitrogen-containing organic buffers, many of which are standard in the art [for example, see Good et al., Biochem , 5 (2), 1966, pp 467-477]. Representative buffers include, but are not limited to, the group consisting of tris(hydroxymethyl)aminoethane·HC1, glycylglycine, N-tris(hydroxmethyl)-methyl-2-aminoethanesulfonic acid and N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid. The first buffer is most preferred. Substantially all of the buffer is located in the spreading zone (described below) of the element of this invention. This means that substantially little buffer is located in other zones of the element. More particularly, at least about 80% of the buffer is located in the spreading zone. The buffer is present in that zone in a suitable amount, depending upon the particular buffer, to maintain the pH of the reaction mixture at the desired pH of 9 or less. These buffering amounts can easily be determined by one of ordinary skill in clinical chemistry, but is generally at least 1 g/m², and preferably at least 2.5 g/m². Other optional reagents can also be added to the element, if desired. For example, metal ion activators can be added to activate the isoenzymes. Such activators include divalent cations such as Mg⁺⁺, Co⁺⁺, Mn⁺⁺, Ca⁺⁺,Zn⁺⁺, Sr⁺⁺ and Fe⁺⁺, available in free or salt form (for example, aspartate, acetate, chloride or sulfate). Alternatively, if the levels of endogenous alkaline phosphatase in the test sample are abnormally high, inhibitors of the enzyme activity may be used. Useful inhibitors include phenylalanine and tetramisole. Such inhibitors advantageously do not affect the activity of some nonhuman alkaline phosphatase isoenzymes. In addition, one or more phosphate acceptors are preferably included in the element to increase the rate of enzyme reaction when phosphate substrates are used. Useful phosphate acceptors include aminoalcohols or derivatives thereof, or aliphatic amines with the amino alcohols being particularly useful. Examples of such compounds are well known in the art. Glycerol or another humectant can be added to one or more zones of the element in order to reduce element curl. The element is divided into at least two zones one of which is the porous spreading zone (described below), and the isoenzyme and substrate are incorporated into individual zones. The two zones can be separate layers, or finite areas within a single layer. They can be composed of the same or different materials and joined by lamination or other standard techniques. The two zones of the element can be self-supporting, that is composed of materials strong enough to maintain its integrity. Preferably, the zones are carried on a support. Such a support can be any suitable dimensionally stable, and preferably, transparent (that is, radiation transmissive) 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 (reflection or transmission spectroscopy). Useful support materials include paper, metal foils, polystyrene, polyesters, polycarbonates, cellulose esters, and others known in the art. The porous spreading zone can be prepared from any suitable fibrous or non-fibrous material or mixtures of either or both. The void volume end average pore size of this zone can be varied depending upon the use intended. For example, if whole blood or other liquid samples containing high molecular weight materials are to be assayed, the void volume and average pore size are generally greater than if serum or urine is to be assayed. Useful spreading zones can be prepared using fibrous materials, either mixed with a suitable binder material or woven into a fabric, as described in U. S. Patent 4,292,272. Alternatively, and preferably, the spreading zone is prepared from polymeric compositions (for example, blush polymers) or particulate materials,as described in U. S. Patents 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 zones as caused by interconnected spaces or pores between particles, fibers or polymeric strands. Most preferably, the porous spreading zone is prepared as a blush polymer layer as described in U.S. Patent 3,992,158, noted above. The elements have two essential zones, at least one of which is a porous spreading zone. The other essential zone can be a reagent zone or a registration zone as those zones are known in the art. The element can have other zones including, but not limited to, additional spreading zones, radiation-blocking or radiation-filtering zones, subbing zones or barrier zones. Preferably, there is a subbing zone located between the two essential zones. The subbing zone helps to insure that the isoenzyme and substrate do not interact prior to the assay. All zones in the element 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 superposed regions of adjacent zones. Preferably, the zones are separately coated layers, although two or more zones can be a single layer. Besides the references noted above, suitable element components are described, for example, in U. S. Patents 4,042,335, 4,132,528 and 4,144,306. A preferred embodiment of this invention is an element comprising a support having thereon, in order and in fluid contact, a layer containing the isoenzyme described herein, a radiation-blocking layer, a subbing layer, and a porous spreading layer which contains a substrate for the isoenzyme and the buffer as described above. The isoenzyme layer can be a porous spreading layer also, but preferably, it is a reagent or registration layer containing one or more hydrophilic binders (for example, gelatin, vinyl pyrrolidone polymers or acrylamide polymers) surfactants, mordants, and other addenda. The subbing layer can comprise one or more subbing materials known to one skilled in the art, for example, vinyl pyrrolidone polymers, acrylamide polymers, and others known in the art. The radiation-blocking layer generally includes one or more binders, surfactants and reflective materials which are known in the art. Optionally, this preferred element can also include a second porous spreading layer which is the outermost layer of the element, and which is generally contiguous to the first porous spreading layer. The second porous spreading layer can be constructed of materials the same as or different than those in the first porous spreading layer containing the isoenzyme substrate. For example, the first spreading layer containing the buffer can comprise blush polymers prepared according to U.S. Patent 3,992,158, noted above, and the second spreading layer can be composed of particulate materials as described above. This second spreading layer can also contain buffer if desired. 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 assay of this invention can be manual or automated. In general, in using the dry elements, theophylline 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, less than 200 µl) of the liquid to be tested. Such contact can be accomplished in any suitable manner, for example, by 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 or heating, that may be desirable to quicken or otherwise facilitate obtaining any test result. The alkaline phosphatase present in the element then catalyses reaction of the substrate at a rate based on the amount of alkaline phosphatase present which is not inhibited by theophylline in the sample. The rate of detectable change (for example, dye formation) due to formation of the reaction product is quantifiable using suitable apparatus for reflection or transmission spectrophotometry. Suitable spectrophotometric apparatus and procedures are known in the art. Other suitable detection means include the use of fluorescence spectrophotometry, radiometry or enzyme labeling. The amount of theophylline is inversely proportional to the measured reaction rate. For example, when p -nithrophenyl phosphate is used as the substrate, the uninhibited enzymatic reaction produces p -nitrophenol which is measurable at 400 nm using a standard spectrophotometer. The rate of the color change can then be directly related to the rate of substrate reaction which, in turn, is indirectly related to the concentration of theophylline in the sample. Examples 1-2: Comparative Examples of Elements These examples are comparisons of the assay and elements of the present invention to an assay and elements similar to those described in EP-A-0 188 372. It can be seen from the data presented below that the present invention exhibits several improved properties. An element of this invention was prepared having the following format and components: A Control element (similar to that described in EP-A-0 188 372, except for the levels of buffer and alkaline phosphatase isoenzyme) was prepared like the element of this invention except that the buffer was omitted from the spreading layer. The rate range of the assay was evaluated by spotting serum containing various levels of theophylline (up to 40 Mg/ml) on the elements and measuring the reflection densities over a seven minute period using a rate analyzer. The results are shown in Table 1 below. A larger rate range is indicative of greater sensitivity in theophylline determination. The elements of this invention exhibit a statistically significant increase in rate range over the Control element. The same elements were evaluated for their keeping properties by measuring the % change from a reference value at two theophylline concentrations. The % change was measured using the following calculation: The elements were used to assay theophylline at 1 and 40 µg/ml concentrations by spotting a 10 µl sample of the calibrator fluid onto the porous spreading layer of the elements. During incubation at 37°C, the rate of enzyme activity was measured by monitoring the absorbance of the resulting dye at 410 nm using a rate analyzer. Table II below shows the keeping data obtained. The elements of this invention showed significantly improved keeping at both theophylline concentrations. This example illustrates the reduced swell observed with the element of this invention (Example 2 above) over a similar Control element having no buffer in the spreading layer Swell in the radiation-blocking layer of the elements was measured by comparing thickness measurements of the layer in both dry and saline wetted states. The difference in thickness values is the amount of swell and is identified as X in micrometers in Table III below. Table III below shows the resulting data. It is apparent that the element of this invention exhibited significantly reduced swell over the Control element.

Claims: A dry analytical element for the determination of theophylline comprising, in fluid contact, a porous spreading zone and at least one additional zone, the element containing an isoenzyme of alkaline phosphatase which is capable of acting on a substrate for the isoenzyme at a pH of 9 or less, and a substrate for the isoenzyme, provided the phosphatase and substrate are in different zones of the element, the element characterized in that it further contains a buffer which is capable of maintaining the pH at 9 or less during the determination, substantially all of which is in the porous spreading zone. The element as claimed in claim 1 wherein the buffer is a nitrogen-containing organic buffer. The element as claimed in either of claims 1 and 2 wherein the buffer is tris(hydroxymethyl)aminomethane·HCl, glycylglycine, N-tris(hydroxymethyl)-methyl-2-aminoethanesulfonic acid or N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid. The element as claimed in any of claims 1 to 3 wherein the phosphatase substrate is in the spreading zone. The element as claimed in any of claims 1 to 4 wherein the substrate is an organic mono- or diester of phosphoric acid and the phosphatase isoenzyme is bovine liver alkaline phosphatase. The analytical element as claimed in any of claims 1 to 5 further comprising a radiation-blocking zone between the additional zone and the porous spreading zone The element as claimed in any of claims 1 to 6 wherein the porous spreading zone is a blush polymer spreading layer. A method for the determination of theophylline comprising the steps of: A. at a pH of 9 or less, contacting a sample of a biological fluid suspected of containing theophylline with the dry analytical element as claimed in any of claims 1 to 7, and B. determining a detectable change resulting from the contact. The method as claimed in claim 8 carried out at a pH of from 7 to 9. The method as claimed in either of claims 8 and 9 wherein the biological fluid is human blood serum or whole blood.

Inventors: FRICKEY, PAUL HENRY, NORTON, GARY EDWARD, Frickey, Paul Henry, c/o Eastman Kodak Company, Norton, Gary Edward, c/o EASTMAN KODAK COMPANY
Applicants: EASTMAN KODAK COMPANY (A NEW JERSEY CORPORATION)
Application year: 1987
Main classification: C12Q1
IPC codes: G01N31, G01N33, C07D473, C12Q1