GLUCOSE-6-PHOSPHATE DEHYDROGENASE CONJUGATES, THEIR PREPARATION AND USE

Abstract: Polyiodothyronine analog conjugates to glucose-6-­ phosphate dehydrogenase (G6PDH) are provided which find use in the determination of polyiodothyronine compounds, particularly thyroxine, normally in physiological fluids, such as serum. The G6PDH enzyme activity can be substantially reduced when bound to antibodies specific to polyiodothyr­ onine as compared to the free or unbound polyiodothyronine analog enzyme conjugates. The polyiodothyronine analog is conjugated to the G6PDH by a linking group having a chain one atom in length.

Description: Thyroxine is an important hormone in the mammalian physiology, being excreted by the thyroid gland. The measurement of thyroxine is an important diagnostic tool in the determination of disease. Various techniques have been used for the determination of thyroxine, including radioimmunoassay, competitive protein binding, chromatography, etc. These techniques suffer from a number of disadvantages in being difficult to carry out and in the case of radioimmunoassay having unstable reagents. U.S. Patent No. 3,817,837 describes enzyme immunoassays. U.S. Patent No. 4,040,907 discloses iodothyronine enzyme conjugates. U.S. Patent No. 4,171,244 discloses enzyme-bound-polyiodothyronine. A polyiodothyronine immunoassay is described in U.S. Patent No. 4,043, 872. U.S. Patent No. 4,121,975 teaches a pretreatment of samples for polyiodothyronine assays. Enzyme immunoassays with glucose-6-phosphate dehydrogenase are described in U.S. Patent No. 3,875,011. A method for the measurement of free thyroxine or 3,5,3'-triiodothyronine in a liquid sample is described in U.S. Patent No. 4,410,633. Iodothyronine immunogens and antibodies are taught in U.S. Patent No. 4,399,121. A thyroxine radioimmunoassay is described in U.S. Patent No. 4,018,883. A radioimmunoassay for measurement of thyroxine and triiodothyronine in blood serum is disclosed in U.S. Patent No. 3,911,096. A radioimmunoasssay method for triiodothyronine and thyroxine is taught in U.S. Patent No. 3,928,553. Polyiodothyronine analogs (PIA) conjugated to glucose-6-phosphate dehydrogenase (G6PDH) are provided for use in immunoassays. The G6PDH conjugates are capable of competing with polyiodothyronine in a sample for antibody binding sites. The binding of the antibody to the PIA-G6PDH conjugate substantially decreases the enzymatic activity of the enzyme conjugate. By determining the enzymatic activity of an assay solution containing the sample in relation to known standards, the amount of polyiodothyronine in the sample can be determined. The polyiodothyronine analogs are conjugated to the G6PDH by a linking group one atom in length. DESCRIPTION OF THE SPECIFIC EMBODIMENTS The compositions of this invention are polyiodothyronine analogs conjugated to G6PDH, where the enzymatic activity of the enzyme conjugate is substantially decreased when bound to an antibody specific for polyiodothyronine. The polyiodothyronine analogs are conjugated to G6PDH by a linking group having a chain one atom in length. The linkage will frequently involve a non-oxo-carbonyl group, including the nitrogen and sulfur analogs thereof, or a sulfonyl group. When a non-oxo-carbonyl group or sulfonyl group is the linking group, it will be bound to amino groups on the G6PDH. (Non-oxo-carbonyl intends a carbonyl group substituted with at least one heteroatom. For the purposes of this disclosure non-oxo-carbonyl shall include the carbonyl group of carboxylic acids the nitrogen containing iminocarbonyl group of amidic acids the sulfur containing thionocarbonyl group of thio acids The sulfonyl group is derived from sulfonic acid Any of the above non-oxo-carbonyl groups or the sulfonyl group may be bonded to an amine to form an amide by displacement of the hydroxyl.) The number of PIAs conjugated to the enzyme will be at least 1, more usually at least 2, generally not exceeding 12, more usually not exceeding 10, and preferably within the range of about 3 to 8 on the average. The PIA differs from thyroxine by the absence of the alanine group and may have 1 to 4 of the iodines replaced by isosteric groups such as bromine and tert-butyl and may have one of the iodines replaced by hydrogen. The enzyme conjugate will be capable of being employed in an immunoassay so that in combination with a receptor, preferably an antibody, and the unknown sample suspected of containing a thyronine derivative, one is able to determine the amount of the thyronine derivative in the unknown by comparison of the enzymatic activity of the assay sample compared to known standards. For the most part, the PIA-G6PDH conjugates will have the following formula: wherein: Z is G6PDH bonded at other than its active site; by "active site" is intended those amino acid units or groups necessary for enzyme activity; m is a number between 1 and 16, usually in the range of 2 to 12, and preferably in the range of 3 to 8; α¹⁻⁴ are usually iodine, bromine or tert-butyl, preferably iodine, although one can be hydrogen; W is carbon or sulfur; X is oxygen when W is sulfur; or when W is carbon, X may be taken together with Y to form a double bond to oxygen, nitrogen, sulfur or carbon wherein the carbon may contain two substituents, each substituent having 1 to 5 atoms selected from carbon, oxygen, nitrogen and sulfur, such as carboxy, lower alkyl or lower alkyl substituted with hydroxy, amino, alkoxy, carboxy, thio, etc., and the like; or X is otherwise hydrogen; and Y is oxygen when W is sulfur; or Y is a substituent having 1 to 5 atoms selected from carbon, oxygen, nitrogen, and sulfur, such as lower alkyl or carboxy, lower alkyl substituted with hydroxy, carboxy, amino, alkoxy, thio, etc., and the like. Exemplary of such compounds are 4-(4-hydroxy-3,5-diiodophenoxy)-3,5-diiodobenzoic acid or 4-(4-hydroxy-3-iodophenoxy)-3,5-diiodobenzoic acid conjugated to G6PDH by means of amide linkages. For the most part, the G6PDH conjugates of this invention will have the following formula: wherein: m and Z have been defined previously; α⁴ is hydrogen or iodine, and A is oxygen or imine. For the most part, the linking group to the G6PDH will be a amide (including the nitrogen analog thereof, i.e., amidine) or sulfonamide, usually an amide and will be derived from an available amino group of lysine, or a terminal amino group. We have found that the PIA-G6PDH conjugates of the present invention provide distinct advantages over closely related polyiodothyronine G6PDH conjugates having longer linking groups when used in enzyme immunoassays. For example, the conjugates of the present invention retain a substantial portion, about 50 - 60% of the original enzyme activity. The retained enzymatic activity can be inhibited by about 40 - 60% upon the binding to the conjugate of an antibody for a polyiodothyronine. Polyiodothyronine analytes which can be detected in an assay employing the conjugates of the present invention for the most part have the following formula: wherein α² and α⁴ may both be iodine or one may be iodine and one hydrogen. Exemplary of such analytes are 0-(4-hydroxy-3,5-diiodophenyl)-3,5-diiodotyrosine and 0-(4-hydroxy-3-iodophenyl)-3,5-diiodotyrosine. The PIA-G6PDH conjugates can be used in a wide variety of immunoassays, either employing a separation step (heterogeneous assays) or not employing a separation step (homogeneous assays). These types of assays have been extensively described in the literature, for example, see "Enzyme-Immunoassay," by Edward T. Maggio, CRC Press, Inc., Boca Raton, Florida, 1980; U.S. Patent Nos. 3,817,837 and 3,935,074 (the disclosures of which are incorporated herein by reference); which listing is not intended to be exhaustive. The assay can be run in at least two different ways. For illustrative purposes, thyroxine will be referred to. In a heterogeneous manner, the PIA-G6PDH conjugate, sample, and antithyroxine antibody are combined in a suitable buffered medium, the mixture incubated for a sufficient time, and the enzyme conjugate bound to antibody separated from the unbound enzyme conjugate by any convenient means. For example, antibodies to anti-thyroxine are employed which aid in a clean separation of the enzyme conjugate-anti-thyroxine complex from the assay medium. The assay medium may then be examined for the enzyme activity of the remaining PIA-G6PDH conjugate. In a homogeneous manner, the PIA-G6PDH conjugate, anti-thyroxine antibody and sample are combined and the combination is incubated for a sufficient time. The enzyme activity in the solution is determined without separation. The amount of thyroxine in the sample is determined by comparing the results of the assay to known standards. For example, samples having known amounts of thyroxine are prepared and the assay carried out and the enzymatic activity determined. The enzymatic activity is then graphed against the thyroxine concentration and the graph used to determine the amount of thyroxine is an unknown. The conditions for the assay will vary depending upon the particular method employed. Where the homogeneous technique is used, the conditions will be selected so as to optimize the change in activity of the enzyme conjugate upon binding by the receptor. Normally, the pH will be in the range of about 5.5 to 10, more usually in the range of about 7 to 9.5, where strong binding between receptor and thyroxine occurs. Moderate temperatures will be employed, normally in the range of about 0° to 45°, more usually about 20° to 40°C. The buffer solution employed will normally be at a concentration to provide in the assay medium, a concentration of from about 0.001 to 0.5M, usually from about 0.01 to 0.2M. Protein will frequently be included to stabilize the enzyme; the protein can be an albumin, such as rabbit serum albumin, and/or gelatin, and will generally be present in about 0.005 to 0.5 weight percent in the final assay mixture, more usually from about 0.01 to 0.2 weight percent. Other additives may be present as found desirable, such as glycerol, Thimerosal, sodium azide, etc. Concentration of the PIA-G6PDH conjugate will vary widely, depending on the concentration of polyiodothyronine of interest. Normally, the PIA-G6PDH conjugate concentration will be from about 10⁻⁵ to 10⁻¹³M, more usually from about 10⁻⁷ to 10⁻¹¹M. The ratio of binding sites to the concentration of conjugated polyiodothyronine will generally be at least about 0.5 and not greater than 1000, more usually being about from 1 to 100. The order of addition of the reagents is not critical. However, it is preferred that the PIA-G6PDH conjugate and receptor not be combined prior to the addition of the sample. The preferred order of addition is the sample and antibody, followed by the addition of enzyme conjugate. The particular substrates for the enzyme may be added as convenient. After each step the assay mixture may be incubated. Usually, incubation periods will be from about 10 seconds to 1 hour. Enzyme activity determinations can be carried out for a duration of from about 5 seconds to 60 minutes, more usually being from about 0.25 to 30 minutes. Rapid determinations in less than two minutes, preferably in less than one minute, are a useful embodiment of this invention. For the most part, spectrophotometric techniques will be employed. However, other techniques include fluorimetry, titrimetry, etc. The PIA-G6PDH conjugates can be prepared by techniques similar to those described in the literature. For example, conjugation of enzymes to low molecular weight compounds is described in U. S. Patent No. 4,040,907, the disclosure of which is incorporated herein by reference. For example, a non-oxo-carbonyl group, e.g., carboxyl group, on the PIA can be activated for reaction with amine groups on the G6PDH by formation of an ester with N-hydroxysuccinimide. The PIA are known compounds. The following examples are offered by way of illustration and not by way of limitation. EXPERIMENTAL (All temperatures not otherwise indicated are in Centigrade. All percents not otherwise indicated are by weight.) EXAMPLE 1 Conjugation of G6PDH to 4-(4-hydroxy-3,5-diiodophenoxy)-3,5-diiodobenzoic acid (HDDA) The N-hydroxysuccinimide (NHS) ester of HDDA was prepared by a conventional method (see U.S. Patent No. 4,040,907). Dicyclohexylcarbodiimide (32 mg) (DCC), HDDA (100mg), and NHS (17.4 mg) were combined in one ml tetrahydrofuran (THF) as a solvent. The ester formed readily at room temperature as indicated by the formation of a white insoluble precipitate--dicyclohexylurea (DCU). The reaction was allowed to continue overnight at room temperature. The DCU precipitate was removed by filtration and the clear filtered ester solution had a slight yellow color. The THF was removed by rotary evaporation. The dry ester was dissolved in dry DMF, refiltered and was then ready for conjugation to G6PDH. Native G6PDH was dialyzed overnight at 4°C with 3 changes of 0.05M phosphate buffer pH 8.5 (no preservatives). The concentration of enzyme used was 4 to 6 mg/ml. DMF was then added to the enzyme solution slowly to a 20% (v/v) ratio. The pH was maintained at 9 during conjugation. The NHS ester of HDDA was added slowly via syringe in small aliquots. Small aliquots of the enzyme were taken after each hapten addition to follow % deactivation and % inhibition. Deactivation should not be allowed to exceed 60%. The HDDA-G6PDH conjugate was chromatographed on Sephadex G-100 column equilibrated with basic buffer (55 mM Tris with NaN₃ and Thimerosal) including 1% Tween-20. Fractions with activity were pooled and any precipitate was removed by filtration using a 0.45 µ Millipore filter. The HDDA-G6PDH conjugate exhibited an enzyme activity of 50% of that of the unconjugated enzyme. On the average each molecule of G6PDH contained 6-10 HDDA units. Anti-T₄ antibodies were prepared in sheep using T₄-BSA immunogen. EXAMPLE 2 Assay Assay Protocol The formulations for reagents employed are listed below: Pretreated sample--serum sample pretreated, in accordance with the procedure described in U.S. Patent No. 4,121,975, with: 0.25M NaOH 0.25% α-cyclodextrin 0.025% Salicylic Acid Calibrators were prepared by dissolving known amounts of thyroxine in thyroxine free human serum at levels of 20, 40, 80, 120, and 200 ng/ml The stepwise testing protocol for the assay was as follows: 1. 40 µl calibrator + 100 µl pretreated sample into beaker #1. 2. 50 µl Reagent A + 300 µl Assay Buffer into beaker #1. 3. 50 µl Reagent B + 300 µl Assay Buffer into beaker #1. 4. Aspirate into a flow cell for the Stasar III spectrophotometer 37°C 340 nm 10 sec delay 30 sec read time The results were as follows: mean = 6.75 µg/dl S.D. = 0.27 µg/dl CV% = 4.07% N = 16 EXAMPLE 3 Conjugation of G6PDH to 4-(4-hydroxy-3-iodophenoxy)-3,5-diiodobenzoic acid (HIDA) Preparation of NHS ester of HIDA was carried out following a procedure similar to the one described above in Example 1 for the preparation of NHS ester of HDDA. The resulting ester derivative was purified by passing through a cellulose column eluted with 20% hexane-CH₂Cl₂. Conjugation to G6PDH enzyme was carried out by a similar procedure as described above except 25% Carbitol was used during conjugation. Purification of HIDA-G6PDH conjugate was achieved through sephadex G-50 column eluted with 0.055 M Tris buffer. Fractions with activity were pooled. The HIDA-G6PDH conjugate exhibited an enzyme activity of around 40% of that of the unconjugated enzyme. Anti-T₃ antibodies were prepared in sheep using T₃-BSA immunogen. EXAMPLE 4 Assay Assay Protocol The formulations for reagents employed are listed below: T₃ Calibrators 0, 25, 50, 75, 100, 200 ng/ml T₃ in 0.05 N NaOH or in serum with 400 ng/ml T₄. The testing protocol for the assay was as follows: 1. 10 µl T₃ calibrator and 100 µl Assay Buffer pipetted into cuvette #1. 2. 5 minute incubation at 25°C. 3. 50 µl Enzyme working reagent (25 µl Reagent B + 25µl Assay Buffer) pipetted into cuvette #1. 4. 9 second incubation at 37°C. 5. 25 µl Reagent A and 250 µl Assay Buffer pipetted into cuvette #1. 6. ABA-100 spectrophotometer analyzer (Abbott Company) set for 5 minute reaction time using a 340/380 nm filter. This assay produces a 0.166 O.D. assay response across the calibration range (0 to 200 ng/ml T₃). The results of the foregoing examples demonstrate that extremely low concentrations, as well as extremely small amounts of polyiodothyronine such as thyroxine can be detected by a method employing the PIA-G6PDH conjugates of the present invention. The method is quite straightforward in requiring few manipulative steps. By combining the reagents in a buffered medium, and optionally incubating the mixture, followed by the addition of the enzyme substrates, one can determine a polyiodothyronine analyte by a spectrophotometric reading over a short period of time. The system allows for automation, so that samples and reagents can be mixed automatically and read. The PIA-G6PDH conjugates offer a distinct advantage over closely related compounds having a linking group between the enzyme and the polyiodothyronine longer than a chain one atom in length. Compounds possessing such longer linking group, when used in the above assay, did not give meaningful results. Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be obvious that certain changes and modifications may be practiced within the scope of the appended claims.

Claims: Claims for the following Contracting States : BE, CH, DE, FR, GB, IT, LI, LU, NL, SE A compound which is a polyiodothyronine analog conjugated to glucose-6-phosphate dehydrogenase through a linking group which is a chain one atom in length, wherein said polyiodothyronine analog is a thyroxine derivative in which the alanine group is absent. The compound of claim 1 wherein the linking group is a non-oxo-carbonyl group. A compound of the formula: wherein: Z is glucose-6-phosphate dehydrogenase; α¹ to α⁴ are iodine, bromine, tert-butyl or hydrogen; wherein no more than one α is hydrogen; W is carbon or sulfur; and X is oxygen when W is sulfur; or is taken together with Y to form a double bond to carbon, oxygen, nitrogen or sulfur when W is carbon or is otherwise hydrogen; Y is oxygen when W is sulfur, or Y is a substituent having 1 to 5 atoms selected from carbon, oxygen, nitrogen and sulfur; and m is a number from 1 to 16. The compound of claim 3 wherein W is carbon. The compound of claim 3 or claim 4 wherein X is taken together with Y to form oxo. The compound of any one of claims 3 to 5 wherein each α¹ to α⁴ is selected from iodine and hydrogen. The compound of any one of claims 3 to 6 wherein α¹ to α⁴ are iodine. A compound of the formula: wherein: α⁴ is iodine or hydrogen; Z is glucose-6-phosphate dehydrogenase; m is a number from 2 to 12; and A is oxygen or imine (NH). The compound of claim 8 wherein α⁴ is iodine and A is oxygen. A method for detecting the presence or amount of a polyiodothyronine in a sample suspected of containing the same, which method comprises: combining in an aqueous medium the sample, the compound of any one of the preceding claims, an antibody specific for the polyiodothyronine, and any other reagents necessary for conducting said detection, and determining the enzyme activity of said medium. A process for preparing a compound according to any one of claims 1 to 9, which process comprises conjugating a polyiodothyronine analog and glucose-6-phosphate dehydrogenase. Claims for the following Contracting State : AT A process for preparing a compound which process comprises conjugating a polyiodothyronine analog and glucose-6-phosphate dehydrogenase, wherein said polyiodothyronine analog is a thyroxine derivative in which the alanine group is absent. A process of claim 1 wherein the polyiodothyronine analog is conjugated to glucose-6-phosphate dehydrogenase through a linking group which is a chain one atom in length. The process of claim 1 or claim 2 wherein the linking group is a non-oxo-carbonyl group. A process for the production of a compound of the formula: wherein: Z is glucose-6-phosphate dehydrogenase; α¹ to α⁴ are iodine, bromine, tert-butyl or hydrogen; wherein no more than one α is hydrogen; W is carbon or sulfur; and X is oxygen when W is sulfur; or is taken together with Y to form a double bond to carbon, oxygen, nitrogen or sulfur when W is carbon or is otherwise hydrogen; Y is oxygen when W is sulfur, or Y is a substituent having 1 to 5 atoms selected from carbon, oxygen, nitrogen and sulfur; and m is a number from 1 to 16. The process of claim 4 wherein W is carbon. The process of claim 4 or claim 5 wherein X is taken together with Y to form oxo. The process of any one of claims 4 to 6 wherein each α¹ to α⁴ is selected from iodine and hydrogen. The process of any one of claims 4 to 7 wherein α¹ to α⁴ are iodine. A process for the production of a compound of the formula: wherein: α⁴ is iodine or hydrogen; Z is glucose-6-phosphate dehydrogenase; m is a number from 2 to 12; and A is oxygen or imine (NH). The process of claim 9 wherein α⁴ is iodine and A is oxygen. A method for detecting the presence or amount of a polyiodothyronine in a sample suspected of containing the same, which method comprises: combining in an aqueous medium the sample, the compound of any one of the preceding claims, an antibody specific for the polyiodothyronine, and any other reagents necessary for conducting said detection, and determining the enzyme activity of said medium.

Inventors: KHANNA, PYARE, SCHULKAMP, KIRK
Applicants: SYNTEX (U.S.A.) INC.
Application year: 1986
Main classification: C12N9
IPC codes: C12Q1, G01N33, C12N9