METHOD AND KIT FOR SERUM PRETREATMENT FOR DIGOXIN ASSAY, AND DIGOXIN CONCENTRATE OBTAINED THEREBY

Abstract: Chromatographic separation is used to obtain a digoxin concentrate from a serum sample for use in an assay for digoxin. Serum is passed through a column containing silica gel alkylated with lower alkyl groups. The column is then washed and the digoxin eluted with aqueous methanol to pro­ vide the digoxin concentrate.

Description: Digoxin, and derivatives of digoxin, find wide use in cardiac treatment, frequently as a component of digitalis. Digoxin is highly potent in its activity and has a narrow therapeutic range. In addition, the drug can produce serious side effects, so that monitoring the digoxin level in blood is important for the well-being of the patient. Since the therapeutic range is from about 0.8 to 2.0 ng/ml, it is necessary not only to measure extremely small amounts of digoxin in serum, but also to be able to distinguish between small differences in concentrations. Depending upon the sensitivity of the assay, the digoxin concentration in serum may be insufficient for detection when diluted into the assay medium. Also, naturally occurring materials in the serum sample may modify the observed signal so as to give false results. It would therefore be desirable to provide for a simple means for pretreatment of a serum sample for a digoxin assay. The pretreatment method should be rapid and efficient and provide an assay sample containing the drug in a concentrated amount free of interfering substances. A radioimmunoassay for determining the digoxin content of a sample is disclosed in US -A- 3,981,982. An enzyme amplification assay is described in US -A-3,817,837. The present invention provides for the use of chromatographic separation in obtaining a digoxin concentrate from a serum sample. In a preferred procedure, serum samples for digoxin assays are pretreated by passing the serum sample through a column, usually a pre-prepared column, containing silica gel that is alkylated with lower alkyl groups. The column is then washed with dilute acid and water, and the digoxin is eluted with an aqueous organic solvent, preferably aqueous methanol, to provide a concentrated sample of digoxin free of interfering substances. The pretreatment method finds particular application in conjunction with assays employing enzyme or fluorescent labels. The present invention also provides a digoxin concentrate as obtainable from a serum sample by chromatographic separation as described herein. Serum samples for digoxin assays are pretreated to provide for at least about a two-fold concentration of digoxin in a medium substantially free of materials present in the serum sample and in a form useful for a digoxin assay determination. It is believed that the present method allows separation of digoxin from endogenous proteins present in serum samples to which proteins the digoxin is bound. The method normally employs reverse phase liquid chromatography using a column containing silica gel alkylated with alkyl groups of from 1-2 carbon atoms, particularly 1 or 2 carbon atoms. The silica gel particles have a size in the range of 30-50 µm, preferably 40 µm. The particles are silanized with methyl or ethyl silyl groups to provide the alkylated silica gel particles. The amount of the column packing, i.e. , alkylated silica gel, employed and the dimensions of the column are dependent on the size of the serum sample to be treated. Generally, for a serum sample of one ml, e.g. 50 to 100 mg of packing will be used. For 50 mg of silica gel, depending upon the manner of packing, as well as the diameter of the column, the height of the column may vary from 7 to 20 mm. The column is packed by introducing the silica gel powder into an appropriate column. The column is pre-prepared, i.e., conditioned, by adding methanol followed by water and removing the solvent by any convenient means, e.g., vacuum, positive pressure, or centrifugation. After the methanol has been removed, the column is then washed with water, preferably deionized water. The water is then removed as described above, and the column is now ready for the sample. Prior to applying the sample to the column, the serum sample may be subjected to other pretreatments. Depending upon the nature of the sample, such as icteric, lipemic or uremic, the sample may be centrifuged, subjected to chromatographic treatment, or filtration. The sample is then added to the column after all the excess methanol and water employed in the pretreatment and washing of the column have been removed. The sample may then be drawn into the column by vacuum or centrifugation, or it may be pushed into the column by positive pressure. The conditions for putting the sample on the column will be generally mild, for example, a vacuum in the range of 3.39-6.77 MPa (10-20 inches Hg) may be used. Various conventional devices can be used, for example, the Vac-Elut vacuum box (Trade Mark: Analytichem International, Harbor City, California 90710). After the sample has been put onto the column, the column is be washed with a small amount of 0.05-0.2 M(N) HCl, more usually 0.1 M(N) HCl. The volume of the acid wash is not critical, usually being one times the column volume, generally less than 5 ml, more usually less than 2 ml and usually at least 1 ml. After the acid wash, the column is then washed with deionized water to ensure the removal of any remaining materials that are not adsorbed onto the column packing material. Again, the amount of water which is used is not critical, usually being one times the column volume, generally being less than 10 ml, more usually being less than 5 ml, and preferably at least 1 ml. After each addition of the wash solution, the wash solution may be drawn through the column as described above for the sample. Usually this will involve, for an initial volume of 1 ml sample, at least 15 seconds and not more than two minutes, generally from 20 seconds to 45 seconds. Any water remaining at the tip of the column may be removed by blotting or other convenient means. The digoxin is then eluted to provide for a digoxin concentrate in purified form to be used in an assay. Elution is accomplished by adding at least 150 µl, preferably at least 250 µl, and preferably, not more than 500 µl of eluent for an initial volume of 1 ml of serum sample. The lower the eluent volume, the greater the concentration of digoxin will be in the final sample. The eluent is generally an aqueous organic solvent wherein the organic solvent generally comprises from 1 to 4 carbon atoms and from 1 to 3 heteroatoms selected from oxygen and nitrogen. Exemplary of such an eluent is 50-100% aqueous methanol, preferably 60% aqueous methanol. Other organic solvents that may be employed are dimethyl formamide and ethanol. In assays involving labels, e.g., enzyme labels, the organic solvent should have little or no detrimental effect on the label activity. In that respect, consideration should also be given to techniques for combining the digoxin concentrate with the assay reagents as this may effect label activity. The aqueous methanol is drawn through the column in the same manner described above. The aqueous methanol may be added to the column in 200-500 µl aliquots, the eluate being drawn through the column before the next addition. Usually, 1 - 3 aliquots, preferably one aliquot, of the aqueous methanol is employed to give an assay sample volume of less than 50%, usually 30-50%, of the initial serum sample volume. The eluate is then collected and is ready to be used in an assay. Serum samples treated to give digoxin concentrates in accordance with the present invention may be assayed for digoxin by a number of assay methodologies. The assays may be heterogeneous or homogeneous involving labels such as enzymes, radioisotopes and fluorescers. The method of the invention is particularly suited for preparing digoxin concentrates for enzyme label assays such as, for example, EMIT assays, and EMIT QST assays (EMIT and QST are trade marks). The invention also includes a kit comprising, in a packaged combination, (1) a prepacked column having dimensions as described above and containing silica gel, alkylated with alkyl groups containing from 1 to 2 carbon atoms, in amounts as described above, to which column the serum sample is to be applied, and (2) 300 µl - 500 µl of 50 - 70% aqueous methanol as an eluent in a suitable container such as a vial made of a suitable material such as glass or plastic. The kit may also include ancillary items such as a device for securing the serum sample or applying the serum sample to the column, wash solutions such as a dilute aqueous mineral acid, e.g., HCl, in amounts and concentrations as described above and deionized water in separate containers. The above kit may be combined with an assay kit for performing a digoxin assay or it may be separate therefrom. EXAMPLE The invention is further demonstrated by the following illustrative example. A Vac-Elut vacuum box (Analytichem International) was used at 4.06-5.08 MPa (12-l5 inches Hg). Up to ten columns (C-1 from Analytichem containing silica gel alkylated with methyl groups) were placed in luer fittings in the lid of the vacuum box and any remaining holes were plugged with stoppers. Each of the columns was filled with spectral grade methanol and the methanol drawn through with a vacuum of 4.40 MPa (13 inches Hg). The column was then rinsed with one column-volume of deionized water to displace the methanol. Onto each column was accurately pipetted 1.0 ml of a serum sample and the sample drawn through with a vacuum of 4.40 MPa (13 inches Hg). The column was rinsed with one column-volume of 0.1 M HCl to remove any extraneous substances. The column was rinsed with one column-volume of deionized water to wash off any remaining materials not adsorbed to the column packing material. Vacuum was applied until no water was left on the column, approximately 30 seconds. (A column-volume equals 1 - 2 ml.) Any water remaining on the column outlets was removed by blotting. A metal collection rack with 10X75 mm test tubes was placed in the vacuum box to collect the eluates. Using an Oxford repeating dispenser (available from, e.g., Lancer Division of Sherwood Medical Co., Foster City, California), either 500 µl of 60% methanol was added to each column or 300 µl in two 150 µl aliquots. The eluent was 60% methanol and was drawn through the column by means of vacuum. The collection tubes were capped and stored until used. In the first study, a number of assays were performed on a Cobas Bio centifugal analyzer (Trade Mark: Roche Analytical Instruments, Inc., New Jersey) according to the operating instructions provided by the manufacturer for conducting an EMIT assay. The parameters for the assay are as follows: The protocol employed the following reagents: 1. Reagent A: 1 to 3500 dilution of neat digoxin antibody with 2 x substrate 2. Reagent B: 1 to 220 dilution of neat digoxin - glucose-6-phosphate dehydrogenase (G6PDH) conjugate in B diluent. (B diluent composition is as follows:) 0.05M Tris HCl 0.45% NaCl 0.05% NaN₃ 1% RSA pH 7.6 3. Assay Buffer: 0.6% NaCl 0.132M Tris 0.006% Thimerosal 0.12% Triton®X-100 0.06% NaN₃ pH = 8.4 4. Working Reagent A: Prepared by combining one volume of Reagent A with 8 volumes of Assay Buffer. 5. Working Reagent B: Prepared by combining 3 volumes of Reagent B with 11 volumes of Assay Buffer. 6. Calibrators: Freon® treated serum was used as a matrix for the calibrators. 7. Calibrator Levels: 8. Calibrators were subjected to a column extraction procedure which was the same as that described above for the serum samples to be assayed. The recovery of digoxin from the 100 mg columns was evaluated with spiked levels of digoxin at 0.7 ng/ml and 3.5 ng/ml. The digoxin was spiked into a pool of digoxin negative sera. For ten samples at 0.7 ng/ml, the mean value was 0.691 ng/ml, standard deviation (SD) was 0.06, percent coefficient of variation (CV) was 8.7, and percent recovery was 98.5%. At the 3.5 ng/ml, the comparable results were 3.46, 0.15, 4.3 and 98.9. The procedure was repeated. In back-to-back comparisons conducted as described above, the Analytichem 100 mg Cl column with a 60% methanol eluent was compared with Analytichem 100 mg C2 column (contained silica gel alkylated with ethyl groups) with 70% methanol as the eluent. The results are reported as the observed change in signal as compared to a sample having no digoxin. The following table indicates the results. The results are comparable except that a better spread seems to be obtained with the Cl column than with the C2 column. In the next study, results were compared using digoxin samples prepared in accordance with the subject invention in an assay as described above and a radioimmunoassay on unextracted samples. The RIA assay was purchased from Clinical Assays, Cambridge, Massachusetts (GammaCoat - Trade Mark [ 125 I] Digoxin Radioimmunoassay Kit). The RIA was performed in accordance with the supplier's instructions. A statistical summary of the results follow: wherein x = RIA performed in accordance with the supplier's instructions. y = EMIT assay performed in accordance with the present invention avg = average SE = standard error SD = standard deviation SEE = standard error of the estimate N = number of samples It is evident from the above results that the subject method greatly enhances the accuracy and reproducibility of digoxin assays, particularly involving enzyme labels. Thus, a sensitive and efficient method is provided for treating samples for digoxin assays which results in accurate determinations of digoxin.

Claims: A method for preparing a serum sample for determination of digoxin in an assay, which method comprises: adding said serum sample to a chromatographic column containing silica gel alkylated with alkyl groups of from 1-2 carbon atoms; washing the column with dilute acid and water; and eluting the digoxin with an aqueous organic solvent having from 50-70 volume percent organic solvent at a volume less than the volume of said serum sample, wherein said organic solvent comprises from 1 to 4 carbon atoms and from 1 to 3 heteroatoms selected from oxygen and nitrogen. The method of claim 1 wherein said alkyl group is methyl. The method of claim 1 or claim 2 wherein said alkyl group is ethyl. The method of claim 1 or claim 2 wherein said organic solvent is methanol. The method of claim 4 wherein said aqueous methanol is 60 volume percent methanol. The method of claim 4 or claim 5 wherein said aqueous methanol is added in a volume of from 30-50% of the volume of said serum sample. A kit for purifying a serum sample to give a digoxin concentrate according to the method of any one of claims 1 to 6, for use in an assay for digoxin, which kit comprises - (a) a column containing silica gel alkylated with alkyl groups containing 1-2 carbon atoms, and (b) an eluent solution which is 50-70% aqueous organic solvent, which organic solvent comprises from 1 to 4 carbon atoms and from 1 to 3 heteroatoms selected from the group consisting of oxygen and nitrogen. The kit of claim 7 wherein said organic solvent is methanol. A digoxin concentrate obtained from a serum sample by the method of any one of claims 1 to 6. A digoxin concentrate as obtainable from a serum sample by chromatographic separation according to the method of any one of claims 1 to 6.

Inventors: ERNST, ROBERTA DOROTHEA, KHANNA, PYARE, STONE, ANNE JENNIFER
Applicants: SYNTEX (U.S.A.) INC.
Application year: 1985
Main classification: G01N33
IPC codes: G01N33