Apparatus and method for conducting a binding assay on an absorbent carrier material

Abstract: Improved apparatus for conducting a binding assay comprises: (a) a first absorbent material having a first reagent immobilized thereon at a first predetermined location; and (b) a second absorbent material in contact with the first absorbent material and having a second reagent releasably immobilized thereon at a second predetermined location, the first absorbent material and the second absorbent material being positioned in juxtaposition to each other so that a flow of liquid through the first absorbent material causes the second reagent to be released from the second absorbent material and flow through the first absorbent material.

Description: The invention relates to an apparatus and a method for carrying out binding assays. The invention also relates in particular to assays which utilize porous carrier materials for transporting reagents. Specific binding assays are valuable tools useful in numerous applications. Such assays involve two or more binding components, one of which is analyte. Examples of specific binding components include antibody and antigen, DNA and/or RNA hybridization, and receptor-ligand interaction. Numerous specific binding assays formats are known in the art, and include competition, sandwich, and agglutination assays. Unfortunately, specific binding is not always observable. Accordingly, labeling techniques have been developed to make the binding either indirectly or directly observable. Labeling typically involves attaching a detectable label to one or both binding components. Radioisotopes, fluorophores, and enzymes are examples of "indirect" labels, since they require instrumentation and/or special treatment for detection. In contrast, "direct" labels are visible to the naked eye and do not require special procedures or instrumentation for detection. Metallic sols, dye sols and colored latex are examples of "direct" labels. Recently, a technology combining binding assay methods and porous carrier materials has been developed. One component of the binding pair (for example, antibody or antigen), is immobilized onto the porous carrier material (for example, a fibrous or porous membrane) by one of the many methods well known in the art, for instance by absorption or covalent linking. For example, in a competition assay, a sample containing analyte is typically mixed with a labeled analyte or labeled analyte analog (the reagent) capable of specifically binding with the immobilized component. The mixture is then applied to the porous carrier material. Migration of the mixture, caused by capillary wicking within the porous carrier material, brings the reagent to a position where it competes for a limited number of binding sites of the immobilized specific binding component. A portion of the labeled reagent specifically binds to the immobilized reagent and in turn becomes immobilized. The amount of bound labeled reagent is in inverse proportion to the amount of analyte in the sample. Excess, unreacted material is removed by washing, for example by applying appropriate volume of an eluant to the porous carrier material. After washing, the label can be developed and/or measured ("indirect" label), or visually evaluated ("direct" label). Labeled reagent can also be impregnated into the porous carrier material so that the labeled reagent becomes mobile when moistened. In such a case, the addition of the sample itself may be sufficient to initiate and run the assay. Deutsch, et al. (U.S. Patent Nos. 4,094,647, 4,235,601 and 4,361,537 discloses a test device for determining a characteristic of a sample (a specific binding assay) in which reagents are mixed and transported in a transportative strip element (also see Baker, et al., WO 87/02774, published May 7, 1987). In one embodiment, a first specific radiolabeled reagent is impregnated into the strip, and a second reagent is immobilized onto the strip downstream of the first reagent via attachment to microbeads. A sample is applied upstream of the first reagent, and the beginning end of the strip is dipped into an eluant. Eluant migrating through the strip picks up the sample. The eluant containing the sample then rehydrates the first reagent, mixes it with the sample, and transports the sample and first reagent to the second reagent. where a specific binding reaction immobilizes a portion of the radiolabeled reagent. Excess radiolabeled reagent is washed away towards the terminal end of the strip. Alternatively, labeled reagent may be applied directly to the strip body. United States Patent No. 4,938,927 issued to Kelton, et al. discloses a rotary fluid manipulator containing a circular chromatographic membrane divided by fluid blocking means into many fluid passages. Each fluid passage contains a labeled specific binding reagent and an immobilized specific binding reagent. A test specimen is applied onto or nearby the labeled reagent. Then, an eluant is applied at a special location close to the center of the membrane. The eluant moves within particular passage towards the outer side of the circle by wicking and centrifugal force. The principles of the specific binding assay itself are identical to those described in Deutsch, et al. United States Patent No. 5,238,652 to Sun, et al. relates to an analytical test device for a competition assay for particular non-protein antigens, such as antigens representing drugs of abuse. Colored latex particles sensitized with detecting antibodies for the non-protein antigen are applied directly to the chromatographic membrane in such a way that latex spheres are immobile when dry and mobile when contacted with liquid. Downstream of the latex, the membrane is impregnated with a permanently immobile drug conjugate probe. When a sample such as body fluid is applied to the membrane, the latex particles are resuspended by the sample and migrate towards the conjugate. If drugs or their metabolites are present in the sample, binding to antibody on the latex will occur during this migration. If the amount of drug or drug metabolite is sufficient to exhaust all antibody binding sites, binding of the colored latex particles to the drug conjugate probe will be prevented. If there are no drugs or drug metabolites in the sample, then colored latex particles will bind to drug conjugate producing a distinct colored band. European Patent No. 0 291 194 B1, to May, et al., discloses an analytical test device comprising a hollow casing containing a chromatographic porous membrane. The device also contains labeled (for example, colored latex) specific binding reagent for an analyte, and an unlabeled specific reagent for the same analyte, permanently immobilized on the membrane. The liquid sample permeates the membrane by capillary action, picking up the labeled reagent and migrating to the zone with the unlabeled specific reagent. The presence of an analyte within the sample is determined by the extent of labeled reagent binding using either a sandwich or a competition format. EP-A 0 525 829 discloses a chromatographic test strip. An inert support foil carries several sheet-like pieces of bibulous material. These sheets are arranged in a linear manner and touch each other pairwise, thus forming a continuous liquid transport pathway. Liquid sample which is introduced to the sheet-like piece on one end of the test strip moves through the consequent pieces until the last piece of sheet material on the opposite end of the strip is reached. Thereby, reagents contained in one or several sheet pieces interact with the analyte contained in the sample liquid and allow its detection. To minimize undesirable interactions between the labeled reagent and the membrane, which may cause irreversible sticking of the labeled reagent to the membrane, it was suggested to apply the labeled reagent as a surface layer. To achieve this, a glazing material, such as aqueous sucrose or cellulose, is applied prior to the application of the labeled reagent to form a glaze layer. The labeled reagent is then applied on top of the glaze. In practice, however, the glazing material penetrates to some extent into the thickness of the membrane, as does the labeled reagent. United States Patent No. 5,275,785, to May, et al., ("May "785") discloses a chromatographic test device incorporating multiple liquid conductive membranes of different wicking ability and a liquid-swellable "switch" material to regulate liquid flow. Different liquid migration speeds in the separate membranes in combination with the liquid-swellable "switch" material (which establishes or breaks contact between the membranes) allow for delivery of reagents to the detection zone in a certain order and/or proportion. Unfortunately, the known technologies referred to above, suffer numerous disadvantages. Two problems of particular concern deal with mobilizing dry binding reagents: the first relates to quantity of reagents mobilized, the second to the fashion of mobilization. Although a goal is to mobilize reagents quantitatively, this is often not possible in practice because reagents tend to irreversibly stick to the porous material. It is preferred to uniformly mobilize reagents to form a continuum with no or minimal concentration gradient. Such a continuum favors a uniform degree of binding (especially critical for the competition assays), and prepossesses a reproducible uniform result band. One known way to achieve mobilization and a uniform concentration throughout solution of the reagents is to premix a sample and a liquid binding reagent, for instance labeled binding reagent, and then apply the mixture to a test device, either directly or via a mediating sample pad. Regrettably, however, this multistep procedure is predisposed to experimental error and increases handling of potentially hazardous samples. One-step devices require only the single step of adding sample to perform a test. The binding reagent is usually embedded into an absorbent material which is in serial contact with the porous carrier material or directly applied to the porous carrier material. However, in such cases the liquid front of a sample non-uniformly mobilizes the dry impregnated binding reagent so that the reagent concentration is highest at the front, thus resulting in a non-uniform concentration pattern which is very sensitive to any variation in the porous carrier material. Even small lot-to-lot differences can cause an adverse affect and precise titration of the labeled reagent is required. Since the titer is chosen with respect to performance at cutoff level of an analyte, it is often difficult to achieve substantial differentiation between no analyte and analyte-at-cutoff signals. Other disadvantages of known one-step systems are non-uniform migration of labeled binding reagent and poor rehydration of latex. The prior art attempted to overcome the problem of poor rehydration and irreversible sticking of latex by applying a binding reagent to the surface layer on top of glazing material (see for example, May, et al., discussed above). However, the viscous compounds which form the glazing material deteriorate migration and cause eddies. This leads to poor reproducibility and a broken result band. In a competitive format, the disadvantages are more acute than in a sandwich assay format. The object of the invention therefore is to provide an assaying device which makes possible to overcome the above-mentioned problems. An assaying device according to the invention is characterized in that it comprises (a) a first absorbent material having a first reagent immobilized thereon at a first predetermined location; and (b) a second absorbent material in contact with and on top of the first absorbent material and having a second reagent releasably immobilized thereon at a second predetermined location, the first absorbent material and the second absorbent material being positioned in juxtaposition to each other so that a flow of liquid through the first absorbent material causes the second reagent to be released from the second absorbent material and flow through the first absorbent material. The main advantages of the closure according to the invention are as follows: The subject invention overcomes the above-mentioned problems and provides an improved specific binding assay through the use of an additional top membrane (a porous carrier material) which contains a specific binding reagent. Through an additional independent membrane impregnated with specific binding reagent, rehydration of the specific reagent and the extent of its dilution by the sample is controlled thereby optimizing the performance of the assay. This unique design achieves a more uniform migration pattern of the labeled reagent, and eliminates undesirable flooding of the membrane body when excessive sample volume is applied. The subject invention can be used with, but is not limited to use with an assaying device for conducting specific binding assays, in particular drug of abuse assays. Such an assaying device is configured and dimensioned to incorporate porous carrier material test strips. A device of this type is disclosed by European Patent Application with publication No. EP-A-0668745. In an assaying device according to the invention the first absorbent material and the second absorbent material are preferably composed of the same material. In an assaying device according to the invention the first absorbent material is preferably selected from the group consisting of bibulous membranes and porous membranes. The first absorbent material is preferably nitrocellulose. In an assaying device according to the invention the second absorbent material is preferably selected from the group consisting of bibulous membranes and porous membranes. The second absorbent material is preferably nitrocellulose. In a preferred embodiment of an assaying device according to the invention the first absorbent material and the second absorbent material are held in contact with each other by a holder, the holder being preferably a band of material, said band of material being preferably a polyester film. In a preferred embodiment of an assaying device according to the invention the second reagent is labeled, the label being preferably a colored latex. In a preferred embodiment of an assaying device according to the invention the first absorbent material has a plurality of reagents immobilized thereon. In a preferred embodiment of an assaying device according to the invention the second absorbent material has a plurality of reagents immobilized thereon. In a preferred embodiment of an assaying device according to the invention the first reagent and the second reagent have an affinity to bind with each other. In a preferred embodiment of an assaying device according to the invention the first reagent and the second reagent each have an affinity to bind with the same substance. In a preferred embodiment of an assaying device according to the invention the wicking rate of the first absorbent material is the same or higher than the wicking rate of the second absorbent material. In a further preferred embodiment of an assaying device according to the invention the wicking rate of the first absorbent material is lower than the wicking rate of the second absorbent material. In a preferred embodiment of an assaying device according to the invention the first reagent is irreversibly immobilized on the first absorbent material. In a preferred embodiment of an assaying device according to the invention the first absorbent material and the second absorbent material each have planar surfaces. In a particularly preferred embodiment the planar surface of the first absorbent material is juxtaposed to the planar surface of the second absorbent material. A method for conducting a binding assay according to the invention is characterized in that it comprises: introducing a liquid sample into the first absorbent material of the assaying device according to the invention to generate a first flow of liquid within the first absorbent material which first flow of liquid splits into two flows when reaching the second absorbent material, thereby introducing the liquid from the first absorbent material into the second absorbent material to generate a second flow of liquid within the second absorbent material while maintaining the first flow of liquid within the first absorbent material and mobilizing the second reagent into the second flow of liquid within the second absorbent material, the second flow, after the mobilization of the second reagent, being in a direction generally parallel to the first flow of liquid within the first absorbent material; and introducing the second flow of liquid and the second reagent from the second absorbent material into the first absorbent material to unite with the first flow of liquid within the first absorbent material. In a preferred embodiment of a method according to the invention, the first flow of liquid within the first absorbent material flows at the same or greater velocity than the second flow of liquid within the second absorbent material. In a preferred embodiment of a method according to the invention, the first flow of liquid within the first absorbent material flows at a lesser velocity than the second flow of liquid within the second absorbent material. An embodiment of the invention will now be described with reference to the accompanying drawings, in which: FIG. 1A depicts a front view of the strip compartment of an assaying device without cover panel. FIG. 1B depicts a side section view of FIG.1A FIG. 1C depicts the cover panel of the assaying device. FIG. 1D depicts a perspective view of the test strip according to the present invention. FIG. 2 depicts a graph of colorimetric density vs. morphine concentration for an assay conducted using the apparatus of the present invention. FIG. 3 depicts a graph of colorimetric density vs. morphine concentration for an assay conducted using a nitrocellulose onto which reagents had been dispensed using an IVEK Digispense 2000™ system. FIG. 4 depicts a graph of colorimetric density vs. Tetrahydrocannabinoids (THC) concentration for an assay conducted using the apparatus of the present invention. FIG. 5 depicts a graph of colorimetric density vs. THC concentration for an assay conducted using a nitrocellulose onto which reagents had been dispersed using an IVEK Digispense 2000™ system. FIG. 6 depicts a graph of colorimetric density vs. THC concentration for an assay conducted using a nitrocellulose onto which reagents had been sprayed using a Paasche airbrush. FIG. 7 depicts a graph of colorimetric density vs. THC concentration for an assay conducted on nitrocellulose that had been sucrose glazed. FIG. 8 depicts a graph of colorimetric density vs. THC concentration for an assay conducted using a glazed nitrocellulose with latex in methyl cellulose solution applied over the sucrose glaze. The subject invention is described below in terms of its preferred embodiments. These embodiments are set forth to aid in understanding the invention, but are not to be construed as limiting. The apparatus and method of the present invention are particularly well suited for use in testing drugs of abuse, such as cocaine, amphetamines, cannabinoids, barbiturates, benzodiazepines, opiates, phencyclidines, propoxyphine, methaqualone, tricyclic antidepressants and methadone, as well as for clinical chemistry, pregnancy, and similar testing. As used throughout the specification, the terms "membrane," "porous carrier material" and "absorbent material" have been used interchangeably, and are to connote a material suitable for the assays described herein. To aid in understanding the invention, several positional terms have been used. The "top" and "main" membranes have been described with the terms "forward" and "rearward." "Forward" is the portion of a membrane which extends beyond the lengthwise midpoint (located in the direction of liquid flow within the membrane). The most forward portion of a membrane is referred to as the "terminal end." "Rearward" is opposite of forward and relates to the portion of the membrane which precedes the lengthwise midpoint in the direction of the end initially contacted by the liquid sample. The most rearward portion of the membrane is referred to as the "beginning end." "Downstream" is the direction in which the liquid flows within the membranes. In contrast, "upstream" relates to a direction opposite the flow of liquid within the membrane. A sample is absorbed at the beginning end of the main membrane and is transported along the length of the main membrane by capillary action. Generally, membrane length is in the range of 1-10 cm, width 0.4-2 cm, and thickness 0.1-1 mm. Typically, the membrane is shaped as an elongated, thin rectangular prism. When the sample flow reaches the reagent-containing top membrane (generally containing a labeled reagent), it splits into two flows: one continues to run in the main membrane, the second permeates into the top membrane, typically in the rearward portion, and mobilizes the reagent located within the top membrane. The reagent within the top membrane is mobile when in a moist state. Although for clarity a single reagent is described as being in the top membrane, it should be appreciated that multiple reagents may also be employed. Likewise, a plurality of top membranes each containing one or more reagents may be utilized. Preferred labels are colored colloidal particles, and colored latex is most preferred. Colored latex is readily visible to the naked eye when bound in the detection zone, therefore no additional developing procedures are required. Procedures for sensitization of latex with specific binding reagents are well known in the art (for example, see Illum, L. and P.D.E. Jones, Attachment of Monoclonal Antibodies to Microspheres, Methods in Enzymology , Vol. 112 , pages 67-84 (1985)) and Galloway, R.J., Development of Microparticle Tests and Immunoassays, Seradyn, Inc. , pages 6-31 (1988)). As alluded to above, multiple reagents may be utilized in one or more top membranes. As such, multiple latex containing membranes or one latex containing membrane sensitized with a plurality of binding reagents, for example antigens and/or antibodies, may be employed. The two parallel flows reunite and mix in the main membrane near or at the forward edge of the top membrane. While migration continues towards the detection site within the main membrane, a binding reaction between the reagent and the analyte takes place. The extent of this binding or simply the presence of the analyte, depending on whether the assay is a competition or a sandwich format, is measured at a detection site, where specific binding with the immobilized reagent occurs. Unbound material continues to migrate downstream towards the terminal end of the main membrane and is partially captured at the control site. No unbound labeled reagent remains in the detection or the control sites. On the main membrane there is a detection site where specific binding reagent (generally unlabeled) is immobilized (that is, the immobilized specific binding reagent cannot be washed away by a developing liquid). Downstream of the detection site there is a control site, where another binding reagent is immobilized. When assembled, the top membrane is in contact with the upper surface of the main membrane upstream of the detection site (see FIG 1D). Optionally, a sample pad can be used to absorb a liquid sample and release the liquid into the beginning end of the main membrane. The sample pad is typically fashioned from a blotting material, and is in contact with the main membrane. The sample pad may optionally contain additional reagents. Optionally, a "sink" pad, fashioned from a blotting material, can be used at the terminal end of the main membrane to absorb excess reagent solution and foster capillary flow in the main membrane for a longer period of time. The membranes and the pads are preferably disposed in a holder. The use of a sample pad and a sink pad are described in U.S. Patent Nos. 4,956,302 and 5,238,652. A skilled artisan having read the present specification would be able to use such pads in connection with the present invention. Examples of suitable top and main membranes include bibulous or fibrous material capable of capillary action, such as thin layer chromatography materials, paper or cellulose chromatography substrates, porous synthetic plastics, etc. Preferred absorbent materials have good wicking ability, a smooth surface which insures good contact between the "main" and "top" membranes, reversible binding of the labeled compound to the membrane to promote quantitative mobilization, ability to transport a labeled reagent by capillary action, availability of wicking characteristics within suitable range, reproducible and convenient reagent application and handling. Preferred membranes include nitrocellulose and nylon. Moreover, membranes precast on plastic are preferred because of handling convenience. Nitrocellulose is desirable because it (i) is able to bind proteins by adsorption without requiring complicated covalent linking, (ii) has excellent wicking characteristics, and (iii) is available in a convenient range of pore sizes. Backed nitrocellulose (for example, precast on a plastic material), is most preferred. Procedures for adsorption of specific proteins on nitrocellulose to form detection and control sites, and blocking of unoccupied binding sites on the nitrocellulose with neutral blocking reagent are well known in the art (see for example, Harvey, M.A., Optimization of Nitrocellulose Membrane-Based Immunoassays, Schleicher & Schuell , No. 557, pp. 18-23, 1991). Migration rates vary with membrane properties, for example, pore size. Selection of flow rate, appropriate membrane type, and pore size is within the knowledge of the skilled artisan, and can be determined with minimal experimentation. Because of the fluid dynamics within the top membrane, reagent (typically labeled) tends to concentrate at the forward edge of the top membrane and seep slowly into the main membrane where it mixes with the first flow. The extent of dilution has been postulated to be controlled by flow dynamics in the present invention rather than by the rate of reagent mobilization, as in known formats. Thus, the subject invention provides the advantage of relatively constant concentration of the top membrane reagent within the reunited flow which occurs downstream in the main membrane. This constant concentration of reagent is maintained as long as the reagent supply in the top membrane lasts. The rate of reagent dilution from the top membrane can be controlled by choosing an appropriate difference in wicking rates between the main membrane and the top membrane. "Wicking rate" refers to the speed of liquid movement within the membrane (distance/time). The top membrane can be made from the same material or other suitable porous carrier material. However, for convenience, the top and the main membranes are generally made from the same material. Such a choice of appropriate membrane materials is readily made by the skilled artisan having read the present specification. For instance, when a higher rate of dilution is required, reagent can be applied via a membrane having slower wicking properties, for example, by using a membrane having a smaller pore size. The wicking rate for the main membrane is preferably higher than or equal to the wicking rate of the top membrane. As a result, reagent from the top membrane permeates into the main membrane behind the liquid front in the main membrane, thus allowing the first flow to prewet the main membrane. Prewetting advantageously (i) facilitates uniform migration of the reagent from the top membrane, (ii) eliminates or at least significantly reduces negative effect of hydrophobicity due to unlabeled binding reagents at the detection site, and (iii) facilitates binding at the detection site. A labeled binding reagent is normally applied to a membrane which has previously been blocked with a blocking compound in order to avoid nonspecific binding of a labeled reagent to the membrane surface. The membrane can be cut and the test strips assembled in any suitable manner. Alternatively, the membrane may be pre-formed to certain dimensional specifications. For instance, it can be convenient to assemble large segments first by securing the top membrane on the surface of the main membrane, which is then cut into strips. The top membrane preferably is the same width as the main membrane, or is more narrow. The length of each membrane is determined empirically, but relatively short membranes with the dimensions just sufficient to accommodate the required amount of a binding reagent are preferred. Exact distance between the membrane and the detection site is determined by balancing the time required for proper mixing of the reagents and completion of a binding reaction, with the deterioration of uniform migration if the distance is too long. Such special parameters can be determined through minimal experimentation. Presently, a distance in the range from 0.5 cm to 5 cm is preferred. Position of the top membrane on the main membrane is upstream of the detection site, but downstream of the sample application site. Lamination of the main and top membranes further prevents occasional membrane flooding due to excess sample traveling outside of the membranes. If a flood wave of sample reaches the location where reagent is imbedded, it may completely distort the test result. This problem is particularly acute in the applications where the sample migrates downwards. Contact between the top and main membranes can be achieved in a number of ways. For instance, the main and the top membranes can be pressed against each other by housing elements, or may be laminated one to another, or both. Lamination is preferred because a reliable "tight" contact is achieved. Moreover, the laminate acts as a dam to prevent flooding. A preferred way to maximize contact between the membranes, involves laminating the top membrane over the main membrane in juxtaposition using a polyester film, such as the mylar tape described in the following example. Currently it is preferred for reagent from the top membrane not to cross the interface between the membranes to any appreciable extent, but rather to be transported completely inside the top membrane towards the forward edge. Figures 1A through 1D, depict a specific device which utilizes chromatographic principles to perform specific binding assays. Three porous carrier membranes testing three different analytes are shown in a plastic holder (102). Such a holder may form a portion of an assaying device of the type disclosed by European Patent Application with publication No. EP-A-0668745. Typically, plastic holder (102) will retain three to five porous carrier membranes. FIG. 1A shows three porous carrier membranes, whereas FIG. 1C shows windows for five porous carrier membranes. Main membrane (101) contains detection site (103) and control site (104). Labeled specific binding reagent (105) is embedded in top membrane (106) which contacts the upper surface of main membrane (101). Top membrane (106) is tightly attached to main membrane (101) with a polyester film with adhesive backing, e.g. with adhesive mylar tape (107). Contacting and preferably slightly overlapping, beginning end (108) of each main membrane (101) is sample pad (109). Sample contacts sample pad (109) through several small holes in the back side of the pad (not shown). A soft rubber seal (110) sits on the top of sample pad (109). The degree of softness needed is readily determined by the skilled artisan. Seal (110) is somewhat dimensionally larger than sample pad (109), so that it covers sample pad (109) and seals it along its borders when squeezed. Sealing is required to prevent leakage of the sample when the chromatographic membranes are installed vertically (sample migrates in a downwards direction). Of course, a seal (110) can also be used when the apparatus is used in other than a vertical position. Absorbent pad (111) at the terminal end (112) of the chromatographic membranes is common to all three membranes. Each detection site can be observed through a cross-shaped result window (113) and the corresponding control site through a circular control window (114) of the cover panel (115). The plastic of the cover panel is typically not transparent. The cross-shaped result window labeled with COC is for an assay for the detection of benzoylecgonine in urine. The cross-shaped result window labeled with THC, which stands for Tetrahydrocannabinoids, is for an assay for the detection of cannabinoids in urine. The cross-shaped result window labeled with MOR is for an assay for the detection of opiates in urine. The appearance of a blue bar in each of the round windows 114 labeled TEST COMPLETE indicates that the corresponding test has run to completion and that the results may be interpreted. These windows 114 expose an area of the membrane strip that is coated with anti-bovine serum albumin antibodies. Latex microparticles are overcoated with bovine serum albumin so that when they migrate past this area of the membrane the antibody binds to the bovine serum albumin and captures the microparticle. The present invention is particularly useful in a competition assay format. In one version of this format, the labeled reagent in the top membrane is either the analyte itself or an analog capable of the same type of specific binding. The labeled reagent is mobilized and diluted by a sample, and migrates with the sample towards the detection site, where unlabeled reagent capable of binding the analyte or its structural analog is immobilized. If analyte is present in a sample, it will compete with the labeled reagent for a limited number of binding sites in the detection zone, and the relative concentration of the analyte can be determined from the extent of reduction of binding of the labeled reagent. In another version, competition is time resolved. In this case, the labeled reagent is a binding partner of the analyte or of its structural analog. The labeled reagent is mobilized and diluted by a sample, and then migrates with the sample towards the detection site, where the unlabeled analyte or its analog is immobilized. If analyte is present in the sample, then during the course of migration it will bind to the labeled reagent. Remaining unsaturated binding sites of the labeled reagent can bind to the immobilized reagent in the detection site. Once again, concentration of the analyte can be determined from the extent of reduction of binding of the labeled reagent. For both versions of the competition format, the ratio of concentrations of binding components in migrating liquid should not fluctuate significantly over the dimensions of the labeled reagent moving zone. Also, uniformity should not decay over the term of migration to the detection site. Deviation in flow uniformity allows competition to fluctuate, and leads to poorly reproducible results. The present invention minimizes non-uniformity, and improves sensitivity by making the labeled reagent more dilute (as described below). Discrimination between zero analyte level and cut-off analyte level can be improved by increasing the total amount of labeled reagent and increasing the volume in which it is presented. As a result, analytical signal at zero drug level becomes stronger, while the signal at cut-off level is kept unchanged, the steeper calibration curve allows better discrimination. The present invention is also useful in a sandwich format. In this format, both the labeled reagent and the unlabeled immobilized reagent are the binding partners of the analyte (though they typically should bind to different epitopes of the analyte). Preferably both the labeled and immobilized unlabeled reagents are present in excess of the analyte. If analyte is present in the sample, then during the course of migration it will fully bind to the labeled reagent. In the detection site, analyte, which is already bound to the labeled reagent, will bind to the immobilized reagent, forming the sandwich. The analyte concentration is directly proportional to the binding at the detection site. Example 1 The following example of a morphine assay is provided to illustrate the invention. It must be pointed out, however, that the present invention is not limited to any particular substance. Morphine derivative - N-[3-[(7,8-Didehydro-4,5-epoxy-6-hydro-17-methyl-morphinan-3-yl)oxy]propoxy]-4-isothiocyanatobenzamide- was prepared as described in European Application with publication No. EP-A-0386644. Conjugation of this morphine derivative to bovine serum albumin (BSA) was performed as follows: To a solution of BSA (approximately 57 mg/mL) in 50 mM potassium phosphate, pH 8, (438 mL) (cooled in an ice bath), dimethylsulfoxide (DMSO) was added dropwise (188 mL). After warming to 25° C, a 5 mg/mL solution of the above mentioned derivative in DMSO (190 ml) was added dropwise. The reaction mixture was stirred at room temperature for 16 hours and then transferred to dialysis tubing and dialyzed first against 15 volumes of 20% DMSO - potassium phosphate buffer (50 mM, pH 8), second against 10% DMSO - potassium phosphate buffer, and third for four times more against potassium phosphate buffer. Colored uniform carboxylate-modified styrene microparticles, e.g. blue latex from Seradyn (0.3 micron particles), was first washed three times at 1% solids by centrifuging in 20 mM, pH 6.1 MES (2-[N-morpholino]ethanesulfonic acid). The washed latex was then adjusted to 5% solids in MES and (i) sensitized with 2 mg/mL anti-morphine monoclonal antibody for 16 hours at room temperature, (ii) blocked with BSA solution in MES for 1 hour at room temperature, then (iii) washed for three times at 1% solids in MES by centrifugation, and (iv) adjusted again to 5% solids. Before use, equal volumes of this latex and 35% w/v sucrose in MES were mixed. Large pore size nitrocelullose cast directly on polyester film, e.g. Mylar backed large pore size nitrocellulose (10-20 micron) from Millipore, was cut into pieces of 10 cm in length and 5 cm in width. Solution of morphine-BSA conjugate (about 5 mg/mL) and anti-BSA monoclonal Ab (about 2 mg/mL), both in 50 mM potassium phosphate buffer pH 7.5, were dispensed using a positive displacement liquid dispensing system, e.g. IVEK Corp. Digispense 2000™ system, at the rate 1 µl/cm onto nitrocellulose at a distance respectively 2 cm and 1 cm from the 10 cm side. Nitrocellulose segments were allowed to dry for about 30 min. at 37°C and then were blocked with 1% w/v polyvinyl alcohol (PVA, m.w. 13,000-23,000) solution in 20 mM Tris, pH 8, for 30 min. at room temperature. The segments were then rinsed in water and dried. Sample pads were prepared by cutting a cellulose filter paper, e.g. BioRad gel blotter, into 0.5 cm 2 pieces. The same cellulose filter paper was used for the sink pads. The same nitrocellulose as described above in this example was used as a separate membrane for latex (top membrane). For this purpose the nitrocellulose, which was previously blocked in the same manner as the main membrane, was cut into 5 mm wide strips and latex was applied using the IVEK dispensing system. After drying for 30 min. at 37°C, this membrane was placed, nitrocellulose surface down, onto the main membrane and laminated to the main membrane with a polyester film with adhesive backing, e.g. with Adhesive Research Inc. adhesive mylar. After this, the segment was cut into 5 mm wide strips, sample pad and sink pad placed respectively at the beginning and terminal ends of the strips, and the calibration curve obtained using solutions containing predetermined amounts of the drug (morphine sulfate). The optimal calibration curve (1.1 µl/cm of latex) is shown in FIG. 2. This curve, and the following curves shown in Figures 3 to 8, were obtained by determining the colorimetric density of the result bands using a tristimulus reflective color analyzer, e.g. Minolta CR-241 Chroma Meter. In the graphs shown in Figures 2 to 8, the vertical axis indicates values measured with Minolta CR-241 Chroma Meter multiplied by 1000. This is indicated by the label "Minolta reading x 1000". This Minolta readings are taken from the colorimetric density, Dx, derived from the X, Y, Z tristimulus values as determined by the CIE (Commission Internationale de l'Eclairage). The tristimulus values are derived from the Yxy color system established by the CIE in 1931, wherein Y is the lightness factor and x and y are the coordinates of the chromaticity diagram. The tristimulus value X is related to the tristimulus value Y by the equation X= Y(x/y). Dx, the colorimetric density of X, is derived from the equation Dx= - log 10 (Xm/Xo) where Xm= the measured X tristimulus value of the result band and Xo = the measured X tristimulus value of the light source. For each strip colorimetric density (D x ) in the red part of spectrum was measured at five points on the conjugate (or result) band, each point 1 mm apart from the neighboring one. Background colorimetric density level was measured off the result band. Average reading, standard deviation and %CV (percent coefficient of variation) were calculated. For comparison, latex was applied directly to the main membrane. Optimal calibration curves (0.5 µl/cm of latex) is shown in FIG. 3. When latex volume larger than 0.5 µl/cm was deposited onto the membrane, inhibition at cut-off concentration (300 ng/ml) was poor. Uniformity of the result band and control band can be characterized by color intensity variation for the five reading points. Table 1 presents %CV in control and result band intensities. Variation (%CV) in result and control band intensity Morphine standard concentration Fig. No. 0 ng/mL 75 ng/ml 150 ng/mL Control anti-BSA 2 8.0 9.3 15.6 5.3 3 24.1 17.1 151.9 22.7 Example 2 This example of a tetrahydrocannabinoid assay further illustrates the invention. Nitrocellulose membranes as described for this invention in Example 1 were prepared using: a tetrahydrocannabinoid-BSA conjugate prepared similarly to the morphine-BSA conjugate, but at a 20:1 drug derivative to BSA ratio; β-cyclodextrin incorporated into the sample pad; and blue latex sensitized with anti-tetrahydrocannabinoid monoclonal antibodies. Analysis of the colored results bands were performed as in Example 1. An optimal calibration curve of 3.0 µl/cm latex is shown in FIG. 4. FIG. 5 and FIG. 6 show optimal THC curves when the latex is applied directly to the main membrane (1 µl/cm) by the IVEK system and a compressed air liquid dispensing system or airbrush, e.g. a Paasche Company airbrush, respectively. Performance of the format, where latex is deposited over a sublayer of sucrose glaze (35% to 66% w/v sucrose in water) was also evaluated. An optimal curve with 60% sucrose sublayer and 2.0 µl/cm latex is presented FIG. 7. A different latex formulation, where latex solution was prepared in 1% methylcellulose, e.g. in 1% Methocel K4M™ methylcellulose (Dow Chemical Company) and 0.6% w/v PVA and applied directly on top of the sucrose sublayer, as was suggested in EP-A-0 291 194, was also evaluated (FIG. 8). More than half of the latex does not even begin to migrate in this case. The sucrose sublayer also slows migration of latex strongly, and migration is not uniform. Uniformity of the result band and control band can be characterized by color intensity variation for the five reading points. Table 2 presents %CV in control and result band intensities. Variation (%CV) in result and control band intensity THC standard concentration Fig. No. 0 ng/mL 12.5 ng/ml 25 ng/mL Control anti-BSA 4 15.5 7.9 12.5 5.3 5 21.4 33.1 37.2 22.7 6 40.8 2L5 52.7 44.3 7 31.8 31.2 54.8 66.0 8 21.6 86.6 22.9 22.6 As is clear from the results of the above two examples, the present apparatus offers superior results when compared with the systems of the prior art.

Claims: An apparatus for conducting a binding assay, which comprises: (a) a first absorbent material (101) having a first reagent immobilized thereon at a first predetermined location; and (b) a second absorbent material (106) in contact with and on top of the first absorbent material (101) and having a second reagent releasably immobilized thereon at a second predetermined location, the first absorbent material (101) and the second absorbent material (106) being positioned in juxtaposition to each other so that a flow of liquid through the first absorbent material (101) causes the second reagent to be released from the second absorbent material (106) and flow through the first absorbent material (101). The apparatus of claim 1, wherein the first absorbent material (101) and the second absorbent material (106) are composed of the same material. The apparatus of claim 1, wherein the first absorbent material (101) selected from the group consisting of bibulous membranes and porous membranes. The apparatus of claim 1, wherein the second absorbent material (106) is selected from the group consisting of bibulous membranes and porous membranes. The apparatus of claim 3, wherein the absorbent material (101) is nitrocellulose. The apparatus of claim 4, wherein the absorbent material (106) is nitrocellulose. An apparatus of claim 1, wherein the first absorbent material (101) and the second absorbent material (106) are held in contact with each other by holder. An apparatus of claim 1, wherein the first absorbent material (101) and the second absorbent material (106) are held in contact with each other by a band of material (107). An apparatus of claim 8, wherein the material (107) is a polyester film. An apparatus of claim 1, wherein the second reagent is labeled. An apparatus of claim 1, wherein the first absorbent material (101) has plurality of reagents immobilized thereon. An apparatus of claim 1, wherein the second absorbent material (106) has a plurality of reagents immobilized thereon. An apparatus of claim 10, wherein the label is a colored latex. An apparatus of claim 1, wherein the first reagent and the second reagent have an affinity to bind with each other. An apparatus of claim 1, wherein the wicking rate of the first absorbent material (101) is the same or higher than the wicking rate of the second absorbent material (106). An apparatus of claim 1, wherein the first reagent is irreversibly immobilized on the first absorbent material (101). An apparatus of claim 1, wherein the first absorbent material (101) and the second absorbent material (106) each have planar surfaces. An apparatus of claim 17, wherein the planar surface of the first absorbent material (101) is juxtaposed to the planar surface of the second absorbent material (106). An apparatus of claim 1, wherein the first reagent and the second reagent each have an affinity to bind with the same substance. A method for conducting a binding assay, which comprises: introducing a liquid sample into the first absorbent material (101) of the apparatus of one of claims 1 to 19 to generate a first flow of liquid within the first absorbent material (101) which first flow of liquid splits into two flows when reaching the second absorbent material (106), thereby introducing the liquid from the first absorbent material (101) into the second absorbent material (106) to generate a second flow of liquid within the second absorbent material (106) while maintaining the first flow of liquid within the first absorbent material (101), and mobilizing the second reagent into the second flow of liquid within the second absorbent material (106), the second flow, after the mobilization of the second reagent, being in a direction generally parallel to the first flow of liquid within the first absorbent material (101); and introducing the second flow of liquid and the second reagent from the second absorbent material (106) into the first absorbent material (101) to unite with the first flow of liquid within the first absorbent material (101). A method of claim 20, wherein the first flow of liquid within the first absorbent material (101) flows at the same or greater velocity than the second flow of liquid within the second absorbent material (106).

Inventors: KLIMOV ALEXEI D, TSAI SHIOW-CHUAN J, KLIMOV, ALEXEI D., TSAI, SHIOW-CHUAN J.
Applicants: HOFFMANN LA ROCHE, F. HOFFMANN-LA ROCHE AG
Application year: 1996
Main classification: G01N33
IPC codes: G01N33, G01N37