Validation of the Immunalysis Microplate ELISA for the Detection of Buprenorphine and Its Metabolite Norbuprenorphine in Urine

Transcription

1 Technical Note I Validation of the Immunalysis Microplate ELISA for the Detection of Buprenorphine and Its Metabolite Norbuprenorphine in Urine Eleanor I. Miller*, Hazel I. Torrance, and John S. Oliver Forensic Medicine and Science Department, University of Glasgow, University Place, Glasgow, G12 8QQ, Scotland Abstract The purpose of this study was to validate the Immunalysis Buprenorphine Microplate enzyme-linked immunosorbent assay (ELISA) for the detection of buprenorpbine in urine samples. Sixty-nine urine samples were obtained from volunteers on the Subutex treatment program and from routine samples submitted to the laboratory for buprenorphine testing. For ELISA analysis, samples were diluted 1:10 with K2HPO4 (0.1M, ph 7.0). The limit of detection was calculated as 0.5 ng/ml buprenorphine. The intra-assay and interday precision was 3.8% (n = 10) and 8.6% (n = 50) respectively at 1 ng/ml buprenorphine. At a low concentration of norbuprenorphine (1 ng/ml), the immunoassay demonstrated a cross-reactivity of 78%. A higher cross-reactivity of 116% was observed at a higher concentration of norbuprenorphine (10 ng/ml). Dihydrocodeine, codeine, tramadol, morphine, propoxypbene, methadone, and EDDP were tested at concentrations of 10 ng/ml and 10,000 ng/ml and demonstrated no cross-reactivity with the assay. For liquid chromatography-tandem mass spectrometry (LC-MS-MS), deuterated internal standard mixture, 1M acetate buffer (ph 5.0), and b-glucuronidase were added to the standards and samples, which were then incubated for 3 h at 60~ After incubation, 3 ml K2HPO 4 (0.1M, ph 6.0) was added and the ph altered to ph 6.0 using 1M KOH. Buprenorphine and norbuprenorphine were subsequently extracted by solid-phase. Twenty-one samples were confirmed positive and 48 samples were confirmed negative by LC-MS-MS. Using a cut-off value of 0.5 ng/ml buprenorphine, the immunoassay demonstrated a sensitivity and specificity of 100%. Introduction Buprenorphine, sold under the tradename Buprenex in the U.S. and Temgesic in Europe, came onto the market in the early 1980s as a prescription treatment for pain relief. It was also used in anesthesiology. It is currently prescribed under the tradename Subutex for * Authr)r t() wh(~m corresljr should be addres~,ed, some individuals as an alternative medication to methadone for the treatment of heroin addiction. High-dose tablets (0.4, 2, and 8 mg) are administered sublingually for this purpose. Subutex is widely prescribed in France, and it is also available in the U.S. and Europe. As a result of increasing black market availability, it is becoming more frequently misused. Buprenorphine-related fatalities have been linked to misuse by intravenous injection and high dosage ingestion. Deaths involving the combined use of Subutex and psychotropic drugs (in particular benzodiazepines) have been reported (1,2). Buprenorphine is a semi-synthetic opioid derivative of thebaine. It has a chemical structure that is closely related to other opiates such as morphine and has strong analgesic properties (25-40 times the potency of morphine) (3). As opioid-binding potential is related to analgesia, buprenorphine binds tightly to the p- and ~r receptors, acting as a partial agonist and antagonist, respectively (4). In na'fve users, buprenorphine demonstrates agonist activity, similar to morphine, whereas in addicts, it acts as an antagonist, producing withdrawal symptoms. Buprenorphine is rapidly metabolized in the liver by the cytochrome P450 CYP3A4 system and excreted in bile and urine. It is N-dealkylated to form its pharmacologically active metabolite, norbuprenorphine. Both drugs form glucuronide conjugates with glucuronic acid. Therapeutic buprenorphine urine levels can be tess than ] ng/ml, requiring sensitive methods of detection (3,5,6). However, concentrations greater than 20 ng/ml have been observed in cases of buprenorphine abuse (5-7). Immunoassay is a fast, automated, and sensitive technique that can be used to screen large numbers of unknown samples prior to confirmation with a highly sensitive and specific technique such as liquid chromatography-tandem mass spectrometry (LC-MS-MS). In the past, radioimmunoassay has been used to screen for buprenorphine in urine (8). However, ELISA microplate systems are currently being used for this purpose and offer a safer, more versatile alternative to radioimmunoassay (5,6). Immunoassay kits consist of microplate wells that are coated with an antibody that has a high affinity for the target analyte. Reproduction (photocopying)of editorial content of this journal is prohibited without publisher's permission. 11 5

2 Journal of Analytical Toxicok~gy, Vol. 30, March 2006 These antibodies display cross-reactivity with related drugs within a drug class. A competitive binding process occurs between the enzyme-labelled drug and the drug in the sample for the antibody binding sites on the wells. The wells are washed after incubation with the enzyme conjugate reagent to remove any unbound sample or residual reagent remaining in the wells. This washing step removes any background before development of the substrate color, effectively improving the assay sensitivity. The addition of stop reagent to stop the catalytic reaction produces a yellow color, which shows greater sensitivity and a better separation of absorbance values than the blue color produced by the substrate. The intensity of color produced is inversely proportional to the concentration of drug in the sample. The Immunalysis Microplate ELISA test has recently become commercially available for the qualitative and semi-quantitative determination of buprenorphine in urine samples. The aim of this study was to validate this assay or buprenorphine detection in urine. Experimental Samples Sixty-nine urine samples were obtained from volunteers on the Subutex treatment program and from routine samples submitted to the lab for buprenorphine testing. The volunteers gave informed consent for collection and testing of the samples. All samples were stored in a refrigerator at 4~ prior to analysis. Apparatus Microplate enzyme immunoassay A Miniprep 75 automatic pipettor, purchased from Tecan (San Jose, CA), was used to dilute samples and to pipette all calibrators and samples into the microplate wells. The plates were washed using a Columbus Plus washer system (Tecan, Gr6dlg, Austria) and read at a wavelength of 450 nm using a Sunrise Remote EIA autoreader from Tecan. Disposable borosilicate glass culture tubes (75 x 12 ram, VWR International, Poole, U.K.) were used for the samples and dilutes. Buprenorphine Direct ELISA kits were purchased from the Immunalysis Corporation (Pomona, CA). The kits contained a 96-well antibodycoated microplate, buprenorphine conjugate labelled with horseradish peroxidase, substrate solution containing 3,3'5,5'- tetramethylbenzidine (TMB), and stop solution containing 1N hydrochloric acid. Phosphate buffer (K~HPO4, 100raM, ph 7.0) was also purchased from Immunatysis. LC-MS-MS All urine samples screened by the ELISA test were confirmed using LC-MS-MS. This consisted of a Surveyor high-performance liquid chromatography (HPLC) system with an LCQ Advantage ion trap MS. HPLC was performed on a Gemini C18 column (150 mmx 2.0 ram, 5-1Jm particle size), fitted with a guard column with identical packing material (4 mm x 2.0 ram, Phenomenex, Torrance, CA). Chemicals and reagents Methanol, acetonitrite, acetone, dichloromethane, and ethyl acetate were all HPLC grade and purchased from BDH (Poole, U.K.). Glacial acetic acid, ammonium hydroxide, formic acid, and potassium dihydrogen phosphate were also purchased from BDH. Ammonium formate and [3-glucuronidase crude solution was obtained from Sigma-Aldrich (Dorset, U.K.). The buprenorphine and norbuprenorphine stock drug standards, used in the preparation of spikes for LC-MS-MS confirmation, were purchased as 0.1 mg/ml methanolic solutions from LGC Promochem (Teddington, U.K.). The internal standards used in LC-MS-MS quantitation of positive ELISA samples were buprenorphine-d4 and norbuprenorphine-d3, purchased as 0.1 mg/ml methanolic solutions from LGC Promochem. Drugs used to test the cross-reactivity of the buprenorphine kit were also purchased from LGC Promochem. Bond Elut Certify LRC columns (130 mg, 10 ml) were produced by Varian and purchased from Crawford Scientific (Strathaven, Scotland). Solutions For 1M sodium acetate buffer (ph 5.0), 42.9 g of sodium acetate trihydrate and 10.4 ml of glacial acetic acid were dissolved in 400 ml of deionized water, the ph was adjusted to 5.0 with 1M acetic acid, and the solution was made up to 500 ml with deionized water. For 0.1M phosphate buffer (ph 6.0), 6.8 g of potassium dihydrogen phosphate was dissolved in 400 ml deionized water, the ph was adjusted to 6.0 using 1M potassium hydroxide solution, and the solution was made up to 500 ml with deionized water. For 3raM ammonium formate and 0.001% formic acid, g of ammonium formate and 10 IJL of concentrated formic acid were made up to 1 L with deionized water. For 0.01M acetic acid (ph 3.0), 286 IJL of 17M glacial acetic acid was placed in a 500-mL volumetric flask and made up to volume with deionized water. Methods ELISA procedure Each ELISA run contained a set of calibrators consisting of a blank and spiked urine samples at a concentration of 0.5, 1, and 5 ng/ml buprenorphine (cut-off value 0.5 ng/ml buprenorphine). The assay is directed towards buprenorphine and cross-reacts 100% with it. These three spiked calibrators were used to construct the semiquantitative curve used to calculate the sample concentrations noted in Table I. A high positive control (10 ng/ml) and a negative control provided with the kit were used to check kit performance. These were distributed to wells at the beginning and end of the plate. Each calibrator, control, and sample was diluted on-line with phosphate buffer (1:10), vortex mixed, and then distributed to the microplate wells. In the ELISA screening procedure for buprenorphine in urine case samples, 20 I~L of diluted calibrator, control, or sample was added to the wells in duplicate. After pipetting all of the sampies, 100 I~L of buprenorphine enzyme conjugate reagent was 116

3 added to each well. The plate was then left in the dark at room temperature for an incubation period of 1 h. Following incubation, the microplate wells were washed six times with deionized water in order to remove any unbound sample or residual conjugate reagent remaining in the wells. TMB substrate (100 t~l) reagent was added to the wells, and the plate was left to incubate in the dark at room temperature for an additional 30 rain. After 30 min, the reaction was stopped by adding 100 IJL of stop reagent to each well. The contents of the wells turned yellow following addition of the stop reagent, and this was to enable detection of the chromophore at 450 nm. A reference wavelength of 650 nm was used for background correction. The absorbance readings were inversely proportional to the concentration of buprenorphine and/or norbuprenorphine present in the sample. LC-MS-MS procedure Urine hydrolysis. Standards were prepared in 1 ml of blank urine at 5, 10, 25, 50, 100, and 200 ng of buprenorphine and norbuprenorphine. A drug-free urine sample containing buprenorphine-d4 and norbuprenorphine-d3 was also prepared, along with a blank with no internal standard. One milliliter of acetate buffer (ph 5.0, 1M) was added to t ml of urine sample along with 40 I~L of [3-glucuronidase crude solution (Helix pomatia). The samples were placed in an oven for 3 h at 60~ and after cooling, 3 ml of phosphate buffer (ph 6.0, 0.1M) was added to each sample. The ph of each sample was adjusted to ph 6.0 using 1M potassium hydroxide. The samples Table I. Immunalysis Buprenorphine Microplate ELISA and LC-MS-MS Positive Urine Results Immunalysis Microplate ELISA Buprenorphine Equivalents LC-MS-MS Sample Concenlration Buprenorphine Norbuprenorphine number (ng/ml) (ng/ml) (ng/ml) Source 1 > Routine 2 > 5 < LOQ 7 Routine 3 > Routine 4 > Routine Routine < LOQ NEG Routine NEG Routine 8 > Routine < LOQ NEG Routine 10 > 5 12 NEG Routine II 0.77 < LOQ NEG Routine 12 > Routine 13 > Crisis 14 > Crisis 15 > Crisis 16 > Crisis NEG Routine Routine 19 > Crisis 20 > Crisis 21 > Crisis were then centrifuged at 2500 rpm for 10 rain. Extraction procedure. Buprenorphine and norbuprenorphine were extracted using Bond Elut Certify LCR cartridges. The cartridges were conditioned using 2 ml methanol, followed by 2 ml phosphate buffer (ph 6.0, 0.1M). The centrifuged samples were applied to the cartridges and allowed to drip through without a vacuum. The cartridges were washed with 1 ml deionized water followed by 0.5 ml acetic acid (ph 3.3, 0.0IM) and allowed to dry under full vacuum for 10 rain. Methanol (50 IJL) was then added to each cartridge, and the cartridges were dried under full vacuum for 2 rain. Finally, the cartridges were washed with 4 ml acetone/dichloromethane (1:1, v/v). The analytes were eluted using ml ethyl acetate with 2% ammonium hydroxide. Tile extracts were blown down under nitrogen at 30~ and reconstituted in 80 IJL of mobile phase. Chromatographic method. The column was maintained at 35~ The mobile phase consisted of 3mM ammonium formate % formic acid in water (A) and acetonitrile (B) at a flow rate of 0.3 mumin. The initial gradient conditions were 75% A, decreasing to 20% A after 15 rain. The final composition of 10% A was reached after a further 2 rain and held for 5 min, then initial conditions were restored and allowed to equilibrate for 5 rain. The total run time was 27 rain. A divert valve taking LC flow to waster was used for the first 3 rain and for the last 7 min of the run to preserve the MS source. Norbuprenorphine eluted from the column at 14.7 rain, and buprenorphine eluted at 23.1 min. MS conditions. All mass spectral data was collected in electrospray positive ion mode. The following settings were applied throughout the analysis: transfer capillary temperature, 280~ probe voltage, 4.5 kv; and sheath and auxiliary gases (nitrogen) were set to 20 and 15 units on the data system arbitrary scale. Internal standard data was collected in selected ion monitoring (SIM) mode and analytes were identified on the basis of their full MS-MS spectra. Buprenorphine was identified by the transition of the precursor ion m/z 468 to the product ion m/z 414 at 36% collision energy; norbuprenorphine was identified by the transition of the precursor ion m/z 414 to the product ion m/z 396 at 32% collision energy. ELISA Limit of detection (LOD) and assay precision. The LOD was calculated from the mean absorbance value for the 48 confirmed negative samples using equation 1. LOD = Ao - 3~ Eq. 1 where Ao is the mean absorbance value and is the standard deviation of the absorbance values. The intra-assay precision of the absorbance readings was calculated by spiking 10 drug-free 117

4 urine samples at 1 ng/ml buprenorphine. These were tested in singlicate on the same day and on the same plate. This process was carried out on five separate days, and an interassay precision of the absorbance readings was calculated. Sensitivity and specificity. The sensitivity and specificity of the assay were calculated by applying equations 2 and 3, respectively, which require the total number of true positives (TP), true negatives (TN), and false positives (FP) to be counted. Sensitivity = (TP x 100)/(TP + FN) Eq. 2 Specificity = (TN x 100)/(TN + FP) Eq. 3 A true positive sample produced both positive screening and confirmation results. A true negative sample produced both negative screening and confirmation results. Although there were no false-positive or false-negative results found in this particular study, a false positive would produce a positive screening and negative confirmation result. Conversely, a false negative would produce a negative screening and positive confirmation result. Cross-reactivity. According to the ELISA kit package insert provided by the manufacturers, norbuprenorphine is the only related drug which cross-reacts with the buprenorphine kit. Blank urine and urine spiked at concentrations of 0.5, 1, and 5 ng/ml buprenorphine and 1, 5, and 10 ng/ml norbuprenorphine was prepared to determine the cross-reactivity of norbuprenorphine at 1 and 10 ng/ml. Other opiate and opioid drugs commonly encountered in our laboratory were tested at concentrations of 10 ng/ml and 10,000 ng/ml drug-free urine. Results As part of the validation for the ELISA screening test, a dose-response curve was generated for drug-free urine spiked at buprenorphine concentrations of 0.5, 1, 10, 25, 50, and 100 ng/ml (Figure 1). The B/Bo (%) values were calculated where B is the absorbance value of bound calibrator and B0 is the absorbance value of the blank calibrator. The optical density values showed a hyperbolic decrease with increasing buprenorphine concentration. This is because the higher to0 20,... lo Concentration (nlvml,) Figure I. ELISA dose-response curve for buprenorphine concentrations ranging from 0.5 to 100 ng/mk 120 the drug concentration in the sample, the less enzyme conjugate binds to the antibody sites, producing a lower optical density value. ELISA LOD and assay precision The LOD of the ELISA assay was calculated to be 0.5 ng/ml urine using equation 1. The intra-assay precision of the absorbance reading for drug-free spiked urine at 1 ng/ml (n = 10) was 3.8%. The interassay precision (n = 50) was determined as 8.6% at 1 ng/ml on five separate days. Cross.reactivity The cross-reactivity values for norbuprenorphine are shown in Table It. The results found were similar to the values provided by the manufacturers in the package inserts (80% and 120% at I and 10 ng/ml norbuprenorphine, respectively). The compounds tested at concentrations of i0 and 10,000 ng/ml did not produce absorbance values in the assay that were equal to or greater than the assay sensitivity ]eve] of 0.5 ng/ml and, therefore, did not cross-react with the immunoassay. The cross-reactivity of the drugs tested is given in Table II. Case sample results Of the urine samples analyzed, 21 of the ELISA results were true positives and 48 were true negatives. There were no falsepositive or false-negative results. The sensitivity and specificity of the Immunalysis Buprenorphine Microplate ELISA, using a cut-off value of 0.5 ng/ml buprenorphine in urine, is 100% for both. Buprenorphine and norbuprenorphine concentrations in positive urine samples ranged from ] to 7550 ng/ml buprenorphine and 7 to 1931 ng/ml for norbuprenorphine. The results are shown in Table I. Discussion The sensitivity and specificity of the Immunalysis Buprenorphine Microplate was 100% (i.e., 100% true results were obtained). A study by Cirimele et al. (5) tested 136 urine samples Table il. Cross-Reactivity of the Immunalysis Buprenorphine Microplate ELISA Concentration % Cross- Compound (ng/mt) Reactivity Norbuprenorphine 1 78 Norbuprenorphine Dihydrocodeine 10 and 10,000 - Codeine 10 and 10,000 - Tramadol 10 and 10,000 - Morphine 10 and 10,000 - Propoxyphene I0 and I0,000 - Methadone I0 and I0,000 - EDDP 10 and 10,

5 using the One-Step TM ELISA test at the same cut-off value used in this study (0.5 ng/ml buprenorphine). The authors found that this test produced 73.5% true results, that is, 68.4% true positives, 5.1% true negatives, 26.5% false positives, and no false negatives using LC-MS as the confirmation technique. The concentrations obtained by the ELISA test for samples 5, 7, 17, and 18 were lower than those obtained by LC-MS-MS. Apart from ELISA being a semiquantitative test, a possible reason for this could be that the ELISA kit does not cross-react with buprenorphine and norbuprenorphine conjugates that may have been present in these particular samples. The ELISA extraction, unlike the LC-MS-MS extraction, did not involve a hydrolysis step. The ELISA assay was not performed after hydrolysis of these samples because a fast extraction method is desirable for the ELISA test as it is only used for screening purposes and the samples screened as positive. Buprenorphine concentrations in samples 2, 6, 9, and 11 could not be quantified by LC-MS-MS. The LC-MS-MS concentrations found for these samples were higher than the LOD (0.4 ng/ml) but lower than the LOQ (1.3 ng/ml). Conclusions The Immunalysis Buprenorphine Microplate ELISA kit is a sufficiently sensitive and specific test for buprenorphine screening in urine using a cut-off concentration of 0.5 ng/ml. The kit demonstrated appropriate cross-reactivity with its metabolite norbuprenorphine. Twenty-one urine samples tested positive at this cut-off concentration, showing that the kit can detect buprenorphine concentrations generally observed after therapeutic administration and in abuse situations. Acknowledgments The authors would like to thank Dr. Gilhooly, Nomaxhosa Mdladlanaa, Dorothy Roarty, and patients at the Turning Point Drugs Crisis, Glasgow for providing samples for this study. It was greatly appreciated. References I. A. Tracqui, P. Kintz, and 8. Ludes. Buprenorphine-related deaths among drug addicts in France: a report on 20 fatalities. J. Anal. Toxicol. 22: (I 998). 2. P. Kintz. Death involving buprenorphine: a compendiurn of French cases. Forensic Sci. Int. 121:65-69 (2001). 3. R.C. Baselt and R.H. Cravey. Disposition of Toxic Drugs and Chemicals in Man, 4th ed. Chemical Toxicology Institute, Foster City, CA, 1995, p M.J. Mycek, R.A. Harvey, and P.C. Champe. Lippincott's Illustrated Reviews: Pharmacology, 2nd ed. Lippincott Williams & Wilkins, Philadelphia, PA, V. Cirimele, S. Etienne, M. Villain, B. Ludes, and P. Kintz. Evaluation of the One-Step TM ELISA kit for the detection of buprenorphine in urine, blood and hair specimens. Forensic Sc'L Int. 143: (2004). 6. V. Cirimele, P. Kintz, S. Lohner, and'b. Ludes. Enzyme immunoassay validation for the detection of buprenorphine in urine. J. Anal. Toxicol. 27: 103-I 05 (2003). 7. F. Vincent, J. Bessard, J. Vacheron, M. Mallaret, and G. Bessard. Determination of buprenorphine and norbuprenorphine in urine and hair by gas chromatography-mass spectrometry. ]. Anal. Toxicol. 23: (1999). 8. L. Debrabandere, M. Van Boven, and P. Daenens. Analysis of buprenorphine in urine specimens. J. Forensic Sci. 37:82-89 (1992). Manuscript received August I, 2005; revision received September 30,

6 Journal of Analytical Toxicology, Vol. 30, September 2006 ATTENTION AUTHORS Submit your manuscripts ONLINE to the Benefits include: Quicker turnaround time Instant acknowledgment of your submission Ability to track status Web-based peer review Visit Erratum The authors of the Technical Note Validation of the Immunalysis Microplate ELISA for Detection of Buprenorphine and Its Metabolite Norbuprenorphine in Urine [E.I. Miller, H.J. Torrance, and J.S. Oliver. J. Anal. Toxicol. 30(2): (2006)] wish to make a correction in their data. The errors occurred in Table I and the Case sample results section. The corrected data and text appear here. Case sample results Of the urine samples analyzed, 21 of the ELISA results were true positives and 48 were true negatives. There were no falsepositive or false-negative results. The sensitivity and specificity of the Immunalysis Buprenorphine Microplate ELISA, using a cut-off value of 0.5 ng/ml buprenorphine in urine, are both 100%. Concentrations in positive urine samples ranged from 1 to 1931 ng/ml for buprenorphine and 7 to 7550 ng/ml for norbuprenorphine. The results are shown in Table I. Table I. Immunalysis Buprenorphine Microplate ELISA and LC MS MS Positive Urine Results Author Guidelines can be found at Campbell Science Corporation South Bluff Rd., Rockton, IL USA Phone (815) Fax (815) campsup@aol.com A complete product listing is available at our Website Gas Chromatography Reagents Acylation Reagents, Silylation Reagents, Chiral and Specialty Reagents, Alkylation Reagents, Silylation Grade Solvents, Siliconizing Fluids. New reagents include: 4-CB, MBHFBA, β-glucuronidase Biochemical Products Amino Acids, Amino Acid Derivatization and Detection, Antibiotics, Biotinylation Reagents, Buffers, Co-Factors, High Purity Detergents (CHAPS, CHAPSO), Inhibitors, Iodination Reagents, Molecular Biology Reagents, Protein Modifiers, Substrates, Vitamins, Cross-Linking Reagents including: Electrophoresis Cross-Linkers, Heterobifunctional Cross-Linkers, Homobifunctional Cross-Linkers, Imidoesters, PEG Cross-Linkers, Photoreactive Phenyl Azides Immunalysis Microplate ELISA Buprenorphine Equivalents LC MS MS Bupre- Norbupre- Sample Concentration norphine norphine Number (ng/ml) (ng/ml) (ng/ml) Source 1 > Routine 2 > 5 < LOQ 7 Routine 3 > Routine 4 > Routine Routine < LOQ NEG Routine NEG Routine 8 > Routine < LOQ NEG Routine 10 > 5 12 NEG Routine < LOQ NEG Routine 12 > Routine 13 > Crisis 14 > Crisis 15 > Crisis 16 > Crisis NEG Routine Routine 19 > Crisis 20 > Crisis 21 > Crisis 4A