Standards of laboratory practice: cardiac drug monitoring. National Academy of Clinical Biochemistry

In this Standard of Laboratory Practice we recommend guidelines for therapeutic monitoring of cardiac drugs. Cardiac drugs are primarily used for treatment of angina, arrhythmias, and congestive heart failure. Digoxin, used in congestive heart failure, is widely prescribed and therapeutically monitored. Monitoring and use of antiarrhythmics such as disopyramide and lidocaine have been steadily declining. Immunoassay techniques are currently the most popular methods for measuring cardiac drugs. Several reasons make measurement of cardiac drugs in serum important: their narrow therapeutic index, similarity in clinical complications and presentation of under- and overmedicated patients, need for dosage adjustments, and confirmation of patient compliance. Monitoring may also be necessary in other circumstances, such as assessment of acetylator phenotypes. We present recommendations for measuring digoxin, quinidine, procainamide (and N-acetylprocainamide), lidocaine, and flecainide. We discuss guidelines for measuring unbound digoxin in the presence of an antidote (Fab fragments), for characterizing the impact of digoxin-like immunoreactive factor (DLIF) and other cross-reactants on immunoassays, and for moni-toring the unbound (free fraction) of drugs that bind to alpha1-acid glycoprotein. We also discuss logistic, clinical, hospital, and laboratory practice guidelines needed for implementation of a successful therapeutic drug monitoring service for cardiac drugs.

Na+-dependent nucleoside transport in liver: two different isoforms from the same gene family are expressed in liver cells

Hepatocytes show a Na+-dependent nucleoside transport activity that is kinetically heterogeneous and consistent with the expression of at least two independent concentrative Na+-coupled nucleoside transport systems (Mercader et al. Biochem. J. 317, 835-842, 1996). So far, only a single nucleoside carrier-related cDNA (SPNT) has been isolated from liver cells (Che et al. J. Biol. Chem. 270, 13596-13599, 1995). This cDNA presumably encodes a plasma membrane protein responsible for Na+-dependent purine nucleoside transport activity. Thus, the liver must express, at least, a second nucleoside transporter which should be pyrimidine-preferring. Homology cloning using RT-PCR revealed that a second isoform is indeed present in liver. This second isoform turned out to be identical to the 'epithelial-specific isoform' called cNT1, which shows in fact high specificity for pyrimidine nucleosides. Although cNT1 mRNA is present at lower amounts than SPNT mRNA, the amounts of cNT1 protein, when measured using isoform-specific polyclonal antibodies, were even higher than the SPNT protein levels. Moreover, partially purified basolateral plasma membrane vesicles from liver were enriched in the SPNT but not in the cNT1 protein, which suggests that the subcellular localization of these carrier proteins is different. SPNT and cNT1 protein amounts in crude membrane extracts from 6 h-regenerating rat livers are higher than in the preparations from sham-operated controls (3.5- and 2-fold, respectively). These results suggest that liver parenchymal cells express at least two different isoforms of concentrative nucleoside carriers, the cNT1 and SPNT proteins, which show differential regulation and subcellular localization.

A case of nondigitalis cardiac glycoside toxicity

A case is presented of cardiac glycoside poisoning in a 1-year-old patient from the plant Nerium oleander (common oleander). The patient had bradycardia, vomiting, altered level of consciousness, and no history of ingestion. Antibody-based digoxin assays may cross-react with other cardiac glycosides nonquantitatively. Chromatographic techniques can be used in the specific diagnosis.

Implementation of a successful on-call system in clinical chemistry

Successful practice of clinical pathology depends on a wide variety of laboratory, clinical, and managerial decisions. The skills needed to make these decisions can most effectively be learned by residents and fellows in pathology using a service-oriented on-call approach. We report our experience implementing an on-call system in the clinical chemistry laboratory at the University of Louisville Hospital (Ky). We detail the guidelines used to establish this system and the elements required for its successful implementation. The system emphasizes a laboratory-initiated approach to linking laboratory results to patient care. From inception of the program during late 1990 through 1995, the number of beeper calls (including clinician contacts) steadily increased and is currently 8 to 20 per week. The on-call system is active 24 hours per day, 7 days per week, thus representing activity on all three laboratory shifts. Types of responses were separated into administrative (12%), analytical (42%), clinical (63%), quality control or quality assurance (12%), and consultation (13%) categories. We also present 6 case reports as examples demonstrating multiple elements in these categories. In 23% of the calls, clinician contact was required and achieved by the fellow or resident on call for the laboratory. The on-call reports are documented and presented informally at weekly on-call report sessions. Emphasis is placed on learning and refinement of investigative skills needed to function as an effective laboratory director. Educational emphasis for the medical staff is in establishing awareness of the presence of the laboratory as an important interactive component of patient care. In addition, we found this program to be beneficial to the hospital and to the department of pathology in fulfilling its clinical service and teaching missions. Our experience may be helpful to other institutions establishing such a program.

Pharmacogenetics: a laboratory tool for optimizing therapeutic efficiency

Pharmacogenetics is the study of the linkage between an individual's genotype and that individual's ability to metabolize a foreign compound. Differences in metabolism of therapeutics can lead to severe toxicity or therapeutic failure by altering the relation between dose and blood concentration of the pharmacologically active drug. Phenotypes exhibiting poor and ultraextensive metabolism result from genetic variance (polymorphism) of enzymes involved in metabolism. Thus, in pharmacogenetic studies one applies genotyping of polymorphic alleles encoding drug-metabolizing enzymes to the identification of an individual's drug metabolism phenotype. This knowledge, when applied to dosing or drug selection, can avoid adverse reactions or therapeutic failure and thus enhance therapeutic efficiency. More than 25 commonly prescribed medicines are metabolized by the cytochrome P-4502D6 (CYP2D6) isoenzyme, and polymorphism of the CYP2D6 gene affects the therapeutic management of up to 17% of individuals in some ethnic groups. In this review, we summarize and update information concerning drug-metabolizing genotypes with emphasis on CYP2D6 genotyping techniques that can be applied by the clinical laboratory for linking human genetics to therapeutic management.

Inhibition of Na,K-ATPase by oleandrin and oleandrigenin, and their detection by digoxin immunoassays

Ingestion of oleander plant, containing the cardiac glycoside oleandrin, has been reported to induce fatal poisonings. Derivatives of oleandrin are structurally similar to digoxin. We investigated the cross-reactivities of oleandrin and its aglycone metabolite, oleandrigenin, in several commercially available digoxin immunoassays; assessed their ability to inhibit Na,K-ATPase catalytic activity; and measured their binding to proteins in serum. As assayed with ACS:180, Stratus, RIA, On-Line, and TDx digoxin assays, oleandrin at 100 micromol/L in digoxin-free serum gave apparent digoxin values of 0, 0.83, 2.24, 2.37, and 5.34 nmol/L, respectively, whereas oleandrigenin at that concentration gave results of 0, 0.52, 0.77, 4.94, and 1.40 nmol/L. Study of Na,K-ATPase inhibition showed IC50 values (micromol/L) of 0.22 for ouabain, 0.62 for oleandrin, 1.23 for oleandrigenin, and 2.69 for digoxin. At 25 degrees C, 96% of oleandrin and 48% of oleandrigenin were bound to serum proteins. Because detection of oleandrin and oleandrigenin by digoxin immunoassays is variable between assays as well as between congeners, assessment of cross-reactivity is warranted for each assay. The inhibition of Na,K-ATPase by oleandrin and oleandrigenin confirms that they likely exert their toxic effects through inhibition of sodium pump activity. In cases of digitalis-like poisoning with suspicion of oleander ingestion, a combination of digoxin immunoassays may be useful to effectively rule out the presence of oleander.

Inhibition of Na,K-ATPase by oleandrin and oleandrigenin, and their detection by digoxin immunoassays

Ingestion of oleander plant, containing the cardiac glycoside oleandrin, has been reported to induce fatal poisonings. Derivatives of oleandrin are structurally similar to digoxin. We investigated the cross-reactivities of oleandrin and its aglycone metabolite, oleandrigenin, in several commercially available digoxin immunoassays; assessed their ability to inhibit Na,K-ATPase catalytic activity; and measured their binding to proteins in serum. As assayed with ACS:180, Stratus, RIA, On-Line, and TDx digoxin assays, oleandrin at 100 micromol/L in digoxin-free serum gave apparent digoxin values of 0, 0.83, 2.24, 2.37, and 5.34 nmol/L, respectively, whereas oleandrigenin at that concentration gave results of 0, 0.52, 0.77, 4.94, and 1.40 nmol/L. Study of Na,K-ATPase inhibition showed IC50 values (micromol/L) of 0.22 for ouabain, 0.62 for oleandrin, 1.23 for oleandrigenin, and 2.69 for digoxin. At 25 degrees C, 96% of oleandrin and 48% of oleandrigenin were bound to serum proteins. Because detection of oleandrin and oleandrigenin by digoxin immunoassays is variable between assays as well as between congeners, assessment of cross-reactivity is warranted for each assay. The inhibition of Na,K-ATPase by oleandrin and oleandrigenin confirms that they likely exert their toxic effects through inhibition of sodium pump activity. In cases of digitalis-like poisoning with suspicion of oleander ingestion, a combination of digoxin immunoassays may be useful to effectively rule out the presence of oleander.

Evidence for presence of a reduced form of digoxin-like immunoreactive factor (dihydro-DLIF) in mammalian tissues

Digoxin-like immunoreactive factor (DLIF) from adrenal glands is an endogenous ligand structurally related to the plant-derived cardiac glycoside digoxin. Cardiac glycosides regulate the activity of the sodium pump and thus play key roles in disease processes involving regulation of ion transport. We now report the discovery of an endogenous dihydro-DLIF analogous to dihydrodigoxin. We used HPLC, ultraviolet spectrophotometry, and cross-reactivity with two antibodies, one specific for digoxin and one for dihydrodigoxin, to support the hypothesis that dihydro-DLIF contains a chemically reduced lactone ring. The spectral absorbance maximum for dihydro-DLIF is at 196 nm, identical to dihydrodigoxin. DLIF and dihydro-DLIF are 975- and 2588-fold less immunoreactive than digoxin and dihydrodigoxin for their respective antibodies. The molar ratio of dihydro-DLIF to DLIF is approximately 5.3 in bovine adrenocortical tissue and approximately 0.38 in human serum. Dihydrodigoxin (reduced lactone ring) added to microsomes isolated from bovine adrenal cortex produced a 4.5-fold increase in digoxin-like immunoreactivity (oxidized lactone ring) after 3 h of incubation. The biotransformation is likely mediated by a cytochrome P-450 NADPH-dependent process. Our findings demonstrate the presence of a dihydro-DLIF in mammals and suggest a metabolic route for synthesis of endogenous DLIF in mammalian tissue.

Evidence for presence of a reduced form of digoxin-like immunoreactive factor (dihydro-DLIF) in mammalian tissues

Digoxin-like immunoreactive factor (DLIF) from adrenal glands is an endogenous ligand structurally related to the plant-derived cardiac glycoside digoxin. Cardiac glycosides regulate the activity of the sodium pump and thus play key roles in disease processes involving regulation of ion transport. We now report the discovery of an endogenous dihydro-DLIF analogous to dihydrodigoxin. We used HPLC, ultraviolet spectrophotometry, and cross-reactivity with two antibodies, one specific for digoxin and one for dihydrodigoxin, to support the hypothesis that dihydro-DLIF contains a chemically reduced lactone ring. The spectral absorbance maximum for dihydro-DLIF is at 196 nm, identical to dihydrodigoxin. DLIF and dihydro-DLIF are 975- and 2588-fold less immunoreactive than digoxin and dihydrodigoxin for their respective antibodies. The molar ratio of dihydro-DLIF to DLIF is approximately 5.3 in bovine adrenocortical tissue and approximately 0.38 in human serum. Dihydrodigoxin (reduced lactone ring) added to microsomes isolated from bovine adrenal cortex produced a 4.5-fold increase in digoxin-like immunoreactivity (oxidized lactone ring) after 3 h of incubation. The biotransformation is likely mediated by a cytochrome P-450 NADPH-dependent process. Our findings demonstrate the presence of a dihydro-DLIF in mammals and suggest a metabolic route for synthesis of endogenous DLIF in mammalian tissue.

Deglycosylated products of endogenous digoxin-like immunoreactive factor in mammalian tissue

Digoxin-like immunoreactive factor (DLIF) from adrenal cortex is an endogenous molecule with structural features remarkably similar to those of digoxin, a plant-derived cardiac glycoside (Shaikh, I. M., Lau, B. W. C., Siegfried, B. A., and Valdes, R., Jr. (1991) J. Biol. Chem. 266, 13672-13678). Two characteristic structural and functional features of digoxin are a lactone ring and three digitoxose sugars attached to a steroid nucleus. Digoxin is known to undergo deglycosylation during metabolism in humans. We now demonstrate the existence of several naturally occurring deglycosylated components of DLIF in human serum. The components are identified as DLIF-genin, DLIF-mono, and DLIF-bis, corresponding to the aglycone, and the aglycone with one and two sugars, respectively. Similar components are produced by acid-induced deglycosylation of DLIF isolated from bovine adrenal cortex. The elution pattern and sequence of DLIF-deglycosylation was identical to that of digoxin suggesting identical sugar stoichiometry. However, analysis of these newly discovered congeners by reverse-phase chromatography, spectrophotometry, antibody reactivity, and kinetics of deglycosylation, demonstrates that subtle structural and physical differences do exist when compared to digoxin. DLIF was chromatographically distinct from digoxin, and interestingly, the mobility of the DLIF-genin was shifted toward increased polarity relative to digoxigenin. DLIF and DLIF-bis, -mono, and -genin congeners have absorbance maxima at 216 nm, whereas digoxin and its congeners absorb at 220 nm. Reaction with specific antibodies directed at the lactone portion of these molecules shows DLIF and its deglycosylated congeners to be 10(3)-fold less reactive than digoxin. Kinetics of sugar removal suggests that DLIF is 8-fold more susceptible to deglycosylation than is digoxin. Two less polar DLIF components produced from the DLIF-genin have lambdamax at 196 nm and are 4-fold less immunoreactive than DLIF. Our data suggest that subtle structural differences exist between DLIF and digoxin at or near the lactone ring as well as in the nature of the sugars. The presence of deglycosylated congeners of DLIF in human serum, including the less polar components, suggests in vivo deglycosylation of these factors. This is the first demonstration of the existence of naturally occurring deglycosylated derivatives of DLIF and establishes the likelihood of active metabolism of DLIF in mammals.

Simultaneous isolation of endogenous digoxin-like immunoreactive factor, ouabain-like factor, and deglycosylated congeners from mammalian tissues

DLIF (digoxin-like immunoreactive factor) and OLF (ouabain-like factor) are endogenous steroid-like ligands (approximately 781 and 595 Da, respectively) with molecular and structural properties similar to the plant-derived cardiac glycosides, digoxin and ouabain. We developed a purification method with a sufficiently wide range of extraction solubility to separate compounds with polarities spanning those of ouabain and digoxin. This technique provides a rapid, reliable, and efficient method for simultaneously isolating DLIF, OLF, and several naturally existing deglycosylated congeners, including three deglycosylated species of DLIF (DLIF-genin, DLIF-mono, and DLIF-bis) and one deglycosylated species of OLF (OLF-genin). Separation is achieved using acid extraction, C-18 reverse-phase HPLC chromatography, and signal detection using two antibodies, one specific for digoxin and one for ouabain. The average extraction efficiency is 400 pmol digoxin equivalent (range 300-500) and 42 pmol ouabain equivalent (range 37-50) per gram of adrenal cortex for DLIF and OLF, respectively. The relative molar immunoreactivity of DLIF is 10(3)-fold less than that of digoxin, whereas that of OLF is unity compared to ouabain, suggesting that OLF is structurally more similar to ouabain than DLIF is to digoxin. Of interest is the presence of a compound reacting with both digoxin and ouabain antibodies. This unique immunoreactive species is liekly to have structural similarity to both digoxin and ouabain and thus may represent a metabolic link between DLIF and OLF.

Serum cardiac troponin T concentrations in hospitalized patients without acute myocardial infarction

The present study was designed to determine whether the measurement of serum levels of cardiac troponin T (cTnT) would establish the presence of myocardial injury in 79 patients, within 12 h of hospitalization, whose admission and final discharge diagnosis were not related to myocardial infarction. Independent classification by clinicians resulted in 35 (44%) patients characterized as having unsuspected myocardial injury and 44 (56%) without myocardial injury. Increased serum cTnT identified 94% of myocardial injury patients, compared to increased serum creatine kinase MB (CK-MB) mass which identified 69%. Negative serum cTnT and CK-MB concentrations respectively excluded 73 and 50% of patients evidently without myocardial injury, respectively. A total of 12 patients, 9 with and 3 without elevated CK-MB concentration, had unexplained cTnT elevations. While our findings show cardiac troponin T to be more efficient than CK-MB in determining and excluding myocardial injury, even in a randomly selected population with a very high probability, unexplained elevations of cardiac troponin T question its role as an absolute cardiospecific marker.

Analytical performance of a monoclonal digoxin assay with increased specificity on the ACS:180

Digoxin metabolites cross-react in the Ciba Corning ACS digoxin assay in proportion to their bioactivity, but have greater (near 100%) cross-reactivity in the Abbott TDx, Baxter Stratus, and Ciba Corning Magic RIA digoxin assays. We studied the analytical performance of the ACS digoxin assay and compared it with these other assays. Coefficients of variation ranged from 5.5% at 3.11 ng/ml to 8.8% at 0.57 ng/ml. Mean analytical recovery was 96.4%. Results on dilutions were linear in the range of 0.6-5.0 ng/ml. We observed no interference by hemoglobin, bilirubin, or triglycerides. Dihydrodigoxin and digitoxin had lower cross-reactivity in the ACS and Stratus assays than in the TDx and Magic assays. Digoxin-like immunoreactive factor (DLIF) in patients' sera was not detected in the ACS assay but was in the TDx, Stratus, and Magic assays. Digibind therapy seemingly did not affect digoxin results by ACS or Stratus, but did for up to 10 days after therapy for TDx and Magic. We compared digoxin results for 121 sera from 49 patients. Deming regression analysis was performed on the first specimen from each patient: ACS = 1.08(TDx)-0.17 ng/ml (r = 0.961, Sy,x = 0.164); ACS = 1.16(Stratus)-0.46 ng/ml (r = 0.973, Sy,x = 0.123); ACS = 1.00(Magic)-0.20 ng/ml (r = 0.982, Sy,x = 0.110). Discrepant results (> 2Sy,x from the regression line) were usually lower by the ACS assay (87%). Nine of 11 patients with discrepant results had renal insufficiency or hepatic disease, conditions commonly associated with increased DLIF. These observations may be explained by the improved specificity of the ACS digoxin assay.

Clinical results of excimer laser photorefractive keratectomy: a multicenter study of 265 eyes

Efficacy, predictability, and safety of excimer laser photorefractive keratectomy were evaluated at centers in Paris and Brest, France. Photoablation was performed with the VISX laser on 265 eyes (151 at the Paris center and 114 at the Brest center). The eyes were clinically and statistically evaluated over a six month follow-up. Initial myopia ranged from -0.7 to -19.4 diopters (D) (mean spherical equivalent [SE] -5.9 D) in the Paris center and from -0.9 to -14.5 D (SE -4.5 D) in the Brest center. At both centers, the mean uncorrected visual acuity was worse than 20/200; over 90% of cases in each center had a best uncorrected visual acuity of 20/100 or worse. Results are reported globally and for subgroups of myopia: Group A, SE better than or equal to -3.0 D; Group B, SE worse than -3.0 D and better than or equal to -7.0 D; Group C, SE worse than -7.0 D. Uncorrected visual acuity was significantly improved in the patients followed for six months; 64% of Paris cases and 62% of Brest cases obtained an uncorrected visual acuity of 20/40 or better. Predictability of the treatment was good; 67% of Paris eyes and 74% of Brest eyes were less than 1.0 D from the intended correction after six months. The data suggest that the initial myopia affected the efficacy and predictability of the treatment; results in the mild to moderate myopia eyes were significantly better than results in the severe myopia eyes. One case of visual acuity regression (less than one line) was observed in the two groups. This was associated with corneal haze of moderate intensity.

Antiinflammatory activity of a Ghanaian antiarthritic herbal preparation: III

alpha-Amyrin palmitate, present in a Ghanaian antiarthritic herbal preparation of Alstonia boonei, Elaies guineensis and Rauvolfia vomitoria, was synthesised and tested on complete Freund's adjuvant-induced arthritic rats. Administered orally at 56 mg/kg body weight (BW) daily for 8 days from days 11 to 18 post adjuvant (acute) or at 66 mg/kg BW every 48 h for 5 days from days 32 to 40 (chronic), the drug returned the increases in serum hyaluronate and blood granulocytes towards non-arthritic levels and corrected the moderate anaemia of adjuvant arthritis. Histological examinations of the proximal interphalangeal foot joints showed reduced synovial proliferation and invasion of joints and reduced leucocyte infiltration of bone marrow and periarticular tissue in treated rats. The results suggest that alpha-amyrin palmitate contributes to the previously shown antiarthritic effect of the herbal preparation.

Treatment of human serum with sulfosalicylic acid structurally alters digoxin and endogenous digoxin-like immunoreactive factor

Pretreatment of human serum with 5-sulfosalicylic acid (SSA) as used in the Abbott TDx digoxin assay produces deglycosylated congeners of digoxin (DIG) and of endogenous digoxin-like immunoreactive factor (DLIF). Using high-performance liquid chromatography analysis, we observed differences in the degree and pattern of DIG breakdown products among five patients. The aglycone digoxigenin was the major product in several samples. Smaller amounts of the bis- and mono-digitoxosides and unidentified products less polar than DIG were sometimes present. Treatment of DLIF-containing plasma with SSA produced similar patterns of DLIF-breakdown products. Incubation of normal plasma containing DIG with SSA for up to 30 min caused little change in measured DIG by TDx and radioimmunoassay (RIA) but decreased to 50% in the ACS DIG assay. These results are consistent with the near 100% cross-reactivities of deglycosylated DIG congeners in the TDx and RIA assays compared to their lower cross-reactivities in the ACS assay. We conclude that the breakdown of DIG and DLIF during treatment of serum with SSA may compromise the accuracy of TDx DIG assays and may explain discrepancies observed in other studies between digoxin immunoassays. This study underscores the importance of understanding the effects of pretreatment strategies used for analytes measured by immunoassay.

A hemoglobin A1C immunoassay method not affected by carbamylated hemoglobin

Hemoglobin A1C (HbA1C) methods based on charge separation of Hb species are subject to interference from carbamylated Hb (carb Hb). Carb Hb adducts are formed via interaction of terminal amino groups of HbA with isocyanic acid, after the spontaneous dissociation of urea to cyanate. It is hypothesized that a new immunoassay method, using a monoclonal antibody that recognizes the N-terminus of the Hb beta-chain and its sugar moiety, should be refractory to cross-reactive interference from carb Hb. To test this hypothesis, Hb was carbamylated in vitro and co-migration of carb Hb assessed with HbA1C using an electrophoretic method. Densitometric scans - post sodium cyanate incubation and electrophoretic separation - showed a 5 to 7 fold elevation of the HbA1C peak only, while HbA1C values obtained using immunoassay were unaffected. Also assessed was carbamylation interference in vivo, and a positive proportional bias with the electrophoretic system (Y) was observed compared to the immunoassay system (X) (y = 1.2x - 0.21 percent). Others have shown that carb Hb may cause a clinically significant false elevation in patient HbA1C values, when methods based on charge separation of Hb species are used. It is our conclusion, however, that while carb Hb may play a role, the differences observed in this study are largely due to calibration.

Digoxin immunoassay with cross-reactivity of digoxin metabolites proportional to their biological activity

Our objective was to identify commercially available digoxin immunoassays whose cross-reactivity with digoxin metabolites paralleled the pharmacological activity of the metabolites. We measured the immunoreactivity of digoxigenin bis- and monodigitoxosides, digoxigenin, and dihydrodigoxin in four immunoassays and compared the immunoactivities with pharmacological activities from studies involving whole-animal and receptor (Na,K-ATPase)-based assays. Correlation coefficients for comparisons of immunoassay reactivity and human heart receptor reactivities were: ACS, 0.96; TDx, 0.60; Stratus, 0.57; and Magic, 0.42. Comparison with other biological assays showed a similar trend. The major difference in metabolite cross-reactivities among the immunoassays was that of digoxigenin (ACS, 0.7%; TDx, 103%; Stratus, 108%; Magic, 153%), which has approximately 10% bioactivity relative to digoxin. Measured recovery of mixtures of digoxin and metabolites confirmed these findings. We conclude that the monoclonal antibody in the ACS digoxin assay closely mimics Na,K-ATPase in detecting digoxin and its metabolites. This finding provides a basis for developing therapeutic drug monitoring immunoassays capable of approximating the true pharmacological activity of a mixture of drug metabolites.

Digoxin immunoassay with cross-reactivity of digoxin metabolites proportional to their biological activity

Our objective was to identify commercially available digoxin immunoassays whose cross-reactivity with digoxin metabolites paralleled the pharmacological activity of the metabolites. We measured the immunoreactivity of digoxigenin bis- and monodigitoxosides, digoxigenin, and dihydrodigoxin in four immunoassays and compared the immunoactivities with pharmacological activities from studies involving whole-animal and receptor (Na,K-ATPase)-based assays. Correlation coefficients for comparisons of immunoassay reactivity and human heart receptor reactivities were: ACS, 0.96; TDx, 0.60; Stratus, 0.57; and Magic, 0.42. Comparison with other biological assays showed a similar trend. The major difference in metabolite cross-reactivities among the immunoassays was that of digoxigenin (ACS, 0.7%; TDx, 103%; Stratus, 108%; Magic, 153%), which has approximately 10% bioactivity relative to digoxin. Measured recovery of mixtures of digoxin and metabolites confirmed these findings. We conclude that the monoclonal antibody in the ACS digoxin assay closely mimics Na,K-ATPase in detecting digoxin and its metabolites. This finding provides a basis for developing therapeutic drug monitoring immunoassays capable of approximating the true pharmacological activity of a mixture of drug metabolites.

Understanding the sodium pump and its relevance to disease

Na,K-ATPase (sodium pump; EC 3.6.1.37) is present in the membrane of most eukaryotic cells and controls directly or indirectly many essential cellular functions. Regulation of this enzyme (ion transporter) and its individual isoforms is believed to play a key role in the etiology of some pathological processes. The sodium pump is the only known receptor for the cardiac glycosides. However, endogenous ligands structurally similar to digoxin or ouabain may control the activity of this important molecular complex. Here we review the structure and function of Na,K-ATPase, its expression and distribution in tissues, and its interaction with known ligands such as the cardiac glycosides and other suspected endogenous regulators. Also reviewed are various disorders, including cardiovascular, neurological, renal, and metabolic diseases, purported to involve dysfunction of Na,K-ATPase activity. The escalation in knowledge at the molecular level concerning sodium pump function foreshadows application of this knowledge in the clinical laboratory to identify individuals at risk for Na,K-ATPase-associated diseases.