New Enzyme-Linked Immunosorbent Digoxin Assay on the ADVIA?? IMS??? 800i System Is Virtually Free from Interference of Endogenous Digoxin-like Immunoreactive Factors

An ultra-sensitive optical aptasensor based on gold nanoparticles/poly vinyl alcohol hydrogel as acceptor/emitter pair for fluorometric detection of digoxin with on/off/on strategy

A novel nanobiosensor was prepared by aptamer and gold nanoparticles conjugate in poly vinyl alcohol hydrogel for sensitive detection of digoxin in human plasma samples. The developed nanobiosensor was characterized by Fourier transform infrared spectroscopy, transmission electron microscopy, and dynamic light scattering instrument. In this sensor the hydrogel acted as a fluorescent probe. The fluorescence intensity of the hydrogel was quenched by aptamer stabilized gold nanoparticles as energy acceptor. Upon addition of digoxin, the aptamer/drug complex was formed and the fluorescence of the hydrogel was restored because of destabilization and aggregation of gold nanoparticles in the presence of salt. The affecting parameters on the nanobiosensor performance were assessed and under the optimized conditions the external and in plasma calibration curves were linear in the 10-1000 ng L^-1 digoxin concentration range with detection limits of 2.9 and 3.1 ng L^-1, respectively. The relative standard deviations for 5 replicate determinations of 50, 250, and 500 ng L^-1 of digoxin, were 7.3, 5.1, and 3.8%, respectively. This nanofluoroprobe was successfully applied for determination of digoxin in spiked plasma samples without any pretreatment procedure.

Role of endogenous digitalis-like factors in the clinical manifestations of severe preeclampsia: a sytematic review

Endogenous digitalis-like factor(s), originally proposed as a vasoconstrictor natriuretic hormone, was discovered in fetal and neonatal blood accidentally because it cross-reacts with antidigoxin antibodies (ADAs). Early studies using immunoassays with ADA identified the digoxin-like immuno-reactive factor(s) (EDLF) in maternal blood as well, and suggested it originated in the feto-placental unit. Mammalian digoxin-like factors have recently been identified as at least two classes of steroid compounds, plant derived ouabain (O), and several toad derived bufodienolides, most prominent being marinobufagenin (MBG). A synthetic pathway for MBG has been identified in mammalian placental tissue. Elevated maternal and fetal EDLF, O and MBG have been demonstrated in preeclampsia (PE), and inhibition of red cell membrane sodium, potassium ATPase (Na, K ATPase (NKA)) by EDLF is reversed by ADA fragments (ADA-FAB). Accordingly, maternal administration of a commercial ADA-antibody fragment (FAB) was tested in several anecdotal cases of PE, and two, small randomized, prospective, double-blind clinical trials. In the first randomized trial, ADA-FAB was administered post-partum, in the second antepartum. In the post-partum trial, ADA-FAB reduced use of antihypertensive drugs. In the second trial, there was no effect of ADA-FAB on blood pressure, but the fall in maternal creatinine clearance (CrCl) was prevented. In a secondary analysis using the pre-treatment maternal level of circulating Na, K ATPase (NKA) inhibitory activity (NKAI), ADA-FAB reduced the incidence of pulmonary edema and, unexpectedly, that of severe neonatal intraventricular hemorrhage (IVH). The fall in CrCl in patients given placebo was proportional to the circulating level of NKAI. The implications of these findings on the pathophysiology of the clinical manifestations PE are discussed, and a new model of the respective roles of placenta derived anti-angiogenic (AAG) factors (AAGFs) and EDLF is proposed.

Therapeutic drug monitoring of digoxin: impact of endogenous and exogenous digoxin-like immunoreactive substances

Digoxin is a cardioactive drug with a narrow therapeutic range. Therapeutic drug monitoring is essential in clinical practice for efficacy as well as to avoid digoxin toxicity. Immunoassays are commonly used in clinical laboratories for determination of serum or plasma digoxin concentrations. Unfortunately, digoxin immunoassays are affected by both endogenous and exogenous compounds. Endogenous compounds are termed 'digoxin-like immunoreactive substances' (DLIS), which are found in elevated concentrations in volume-expanded patients. Exogenous compounds that interfere with digoxin assays are various drugs such as spironolactone, potassium canrenoate as well as Digibind (Fab fragment of antidigoxin antibody), which is used in treating life-threatening digoxin overdose. Moreover, various Chinese medicines such as Chan Su, Lu-Shen Wan and oleander-containing herbal preparations also interfere with serum digoxin measurements by immunoassays. Monitoring unbound (free) digoxin concentration may under certain circumstances eliminate such interferences. Clinicians should be aware of limitations of therapeutic drug monitoring of digoxin using immunoassays.

Usefulness of monitoring free (unbound) concentrations of therapeutic drugs in patient management

Drugs are bound to various serum proteins in different degrees and only unbound or free drug is pharmacologically active. Although free drug concentration can be estimated from total concentration, for strongly bound drugs, prediction of free level is not always possible. Conditions like uremia, liver disease and hypoalbuminemia can lead to significant increases in free drug resulting in drug toxicity even if the concentration of total drug is within therapeutic range. Drug-drug interactions may also lead to a disproportionate increase in free drug concentrations. Elderly patients may have increased free drug concentrations due to hypoalbuminemia. Elevated free phenytoin concentrations have also been reported in patients with AIDS and pregnancy. Currently free drug concentrations of anticonvulsants such as phenytoin, carbamazepine and valproic acid are widely measured in clinical laboratories. Newer drugs such as mycophenolic acid mofetil and certain protease inhibitors are also considered as candidates for monitoring free drug concentration.

A new enzyme-linked chemiluminescent immunosorbent digoxin assay is virtually free from interference of spironolactone, potassium canrenoate, and their common metabolite canrenone

Spironolactone and potassium canrenoate (aldosterone antagonist diuretics) are sometimes used in conjunction with digoxin for patient management. Spironolactone, potassium canrenoate, and their common metabolite canrenone interfere with serum digoxin measurement using various immunoassays. Recently a new enzyme-linked chemiluminescent immunosorbent digoxin assay (ECLIA-Digoxin) became commercially available for application on the ADVIA IMS 800i modular system (Bayer HealthCare, Tarrytown, NY). We investigated the potential interference of spironolactone and related compounds in this assay by comparing the results with the fluorescence polarization immunoassay (FPIA), which is known to have significant cross-reactivity with these compounds as well as a turbidimetric assay for digoxin with no known cross-reactivity with spironolactone and related compounds. Aliquots of drug free serum were supplemented with therapeutic and above therapeutic concentrations of spironolactone, canrenone, and potassium canrenoate, and apparent digoxin concentrations were measured. No apparent digoxin concentration was observed using the ECLIA-Digoxin or turbidimetric assay. When serum pools prepared from patients receiving digoxin were further supplemented with these compounds, we observed no significant change in digoxin concentrations in the presence of these compounds with the ECLIA-Digoxin. We conclude that this assay is virtually free from interferences from spironolactone, potassium canrenoate and their common metabolite canrenone.

Suspected DLIS Interference in the Dimension DGNA Digoxin Assay Method and the Clinical Application of the Revised Digoxin Target Range

The authors present a case of an elderly female patient with heart failure and renal dysfunction treated with digoxin, where 2 commercial immunoassay methods (DRI, Microgenics, and DGNA, Dade Behring) showed a clinically very significant discrepancy on the same plasma sample, viz. 0.5 and 2.3 nmol/L, respectively. The sample was also referred to a third external laboratory that returned a result of 0.9 nmol/L using mFPIA (AxSYM, Abbott). Subsequent ultrafiltration (30,000 Dalton) on the sample essentially eliminated the difference, suggesting an interference from a large molecular weight compound(s), potentially the well-described digoxin-like immunoreactive substance(s) (DLIS). Although further study is required to verify that the DLIS implicated was indeed the interfering species, it does again highlight the importance of careful method selection in the clinical therapeutic drug monitoring laboratory to ensure that such well-established potential problems do not result in inappropriate dosage reduction with consequent lack of adequate drug exposure and serious clinical sequelae.