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

Digoxin and its related endogenous factors

The digitalis drugs are plant-derived cardenolide compounds used medicinally for several hundred years. These drugs elicit inotropic and chronotropic effects on the heart, but they also affect many other tissues. The mechanism of action involves inhibition of the ion-transport activity of a membrane-associated protein called Na, K-ATPase (sodium pump). Present theory holds that the sodium pump is the principal molecular receptor for the digitalis drugs. Recent evidence indicates the presence of naturally occurring digitalis-like compounds in mammals. It is believed these compounds, collectively known as either digitalis-like (DLF) or ouabain-like (OLF) factors, may be endogenous hormones regulating the biological activity of the sodium pump and its isoforms. The presence of deglycosylated and other congeners of one specific DLF, the digoxin-like immunoreactive factor (DLIF), has very recently been described in humans. Digoxin as a drug is the most widely prescribed digitalis in the U.S., and its measurement in serum has established a model for present-day therapeutic drug monitoring (TDM). Historically, the accurate measurement of digoxin in blood has been difficult. This article focuses on the present understanding of the clinical use of digoxin, factors that affect the accuracy of measuring digoxin, the principle of measuring metabolically active species of digoxin, and the effects of DLIF and other interfering substances in digoxin immunoassay.

Multicenter study of Abbott AxSYM® Digoxin II assay and comparison with 6 methods for susceptibility to digoxin-like immunoreactive factors

Performance characteristics of the Abbott nonpretreatment AxSYM® Digoxin II assay were evaluated for quantification of digoxin at four independent sites. Correlation of digoxin measurements with the Abbott pretreatment AxSYM®, Baxter Stratus® II, Abbott TDx/TDxFLx II®, Abbott IMx®, Emit® 2000, and Beckman Synchron CX® digoxin assays showed acceptable agreement, as indicated by: slope values >0.84, r >0.90, y-intercepts for all comparisons at or below the assay detection limit, and Sy|x ranging between 7.5% and 15.4% of the average digoxin value. Susceptibility to interference from digoxin-like immunoreactive factors (DLIFs) was examined in 233 samples from renal patients, liver disease patients, cord blood, and third-trimester pregnancies; the AxSYM Digoxin II assay demonstrated the least DLIFs interference. DLIF susceptibility for four of the methods was significantly greater (P <0.05) than in the AxSYM Digoxin II assay; susceptibilities of the Stratus II and Emit 2000 methods were similar to the AxSYM Digoxin II assay.

Digoxinlike immunoreactive factor isolated from human pleural fluid is structurally similar to digoxin

To further define the chemical structure of human endogenous digoxinlike immunoreactive factors (DLIF) we used human pleural effusions as a source of the substance. Digoxinlike immunoreactive factor activity was detected by radioimmunoassay in the pleural fluid of each of four patients; average concentration was 0.35 ng/mL. The chemical profile of DLIF was determined by initial extraction and concentration of DLIF by ion exchange chromatography followed by reverse phase-high-pressure liquid chromatography (RP-HPLC) separation and purification. Using high-pressure liquid chromatography cochromatography of DLIF, together with several radioactively marked glycosides, we observed a single peak of DLIF activity that was chromatographically identical to digoxin. The present study further supports the recent finding that DLIF is related structurally to the cardiac glycosides, and for the first time it has been proven that DLIF is present in pleural fluids.

Enzyme-multiplied immunoassay (EMIT 2000) digoxin assay compared with fluorescence polarization immunoassay and amerlex 125I-radioimmunoassay at two Australian centers

Digoxin assays in plasma from patients treated with the drug have played an integral role in its therapeutic management. Commercial digoxin immunoassays have been criticized for poor performance owing to various interferences and limited sensitivity. The present study compared the performance of a new enzyme-multiplied immunoassay technique (EMIT 2000) to fluorescence polarization immunoassay (FPIA) and radioimmunoassay (RIA) in two separate Australian centers. Comparisons were made using standard indices of precision and accuracy, samples taken from patients, quality-assurance samples, and cord blood samples from neonates, in which high concentrations of digoxin-like immunoreactive substances (DLIS) would be anticipated. The results confirmed satisfactory precision and accuracy for therapeutic drug monitoring purposes, a sensitivity of < 0.1 microgram/L, and very low DLIS interference, as assessed by assay of neonatal cord blood samples.

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.

Estimating concentrations of total digoxin and digoxin-like immunoreactive substances in volume-expanded patients being treated with digoxin

High concentrations of digoxin-like immunoreactive substances (DLIS) artificially increase serum digoxin concentrations. However, DLIS are absent in the protein-free ultrafiltrate because of their strong binding with serum macromolecules, whereas approximately 75% of digoxin can be found in the ultrafiltrate. Using regression analysis, we devised equations by which total digoxin concentration can be calculated from free digoxin and albumin concentrations in serum. We used two different assays, fluorescence polarization and chemiluminescence, for measuring total and free-digoxin concentrations in sera. Both equations were very similar. Because measured concentrations of digoxin in the serum exhibit DLIS interferences, the measured concentrations were sometimes higher in volume-expanded patients than the calculated digoxin concentrations. We also estimated the extent of interferences from DLIS by subtracting the calculated digoxin concentration from the measured digoxin concentration in volume-expanded patients.