Mass spectrometry studies of a novel digoxin-like substance (DLIS-2) isolated from human plasma ultrafiltrate

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.

Endogenous and exogenous digoxin-like immunoreactive substances: impact on therapeutic drug monitoring of digoxin

Endogenous digoxin-like immunoreactive substance (DLIS) was first reported in volume-expanded dogs. Its presence has been confirmed in blood, urine, and other body fluids. Elevated DLIS concentrations are encountered in patients with volume-expanded conditions such as uremia, essential hypertension, liver disease, and preeclampsia. DLISs cross-react with antidigoxin antibodies and falsely elevate serum digoxin concentrations, interfering in interpretation of results for therapeutic digoxin monitoring. Falsely lower digoxin values due to the presence of DLISs have been reported. The association of DLISs with volume expansion led to speculation that they could be natriuretic hormones. Several structures have been proposed for DLISs, including nonesterified fatty acid, phospholipid, lysophospholipid, bile acid, bile salt, and steroid. Exogenous DLISs can be found in serum after ingestion of various Chinese medicines and therapy with spironolactone, canrenone, or potassium canrenoate. Like endogenous DLISs, exogenous DLISs interfere with serum digoxin assays, complicating therapeutic digoxin monitoring. However, most reported endogenous and exogenous DLISs are strongly protein-bound while digoxin is weakly protein-bound. Therefore, interference of both endogenous and exogenous DLISs in serum digoxin measurement can be eliminated by monitoring digoxin concentrations in the protein-free ultrafiltrates.

Isolation of an endogenous Na-pump specific inhibitor from normal pig urine: characterization and comparison with the inhibitor purified from bovine adrenal glands

An endogenous Na-pump specific inhibitor has been purified to homogeneity from normal pig urine using Amberlite XAD-2 adsorption chromatography followed by five steps of reverse phase HPLC. Although most of the dose response curves for this purified Na-pump inhibitor, designated uroxin, in the various assay systems paralleled those of authentic ouabain and the specific Na-pump inhibitor previously purified from bovine adrenal glands (designated adrexin C), the cross-reactivity curve with anti-ouabain antibodies did not. The retention times of uroxin on various types of reverse phase HPLC columns were also different from those of plant-derived cardiotonic steroids and adrexin C. The cross-reaction curve of adrexin C was superimposable with that of ouabain, and adrexin C coeluted with ouabain from all of the HPLC columns tested. The results from physical and chemical characterization of both purified inhibitors suggest that uroxin is a novel Na-pump inhibitor which is structurally different from any of the known cardiotonic steroids or other substances previously reported to exhibit Na-pump inhibitory activity. The results also indicate that adrexin C is indistinguishable from ouabain. These results suggest that there are at least two different types of endogenous Na+,K(+)-ATPase inhibitors in the mammalian body.

Digoxin-like immunoreactivity in cord blood plasma extracts is not only due to endogenous corticosteroids

Methods for quantitative extraction and enrichment of digoxin-like immunoreactive substances (DLIS) from human cord blood plasma (CBP) using organic solvents combined with a solid state absorbent are presented. Sephadex LH-20 column chromatography is more suitable than sephadex G-25 for DLIS separation and purification from CBP extract. The elution pattern of sephadex LH-20 chromatography of deproteinized and desalted CBP shows two distinct DLIS peaks. The first peak coelutes with the steroids aldosterone, cortisol, progesterone, 17-OH-progesterone, testosterone and estradiol, while the second DLIS peak coelutes with dehydroepiandrosterone sulfate (DHEA-S). The Na+,K(+)-ATPase inhibitory activity peak partially overlaps with the first DLIS peak. Coeluted steroids could account only partially for digoxin-like immunoreactivity in the first DLIS peak; however, they did not contribute to the Na+,K(+)-ATPase inhibitory activity, whereas the second DLIS peak could be ascribed to endogenous DHEA-S.

pH-dependent column fractionation for characterization of endogenous digoxin-like immunoreactive factors in pregnant urine

Samples of pregnant urine were treated with three diatomaceous earth columns to fractionate organic solvent-soluble components. The first column was pre-injected with the urine sample. The second and the third columns were pre-injected with sodium bicarbonate and sodium hydroxide solutions, respectively; these played an important role in capturing acidic components in the urine extract from the first column. Neutral components could be eluted out from the columns with the organic solvent, while the basic components were retained in the first column. Once strongly and weakly acidic components were captured on the alkaline columns, they were separately treated with an acidic mobile phase and recovered. The digoxin-like activities of each fraction were examined for cross-reactivity to antidigoxin antibody and inhibitory effects on dog kidney Na+, K(+)-ATPase. Both types of reactivity were found in all three fractions and decreased in the following order: strongly acidic, neutral and weakly acidic. The neutral fraction of pregnant urine showed significantly greater cross-reactivity than that of the non-pregnant urine. Following hydrolysis of the pregnant urine with beta-D-glucuronidase, the cross-reactivity of the neutral fraction increased significantly. High performance liquid chromatographic analysis showed that several components in the neutral fraction had apparently increased after the hydrolysis, showing the possible presence of unknown digoxin-like components in the pregnant urine.

Digoxin-like immunoreactivity: is it still worth measuring?

On the assumption that digoxin-like immunoreactivity may represent digitalis-like sodium pump inhibitors in the mammalian body, many investigators have used radioimmunoassay for digoxin to monitor such factors during the past decade. The presence of digoxin-like immunoreactivity has been confirmed by numerous studies using biochemical, immunological or morphological methods. Very recently, ouabain or a very similar substance, which did not cross-react with antidigoxin antibodies, was identified from the human plasma as the long-sought sodium pump inhibitor. However, it is yet to be determined whether sodium pump inhibitory activity in the circulation results from one substance or several. Some researchers still insist on the possible physiological roles of digoxin-like immunoreactivity which may or may not be related to the regulation of sodium pump. These issues are critically reviewed in this article.