Tetra-primer ARMS-PCR combined with GoldMag lateral flow assay for genotyping: simultaneous visual detection of both alleles

Rapid and simple detection of single nucleotide polymorphism (SNP) is vital for individualized diagnosis and eventual treatment in the current clinical setting. In this study, we developed a tetra-primer ARMS-PCR combined lateral flow assay (T-ARMS-PCR-LFA) method for simultaneous visual detection of two alleles. By using four primers labeled with digoxin, biotin and Cy5 separately in one PCR reaction, the amplified allele-specific products could be captured by streptavidin and the anti-Cy5 antibody on two separated test lines of a LFA strip, which allows the presentation of both alleles within the single LFA strip. Both DNA and whole blood can be used as templates in this genotyping method in which the whole detection process is completed within 75 minutes. The performance assay of T-ARMS-PCR-LFA demonstrates the accuracy, specificity and sensitivity of this method. One hundred human whole blood samples were used for MTHFR C677T genotyping in T-ARMS-PCR-LFA. The concordance rate of the results detected was up to 100% when compared with that of the sequencing results. Collectively, this newly developed method is highly applicable for SNP screening in clinical practices.

Clinical judgement perplexed by initially undisclosed use of herbal medicine and unexpected cross-reactivity of immunoassay

We report a case of symptomatic bradycardia caused by consumption of a Chinese herbal medicine which was initially undisclosed to the attending emergency physician. The scientific name of the herb is Panax japonicus. Electrocardiogram revealed sinus bradycardia. Laboratory tests were normal except for the detection of a high serum digoxin level. Further interrogation of the patient eventually disclosed ingestion of the herb which, however, did not contain any digoxin. Other active ingredients in the herb include various types of ginsenoside. These are digoxin-like substances that had caused the observed false-positive detection of digoxin by fluorescence polarization immunoassay due to cross-reactivity. Our case-report provides an important insight about a blind-spot in the field of laboratory medicine (clinical pathology), namely, the false positive detection of digoxin due to crossreactivity in the immunoassay when we come across digoxin-like substances in clinical scenarios, which has barely received attention in the medical literature. It also conveys a clear educational message that with full understanding of the laboratory methodology and its mechanistic rationale there are actually some tricks-of-the-trade that allow us to optimize the specificity of the biochemical tests and the treatment of digoxin-like substances overdose.

Recurrence of digoxin toxicity following treatment with digoxin immune fab in a patient with renal impairment

In patients with chronic digoxin toxicity, especially in the presence of renal impairment, a prolonged duration of continuous monitoring is required with consideration given to further doses of immune fab if necessary for re-emergence of toxicity.

[Focus on digitalis intoxication in the elderly. Report of a case treated with digoxin-specific Fab antibody fragments]

The indications for digoxin are currently limited to rare cases of heart failure and/or atrial fibrillation. Its use should be even more rare in geriatrics its pharmacological characteristics, associated with age-related changes and comorbidities, particularly increase the risk of digoxin poisoning in the elderly. However, at least a third of aged patients suffering from heart failure and/or atrial fibrillation is treated by digitalis. Digoxin intoxication can provoke gastrointestinal troubles, neurological disturbances and, above all, cardiac conduction impairment and dysrythmias, which explain its severity and high mortality rate. Presently, first-line therapy is the administration of digoxin specific antibodies. Poor prognosis factors, frequently found in digoxin intoxications in the elderly, have been established for guiding the prescription of antibodies and their dosage. It is important for geriatricians to be able to recognize poisoning signs and the conditions in which an antidote treatment is necessary. This will permit a more effective management of the case, with the support of a poison control center and possible referral of the patient to an intensive care unit.

Digoxin--'a friend or foe'

The authors report about a patient who was admitted after developing nausea, vomiting, change in vision and lethargy. She was on digoxin 250 mcg once daily among all her other medications in the wake of a recent stroke that was accompanied by atrial fibrillation (AF). Her digitalis levels shortly before and on admission were 3.4 and 2.9 ng/ml, respectively. Her admission rhythm was slowly conducted AF at an average of 35 bpm. After a careful assessment by the cardiology consultant in charge, she received Digibind infusion for a chronic digitalis toxicity with the digoxin immune Fab dose based on the formula recommended in the product literature.(3) A few days observation on the ward ensured that her resting heart rate rose to 65 bpm and that she did not need a pacemaker for a slow AF. Her functional status remained reasonably good as she enjoyed a satisfactory recovery postthrombolysis for her recent stroke.

Visual detection of Hg²⁺ in aqueous solution using gold nanoparticles and thymine-rich hairpin DNA probes

We report a sensitive method for visual detection of mercury ions (II) (Hg²⁺) in aqueous solution by using gold nanoparticles (Au-NPs) and thymine (T)-rich hairpin DNA probes. The thiolated hairpin DNA probe was immobilized on the Au-NP surface through a self-assembling method. Another thymine-rich, digoxin-labeled DNA probe was introduced to form DNA duplexes on the Au-NP surface with thymine-Hg²⁺-thymine (T-Hg²⁺-T) coordination in the presence of Hg²⁺. The Au-NPs associated with the formed duplexes were captured on the test zone of a lateral flow strip biocomponent (LFSB) by immunoreaction events between the digoxin on the duplexes and anti-digoxin antibodies on the LFSB. The accumulation of Au-NPs produced a characteristic red band on the test zone, enabling visual detection of Hg²⁺ without instrumentation. A detection limit of 0.1 nM was obtained under optimal experimental conditions. This method provides a simple, rapid, sensitive approach for the detection of Hg²⁺ and shows great promise for point-of-care and in-field detection of environmentally toxic mercury.

Digoxin: clinical highlights: a review of digoxin and its use in contemporary medicine

Digoxin is the oldest cardiac medication used in contemporary medicine. With a complex pharmacokinetic profile and narrow therapeutic index, its use in managing patients with atrial arrhythmias or heart failure can present a challenge to today's clinicians. Digoxin dosing based on patient-specific factors such as age, lean body weight, and renal function will allow practitioners to minimize drug toxicity while maintaining clinical efficacy. The ability to recognize digoxin overdose, which can manifest in both the acute and chronic settings, helps guide the appropriate dosing of digoxin immune globulins to reverse toxicity. Understanding this unique medication is essential for clinicians to ensure digoxin is used safely and effectively in practice.

An outbreak of foxglove leaf poisoning

Comfrey (Symphytum officinale) leaves resemble those of foxglove (Digitalis purpurea) when the plant is not in bloom and, therefore, cardiac glycoside poisoning may occur when people confuse foxglove with comfrey. We report an outbreak of foxglove leaf poisoning following the use of alleged "comfrey" herbal tea. Nine patients were involved and initially presented with nausea, vomiting, diarrhea and dizziness. Significant cardiotoxicity developed later among the 3 patients who also had mild hyperkalemia. Peak serum digoxin concentration measured by immunoassay was elevated in all patients and ranged from 4.4 ng/mL to 139.5 ng/mL. Patients with severe cardiotoxicity were treated with temporary cardiac pacing. Moreover, 40-80 mg of digoxin-specific antibody therapy was given without any effect. All patients recovered uneventfully. Our report highlights the potential risk of misidentification of herbs; in this case, D. purpurea was mistaken for S. officinale. Physicians should be aware that cardiac glycoside poisoning could arise from such misidentification. Public education about the toxicity of D. purpurea poisoning may reduce the risk of misidentification and subsequent poisoning.

Detection and quantification of affinity ligand leaching and specific antibody fragment concentration within chromatographic fractions using surface plasmon resonance

Rapid analyses of chromatographic steps within a biopharmaceutical manufacturing process are often desirable to evaluate column performance, provide mass balance data and to permit accurate calculations of yields and recoveries. Using SPR (surface plasmon resonance) biosensor (Biacore) technology, we have developed a sandwich immunoassay to quantify polyclonal anti-digoxin Fab fragments used for the production of the FDA (Food and Drug Administration)-approved biotherapeutic DigiFab. The results show that specific Fab may be quantified in all affinity process streams and accurate yield and mass balance data calculated. Control experiments using sheep Fab and Fc indicate that the assay is specific to DigiFab. The quantification of potential leached ligand within chromatographic fractions may also be technically challenging, particularly when low-molecular-mass ligands are covalently coupled with an affinity absorbent. Typical methods to assess ligand leakage such as DDMA (digoxin-dicarboxymethoxylamine; digoxin analogue) often involve the use of labelled ligands and relatively complex and labour-intensive analytical techniques. Using the same analytical methodologies, an assay to detect leached or eluted ligand off the column was developed. The results indicate minimal levels of leached ligand in all chromatographic fractions, with total levels of leached DDMA calculated to be 1.52 microg. This is less than 0.01% of the total amount of DDMA coupled with the laboratory-scale affinity column. The SPR methods described in the present study may be applicable for the rapid in-process analysis of specific polyclonal Fab fragments (within a polyclonal mixture) and to rapidly assess leakage of small molecule ligands covalently attached to chromatographic supports.

Modification and identification of a vector for making a large phage antibody library

BACKGROUND

The large phage antibody library is used to obtain high-affinity human antibody, and the Loxp/cre site-specific recombination system is a potential method for constructing a large phage antibody library. In the present study, a phage antibody library vector pDF was reconstructed to construct diabody more quickly and conveniently without injury to homologous recombination and the expression function of the vector and thus to integrate construction of the large phage antibody library with the preparation of diabodies.

METHODS

scFv was obtained by overlap polymerase chain reaction (PCR) amplification with the newly designed VL and VH extension primers. loxp511 was flanked by VL and VH and the endonuclease ACC III encoding sequences were introduced on both sides of loxp511. scFv was cloned into the vector pDF to obtain the vector pDscFv. The vector expression function was identified and the feasibility of diabody preparation was evaluated. A large phage antibody library was constructed in pDscFv. Several antigens were used to screen the antibody library and the quality of the antibody library was evaluated.

RESULTS

The phage antibody library expression vector pDscFv was successfully constructed and confirmed to express functional scFv. The large phage antibody library constructed using this vector was of high diversity. Screening of the library on 6 antigens confirmed the generation of specific antibodies to these antigens. Two antibodies were subjected to enzymatic digestion and were prepared into diabody with functional expression.

CONCLUSIONS

The reconstructed vector pDscFv retains its recombination capability and expression function and can be used to construct large phage antibody libraries. It can be used as a convenient and quick method for preparing diabodies after simple enzymatic digestion, which facilitates clinical trials and application of antibody therapy.

Omeprazole-associated digoxin toxicity

Omeprazole is a commonly prescribed inhibitor of the gastric proton pump and has numerous indications in the treatment of gastrointestinal diseases. It is primarily metabolized through the CYP2C19 enzyme, a member of the P450 mixed-function oxidase group, although a minor pathway of metabolism is through CYP3A4, another P450 enzyme. Digoxin is primarily metabolized outside the P450 system, but a minor pathway of metabolism is by CYP3A4. To our knowledge, this is the first known case of digoxin toxicity associated with omeprazole. The possible pathways for such an interaction are reviewed, including increased stomach absorption, p-glycoprotein activity and interactions in the P450 system.

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.

Homogeneous electrogenerated chemiluminescence immunoassay for the determination of digoxin employing Ru(bpy)(2)(dcbpy)NHS and carrier protein

A highly sensitive homogeneous electrogenerated chemiluminescence (ECL) immunoassay for the determination of anti-digoxin antibody and digoxin hapten was developed employing Ru(bpy)(2)(dcbpy)NHS (bpy = 2,2'-bipyridyl; dcbpy = 2,2'-bipyridine-4,4'-dicarboxylic acid; NHS = N-hydroxysuccinimide ester) as an electrochemiluminescent label and bovine serum albumin (BSA) as a carrier protein. A digoxin hapten was indirectly heavily labelled with Ru(bpy)(2)(dcbpy)NHS through BSA to form Ru(bpy)(2)(dcbpy)NHS-BSA-digoxin conjugate. The ECL intensity of the immunocomplex of the conjugate with anti-digoxin antibody markedly decreased when the immunoreaction between Ru(bpy)(2)(dcbpy)NHS-BSA-digoxin conjugate and anti-digoxin antibody took place. Two formats, direct homogeneous immunoassay for anti-digoxin antibody and competitive immunoassay for digoxin, were developed to determine anti-digoxin antibody and digoxin, respectively. The anti-digoxin antibody concentration in the range 7.6 x 10(-8)-7.6 x 10(-6) g/mL was determined by direct homogeneous format. Digoxin hapten was determined throughout the range 4.0 x 10(-10)-1.0 x 10(-7) g/mL with a detection limit of 1.0 x 10(-10) g/mL by competitive format. The relative standard derivation for 6.0 x 10(-9) g/mL was 4.3%. The method has been applied to assaying digoxin in control human serum.

Endogenous Na/K-ATPase inhibitors in patients with preeclampsia

Although preeclampsia (PE) is a major cause of maternal and fetal mortality, its pathogenesis is not fully understood. Digitalis-like cardiotonic steroids (CTS) are believed to be involved in the pathophysiology of PE, as illustrated by clinical observations that DIGIBIND, a digoxin antibody which binds CTS, lowers blood pressure in PE. Recently we reported that plasma levels of marinobufagenin (MBG), a vasoconstrictor CTS, are increased fourfold in patients with severe PE. In the present study, we tested whether anti-MBG, or anti-ouabain antibodies, or DIGIBIND can reverse inhibition of erythrocyte Na/K-ATPase (NKA) from patients with mild PE (blood pressure, 149 +/- 3/93 +/- 3 mm Hg; age, 28 +/- 2 years; gestational age, 37 +/- 1 weeks). Development of PE was associated with twofold rise in plasma MBG levels (1.58 +/- 0.15 vs. 0.80 +/- 0.11 nmol/L; P<0.01). The activity of erythrocyte NKA in 12 patients with PE was lower than in 6 normotensive gestational age-matched subjects (1.56 +/- 0.18 vs. 3.11 +/- 0.16 micromol Pi/ml/hr; P<0.001). In vitro treatment of erythrocytes from PE patients with anti-MBG antibody fully restored the NKA activity (3.26 +/- 0.41 micromol Pi/ml/hr; P<0.01). The effects of DIGIBIND was marginally significant (2.53 +/- 0.32 micromol Pi/ml/hr), while the anti-ouabain antibody was not effective (2.25 +/- 0.25 micromol Pi/ml/hr, P>0.5). The present observations provide evidence for a role for MBG in the pathogenesis of PE, and suggest that antibodies against MBG may be useful in the treatment of this syndrome.

Digoxin use and digoxin toxicity in the post-DIG trial era

BACKGROUND

The advent of medical therapies for congestive heart failure that have proven survival benefits, specifically angiotensin-converting enzyme (ACE) inhibitors, beta-adrenergic antagonists, and the aldosterone antagonists, have called into question the use of digoxin for patients with normal sinus rhythm, left ventricular dysfunction, and symptomatic heart failure. This issue appears to have been heightened after the publication of the results of the Digitalis Investigation Group (DIG) Trial in 1997 that did not demonstrate a statistically significant impact of digoxin on mortality.

METHODS AND RESULTS

We used data from a large heart failure registry to examine digoxin use at the time of hospital admission for heart failure, a surveillance system for recording toxic drug exposures to describe patterns in digoxin toxicity and industry estimates for the use of digoxin antibody. Digoxin use has decreased significantly from 31.4% in late 2001 to 23.5% in late 2004 (P < .00001) independent of patient age, gender, or baseline creatinine. Conversely, the number of toxic or potentially toxic exposures to digoxin requiring hospitalization has not decreased.

CONCLUSION

Digoxin use is decreasing but there has not been a similar decline in cases of toxicity. Further analyses are required to delineate the reasons underlying these trends and the appropriateness of prescribing practices for both digoxin and its antidote.

Analysis of ligand binding by bioaffinity mass spectrometry

BACKGROUND

Ligand binding is commonly analyzed using various immunoassays that are generally time-consuming and some may require secondary antibodies or gel electrophoresis which are also time-consuming and sometimes subjective. We introduced various examples for a more rapid approach using pre-activated surface chips which are analyzed by surface enhanced laser desorption/ionization-time of flight mass spectrometry (SELDI-TOF MS). Specific applications presented in this study include immobilization of antigen, antibody or oligo DNA on pre-activated chips with subsequent identification of the binding antibodies, antigens or DNA binding proteins to demonstrate the universal utility of this novel approach.

METHODS

BSA-digoxin conjugate (BSA-Dig), anti-digoxin antibody, anti-urinary trypsin inhibitor (uTi) antibody, or a double stranded oligo nucleotide based on the nucleotide sequence between -91 and -10 of the human CYP 450 2E1 promoter were immobilized on the Ciphergen pre-activated surface chips. Anti-digoxin antibody, BSA-digoxin conjugate, uTi, and CYP450 2E1 promoter binding protein were captured on the chip and identified by SELDI-TOF MS.

RESULTS

A protein with 141kDa was identified from anti-digoxin serum using BSA-Dig chips. This binding was competitively inhibited by addition of digoxin. Using anti-digoxin antibody, a peak at approximately 66kDa was detected in the preparation of BSA-Dig. This peak was also inhibited by free digoxin, suggesting BSA-Dig is detected. uTi fragments with approximately 3kDa to approximately 30kDa in the standard and urine samples were captured on the chip by anti-uTi antibody. Finally, we identified a 95-kDa CYP 450 2E1 promoter binding protein in HeLa cells nuclear extracts.

CONCLUSIONS

Bioaffinity SELDI-TOF MS is a powerful and versatile approach for analysis of ligands. It eliminates tracer-labeled secondary antibodies and allows for determination of molecular weights of binding proteins and their ligands directly. This approach may also be considered for the detection of enzymes, receptors, or any other specific ligands.

Successful treatment of oleander intoxication (cardiac glycosides) with digoxin-specific Fab antibody fragments in a 7-year-old child

A 7-year-old girl presented six hours after ingestion of a yellow oleander seed (Thevetia peruviana) with severe emesis, change in colour vision and complete heart block. Initial treatment with phenytoin and isoprenalin infusion led to intermittent supraventricular and ventricular tachycardia. The patient was then treated with two intravenous doses of 190 mg of digoxin-specific Fab antibody fragments (Digibind). Subsequently the patient's rhythm reverted to sinus rhythm and the symptoms resolved within 2 hours.

CONCLUSION

administration of digoxin-specific Fab antibody fragments in an otherwise healthy child after oleander intoxication is safe and without adverse reactions.

Production of human single-chain variable fragment (scFv) antibody specific for digoxin by ribosome display

Ribosome display was applied in vitro to select single-chain variable fragment (scFv) antibody specific for digoxin from a human non-immune naive scFv library. A cell-free system was used to produce stable antibody-ribosome-mRNA (ARM) complexes to provide the linkage of genotype and phenotype, allowing simultaneous selection of a desired antibody together with its encoding mRNA. The mRNA was then recovered and amplified as DNA by reverse transcriptase-polymerase chain reaction (RT-PCR). Repeating the display cycle enriched the selected molecules, enabling rare species to be isolated. In this study, digoxin-binding segments were selected over four cycles of ARM display and the selected DNA was cloned and expressed as a single-chain variable fragment antibody (the best scFv, A3) in Escherichia coli. The affinity (equilibrium dissociation constant Kd) of digoxin was 8.3 x 10(-8) M for A3, which validated construction of the naïve library and the power of ribosome display lending to the evolution of functional characteristics, such as potency of leading candidate antibodies to provide therapeutic antibodies. A3 was purified using affinity chromatography and determined by Western blot. The results indicate that ribosome display technnique can be efficiently used to isolate specific antibody fragments from a naive library.

[Preparation of humanized anti-digoxin scFv and diabody]

AIM

To prepare humanized anti-digoxin(Dig) antibodies(single chain Fv, scFv) from a human phage antibody library and construct anti-Dig diabody vector.

METHODS

A human phage antibody library was panned against immobilized Dig for four times, and the specificity of the selected scFv expression clones was identified by ELISA. Positive clones against Dig were analyzed by DNA fingerprint and sequencing. A clone which had high affinity to Dig was selected to construct a diabody vector. Diabody was secreted from E. coli by IPTG induction, and the specificity was also identified by ELISA.

RESULTS

After four rounds of panning, four specific humanized anti-Dig antibodies(scFv) were obtained. DNA fingerprint and sequencing analysis proved that they had different Ab-encoding genes. The V(L) of the 4 expression clones belonged to lambda subgroup 1 and V(H) belonged to 3 and 4 subgroups, respectively. A scFv clone was picked out to construct diabody vector. The prepared humanized diabody reacted with Dig specifically.

CONCLUSION

Humanized anti-Dig antibodies(scFv) have been got by using phage display. A diabody has been obtained from scFv. These humanized anti-Dig antibodies may be used for the diagnosis and therapy of Dig toxication.

Effect of Brazilian, Indian, Siberian, Asian, and North American ginseng on serum digoxin measurement by immunoassays and binding of digoxin-like immunoreactive components of ginseng with Fab fragment of antidigoxin antibody (Digibind)

We compared Brazilian, Indian, Siberian, Asian, and North American ginseng for potential interference with 3 digoxin immunoassays: fluorescence polarization (FPIA), microparticle enzyme (MEIA), and Tina-quant (Roche Diagnostics, Indianapolis, IN). We supplemented aliquots of a drug-free serum pool with ginseng extracts representing expected in vivo concentrations and overdose. We observed apparent digoxin-like immunoreactivity with FPIA, modest immunoreactivity with MEIA, and no apparent digoxin immunoreactivity with the Tina-quant with all ginsengs except Brazilian, which showed no immunoreactivity with any assay. When aliquots of serum pools prepared from patients receiving digoxin were supplemented with ginsengs, we observed falsely elevated digoxin values with FPIA, falsely lower digoxin values (negative interference) with MEIA, and no interference with the Tina-quant. Digoxin-like immunoreactive components of various ginsengs have moderate protein binding; monitoring free digoxin concentrations does not eliminate such interference. We also observed that Digibind (Burroughs Wellcome, Research Triangle Park, NC) can bind free digoxin-like immunoreactive components of ginsengs; such effects can be monitored by measuring apparent free digoxin concentrations. Indian, Asian, and North American ginsengs interfere with serum digoxin measurement by FPIA and MEIA; the Tina-quant is free of such interference. Digibind can bind free digoxin-like immunoreactive components of ginseng.

Endogenous digitalis-like factors. An historical overview

The sodium pump is a ubiquitous cell surface enzyme, a Na/K-ATPase, that maintains ion gradients between cells and the extracellular fluid. The extracellular domain of this enzyme contains a highly conserved receptor for a plant-derived family of compounds, the digitalis glycosides, used in the treatment of congestive heart failure, and certain cardiac arrhythmias. The concept that an endogenous modulator of this enzyme, analogous to the cardiac glycosides, emerged from work on two separate areas: the regulation of extracellular fluid (ECF) volume by a natriuretic hormone (NH), and the regulation of peripheral vascular resistance by a circulating inhibitor of vascular Na/K-ATPase. These two areas merged with the hypothesis that natriuretic hormone and the vascular Na/K-ATPase inhibitor were the same factor, and furthermore, that this factor played a causative role in the pathophysiology of certain types of hypertension. In this communication, the development of this field from its beginnings is traced; evidence for the existence of and efforts to identify the structure of this factor are briefly reviewed, and suggestions for future development of the field are put forward.