The bioavailability of digoxin from three oral formulations measured by a specific h.p.l.c. assay

Medicago sativa and M. tunetana reveal contrasting physiological and metabolic responses to drought

Alfalfa production is frequently constrained by drought, indicating the importance of assessing species biodiversity in endemic close relatives to enhance forage production under future global change conditions. In the present study, plants of two ecotypes of M. tunetana, native to Tunisia, and four commercial cultivars of M. sativa were subjected to two water regimes (control vs drought [15% field capacity]). Physiological, isotopic and metabolic analyses were used to characterize leaf and nodule profiles of the plants. Biomass, gas exchange and the maximum carboxylation rate (Vc_max) indicated significant decreases in photosynthetic capacity under drought in M. sativa cultivars. However, M. tunetana ecotypes maintained photosynthetic performance and aboveground biomass under drought conditions. Furthermore, nitrogen isotope composition (δ¹⁵N) in nodules and leaves was significantly decreased, which reveals a reduction in the N₂ fixing activity of nodules under drought conditions that was not translated into lower leaf N content but was probably due to lower N demand. Analyses of starch, soluble sugar, and amino acid content in leaves and nodules have clearly proven the ability of Medicago spp. cultivars to increase the accumulation of osmo-protectors under drought. This study demonstrated the genetic variability of the strategy adopted among the studied cultivars in response to drought. In this sense, M. tunetana, and in part the M. sativa cultivar adapted to Mediterranean conditions, seem capable of maintaining adequate biomass, photosynthesis and biological N₂ fixation in comparison to the other M. sativa cultivars.

Geminal Diheteroatomic Motifs: Some Applications of Acetals, Ketals, and Their Sulfur and Nitrogen Homologues in Medicinal Chemistry and Drug Design

Acetals and ketals and their nitrogen and sulfur homologues are often considered to be unconventional and potentially problematic scaffolding elements or pharmacophores for the design of orally bioavailable drugs. This opinion is largely a function of the perception that such motifs might be chemically unstable under the acidic conditions of the stomach and upper gastrointestinal tract. However, even simple acetals and ketals, including acyclic molecules, can be sufficiently robust under acidic conditions to be fashioned into orally bioavailable drugs, and these structural elements are embedded in many effective therapeutic agents. The chemical stability of molecules incorporating geminal diheteroatomic motifs can be modulated by physicochemical design principles that include the judicious deployment of proximal electron-withdrawing substituents and conformational restriction. In this Perspective, we exemplify geminal diheteroatomic motifs that have been utilized in the discovery of orally bioavailable drugs or drug candidates against the backdrop of understanding their potential for chemical lability.

Expression, regulation and function of intestinal drug transporters: an update

Although oral drug administration is currently the favorable route of administration, intestinal drug absorption is challenged by several highly variable and poorly predictable processes such as gastrointestinal motility, intestinal drug solubility and intestinal metabolism. One further determinant identified and characterized during the last two decades is the intestinal drug transport that is mediated by several transmembrane proteins such as P-gp, BCRP, PEPT1 and OATP2B1. It is well-established that intestinal transporters can affect oral absorption of many drugs in a significant manner either by facilitating their cellular uptake or by pumping them back to gut lumen, which limits their oral bioavailability. Their functional relevance becomes even more apparent in cases of unwanted drug-drug interactions when concomitantly given drugs that cause transporter induction or inhibition, which in turn leads to increased or decreased drug exposure. The longitudinal expression of several intestinal transporters is not homogeneous along the human intestine, which may have functional implications on the preferable site of intestinal drug absorption. Besides the knowledge about the expression of pharmacologically relevant transporters in human intestinal tissue, their exact localization on the apical or basolateral membrane of enterocytes is also of interest but in several cases debatable. Finally, there is obviously a coordinative interplay of intestinal transporters (apical-basolateral), intestinal enzymes and transporters as well as intestinal and hepatic transporters. This review aims to give an updated overview about the expression, localization, regulation and function of clinically relevant transporter proteins in the human intestine.

Modeling the kinetics of digoxin absorption: enhancement by P-glycoprotein inhibition

An increase in the area under the curve (AUC) after oral digoxin due to coadministration of drugs known as P-glycoprotein (P-gp) inhibitors has been reported in several studies, but there is very little information on the rate of absorption after P-gp inhibition. Based on an inverse Gaussian density absorption model and using a population approach, the authors reanalyzed data showing an increase in oral digoxin AUC in healthy volunteers after coadministration of talinolol. The model fitted the data well, and the results revealed that the maximum rate of digoxin absorption increased nearly 2-fold, whereas bioavailability increased only by 21%. It is concluded that the increase in the rate of absorption seems to be a better indicator of intestinal P-gp inhibition than the increase in extent of absorption. Furthermore, the authors use a simulation study to demonstrate the ability of the method to estimate bioavailability based on the population characteristics of digoxin disposition kinetics obtained from a different group of healthy volunteers.

Undesirable effects of citrus juice on the pharmacokinetics of drugs: focus on recent studies

It is well known that intake of grapefruit juice affects the pharmacokinetics of various kinds of drugs. It has been reported that other citrus juices also interact with certain drugs. To re-evaluate citrus juice-drug interactions based on currently available evidence, a literature search was conducted for new and updated information since the grapefruit juice-drug interaction was last reviewed in 1998. MEDLINE (1998-October 2004) was accessed and more than 200 reports were found. The effects of grapefruit juice ingestion on the pharmacokinetics of orally administered drugs have been reported for 40 drugs since the reviews published in 1998. Increases in either area under the concentration-time curve (AUC) or maximum plasma concentration (C(max)) were found with 34 of these, the major mechanism being considered to be inactivation of intestinal cytochrome P450 3A4, a so-called mechanism-based inhibition. Although recent reports point to the inhibitory effects of grapefruit juice on the function of P-glycoprotein, which transports substrates from enterocytes back into the lumen, the contribution to the bioavailability of drugs that are substrates of P-glycoprotein has not been established yet. Dramatic decreases in AUC and C(max) for two drugs in association with grapefruit juice ingestion has been reported and, in these cases, inhibitory effects on organic anion transporting polypeptide, which mediates absorption from the intestinal lumen to enterocytes, might be involved. Other citrus juices such as Seville (sour) orange juice and commonly ingested varieties of orange juice also showed significant effects on the AUC and C(max) of some drugs. Although the situation is complex and uncertainties remain, we recommend that patients avoid citrus juice intake while taking medications and that healthcare providers advise against citrus juice intake in this setting until any interactions with subject drugs can be clarified in clinical studies.

Various Dosing Weights and Correction to Serum Digoxin Assays in Hemodialysis Patients

Background:

Digoxin is distributed in skeletal muscles in high concentrations. The most reliable parameter to measure the distribution of digoxin in patients undergoing hemodialysis is not known.

Objective:

To estimate which distribution parameter—estimated lean body mass (E-LBM) calculated by subtracting the fat mass from the dry weight, lean body mass, dry weight, and ideal body weight—is the most reliable predictor for assessing the accuracy of a digoxin dosing regimen in patients undergoing hemodialysis.

Methods:

A retrospective study was conducted to evaluate 21 patients undergoing hemodialysis who were administered digoxin. The patients were divided into 2 groups: digoxin 0.125 mg administered twice a week (low-dose group) or 3 times per week (high-dose group). The differences between E-LBM, lean body mass, dry weight, and ideal body weight for the low- and high-dose groups were determined. The relationships between serum digoxin concentrations and the weekly digoxin dose per E-LBM, lean body mass, dry weight, and ideal body weight were also determined.

Results:

E-LBM, lean body mass, dry weight, and ideal body weight in the high-dose group were significantly larger than those in the low-dose group (p = 0.021, 0.015, 0.024, and 0.0029, respectively), although no significant difference in serum digoxin concentrations was evident. Significant correlation was found between serum digoxin concentrations and the weekly digoxin dosage per E-LBM, dry weight, lean body mass, and ideal body weight (r = 0.746, p < 0.0001; r = 0.638, p = 0.0014; r = 0.645, p < 0.0011; r = 0.553, p = 0.0083, respectively).

Conclusions:

E-LBM appears to reflect the best parameter for predicting serum digoxin concentrations. The use of the dry weight parameter could be generally useful for adjusting the dosage of digoxin in patients undergoing hemodialysis.

Digoxin-carvedilol interactions in children

Digoxin is often coadministered with carvedilol in children with severe ventricular failure. In eight children (age 2 weeks to 8 years), the oral clearance of digoxin decreased by half with carvedilol, and two of them had digoxin toxicity. Carvedilol increases serum concentrations of digoxin in children, and its dose may need to be reduced to avoid toxicity.

The effects of tegaserod (HTF 919) on the pharmacokinetics and pharmacodynamics of digoxin in healthy subjects

Tegaserod (HTF 919) is a highly specific 5-HT4 receptor partial agonist that exhibits promotile activity throughout the gastrointestinal tract and is under development for the treatment of functional gastrointestinal motility disorders. The present study was designed to assess the effect of multiple doses of tegaserod on the single-dose pharmacokinetics and pharmacodynamics of digoxin, a commonly prescribed agent for congestive heart failure. The study was an open-label, randomized, two-period crossover design of 12 healthy subjects. One treatment included digoxin treatment alone; the other treatment included a combined digoxin and tegaserod treatment. On day 1 of the digoxin treatment period, subjects received a single 1 mg oral dose of digoxin. In the combined tegaserod/digoxin treatment period, subjects received a single oral dose of 1 mg digoxin after 3 days of tegaserod (6 mg bid). After coadministration of tegaserod, systemic exposure to digoxin was decreased; mean AUC decreased by 11.9% (p < 0.05) relative to digoxin alone. Cmax was decreased by about 15% (p < 0.05). The 0.5-hour difference in the median tmax between the two treatments was not statistically significant. Because the steady-state trough concentration of digoxin (C(SS,min)) correlates with pharmacological effects, C(SS,min) for digoxin alone and in combination with tegaserod was simulated based on both parametric compartmental modeling and nonparametric superpositioning approaches. The predicted arithmetic mean C(SS,min) for combination therapy is 86% to 89% of that following digoxin alone. Likewise, the predicted arithmetic mean steady-state peak concentration (C(SS,min)) and AUC at steady state during a dosing interval (AUC(SS,tau)) have a similar decrease. This extent of decrease in systemic exposure of digoxin at steady state is unlikely to be clinically relevant. Administration of tegaserod (6 mg bid) was well tolerated, both alone and in combination with a single dose of digoxin. There were no pharmacodynamic changes in ventricular rate and QT interval following coadministration of tegaserod with digoxin. The 1.5-hour and 2-hour postdose plasma concentrations of tegaserod on days 3 and 4 confirmed adequate exposure. In conclusion, dose adjustment for digoxin is unlikely to be needed when tegaserod is coadministered.

Specific and sensitive determination of digoxin and metabolites in human serum by high performance liquid chromatography with cyclodextrin solid-phase extraction and precolumn fluorescence derivatization

A precolumn fluorescence derivatization high performance liquid chromatographic method has been developed for the simultaneous determination of digoxin and its metabolites digoxigenin bisdigitoxoside, digoxigenin monodigitoxoside digoxigenin, and dihydrodigoxin (20-R and 20-S epimers) in human serum. Digoxin and its metabolites were extracted from serum samples (containing digitoxin as internal standard) with a cyclodextrin solid-phase extraction (SPE) column. Fluorescent derivatives were formed by reaction of the analytes with 1-naphthoyl chloride in the presence of 4-dimethylaminopyridine under a nitrogen atmosphere in a glove box with controlled relative humidity (26% r.h. or less). The derivatives were isolated using cyclodextrin and C1 SPE columns sequentially, and determined by HPLC using silica column separation and fluorescence detection. Calibration curves were linear over the concentration range from 0.25 to 4.0 ng ml-1. Recoveries of digoxin and its metabolites from serum ranged from 62 to 86%, and coefficients of variation from repetitive analyses ranged from 6.9 to 20.9% and from 5.8 to 12.2% at 0.5 ng ml-1 and 2.0 ng ml-1, respectively. This method has been shown capable of specifically determining digoxin and its major metabolites in serum, and has been successfully used in the determination of digoxin and its metabolites in serum samples collected from patients undergoing digoxin therapy. This method thus permits the investigation of digoxin metabolism and pharmacokinetics after the administration of commercial dosage forms.

The influence of reduced dietary fat absorption induced by orlistat on the pharmacokinetics of digoxin in healthy volunteers

To assess the influence of an orlistat-induced reduction in dietary fat absorption on the pharmacokinetics of digoxin, an open-label, placebo-controlled, randomized, two-way crossover study was performed in 12 healthy volunteers. Each subject received single 0.4-mg doses of digoxin (soft gelatin capsules) administered orally on the fourth day of orlistat (120 mg three times daily for 6 days) and placebo (three times daily for 6 days) treatment, separated by at least an 11-day washout period. Serial blood samples were collected before and at appropriate intervals after each digoxin dose to determine plasma concentrations of unchanged digoxin. The 90% confidence intervals for the ratio of geometric least-squares means (for Cmax, AUC0-48, AUC0-t, and AUC) and for the difference of arithmetic least-squares means (for tmax and lambda z) indicate that the pharmacokinetics of digoxin was not altered by treatment with orlistat. This results suggests that a approximately 30% reduction in dietary fat absorption will not change the efficacy of digoxin in cardiac patients.