Vitamin D bioavailability: state of the art.

There has been renewed interest in vitamin D since numerous recent studies have suggested that besides its well-established roles in bone metabolism and immunity, vitamin D status is inversely associated with the incidence of several diseases, e.g., cancers, cardio-vascular diseases, and neurodegenerative diseases. Surprisingly, there is very little data on factors that affect absorption of this fat-soluble vitamin, although it is acknowledged that dietary vitamin D could help to fight against the subdeficient vitamin D status that is common in several populations. This review describes the state of the art concerning the fate of vitamin D in the human upper gastrointestinal tract and on the factors assumed to affect its absorption efficiency. The main conclusions are: (i) ergocalciferol (vitamin D2), the form mostly used in supplements and fortified foods, is apparently absorbed with similar efficiency to cholecalciferol (vitamin D3, the main dietary form), (ii) 25-hydroxyvitamin D (25OHD), the metabolite produced in the liver, and which can be found in foods, is better absorbed than the nonhydroxy vitamin D forms cholecalciferol and ergocalciferol, (iii) the amount of fat with which vitamin D is ingested does not seem to significantly modify the bioavailability of vitamin D3, (iv) the food matrix has apparently little effect on vitamin D bioavailability, (v) sucrose polyesters (Olestra) and tetrahydrolipstatin (orlistat) probably diminish vitamin D absorption, and (vi) there is apparently no effect of aging on vitamin D absorption efficiency. We also find that there is insufficient, or even no data on the following factors suspected of affecting vitamin D bioavailability: (i) effect of type and amount of dietary fiber, (ii) effect of vitamin D status, and (iii) effect of genetic variation in proteins involved in its intestinal absorption. In conclusion, further studies are needed to improve our knowledge of factors affecting vitamin D absorption efficiency. Clinical studies with labeled vitamin D, e.g., deuterated or (13)C, are needed to accurately and definitively assess the effect of various factors on its bioavailability.

ESPEN Guidelines on Parenteral Nutrition: adult renal failure.

Among patients with renal failure, those with ARF and critical illness represent by far the largest group undergoing artificial nutrition. ARF, especially in the ICU, seldom occurs as isolated organ failure but rather is a component of a much more complex metabolic environment, in the setting of the multiple organ failure. Nutritional programs for ARF patients must consider not only the metabolic derangements peculiar to renal failure and with the underlying disease process/associated complications, but also the relevant derangements in nutrient balance due to renal replacement therapies, especially when highly efficient renal replacement therapies (RRT) are used, such as continuous veno-venous hemofiltration (CVVH), or prolonged intermittent modalities such as sustained low-efficiency dialysis (SLED). Finally it is to be taken into account that nutrient requirements can change considerably during the course of illness itself (see also guidelines on PN in intensive care). From a metabolic point of view, patients with CKD or on chronic HD who develop a superimposed acute illness should be considered to be similar to patients with ARF. The same principles in respect of PN should therefore be applied.

Nutritional depletion in patients on long-term oxygen therapy and/or home mechanical ventilation.

The purpose of this study was to estimate the prevalence of malnutrition in outpatients on long-term oxygen therapy or home mechanical ventilation, to determine the relationships between malnutrition and impairment/disability and smoking and also to identify relevant tools for routine nutritional assessment. In 744 patients (M:F 1.68, aged 65+/-15 yrs) with chronic obstructive pulmonary disease (COPD, 40%), restrictive disorders (27%), mixed respiratory failure (15%), neuromuscular diseases (13%) and bronchiectasis (5%), body mass index (BMI), fat-free mass (FFM), serum albumin, transthyretin, 6-min walking test, forced vital capacity (FVC), forced expiratory volume in one second (FEV1) and blood gases were recorded. FFM was the most sensitive parameter for detecting malnutrition, being abnormal in 53.6% of patients, while BMI was <20 in 23.2%, serum albumin <35 g x L(-1) in 20.7%, and serum transthyretin <200 mg x L(-1) in 20%. FFM depletion predominated in neuromuscular, bronchiectasis and restrictive disorders. BMI and FFM were correlated with FEV1, FVC and 6-min walking test. In multivariate analysis a BMI<20 was related to FEV1 and smoking habits, and a low FFM to smoking, FEV1 and female sex. Malnutrition is highly prevalent in home-assisted respiratory patients and is related to causal disease, forced expiratory volume in one second, smoking and disability. Fat-free mass appeared to be the most sensitive and relevant nutritional parameter according to impairment and disability.

Dose-dependent reversal of digoxin-inhibited activity of an in vitro Na+K+ATPase model by digoxin-specific antibody.

We investigated the potency of digoxin-specific Fab fragments to reverse digoxin-induced Na+K+ATPase inhibition in rat brain microsomes according to (a) the extent of initial inhibition of Na+K+ATPase and (b) the neutralizing dose of antibody. Mathematical analysis of the digoxin concentration-Na+K+ATPase inhibition curve supports the existence of 2 digoxin sensitive Na+K+ATPase isoforms. The IC50 was 1.3 x 10(-4) M and 2.5 x 10(-8) M for the low (alpha 1) and high (alpha 2) digoxin affinity isoenzyme, respectively. The reversal of digoxin-induced Na+K+ATPase inhibition was dependent on the digoxin-specific Fab concentration. The maximal effect was observed when the Fab:digoxin ratio was stoichiometrical and addition of an excess of antibodies did not result in a complete reversal of inhibition at the 4 digoxin concentrations studied. This simple and rapid in vitro model will be a useful tool to predict the efficacy of a new generation of antibodies.

Affinity and dose-dependent digoxin Na+K+ATPase dissociation by monoclonal digoxin-specific antibodies.

The effect of three monoclonal digoxin-specific antibodies and of polyclonal Digidot as reference on digoxin dissociation from rat brain Na+K+ATPase microsomes was studied to determine the role of the affinity constant (Ka) and dose of the antibody on the rate of digoxin dissociation from Na+K+ATPase. Stoichiometrical doses of 1C10, 6C9, 9F5 IgG, and Digidot (Ka = 6 10(9), 3.1 10(8), 2.5 10(7), and 8.5 10(9) M-1, respectively) resulted in digoxin dissociation related to Ka. When the IgG:digoxin molar ratio increased from 0.25 to 10, digoxin dissociation from Na+K+ATPase sites also increased according to the Hill equation, allowing comparative parameters among the three antibodies to be determined. 1C10 IgG was 2- and 10-fold more efficacious than 6C9 and 9F5, respectively. This in vitro model appears to be a useful predictive screening assay before in vivo experimentation.

Monoclonal digoxin-specific antibodies induce dose- and affinity-dependent plasma digoxin redistribution in rats.

The effect of three monoclonal digoxin-specific antibodies on total and free digoxin plasma disposition was studied in rats in order to determine the role of affinity constant (Ka) and dose. Thirty minutes after digoxin infusion, administration of a stoichiometrical dose of the ICIO, 6C9 and 9F5 IgG (Ka = 6 10(9), 3.1 10(8) and 2.5 10(7) M-1, respectively) resulted in a plasma digoxin increase linearly related to Ka. The mean free plasma digoxin was 0.6 +/- 0.4, 7.8 +/- 3.3 and 43 +/- 22% respectively after 1C10, 6C9, and 9F5 IgG infusion in comparison to 70 +/- 9% in the control group. When the IgG:digoxin ratio increased from 1 to 5, plasma digoxin Cmax and AUCT also increased as a function of both affinity (Ka) and dose (N), but not linearly. The product of NKa defined an immunoreactivity factor that was well fitted to the digoxin redistribution parameters (Cmax and AUCT) by a Hill equation.

Development of a sensitive radioimmunoassay for Fab fragments: application to Fab pharmacokinetics in humans.

Anti-sheep Fab fragment antisera were produced in rabbits using sheep digoxin-specific Fab fragments (Digidot) as immunogen. These antisera were used for the development of a radioimmunoassay (RIA) of sheep Fab fragments in human plasma and urine using 125I-labeled Fab fragments. Interference in the assays by digoxin, human proteins, and antibodies from different species was insignificant, but cross-reactivity between anti-sheep Fab antisera and goat IgG or Fab fragments was 22 to 67%. The limit of detection was 0.1 microgram/mL and the assay was linear over a 0.6-28 micrograms/mL range of Fab fragments. Intra- and interassay coefficients of variation were less than 6.9 and 10.5%, respectively. Accuracy of plasma and urine assays at various Fab fragment levels ranged from 96 to 106%. RIA was applied to the pharmacokinetic study of sheep digoxin-specific Fab fragments in one patient acutely intoxicated by digitoxin and treated with Digidot. The Fab elimination half-life was 12.1 hr. Steady-state volume of distribution and total-body clearance were 10.8 L and 23.4 mL/min, respectively. Unchanged Fab fragments (50 kD) and degradation products (25 kD) isolated by gel filtration chromatography of a urine sample cross-reacted with the anti-Fab antiserum.

Fab-bound colchicine appears to adopt Fab fragment disposition in rats.

The disposition of colchicine-specific Fab fragments and the effect of Fab fragment administration on the disposition of colchicine were studied in anaesthetized bile duct-cannulated rats. One group of rats (n = 6) received a 125I-Fab dose of 38 mg kg-1 i.v. The plasma disposition was characterized by a volume of distribution of 179 +/- 48 mL kg-1, total body clearance of 1.02 +/- 0.07 mL min-1 kg-1, t1/2 alpha of 0.17 +/- 0.03 h and t1/2 beta of 1.3 +/- 0.3 h. Fab fragments were in part excreted by the renal route (15.6 +/- 6% of the Fab dose), while biliary excretion was a minor route (< 2% of the Fab dose). Two other groups of rats received 15 micrograms kg-1 colchicine (n = 6) or 15 micrograms kg-1 colchicine plus 38 mg kg-1 colchicine-specific Fab fragments (n = 6) by intravenous infusion. Pharmacokinetics of colchicine was markedly altered in the Fab-colchicine-treated rats. In this group, distribution volume and total body clearance of colchicine were decreased by factors of 22 and 10, respectively, compared with the values in the colchicine-treated group and were very similar to those of Fab fragments. An 80% reduction of cumulative biliary excretion of colchicine was observed in Fab-colchicine-treated rats (P < 0.01). The fraction of colchicine dose excreted by the urinary route was 38 +/- 6.9 and 9 +/- 0.7% respectively in Fab-colchicine- and colchicine-treated groups (P < 0.01). These data show that during Fab treatment, colchicine followed the elimination kinetics of Fab fragments.(ABSTRACT TRUNCATED AT 250 WORDS)

Analytical control procedures of immunoreactivity for IgG and Fab fragments specific to haptens.

This study investigates immunoreactivity control procedures, i.e., specificity, affinity constant (Ka), and specific active binding sites (SABS), for polyclonal anticolchicine, monoclonal antidigitoxin IgG and Fab fragments, and antidigoxin Fab fragments (Digidot). Preliminary control procedures for IgG and Fab fragment purity indicated that all reagents were immunologically pure. All IgG and Fab fragments exhibited similar cross-reactivity and Ka. No decrease in percentage of Fab fragment SABS was observed after papain cleavage of anticolchicine and antidigitoxin IgG. Nevertheless, only 4.3 +/- 1.2% of nonimmunopurified anticolchicine polyclonal Fab fragments and 76.2 +/- 2.3 to 88.7 +/- 2.5% of different batches of immunopurified anti-digoxin Fab (Digidot) were active, the latter percentage being in the range of the 85% specified by the manufacturer. Only 58 +/- 3% of digitoxin-specific monoclonal IgG was active and 67 +/- 7% of its Fab fragments. Results show the importance of determining the ratio of SABS to presumed total specific binding sites for pharmaceutical monoclonal and polyclonal antibody preparations against haptens.

Colchicine-specific Fab fragments alter colchicine disposition in rabbits.

High-affinity goat antibodies and Fab fragments (Ka = 1.1 x 10(10) M(-1) specific to colchicine were prepared to study their effect on colchicine pharmacokinetics in rabbits. First, colchicine disposition kinetics were investigated in four control rabbits after administration of 0.1 mg/kg i.v. Total and free plasma and urine colchicine were assayed by specific radioimmunoassay. The mean elimination half-life of total plasma colchicine was 16 +/- 2.9 h. Colchicine has a large volume of distribution (8.8 +/- 1.8 l/kg) and a low systemic clearance (114.6 +/- 3.4 ml.h-1.kg-1). Renal clearance represented 30.7 +/- 1.9% of total body clearance. The free plasma colchicine fraction was 70% after equilibrium dialysis. Second, 1.5 h after injection of 0.1 mg/kg colchicine, four rabbits were infused over 0.25 h with colchicine-specific Fab fragments at a half-stoichiometrically equivalent dose compared to the colchicine dose. Within 15 min after Fab infusion, total colchicine concentrations increased 10- to 16-fold. Mean area under the plasma concentration-time curves increased 20-fold compared to controls. The free plasma fraction decreased to an undetectable level over a period of 2 h. The Fab fragment administration also produced, respectively, a 24- and 17-fold decrease in the volume of distribution and systemic clearance. Colchicine recovered in urine was significantly higher than in the control group: 44.7 +/- 2.3 and 30.9 +/- 2% of the dose, respectively (P less than .05). These data suggest that high-affinity colchicine-specific Fab fragments can sequestrate and extract colchicine from tissues to the vascular compartment with subsequent colchicine excretion by the renal route.