Protocol development for biological tracer studies

Synthesis, in Vitro Covalent Binding Evaluation, and Metabolism of (14)C-Labeled Inhibitors of 11β-HSD1

In this letter, we reported the design and synthesis of three potent, selective, and orally bioavailable 11β-HSD1 inhibitors labeled with (14)C: AMG 456 (1), AM-6949 (2), and AM-7715 (3). We evaluated the covalent protein binding of the labeled inhibitors in human liver microsomes in vitro and assessed their potential bioactivation risk in humans. We then studied the in vitro mechanism of 2 in human hepatocytes and the formation of reactive intermediates. Our study results suggest that 1 and 3 have low potential for metabolic bioactivation in humans, while 2 has relatively high risk.

Carbon isotopes profiles of human whole blood, plasma, red blood cells, urine and feces for biological/biomedical 14C-accelerator mass spectrometry applications

Radiocarbon ((14)C) is an ideal tracer for in vivo human ADME (absorption, distribution, metabolism, elimination) and PBPK (physiological-based pharmacokinetic) studies. Living plants peferentially incorporate atmospheric (14)CO(2) versus (13)CO(2) versus (12)CO(2), which result in unique signature. Furthermore, plants and the food chains they support also have unique carbon isotope signatures. Humans, at the top of the food chain, consequently acquire isotopic concentrations in the tissues and body fluids depending on their dietary habits. In preparation of ADME and PBPK studies, 12 healthy subjects were recruited. The human baseline (specific to each individual and their diet) total carbon (TC) and carbon isotope (13)C (δ(13)C) and (14)C (F(m)) were quantified in whole blood (WB), plasma, washed red blood cell (RBC), urine, and feces. TC (mg of C/100 μL) in WB, plasma, RBC, urine, and feces were 11.0, 4.37, 7.57, 0.53, and 1.90, respectively. TC in WB, RBC, and feces was higher in men over women, P < 0.05. Mean δ(13)C were ranked low to high as follows: feces < WB = plasma = RBC = urine, P < 0.0001. δ(13)C was not affected by gender. Our analytic method shifted δ(13)C by only ±1.0 ‰ ensuring our F(m) measurements were accurate and precise. Mean F(m) were ranked low to high as follows: plasma = urine < WB = RBC = feces, P < 0.05. F(m) in feces was higher for men over women, P < 0.05. Only in WB, (14)C levels (F(m)) and TC were correlated with one another (r = 0.746, P < 0.01). Considering the lag time to incorporate atmospheric (14)C into plant foods (vegetarian) and or then into animal foods (nonvegetarian), the measured F(m) of WB in our population (recruited April 2009) was 1.0468 ± 0.0022 (mean ± SD), and the F(m) of WB matched the (extrapolated) atmospheric F(m) of 1.0477 in 2008. This study is important in presenting a procedure to determine a baseline for a study group for human ADME and PBPK studies using (14)C as a tracer.

Human radiolabeled mass balance studies: objectives, utilities and limitations

The determination of metabolic pathways of a drug candidate through the identification of circulating and excreted metabolites is vitally important to understanding its physical and biological effects. Knowledge of metabolite profiles of a drug candidate in animals and humans is essential to ensure that animal species used in toxicological evaluations of new drug candidates are appropriate models of humans. The recent FDA final guidance recommends that human oxidative metabolites whose exposure exceeds 10% of the parent AUC at steady-state should be assessed in at least one of the preclinical animal species used in toxicological assessment. Additional toxicological testing on metabolites that have higher exposure in humans than in preclinical species may be required. The metabolite profiles in laboratory animals and humans are generally accomplished by mass balance and excretion studies in which radiolabeled drugs are administered to these species. The biological fluids are collected, analysed for total radioactivity and evaluated for a quantitative profile of metabolites. Thus, these studies not only determine the rates and routes of excretion but also provide very critical information on the metabolic pathways of drugs in preclinical species and humans. In addition, these studies are required by regulatory agencies for the new drug approval process. Despite the usefulness of these radiolabeled mass balance studies, there is little concrete guidance on how to perform or assess these complex studies. This article examines the objectives, utilities and limitations of these studies and how these studies could be used for the determination of the metabolite exposure in animals and humans.

Mass Balance Studies, with a Focus on Anticancer Drugs

The importance of in-depth knowledge about the pharmacokinetics of a drug is evident because pharmacokinetic behaviour may correlate with activity and toxicity. The most elaborate pharmacokinetic investigation is a so-called mass balance study employing a radioactive tracer. A mass balance study investigates the plasma pharmacokinetics and excretion of both the unchanged drug and the total radioactivity (drug and metabolites), and allows elucidation of the metabolic fate of a drug. The main objective is the maximum recovery of the radioactive dose in urine and faeces. However, there is little concrete guidance on how to perform or assess such a mass balance study. Based on a critical review of the available literature and regulation, we discuss the design, conduct and evaluation of mass balance studies. The study must be preceded by pre-clinical studies to optimise the design. Compliance with available regulation (US FDA is more specific than European guidelines), choice of radioisotope (tritium may be lost and is consequently used less often), selection of subjects and safe administration of a relevant dose are critical. The drug (both the labelled and the non-labelled portion of the dose) must be prepared, characterised and administered, and the biological samples must be collected and analysed properly to obtain reliable and meaningful results. A recent development is the use of highly sensitive accelerator mass spectrometry for (14)C-detection, which allows the use of much lower amounts of radioactivity. Calculating the mass balance requires quantification of any possible losses of radioactivity during the study, e.g. during administration (adsorption to tubing). Total recovery should be at least 90% of the administered dose. A lower recovery, which is not uncommon, must be explained by biological factors such as a long decay half-life, irreversible binding to tissue components, or loss through expiration. The extent and relevance of metabolism is determined by comparing the pharmacokinetics of the unchanged drug and the total radioactivity. Metabolic profiling supplements existing knowledge of metabolism. We illustrate the important aspects of a mass balance study with literature on anticancer drugs. This review could serve as guidance for future mass balance studies.

Kinetics of 14C distribution after tracer dose of 14C-lutein in an adult woman

Lutein is an oxygenated carotenoid (xanthophyll) found in dark green leafy vegetables. High intakes of lutein may lower the risk of age-related macular degeneration. Current understanding of human lutein metabolism as it might occur in vivo is incomplete. Therefore, we conducted a feasibility study where we dosed a normal adult woman with 14C-lutein (125 nmol, 36 nCi 14C), dissolved in olive oil (0.5 g/kg body weight) and mixed in a banana shake. Blood, urine, and feces collected before the dose was administered served to establish baseline values. Thereafter, blood was collected for 63 d following the dose, while feces and urine were collected for 2 wk post-dose. The 14C contents in plasma, urine, and feces were measured by accelerator MS. The 14C first appeared in plasma 1 h after dosing and reached its highest level, approximately2.08% of dose/L plasma, at 14 h post-dose. The plasma pattern of 14C did not include a chylomicrons/VLDL (intestinal) peak like that when the same subject received 14C-beta-carotene (a previous test), suggesting that lutein was handled differently from beta-carotene by plasma lipoproteins. Lutein had an elimination half-life (t1/2) of approximately10 d. Forty-five percent of the dose of 14C was eliminated in feces and 10% in urine in the first 2 d after dosing. Quantifying human lutein metabolism is a fertile area for future research.

Carotenoid and retinoid metabolism: insights from isotope studies

Use of isotopes as tracers has had an important role in elucidating key features of vitamin A and retinoid metabolism in animal models and humans. Their use has shown that beta-carotene absorption is variable, and that the appearance of beta-carotene and its metabolites in the blood by time since dosing follows characteristic patterns. Retinol formed from beta-carotene shows a different pattern, as does lutein. In this article, we summarize and discuss insights and some surprises into the absorption and metabolism of vitamin A, beta-carotene, and lutein that were gained with the use of isotope tracers in humans, rats, and cells as models.

Profiling and in vivo Quantification of Proteins by High Resolution Mass Spectrometry: The Example of Goserelin, an Analogue of Luteinizing Hormone-Releasing Hormone

Proteins are essential biomolecules which are frequently involved in major pathological syndromes and are widely used as diagnostic markers or therapeutic agents. The emergence of proteomics will doubtless further increase the significance of proteins both in the clinic and in the life sciences in general. Our main objective is to offer innovative solutions to what we like to call the "post-proteomics era". To achieve our goal, we intend to develop novel approaches and technologies for in vivo metabolic studies of proteins using mass spectrometry (MS), focusing on pharmacokinetics and pharmacodynamics. Using goserelin as a model, we have successfully developed and validated a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the detection and quantification of an intact analogue of luteinizing hormone-releasing hormone (LHRH) in small volumes of rat plasma samples at concentrations ranging from 0.3 to 405.0 microg/l. To this end, a microbore reversed-phase-HPLC system was coupled on-line to a tandem high resolution quadrupole time-of-flight (Q-TOF) instrument fitted with an electrospray ion source and operated in LC-MS/MS mode. External calibration was used and the high resolution was crucial to discard contaminating signals, which would not have been possible with the more conventional triple quadrupole mass spectrometers operated in a static mode. For low sample amounts, calibration curves were constructed corresponding to rat plasma levels of 0.3 to 16.4 microg/l and found to be of third order with a coefficient of determination greater than 0.999. The relative standard deviation was found to be lower than 15%. A lower limit of detection (LLOD) of 0.17 microg/l and a lower limit of quantification (LLOQ) of 0.3 microg/l were determined.

Isotopic tracer techniques for studying the bioavailability and bioefficacy of dietary carotenoids, particularly beta-carotene, in humans: a review

Vitamin A deficiency is a serious health problem in many developing countries. Provitamin A carotenoids in fruit and vegetables are the major source of vitamin A for a large proportion of the world's population. However, the contribution of plant foods is substantial only when both the consumption and provitamin A content of such food is high and, at the same time, the bioefficacy of the provitamin A is high. With respect to provitamin A carotenoids, the term bioefficacy is defined as the product of the fraction of the ingested amount that is absorbed (bioavailability) and the fraction of that which is converted to retinol in the body (bioconversion). Isotopic tracer techniques can meet the need for accurate and precise estimates of the bioavailability, bioconversion, and bioefficacy of dietary carotenoids in humans. Use of such techniques will enable proper evaluation of food-based approaches to eliminating vitamin A deficiency. In addition, the putative antioxidant capacities of carotenoids can be better understood if their bioavailability is known. Here, we discuss how tracer techniques can be applied to obtain reliable and representative data. A step-by-step discussion of aspects related to these techniques is provided, including study design, choice of isotopic tracers, dosing regimen, collection of samples, chemical analysis of samples, and data analysis.

In vivo kinetics of folate metabolism

Investigation of the in vivo kinetics of folate metabolism provides information that contributes to a better understanding of the manner in which this vitamin is processed in vivo. Kinetic studies can yield insight into the requirements for folate, especially with respect to factors that may lead to increased requirements. This review considers the strengths and weaknesses of various approaches to the study of folate kinetics and resulting data, followed by a summary and interpretation of existing data.

Variability in conversion of beta-carotene to vitamin A in men as measured by using a double-tracer study design

BACKGROUND

The vitamin A activity of beta-carotene is variable and surprisingly low in women. The reasons for this are not well understood. The vitamin A activity of beta-carotene in men is still uncertain. Contributions of dietary factors compared with individual traits are largely unknown.

OBJECTIVE

Our objective was to measure the intrinsic variability in the vitamin A activity of beta-carotene among healthy, well-fed men living in a controlled environment.

DESIGN

We used a double-tracer test-retest design. We dosed 11 healthy men orally with 30 micromol hexadeuterated (D6) retinyl acetate (all-trans-19,19,19,20,20,20-[2H6]retinyl acetate) and then with 37 micromol D6 beta-carotene (19,19,19,19',19',19'-[2H6]beta-carotene) 1 wk later. Doses were taken with breakfasts containing 16 g fat. We measured D6 retinol, D6 beta-carotene, and trideuterated (D3) retinol (derived from D6 beta-carotene) concentrations in plasma. Areas under the plasma concentration x time since dosing curves (AUCs) were determined for D6 retinol, D6 beta-carotene, and D3 retinol.

RESULTS

All men had detectable D6 retinol concentrations in plasma. The mean (+/-SE) absorption of D6 beta-carotene in all subjects was 2.235 +/- 0.925%, and the mean conversion ratio was 0.0296 +/- 0.0108 mol retinol to 1 mol beta-carotene. Only 6 of 11 men had sufficient plasma concentrations of D6 beta-carotene and D3 retinol that we could measure. The mean absorption of D6 beta-carotene in these 6 subjects was 4.097 +/- 1.208%, and the mean conversion ratio was 0.0540 +/- 0.0128 mol retinol to 1 mol beta-carotene.

CONCLUSION

The vitamin A activity of beta-carotene, even when measured under controlled conditions, can be surprisingly low and variable.

Variability of the conversion of beta-carotene to vitamin A in women measured by using a double-tracer study design

BACKGROUND

Blood beta-carotene and vitamin A responses to oral beta-carotene are variable in humans. Some individuals are characterized as responders and others as low- or nonresponders. A better understanding of the conditions that produce the variability is important to help design public health programs that ensure vitamin A sufficiency.

OBJECTIVE

Our objective was to assess variability in absorption and conversion of beta-carotene to vitamin A in vivo in humans by using a novel double-tracer ¿hexadeuterated (D(6)) beta-carotene and D(6) retinyl acetate approach.

DESIGN

Eleven healthy women were housed at the US Department of Agriculture Western Human Nutrition Research Center metabolic unit for 44 d, where they consumed diets adequate in vitamins and minerals except for carotenoids. After an adaptation period, the women were given 30 micromol D(6) retinyl acetate orally, followed 1 wk later with 37 micromol D(6) beta-carotene (approximately equimolar doses). Time-dependent plasma concentration curves were determined for D(6) retinol, D(6) beta-carotene, and trideuterated (D(3)) retinol (derived from D(6) beta-carotene).

RESULTS

Mean (+/-SE) absorption of D(6) beta-carotene was 3.3 +/- 1.3% for all subjects. The mean conversion ratio was 0.81 +/- 0.34 mol D(3) retinol to 1 mol D(6) beta-carotene for all subjects. However, only 6 of the 11 subjects had plasma D(6) beta-carotene and D(3) retinol concentrations that we could measure. The mean absorption of D(6) beta-carotene in these 6 subjects was 6.1 +/- 0.02% and their conversion ratio was 1.47 +/- 0.49 mol D(3) retinol to 1 mol D(6) beta-carotene. The remaining 5 subjects were low responders with </=0.01% absorption and a mean conversion ratio of 0.014 +/- 0.004 mol D(3) retinol to 1 mol D(6) beta-carotene.

CONCLUSION

Variable absorption and conversion of beta-carotene to vitamin A both contribute to the variable response to consumption of beta-carotene. Our double-tracer approach is adaptable for identifying efficient converters of carotenoid to retinoid.