Imaging of Gastrointestinal Absorption and Biodistribution of an Orally Administered Probe Using Positron Emission Tomography in Humans

Evidence of a Gastro-Duodenal Effect on Adipose Tissue and Brain Metabolism, Potentially Mediated by Gut-Liver Inflammation: A Study with Positron Emission Tomography and Oral 18FDG in Mice

Interventions affecting gastrointestinal (GI) physiology suggest that the GI tract plays an important role in modulating the uptake of ingested glucose by body tissues. We aimed at validating the use of positron emission tomography (PET) with oral ¹⁸FDG administration in mice, and to examine GI effects on glucose metabolism in adipose tissues, brain, heart, muscle, and liver, and interfering actions of oral lipid co-administration. We performed sequential whole-body PET studies in 3 groups of 10 mice, receiving i.p. glucose and ¹⁸FDG or oral glucose and ¹⁸FDG ± lipids, to measure tissue glucose uptake (GU) and GI transit, and compute the absorption lumped constant (LCa) as ratio of oral ¹⁸FDG-to-glucose incremental blood levels. GI and liver histology and circulating hormones were tested to generate explanatory hypothesis. Median LCa was 1.18, constant over time and not significantly affected by lipid co-ingestion. Compared to the i.p. route, the oral route (GI effect) resulted in lower GU rates in adipose tissues and brain, and a greater steatohepatitis score (+17%, p = 0.03). Lipid co-administration accelerated GI transit, in relation to the suppression in GIP, GLP1, glucagon, PP, and PYY (GI motility regulators), abolishing GI effects on subcutaneous fat GU. Duodenal crypt size, gastric wall ¹⁸FDG uptake, and macro-vesicular steatosis were inversely related to adipose tissue GU, and positively associated with liver GU. We conclude that ¹⁸FDG-PET is a suitable tool to examine the role of the GI tract on glucose transit, absorption, and bio-distribution. The GI effect consists in the suppression of glucose metabolism selectively in organs responsible for energy intake and storage, and is blunted by lipid ingestion. Modulation of gut and liver inflammation, as reflected by high GU, may be involved in the acute signalling of the energy status.

PET imaging of 11C-labeled thiamine tetrahydrofurfuryl disulfide, vitamin B1 derivative: First-in-human study

Vitamine B₁ thiamine is an essential component for glucose metabolism and energy production. The disulfide derivative, thiamine tetrahydrofurfuryl disulfide (TTFD), is more absorbent compared to readily-available water-soluble thiamine salts since it does not require the rate-limiting transport system required for thiamine absorption. However, the detailed pharmacokinetics of thiamine and TTFD under normal and pathological conditions were not clarified yet. Recently, ¹¹C-labeled thiamine and TTFD were synthesized by our group, and their pharmacokinetics were investigated by PET imaging in normal rats. In this study, to clarify the whole body pharmacokinetics of [¹¹C]TTFD in human healthy volunteers, we performed first-in-human PET imaging study with [¹¹C]TTFD, along with radiation dosimetry of [¹¹C]TTFD in humans.

METHODS

Synthesis of [¹¹C]TTFD was improved for clinical study. Dynamic whole-body PET images were acquired on three young male normal subjects after intravenous injection of [¹¹C]TTFD. VOIs were defined for source organs on the PET images to measure time-course of [¹¹C]TTFD uptake as percentage injected dose and the number of disintegrations for each organ. Radiation dosimetry was calculated with OLINDA/EXM.

RESULTS

We succeeded in developing the improved synthetic method of [¹¹C]TTFD for the first-in-human PET study. In the whole body imaging, uptake of [¹¹C]TTFD by various tissues was almost plateaued at 10 min after intravenous injection, afterward gradually increased for the brain and urinary bladder (urine). %Injected dose was high in the liver, kidney, urinary bladder, heart, spine, brain, spleen, pancreas, stomach, and salivary glands, in this order. %Injected dose per gram of tissue was high also in the pituitary. By dosimetry, the effective radiation dose of [¹¹C]TTFD calculated was 5.5 μSv/MBq (range 5.2-5.7).

CONCLUSION

Novel synthetic method enabled clinical PET study with [¹¹C]TTFD, which is a safe PET tracer with a dosimetry profile comparable to other common ¹¹C-PET tracers. Pharmacokinetics of TTFD in the pharmacological dose and at different nutritional states could be further investigated by future quantitative PET studies. Noninvasive in vivo PET imaging for pathophysiology of thiamine-related function may provide diagnostic evidence of novel information about vitamin B₁ deficiency in human tissues.

In Vitro Sensitivity Analysis of the Gastrointestinal Dissolution Profile of Weakly Basic Drugs in the Stomach-to-Intestine Fluid Changing System: Explanation for Variable Plasma Exposure after Oral Administration

An in vitro methodology for simulating the change in the pH and composition of gastrointestinal fluid associated with the transition of orally administered drugs from the stomach to the small intestine was developed (the stomach-to-intestine fluid changing system (the SIFC system)). This system was applied to in vitro sensitivity analysis on the dissolution of weakly basic drugs, and the obtained results were discussed in relation to the intrasubject variability in the plasma exposure in human bioequivalence (BE) study. Three types of protocols were employed (steep pH change: pH 1.6 FaSSGF → pH 6.5 FaSSIF, gradual pH change: pH 1.6 FaSSGF → pH 6.5 FaSSIF, and high gastric pH: pH 4.0 FaSSGF → pH 6.5 FaSSIF). Regardless of the protocols and the forms of drug applied in active pharmaceutical ingredient powder or formulation, dissolution profiles of pioglitazone after fluid shift were similar and the final concentrations in FaSSIF were approximately equal to the saturation solubility in FaSSIF, supporting its small intrasubject variance in human BE study. In contrast, dissolved concentration of terbinafine in the SIFC system became less than half in the high gastric pH protocol than that in other protocols, suggesting the fluctuation of gastric pH as one of the factors of high intrasubject variance of terbinafine in human. Plasma exposure of telmisartan was highly variable especially at the high dose. Although the dissolution of telmisartan in the SIFC system was greatly improved by formulation, it considerably fluctuated during fluid shift especially at the high dose, which corresponds well to in vivo results.

68Ga-NOTA PET imaging for gastric emptying assessment in mice

BACKGROUND

Positron emission tomography (PET) has the potential for visualization and quantification of gastric emptying (GE). The traditional Chinese medicine (TCM) has been recognized promising for constipation. This study aimed to establish a PET imaging method for noninvasive GE measurement and to evaluate the efficacy of a TCM on delayed GE caused by constipation using PET imaging.

METHODS

[⁶⁸Ga]Ga-NOTA was synthesized as the tracer and sesame paste with different viscosity were selected as test meals. The dynamic PET scans were performed after [⁶⁸Ga]Ga-NOTA mixed with test meals were administered to normal mice. Two methods were utilized for the quantification of PET imaging. A constipation mouse model was treated with maren chengqi decoction (MCD), and the established PET imaging scans were performed after the treatment.

RESULTS

[⁶⁸Ga]Ga-NOTA was synthesized within 20 min, and its radiochemical purity was > 95%. PET images showed the dynamic process of GE. %ID/g, volume, and total activity correlated well with each other. Among which, the half of GE time derived from %ID/g for 4 test meals were 3.92 ± 0.87 min, 13.1 ± 1.25 min, 17.8 ± 1.31 min, and 59.7 ± 3.11 min, respectively. Constipation mice treated with MCD showed improved body weight and fecal conditions as well as ameliorated GE measured by [⁶⁸Ga]Ga-NOTA PET.

CONCLUSIONS

A PET imaging method for noninvasive GE measurement was established with stable radiotracer, high image quality, and reliable quantification methods. The efficacy of MCD on delayed GE was demonstrated using PET.

Oral uptake and persistence of the FnAb-8 protein characterized by in situ radio-labeling and PET/CT imaging

The absorption of peptides and proteins delivered orally is minimum because of the intestine epithelial barrier. There are few known active transport mechanisms for macromolecules including the neonatal Fc Receptor (FcRn) for the absorption and secretion of IgGs in infant and adult intestine. We had previously described the FnAb-8 protein that could bind to hFcRn tightly at pH 6.0 but barely at pH 7.4. In this study, we examined its uptake, biodistribution and pharmacokinetics after peroral administration in both wild-type and human FcRn transgenic (Tg) mice. FnAb-8 was modified to contain trans-cyclooctene (TCO) which could interact with ¹⁸F labeled tetrazine in situ via the bioorthogonal inverse-electron-demand Diels−Alder reaction. We showed that FnAb-8 had a tendency to distribute and persist in the Tg mice intestine for an extended duration of time. It could also be absorbed into the circulation and distributed systemically over a long period of time up to 172 h. The improvement in oral uptake and concentration in the intestine tissue may be valuable for designing oral delivery of biopharmaceuticals, especially for diseases involving the gastric intestinal tissue.

Bottom-Up Physiologically Based Oral Absorption Modeling of Free Weak Base Drugs

In this study, we systematically evaluated "bottom-up" physiologically based oral absorption modeling, focusing on free weak base drugs. The gastrointestinal unified theoretical framework (the GUT framework) was employed as a simple and transparent model. The oral absorption of poorly soluble free weak base drugs is affected by gastric pH. Alternation of bulk and solid surface pH by dissolving drug substances was considered in the model. Simple physicochemical properties such as pKa, the intrinsic solubility, and the bile micelle partition coefficient were used as input parameters. The fraction of a dose absorbed (Fa) in vivo was obtained by reanalyzing the pharmacokinetic data in the literature (15 drugs, a total of 85 Fa data). The AUC ratio with/without a gastric acid-reducing agent (AUCr) was collected from the literature (22 data). When gastric dissolution was neglected, Fa was underestimated (absolute average fold error (AAFE) = 1.85, average fold error (AFE) = 0.64). By considering gastric dissolution, predictability was improved (AAFE = 1.40, AFE = 1.04). AUCr was also appropriately predicted (AAFE = 1.54, AFE = 1.04). The Fa values of several drugs were slightly overestimated (less than 1.7-fold), probably due to neglecting particle growth in the small intestine. This modeling strategy will be of great importance for drug discovery and development.

Use of imaging to assess the activity of hepatic transporters

Introduction: Membrane transporters of the SLC and ABC families are abundantly expressed in the liver, where they control the transfer of drugs/drug metabolites across the sinusoidal and canalicular hepatocyte membranes and play a pivotal role in hepatic drug clearance. Noninvasive imaging methods, such as PET, SPECT or MRI, allow for measuring the activity of hepatic transporters in vivo, provided that suitable transporter imaging probes are available.Areas covered: We give an overview of the working principles of imaging-based assessment of hepatic transporter activity. We discuss different currently available PET/SPECT radiotracers and MRI contrast agents and their applications to measure hepatic transporter activity in health and disease. We cover mathematical modeling approaches to obtain quantitative parameters of transporter activity and provide a critical assessment of methodological limitations and challenges associated with this approach.Expert opinion: PET in combination with pharmacokinetic modeling can be potentially applied in drug development to study the distribution of new drug candidates to the liver and their clearance mechanisms. This approach bears potential to mechanistically assess transporter-mediated drug-drug interactions, to assess the influence of disease on hepatic drug disposition and to validate and refine currently available in vitro-in vivo extrapolation methods to predict hepatic clearance of drugs.

Human Radiation Dosimetry for Orally and Intravenously Administered 18F-FDG

Intravenous access is difficult in some patients referred for ¹⁸F-FDG PET imaging. Extravasation at the injection site and accumulation in central catheters can lead to limited tumor ¹⁸F-FDG uptake, erroneous quantitation, and significant image artifacts. In this study, we compared the human biodistribution and dosimetry for ¹⁸F-FDG after oral and intravenous administrations sequentially in the same subjects to ascertain the dosimetry and potential suitability of orally administered ¹⁸F-FDG as an alternative to intravenous administration. We also compared our detailed intravenous ¹⁸F-FDG dosimetry with older dosimetry data. Methods: Nine healthy volunteers (6 male and 3 female; aged 19-32 y) underwent PET/CT imaging after oral and intravenous administration of ¹⁸F-FDG. Identical preparation and imaging protocols (except administration route) were used for oral and intravenous studies. During each imaging session, 9 whole-body PET scans were obtained at 5, 10, 20, 30, 40, 50, 60, 120, and 240 min after ¹⁸F-FDG administration (370 ± 16 MBq). Source organ contours drawn using CT were overlaid onto registered PET images to extract time-activity curves. Time-integrated activity coefficients derived from time-activity curves were given as input to OLINDA/EXM for dose calculations. Results: Blood uptake after orally administered ¹⁸F-FDG peaked at 45-50 min after ingestion. The oral-to-intravenous ratios of ¹⁸F-FDG uptake for major organs at 45 min were 1.07 ± 0.24 for blood, 0.94 ± 0.39 for heart wall, 0.47 ± 0.12 for brain, 1.25 ± 0.18 for liver, and 0.84 ± 0.24 for kidneys. The highest organ-absorbed doses (μGy/MBq) after oral ¹⁸F-FDG administration were observed for urinary bladder (75.9 ± 17.2), stomach (48.4 ± 14.3), and brain (29.4 ± 5.1), and the effective dose was significantly higher (20%) than after intravenous administration (P = 0.002). Conclusion: ¹⁸F-FDG has excellent bioavailability after oral administration, but peak organ activities occur later than after intravenous injection. These data suggest PET at 2 h after oral ¹⁸F-FDG administration should yield images that are comparable in biodistribution to conventional clinical images acquired 1 h after injection. Oral ¹⁸F-FDG is a palatable alternative to intravenous ¹⁸F-FDG when venous access is problematic.

Imaging in Pharmacogenetics

An increasing collection of imaging technologies makes it possible to differentiate treatment responders from nonresponders based on genetic variation. This chapter will review some of the imaging technologies currently available in nuclear medicine to visualize drug absorption, distribution, metabolism, and elimination. Some of the commonly used techniques to detect radiation-emitting compounds are the two-dimensional scintigraphy and the three-dimensional single-photon emission computed tomography (SPECT) which both detect photons using a gamma camera, and the three-dimensional positron emission tomography (PET), which detect the decay of positron-emitting radionuclides. Current examples include visualization of functional effects of genetic variants, and these provide proof of concept for imaging in pharmacogenetics as a tool to improve efficacy and safety of drugs.

Comparison of pharmacokinetics of newly discovered aromatase inhibitors by a cassette microdosing approach in healthy Japanese subjects

The aim of the present study is to investigate the pharmacokinetics of our newly developed aromatase inhibitors (cetrozole and TMD-322) in healthy subjects by a cassette microdose strategy. A cocktail of cetrozole and TMD-322 was administered intravenously or orally (1.98 μg for each drug) to six healthy volunteers in a crossover fashion. Anastrozole (1.98 μg) was also included in the oral cocktail. Total body clearance and bioavailability were 12.1 ± 7.1 mL/min/kg and 34.9 ± 32.3% for cetrozole, and 16.8 ± 3.5 mL/min/kg and 18.4 ± 12.2% for TMD-322, respectively. The area under the plasma concentration-time curves of cetrozole and TMD-322 after oral administration was markedly lower than that of anastrozole because of their high hepatic clearance. Two subjects out of six exhibited 4- and 17-fold larger exposure of cetrozole than the others following intravenous and oral administration, respectively. Such variation was not observed for TMD-322 and anastrozole. Extensive metabolism of cetrozole and TMD-322 was observed in the CYP2C19 expression system among the test CYP isoforms (CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4). We report the first clinical investigation of our aromatase inhibitors by a cassette microdose strategy in healthy Japanese subjects. This strategy offers an optional approach for candidate selection as a phase zero study in drug development.

A Novel Intestine Model Apparatus for Drug Dissolution Capable of Simulating the Peristaltic Action

A novel dissolution apparatus has been proposed as an alternative apparatus for dissolution testing. In this study, we evaluated the performance of the new intestine model for simulating the peristaltic action (IMSPA), generating the movement that closely mimics peristaltic contractions of the small intestine. Two polyethylene oxide matrix tablet formulations, containing a model drug belonging to class III of the Biopharmaceutics Classification System, were tested. Dissolution was also performed in the USP2 apparatus. The release profiles were further compared to the in vivo data to evaluate the in vivo relevance of the new apparatus. The results demonstrated that the novel apparatus showed good discriminatory power between different polyethylene oxide formulations. Moreover, a better relation to the in vivo data was established by the IMSPA as compared to the USP2 apparatus. In conclusion, the model parameters were efficiently controlled to ensure the dissolution conditions crucial for evaluating the in vivo release performance of the tested formulations.

Investigation of Biodistribution and Speciation Changes of Orally Administered Dual Radiolabeled Complex, Bis(5-chloro-7-[131I]iodo-8-quinolinolato)[65Zn]zinc

Many zinc (Zn) complexes have been developed as promising oral antidiabetic agents. In vitro assays using adipocytes have demonstrated that the coordination structures of Zn complexes affect the uptake of Zn into cells and have insulinomimetic activities, for which moderate stability of Zn complexes is vital. The complexation of Zn plays a major role improving its bioavailability. However, investigation of the speciation changes of Zn complexes after oral administration is lacking. A dual radiolabeling approach was applied in order to investigate the speciation of bis(5-chloro-7-iodo-8-quinolinolato)zinc complex [Zn(Cq)₂], which exhibits the antidiabetic activity in diabetic mice. In the present study, ⁶⁵Zn- and ¹³¹I-labeled [Zn(Cq)₂] were synthesized, and their biodistribution were analyzed after an oral administration using both invasive conventional assays and noninvasive gamma-ray emission imaging (GREI), a novel nuclear medicine imaging modality that enables analysis of multiple radionuclides simultaneously. The GREI experiments visualized the behavior of ⁶⁵Zn and [¹³¹I]Cq from the stomach to large intestine and through the small intestine; most of the administered Zn was transported together with clioquinol (5-chloro-7-iodo-8-quinolinol) (Cq). Higher accumulation of ⁶⁵Zn for [Zn(Cq)₂] than ZnCl₂ suggests that the Zn associated with Cq was highly absorbed by the intestinal tract. In particular, the molar ratio of administered iodine to Zn decreased during the distribution processes, indicating the dissociation of most [Zn(Cq)₂] complexes. In conclusion, the present study successfully evaluated the speciation changes of orally administered [Zn(Cq)₂] using the dual radiolabeling method.

Reply to "Comment on 'In Silico Modeling of Gastrointestinal Drug Absorption: Predictive Performance of Three Physiologically Based Absorption Models'"

This is a reply to the comment on "In Silico Modeling of Gastrointestinal Drug Absorption: Predictive Performance of Three Physiologically Based Absorption Models" by Turner and other Simcyp associates. In the reply we address the major concerns raised by Turner et al. regarding the methodology to compare the predictive performance of the different absorption models and at the same time ensure that the systemic pharmacokinetic input was exactly the same for the different models; the selection of the human effective permeability value of fexofenadine; the adoption of model default values and settings; and how supersaturation/precipitation was handled. In addition, we also further discuss aspects related to differences in in silico models and the potential implications of such differences. Our original report should be viewed as the starting point in a thorough and transparent review of absorption prediction models with the overall aim of improving their application as validated tools for bridging studies of active pharmaceutical ingredients from various sources and origins in a regulatory context. With this reply we encourage other independent investigators to perform further model evaluations of commercial as well as other existing or recently implemented models. This will boost the overall progression of physiologically based biopharmaceutical models for predicting and simulating intestinal drug absorption both in research and development and in a regulatory context.

Exploration of Antiemetics for Osteoporosis Therapy-Induced Nausea and Vomiting Using PET Molecular Imaging Analysis to Gastrointestinal Pharmacokinetics

PURPOSE

To select appropriate antiemetics relieving teriparatide-induced nausea and vomiting during osteoporosis treatment using PET molecular imaging and pharmacokinetic analysis.

METHODS

Rats were pretreated with subcutaneous teriparatide, followed by oral administration of antiemetics with different pharmacological effects. The pharmacokinetics of antiemetics were assessed by oral administration of 2-deoxy-2-[(18)F]fluoro-D-glucose ([(18)F]FDG) under free moving conditions in vivo. The effect of teriparatide on the permeability of Caco-2 cell membranes to [(18)F]FDG was assessed in vitro. The effects of antiemetics on teriparatide-induced suppression of gastrointestinal motility in vivo was assayed by positron emission tomography (PET) using orally administered [(18)F]FDG.

RESULTS

Teriparatide delayed the time-radioactivity profile of [(18)F]FDG in blood and significantly reduced its absorption rate constant (k a ), determined from non-compartmental analysis, to 60% of control. In contrast, co-administration of granisetron or mosapride restored the time-radioactivity profile and k a of [(18)F]FDG to control levels. Teriparatide had no effect on Caco-2 membrane permeability to [(18)F]FDG. Pharmacokinetic PET imaging data analysis quantitatively showed the pharmacological effects of teriparatide-induced suppression of upper gastrointestinal motility and its restoration by granisetron and mosapride.

CONCLUSIONS

Teriparatide-induced abdominal discomfort might be attributed to GI motility, and PET imaging analysis is a useful tool to for the selection of appropriate antiemetics.

Effects of gastric pH on oral drug absorption: In vitro assessment using a dissolution/permeation system reflecting the gastric dissolution process

The aim of the present study was to evaluate the effects of gastric pH on the oral absorption of poorly water-soluble drugs using an in vitro system. A dissolution/permeation system (D/P system) equipped with a Caco-2 cell monolayer was used as the in vitro system to evaluate oral drug absorption, while a small vessel filled with simulated gastric fluid (SGF) was used to reflect the gastric dissolution phase. After applying drugs in their solid forms to SGF, SGF solution containing a 1/100 clinical dose of each drug was mixed with the apical solution of the D/P system, which was changed to fasted state-simulated intestinal fluid. Dissolved and permeated amounts on applied amount of drugs were then monitored for 2h. Similar experiments were performed using the same drugs, but without the gastric phase. Oral absorption with or without the gastric phase was predicted in humans based on the amount of the drug that permeated in the D/P system, assuming that the system without the gastric phase reflected human absorption with an elevated gastric pH. The dissolved amounts of basic drugs with poor water solubility, namely albendazole, dipyridamole, and ketoconazole, in the apical solution and their permeation across a Caco-2 cell monolayer were significantly enhanced when the gastric dissolution process was reflected due to the physicochemical properties of basic drugs. These amounts resulted in the prediction of higher oral absorption with normal gastric pH than with high gastric pH. On the other hand, when diclofenac sodium, the salt form of an acidic drug, was applied to the D/P system with the gastric phase, its dissolved and permeated amounts were significantly lower than those without the gastric phase. However, the oral absorption of diclofenac was predicted to be complete (96-98%) irrespective of gastric pH because the permeated amounts of diclofenac under both conditions were sufficiently high to achieve complete absorption. These estimations of the effects of gastric pH on the oral absorption of poorly water-soluble drugs were consistent with observations in humans. In conclusion, the D/P system with the gastric phase may be a useful tool for better predicting the oral absorption of poorly water-soluble basic drugs. In addition, the effects of gastric pH on the oral absorption of poorly water-soluble drugs may be evaluated by the D/P system with and without the gastric phase.

Visualization of drug translocation in the nasal cavity and pharmacokinetic analysis on nasal drug absorption using positron emission tomography in the rat

We performed positron emission tomography (PET) using 2-deoxy-2-[(18)F]fluoro-D-glucose ([(18)F]FDG) to evaluate the pharmacokinetics of nasal drug absorption in the rat. The dosing solution of [(18)F]FDG was varied in volume (ranging from 5 to 25 μl) and viscosity (using 0% to 3% concentrations of hydroxypropylcellulose). We modeled the pharmacokinetic parameters regarding the nasal cavity and pharynx using mass balance equations, and evaluated the values that were obtained by fitting concentration-time profiles using WinNonlin® software. The regional nasal permeability was also estimated using the active surface area derived from the PET images. The translocation of [(18)F]FDG from the nasal cavity was visualized using PET. Analysis of the PET imaging data revealed that the pharmacokinetic parameters were independent of the dosing solution volume; however, the viscosity increased the absorption rate constant and decreased the mucociliary clearance rate constant. Nasal permeability was initially higher but subsequently decreased until the end of the study, indicating regional differences in permeability in the nasal cavity. We concluded that the visualization of drug translocation in the nasal cavity in the rat using PET enables quantitative analysis of nasal drug absorption, thereby facilitating the development of nasal formulations for human use.

Metabolic imaging in obesity: underlying mechanisms and consequences in the whole body

Obesity is a phenotype resulting from a series of causative factors with a variable risk of complications. Etiologic diversity requires personalized prevention and treatment. Imaging procedures offer the potential to investigate the interplay between organs and pathways underlying energy intake and consumption in an integrated manner, and may open the perspective to classify and treat obesity according to causative mechanisms. This review illustrates the contribution provided by imaging studies to the understanding of human obesity, starting with the regulation of food intake and intestinal metabolism, followed by the role of adipose tissue in storing, releasing, and utilizing substrates, including the interconversion of white and brown fat, and concluding with the examination of imaging risk indicators related to complications, including type 2 diabetes, liver pathologies, cardiac and kidney diseases, and sleep disorders. The imaging modalities include (1) positron emission tomography to quantify organ-specific perfusion and substrate metabolism; (2) computed tomography to assess tissue density as an indicator of fat content and browning/ whitening; (3) ultrasounds to examine liver steatosis, stiffness, and inflammation; and (4) magnetic resonance techniques to assess blood oxygenation levels in the brain, liver stiffness, and metabolite contents (triglycerides, fatty acids, glucose, phosphocreatine, ATP, and acetylcarnitine) in a variety of organs.

Quantification of gastrointestinal liquid volumes and distribution following a 240 mL dose of water in the fasted state

The rate and extent of drug dissolution and absorption from solid oral dosage forms is highly dependent upon the volumes and distribution of gastric and small intestinal water. However, little is known about the time courses and distribution of water volumes in vivo in an undisturbed gut. Previous imaging studies offered a snapshot of water distribution in fasted humans and showed that water in the small intestine is distributed in small pockets. This study aimed to quantify the volume and number of water pockets in the upper gut of fasted healthy humans following ingestion of a glass of water (240 mL, as recommended for bioavailability/bioequivalence (BA/BE) studies), using recently validated noninvasive magnetic resonance imaging (MRI) methods. Twelve healthy volunteers underwent upper and lower abdominal MRI scans before drinking 240 mL (8 fluid ounces) of water. After ingesting the water, they were scanned at intervals for 2 h. The drink volume, inclusion criteria, and fasting conditions matched the international standards for BA/BE testing in healthy volunteers. The images were processed for gastric and intestinal total water volumes and for the number and volume of separate intestinal water pockets larger than 0.5 mL. The fasted stomach contained 35 ± 7 mL (mean ± SEM) of resting water. Upon drinking, the gastric fluid rose to 242 ± 9 mL. The gastric water volume declined rapidly after that with a half emptying time (T50%) of 13 ± 1 min. The mean gastric volume returned back to baseline 45 min after the drink. The fasted small bowel contained a total volume of 43 ± 14 mL of resting water. Twelve minutes after ingestion of water, small bowel water content rose to a maximum value of 94 ± 24 mL contained within 15 ± 2 pockets of 6 ± 2 mL each. At 45 min, when the glass of water had emptied completely from the stomach, total intestinal water volume was 77 ± 15 mL distributed into 16 ± 3 pockets of 5 ± 1 mL each. MRI provided unprecedented insights into the time course, number, volume, and location of water pockets in the stomach and small intestine under conditions that represent standard BA/BE studies using validated techniques. These data add to our current understanding of gastrointestinal physiology and will help improve physiological relevance of in vitro testing methods and in silico transport analyses for prediction of bioperformance of oral solid dosage forms, particularly for low solubility Biopharmaceutics Classification System (BCS) Class 2 and Class 4 compounds.