Introduction We investigated the relationship between gut microbiota composition and osteoporosis/fracture risk in Japanese postmenopausal women using 16S rRNA gene sequencing, FRAX, bone mineral density, biochemical bone parameters, and a self-administered questionnaire. Variation in abundance of specific microbiota was found to be significantly associated with fracture risk and vitamin K levels. Gut microbiota data with respect to bone metabolism and fracture risk is limited. Vitamin K is produced by certain intestinal bacteria and has been reported to play a role in maintaining bone quality. Purpose We investigated relationships among gut microbiota composition, bone metabolism, and fracture risk in postmenopausal Japanese women. Methods Bone mineral density (BMD) was evaluated in 38 postmenopausal women (mean age 62.9 years) using forearm dualenergy X-ray absorptiometry. We collected and analyzed serum bone turnover markers (vitamin K fraction and tartrate-resistant acid phosphatase 5b; TRACP-5b), gut microbiota profiling (16S rRNA gene sequencing), and self-administered questionnaire data, including fracture history and vitamin K intake. Vitamin K2, BMD, and TRACP-5b data were divided into high-and lowlevel groups using cutoff values of 0.06 ng/mL, 87.05%, and 420 mU/dL, respectively; the proportions of bacteria were analyzed. Fracture incidence and relative risk were investigated for each bacterium. Results The genus Bacteroides was predominant in the high vitamin K2 group (29.73% vs 21.58%, P = 0.022). Fracture incidence was significantly higher in the low Bacteroides group, with a 5.6-times higher risk ratio of fracture history. The family Rikenellaceae was more abundant in the low BMD group and more abundant in the high TRACP-5b group (2.15% vs 0.82%, P = 0.004; 2.38% vs 1.12%, P = 0.013, respectively). Conclusion Bacteroides and Rikenellaceae may be involved in bone metabolism and fracture risk. Further investigations of the underlying microbiota-related pathways in bone metabolism may reveal treatment strategies, and facilitate the prevention of osteoporosis.

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To develop potent drugs for oral use, information on their pharmacokinetic (PK) properties after oral administration is of great importance. We have recently reported the utility of positron emission tomography (PET) for the analysis of gastrointestinal (GI) absorption of radiolabeled compounds. In this study, PET image analysis was performed in rats using a novel PET probe, [(18)F]deoxyfluoropoly(ethylene glycol)s, with an average molecular weight of 2 kDa ([(18)F]FPEG), as a nonabsorbable marker to elaborate the GI physiology in more detail, such as segmental transition of the administered water, and fluid volume and distribution in the intestine. After oral administration of [(18)F]FPEG solution to rats, a 90 min PET scan with continuous blood sampling was performed, and then the disposition of radioactivity in each part of GI tract was investigated. From blood PK analysis, it was confirmed that the bioavailability of [(18)F]FPEG was quite low in rats. PET image analysis showed that the radioactivity after oral administration of [(18)F]FPEG solution rapidly passed through the stomach, spread into the proximal small intestine, and then transited toward the distal region of the small intestine without decreasing the radioactivity during GI transition. Radiometabolite analysis revealed that the radioactivity in intestinal mucosal tissues, blood, and urine was mainly derived from unchanged [(18)F]FPEG. It was also found that the volume of interest (VOI) after oral administration of the radiotracer enables an understanding of the time-dependent manner of effective fluid volume changes in the stomach and the small intestine. In addition, the rate constant of the intestinal transition of radioactivity in each intestinal segment was calculated by kinetic model analysis, which revealed that PET analysis enables us to determine the GI transit from the same individuals and that it is applicable to determine site-specific intestinal absorption. In conclusion, we demonstrated the high potency of PET imaging technique to elucidate the distribution of orally administered solution in the GI tract in vivo.

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Amino acid transporters are upregulated in many cancer cells, and system L amino acid transporters (LAT1–4), in particular, LAT1, which preferentially transports large, neutral, and branched side-chain amino acids, are considered a primary target for cancer positron emission tomography (PET) tracer development. Recently, we developed a 11C-labeled leucine analog, l-α-[5-11C]­methylleucine ([5-11C]­MeLeu), via a continuous two-step reaction of Pd0-mediated 11C-methylation and microfluidic hydrogenation. In this study, we evaluated the characteristics of [5-11C]­MeLeu and also compared the sensitivity to brain tumors and inflammation with l-[11C]­methionine ([11C]­Met) to determine its potential for brain tumor imaging. Competitive inhibition experiments, protein incorporation, and cytotoxicity experiments of [5-11C]­MeLeu were performed in vitro. Further, metabolic analyses of [5-11C]­MeLeu were performed using a thin-layer chromatogram. The accumulation of [5-11C]­MeLeu in tumor and inflamed regions of the brain was compared with [11C]­Met and 11C-labeled (S)-ketoprofen methyl ester by PET imaging, respectively. Transporter assay with various inhibitors revealed that [5-11C]­MeLeu is mainly transported via system L amino acid transporters, especially LAT1, into A431 cells. The protein incorporation assay and metabolic assay in vivo demonstrated that [5-11C]­MeLeu was neither used for protein synthesis nor metabolized. These results indicate that MeLeu is very stable in vivo. Furthermore, the treatment of A431 cells with various concentrations of MeLeu did not change their viability, even at high concentrations (∼10 mM). In brain tumors, the tumor-to-normal ratio of [5-11C]­MeLeu was more elevated than that of [11C]­Met. However, the accumulation levels of [5-11C]­MeLeu were lower than those of [11C]­Met (the standardized uptake value (SUV) of [5-11C]­MeLeu and [11C]­Met was 0.48 ± 0.08 and 0.63 ± 0.06, respectively). In brain inflammation, no significant accumulation of [5-11C]­MeLeu was observed at the inflamed brain area. These data suggested that [5-11C]­MeLeu was identified as a stable and safe agent for PET tracers and could help detect brain tumors, which overexpress the LAT1 transporter.