Although 4-borono-l-phenylalanine (4-BPA) is currently the only marketed agent available for boron neutron capture therapy (BNCT), its low water solubility raises concerns. In this study, we synthesized 3-borono-l-phenylalanine (3-BPA), a positional isomer of 4-BPA, with improved water solubility. We further evaluated its physicochemical properties, tumor accumulation, and biodistribution. The water solubility of 3-BPA was 125 g/L, which is more than 100 times higher than that of 4-BPA. Due to the high water solubility, we prepared the administration solution of 3-BPA without a solubilizer sugar, which is inevitably added to 4-BPA preparation and has adverse effects. In in vitro and in vivo experiments, boron accumulation in cancers after administration was statistically equivalent in both sugar-complexed 3-BPA and 4-BPA. Furthermore, the biodistribution of 3-BPA was comparable with that of sugar-complexed 3-BPA. Since 3-BPA has high water solubility and tumor targetability equivalent to 4-BPA, 3-BPA can replace 4-BPA in future BNCT.

Since membrane type‐1 matrix metalloproteinase (MT1‐MMP) plays pivotal roles in tumor progression and metastasis and holds great promise as an early biomarker for malignant tumors, a method of evaluating MT1‐MMP expression levels would be valuable for molecular biological and clinical studies. Although we have previously developed a ^99mTc‐labeled anti‐MT1‐MMP monoclonal IgG (^99mTc‐MT1‐mAb) as an MT1‐MMP imaging probe by nuclear medical techniques for this purpose, slow pharmacokinetics were a problem due to its large molecular size. Thus, in this study, our aim was to develop miniaturized antibodies, a single chain antibody fragment (MT1‐scFv) and a dimer of two molecules of scFv (MT1‐diabody), as the basic structures of MT1‐MMP imaging probes followed by in vitro and in vivo evaluation with an ¹¹¹In radiolabel. Phage display screening successfully provided MT1‐scFv and MT1‐diabody, which had sufficiently high affinity for MT1‐MMP (K_D = 29.8 and 17.1 nM). Both ¹¹¹In labeled miniaturized antibodies showed higher uptake in MT1‐MMP expressing HT1080 cells than in non‐expressing MCF7 cells. An in vivo biodistribution study showed rapid pharmacokinetics for both probes, which exhibited >20‐fold higher tumor to blood radioactivity ratios (T/B ratio), an index for in vivo imaging, than ^99mTc‐MT1‐mAb 6 h post‐administration, and significantly higher tumor accumulation in HT1080 than MCF7 cells. SPECT images showed heterogeneous distribution and ex vivo autoradiographic analysis revealed that the radioactivity distribution profiles in tumors corresponded to MT1‐MMP‐positive areas. These findings suggest that the newly developed miniaturized antibodies are promising probes for detection of MT1‐MMP in cancer cells.

Positron emission tomography with ¹⁵O-labeled gases (¹⁵O-PET) is important for in vivo measurement of cerebral oxygen metabolism both in clinical and basic settings. However, there are currently no reports concerning ¹⁵O-PET in mice. Here, we developed an ¹⁵O-PET method applicable to mice with spontaneous respiration of ¹⁵O-gas without a tracheotomy catheter. Sequential ¹⁵O-PET was also performed in a mouse model of chronic cerebral hypoperfusion with bilateral common carotid artery stenosis (BCAS) induced by placement of microcoils. ¹⁵O-gas with isoflurane was supplied to the nose of mouse with evacuation of excess ¹⁵O-gas surrounding the body. ¹⁵O-PET was performed on days 3, 7, 14, 21, and 28 after surgery. Cerebral blood flow (CBF), cerebral blood volume, oxygen extraction fraction (OEF), and cerebral metabolic rate of oxygen (CMRO₂) were calculated in whole brains. A significant decrease in CBF and compensatory increase in OEF in the BCAS group produced CMRO₂ values comparable to that of the sham group at three days post-operation. Although CBF and OEF in the BCAS group gradually recovered over the first 28 days, the CMRO₂ showed a gradual decrease to 68% of sham values at 28 days post-operation. In conclusion, we successfully developed a noninvasive ¹⁵O-PET method for mice.

As matrix metalloproteinases (MMPs), especially MMP-9 and MMP-12 are involved in the pathological processes associated with chronic obstructive pulmonary disease (COPD), we developed a novel radiofluorinated probe, ¹⁸F-IPFP, for MMPs-targeted positron emission tomography (PET). ¹⁸F-IPFP was designed by iodination of MMP inhibitor to enhance the affinity, and labelled with a compact prosthetic agent, 4-nitrophenyl 2-¹⁸F-fluoropropionate (¹⁸F-NFP). As a result, IPFP demonstrated the highest affinity toward MMP-12 (IC₅₀ = 1.5 nM) among existing PET probes. A COPD model was employed by exposing mice to cigarette smoke and the expression levels of MMP-9 and MMP-12 were significantly increased in the lungs. Radioactivity accumulation in the lungs 90 min after administration of ¹⁸F-IPFP was 4× higher in COPD mice than normal mice, and 10× higher than in the heart, muscle, and blood. Ex vivo PET confirmed the radioactivity distribution in the tissues and autoradiography analysis demonstrated that accumulation differences in the lungs of COPD mice were 2× higher than those of normal mice. These results suggest that ¹⁸F-IPFP is a promising probe for pulmonary imaging and expected to be applied to various MMP-related diseases for early diagnosis, tracking of therapeutic effects, and new drug development in both preclinical and clinical applications.