Background/Objectives: Integrin α_Vβ₃ plays a crucial role in tumor angiogenesis and cancer progression, making it a key target for radiolabeled probes used in imaging and therapy. A previously developed probe, [¹²⁵I]bcRGD, exhibited high selectivity for α_Vβ₃ but limited tumor accumulation due to rapid blood clearance. This study aimed to address this issue through two strategies: (1) conjugating albumin-binding molecules to enhance systemic circulation and (2) dimerizing RGD peptides to improve binding affinity via multivalency effects. Methods: Three [¹²⁵I]bcRGD derivatives were synthesized: [¹²⁵I]bcRGD_pal (with palmitic acid), [¹²⁵I]bcRGD_iba (with 4-(p-iodophenyl)butyric acid), and [¹²⁵I]bcRGD_dimer (a dimeric bicyclic RGD peptide). Their physicochemical properties, α_Vβ₃-selectivity, albumin-binding capacity, and biodistribution were assessed in vitro and in vivo using tumor-bearing mice. Tumor models included α_Vβ₃-high U-87 MG and α_Vβ₃-low A549 xenografts. Results: [¹²⁵I]bcRGD_pal and [¹²⁵I]bcRGD_iba exhibited prolonged blood retention (30-fold and 55-fold vs. [¹²⁵I]bcRGD, respectively) and increased tumor accumulation (3.9% ID/g and 3.6% ID/g at 2 h, respectively). Despite improved systemic circulation, tumor-to-blood ratios remained low (<1), indicating limited tumor retention. [¹²⁵I]bcRGD_dimer achieved significantly greater tumor accumulation (4.2% ID/g at 2 h) and favorable tumor-to-blood (22) and tumor-to-muscle (14) ratios, with a 5.4-fold higher uptake in U-87 MG tumors compared to A549 tumors. Conclusions: Dimerization was more effective than albumin binding in enhancing bcRGD’s tumor-targeting potential. The dimeric probe demonstrated improved tumor accumulation, favorable pharmacokinetics, and preserved integrin selectivity. These findings provide a foundation for further structural optimization of bicyclic RGD peptides for integrin α_Vβ₃-targeted imaging and therapy applications.

In boron neutron capture therapy (BNCT), the intracellular localization and concentration of boron-10 atoms significantly influence therapeutic efficacy. Although various boronic-acid-targeted fluorescent probes have been developed to evaluate BNCT agents, most of these probes emit at short wavelengths and are, therefore, incompatible with common nuclear-staining reagents such as Hoechst 33342 and 4′,6-diamidino-2-phenylindole (DAPI). While our previously reported probe, BS-631, emitted fluorescence above 500 nm, it exhibited limitations in terms of reaction rate and fluorescence intensity. To address these issues, we developed a boronic-acid-targeted fluorescent probe with a longer emission wavelength, rapid reactivity, and strong fluorescence intensity. Herein, we designed and synthesized BTTQ, a probe based on a 2-(2-hydroxyphenyl)benzothiazole core structure. BTTQ exhibited immediate fluorescence upon reaction with 4-borono-L-phenylalanine (BPA), with an emission wavelength of 567 nm and a sufficiently high fluorescence quantum yield for detection. BTTQ quantitatively detected BPA with high sensitivity (quantification limit of 10.27 µM), suitable for evaluating BNCT agents. In addition, BTTQ exhibited selective fluorescence for BPA over metal cations. Importantly, BTTQ enabled fluorescence microscopic imaging of intracellular BPA distribution in living cells co-stained with Hoechst 33342. These results suggest that BTTQ is a promising fluorescent probe for the evaluation of future BNCT agents.

Background/Objectives: The kinase p38α, a member of the mitogen-activated protein kinase (MAPK) family, is activated by external stimuli and plays a crucial role in inflammation, tumor growth, and metabolic disorders. In particular, p38α is involved in thermogenesis and the metabolism of glucose in brown adipose tissue (BAT), and it contributes to the suppression of obesity and diabetes. The noninvasive imaging of activated p38α could help elucidate diverse pathological processes, including metabolic and inflammatory conditions. This study aimed to develop and evaluate a novel fluorine-18-labeled positron emission tomography (PET) probe for imaging activated p38α in vivo. Methods: We designed 6-(4-[¹⁸F]fluoro-2-fluorophenoxy)-8-methyl-2-(tetrahydro-2H-pyran-4-ylamino)-pyrido[2,3-d]pyrimidin-7(8H)-one ([¹⁸F]R1487) by replacing a fluorine atom in R1487, which is a highly selective p38α inhibitor, with ¹⁸F. A tributylstannyl precursor was reacted with [¹⁸F]KF in the presence of a copper catalyst to synthesize [¹⁸F]R1487. Biodistribution studies and PET/computed tomography (CT) were performed on normal mice to evaluate the in vivo potential of [¹⁸F]R1487. Results: [¹⁸F]R1487 was obtained with a decay-corrected radiochemical conversion of 30.6 ± 5.6% and a decay-corrected radiochemical yield of 6.9 ± 3.6% with a radiochemical purity of >99% after reversed-phase high-performance liquid chromatography purification. The biodistribution study demonstrated high and rapid radioactivity accumulation in BAT (16.3 ± 2.7 %ID/g at 5 min post-injection), with a consistently high BAT-to-blood ratio (>5 over 2 h post-injection). PET/CT imaging successfully visualized BAT with high contrast. Conclusions: These results suggest that [¹⁸F]R1487 is a promising PET probe for imaging activated p38α in vivo, which has potential applications for pathophysiological conditions such as inflammation, cancer, and metabolic disorders.

Background/Objectives: Boron neutron capture therapy (BNCT) relies on the selective delivery of boron-10 to tumor cells. Although 4-[¹⁰B]borono-L-phenylalanine (BPA) is currently the only clinically approved BNCT agent, it is limited by poor L-type amino acid transporter 1 (LAT1)/LAT2 selectivity and aqueous solubility. We previously developed 3-borono-5-fluoro-α-methyl-L-phenylalanine (5F-αMe-3BPA), a novel BPA derivative designed to be a LAT1-targeted BNCT/positron emission tomography theranostic agent. This study comprehensively characterizes its pharmacological profile and explores its pharmacokinetic optimization by modulating renal organic anion transporter 1 (OAT1). Methods: Transport kinetics of BPA, related analogs, and 5F-αMe-3BPA were analyzed in HEK293 cells stably expressing LAT1 or LAT2 using Michaelis–Menten analysis. Time-dependent cellular uptake and intracellular retention of BPA and 5F-αMe-3BPA were evaluated in T3M-4 pancreatic cancer cells with or without the LAT1 inhibitor JPH203. In vivo biodistribution was examined in T3M-4 tumor-bearing mice after intravenous administration of 5F-αMe-3BPA or BPA, with assessment of probenecid pretreatment. Results: 5F-αMe-3BPA retained LAT1 affinity comparable to that of BPA while showing markedly reduced LAT2-mediated transport, indicating improved LAT1/LAT2 selectivity. In T3M-4 cells, 5F-αMe-3BPA showed stronger LAT1 dependence, higher steady-state accumulation, and better intracellular retention than BPA under amino acid-containing conditions. Although 5F-αMe-3BPA achieved favorable tumor-to-plasma and tumor-to-muscle ratios in vivo, it was rapidly cleared from circulation. Probenecid pretreatment increased plasma exposure, reduced early renal accumulation, and significantly enhanced tumor boron accumulation, reaching approximately twofold higher levels than control. Conclusions: These findings establish 5F-αMe-3BPA as a highly LAT1-selective BNCT candidate and identify probenecid pretreatment as a clinically translatable pharmacokinetic strategy for maximizing therapeutic boron delivery.