Trans-1-amino-3-18 F-fluorocyclobutanecarboxylic acid (anti-18 F-FACBC) is an amino acid PET tracer that has shown promise for visualizing prostate cancer. Therefore, we aimed to clarify the anti-18 F-FACBC transport mechanism in prostate cancer cells. We also studied the fate of anti-18 F-FACBC after it is transported into cells. Methods: For convenience, because of their longer half-lives, 14 C compounds were used instead of 18 F-labeled tracers. Trans-1-amino-3-fluoro-1-14 C-cyclobutanecarboxylic acid ( 14 C-FACBC) uptake was examined in human prostate cancer DU145 cells with the following substrates of amino acid transporters: a-(methylamino) isobutyric acid (a system A-specific substrate) and 2-amino-2-norbornanecarboxylic acid (a system L-specific substrate). The messenger RNA expression of amino acid transporters in human prostate cancer specimens was analyzed by complementary DNA microarray and quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR). Gene expression in DU145 cells was analyzed by qRT-PCR. We also examined the knockdown effect of the amino acid transporters system ASC transporter 2 (ASCT2) and sodium-coupled neutral amino acid transporter 2 (SNAT2) on 14 C-FACBC uptake. In addition, the possibility of 14 C-FACBC incorporation into proteins was examined. Results: 14 C-FACBC uptake by DU145 cells was markedly decreased to approximately 20% in the absence of Na 1 , compared with that in its presence, indicating that Na 1 -dependent transporters are mainly responsible for the uptake of this tracer. Moreover, 2-amino-2-norbornanecarboxylic acid inhibited the transport of 14 C-FACBC to the basal level in Na 1 -free buffer. In contrast, a-(methylamino) isobutyric acid did not inhibit 14 C-FACBC accumulation in DU145 cells. Human prostate tumor specimens and DU145 cells had similar messenger RNA expression patterns of amino acid transporter genes. Although SNAT2 and ASCT2 are 2 major amino acid transporters expressed in prostate tumor tissues and DU145 cells, ASCT2 knockdown using small interfering RNA was more effective in lowering 14 C-FACBC transport than SNAT2. Almost all intracellular 14 C-FACBC was recovered from the nonprotein fraction. Conclusion: ASCT2, which is a Na 1 -dependent amino acid transporter, and to a lesser extent Na 1 -independent transporters play a role in the uptake of 14 C-FACBC by DU145 cells. Among the Na 1 -independent transporters, system L transporters are also involved in the transport of 14 C-FACBC. Moreover, 14 C-FACBC is not incorporated into proteins in cells. These findings suggest a possible mechanism of anti-18 F-FACBC PET for prostate cancer.
The a v b 3 integrin plays a pivotal role in angiogenesis and tumor metastasis. Angiogenic blood vessels overexpress a v b 3 integrin, as in tumor neovascularization, and a v b 3 integrin expression in other microvascular beds and organs is limited. Therefore, a v b 3 integrin is a suitable receptor for tumor-targeting imaging and therapy. Recently, tetrameric and dimeric RGD peptides have been developed to enhance specificity to a v b 3 integrin. In comparison to the corresponding monomeric peptide, however, these peptides show high levels of accumulation in kidney and liver. The purpose of this study is to evaluate tumor-targeting properties and the therapeutic potential of 111 In-and 90 Y-labeled monomeric RGD peptides in BALB/c nude mice with SKOV-3 human ovarian carcinoma tumors. DOTA-c(RGDfK) was labeled with 111 In or 90 Y and purified by HPLC. A biodistribution study and scintigraphic images revealed the specific uptake to a v b 3 integrin and the rapid clearance from normal tissues. These peptides were renally excreted. At 10 min after injection of tracers, 111 In-DOTA-c(RGDfK) and 90 Y-DOTA-c(RGDfK) showed high uptake in tumors (7.3 6 0.6% ID/g and 4.6 6 0.8% ID/g, respectively) and gradually decreased over time (2.3 6 0.4% ID/g and 1.5 6 0.5% ID/g at 24 hr, respectively). High tumor-to-blood and -muscle ratios were obtained from these peptides. In radionuclide therapeutic study, multipledose administration of 90 Y-DOTA-c(RGDfK) (3 3 11.1 MBq) suppressed tumor growth in comparison to the control group and a single-dose administration ( The a v b 3 integrin recognizes the amino acid sequence of arginine-glycine-aspartic acid (RGD peptide). On the basis of the RGD peptide, many peptidomimetic compounds and peptides have been designed to antagonize the a v b 3 integrin. 4 These compounds and anti-a v b 3 integrin monoclonal antibodies have been reported to inhibit angiogenesis without affecting preexisting vessels. 2,5,6 Because of its restricted expression, the a v b 3 integrin is an attractive targeting molecule for tumor imaging and therapy, leading to decreased side effects compared to conventional chemotherapy.The expression of the a v b 3 integrin has been reported to be associated with metastatic potential in melanoma, breast cancer, and colon cancer. 7-9 The development of radiopharmaceuticals for targeting the a v b 3 integrin would be clinically beneficial, not only for screening and for treating patients with a v b 3 integrin-positive tumors but also for monitoring therapeutic efficacy.Many RGD peptides labeled with gamma-emitting and positron-emitting nuclides ( 18 F, 64 Cu, 99m Tc, 125 I, etc.) have been reported as angiogenesis-imaging agents. 10 A recent trend in the development of RGD peptides is the multimerization of RGD peptides to improve the high tumor accumulation and retention of RGD peptides. 11 However, this also leads to the enhancement of radioactive accumulation in nontargeted organs such as kidney and liver. 12 Compared with antibody or multimeric RGD peptides, monomeric RGD ...
Activation of the PI3K-Akt pathway is known to induce tumor radioresistance. In the current study, we examined the ability of 17AAG, which decreases the levels of Hsp90 client proteins including components of the PI3K-Akt pathway, to sensitize radioresistant human squamous cell carcinoma cells to Xirradiation. Human squamous cell carcinoma cell lines (SQ20B, SCC61 and SCC13) were incubated for 16 h at 37°°°°C in medium containing 17AAG. Radiation sensitivity was determined by clonogenic assays, and protein levels were examined by western blotting. Apoptosis was determined in monolayer cells by AO/EB double staining and in spheroids using the TdT-mediated dUTP nick end labeling assay. 17AAG (0.2 µ µ µ µM) enhanced the radiosensitivity more effectively in radioresistant SQ20B and SCC13 cells than in radiosensitive SCC61 cells. However, in all three cell lines, 17AAG increased radiation-induced apoptosis by reducing the expression of EGFR and ErbB-2 and inhibiting the phosphorylation of Akt. Furthermore, 17AAG (1 µ µ µ µM) sensitized SQ20B spheroids to radiation, and inhibition of Akt activation by 17AAG increased radiation-induced apoptosis in spheroids. The findings suggest that 17AAG effectively sensitizes radioresistant cells to radiation by inhibiting the PI3K-Akt pathway. Targeting the PI3K-Akt pathway with 17AAG could be a useful strategy for radiosensitization of carcinomas. (Cancer Sci 2005; 96: 911-917) R adiation therapy is commonly used for squamous cell carcinoma of the head and neck because of its minimal cosmetic effects and its ability to preserve voice in laryngeal cancer. However, treatment failure correlates with a poorer prognosis and decreased patient survival.The EGFR family contributes to resistance to radiation and chemotherapy in many human tumors, including head and neck cancer.(1-14) ErbB-2, a member of the EGFR family, is also overexpressed in 30% of breast tumors as well as in a significant number of other cancers. (15,16) Blockade of the EGFRmediated signaling pathway enhances the sensitivity of tumor cells to radiation and several chemotherapeutic agents.The PI3K-Akt pathway is a major downstream pathway initiated by activation of the EGFR family members. (17)(18)(19)(20)(21) This pathway plays an important role in cell growth and survival. (22,23) The serine/threonine kinase Akt (also known as protein kinase B) is thought to be a downstream target of PI3K. Akt activation is responsible for desensitizing cells to apoptotic stimuli. This occurs by regulation of the transcriptional activity of both Forkhead family members (24,25) and NF-κB, (26,27) and through phosphorylation and inactivation of the apoptotic machinery including Bcl-2 homolog, Bad (28,29) and caspase-9.(30) Furthermore, Akt is amplified or overexpressed in a wide variety of human tumors.(31-33) PTEN, a tumor suppressor gene, is thought to negatively regulate the PI3K-Akt pathway.(34-36) Mutations causing PTEN to be functionally inactive are frequently detected in many human cancers and enhance Akt activity.(37) Akt acti...
In boron neutron capture therapy (BNCT), 10B‐4‐borono‐L‐phenylalanine (BPA) is commonly used as a 10B carrier. PET using 4‐borono‐2‐18F‐fluoro‐phenylalanine (18F‐FBPA PET) has been performed to estimate boron concentration and predict the therapeutic effects of BNCT; however, the association between tumor uptake of 18F‐FBPA and boron concentration in tumors remains unclear. The present study investigated the transport mechanism of 18F‐FBPA and BPA, and evaluated the utility of 18F‐FBPA PET in predicting boron concentration in tumors. The transporter assay revealed that 2‐aminobicyclo‐(2.2.1)‐heptane‐2‐carboxylic acid, an inhibitor of the L‐type amino acid transporter, significantly inhibited 18F‐FBPA and 14C‐4‐borono‐L‐phenylalanine (14C‐BPA) uptake in FaDu and LN‐229 human cancer cells. 18F‐FBPA uptake strongly correlated with 14C‐BPA uptake in 7 human tumor cell lines (r = .93; P < .01). PET experiments demonstrated that tumor uptake of 18F‐FBPA was independent of the administration method, and uptake of 18F‐FBPA by bolus injection correlated well with BPA uptake by continuous intravenous infusion. The results of this study revealed that evaluating tumor uptake of 18F‐FBPA by PET was useful for estimating 10B concentration in tumors.
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