(70bj) Gadolinium-Containing Lipid Nanoparticles for Neutron Capture Therapy

Gadolinium neutron-capture therapy (GdNCT) is a cancer therapy, which uses radiation emitted from gadolinium-157 (157Gd) as a result of its neutron capture reaction with thermal neutrons. This therapy has certain advantages over traditional cancer chemotherapy. Unlike chemotherapy that uses antitumor drugs, NCT does not need to use pharmacologically active substances in a traditional sense, since it is the neutron-capture element itself that contributes to tumor inactivation. Therefore, the administration of a large amount of the radiosensitizer becomes possible, provided that the elements are modified so as to be non-toxic compounds. Thus, severe side effects, which are often experienced in cancer chemotherapy, are not a major concern in NCT. The 157Gd concentration in the tumor required to obtain significant tumor growth suppression was estimated to be at least 100 μg/g tissue based on the result of our in vivo GdNCT trial, which was performed by an i.t. injection of Gd-loaded chitosan nanoparticles for SCC-VII tumor-bearing mice. These results motivated us to develop a functional Gd delivery device to obtain sufficient Gd concentration in tumors. Gadolinium-incorporating lipid-nanoemulsions (Gd-nanoLE) for neutron-capture therapy were first prepared. The Gd-nanoLEs had a core of soybean oil and a surface layer containing hydrogenated egg yolk phosphatidylcholine (HEPC). The Gd-nanoLE was prepared by a thin-layer hydration method combined with a sonication method. Briefly, soybean oil, HEPC, gadolinium-diethylenetriaminepentaacetic acid-distearylamide (Gd-DTPA-SA), and HCO-60, a cosurfactant, were dissolved in an appropriate amount of chloroform. To form thin-films, the solvent was removed by a rotary evaporator and dried in a vacuum for 3-4 h at room temperature. The dried films were allowed to hydrate in distilled water at 55-60°C for 5 min. By sonicating the resultant hydrated mixture with a bath-type sonicator and vortexing for 60 min, the Gd-nanoLE was obtained. For biodistribution studies, hamsters bearing Greene's melanoma (melanotic No.179 cell, D1179) were obtained by subcutaneous transplantation of the melanoma cell fragments on the left thigh 10 days before use. As a more convenient administration route than the intraperitoneal (i.p.) injection previously reported, the intravenous (i.v.) injections in tumor-bearing hamsters were carried out at an administration volume of 1 ml, which was the maximum tolerable injection volume of an i.v. injection, a half of that of an i.p. injection. When the standard-Gd-nanoLE of 1.5 mg Gd/ml was administered, the absolute bioavailability in the previous i.p. injection was found to be 57%, probably resulting from incomplete absorption from the peritoneal cavity into the blood stream. The biodistribution data revealed that the i.v. injection had three advantages over the i.p. injection, namely, a faster accumulation of Gd-nanoLE, a higher accumulation, and a more extended retention time in the tumor. Two i.v. injections of the standard-Gd-nanoLE with a 24 h interval doubled the tumor accumulation of Gd, resulting in 49.7 µg Gd/g wet tumor 12 h after administration. By using a two-fold Gd-enriched formulation (High-Gd-nanoLE) of 3.0 mg Gd/ml in the repeated administration schedule, the accumulation was doubled again, reaching 101 µg Gd/g wet tumor. This level was comparable to the maximum level in the single i.p. injection previously reported. These results demonstrated that i.v. injection could be an alternative to i.p. injection as an administration route. In order to simplify the formulation and reduce the particle size, micellar nanoparticles (Gd-nanoMIC-SA) were secondly prepared without soybean oil and HEPC. Gd-nanoMIC-SA consisted of 300 mg of Gd-DTPA-SA, 1200 mg of HCO-60 and 10 mg of stearic acid in 10 ml of water with a particle size of 83 nm. Subsequently heating at 70ºC for 20 min reduced the size of Gd-nanoMIC-SA to 38 nm. The Gd-nanoLE or Gd-nanoMIC-SA suspension (4.5 mg Gd/ml) was twice injected intravenously at 24-hour interval at a dose of 1ml per hamster each or continuously infused for 5 min at the same total dose. After twice repeated injection, biodistribution of Gd was determined. Then, the profiles of Gd concentration in blood indicated a faster elimination with Gd-nanoMIC-SA, compared with Gd-nanoLEs. In tumor, Gd concentration was 127 μg/g wet tissue with Gd-nanoMIC-SA and 189 μg/g wet tissue with Gd-nanoLE at 12 hours. The tumor/blood (T/B) ratio at 12 hours was 0.9 with Gd-nanoLE, and 2.6 with Gd-nanoMIC-SA. But, T/B ratio of Gd-nanoMIC-SA was increased to 6.2 at 24 hours. The infusion of Gd-nanoMIC-SA exhibited biodistribution comparable with the repeated administration. These results suggested that Gd-nanoMIC-SA, which was prepared in a simpler formulation with a smaller particle size, making it possible to effectively modify the particle-surface without excessive size-enlargement, might be an alternative to Gd-nanoLE.