No abstract

Heating properties of magnetite cationic liposomes (MCL) were investigated in ex vivo experiments using implanted cell pellets. The cell pellets, which consisted of rat glioma T9 cells into which MCL had been incorporated in a petri dish, were implanted subcntaneously in the left femoral region of female F344 rats. The rats were placed in a magnetic field generating coil and irradiated with an alternating magnetic field (384 Oe, 118 kHz) for 60 min. The cell pellets were heated to over 43°C by MCL in the magnetic field, hut other body parts of the rats were not heated. After 3 cycles of magnetic heating, all glioma cells were killed and no tumor take was observed.

Induction of antitumor immunity to T-9 rat glioma by intracellular hyperthermia using functional magnetic particles was investigated. Magnetite cationic liposomes (MCLs), which have a positive surface charge, were used as heating mediators for intracellular hyperthermia. Solid T-9 glioma tissues were formed subcutaneously on both femurs of female F344 rats, and MCLs were injected via a needle only into the left solid tumors (treatment side). The rats were then divided into two groups, which received no irradiation, or irradiation for 30 min given three times at 24-h intervals with an alternating magnetic field (118 kHz, 384 Oe). On the treatment side, the tumor tissue disappeared completely in many rats exposed to the magnetic field. The tumor tissue on the opposite side also disappeared completely, even though MCLs were not injected into the right solid tumors. To examine whether a long-lasting and tumor-specific immunity could be generated, the rats that had been cured by the hyperthermia treatment were rechallenged with T-9 cells 3 months later. After a period of transient growth, all tumors disappeared. Furthermore, immunocytochemical assay revealed that the immune response induced by the hyperthermia treatment was mediated by both CD8 Various methods have been employed, such as whole body hyperthermia, 2) radiofrequency hyperthermia, 3) and inductive hyperthermia using microwave antenna 4) or implantable needles. 5) However, it is always difficult to achieve uniform heating of the tumor region to the desired temperature without damaging normal tissue. Therefore, some researchers have proposed intracellular hyperthermia and developed submicron magnetic particles for this purpose.6-8) These magnetic particles are easily incorporated into cells and generate heat under an alternating magnetic field through hysteresis loss. 9) We have also developed 'magnetite cationic liposomes' (MCLs) for intracellular hyperthermia.10, 11) MCLs were developed to improve adsorption and accumulation into the tumor cells and show ten-fold higher affinity for tumor cells than neutrally charged magnetoliposomes, 10) owing to electrostatic interaction with the negatively charged cell membrane. 12,13) The hyperthermic effect of the MCLs was examined in vivo.14) MCLs were injected into solid tumors formed subcutaneously in F344 rats and the rats were irradiated three times for 30 min with an alternating magnetic field. Histological observations were carried out just after the irradiation and showed that some tumor cells survived, especially in the peripheral area. However, complete tumor regression was observed one month after the irradiation. We were interested in the possibility that antitumor immunity had been induced by the hyperthermic treatment using MCLs.In the present paper, it is demonstrated that our hyperthermia system can induce an antitumor immune response and the acquired immunity is long-lasting. MATERIALS AND METHODS MaterialsDilauroylphosphatidylcholine and dioleoylphosphatidylethanolamine were purchased from Sigma Ch...

‘Magnetite cationic liposomes (MCL)’ were developed as a means to generate intracellular hyperthermia. Affinity of the MCL to glioma cells was ten times higher than that of magnetite‘neutral’ liposomes due to the electrostatic interaction based on the positive charge of the MCL. Heat generation of the MCL was studied using agar phantoms and small pellets of rat glioma cells. When a high‐frequency magnetic field, 118 kHz, 384 Oe was applied to glioma cells in the presence of MCL, the glioma cell pellet of 80 μl (5.4 mm in diameter) was heated to over 43°C and all the cells died after 40 min irradiation owing to the hyperthermic effect. The terminal temperature of the cell pellet was proportional to the pellet volume when other parameters were constant. It thus appears that the MCL can heat a tumor of more than 80 μl in volume to above 42°C.

The effect of hyperthermia on solid glioma tissue formed subcutaneously in the left femoral region of female F344 rats was investigated. Magnetite cationic liposomes (MCLs), which have a positive surface charge, were used as heating mediators for intracellular hyperthermia. MCLs were injected into the solid tumors, which were then subjected to irradiation by an alternating magnetic field (118 kHz, 384 Oe). The rats were divided into four groups, which received no irradiation (control: group I), or irradiation for 30 min given once (group II), twice (group III) or three times (group IV), and the hyperthermic effect on tumor growth was evaluated. Complete tumor regression was observed in 87.5% of the rats in group IV. In the other groups, tumors completely regressed in 20 and 60% of the rats in groups II and III, respectively. Histological observations showed that in group I tumors, MCLs were localized only around the point where they were injected, while in group II tumors they were a little more dispersed. In the cases of group III and IV tumors, however, the distribution of the MCLs was found to be widespread, and necrotic cells were observed throughout almost the entire tumor tissue. The high percentage of complete regression of group IV is considered to be due to this wide distribution of the MCLs. Furthermore, the treated rats showed no severe side-effects. These results suggest that our magnetic particles are potentially effective tools for the treatment of solid tumors.