Thermal Behavior of 19F Nuclear Magnetic Resonance Signal of 19F-Containing Compound in Lipid Nano-Emulsion for Potential Tumor Diagnosis.

We developed carriers of a 19F magnetic resonance imaging (19F MRI) agent, capable of responding to the temperature difference for cancer diagnosis. The carriers were based on high melting point (mp) neutral lipids, namely, tripalmitin (TPT) and tristearin (TSR) and triarachidin (TAC). Lipid nano-emulsions (LNEs) containing a fluorine compound, i.e., a modified α-tocopherol (19F-TP), were respectively prepared as TPT-LNE, TSR-LNE, TAC-LNE1, and TAC-LNE2 and studied by 19F NMR spectroscopy. In LNE prepared with soybean oil as a control, the full width at half maximum (FWHM) values of the 19F NMR signal of 19F-TP remained constant at 25, 37, and 42°C, while those of the LNEs prepared from a neutral lipid with a high mp showed a sharp decrease between 25 and 37°C. The magnitude of the decrease followed the order: TPT-LNE < TSR-LNE < TAC-LNE1. However, TAC-LNE2, for which the amount of encapsulated 19F-TP was one third less than that of TAC-LNE1, showed a sharp decline in the FWHM between 37 and 42°C. To examine these changes, the 19F spin-lattice (T1) and spin-spin (T2) relaxation times of 19F-TP were measured. TAC-LNE2 in particular showed a substantial change in its T2 value between 37 and 42°C compared with the change of its T1 value. This result was attributed to activation of the molecular motion of 19F-TP in TAC-LNE2 from 37 to 42°C. Thus, TAC-LNE showed potential for use as a carrier for cancer diagnosis using 19F MRI.