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

We developed carriers of a ¹⁹ F magnetic resonance imaging ( ¹⁹ F 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 ( ¹⁹ F-TP), were respectively prepared as TPT-LNE, TSR-LNE, TAC-LNE1, and TAC-LNE2 and studied by ¹⁹ F NMR spectroscopy. In LNE prepared with soybean oil as a control, the full width at half maximum (FWHM) values of the ¹⁹ F NMR signal of ¹⁹ F-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 ¹⁹ F-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 ¹⁹ F spin-lattice (T ₁ ) and spin-spin (T ₂ ) relaxation times of ¹⁹ F-TP were measured. TAC-LNE2 in particular showed a substantial change in its T ₂ value between 37 and 42°C compared with the change of its T ₁ value. This result was attributed to activation of the molecular motion of ¹⁹ F-TP in TAC-LNE2 from 37 to 42°C. Thus, TAC-LNE showed potential for use as a carrier for cancer diagnosis using ¹⁹ F MRI.