Formulation and characterisation of magnetic resonance imageable thermally sensitive liposomes for use with magnetic resonance-guided high intensity focused ultrasound.

Purpose: Objectives of this study were to: 1) develop iLTSL, a low temperature sensitive liposome co-loaded with an MRI contrast agent (ProHance® Gd-HP-DO3A) and doxorubicin, 2) characterise doxorubicin and Gd-HP-DO3A release from iLTSL and 3) investigate the ability of magnetic resonance-guided high intensity focused ultrasound (MR-HIFU) to induce and monitor iLTSL content release in phantoms and in vivo.

Methods: iLTSL was passively loaded with Gd-HP-DO3A and actively loaded with doxorubicin. Doxorubicin and Gd-HP-DO3A release was quantified by fluorescence and spectroscopic techniques, respectively. Release with MR-HIFU was examined in tissue-mimicking phantoms containing iLTSL and in a VX2 rabbit tumour model.

Results: iLTSL demonstrated consistent size and doxorubicin release kinetics after storage at 4°C for 7 days. Release of doxorubicin and Gd-HP-DO3A from iLTSL was minimal at 37°C but fast when heated to 41.3°C. The magnitude of release was not significantly different between doxorubicin and Gd-HP-DO3A over 10 min in HEPES buffer and plasma at 37°, 40° and 41.3°C (p > 0.05). Relaxivity of iLTSL increased significantly (p < 0.0001) from 1.95 ± 0.05 to 4.01 ± 0.1 mMs⁻¹ when heated above the transition temperature. Signal increase corresponded spatially and temporally to MR-HIFU-heated locations in phantoms. Signal increase was also observed in vivo after iLTSL injection and after each 10-min heating (41°C), with greatest increase in the heated tumour region.

Conclusion: An MR imageable liposome formulation co-loaded with doxorubicin and an MR contrast agent was developed. Stability, imageability, and MR-HIFU monitoring and control of content release suggest that MR-HIFU combined with iLTSL may enable real-time monitoring and spatial control of content release.