Non-ionic Gd-based MRI contrast agents are optimal for encapsulation into phosphatidyldiglycerol-based thermosensitive liposomes

Abstract Thermosensitive liposomes (TSL) with encapsulated magnetic resonance imaging (MRI) longitudinal relaxation time (T 1 ) contrast agents (CAs) have been proposed for MRI assisted interventional thermotherapy in solid tumors. Here the feasibility of 6 clinically approved CAs (Gd-DTPA, Gd-BOPTA, Gd-DOTA, Gd-BT-DO3A, Gd-DTPA-BMA, and Gd-HP-DO3A) for formulation into TSL was investigated. CAs were passively encapsulated with 323 mOs kg − 1 into 1,2-dipalmitoyl- sn -glycero-3-phosphocholine/1,2-distearoyl- sn -glycero-3-phosphocholine/1,2-dipalmitoyl- sn -glycero-3-phosphodiglycerol 50/20/30 (mol/mol) TSL (DPPG 2 -TSL) to obtain stable formulations. T 1 relaxivity (r 1 ) and diffusive permeability to water (P d ) across the membrane were determined. Shelf life at 4 °C was investigated by determining lysolipid content up to 10 weeks after preparation. All preparations were monodispersed with comparable small vesicle sizes (~ 135 nm). Neither zeta potential nor phase transition temperature (T m ) was affected by the CA. The formulations showed an increase in r 1 in the temperature range between 38 and 44 °C. This correlated with the phase transition. Change in r 1 (Δr 1  = r 1 (45.3 °C) − r 1 (37.6 °C)) and r 1 (T  m ) depended on the encapsulated CA concentration. P d at T ≤ 37.6 °C was lower for DPPG 2 -TSL encapsulating non-ionic Gd-BT-DO3A, Gd-DTPA-BMA, and Gd-HP-DO3A. All CAs except Gd-DTPA-BMA induced phospholipid hydrolysis, which resulted in unwanted CA leakage. The serum proteins HSA and IgG both contributed to the increase of MRI signal at 30 °C by increasing P d . A high concentration of encapsulated CA is a prerequisite to achieve a sufficiently high Δr 1 during heat triggered CA release combined with a low r 1 at 37 °C. Hence, the optimal CA is characterized by a non-ionic structure and a low contribution to osmolality.