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pH-sensitive paramagnetic liposomes for MRI: assessment of stability in blood
Knut-Egil Løkling1, Roald Skurtveit, Sigrid L Fossheim
1Department of Medicinal Chemistry, School of Pharmacy, University of Oslo, Norway. knut-egil.lokling@amersham.com
Abstract:
The pH-dependent stability of dipalmitoyl phosphatidyl ethanolamine/palmitic acid (DPPE/PA) liposomal GdDTPA-BMA was investigated in human blood and after exposure to selected blood components. Relaxometry, visual observations and cryo-transmission electron microscopy (cryo-TEM) were employed for the assessment of stability. The liposomes were stable in buffer at physiological pH and the T(1)-relaxivity (r(1)) of the system was significantly lowered compared to that of non-liposomal GdDTPA-BMA, which could be explained by an exchange limited relaxation process. Lowering the pH, however, gave a marked increase in r(1), due to liposome aggregation and subsequent leakage of GdDTPA-BMA. After a few minutes incubation in human blood the liposomes were destabilised and leaky at both high and low pH, and blood components likely to cause the instability were studied. Physiological level of Na(+) (150 mM) did not affect the relaxometric behavior of the liposomes at pH 7.4, but shifted the pH-r(1) profile laterally to higher pH-values compared to a level of 50 mM Na(+). Increased screening of the surface charges and, concomitantly, a lowering of the energy-barrier against aggregation is a plausible explanation for this phenomenon. In contrast, both Ca(2+) and Mg(2+) (physiological level, both 2 mM) caused massive aggregation of the liposomes and leakage of contents, and were therefore much more detrimental to the stability of the liposomes than a physiological level of Na(+). This could be due to the higher screening ability of divalent cations, but aggregation could also be induced through an inter-liposomal "bridging" effect. Physiological level of both Na(+) and Ca(2+) caused less leakage than for lower Na(+) concentration (50 mM Na(+) and 2 mM Ca(2+)), probably due to competition for the negative surface charges. Albumin also destabilised the liposomes, and it was shown to be due to an interaction between albumin and PA in the liposomal membrane.
Insights
Dipalmitoyl phosphatidyl ethanolamine/palmitic acid (DPPE/PA) liposomes containing GdDTPA-BMA are unstable in human blood, with divalent cations like Ca(2+) and Mg(2+) causing significant aggregation and leakage. Albumin also destabilizes these liposomes through membrane interaction.
Area of Science:
- Biomedical Engineering
- Materials Science
- Radiochemistry
Background:
- Liposomal formulations of contrast agents like GdDTPA-BMA are crucial for MRI.
- Understanding the stability of liposomes in biological environments is essential for their clinical application.
- Dipalmitoyl phosphatidyl ethanolamine/palmitic acid (DPPE/PA) liposomes are investigated for their potential as drug/contrast agent carriers.
Purpose of the Study:
- To investigate the pH-dependent stability of DPPE/PA liposomal GdDTPA-BMA in human blood and in the presence of specific blood components.
- To elucidate the mechanisms underlying liposome destabilization by various ions and proteins.
Main Methods:
- Relaxometry was used to assess the T(1)-relaxivity (r(1)) changes.
- Visual observations and cryo-transmission electron microscopy (cryo-TEM) were employed to evaluate liposome aggregation and integrity.
- Incubation studies were performed in human blood and with specific blood components (Na+, Ca2+, Mg2+, albumin) at physiological conditions.
Main Results:
- DPPE/PA liposomes were stable at physiological pH in buffer but aggregated and leaked GdDTPA-BMA at lower pH.
- In human blood, liposomes destabilized rapidly at both high and low pH.
- Divalent cations (Ca2+, Mg2+) caused significant aggregation and leakage, more so than Na+.
- Albumin destabilized liposomes via interaction with the PA component.
Conclusions:
- DPPE/PA liposomal GdDTPA-BMA exhibits limited stability in human blood, particularly in the presence of divalent cations and albumin.
- The stability is pH-dependent, with aggregation and leakage occurring at acidic pH.
- Understanding these interactions is critical for the development of stable liposomal MRI contrast agents.