Materials characterization of the low temperature sensitive liposome (LTSL): effects of the lipid composition (lysolipid and DSPE–PEG2000) on the thermal transition and release of doxorubicin

Abstract

This paper describes how we have used material science, physical chemistry, and some luck, to design a new thermal-sensitive liposome (the low temperature sensitive liposome (LTSL)) that responds at clinically attainable hyperthermic temperatures releasing its drug in a matter of seconds as it passes through the microvasculature of a warmed tumor. The LTSL is composed of a judicial combination of three component lipids, each with a specific function and each affecting specific material properties, including a sharp thermal transition and a rapid on-set of membrane permeability to small ions, drugs and small dextran polymers. Experimentally, the paper describes how bilayer-concentration changes involving the lysolipid and the presence or absence of DSPE-PEG2000 affect both the lipid transition temperature and the drug release. While the inclusion of 4 mol% DSPE-PEG2000 raises the transition temperature peak (T(m)) by about 1 degrees C, the inclusion of 5.0, 9.7, 12.7 and 18.0 mol% MSPC slightly lowered this peak back to 41.7 degrees C, while not further broadening the peak breadth. As for drug release, in the absence of MSPC, the encapsulated doxorubicin-citrate is hardly released at all. Increasing the composition of MSPC in the lipid mixture (5.0, 7.4, 8.5 and 9.3 mol% MSPC) shows faster and faster initial doxorubicin release rates, with 8.5 and 9.3 mol% MSPC formulations giving 80% of encapsulated drug released in 4 and 3 min, respectively. The Thermodox formulation (9.7 mol% MSPC) gives 60% released in the first 20 s. The presence of PEG-lipid is found to be essential in order for the lysolipid-induced permeability to reach these very fast times. From drug and dextran release experiments, and estimates of the molecular and pore size, the conclusions are that: in order to induce lasting nanopores in lipid bilayers -10 nm diameter, they initially require the presence (from the solid phase structure) of grain boundary defects at the DPPC transition and the permeabilizing component(s) can either be a pore forming lysolipid/surfactant plus a PEG-lipid, or can be generated by a PEG-surfactant incorporated at -4-5 mol%. The final discussion is centered around the postulated defect structures that result in membrane leakage and the permeability of doxorubicin and H+ ions.

Lipid-Based Drug Delivery Systems in Cancer Therapy: What Is Available and What Is Yet to Come

“…However, the efficacy of this formulation was not as great in extending the tumor growth inhibition time of these animals relative to the temperature-sensitive liposomes (Needham et al, 2000). Similarly, the inclusion of MSPC into the liposomal formulation led to a rapid release of liposomal contents under mild hyperthermic conditions (Needham et al, 2013). In fact, incorporation of MSPC into the liposomal formulation at 5.0, 7.4, 8.5, and 9.3 mol% accelerated the initial DOX release rate, with the 8.5 and 9.3 mol% formulations releasing 80% of their content within 4 and 3 minutes, respectively (Needham et al, 2013).…”

“…The rapid release of encapsulated content from temperature-sensitive liposomes containing lysolipids can be explained by the fact that these single chain lipids can increase the fluidity of the liposomal membrane and reduce the PTT slightly by ;1°C (Needham et al, 2013). More importantly, lysolipids can form porous defects that are stabilized by DSPE-PEG 2000Da within the liposomal bilayer along the grain boundary between solid and liquid lipid at the PTT (Needham et al, 1997;Zhelev, 1998;Mills and Needham, 2005;Needham et al, 2013).…”

“…This led to the development of ThermoDox, a temperature-sensitive liposomal DOX formulation. ThermoDox (Celsion Corporation, Lawrenceville, NJ) is composed of DPPC:MSPC: DSPE-PEG 2000Da at 86.5:9.7:3.8 mol% (Needham et al, 2013). In rabbits bearing the Vx2 tumor, the use of ThermoDox resulted in a 3-to 4-fold increase in DOX levels within heated tumors compared to nonheated tumors and an 8-fold increase in comparison to free DOX (Ranjan et al, 2012).…”

“…More importantly, lysolipids can form porous defects that are stabilized by DSPE-PEG 2000Da within the liposomal bilayer along the grain boundary between solid and liquid lipid at the PTT (Needham et al, 1997;Zhelev, 1998;Mills and Needham, 2005;Needham et al, 2013).…”

“…Moreover, the temperature required to trigger release was marginally higher than that achievable clinically (Kong and Dewhirst, 1999). As a result, micelle-forming lysolipids, such as 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (MPPC) and 1-steroyl-2-hydroxy-snglycero-3-phosphocholine (MSPC), have been incorporated into temperature-sensitive liposome formulations to increase the permeability of the liposomal membrane to their encapsulated contents Needham et al, 2013).…”