Thursday, March 27, 2008

Is a Dried Liposome Really a Liposome?

We all know that in order for phospholipids to organize into the familiar bilayer sheets that water must be present. The general definition of a liposome is a vesicle composed of a phospholipid bilayer suspended in an aqueous medium which encloses an aqueous space. So if we remove water from the liposome suspension, is it still a liposome? Generally the answer is NO. What remains is dried lipid perhaps with some bilayer organization present and when water is reintroduced into the system, multilamellar liposomes will reform. If a water-soluble compound was entrapped inside the liposome and the unencapsulated compound removed prior to drying, the compound would re-equilibrate with compound now both on the inside and the outside of the newly formed liposomes (the encapsulated compound would leak out). Also if the liposomes were extruded or homogenized to form predominantly unilamellar vesicles prior to the water being removed, the liposomes would revert to a typical population of multilamellar vesicles when the water is added back to the dried lipid as if sizing had never been performed.

John and Lois Crowe along with others performed many of the definitive experiments on how to lyophilize (freeze-dry) liposomes while retaining the liposome structure and encapsulated contents upon reconstitution (addition of water). The key is adding a large amount of sugar (at least 3 molar) to both the internal and external aqueous medium of the liposomes prior to lyophilization. The hydroxyl groups on the sugar molecules appear to substitute for water in hydrogen-bonding to the phosphate headgroup thus stabilizing the bilayer organization as the water is removed. The presence of the sugar molecules also prevent the liposomes from fusing as the water is removed therefore the initial size of the liposomes is maintained. The combination of stabilizing the headgroups of the lipids in the bilayer along with preventing fusion of the liposomes as the water is removed also allows the liposome to maintain the encapsulation of water-soluble compounds even after reconstitution.

This is a very general and simplistic explanation of the issues involved with lyophilizing liposomes. Anyone with experience in the field understands just how many variables must be determined experimentally in order to successfully lyophilize and reconstitute any particular liposome formulation with a compound either encapsulated into the aqueous space or incorporated into the membrane. Additionally, on a commercial scale, lyophilization is a very expensive process. Scientists who are designing liposome formulations for potential commercial use should carefully evaluate the need to lyophilize the liposome product versus packaging and storing the liposome suspension as a liquid. This evaluation should include the extended research and development time required to formulate the lyophilized product versus the liquid product and whether or not having a large amount of sugar in the product is desirable or even feasible.