Multivesicular liposomes utilizing neutral lipids to modify in vivo release

#This invention relates to liposomal formulations of compounds such as drugs. More particularly this invention relates to methods of modifying the in vivo rate of release of encapsulated compounds from multivesicular liposomes by the choice of the neutral lipid in the liposomal formulation.

#When phospholipids and many other amphipathic lipids are dispersed gently in an aqueous medium they swell, hydrate, and spontaneously form multilamellar concentric bilayer vesicles with layers of aqueous media separating the lipid bilayers. These systems are commonly referred to as multilamellar liposomes or multilamellar vesicles (MLV), and usually have diameters of from 0.2 to 5 .mu.m. Sonication of MLV results in the formation of small unilamellar vesicles (SUV) bounded by a single lipid bilayer with diameters usually in the range of from 20 to 100 nm, containing an aqueous solution. Multivesicular liposomes (MVL) differ from MLV and SUV in the way they are manufactured, in the random, non-concentric arrangement of aqueous-containing chambers within the liposome, and in the inclusion of neutral lipids necessary to form the MVL.

#Various types of lipids differing in chain length, saturation, and head group have been used in liposomal drug formulations for years, including the unilamellar, multilamellar, and multivesicular liposomes mentioned above. The neutral lipids used in the manufacture of multivesicular liposomes to date have been primarily limited to triolein and tricaprylin.

#FIG. 1 is a graph comparing the release rates of recombinant, human granulocyte colony stimulating factor (rhu G-CSF) from multivesicular liposomes (MVL) formulated with different molar ratios of triolein to tricaprylin (.gradient., 100:0; .DELTA..DELTA., 50:50; .largecircle., 25:75; .quadrature., 10:90; .diamond., 0:100) as the neutral lipid when incubated at 37.degree. C. in 60% human plasma in saline solution.

#FIG. 2 is a graph showing the results of a pharmacodynamic study comparing the duration of the rhu G-CSF dependent elevation in peripheral blood leucocyte number of golden Syrian hamsters following subcutaneous injection of 100 .mu.g/kg of rhuG-CSF in saline solution (.gradient.), in a triolein-only, 100:0, MVL formulation (.largecircle.), in a triolein-only MVL formulation solubilized pre-injection with Tween.TM. 20 (.DELTA.), and in a tricaprylin-only, 0:100, MVL formulation (.quadrature.)

#FIG. 3 is a graph comparing the release rates of rhu GM-CSF from MVL formulated with different molar ratios of triolein to tricaprylin (.DELTA., 100:0; .largecircle., 25:75; .quadrature., 10:90) as the neutral lipid when incubated at 37.degree. C. in 60% human plasma in saline solution.

#A method is provided for modifying the rate of release of a biologically active compound, such as a drug, encapsulated in a multivesicular liposomal formulation by selection of the neutral lipid, or combination of neutral lipids, used to manufacture the multivesicular liposomes (MVL).

#There are at least three types of liposomes. The term "multivesicular liposomes (MVL)" as used throughout the specification and claims means man-made, 1-200 .mu.m particles partially comprised of lipid membranes enclosing multiple non-concentric aqueous chambers. In contrast, "multilamellar liposomes or vesicles" (MLV) have multiple "onion-skin" concentric membranes, in between which are shell-like concentric aqueous compartments. Multilamellar liposomes characteristically have mean diameters in the micrometer range, usually from 0.5 to 25 .mu.m. The term "unilamellar liposomes or vesicles (ULV)" as used herein refers to liposomal structures having a single aqueous chamber, usually with a mean diameter range from about 20 to 500 nm.

#Multilamellar and unilamellar liposomes can be made by several relatively simple methods. The prior art describes a number of techniques for producing ULV and MLV (for example U.S. Pat. No. 4,522,803 to Lenk; U.S. Pat. No. 4,310,506 to Baldeschweiler; U.S. Pat. No. 4,235,871 to Papahadjopoulos; U.S. Pat. No. 4,224,179 to Schneider, U.S. Pat. No. 4,078,052 to Papahadjopoulos; U.S. Pat. No. 4,394,372 to Taylor U.S. Pat. No. 4,308,166 to Marchetti; U.S. Pat. No. 4,485,054 to Mezei; and U.S. Pat. No. 4,508,703 to Redziniak).