Liposomes are closed vesicles having at least one lipid bilayer surrounding an aqueous core. The intra-liposomal space and lipid layer(s) can entrap a wide variety of substances including drugs, cosmetics, diagnostic reagents, genetic material and bioactive compounds. Since non-toxic lipids act as the basis for liposomes, they generally exhibit low toxicity. The low toxicity coupled with the ability of liposomes to increase the plasma circulation lifetime of agents gives rise to liposomes as vehicles particularly useful for delivering pharmaceutically active agents. In many cases, liposome-delivered drugs result in superior clinical efficacy paired with reduced toxicity.
The practical application of liposomal preparations as drug delivery vehicles is limited by the chemical and physical stability of the preparation. Commercialization requires long term stability at both the chemical and physical levels. The use of frozen or freeze-dried (lyophilized) preparations to avoid breakdown of labile drug and/or lipid components provides some improvement in stability. However, during the lyophilization process, ice crystal formation can lead to mechanical rupture, liposome aggregation and fusion (resulting in increased liposome size). Moreover, when liposomes containing drug are lyophilized and then reconstituted at room temperature, changes often occur in the structure of their bilayer(s) which gives rise to accelerated drug leakage.
Prior attempts at preparing lyophilized liposomal compositions have relied on conventional liposomes which are typically in a liquid phase at body temperature where movement of the lipids is fluid and uncontrolled. Such conventional liposomes fall into two categories. The first are maintained in a liquid state because they comprise lipids wherein the gel-to-liquid crystalline temperature (Tc) is below body temperature (i.e., they will be in the liquid phase at body temperature). These liposomes are routinely used in the art however; the downside of being fluid is poor drug retention for many encapsulated agents.
The second type of conventional liposomes never undergo a liquid to gel transition because they include high amounts of membrane rigidification agents, such as cholesterol (e.g., 30-45 mol %). Cholesterol acts to increase bilayer thickness and fluidity while decreasing membrane permeability, protein interactions, and lipoprotein destabilization of the liposome. These high amounts of cholesterol are most frequently used in liposomal studies and historically have been taught as necessary for adequate serum stability and drug retention in vivo, though not all drugs can be sufficiently retained. Certain drugs exhibit better drug retention both in vitro and in vivo in liposomes containing substantially no cholesterol. See, e.g., Dos Santos, et al., Biochim. Biophs. Acta, (2002) 1561:188-201.
On the other hand, liposomes in the gel-phase are more stable and exhibit improved drug retention. The invention takes advantage of liposomes which are in the gel phase at body temperature (i.e., body temperature is below the Tc of the liposomes). Gel-phase liposomes can be prepared with a number of lipids; however, those made with more saturated acyl side chain phosphatidyl lipids, such hydrogenated soy PC, dipalmitoyl phosphatidylcholine (DPPC) or distearoyl phosphatidylcholine (DSPC) are required to have less than 30% cholesterol in order to achieve gel-phases at body temperature. One example of conventional liposomes that do not exhibit gel-phases at body temperature are those made of egg phosphatidylcholine (EPC) which are significantly leaky.
Prior attempts at preparing lyophilized liposomal compositions using conventional liposomes have involved either empty liposomes or liposomes containing only a single agent. They may employ a cryoprotectant, typically a saccharide, both inside and outside of the liposomes or a large osmotic gradient across the liposomal membrane.
For example, cryoprotectants were used to protect against freeze/thaw damage to ‘liquid’ EPC liposomes encapsulating a single agent when present in sufficient amounts both on the inside and outside of the liposomes, ideally when these amounts are equal. See, e.g., U.S. Pat. Nos. 5,077,056 and 4,883,665. The presence of 1%-10% cryoprotectant in the internal liposomal medium preserves a lyophilized EPC liposome-encapsulated doxorubicin formulation where preferably the internal osmolarity is near physiological osmolarity. See, e.g., U.S. Pat. No. 4,927,571. Failure to include a cryoprotectant in the liposome interior has been shown to result in a loss of liposome integrity upon reconstitution, particularly with regard to retaining an encapsulated agent. As described, “prevention of leakage requires the sugar be present both inside and outside the liposome” (Lowery, M. (June 2002) Drug Development and Delivery, Vol. 2, No. 4).
In one case, protection from vesicle aggregation and fusion, as well as against loss of an entrapped drug, has also been reported for hydrogenated soy PC:cholesterol:DSPE-mPEG (51:44:5 molar ratio) liposomes where the liposome preparation contains 44 mol % cholesterol as well as a cryoprotectant and a high concentration of salt in the external medium. The presence of 44% cholesterol means that the liposomes will be in the liquid phase at or below body temperature. Furthermore, the protective effect is only realized if a large osmotic gradient exists across the membrane such that the outer liposome osmolarity is significantly higher than the internal osmolarity. See, e.g., WO01/05372.
Membrane-bound cryoprotectants also further improve resistance to freezing and lyophilizing of these non-gel phase liposomes. In particular, sugars grafted onto EPC or EPC:cholesterol (1:1 molar ratio) liposomal membrane surfaces via oligo(ethylene oxide) linkers consisting of one to three repeating units have been reported to be cryoprotective for liposomes containing a fluorescent probe. See, e.g., Bendas, et al., Eur. J. Pharm. Sci. (1996) 4:211-222; Goodrich, et al., Biochem. (1991) 30:5313-5318; U.S. Pat. No. 4,915,951. Baldeschwieler, et al., reported that in the absence of the terminal sugar group, liposomes prepared with the oligoethylene oxide linker itself were unable to protect against fusion subsequent to freezing. U.S. Pat. No. 4,915,951.
Trehalose in the external medium of a PC liposome formulation encapsulating a single agent provides resistance to liposome aggregation and fusion. U.S. Pat. No. 6,319,517. Other methods of producing small liposomes stabilized against aggregation require the formation of empty PC:Cholesterol (1:1 molar ratio) liposomes to which a solution of sugar and a single reagent are added and then subsequently dried. During the drying process a percentage of the reagent is entrapped within the liposome. These liposomes are reportedly more stable upon storage than in the absence of sugar. See, e.g., WO99/65465.
As stated previously, most previous techniques for lyophilization focused on lyophilization of either empty liposomes or liposomes encapsulating a single agent. Lyophilization with retention of integrity is more challenging where two or more agents are encapsulated, especially if the agents differ in solubility characteristics. Encapsulating two or more agents is often useful since many life-threatening diseases such as cancer, are influenced by multiple molecular mechanisms and due to this complexity, achieving cures with a single agent has been met with limited success. Therefore, almost all cancer treatments involve combinations of more than one therapeutic agent. This is true of treatment of other conditions as well, including infections and chronic diseases.
PCT publication WO03/041681, incorporated herein by reference, reports that gel-phase liposomes with transition temperatures of 38° C. or greater can be prepared using saturated phosphatidyl lipids such as DPPC and DSPC and lower amounts (0-20%) of cholesterol if at least 1 mol % of phosphoinositol (PI) or phosphatidylglycerol (PG) are included in the compositions. These liposomes, when containing combinations of encapsulated irinotecan and floxuridine (FUDR) were shown to be stable to freezing at −20° C. Simple freezing is generally less harsh and less destructive to liposome integrity than lyophilization.
The use of liposomes as delivery vehicles for these combinations is advantageous, particularly if the liposomes include, and are capable of maintaining, ratios of the agents that are non-antagonistic. This general approach is described in detail in U.S. Pat. No. 7,850,990, incorporated herein by reference. This patent teaches how to determine non-antagonistic or synergistic ratios of various therapeutic agents, including antineoplastic agents that maintain such non-antagonism or synergy over a wide range of concentrations. The patent also teaches that it is essential to deliver the drugs in the administered ratio and maintaining that ratio by letting delivery vehicles control the pharmacokinetics. Exemplified in this patent are liposomes that contain, and maintain the ratio of, non-antagonistic or synergistic ratios of two or more therapeutic agents, including irinotecan and FUDR. Such combinations encapsulated in liposomes would benefit from the advantages of being stored in lyophilized form if, upon reconstitution, the integrity of the liposomes and the concentration of the agents and their ratios are maintained. A particularly useful such combination of cytarabine and daunorubicin encapsulated in liposomes is described in U.S. Pat. No. 8,022,279, also incorporated herein by reference.
The use of these combinations in therapeutic protocols with surprisingly good results is described in PCT publication WO2007/050784 and PCT publication WO2008/101214. Additional formulations with liposomal encapsulation of desired drug delivery options are described in WO2009/097011 and WO2009/070761, as well as WO2010/043050. These formulations are simply exemplary of useful compositions wherein two or more therapeutic agents are contained in liposomes for delivery to the patient.
As described above, preparing stable lyophilized compositions of liposomes in general that maintain their integrity upon reconstitution has been difficult and unpredictable. Obtaining such stable liposomal compositions for combinations of two or more agents is even more challenging. Thus, the success of the method of the invention in obtaining lyophilized liposomes wherein the liposomes contain two or more therapeutic or diagnostic agents, and wherein they maintain their integrity upon reconstitution, is a remarkable achievement.