Bi-directionally crosslinked liposomes and method of making same

1. A method of producing bi-directionally crosslinked unilamellar liposomes, comprising the steps of:

providing a lipid composition comprising a plurality of reactive lipids, wherein each of the reactive lipids comprises a reactive hydrophobic group, a reactive hydrophilic group, or a reactive hydrophilic group and a reactive hydrophobic group;

forming an un-crosslinked liposome comprising the reactive lipids; and

crosslinking at least a portion of the reactive hydrophobic groups, at least a portion of the reactive hydrophilic groups, or at least a portion of the reactive hydrophobic groups and at least a portion of the hydrophilic groups;

thereby producing the bi-directionally crosslinked liposomes,

wherein the bi-directionally crosslinked liposomes are radially crosslinked and circumferentially crosslinked,

wherein the radial crosslinks comprise covalent bonds between two layers of the lipid bilayer, and

wherein the circumferential crosslinks comprise covalent bonds between lipids in the same layer of the lipid bilayer.

2. The method of claim 1 , wherein the reactive lipids in the un-crosslinked liposome are arranged in a lipid bilayer having at least two leaflets and wherein:

crosslinking the reactive hydrophobic groups comprises forming covalent bonds between the reactive hydrophobic groups of adjacent reactive lipids within the same leaflet;

crosslinking the reactive hydrophobic groups comprises forming covalent bonds between the reactive hydrophobic groups of adjacent reactive lipids across different leaflets; or

crosslinking the reactive hydrophobic groups comprises forming covalent bonds between the reactive hydrophobic groups of adjacent reactive lipids within the same leaflet and between the reactive hydrophobic groups of adjacent reactive lipids across different leaflets.

3. The method of claim 1 , wherein:

the reactive hydrophobic groups comprise one or more carbon-carbon double bonds, one or more carbon-carbon triple bonds, one or more azide groups, one or more thiol groups, or a combination thereof;

the reactive hydrophilic groups comprise one or more carbon-carbon double bonds, one or more carbon-carbon triple bonds, one or more azide groups, one or more thiol groups, or a combination thereof; or

the reactive hydrophobic groups and the reactive hydrophilic groups comprise one or more carbon-carbon double bonds, one or more carbon-carbon triple bonds, one or more azide groups, one or more thiol groups, or a combination thereof.

4. The method of claim 1 , wherein each reactive lipid is independently selected from the group consisting of 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine; 1-palmitoyl-2-(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine; 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphoethanolamine; and 1-palmitoyl-2-(10,12-tricosadiynoyl)-sn-glycero-3-phosphoethanolamine.

5. The method of claim 1 , wherein:

crosslinking the reactive hydrophobic groups of the component lipids comprises contacting the reactive hydrophobic groups with a non-lipid crosslinking reagent;

crosslinking the reactive hydrophilic groups of the component lipids comprises contacting the reactive hydrophilic groups with a non-lipid crosslinking reagent; or

crosslinking the reactive hydrophobic groups of the component lipids comprises contacting the reactive hydrophobic groups with a non-lipid crosslinking reagent and crosslinking the reactive hydrophilic groups of the component lipids comprises contacting the reactive hydrophilic groups with a non-lipid crosslinking reagent.

6. The method of claim 5 , wherein the non-lipid crosslinking reagent comprises one or more carbon-carbon double bonds, one or more carbon-carbon triple bonds, one or more azide groups, or a combination thereof.

7. The method of claim 5 , wherein the crosslinking reagent is selected from the group consisting of an amine-reactive homobifunctional crosslinker, a thiol-reactive homobifunctional crosslinker, an amine- and thiol-reactive heterobifunctional crosslinker, and combinations thereof.

8. The method of claim 1 , wherein the lipid composition and the liposomes further comprise a plurality of non-reactive lipids.

9. The method of claim 8 , wherein each of the non-reactive lipids is independently selected from the group consisting of a phosphatidylcholine, a PEGylated phosphatidylethanolamine, and a sterol.

10. The method of claim 8 , wherein the lipid composition and the liposomes are substantially free of cholesterol or cholesterol derivatives.

11. The method of claim 1 , wherein:

the bi-directionally crosslinked liposomes have an average diameter of 100 nanometers or less;

the bi-directionally crosslinked liposomes have a polydispersity index of 0.20 or less; or

the bi-directionally crosslinked liposomes have an average diameter of 100 nanometers or less and a polydispersity index of 0.20 or less.

12. A population of bi-directionally crosslinked unilamellar liposomes produced by the method of claim 1 , wherein the bi-directionally crosslinked unilamellar liposomes are radially crosslinked and circumferentially crosslinked, wherein the radial crosslinks comprise covalent bonds between two layers of the lipid bilayer, and wherein the circumferential crosslinks comprise covalent bonds between lipids in the same layer of the lipid bilayer.

13. The population of bi-directionally crosslinked liposomes according to claim 12 , further comprising a therapeutic agent, diagnostic agent, or a combination thereof.

14. A population of bi-directionally crosslinked unilamellar liposomes comprising crosslinked lipids arranged in a lipid bilayer having two leaflets,

wherein the crosslinked lipids comprise a crosslinked hydrophobic group, a crosslinked hydrophilic group, or a crosslinked hydrophilic group and a crosslinked hydrophobic group,

wherein the bi-directionally crosslinked liposomes are radially crosslinked and circumferentially crosslinked,

wherein the radial crosslinks comprise covalent bonds between two layers of the lipid bilayer, and

wherein the circumferential crosslinks comprise covalent bonds between lipids in the same layer of the lipid bilayer.

15. The population of bi-directionally crosslinked liposomes according to claim 14 , wherein each of the crosslinked lipids is independently selected from the group consisting of crosslinked 2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine; crosslinked 1-palmitoyl-2-(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine; crosslinked 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphoethanolamine; and crosslinked 1-palmitoyl-2-(10,12-tricosadiynoyl)-sn-glycero-3-phosphoethanolamine.

16. The population of bi-directionally crosslinked liposomes according to claim 14 , further comprising a plurality of un-crosslinked lipids.

17. The population of bi-directionally crosslinked liposomes according to claim 16 , wherein each of the un-crosslinked lipids is independently selected from the group consisting of a phosphatidylcholine and a PEGylated phosphatidylethanolamine.

18. The population of bi-directionally crosslinked liposomes according to claim 17 , which are substantially free of cholesterol or cholesterol derivatives.

19. The population of bi-directionally crosslinked liposomes according to claim 14 , wherein:

the bi-directionally crosslinked liposomes have an average diameter of 100 nanometers or less;

the bi-directionally crosslinked liposomes have a polydispersity index of 0.20 or less; or

the bi-directionally crosslinked liposomes have an average diameter of 100 nanometers or less and a polydispersity index of 0.20 or less.

20. The population of bi-directionally crosslinked liposomes according to claim 14 , further comprising a therapeutic agent, diagnostic agent, or a combination thereof.