Branched lipid compouds

A compound of formula (I):

• wherein:
• R21 is selected from the group consisting of H, —CH2OH, —CH2CH2OH, —CH(CH3)OH, —CH2OPO(OH)2, CH2C(═O)NH2, —CH2CH2C(═O)OH and —CH2CH2C(═O)OR8, wherein any one of the alkyl hydrogens can be replaced with a halogen;
• R22 is H, C1-C6 aliphatic, an amino protecting group, L3—C(═O)—, or A2;
• L1 and L2 are each independently C6-C21 aliphatic or C5-C20 heteroaliphatic;
• L3 is C1-C21 aliphatic or C2-C20 heteroaliphatic;
• A2 is an amino acid or a peptide;
• R23 is H or C1-C6 aliphatic;
• R24a and R25a are each independently selected from C1-C6 aliphatic and C1-C6 heteroaliphatic and R24b and R25b are each independently selected from H, C1-C6 aliphatic and C1-C6 heteroaliphatic, or R24a and R24b together with the carbon atom to which they are attached form a C3-8cycloalkyl or 3-8 membered heterocyclyl group, and/or R25a and R25b together with the carbon atom to which they are attached form a C3-8cycloalkyl or 3-8 membered heterocyclyl group;
• X is selected from —S—, —S(═O)— and —S(═O)2—;
• v is an integer from 1-3;
• R26 and R27 are each independently selected from H and C1-C6 aliphatic;
• Z1 and Z2 are independently selected from the group consisting of —C(═O)O—, —OC(═O)—, —C(═O)NR—, —NRC(═O)—, —C(═O)S—, —SC(═O)—, —OC(═O)O—, —NRC(═O)O—, —OC(═O)NR—, and —NRC(═O)NR—;
• wherein each R is independently H or C1-C6 aliphatic;
• PEG is a polyethylene glycol;
• wherein any aliphatic, heteroaliphatic, cycloalkyl and heterocyclyl present in any of R21, R22, R23, R24a, R24b, R25a, R25b, R26, R27, L1, L2 and L3 is optionally substituted or a pharmaceutically acceptable salt, solvate or prodrug thereof.