A compound of the formula I <;CHEM>; wherein: nuc is the residue of a nucleoside analogue bonded through its single hydroxy group on the cyclic or acyclic saccharide moiety, R1 is hydroxy, amino or carboxy; optionally having esterified/amide bonded thereon a C4-C22 saturated or unsaturated, optionally substituted fatty acid or alcohol, or an aliphatic L-amino acid; R2 is the residue of an aliphatic L-amino acid; L1 is a trifunctional linker group; L2 is absent or a difunctional linker group; and pharmaceutically acceptable salts thereof. <;IMAGE>;