A compound of the general formula (III): wherein X is O, S, NH or CH2; Y is O, S or NH; Z is O, S or CH2; R1 is C1-8 alkyl, especially C1-6 alkyl, preferably n-alkyl, e.g., n-pentyl or n-hexyl; at least one of R2 and R3 is H—[R4-R5]n—R6—, in which: H—[R4-R5]n— comprises an oligopeptide, R4 being an amino acid and R5 being an amino acid selected from proline, alanine, hydroxyproline, dihydroxyproline, thiazolidinecarboxylic acid (thioproline), dehydroproline, pipecolic acid (L-homoproline), azetidinecarboxylic acid, aziridinecarboxylic acid, glycine, serine, valine, leucine, isoleucine and threonine, R6 is a neutral, non-polar amino acid moiety that is bonded to R5 by a peptide bond, and n is 1, 2, 3, 4 or 5; and the other of R3 and R2 is H—[R4-R5]n-R6— or H; or a pharmaceutically acceptable salt thereof.