Compounds of formula II wherein each R.sup.1 and R.sup.2 is independently selected from H and --P(O)(R.sup.3)(R.sup.4), provided that at least one of R.sup.1 and R.sup.2 is --P(O)(R.sup.3)(R.sup.4); each R.sup.3 and R.sup.4 is independently selected from the group consisting of H, --OH, C.sub.1-C.sub.6 alkyl and C.sub.1-C.sub.6 alkoxy;or a pharmaceutically acceptable salt thereof. Pharmaceutical compositions comprising a compound of formula II, and methods of administering such compounds and compositions.