The present invention provides a compound of the following formula I: R.sup.3--NH--C(.dbd.X)--P(.dbd.O)OR.sup.1OR.sup.2 including pharmaceutically acceptable salts, solvates, hydrates and polymorphs of the compounds of formula I, as well as geometrical isomers and optically active forms of the compounds of formula I and pharmaceutically acceptable salts, solvates, hydrates and polymorphs of said isomers and forms, wherein R.sup.1 and R.sup.2 may be the same or different and are each selected from hydrogen, acyloxyalkyl and aryl or R.sup.1 and R.sup.2 may form together with the oxygen and phosphorus atoms a dioxaphosphacycloalkane ring; X is O or S; and R.sup.3 is selected, when X is O, from bicycloalkyl, cycloalkylalkyl and substituted cycloalkyl by at least one of alkyl, amino, amidino and guanidino; and R.sup.3 is selected, when X is S, from bicycloalkyl, cycloalkylalkyl and cycloalkyl optionally substituted by at least one of alkyl, amino, amidino and guanidino; with the proviso that: when X is O, R.sup.3 is not cyclohexylmethyl, and when X is S, R.sup.3 is not cyclohexyl.The invention further provides pharmaceutical compositions comprising the above compounds and their use in medicine.