An isomer, enantiomer, diastereoisomer or tautomer of a compound, represented by formula I: ##STR00001## wherein wherein A, B, R.sup.2, R.sup.3, M.sup.1, M.sup.2, M.sup.3, M.sup.4, Y.sup.1 and Z are as defined in claim 1, or a salt thereof, as an inhibitor of HCV NS5B polymerase.