Compounds of the formula I, in which Het is an unsubstituted or R6-and/or R7-sustituted pyrrolyl, furanyl, thiophenyl, pyrazolyl, imidazolyl, oxazolyl, isoxazolyl, thiazolyl, isothiazolyl, thiadiazolyl, pyrimidinyl, pyrazinyl or pyridazinyl radical, or an unsubstituted or R6-and/or R7-substituted fused bi-or tricyclic ring system comprising at least one aromatic ring and up to 4 heteroatoms-selected from the group consisting of O(oxygen), S(sulphur) or N(nitrogen)-which is bonded to the phenanthridinyl radical via one of the rings comprising one or more heteroatoms, areactive PDE4 inhibitors.