The invention concerns a compound having formula (I):

wherein R1 represents a radioactive isotope of an element which radioactive isotope is detectable by positron emission tomography (PET) or wherein R1 represents any nonradioactive isotope of said element, R2 represents -CN or -CF 3 , wherein R3 represents

wherein * represents C or N and R4 represents one of the moieties of a group consisting of alkyl, alkyl ether, alkyl nitrile, alkyl alcohol, alkyl halide, O-alkyl, S-alkyl, halogen, nitrile, trifluoromethyl, amino, aminoalkyl, aminodialkyl, phenyl, substituted phenyl, heterocyclic aromatic or a substituted heterocyclic aromatic, dioxolanyl, methyldioxolanyl, and dioxanyl.