Use of a compound for the detection of senescent cells, wherein the compound is of the general formula or , shown below: wherein: R 1 and R 2 are each independently selected from: i) hydrogen, with the proviso that R 2 is not hydrogen in general structure above and only one of R 1 and R 2 can be hydrogen in general structure above. ii) an optionally substituted (1-10C)alkyl group; iii) an optionally substituted aryl group; iv) an optionally substituted (1-10C)alkyl-aryl group; v) an optionally substituted aryl-(1-10C)alkyl group; or vi) R 1 and R 2 are linked so as to form part of a spiranic cycloalkane group, selected from cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl or adamantanyl; R 3 is hydrogen or (1-10C)alkyl group; R 4 is hydrogen, or one or more of the following substituents: i) a halogen selected from F, Cl, Br and I; ii) NO 2 ; iii) CF 3 ; iv) SCH 3 ; v) an optionally substituted (1-5C)alkyl group; vi) a (1-10C)alkoxy group which optionally comprises one or more heteroatoms; and Z is either Z 1 or -Ar-Zi- and Z 1 is selected from OH, NH 2 , O(CH 2 ) n CH 2 OH, (CH 2 ) q OH or COOH, wherein n is an integer selected from 1 to 9, and wherein q is an integer selected from 1 to 4, and wherein Ar is an aryl group optionally substituted with one or more of the following substituents halogen, (1-6C)alkyl, (1-6C)alkenyl or (1-5C)alkoxy. R 1 and R 2 are each independently selected from: i) hydrogen, with the proviso that R 2 is not hydrogen in general structure above and only one of R 1 and R 2 can be hydrogen in general structure above. ii) an optionally substituted (1-10C)alkyl group; iii) an optionally substituted aryl group; iv) an optionally substituted (1-10C)alkyl-aryl group; v) an optionally substituted aryl-(1-10C)alkyl group; or vi) R 1 and R 2 are linked so as to form part of a spiranic cycloalkane group, selected from cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl or adamantanyl; i) hydrogen, with the proviso that R 2 is not hydrogen in general structure above and only one of R 1 and R 2 can be hydrogen in general structure above. ii) an optionally substituted (1-10C)alkyl group; iii) an optionally substituted aryl group; iv) an optionally substituted (1-10C)alkyl-aryl group; v) an optionally substituted aryl-(1-10C)alkyl group; or vi) R 1 and R 2 are linked so as to form part of a spiranic cycloalkane group, selected from cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl or adamantanyl; R 3 is hydrogen or (1-10C)alkyl group; R 4 is hydrogen, or one or more of the following substituents: i) a halogen selected from F, Cl, Br and I; ii) NO 2 ; iii) CF 3 ; iv) SCH 3 ; v) an optionally substituted (1-5C)alkyl group; vi) a (1-10C)alkoxy group which optionally comprises one or more heteroatoms; and i) a halogen selected from F, Cl, Br and I; ii) NO 2 ; iii) CF 3 ; iv) SCH 3 ; v) an optionally substituted (1-5C)alkyl group; vi) a (1-10C)alkoxy group which optionally comprises one or more heteroatoms; and Z is either Z 1 or -Ar-Zi- and Z 1 is selected from OH, NH 2 , O(CH 2 ) n CH 2 OH, (CH 2 ) q OH or COOH, wherein n is an integer selected from 1 to 9, and wherein q is an integer selected from 1 to 4, and wherein Ar is an aryl group optionally substituted with one or more of the following substituents halogen, (1-6C)alkyl, (1-6C)alkenyl or (1-5C)alkoxy. <EOS>