The present invention relates to the compound of formula (I): in which R1 represents a hydrogen atom, an optionally labelled halogen, a radionuclide or a Sn[(C1-C4)alkyl]3 group, Ar represents an aryl group or a heteroaryl group, R9 represents a hydrogen atom, a (C1-C4)alkyl group or forms together with the group R1—Ar a ring fused with the Ar group, A represents a group of formula (β) or (δ): R3 and R4 independently represent a hydrogen atom, a (C1-C6)alkyl group, a (C1-C6)alkenyl group or a group of formula (γ): —Y—Z—W—R11  (γ) wherein R11 represents an optionally labelled halogen, a radionuclide, an aryl or heteroaryl group optionally substituted by an optionally labelled halogen, a radionuclide, a —NO2 group, a —NR5R6 group, a —N+R5R6R7X− group, or a —OSO2R12 group, and their addition salts with pharmaceutically acceptable acids. The present invention also relates to pharmaceutical compositions comprising them and to their use in diagnosis, in particular with SPECT or PET imaging and in therapy of melanoma via targeted radionuclide therapy.