Abstract: The present invention relates to a endothelin receptor antagonist conjugate of the formula (I) wherein R2 is an alkoxy group and one of R1 and R3 represents an alkoxy group and the other represents a group of the formula: (OCH2CH2)n—NH—X, wherein n is an integer of 1 to 100 and X represents a fluorescent dye and tautomers thereof. Furthermore, the present invention relates to a diagnostic composition comprising the compounds of the invention. The present invention also relates to the use of the compounds of the invention for the preparation of a diagnostic composition for the diagnosis of cancer, the evaluation of cancer biology and/or monitoring of anticancer therapy. In a further aspect, the present invention relates to kits comprising the compounds of the invention.