The present invention is a polypeptide, which is an exendin-4 peptide derivative, or a salt thereof. Said polypeptide comprises an amino acid sequence represented by Y-QMEEEAVRLFIEWLKNGGPSSGAPPPS-NH2 (I). In formula (I), Y- represents an N-terminal [alpha]-amino group being attached to a C-terminal carboxyl group of an amino acid sequence represented by any one of B-DLSK (II), H2N-DLSX (III), B-HGEGTFTSDLSK (IV) or H2N-HGEGTFTSDLSX (V). In formulas (II) and (IV), B- represents an N-terminal [alpha]-amino group modified by a modifying group having a radioactive gallium binding site. In formulas (III) and (V), X represents a lysine residue modified by a modifying group wherein a side chain amino group has a radioactive gallium binding site. The N-terminal [alpha]-amino group of the aforementioned B- and the side chain amino group of the lysine residue of the aforementioned X are each a polypeptide or a salt thereof, modified by a modifying group having a radioactive gallium binding site represented by the prescribed general formula.本發明的多肽或其鹽,係exendin-4之肽衍生物的多肽或其鹽;其中,該多肽係由「Y-QMEEEAVRLFIEWLKNGGPSSGAPPPS-NH2(I)」所示胺基酸序列形成;式(I)中,「Y-」係表示N端α-胺基、與「B-DLSK(II)」、「H2N-DLSX(III)」、「B-HGEGTFTSDLSK(IV)」或「H2N-HGEGTFTSDLSX(V)」中之任一者所示胺基酸序列的C端羧基形成肽鍵;式(II)及(IV)中,「B-」係表示利用具有放射性鎵結合部位的修飾基,進行修飾之N端α-胺基;式(III)及(V)中,「X」係表示側鏈胺基利用具有放射性鎵結合部位的修飾基,進行修飾之離胺酸殘基;上述「B-」的N端α-胺基、及上述「X」的離胺酸殘基之側鏈胺基,係利用具有既定一般式所示放射性鎵結合部位的修飾基進行修飾。
