Synthesis of a novel pentagastrin-drug conjugate for a targeted tumor therapy

The synthesis of the novel pentagastrin seco-CBI conjugate 3, which is based on the highly cytotoxic antitumor antibiotic (+)-duocarmycin SA (1), is reported. A key step in the synthesis is the palladium-catalyzed carbonylation of aryl bromide 7 to give the benzyl ester 16, which is transformed into the new seco-CBI derivative 21 bearing a carboxylic acid ester moiety. Subsequent transformation of 21 into an activated ester followed by the introduction of beta-alanine and tetragastrin led to the new pentagastrin drug 3 that contains a peptide moiety for targeting cancer cells expressing CCK-B/gastrin receptors.