Molecular Requirements for Targeting the Polyamine Transport System. Synthesis and Biological Evaluation of Polyamine−Anthracene Conjugates

A series of nine N -(9-anthracenylmethyl)tetraamines (e.g., Ant-4,4,4-tetraamine) were synthesized and evaluated for cytotoxicity in L1210, α-difluoromethylornithine (DFMO)-treated L1210, Chinese hamster ovary (CHO), and CHO-MG cell lines. Surprisingly, the 3,3,4- and 3,4,3-tetraamine motifs had the same or decreased cytotoxicity in DFMO-treated L1210 cells, whereas the rest of the tetraamine systems were usually more cytotoxic and gave lower IC50 values in this treated cell line. The most sensitive derivatives to DFMO treatment were the Ant-4,4,3- and Ant-4,4,4-tetraamine analogues, which were 7 and 5 times more cytotoxic in DFMO-treated L1210 cells, respectively. K i values for each of the anthracenylmethyl(Ant)-polyamine conjugates were determined in L1210 cells and revealed that these systems are high-affinity ligands for the polyamine transporter (PAT). Mixed results were observed in the CHO and CHO-MG assays. The 4,4,4- and 5,4,4-tetraamine motifs were 3 times more toxic to CHO cells with active polyamine transporters. For example, the Ant-4,4,4-tetraamine conjugate displayed IC50 values of 11 μM in CHO cells and 33 μM in CHO-MG cells, a PAT-deficient cell line. This suggested that these derivatives used the PAT in part to access cells. However, most of the other tetraamine derivatives had similar potencies in both the CHO and CHO-MG cell lines. In terms of vector design, higher affinity for the PAT (lower K i values) did not translate into higher potency for the tetraamine conjugate. In contrast, the related triamine systems, which had micromolar K i values in L1210 cells, were more efficacious and selective. In one case, the 4,4-triamine motif imparted 150-fold higher potency in CHO cells than the CHO-MG mutant. A deconvolution microscopy study in A375 melanoma cells revealed a rapid internalization of the Ant-4,4-triamine as fluorescent vesicles, whereas the Ant-4,4,4-tetraamine remained mostly at the cell surface. These findings help define the key characteristics required for selective delivery of polyamine−drug conjugates into cell types with active polyamine transporters.