MODULATION OF MELPHALAN RESISTANCE IN GLIOMA-CELLS WITH A PERIPHERAL BENZODIAZEPINE RECEPTOR-LIGAND MELPHALAN CONJUGATE

Peripheral benzodiazepine receptors (PBRs) are located on the outer me mbrane of mitochondria, and their density is increased in brain tumors . Thus, they may serve as a unique intracellular and selective target for antineoplastic agents. A PBR ligand-melphalan conjugate (PBR-MEL) was synthesized and evaluated for cytotoxicity and affinity for PBRs. PBR-MEL (9) (i.e., 670 amu) was synthesized by coupling of two key int ermediates: 4-[bis(2-chloroethyl)amino]-L-phenylalanine ethyl ester tr ifluoroacetate (6) and ro-1,3-dihydro-5-phenyl-2H-1,4-benzodiazepin-2- one (8). On the basis of receptor-binding displacement assays in rat b rain and glioma cells, 9 had appreciable binding affinity and displace d a prototypical PBR ligand, Ro 5-4864, with IC50 values between 289 a nd 390 nM. 9 displayed differential cytotoxicity to a variety of rat a nd human brain tumor cell lines. In some of the cell lines tested incl uding rat and human melphalan-resistant cell lines, 9 demonstrated app reciable cytotoxicity with IC50 values in the micromolar range, lower than that of melphalan alone. The enhanced activity of 9 may reflect i ncreased membrane permeability, increased intracellular retention, or modulation of melphalan's mechanisms of resistance. The combined data support additional studies to determine how 9 may modulate melphalan r esistance, its mechanisms of action, and if target selectivity can be achieved in in vivo glioma models.