The antifolate drug methotrexate (MTX) can serve as a dual-functional ligand in antitumoral drug delivery, inducing both a folate receptor mediated cellular uptake and an intracellular cytotoxic action. Bioactivity of MTX however changes by conjugation; the activity can be affected by the hampered intracellular conversion to more potent poly-γ-glutamyl derivatives. Therefore, in a cancer combination therapy approach for the codelivery of cytotoxic dsRNA polyinosinic-polycytidylic acid poly(I:C), a set of molecularly precise oligo(ethanamino)amides were synthesized comprising poly(ethylene glycol) conjugated MTX ligands. The conjugates differed in the number of additional glutamic acid residues to investigate the effect of different degrees of synthetic "a priori" polyglutamylation. The bioactivity of these compounds concerning dihydrofolate reductase (DHFR) inhibition, cytotoxicity, nucleic acid binding potency, cellular uptake of poly(I:C) polyplexes, and combined antifolate/poly(I:C) toxicity was investigated. Synthetic polyglutamylation had a crucial impact on several stages of efficient poly(I:C) delivery and combined MTX cytotoxicity. DHFR inhibition of the conjugates significantly increased with increasing polyglutamate chain length. The library member with highest glutamylation degree even outperformed free MTX in direct comparison. Studies in KB cells showed the corresponding enhanced cytotoxicity by polyglutamylation. Also poly(I:C) polyplexes of the glutamylated MTX variants exhibited higher cellular uptake in the folate receptor positive cell line. Finally, a synergistic combined cytotoxicity of polyglutamylated MTX ligands and complexed poly(I:C) cargo was observed in transfected KB cells. The present structure-activity relationship study of MTX-based ligands pinpoints the concept of synthetic polyglutamylation as a promising approach for optimizing bioactivity of antifolate conjugates, which might be considered as a useful tool also in context of other drug delivery systems.
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