Exploring tryptamine conjugates as pronucleotides of phosphate-modified 7-methylguanine nucleotides targeting cap-dependent translation

Eukaryotic translation initiation factor 4E (eIF4E) is overexpressed in many cancers deregulating translational control of the cell cycle. mRNA 5′ cap analogs targeting eIF4E are small molecules with the potential to counteract elevated levels of eIF4E in cancer cells. However, the practical utility of typical cap analogs is limited because of their reduced cell membrane permeability. Transforming the active analogs into their pronucleotide derivatives is a promising approach to overcome this obstacle. 7-Benzylguanosine monophosphate (bn⁷GMP) is a cap analog that has been successfully transformed into a cell-penetrating pronucleotide by conjugation of the phosphate moiety with tryptamine. In this work, we explored whether a similar strategy is applicable to other cap analogs, particularly phosphate-modified 7-methylguanine nucleotides. We report the synthesis of six new tryptamine conjugates containing N⁷-methylguanosine mono- and diphosphate and their analogs modified with thiophosphate moiety. These new potential pronucleotides and the expected products of their activation were characterized by biophysical and biochemical methods to determine their affinity towards eIF4E, their ability to inhibit translation in vitro, their susceptibility to enzymatic degradation and their turnover in cell extract. The results suggest that compounds containing the thiophosphate moiety may act as pronucleotides that release low but sustainable concentrations of 7-methylguanosine 5′-phosphorothioate (m⁷GMPS), which is a translation inhibitor with in vitro potency higher than bn⁷GMP.