Antisense oligonucleotides conjugated with boron clusters (B-ASOs) have been described as potential gene expression inhibitors and carriers of boron for boron neutron capture therapy (BNCT), providing a dual-action therapeutic platform. In this study, we tested the nucleolytic stability of DNA oligonucleotides labeled with metallacarborane [(3,3'-iron-1,2,1',2'-dicarbollide)(-1)]ate [Fe(C2B9H11)2] (FESAN) against snake venom phosphodiesterase (svPDE, 3'→5'-exonuclease). Contrary to the previously observed protective effect of carborane (C2B10H12) modifications, the B-ASOs containing a metallacarborane moiety at the 5'-end of the oligonucleotide chain were hydrolyzed faster than their parent nonmodified oligomers. Interestingly, an enhancement in the hydrolysis rate was also observed in the presence of free metallacarborane, and this reaction was dependent on the concentration of the metallacarborane. Microscale thermophoresis (MST) analysis confirmed the high affinity (Kd nM range) of the binding of the metallacarborane to the proteins of crude snake venom and the moderate affinity (Kd µM range) between the metallacarborane and the short single-stranded DNA. We hypothesize that the metallacarborane complex covalently bound to B-ASO holds DNA molecules close to the protein surface, facilitating enzymatic cleavage. The addition of metallacarborane alone to the ASO/svPDE reaction mixture provides the interface to attract freely floating DNA molecules. In both cases, the local DNA concentration around the enzymes increases, giving rise to faster hydrolysis. It was experimentally shown that an allosteric effect, possibly attributable to the observed boost in the 3´→5´-exonucleolytic activity of snake venom phosphodiesterase, is much less plausible.
Keywords: BNCT; MST; antisense oligonucleotide; binding affinity; boron cluster; exonuclease; metallacarborane; phosphodiesterase; snake venom.