Silicon analogues of the retinoid agonists TTNPB and 3-methyl-TTNPB, disila-TTNPB and disila-3-methyl-TTNPB: chemistry and biology.

Twofold silo-substitution (C/Si exchange) in the saturated ring of the tetrahydronaphthalene skeleton of the retinoid agonists TTNPB (1a) and 3-methyl-TTNPB (2 a) leads to disila-TTNPB (1b) and disila-3-methyl-TTNPB (2b), respectively. The silicon compounds 1b and 2b were synthesized in multiple steps, and their identities were established by elemental analyses, multinuclear NMR experiments, and single-crystal X-ray diffraction studies. Like TTNPB (1a) and 3-methyl-TTNPB (2a), the analogous silicon-based arotinoids 1b and 2b are strong pan-RAR agonists and display the some strong differentiation and opoptosis-inducing activity in NB4 promyelocytic leukemia cells as the parent carbon compounds. These results are in keeping with the nearly isomorphous structures of 1a and 1b bound to the complex of the RAR beta ligand-binding domain with the nuclear receptor (NR) box 2 peptide of the SRC-1 coactivator. The contacts within the ligand-binding pocket are identical except for helix H11, for which two turns are shifted in the disila-TTNPB (1b) complex. This study represents the first comprehensive structure-function analysis of a carbon/silicon switch in a signaling molecule and demonstrates that silicon analogues can have the some biological functionalities and conserved structures as their parent carbon compounds, and it illustrates at the same time that silicon analogues of biologically active compounds have the potential to induce alternative allosteric effects, as in the case of helix H11, which might allow for novel options in drug design.