Chemoenzymatic synthesis of 2,6-disubstituted tetrahydropyrans with high σ1 receptor affinity, antitumor and analgesic activity

Abstract

1,3-Dioxanes 1 and cyclohexanes 2 bearing a phenyl ring and an aminoethyl moiety in 1,3-relationship to each other represent highly potent σ₁ receptor antagonists. In order to increase the chemical stability of the acetalic 1,3-dioxanes 1 and the polarity of the cyclohexanes 2, tetrahydropyran derivatives 3 equipped with the same substituents were designed, synthesized and pharmacologically evaluated. The key step of the synthesis was a lipase-catalyzed enantioselective acetylation of the alcohol (R)-5 leading finally to enantiomerically pure test compounds 3a-g. With respect to σ₁ receptor affinity and selectivity over a broad range of related (σ₂, PCP binding site) and further targets, the enantiomeric benzylamines 3a and cyclohexylmethylamines 3b represent the most promising drug candidates of this series. However, the eudismic ratio for σ₁ binding is only in the range of 2.5-3.3. Classical molecular dynamics (MD) simulations confirmed the same binding pose for both the tetrahydropyran 3 and cyclohexane derivatives 2 at the σ₁ receptor, according to which: i) the protonated amino moiety of (2S,6R)-3a engages the same key polar interactions with Glu172 (ionic) and Phe107 (π-cation), ii) the lipophilic parts of (2S,6R)-3a are hosted in three hydrophobic regions of the σ₁ receptor, and iii) the O-atom of the tetrahydropyran derivatives 3 does not show a relevant interaction with the σ₁ receptor. Further in silico evidences obtained by the application of free energy perturbation and steered MD techniques fully supported the experimentally observed difference in receptor/ligand affinities. Tetrahydropyrans 3 require a lower dissociative force peak than cyclohexane analogs 2. Enantiomeric benzylamines 3a and cyclohexylmethylamines 3b were able to inhibit the growth of the androgen negative human prostate cancer cell line DU145. The cyclohexylmethylamine (2S,6R)-3b showed the highest σ₁ affinity (K_i(σ₁) = 0.95 nM) and the highest analgesic activity in vivo (67%).

Keywords: 2,6-Disubstituted tetrahydropyrans; Analgesic activity; Androgen negative human prostate cancer cell line DU145; Antiallodynic activity; Antitumor activity; CD spectroscopy; Chemoenzymatic synthesis; Chiral HPLC; Docking studies; Kinetic resolution; Molecular dynamics; Neuropathic pain; Selectivity; Steered molecular dynamics; Stereochemistry; Structure affinity relationships; σ(1) receptor; σ(1) receptor affinity.