Novel σ1 antagonists designed for tumor therapy: Structure - activity relationships of aminoethyl substituted cyclohexanes

Abstract

Depending on the substitution pattern and stereochemistry, 1,3-dioxanes 1 with an aminoethyl moiety in 4-position represent potent σ₁ receptor antagonists. In order to increase the stability, a cyclohexane ring first replaced the acetalic 1, 3-dioxane ring of 1. A large set of aminoethyl substituted cyclohexane derivatives was prepared in a six-step synthesis. All enantiomers and diastereomers were separated by chiral HPLC at the stage of the primary alcohol 7, and their absolute configuration was determined by CD spectroscopy. Neither the relative nor the absolute configuration had a large impact on the σ₁ affinity. The highest σ₁ affinity was found for cis-configured benzylamines (1R,3S)-11 (K_i = 0.61 nM) and (1S,3R)-11 (K_i = 1.3 nM). Molecular dynamics simulations showed that binding of (1R,3S)-11 at the σ₁ receptor is stabilized by the typical polar interaction of the protonated amino moiety with the carboxy group of E172 which is optimally oriented by an H-bond interaction with Y103. The lipophilic interaction of I124 with the N-substituent also contributes to the high σ₁ affinity of the benzylamines. The antagonistic activity was determined in a Ca²⁺ influx assay in retinal ganglion cells. The enantiomeric cis-configured benzylamines (1R,3S)-11 and (1S,3R)-11 were able to inhibit the growth of DU145 cells, a highly aggressive human prostate tumor cell line. Moreover, cis-11 could also inhibit the growth of further human tumor cells expressing σ₁ receptors. The experimentally determined logD_7.4 value of 3.13 for (1R,3S)-11 is in a promising range regarding membrane penetration. After incubation with mouse liver microsomes and NADPH for 90 min, 43% of the parent (1R,3S)-11 remained unchanged, indicating intermediate metabolic stability. Altogether, nine metabolites including one glutathione adduct were detected by means of LC-MS analysis.

Keywords: Aminoethylcyclohexanes; Antagonistic activity; Biotransformation; CD spectroscopy; Ca(2+) influx assay; Calculated free energy of binding; Chiral HPLC; DU145 tumor cells; Inhibition of human prostate tumor cell growth; Lipophilicity; Molecular dynamics simulations; Molecular interactions; Selectivity; Stereochemistry; Structure affinity relationships; Voltage gated Ca(2+) channel; per-residue binding free energy; σ receptors; σ(1) receptor affinity.