Acyclic Cucurbit[n]uril-Type Containers as Receptors for Neuromuscular Blocking Agents: Structure-Binding Affinity Relationships

Abstract

Acyclic cucurbit[n]uril molecular containers 1 and 2C3 have previously been shown to strongly bind to the neuromuscular blocking agents rocuronium, vecuronium, pancuronium, and cisatracurium in vitro by optical methods and to reverse neuromuscular block in vivo in rats. In this paper we study the in vitro binding of a panel of acyclic CB[n]-type receptors toward the four neuromuscular blocking agents and acetylcholine to develop structure-binding affinity relationships. The selected variants include those with different aromatic sidewalls (e.g. 1Me₄ with dimethyl o-xylylene walls; 3 with 1,8-linked naphthalene walls), with different glycoluril oligomer lengths (e.g. 4 and 5 based on glycoluril trimer), and with different linker lengths between aromatic wall and SO₃ ^- solubilizing group (e.g. 2C2 - 2C4). Based on the analysis of complexation induced changes in ¹H NMR chemical shift we conclude that the hydrophobic regions of the guests bind in the hydrophobic cavity of the hosts with the cationic moieties of the guest binding at the ureidyl C=O portals by ion-dipole and ion-ion interactions. The thermodynamic parameters of binding were determined by direct and competition isothermal titration calorimetry experiments. We find that hosts 4 and 5 based on glycoluril trimer form significantly weaker complexes with the streroidal NMBAs than with the analogues hosts based on glycoluril tetramer (1 and 2C3). Similarly, hosts 1Me₄ and 3 with different length and height aromatic walls do not exhibit the extreme binding constants displayed by 2C3 but rather behave similarly to 1. Finally, we find that hosts 2C2 and 2C4 bind only slightly more weakly to the NMBAs than 2C3, but retain the ability to discriminate against acetylcholine, and possess higher inherent water solubility than 2C3. Host 2C4, in particular, holds potential for future in vivo applications.

Keywords: Cucurbituril; drugs; molecular container; neuromuscular blocking agent; reversal agent.

Figure 1.

Chemical structures of Sugammadex, CB[n], and guest.

Figure 2.

Chemical structures of acyclic CB[n]-type receptors used in this study.

Figure 3.

¹H NMR spectra recorded (400 MHz, RT, D₂O) for solutions containing: a) cis (1 mM), b) cis (1 mM) and 2C2 (1 mM), c) 2C2 (1 mM), d) vec (1 mM), e) 1Me₄ (1 mM) and vec (2 mM), f) 1Me₄ (1 mM) and vec (2 mM), and g) 1Me₄ (1 mM).

Figure 4.

Cross-eyed stereoview of the MMFF minimized geometry of 1Me₄•vec. Color code: C, grey; H, white; N, blue; O, red; S, yellow.

Figure 5.

Thermograms recorded (298.0 K, 20 mM sodium phosphate buffered H₂O, pH 7.4) during the titration of: a) a solution of 2C4 (70 μM) with a solution of CHDA (1 mM) in the syringe, and c) a solution of 2C4 (100 μM) and CHDA (500 μM) in the cell with a solution of Roc (1.08 mM) in the syringe. The data was fitted using the PEAQ-ITC analysis software to: b) a one-set-of-sites binding model to extract the thermodynamic parameters for 2C4•CHDA, and d) to a competitive binding model using the thermodynamic parameters for 2C4•CHDA as inputs to determine the thermodynamic parameters for 2C4•Roc.

Figure 6.

Bar chart depicting the thermodynamic parameters of binding (Δ rG, red bars; ΔrH, green bars; -TΔrS, blue bars) in kcal mol-1 for complexes of roc, vec, and pan toward hosts 1, 4, 1Me4, 2C2, 2C3, 2C4, 5, and 3.