Cyclic Peptides as T-Type Calcium Channel Blockers: Characterization and Molecular Mapping of the Binding Site

T-Type Calcium Channels: A Mixed Blessing

The role of T-type calcium channels is well established in excitable cells, where they preside over action potential generation, automaticity, and firing. They also contribute to intracellular calcium signaling, cell cycle progression, and cell fate; and, in this sense, they emerge as key regulators also in non-excitable cells. In particular, their expression may be considered a prognostic factor in cancer. Almost all cancer cells express T-type calcium channels to the point that it has been considered a pharmacological target; but, as the drugs used to reduce their expression are not completely selective, several complications develop, especially within the heart. T-type calcium channels are also involved in a specific side effect of several anticancer agents, that act on microtubule transport, increase the expression of the channel, and, thus, the excitability of sensory neurons, and make the patient more sensitive to pain. This review puts into context the relevance of T-type calcium channels in cancer and in chemotherapy side effects, considering also the cardiotoxicity induced by new classes of antineoplastic molecules.

Conceptual DFT, QTAIM, and Molecular Docking Approaches to Characterize the T-Type Calcium Channel Blocker Anandamide

The anandamide is a relevant ligand due to its capacity of interacting with several proteins, including the T-type calcium channels, which play an important role in neuropathic pain and depression disorders. Hence, a detailed characterization of the chemical properties and conformational stability of anandamide may provide valuable information to understand its behavior in a biological context. Herein, conceptual DFT and QTAIM analyses were performed to theoretically characterize the chemical reactivity properties and the structural stability of conformations of anandamide, using the BP86/cc-pVTZ level of theory. Global reactivity description, based on conceptual DFT, indicates that the hardness increases and the electrophilicity index decreases for both, the hairpin and U-shape conformers relative to the extended conformers. Also, an increase in the chemical potential value and a decrease in the electronegativity and the electrophilicity index is observed in the ethanolamide open ring conformers in comparison with the corresponding closed ring structures. In addition, regarding the characterization of local reactivity descriptors, the maximum values of the Fukui and Parr functions indicate that the most probable location for a nucleophilic attack is either the hydroxyl oxygen located in the ethanolamide closed ring conformers or the carbonyl oxygen present in the open ring conformers. The most probable location for an electrophilic attack is in the alkyl double bond region in all anandamide conformers. According to the QTAIM results, the intramolecular hydrogen bond formation stabilizing the structure of anandamide has interaction energy values for the closed ring conformations of 12.33–12.46 kcal mol⁻¹, indicating a strong interaction. Lastly, molecular docking calculations determined that a region in the pore, denominate as pore-blocking, is a probable site for the interaction of anandamide with the human Ca_v3.2 isoform of the T-type calcium channel family. The pore-blocking site contains hydrophobic residues where the non-polar part in the final alkyl region of anandamide established mainly alkyl-alkyl interactions, while the polar part (the ethanolamide group) interacts with the polar residue S900. The information based on conceptual DFT presented may aid in the design of drugs with similar chemical characteristics as those identified in anandamide so as to bind anandamide-interacting proteins, including the T-type calcium channels.

Identification of a novel class of cyclic penta-peptides against hepatitis C virus as p7 channel blockers

The hepatitis C virus (HCV) p7 viroporin protein is essential for viral assembly and release, suggesting its unrealised potential as a target for HCV interventions. Several classes of small molecules that can inhibit p7 through allosteric mechanisms have shown low efficacy. Here, we used a high throughput virtual screen to design a panel of eight novel cyclic penta-peptides (CPs) that target the p7 channel with high binding affinity. Further examination of the effects of these CPs in viral production assays indicated that CP7 exhibits the highest potency against HCV among them. Moreover, the IC₅₀ efficacy of CP7 in tests of strain Jc1-S282T suggested that this cyclopeptide could also effectively inhibit a drug-resistant HCV strain. A combination of nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations revealed that CP7 blocking activity relies on direct binding to the p7 channel lumen at the N-terminal bottleneck region. These findings thus present a promising anti-HCV cyclic penta-peptide targeting p7 viroporin, while also describing an alternative strategy for designing a new class of p7 channel blockers for strains resistant to direct-acting antiviral agents (DAA).