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. 2021 Jun 30;16(4):341-357.
doi: 10.4103/1735-5362.319573. eCollection 2021 Aug.

Design, synthesis, and evaluation of novel racecadotril-tetrazole-amino acid derivatives as new potent analgesic agents

Mehdi Asadi  1 , Maryam Mohammadi-Khanaposhtani  2 , Faezeh Sadat Hosseini  1 , Mahdi Gholami  3 , Ahmad Reza Dehpour  4   5 , Massoud Amanlou  1   5
Affiliations

Design, synthesis, and evaluation of novel racecadotril-tetrazole-amino acid derivatives as new potent analgesic agents

Mehdi Asadi et al. Res Pharm Sci. .

Abstract

Background and purpose: Although pain is one of the most common symptoms of diseases, it is often mismanaged due to limited access to painkillers and ineffectiveness, unacceptable side effects, or the possibility of abuse. However, an alternative approach to existing analgesics is to indirectly increase endogenous pain relief pathways by neprilysin (an enkephalinase) inhibitors. This enzyme breaks down and inactivates enkephalin, dynorphin, endorphins, and their derivatives.

Experimental approach: In this project, a new series of racecadotril-tetrazole-amino acid derivatives 15a-l was synthesized and characterized on the basis of IR, 1H and 13C NMR, mass spectrometry, and elemental analysis. The antinociceptive activity of synthesized compounds was assessed by a hot plate, tail-flick, and formalin assays in mice. Docking was used to identify the possible interactions between neprilysin and synthesized compounds. 15a-l was synthesized and characterized on the basis of IR, 1H and 13C NMR, mass spectrometry, and elemental analysis. The antinociceptive activity of synthesized compounds was assessed by a hot plate, tail-flick, and formalin assays in mice. Docking was used to identify the possible interactions between neprilysin and synthesized compounds.

Findings/results: Most of the synthesized compounds showed moderate to good analgesic effects in hot plat and tail-flick test in comparison to morphine and racecadotril. Compounds 15l and 15j were the most potent compounds. The synergistic analgesic effect of compounds 15l and 15j with morphine and the antagonistic effect of naloxone on the activity of these compounds confirm that the analgesic effect of compounds 15l and 15j could be mediated through the opioidergic system. The negative and high binding energy of docking simulation of the most potent compounds in the catalytic site of neprilysin was also in good agreement with the inhibitory activity of test compounds.

Conclusion and implications: Racecadotril-tetrazole-amino acid derivatives, as potential antinociceptive agents, demonstrated moderate to good antinociceptive activities comparable with morphine and higher than racecadotril.

Keywords: Antinociceptive activity; Enkephalinase; Molecular docking simulation; Racecadotril; Tetrazole; Thiorphan.

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Conflict of interest statement

The authors declared no conflict of interest in this study.

Figures

Fig. 1.
Fig. 1.
Design strategy for racecadotril-tetrazole-amino acid derivatives 15a-l.
Scheme 1.
Scheme 1.
Reagents and conditions for the synthesis of compounds 15a-l: (a) EtOAC, Et2NH, 0 °C, 3 h; (b) pH adjust to 1 (HCl 1N); (c) CH2Cl2, reflux, 3 h, yield = 58%; (d) NH4Cl, NaN3, DMF, 80 °C, 8 h, yield = 80%; (e) EtOH, reflux, 3 h; (f) Et3N, tetrahydrofuran, -10 °C, 1 h; (g) H2O, tetrahydrofuran, RT, 3 h; (h) trifluoroacetic acid, tetrahydrofuran, H2O, RT, overnight; (i) EDC/HOBT, DMF, Et3N, RT, 16 h.
Fig. 2
Fig. 2
Evaluation of synergic effects of racemic racecadotril and compounds 15j and 15l with morphine and antagonist effect of naloxone on racecadotril and compounds 15j and 15l in the hot plate test. The values represent mean ± SEM, n = 6. *P < 0.05 and **P < 0.01 indicate significant differences from saline as the vehicle. M, Morphine; rac, racecadotril.
Fig. 3
Fig. 3
Evaluation of synergic effects of racecadotril and compounds 15j and 15l with morphine and antagonist effect of naloxone on racecadotril and compounds 15j and 151 in the tail-flick test. The values represent mean ± SEM, n = 6. * P <0.05 and ** P < 0.01 indicate significant differences from saline as vehicle. M, Morphine; N, naloxone; rac, racecadotril.
Scheme 2
Scheme 2
Possible hydrolytic metabolism of compounds 15a-l to active metabolites 16a-l.
Fig. 4
Fig. 4
Superimposed position of the most analgesic compounds in the catalytic site of NEP. The close interaction mode with residues and Zn ion in 3D and 2D displayed of the parent form of 15j (A, E), 15l (B, F), also hydrolyzed form of 15j (C, G), and 15l (D, H).
Fig. 5
Fig. 5
Superimpose of co-crystal ligand (cyan) and re-docked ligand (green). The position of S’1 subsite of neprilysin and residue His 583, His 587, and Glu 646 which are coordinated with the zinc ion.
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