β2-Homo-Amino Acid Scan of µ-Selective Opioid Tetrapeptide TAPP

Abstract

TAPP (H-Tyr-d-Ala-Phe-Phe-NH2) is a potent, µ-selective opioid ligand. In order to gain further insights into pharmacophoric features of this tetrapeptide, we have performed a β2-Homo-amino acid (β2hAA) scan of the TAPP sequence. To this aim, 10 novel analogues have been synthesized and evaluated for µ-opioid and δ-opioid receptor affinity as well as for stability in human plasma. The derivatives included compounds in which a (R)- or (S)-β2-Homo-Homologue replaced the amino acids in the TAPP sequence. The derivatives with (R)- or (S)-β2hPhe4 turned out to bind µOR with affinities equal to that of the parent. β2hAAs in position 1 and 3 resulted in rather large affinity decreases, but the change differed depending on the stereochemistry. β2-Homologation in the second position gave derivatives with very poor µOR binding. According to molecular modelling, the presented α/β-peptides adopt a variety of binding poses with their common element being an ionic interaction between a protonable amine of the first residue and Asp147. A feature required for high µOR affinity seems the ability to accommodate the ring in the fourth residue in a manner similar to that found for TAPP. Contrary to what might be expected, several compounds were significantly less stable in human plasma than the parent compound.

Antinociceptive and Cytotoxic Activity of Opioid Peptides with Hydrazone and Hydrazide Moieties at the C-Terminus

“…TAPP-based hydrazone (3a; TAPP = H-Tyr-D-Ala-Phe-Phe-NH 2 ) binds with IC 50 = 90.04 nM, which is clearly a higher value than that recently determined for the tetrapeptide in our laboratory (IC 50 = 5.1 nM [38]). Regarding compound 4a, a similar comparison is not straightforward, as we are not aware of any MOR binding data for amide H-Tyr-D-Ala-Trp-NH 2 .…”

“…Regarding the TAPP-based analogues ( 3a and 3b ), the starting position of the peptide sequence (H-Tyr-D-Ala-Phe-Phe-) was based on the position of TAPP reported in our recent paper, which focused on TAPP derivatives [ 38 ]. Again, as a result of the local docking search performed for the novel analogues, only minor displacements occurred in the peptide part ( Figure 5 A).…”

“…The first and the third features correspond to analogous elements of the binding modes of small molecular MOR agonist BU72 (as found in crystal structure 5C1M [46]) or fentanyl (as found by modelling [42,47]). Regarding the TAPP-based analogues (3a and 3b), the starting position of the peptide sequence (H-Tyr-D-Ala-Phe-Phe-) was based on the position of TAPP reported in our recent paper, which focused on TAPP derivatives [38]. Again, as a result of the local docking search performed for the novel analogues, only minor displacements occurred in the peptide part ( Figure 5A).…”

“…The initial binding poses of the compounds in the MOR binding site were prepared manually by building the N-acylhydrazone-, N -acylhydrazide-, or N -acetylhydrazide-based fragments into the peptide structures. The positions of the peptide sequences were based on the experimental positions [43] of corresponding structural elements in DAMGO (1a-d) or based on the results of molecular modelling [38] for TAPP tetrapeptide amide (3a and 3b) and for other derivatives (2a-b, 4a-b, 5a, 6a).…”

In Vivo, In Vitro and In Silico Studies of the Hybrid Compound AA3266, an Opioid Agonist/NK1R Antagonist with Selective Cytotoxicity

“…At MOR, the most populated cluster (with respect to the positioning of the opioid part) closely resembles the experimental positioning of DAMGO in this receptor. Similar binding poses were also proposed based on molecular modelling results for the opioid fragment of other opioid/antitachykinin hybrids [ 17 ] or some Tyr- d -Ala-Phe-Phe (TAPP) derivatives [ 66 ]. Aromatic rings of BU72 (experiment [ 67 ]), fentanyl (modelling [ 68 ]) cyclic opioid derivatives (modelling [ 69 ]), or linear peptide opioids (modelling [ 70 ]) were found to be located similarly as the Phe 4 ring in MOR-1 pose.…”