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In previous work we have obtained a dimeric dipeptide mimetic of 4th loop of BDNF - hexamethylenediamide bis-(N-monosuccinil-L-seryl-L-lysine) (GSB-106), having a neuroprotective activity in vitro in a concentration range 10(-5)-10(-8) M and an antidepressant activity in vivo at doses 0.1 and 1 mg/kg i.p. in rats. We have investigated the structural and functional relationships among analogues of GSB-106. Glycine scan was performed and a number of appropriate compounds were synthesized: GT-105 (here lysine is replaced by glycine), GT-107 (here serine is replaced by glycine), GT-106Ac (here monosuccinic radical is replaced by acetyl group). We have studied the dependence of activity of following compounds from the configuration of amino acid residues: GT-107D (D-enantiomer of the GT-107), GT-106DL (L-serine was replaced by D-serine), GT-106LD (L-lysine was replaced by D-lysine). The investigation of these compounds using the HT22 cell culture in conditions of oxidative stress has approved only two analogues of GSB-106 to have a neuroprotective effect: in the case of replacement of serine to glycine and of replacment of succinic radical to acetic group. A disappearance of this effect was observed in event of the replacement of lysine residue to glycine in GT-105, L-lysine residue to D-lysine and also by conversion of serine configuration. These results show that lysine residue is crucial for the neuroprotective activity of GSB-106. L-Configuration of the lysine and serine residues required. Configuration of lysine residue becomes critical in absence of serine side group. Thus, the the following fragment is a minimum pharmacophore of beta-turn of 4 loop of BDNF: HOOC-CH2-CH-CO-NH-(S)-CH(CH2OH)-CO-NH-(S)-CH((CH2)4NHz)-CO-NH-(CH2)3-. Only one (GT-106Ac) out of two analogues of GSB-106 with neuroprotective activity possesses antidepressant activity too. This fact indicates about a necessity of more stringent structural requirements for exposure of antidepressant activity. The results obtained can be useful for designing of new active mimetics of BDNE

Dimeric dipeptide GSB-106, a novel structural analogue of neurotrophin BDNF, in doses from 10(-5) to 10(-8) M protected cultured immortalized mouse hippocampal HT-22 neurons from H2O2 or glutamate damage. The neuroprotective effect of GSB-106 was also shown on cultured SH-SY5Y human neuroblastoma cells after treatment with neurotoxin 6-hydroxidopamine. These data attest to functional similarity of GSB-106 and neurotrophin BDNF.

A dimeric dipeptide mimetic of nerve growth factor (NGF), bis-(N-monosuccinyl-L-glutamyl-L-lysine) hexamethylenediamide (GK-2), was previously developed at V. V. Zakusov State Institute of Pharmacology, activated specific TrkA receptors, and exhibited neuroprotective activity in vitro (10–5 – 10–9 M) and in vivo (0.1 – 10 mg/kg i.p. and p.o.). GK-2 was designed based on the beta-turn (-Asp94-Glu95-Lys96-Gln97-) of the NGF 4th loop and preserved the central dipeptide fragment (-Glu95-Lys96-). The Asp94 residue was replaced by its monosuccinyl bioisostere. The dimeric structure of NGF was reproduced using a bivalent hexamethylenediamine spacer. The structure—activity (neuroprotective) relationship for GK-2 was studied in the present work using a glycine scan, i.e., successive replacement of the peptide side groups by H. The bis-(N-acetyl-L-glutamyl-L-lysine) (GK-2Ac), bis-(N-monosuccinylglycyl-L-lysine) (GK-2-Gly1), and bis-(N-monosuccinyl-L-glutamylglycine) hexamethylenediamides (GK-2-Gly2) were less active with neuroprotective activity in vitro under oxidative stress for HT22 cells at concentrations 10 – 100 times greater than GK-2. The conclusion was drawn that each side radical of GK-2 was important for manifestation of the full neuroprotective activity of dimeric dipeptide GK-2, a mimetic of the NGF 4th loop. However, removal of any of the side radicals would probably not change the active structure of the beta-turn so that the two remaining side radicals should retain the ability to bind to their TrkA subsites. This could explain the retention of neuroprotective activity in the GK-2 glycine analogs. [ABSTRACT FROM AUTHOR]