This study aimed at developing nerve growth factor (NGF) mimetics that selectively activate specific biological signals and, as a result, lack the side effects of the full-length protein. Two dimeric dipeptides, bis-(N-aminocaproyl-glycyl-L-lysine) hexamethylenediamide (GK-6) and bis(N-succinyl-L-glutamyl-L-lysine) hexamethylenediamide (GK-2), were designed based on the most exposed outside fragments of NGF, namely, the loop 1 and loop 4 β-turn sequences, respectively. These dipeptides exhibited neuroprotective activity in vitro at micro-nanomolar concentrations. Studies on the mechanism of action revealed that both compounds elevate the level of tyrosine kinase A (TrkA) receptor phosphorylation and that they each have different postreceptor signaling patterns. GK-6 increases the levels of extracellular signal-regulated kinase (ERK) and AKT kinase phosphorylation, whereas GK-2 only increases the level of AKT phosphorylation. Apart from the neuroprotective activity, GK-6 promoted differentiation in PC12 cells, whereas GK-2 did not. Furthermore, it was established that the neuroprotective activity of GK-2 was completely abolished by a selective inhibitor of phosphatidylinositol 3-kinase (LY294002) but not by a specific inhibitor of mitogen-activated protein kinases MEK1 and MEK2 (PD98059). In vivo experiments demonstrated that GK-2 did not induce hyperalgesia, which is one of the primary adverse effects of NGF. By contrast, GK-6 produced a significant decrease in the pain threshold of rats as determined by the tail flick test. The data obtained suggest that dimeric dipeptide NGF mimetics are promising candidates in the development of pharmacological agents with NGF-like activity that are free of the main side effect of NGF.

Background: Two dimeric dipeptides, bis-(N-monosuccinyl-L-seryl-L-lysine)hexamethy­lenediamide (GSB-106) and bis-(N-monosuccinyl-L-methionyl-L-serine) heptamethylenediamide (GSB-214), were designed based on the brain-derived neurotrophic factor (BDNF) loop 4 and loop 1 β-turn sequences, respectively. Earlier, both of these dipeptides were shown to exhibit neuroprotective activity in vitro (10^-5–10^-8 M). The present study aimed to investigate the mechanisms of action of these peptides and their neuroprotective activity in an experimental stroke model. Methods: We used western blot and HT-22 hippocampal neuronal cell line to investigate whether these peptides induced phosphorylation of the TrkB receptor and the AKT and ERK kinases. Rat middle cerebral artery occlusion (MCAO) was used as a stroke model. GSB-106 and GSB-214 were administered intraperitoneally (0.1 mg (1.3×10^-7 mol)/kg) 4 hours after MCAO and daily for 7 days. The cerebral infarct volumes were measured with 2,3,5-triphenyltetrazolium chloride staining 21 days after MCAO. Results: Both compounds were shown to elevate the TrkB phosphorylation level while having different post-receptor signaling patterns. GSB-106 activated the PI3K/AKT and MAPK/ERK pathways simultaneously, whereas GSB-214 activated the PI3K/AKT only. In experimental stroke, the reduction of cerebral infarct volume by GSB-106 (~66%) was significantly greater than that of GSB-214 (~28% reduction), which could be explained by the fundamental role of the MAPK/ERK pathway in neurogenesis and neuroplasticity. Notably, between these two dipeptides, only GSB-106 exhibited antidepressant activity, as was found previously.
Conclusion: The results provided support for the beneficial pharmacological properties of BDNF loop 4 mimetic GSB-106, thereby suggesting a potential role for this dipeptide as a therapeutic agent.

Low-molecular-weight mimetics of loops 1 and 4 of the brain-derived neurotrophic factor (BDNF) have been designed and synthesized. The compounds represent monomeric and dimeric amides of N-acyldipeptides. Their dipeptide fragments coincide in sequence with the central regions of beta-turns of the corresponding neurotrophin loops, and acyl groups are the bioesosteres of preceding amino acid residues. Hexa- or heptamethylenediamines were used as spacers to link the C-terminal regions of dipeptides in dimeric mimetics of BDNF. These compounds were synthesized by classical methods of peptide synthesis in solution and received the laboratory codes GSB-104 (HO-Suc-Ser-Lys-NH₂), GSB-106 {[HO-Suc-Ser-Lys-NH-(CH₂)₃−]₂}, GSB-207 (HO-Suc-Met-Ser-NH₂), and GSB-214 ([HO-Suc-Met-Ser-NH-(CH₂)_7/2-]₂). It was shown using immortalized hippocampal cells of the HT22 line under conditions of oxidative stress that the dimeric mimetics of both loops at concentrations of 10⁻⁵−10⁻⁸ M possess a neuroprotective activity. The monomeric loop 1 mimetic GSB-207 in the same concentration range is inactive, and the monomeric loop 4 mimetic GSB-104 at a concentration of 10⁻⁷ impairs the survival of neurons. The finding that only dimeric mimetics possess the neuroprotective activity is consistent with the data indicating that BDNF is active in the homodimeric form. As opposed to the dimeric loop 1 mimetic GSB-214, the dimeric loop 4 mimetic GSB-106 exhibits the antidepressant activity typical for BDNF in the Porsolt test on rats at doses of 0.1 and 1 mg/kg injected intraperitoneally. This suggests that the antidepressant activity of BDNF is related to its 4th loop. We believe that the compounds obtained will be useful in studies of the mechanism of action of BDNF and may form the basis for the design of a novel group of drugs with antidepressant and neuroprotective activities.

The search for new highly-effective, fast-acting antidepressant drugs is extremely relevant. Brain derived neurotrophic factor (BDNF) and signaling through its tropomyosin-related tyrosine kinase B (TrkB) receptor, represents one of the most promising therapeutic targets for treating depression. BDNF is a key regulator of neuroplasticity in the hippocampus and the prefrontal cortex, the dysfunction of which is considered to be the main pathophysiological hallmark of this disorder. BDNF itself has no favorable drug-like properties due to poor pharmacokinetics and possible adverse effects. The design of small, proteolytically stable BDNF mimetics might provide a useful approach for the development of therapeutic agents. Two small molecule BDNF mimetics with antidepressant-like activity have been reported, 7,8-dihydroxyflavone and the dimeric dipeptide mimetic of BDNF loop 4, GSB-106. The article reflects on the current literature on the role of BDNF as a promising therapeutic target in the treatment of depression and on the current advances in the development of small molecules on the base of this neurotrophin as potential antidepressants.

Noopept (N-phenyl-acetyl-L-prolylglycine ethyl ester) was constructed as a dipeptide analog of the standard cognition enhancer, piracetam. Our previous experiments have demonstrated the cognition restoring effect of noopept in several animal models of Alzheimer disease (AD). Noopept was also shown to prevent ionic disbalance, excitotoxicity, free radicals and pro-inflammatory cytokines accumulation, and neurotrophine deficit typical for different kinds of brain damages, including AD. In this study, we investigated the neuroprotective action of noopept on cellular model of AD, Aβ25–35-induced toxicity in PC12 cells and revealed the underlying mechanisms.