Amyloid peptide aggregation has been implicated in diverse neurodegenerative diseases, including Parkinson's, Alzheimer's, Huntington's, and prion diseases, affecting hundreds of millions of people worldwide [1]. Alzheimer's disease (AD) is the main disorder responsible for behavioral and physical decline, leading to dementia in elderly people [2,3]. It has deserved much attention in the last decade [[4], [5], [6], [7]], and different hypotheses and strategies are being tested to identify pathological conditions as early as possible and to achieve more effective therapeutic treatments. Several hypotheses have been proposed to explain its pathology, such as abnormally phosphorylated and β-folded τ protein [8], cholinergic impairment [9], metal dyshomeostasis, [10], and oxidative stress [11], in addition to amyloid plaques formation [6].

The currently approved drugs for AD are mostly based on the cholinergic system, which plays an important role in the regulation of learning and memory processes [12]. Drugs such as rivastigmine, donepezil, and galantamine act as acetylcholinesterase inhibitors, entering clinical tests and commercial market. But they do not deal with the pathology of the disease, rather combating the symptoms.

In the literature, molecular mechanisms involved in the formation of amyloid fibrils from soluble proteins have been extensively investigated and elucidated in vitro [7]. Investigations on the toxicity of soluble Aβ oligomers pointed toward a more complex scenario, where metals and reactive oxygen species (ROS) play an important role [[13], [14], [15], [16]].

Based on evidence that metal ions are implicated in most of the processes related to the progress of AD (oxidative stress, neurotoxicity, induction of autophagy or apoptosis, synaptic plasticity) [17,18], many studies focused on the development of multifunctional molecules containing different metal binding moieties, besides having other fitting properties for potential therapeutic strategies [19,20]. Additionally, since the 90's, some antibodies have been reported as targeting amyloid aggregation and causing its inhibition [21,22]. More recently, amyloid structures have been used to design inhibitors of fibril aggregation, especially de novo miniproteins that also prevent the seeded aggregation and the toxicity of fibrils in cells [23].

However, by what means the different steps and factors that can modulate the disease in vivo still remain to be explained [24]. Initial studies to better understand this disease described neuronal dysfunctions because of fibrillary tangles and senile plaques formation, probably due to aggregation of amyloid peptides [25,26]. Despite the correlation between amyloid aggregation and toxicity has not been clarified yet, many studies indicated that pathogenic fibril formation contributes to cellular dysfunction and death. Further, soluble amyloid assemblies have also been identified in other degenerative diseases [27] and metal-amyloid peptide species were reported, especially with copper ions, in a stoichiometric ratio of 1:1 [28]. Metal coordination to Aβ peptide seems to regulate the amyloid fibril assemblies, and their toxicity [[29], [30], [31]]. Consequently, different approaches were suggested and developed to prevent the oligomerization of Aβ into neurotoxic species [32]. Many studies focused the metal binding affinity of amyloid peptides, depending on the peptide considered, being Aβ1–16, Aβ1–28, and Aβ1–40 the most investigated [[33], [34], [35]].

In a rational therapeutic approach, some efficient metal chelating ligands, like clioquinol (CQ) and pyrrolidine dithiocarbamate, have been used to interfere in copper homeostasis in animal models of AD, since elevated extracellular and low intracellular copper levels were detected in AD brains [36]. Engineered carbohydrate-containing ligands were also reported as promising agents for AD therapy [37], as well as some tacrine-coumarin hybrid molecules [38] or designed thioflavin-derived molecules that contain both amyloid binding and metal chelating properties [39].

In a unique and alternative proposal, some tetradentate bis(thiosemicarbazonate) complexes of copper(II) and zinc(II) were reported as potential agents for AD therapy [40]. Those complexes were able to dissociate intracellularly, and increased metal levels in Chinese hamster ovary cells overexpressing amyloid precursor protein (APP-CHO), with subsequent effect on extracellular levels of Aβ. They interfere in the metabolism of such peptides, activating some kinase-dependent pathways, through relatively large changes in metal levels (100–200-fold for copper, and 10-fold for zinc). Another investigation reported the efficient binding of the iron(III) 2,17-bis-sulfonato-5,10,15- tris(pentafluorophenyl)corrole complex to the Aβ peptide, and its ability in acting as catalytic antioxidant toward oxidative action of copper ions restraining the formation of large peptide aggregates [41].

Particularly, a class of phenylhydrazone derivatives of indoles has been previously studied as inhibitors of amyloid peptides (Aβ1–40) aggregation. These derivatives were shown to be effective in reducing its fibrillization process [42,43]. Further, tacrine-isatin hybrids showed a good inhibition potential for acetylcholinesterase (AChE). The study also showed a mixed-type inhibition in Lineweaver-Burk plots which, supported by docking studies, indicates a possible affinity of the compounds with the peripheral anionic site (PAS) and the catalytic anionic site (CAS) portions of the enzyme AChE [44]. Isatin thiosemicarbazone as well exhibited a protective effect on hippocampal neuronal cells against neurotoxicity caused by the fibrillization of peptides Aβ40 and Aβ42. Additionally, it was shown that this class of compounds can interact with the more aggressive forms of amyloid peptides and inhibit its self-aggregation, preventing damage to the cells caused by ROS [45].

Based on previous studies in our lab on oxindolimines and corresponding metal complexes as promising pharmacological agents [46], some oxindole derivatives as hydrazones (isahim and isahpy) and Schiff bases (isapn and misapn) were prepared and their ability to interact with amyloid peptides was investigated (Fig. 1). Oxindolimine complexes can enter the cells, having DNA and mitochondria as main targets, causing oxidative damage by the generation of ROS and inducing apoptosis [47]. Further, they can inhibit cyclin-dependent kinases, by interacting remarkably at ATP-binding site [48] and interact efficiently with topoisomerase IB, inhibiting the DNA cleavage step [49]. Therefore, an investigation into the reactivity of such oxindole derivatives toward amyloid peptides was initiated.

The main purpose of the present study was to verify the ability of oxindole-derived compounds, Schiff bases or hydrazones (shown in Fig. 1), in competing with Aβ peptides to coordinate metal ions, particularly copper(II) and zinc(II), and to attest its effects in aggregation processes involving those peptides. The results indicated strong interactions between the designed ligands and the peptides, in addition to competitive performance in equilibria of metal-coordinated species formation.