Alzheimer's disease (AD) is considered a neuro-dementing disorder associated with cognitive dysfunction and memory loss [1]. According to the World Alzheimer's Report 2021, 55 million people are suffering from AD worldwide which is expected to reach 78 million by 2030 and 150 million by 2050 [2]. AD is the most common form of dementia which contributes about 60–70 % of total dementia cases. It is believed that more than 90 % of people are living in low to middle-income nations undiagnosed because of a lack of knowledge or attentiveness [3]. These exponential growths of AD patients and the lack of specific treatments put a socio-economic burden, especially in developing countries [4]. The 70 % genetic and 30 % environmental factors are responsible for the AD. Considering the genetic factors, AD is mainly classified into two types i.e. familial and sporadic AD. Familial AD is frequently accompanied by several gene alterations, including amyloid precursor protein (APP) and presenilin (PSEN1 and PSEN2 genes) which make up < 1 % of AD patients [5,6]. While, changes at the molecular level (methylation, oxidative damage) in certain genes including the apolipoproteins (APOE) gene which, in the absence of an efficient repair system in an aging organism leads to sporadic AD (considered the most common form of AD) [7,8].

Due to the complex etiology involved in the progression of disease, the precise cause and course of AD are yet unknown [9]. However, various pathophysiological hallmarks like ACh dysregulation in the synaptic cleft, neuro-inflammation, over-activation of N-methyl-d-aspartate receptor (NMDAR), Aβ and Tau aggregation, oxidative stress, CREB signaling modulation, biometal dysbalance, pathogenic contribution, and neuronal vascular malfunction are believed to be some of the key players in AD progression [[10], [11], [12], [13], [14], [15], [16], [17], [18]].

Amongst the several pathophysiologies proposed for AD development and its progression, we mainly considered cholinergic and Aβ hypothesis to work around [19]. The cholinergic hypothesis involves the serine hydrolase cholinesterase (ChE) enzymes including Acetylcholinesterase (AChE) and Butyrylcholinesterase (BuChE) that catalyze the hydrolysis of ACh into choline and acetate [20]. Pathologically reduced level of ACh in the brain leads to neuro-signaling impairment. Therefore, the inhibition of ChE via modulating its native hydrolytic degradation is considered to be the most promising strategy to upregulate ACh in synaptic cleft and so as cholinergic neurotransmission [21]. The second most accepted hypothesis in AD progression is the Aβ hypothesis that states proteolytic cleavage of APP under the influence of BACE-1 enzymes which leads to the formation of Aβ insoluble plaques [22]. Formation of Aβ plaques leads to neuro-inflammation, hyper activation of its associated pathways and also produces reactive oxygen species (ROS) causing oxidative stress in the neuronal cells [23]. It is also observed that elevated level of ChE in the brain triggers Aβ aggregation [9].

In the last 3-4 decades, continuous efforts have been made by research scientists to develop a therapy to overcome AD progression. The most common therapy for AD includes Food and Drugs Administration (FDA) approved drugs such as memantine (NMDAR antagonist), galantamine, donepezil, and rivastigmine (AChE inhibitors), which only provide symptomatic relief with a slight improvement in learning and cognition instead of halting the progression of the disease [24]. Recently, some monoclonal antibodies have been approved as disease-modifying agents like Lecanemab and Aducanumab (recently approved by the FDA in an accelerated approval pathway) however, their use are still debated [25,26].

Our previous work demonstrated that compound AV-2 (with quinazoline moiety) had the potential to inhibit multiple targets. Considering the multi-target directed ligands (MTDLs) properties of the compound AV-2 it was further optimized for better inhibitory activities against various targets associated with AD progression. The rigorous literature survey of compounds containing quinazoline moiety revealed that quinazoline derivatives played a significant role in inhibiting various targets responsible for the progression of AD. The various recently reported synthetic (compounds 1–10) and natural (Deoxyvasicine, Vasicine, Vasicinone, Deoxyvasicinone) quinazoline compounds along with their inhibitory activities were discussed in Fig. 1, Fig. 2 [[27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37]].

Based on the above facts and the multi-faceted nature of the disease, a new series of quinazoline derivatives (AK-1 to AK-14) was rationally designed based on the lead optimization technique. We introduced various substituted piperazines at the C4 position of quinazoline considering a bioisosteric replacement for N-benzylpiperidine ring (pharmacophoric feature of FDA-approved drug donepezil). Further, to improve binding in the peripheral anionic site (PAS) region of AChE, the methoxy (-OCH3) groups were introduced at the C6 and C7 positions of the quinazoline ring.

The designed derivatives were synthesized and characterized by various state-of-the-art spectroscopic techniques. These derivatives were biologically evaluated for their inhibitory activity against hChE and hBACE-1 enzymes. Amongst them, the screened compounds were analyzed for their blood-brain barrier (BBB) permeability and binding ability towards the PAS of hAChE via peripheral artificial membrane permeability (PAMPA)-BBB and propidium iodide (PI) displacement assays, respectively. The enzyme kinetic and reversibility study of the most promising compound against hAChE were performed to check the type of inhibition, and reversible or irreversible nature of the compound. The neurotoxic and neuroprotective properties of the most promising compound were estimated via MTT assay on differentiated neuroblastoma SH-SY5Y cell lines. The Aβ aggregation inhibition potential of compound AK-2 was checked through self- and AChE-induced Aβ aggregation thioflavin T-based assay and supported with the microscopic studies including AFM and confocal microscopy. The in-vivo Morris water maze test and Drosophila AD models were studied to examine the amelioration of cognitive impairment and rescued eye phenotype in both the respective models post-treatment. The ex-vivo studies including immunohistochemistry (IHC) were also performed to estimate the molecular levels of BACE-1 and Aβ in the hippocampal brain. The in-silico studies including, molecular docking, dynamic simulation, and density-functional theory (DFT) of the most active compound were performed to predict the ligand-protein binding profile and stability. Finally, the in-vivo brain permeability and pharmacokinetic profile of the most promising compound AK-2 were evaluated to quantify the amount of compound reached into the brain and its oral absorption, respectively.