Drug repurposing is a method of identify new clinical indications for drugs already in clinical practice [1]. Predominantly, drug repurposing arises from two different principles. Primarily multiple drugs that have enigmatic biological roles. Secondly, various diseases commonly share the same molecular pathways and origin [2]. Subsequently, considering both principles gives rise to promising drug moieties with new indications and the approach known as Drug repurposing [3]. This approach contrasts with traditional drug development, which involves developing new moieties from roots and typically takes 10–15 years for final regulatory approval [[4], [5], [6]]. The overall cost for marketing a new drug through this approach exceeds 2 billion dollars and requires consistently 15 years [7]. Additionally, after the cumbersome process of traditional drug development, the failure rate is very high. Indeed, only about 10 % of drugs cross the preclinical studies and enter clinical trials, eventually gaining approval to be in the market [8,9]. Therefore, a more protracted process of drug discovery and a low success rate can lead to emerging health crises and unmet medical needs.

In the present research, we focused on the implications of drug repurposing applications to find a potent lead to treat Alzheimer's disease (AD) from the pool of USFDA approved via the inhibition of the NMDA receptor [[10], [11], [12]]. AD, in general, is a type of physical illness that impairs an individual's cognitive ability. AD is characterized by the reduced chemical level of acetylcholine, which correlates well with the decline in mental power and memory. The disease is also marked by peculiar features, including the formation of β-amyloid (Aβ) plaques followed by neurofibrillary tau tangles (NFTs), causing further cognitive decline [13,14]. As per the report, AD is known to affect 6.9 million Americans aged 65 and above in 2024, which is to rise gradually [15,16]. So far, five drugs are on the market that have been approved to treat the condition. This includes the first approved drug, Tacrine (Acetylcholinesterase (AChE)), followed by Donepezil (AChE), Galantamine (Nicotinic acetylcholine receptors (nAChRs) and AChE), Rivastigmine (AChE and butyrylcholinesterase (BuChE)), and Memantine (N-methyl-D-aspartate (NMDA) receptor) [17,18]. The chemical structures of the drugs are represented in Fig. 1A).

Even though a lot of drug candidates have been developed, efforts to find a powerful therapeutic compound have consistently failed over the past few decades. This is primarily owing to the multifacet role of AD progression, which is orchestrated through numerous receptor and signaling molecules, evolving resistance mechanisms, etc. To counteract this, countless studies are in progress in the clinical arena to find a cure for AD via drug repurposing. The crucial studies include repurposing nilotinib (NCT05143528), an anticancer drug presently in phase 3 trials for AD [19,20]. Leuprolide is another approved anticancer drug, which has also been explored in phase 2 trials for its use in managing AD [21].

Another study reports the synthetic analog of delta-9-tetrahydrocannabinol, Dronabinol, used in chemotherapy-induced nausea and vomiting for its repositioning in AD (NCT02792257). It is currently in phase trial 2. [22]. Besides this, antidiabetic drugs that include Metformin (Phase 3; NCT04098666), Semaglutide (Phase 3; NCT04777409), and dapagliflozin (Phase 2; NCT03801642) are also undergoing repurposing to treat and manage AD [23]. Recent reports portray that the desensitization of insulin signaling is associated as one of the critical risk factors with AD [24,25]. In this league of repurposing one antiepileptic drug, levetiracetam is also explored for its use in AD (Phase 2; NCT04004702) [26]. Besides this, a few monoclonal antibodies, including canakinumab and daratumumab, have also been explored for the management of AD. The chemical structures of the approved drugs repurposed for use in AD are sketched in Fig. 1B.

To address the situation, we herein focused on the drug repurposing strategy on NMDA (N-methyl-D-aspartate) receptors [27,28]. NMDA receptors are essential in AD drug discovery because of their role in synaptic plasticity and cognitive function, which are reduced in AD [29]. Chronic glutamate overstimulation of NMDA receptors causes excitotoxicity and neuronal death, which contributes to Alzheimer's disease cognitive loss. N-methyl-D-Aspartate receptors (NMDAR1) are a subunit of heterotetrameric complexes. NMDARs are present at either synaptic or extrasynaptic membranes throughout the central nervous system (CNS). NMDAR is a ligand-gated ion channel, and regulation is multifactorial and facilitates the precise movement of ions in a physiological state. This plays a vital role in synaptic plasticity, which helps with memory [30]. Since NMDAR mediates excitatory neural transmission and its appropriate activation is essential for physiological function, inappropriate activation causes excitotoxicity. Over-activation of NMDAR causes more Ca2+ influx into neurons and triggers several processes responsible for neurotoxicity [31]. Overactivation and dysregulated activation of NMDAR are associated with several pathological conditions, including seizure or ischemic stroke, neurodegenerative disorders such as AD, Huntington's, Parkinson's disease, neuropsychiatric conditions such as depression, schizophrenia, addiction, anxiety, and neurodevelopmental disorders such as autism. Modulation of NMDAR has excellent therapeutic potential in various neuronal diseases. [32,33]. NMDRA antagonist is effective against the neurotoxicity mediated by NMDRA and is clinically used in multiple neurological alignments like Alzheimer's disease, dementia, glaucoma, and severe neuropathic pain.

In summary, we explored the database of 1827 approved USFDA drugs. The high throughput virtual screening (HTVS) enabled us to identify 177 plausible hits that could interact with the NMDA receptor. To overcome the outliers, 177 drugs were further subjected to molecular docking via standard precision (SP) and Xtra Precision (XP) modes of Schrodinger software to identify 50 probable compounds (based on dock score) that could elicit a possible affinity with the NMDA receptor. Next, the molecular mechanics (MM-GBSA analysis) enabled us to shortlist the ten compounds, which were further correlated with optimal physiochemical parameters, allowing us to fetch the two best leads, Ertugliflozin and Selpercatinib from the pool of USFDA-approved drugs. Both the leads were further corroborated using the simulation-based molecular dynamics (MD) approach. The analysis here revealed that during the course of the simulation, ertugliflozin was more stable than selpercatinib. This was further validated using in vitro assays and immunoblotting, corroborating the in silico findings.

The present work, therefore, presents a successful implication of a drug repurposing approach for identifying a plausible NMDA receptor inhibitor. The approach could be applied to other biological targets as well to identify plausible regimens in both a time and cost-saving manner.