Alzheimer's disease (AD) which is the most prevailing class of dementia, accounting for nearly two-thirds of dementia instances universally in adults aged 65 and older. (Kumar et al., 2023). This condition is marked by behavioral, language, reasoning, comprehension, attention, judgment and cognitive impairment, encompassing issues such as memory dysfunction, and is classified as a neurodegenerative disease (Kumar et al., 2023). On a pathophysiological level, the ailment is distinguished by the existence of neurofibrillary tangles (NFTs) and amyloid plaques (Aß) within the brain (Kumar et al., 2023). The global statistics for Alzheimer's Disease is alarming as its prevalence currently is estimated to be around 24 million, Out of which around 10 million current cases are diagnosed each year, and the given number has been estimated to rise 4 times upto the year 2050 (Kumar et al., 2023; Naheed et al., 2023). In South-Asian countries, this number is expected to rise to 24.7 million, with 11.4 million cases in India, 2.0 million in Bangladesh and 1.4 million in Pakistan (Naheed et al., 2023).

Currently, the US FDA has approved three inhibitors of acetylcholinesterase (specifically Rivastigmine (Exelon), Galantamine (Razadyne), Donepezil (Aricept) and an antagonist of NMDA receptor memantine (Ebixa) against Alzheimer's Disease (Kumar et al., 2023). In addition, more recently, the FDA has given its approval to aducanumab and lecanemab, both of which are monoclonal antibodies designed to target amyloid deposits in brain for the treatment for Alzheimer's Disease(Cummings, 2023). However, it is crucial to emphasize that these therapeutic alternatives primarily offer symptomatic relief or a slowing of the disease's progression(Cummings, 2023).

Accumulation of amyloid plaques and NFTs also contribute to impairment in synaptic integrity and cause alteration in neurotransmission, with imbalances observed in the levels of acetylcholine, GABA, and serotonin (Rajmohan and Reddy, 2017). Despite this observed correlation, acetylcholinesterase inhibitors (AChEIs) like donepezil have not proved sufficient in preventing neuronal death caused by the accumulation of Aβ (Lin et al., 2021). Hence, it is imperative to develop an add-on therapy that can be given along with the standard drugs for AD so as to both protect against neuronal damage and regulate neuronal wiring by balancing acetylcholine levels.

Evidences suggest that vascular dysfunction may occur in the advance stages of Alzheimer's pathophysiology for e.g. hypoxia because of inadequate blood supply might trigger the accumulation of amyloid plaques and NFTs, leading to progression of the neurodegenerative process as well as cognitive decline (Kelleher and Soiza, 2013). Auto-regulation is the mechanism of the cerebral blood flow (CBF), where regulation occurs via the release of vasoactive agents like angiotensin converting enzymes (ACEs), vascular endothelial growth factor (VEGF), and endothelin 1 (ET1), (Sharma et al., 2021). ET1, with its potent and prolonged vasoconstrictive properties, plays a role in diminishing cerebral blood flow. (Barker et al., 2014). An elevation in its levels has also been observed in the temporal cortex of the brains of individuals with Alzheimer's disease (Palmer et al., 2012; Sharma et al., 2021). Decrease in the blood flow of cerebral region can lead to the apoptosis as well as microinfarcts, ultimately causing neuronal damage/death, which represents an early development in the progression of Alzheimer's disease (Sharma et al., 2021).

The biological effects of endothelin peptides are exerted through the stimulation of two separate G-protein coupled receptors (GPCRs) known as ETA and ETB receptors. (Gulati et al., 2018). Within the central nervous system, these receptors play a crucial role in maintaining homeostasis by overseeing the sympathetic nervous system, cerebral blood flow (CBF), as well as processes such as neuronal- migration, apoptosis and proliferation. (Gulati et al., 2018). In recent years, studies have revealed that the adult brain also comprise of specific niches containing neuronal progenitor cells (specifically in dentate gyrus and subventricular zone) throughout their life, that are essential for repair and restoration process in case of trauma or CNS related diseases (Eriksson et al., 1998; Spalding et al., 2013). Endothelin-B (ET-B) play a very important role in the development of central nervous system as they have a role in regulating the differentiation, migration of neurons, proliferation of the melanocytes and glia of both the enteric and central nervous systems (Druckenbrod et al., 2008). Expression level of these receptors is higher in the human cerebellum, cerebral cortex hippocampus,brainstem, hypothalamus, striatum, lungs and olfactory bulb. (Ap et al., 2016).

A synthetic analog of ET-1 is IRL-1620 [N-Succinyl-[Glu9, Ala11,15] endothelin 1], that is highly selective for ET-B receptors and have been used in numerous studies involving hepatic, endocrine, gastrointestinal, renal,pulmonary and dermatological systems (Gulati et al., 2018). In the aged rat model of the cerebral ischemia, there is stimulation of the endothelin-B (ET-B) receptors by intravenous administration (i.v) of IRL-1620, showed better performance in both motor functions as well as neurological by enhancing neurogenesis along with angiogenesis (Leonard et al., 2011, Leonard et al., 2012; Leonard and Gulati, 2013). The drug was also found to show improvement in memory and learning in the rat model of the Alzheimer's Disease (AD) (Briyal et al., 2015).

Owing to the promising results IRL-1620 has shown in the preclinical studies for various CNS related diseases, the present study aimed at observing its effect in an Alzheimer's Disease rat model when administered either alone or in combination with donepezil, the standard drug prescribed for treatment of AD. The neurobehavioral parameters (Morris water maze, elevated plus maze), and biochemical test (MDA, GSH, SOD, and acetylcholinesterase level) were performed. Further, the effect on the gene expression (ET-B, NGF and VEGF) and, correlated with immunohistochemistry was evaluated. The study investigated the possibility of developing a treatment regimen that has the potential to combat the neurological and functional features associated with the AD. Moreover, the present study evaluated the effect of IRL-1620 when administered at different doses (5 μg/kg,7 μg/kg, 9 μg/kg).