Alzheimer's disease (AD), the most common form of dementia, is characterized by progressive neurodegeneration, memory loss, and cognitive impairment, with no definitive cause or cure. Its growing social and economic burden highlights the need for effective disease management [1]. AD pathophysiology is multifactorial, involving genetic, aging, lifestyle, and environmental factors [2], with hallmark features including amyloid beta (Aβ) plaques and neurofibrillary tangles of hyperphosphorylated Tau, which induce neuroinflammation, oxidative stress, synaptic dysfunction, and neurotransmitter deficits [3].

Epigenetic modifications, such as histone acetylation and DNA methylation, regulate gene expression via chromatin remodeling [4]. Histone acetylation is catalyzed by histone acetyltransferases and deacetylation by histone deacetylases (HDACs) [5]. Dysregulated histone acetylation contributes to AD pathology, and HDAC inhibitors are being explored as potential therapies [6]. Among the 18 HDACs, HDAC6 is cytoplasmic and deacetylates α-tubulin, affecting microtubule stability and intracellular transport; its inhibition increases α-tubulin acetylation, reduces Tau phosphorylation, and improves cognition in animal models [[7], [8], [9], [10]].

Neuroinflammation is another key feature of AD, driven by astrocytes and microglia releasing cytokines and activating inflammatory pathways, including caspase, nitric oxide, and cyclooxygenases (COX). The JAK/STAT pathway, particularly STAT3, promotes neuroinflammation and AD progression [11]. Microglia-mediated Aβ deposition further amplifies inflammation, stimulating cytosolic PLA2, COX-1, and COX-2. COX-2, expressed in neurons and microglia, drives prostaglandin production, neuronal injury, synaptic dysfunction, memory loss, and Tau hyperphosphorylation [12,13]. Thus, COX-2 inhibition may reduce neuroinflammation and prevent AD progression [14].

Pyrazole scaffolds offer two key advantages for developing AD therapeutics. First, as a bidentate heterocycle, pyrazole can act as both a hydrogen-bond donor and acceptor, facilitating complementary interactions with peptide backbones. Second, stacking interactions with phenylalanine residues enhances binding affinity [15]. Pyrazole-containing compounds are established COX-2 inhibitors used in cancer and inflammatory diseases [16,17], and the 1,5-diarylpyrazole framework, exemplified by the FDA-approved celecoxib (I), provides potent and selective COX-2 inhibition with improved gastrointestinal safety. Selective COX-2 inhibitors are increasingly explored for AD because they modulate neuroinflammation, helping prevent neuronal injury and cognitive decline [18].

Designing multitarget hybrid molecules has emerged as a promising strategy to enhance efficacy and minimize adverse effects. These single compounds incorporate multiple pharmacophores, enabling simultaneous interaction with several targets. The structural flexibility of HDAC inhibitor cap groups and the synergistic potential of HDAC inhibition have driven the development of HDAC-based multitarget ligands (MTLs) to address key AD pathological mechanisms, including Aβ plaques, neurofibrillary tangles, and neuroinflammation [[19], [20], [21]]. Modern therapeutic strategies for AD focus on agents that act on multiple interconnected pathways to better manage the disease's complex etiology [22,23].

Several studies highlight the diverse biological activities of ACY-1215(IV), a tricyclic HDAC inhibitor with a common zinc-binding group (ZBG), which inhibits HDAC6 (IC50 = 4 nM) and HDAC1 (IC50 = 3790 nM) and suppresses inflammatory signaling via STAT3 and NF-κB, reducing IL-1β and IL-6 expression [24,25]. Ruxolitinib (V) (FDA-approved JAK inhibitor) has been hybridized with hydroxamic acid to generate dual JAK/HDAC6 inhibitors (VI) [26]. Recently, we reported a series of celecoxib-based hybrids with selective COX-2 inhibition; among them, compound VII showed an IC50 of 12.5 μM and a selectivity index of 16 [27] (Fig. 1).

The 1,5-diarylpyrazole scaffold, exemplified by the FDA-approved drug celecoxib (I), is a well-established pharmacophore for selective COX-2 inhibition, providing potent anti-inflammatory activity with an improved gastrointestinal safety profile [18]. Selective COX-2 inhibitors have recently attracted attention as potential neuroprotective agents in AD due to their ability to modulate neuroinflammatory cascades, a key pathological hallmark of disease progression. Concurrently, HDAC6 inhibition has emerged as a promising therapeutic strategy, as it regulates protein degradation, axonal transport, autophagy, and neuroinflammation [28].

Guided by these principles, a fragment-based molecular hybridization approach was employed to design multitarget-directed ligands (MTDLs) capable of modulating both COX-2 and HDAC6. This study drew inspiration from the pharmacophoric framework of HDACIs, exemplified by the FDA-approved drug belinostat (II). The 1,5-diarylpyrazole scaffold served as the cap region for optimal hydrophobic interactions within the COX-2 active site. Diverse zinc-binding groups (ZBGs), including hydroxamic acid derivatives and ethyl glycinate, were appended through suitable linkers to engage the catalytic zinc ion of HDAC6, thereby imparting HDAC inhibitory activity. Substituent variations at the R1 position were introduced to optimize electronic and lipophilic properties and enhance dual target affinity.

This design strategy aimed to produce compounds that simultaneously attenuate neuroinflammation and modulate epigenetic mechanisms. The synthesized hybrids were evaluated for COX-2 and HDAC6 inhibition, their effects on α-tubulin and histone H3 acetylation, cytokine expression (IL-1β, IL-6, TNF-α), Aβ clearance, Tau phosphorylation, and preservation of neuronal and synaptic integrity. Behavioral studies in a scopolamine-induced AD mouse model assessed cognitive performance and BDNF signaling restoration. Molecular dynamics simulations further confirmed the structural stability and favorable binding of the lead compound in both enzyme active sites. Collectively, this approach provides a rationale for developing dual COX-2/HDAC6 inhibitors as potential disease-modifying agents against AD (Fig. 2) 29,30].