Molecular hybridization is a well-exploited medicinal chemistry strategy that aims to combine two molecules (or parts of them) in a new, single chemical entity. Recently, it has been recognized as an effective approach to design ligands able to modulate multiple targets of interest. Hybrid compounds can be obtained by linking (presence of a linker) or framework integration (merging or fusing) strategies. Although very promising to combat the multifactorial nature of complex diseases, the development of molecular hybrids faces the critical issues of selecting the right target combination and the achievement of a balanced activity towards them, while maintaining drug-like-properties. In this review, we present recent case histories from our own research group that demonstrate why and how molecular hybridization can be carried out to address the challenges of multitarget drug discovery in two therapeutic areas that are Alzheimer’s and parasitic diseases. Selected examples spanning from linker- to fragment- based hybrids will allow to discuss issues and consequences relevant to drug design.

The continued drug discovery failures in complex neurodegenerative diseases, including Alzheimer's disease (AD), has raised questions about the classical paradigm "one-drug, one-target, one-disease." In parallel, the ever-increasing awareness of the multiplicity of the underlying pathways has led to the affirmation of polypharmacological approaches. Polypharmacology, which broadly embodies the use of pharmaceutical agents acting on multiple targets, seems to be the best way to restore the complex diseased network and to provide disease-modifying effects in AD. In this review, our aim is to provide a roadmap into a world that is still only partly explored and that should be seen as a continuum of pharmacological opportunities, from drug combinations to multitarget-directed ligands (both codrugs and hybrids). Each modality has unique features that can be effectively exploited by medicinal chemists. We argue that understanding their advantages and drawbacks is very helpful in choosing a proper approach and developing successful AD multitarget drug-discovery endeavors. We also briefly dwell on (co)target validation, an aspect that is quite often neglected, but critical for an efficient clinical translation. We substantiate our discussion with instructive examples taken from the recent literature. Our wish is that, in spite of the specter of the high attrition rates, best researchers preferring to enter, stay, and progress in the field would help grow the sector and develop AD polypharmacology to full potential.

Despite Alzheimer’s disease (AD) incidence being projected to increase worldwide, the drugs currently on the market can only mitigate symptoms. Considering the failures of the classical paradigm “one target-one drug-one disease” in delivering effective medications for AD, polypharmacology appears to be a most viable therapeutic strategy. Polypharmacology can involve combinations of multiple drugs and/or single chemical entities modulating multiple targets. Taking inspiration from an ongoing clinical trial, this work aims to convert a promising cromolyn–ibuprofen drug combination into single-molecule “codrugs.” Such codrugs should be able to similarly modulate neuroinflammatory and amyloid pathways, while showing peculiar pros and cons. By exploiting a linking strategy, we designed and synthesized a small set of cromolyn–ibuprofen conjugates (4–6). Preliminary plasma stability and neurotoxicity assays allowed us to select diamide 5 and ethanolamide 6 as promising compounds for further studies. We investigated their immunomodulatory profile in immortalized microglia cells, in vitro anti-aggregating activity towards Aβ42-amyloid self-aggregation, and their cellular neuroprotective effect against Aβ42-induced neurotoxicity. The fact that 6 effectively reduced Aβ-induced neuronal death, prompted its investigation into an in vivo model. Notably, 6 was demonstrated to significantly increase the longevity of Aβ42-expressing Drosophila and to improve fly locomotor performance.

Polypharmacology is a new trend in amyotrophic lateral sclerosis (ALS) therapy and an effective way of addressing a multifactorial etiology involving excitotoxicity, mitochondrial dysfunction, oxidative stress, and microglial activation. Inspired by a reported clinical trial, we converted a riluzole (1)–rasagiline (2) combination into single-molecule multi-target-directed ligands. By a ligand-based approach, the highly structurally integrated hybrids 3–8 were designed and synthesized. Through a target- and phenotypic-based screening pipeline, we identified hit compound 6. It showed monoamine oxidase A (MAO-A) inhibitory activity (IC50 = 6.9 μM) rationalized by in silico studies as well as in vitro brain permeability. By using neuronal and non-neuronal cell models, including ALS-patient-derived cells, we disclosed for 6 a neuroprotective/neuroinflammatory profile similar to that of the parent compounds and their combination. Furthermore, the unexpected MAO inhibitory activity of 1 (IC50 = 8.7 μM) might add a piece to the puzzle of its anti-ALS molecular profile.

Thanks to the widespread use and safety profile of donepezil (1) in the treatment of Alzheimer's disease (AD), one of the most widely adopted multi‐target‐directed ligand (MTDL) design strategies is to modify its molecular structure by linking a second fragment carrying an additional AD‐relevant biological property. Herein, supported by a proposed combination therapy of 1 and the quinone drug idebenone, we rationally designed novel 1‐based MTDLs targeting Aβ and oxidative pathways. By exploiting a bioisosteric replacement of the indanone core of 1 with a 1,4‐naphthoquinone, we ended up with a series of highly merged derivatives, in principle devoid of the “physicochemical challenge” typical of large hybrid‐based MTDLs. A preliminary investigation of their multi‐target profile identified 9, which showed a potent and selective butyrylcholinesterase inhibitory activity, together with antioxidant and antiaggregating properties. In addition, it displayed a promising drug‐like profile.