Abstract: Vorinostat, also known as suberoylanilide hydroxamic acid (SAHA), is a first-generation pan-histone deacetylase (HDAC) inhibitor originally approved for the treatment of cutaneous T-cell lymphoma. While its primary applications have historically been in oncology, emerging evidence highlights its significant potential in neurodegenerative disease research and neurotrauma. SAHA possesses the critical ability to cross the blood-brain barrier, enabling direct action within the central nervous system (CNS). Preclinical and clinical investigations demonstrate that HDAC inhibition via SAHA can modulate neuroinflammation, prevent glutamate excitotoxicity, and promote functional motor recovery in models of spinal cord injury and autoimmune encephalomyelitis. Furthermore, SAHA is currently being investigated in clinical trials for Alzheimer's disease. This review synthesizes the current literature on Vorinostat within the context of neurodegenerative and neurological research, detailing its pharmacological activity, molecular mechanisms, structure-activity relationships, current limitations, and future therapeutic perspectives.
1. Introduction
Epigenetic modulation, particularly the balance between histone acetyltransferases (HATs) and histone deacetylases (HDACs), plays a pivotal role in the regulation of gene expression and cellular homeostasis [5]. Vorinostat (suberoylanilide hydroxamic acid, SAHA) is a first-generation pan-HDAC inhibitor that was approved by the United States Food and Drug Administration (FDA) in 2006 for the treatment of cutaneous T-cell lymphoma (CTCL) [2]. While SAHA has established multipurpose use in various cancers, its therapeutic scope is continuously expanding into non-oncological conditions. Notably, SAHA has garnered significant attention in neurodegenerative disease research and central nervous system (CNS) disorders. The compound is currently being investigated in clinical trials for Alzheimer's disease (e.g., NCT03056495) [1]. Furthermore, preclinical models of traumatic and non-traumatic spinal cord injury (SCI) and autoimmune encephalomyelitis have demonstrated the neuroprotective and immunomodulatory potential of HDAC inhibitors like SAHA [4] [11].
2. Pharmacological Activity
A major pharmacological advantage of SAHA in the context of neurodegenerative and neurological research is its ability to cross the blood-brain barrier, a property that not only prevents the formation of brain metastases in oncology but also allows for direct therapeutic action within the CNS [2]. In preclinical models of spinal cord injury, HDAC inhibitors have been associated with significant neurobehavioural improvements, including functional motor recovery and the amelioration of anxiety and pain scores [4]. SAHA has also been shown to attenuate experimental autoimmune encephalomyelitis, a common model for neuroinflammatory and neurodegenerative conditions like multiple sclerosis [11]. Additionally, HDAC inhibitors demonstrate neuroprotective properties by increasing the preservation of myelinated axons and improving neuronal conduction following traumatic brain and spinal cord injuries [4].
3. Molecular Mechanism of Action
SAHA functions as a pan-inhibitor that targets multiple HDACs in Class I (HDAC1, 2, 3, and 8) and Class II (HDAC4, 5, 6, 7, 9, and 10) [4] [7]. By inhibiting the removal of acetyl groups from the N-terminal tails of histone and non-histone proteins, SAHA prevents chromatin condensation, thereby promoting an open chromatin structure and active gene transcription [2] [4]. In the context of neurodegeneration and CNS injury, the secondary phase of injury involves intracellular Ca2+ dysregulation, glutamate excitotoxicity, and free radical release, which hinder neuronal regeneration [4]. SAHA mitigates these effects through potent anti-inflammatory and neuroprotective mechanisms. For instance, HDAC inhibition reduces the expression of pro-inflammatory cytokines such as IL-6, TNF-α, and IL-1β, while simultaneously increasing the expression of the immunosuppressive cytokine IL-10 [4]. Furthermore, HDAC inhibition modulates microglia and macrophage polarization, shifting the immune response away from a neuro-destructive phenotype [4].
4. Structure-Activity Relationship (SAR)
Vorinostat belongs to the hydroxamic acid class of HDAC inhibitors, which is the most diverse class of inhibitors with a high affinity for HDAC enzymes [7]. The structural framework of SAHA is characterized by three distinct functional domains: a zinc-binding hydroxamic acid group that coordinates directly with the zinc ion located in the active catalytic site of the HDAC enzyme, a hydrophobic linker region that transverses the active site channel, and a capping group that interacts with the external surface of the enzyme [2] [3]. As a polar-planar compound of the second generation, this structural conformation allows SAHA to effectively block the catalytic activity of both Class I and Class II HDACs at sub-micromolar concentrations [7].
5. Current Limitations
Despite its therapeutic promise, the clinical application of SAHA is limited by its toxicity profile. In clinical trials, SAHA has been associated with adverse events such as fatigue, dyspnea, nausea, and anemia, which frequently necessitate dose reductions or early withdrawal from treatment [5]. Furthermore, because SAHA is a pan-HDAC inhibitor, its lack of selectivity can lead to conflicting biological effects. For example, while inhibiting Class I HDACs (like HDAC3) is generally neuroprotective and anti-inflammatory, the inhibition of Class IIa HDACs has been shown in some models to promote inflammation at the injury site by shifting macrophage polarization towards the pro-inflammatory M1-type [4]. This dual action may explain why pan-HDAC inhibitors like SAHA do not always yield definitive or consistent improvements in functional outcomes in all neurological models [4].
6. Future Perspectives
The future of HDAC inhibitors in neurodegenerative disease research lies in the development and application of isoform-selective inhibitors. Targets such as HDAC3 (Class I) and HDAC6 (Class IIb) appear particularly promising for CNS applications, as their specific inhibition may provide the neuroprotective and immunomodulatory benefits of SAHA without the counterproductive inflammatory effects associated with Class IIa inhibition [4] [8]. Additionally, because the toxicity profile of FDA-approved SAHA is already well-understood, it remains a strong candidate for drug repurposing. Future translational clinical trials are essential to determine if the neurobehavioural improvements, such as reduced anxiety and pain sensitization observed in preclinical models, can be successfully replicated in human patients suffering from Alzheimer's disease, spinal cord injuries, and other neurodegenerative conditions [1] [4].
