Abstract: Vorinostat, also known as suberoylanilide hydroxamic acid (SAHA), is a potent, first-generation pan-inhibitor of histone deacetylases (HDACs). Initially approved for the treatment of cutaneous T-cell lymphoma, SAHA has demonstrated broad epigenetic modulatory capabilities by reversing chromatin condensation and altering gene transcription. While extensively investigated in oncology, its ability to modulate epigenetic silencing has positioned it as a promising candidate in HIV latency reversal research. This review synthesizes the pharmacological activity, molecular mechanisms, and structure-activity relationships of SAHA, highlighting its emerging therapeutic potential in activating latent HIV reservoirs, as evidenced by ongoing clinical trials. Furthermore, current limitations such as drug resistance and toxicity are discussed alongside future perspectives on novel delivery systems and combination therapies.
1. Introduction
Vorinostat (SAHA) is a first-generation histone deacetylase inhibitor (HDI) belonging to the hydroxamic acid class [16]. In 2006, it became the first HDI approved by the United States Food and Drug Administration (FDA) for the treatment of cutaneous T-cell lymphoma (CTCL) [2]. By inhibiting HDAC enzymes, SAHA acts as a critical epigenetic modulator, influencing both histone and non-histone proteins to regulate gene expression, cell cycle progression, and apoptosis [2]. Although the majority of clinical and preclinical studies have focused on its antineoplastic properties across various malignancies, the fundamental mechanism of chromatin remodeling has expanded its multipurpose use into virology [1]. Specifically, epigenetic silencing is a primary mechanism maintaining HIV latency, and HDAC inhibitors have been identified as agents capable of reversing this latency. Consequently, SAHA is currently being investigated in multiple clinical trials (e.g., NCT01365065, NCT02707900, NCT03803605) for its potential to activate latent HIV infections [1].
2. Pharmacological Activity
SAHA exhibits a wide array of pharmacological activities, primarily characterized by its ability to induce cell cycle arrest, apoptosis, and autophagy, while inhibiting proliferation and angiogenesis [13][18]. A notable pharmacological advantage of SAHA is its ability to cross the blood-brain barrier [14]. While this property is highly beneficial for preventing brain metastases in oncology, it is equally critical for HIV latency reversal, as the central nervous system serves as a major anatomical reservoir for latent HIV. In the context of viral latency, the pharmacological rationale for using HDAC inhibitors is well-supported; for example, related class I selective HDAC inhibitors have been shown to successfully activate HIV production from latently infected primary T cells [9]. By translating these epigenetic effects, SAHA aims to force the transcription of dormant viral genomes, making the infected cells susceptible to immune clearance or antiretroviral therapy.
3. Molecular Mechanism of Action
The molecular mechanism of SAHA is rooted in its function as a pan-HDAC inhibitor, effectively suppressing both Class I (HDAC1, 2, 3, and 8) and Class II (HDAC4, 5, 6, 7, 9, and 10) histone deacetylases [13]. SAHA exerts its inhibitory effect by directly binding to the zinc ion located within the catalytic domain of the HDAC enzymes [16]. Under normal conditions, HDACs act as transcriptional repressors by removing acetyl groups from lysine residues on histone tails, leading to chromatin condensation and gene silencing [2]. By inhibiting this deacetylation, SAHA promotes the accumulation of acetylated histones, resulting in an open, transcriptionally active chromatin structure [2]. This mechanism is directly responsible for the reactivation of silenced viral promoters in latent HIV reservoirs. Additionally, SAHA modulates the acetylation state of non-histone targets, including transcription factors and signaling mediators, which further influences cellular pathways and viral protein expression [2].
4. Structure-Activity Relationship (SAR)
The structural design of zinc-dependent HDAC inhibitors like SAHA is characterized by three distinct functional domains: a cap group (or surface recognition unit), a zinc-binding domain (ZBD), and a linker domain that connects the two [71]. The cap group interacts with the external surface of the HDAC enzyme, while the linker domain transverses the active site channel [18]. These two domains are primarily responsible for ligand-receptor interactions and dictate the selectivity of the inhibitor [71]. The ZBD in SAHA is a hydroxamic acid moiety, which is crucial for its pharmacological activity as it coordinates directly with the zinc ion in the active site, effectively neutralizing the enzyme's catalytic function [18][71]. The hydroxamic acid group provides a high affinity for HDACs, making SAHA a potent pan-inhibitor across multiple HDAC classes [13].
5. Current Limitations
Despite its therapeutic potential, the clinical application of SAHA is hindered by several limitations, including toxicity and the development of drug resistance. Clinical trials have reported unexpected toxicities requiring dose reductions or discontinuation, with some studies linking these adverse events to elevated levels of interleukin-6 (IL-6) [30]. Furthermore, cells can develop resistance to HDAC inhibitors through multiple mechanisms. One primary mechanism is the upregulation of P-glycoprotein (P-gp), an ATP-dependent efflux pump that expels the drug from the cell, thereby reducing its intracellular concentration and efficacy [75]. Additionally, resistance can manifest through the compensatory upregulation of HDAC enzymes or the downregulation of histone acetyltransferases (HATs), which counteracts the hyperacetylation induced by SAHA [24].
6. Future Perspectives
To overcome the current limitations of SAHA, future research is heavily focused on combination therapies and advanced drug delivery systems. The integration of SAHA into nanomaterial-based drug delivery systems, such as co-prodrug nanomicelles, has shown promise in providing sustained drug release and reducing resistance mechanisms like P-gp efflux [88]. Additionally, the development of chimeric HDAC-based inhibitors—molecules that combine the pharmacophore of an HDAC inhibitor with another active agent—offers a strategy to simultaneously target multiple pathways without the additive toxicity often seen in standard combination therapies [89]. In the context of HIV latency reversal, the ongoing clinical trials (NCT01365065, NCT02707900, NCT03803605) will be critical in determining the optimal dosing regimens and combination strategies required to safely and effectively purge latent viral reservoirs [1].
