Abstract: Romidepsin (FK228) is a potent, bicyclic depsipeptide and a selective class I histone deacetylase inhibitor (HDACi) that has garnered significant attention in the field of HIV-1 research. Originally approved for the treatment of specific T-cell lymphomas, romidepsin is currently being investigated as a latency-reversing agent (LRA) to combat HIV-1 latency—the primary barrier to a definitive cure. By acting as a prodrug that undergoes intracellular reduction, romidepsin releases a thiol group that chelates zinc ions within the HDAC active site, leading to chromatin hyperacetylation and the subsequent reactivation of dormant proviral DNA. Clinical and transcriptomic studies demonstrate that romidepsin is highly effective at inducing HIV-1 transcription in vivo, modulating thousands of host genes associated with cell activation and signaling. However, its clinical utility as a monotherapy is limited by off-target toxicities, adverse events, and an inability to significantly reduce the latent viral reservoir on its own. Consequently, current research is heavily focused on integrating romidepsin into combination "shock-and-kill" strategies alongside immunotherapies or therapeutic vaccines to achieve a functional HIV-1 cure.
1. Introduction
Despite the immense success of antiretroviral therapy (ART) in suppressing the human immunodeficiency virus type 1 (HIV-1) and preventing disease progression, ART cannot eradicate the virus. The virus integrates its genome into the host's DNA, establishing a latent reservoir primarily within long-lived memory CD4+ T cells [1][2]. Because these latently infected cells are transcriptionally silent, they evade immune detection and clearance, necessitating lifelong ART to prevent viral rebound [2]. To overcome this barrier, researchers have proposed the "shock-and-kill" (or "kick-and-kill") strategy. This approach utilizes latency-reversing agents (LRAs) to reactivate viral transcription in dormant cells (the "shock"), thereby exposing them to immune-mediated clearance or viral cytopathic effects (the "kill") [1][2].
Among the various classes of LRAs, chromatin modulators—specifically histone deacetylase inhibitors (HDACis)—have been extensively studied. Romidepsin (also known as FK228 or depsipeptide) is an FDA-approved epigenetic drug originally developed for the treatment of peripheral and cutaneous T-cell lymphomas [4][8][10]. Due to its potent ability to alter the chromatin environment and promote gene transcription, romidepsin has been repurposed and rigorously evaluated in clinical trials as a leading LRA candidate for HIV-1 latency reversal [1][2].
2. Pharmacological Activity
Romidepsin exhibits profound pharmacological activity as an LRA, operating effectively in the nanomolar range. In vitro assays demonstrate that romidepsin has an IC50 of 36 nM for HDAC1 and 47 nM for HDAC2 [10], making it significantly more potent than other HDACis like suberanilohydroxamic acid (SAHA/vorinostat), which operates in the micromolar range [1].
In clinical studies involving HIV-1 infected individuals on suppressive ART, romidepsin administration has been shown to safely and robustly induce HIV-1 transcription. This is evidenced by significant fold-change increases in cell-associated unspliced (CA US) HIV-1 RNA, often peaking within 24 hours of treatment initiation, and resulting in detectable plasma HIV-1 RNA [1][2]. Transcriptomic reanalysis of CD4+ T cells treated with LRAs revealed that romidepsin is one of the most prolific modulators of host gene expression. In one study, it modulated 5,798 genes (representing 33.7% of all expressed genes in the samples), significantly upregulating pathways related to cell activation, differentiation, and signaling, including the MAPK and Rho-GTPase pathways [1]. Furthermore, romidepsin downregulates known HIV-1 repressor genes (such as IL16, MX2, and PIK3CG) while modulating long non-coding RNAs (lncRNAs) like NRON, creating an intracellular milieu highly favorable for viral reactivation [1].
3. Molecular Mechanism of Action
The molecular mechanism of romidepsin centers on the epigenetic regulation of gene expression. Under latent conditions, histone deacetylases (HDACs) remove acetyl groups from lysine residues on histone tails, leading to chromatin compaction and the transcriptional repression of the integrated HIV-1 provirus [10]. Romidepsin acts as a highly selective inhibitor of class I HDACs, specifically targeting HDAC1 and HDAC2 [4][9].
Romidepsin is administered as a prodrug. Upon entering the intracellular environment, it undergoes reductive activation to form an active metabolite containing a crucial thiol group. This thiol group acts as a potent chelator, binding to the essential zinc ion (Zn2+) located within the active catalytic site of the HDAC enzyme [6][10]. By blocking access to the active site, romidepsin prevents the enzyme from deacetylating histones. This inhibition shifts the balance toward histone acetyltransferases (HATs), resulting in chromatin hyperacetylation. The chromatin subsequently remodels into a relaxed, euchromatic state, allowing host transcription factors to access the HIV-1 promoter and initiate proviral transcription [2][10]. Additionally, romidepsin induces the hyperacetylation of non-histone chaperone proteins like HSP90 and upregulates the p21 tumor-suppressor gene, which contributes to cell cycle arrest [4].
4. Structure-Activity Relationship (SAR)
Structurally, romidepsin (FK228, FR901228) belongs to the cyclic tetrapeptide (specifically, bicyclic depsipeptide) class of HDAC inhibitors [6][8]. It is a natural fermentation product originally isolated from the bacterium Chromobacterium violaceum. The defining structural feature responsible for its biological activity is its internal disulfide bond. This disulfide bridge serves as a stable prodrug delivery mechanism. Once inside the reducing environment of the cell, the disulfide bond is cleaved to expose a free sulfhydryl (thiol) group. This specific structural transformation is strictly required for the molecule to interact with and chelate the zinc atom in the HDAC active pocket [6]. Romidepsin shares this unique structural mechanism with other naturally occurring depsipeptides, such as FR901375 and largazole, which also rely on a hidden thiol group for their epigenetic modulating effects [7].
5. Current Limitations
Despite its potent latency-reversing capabilities, the clinical application of romidepsin for HIV-1 cure strategies faces several significant limitations:
Toxicity and Adverse Events: Because romidepsin modulates over 30% of expressed genes in CD4+ T cells, its lack of specificity can lead to generalized cellular toxicity and off-target immune activation [1]. In clinical trials, romidepsin is frequently associated with adverse events (AEs). The most commonly reported AEs include gastrointestinal symptoms (nausea, vomiting, abdominal pain), fatigue, headache, and hematological toxicities such as thrombocytopenia and neutropenia [2][4].
Failure to Reduce Reservoir Size: While romidepsin successfully induces the "shock" phase by reactivating viral RNA, it generally fails to induce the "kill" phase. Clinical studies have shown that romidepsin monotherapy does not lead to a significant reduction in the size of the latent HIV-1 reservoir, nor does it prolong the time to viral rebound during analytical treatment interruption (ATI) [1][2].
Post-Transcriptional Blocks: The inability of romidepsin to clear the reservoir may be partly due to post-transcriptional blocks. Even though proviral DNA is transcribed into RNA, these blocks can prevent the de novo production and translation of viral proteins, thereby preventing the infected cell from being recognized and destroyed by cytotoxic T lymphocytes [1].
6. Future Perspectives
The future of romidepsin in HIV-1 research lies in combination therapies. Because transcriptional reactivation alone is insufficient to eliminate latently infected cells, romidepsin must be paired with agents that enhance immune-mediated clearance. Early clinical trials exploring these combinations have shown promise. For instance, a phase 1B/2A trial combining romidepsin with an experimental HIV-1 vaccine (Vacc-4x and rhuGM-CSF) resulted in a 38% reduction in the size of the HIV-1 reservoir, although it did not significantly delay viral rebound upon ART interruption [1][2]. Other ongoing studies are evaluating romidepsin in tandem with broadly neutralizing antibodies (bNAbs), such as 3BNC117, to facilitate the targeted killing of reactivated cells [2].
Furthermore, a deeper understanding of the transcriptomic pathways modulated by romidepsin—such as the MAPK, Wnt, and Rho-GTPase pathways—could guide the development of more refined, synergistic LRA combinations. By mapping these pathways, researchers hope to design interventions that maximize viral reactivation while mitigating off-target gene expression and associated toxicities, ultimately moving closer to a functional HIV-1 cure [1].
