LSD1 inhibitors are a class of experimental drugs designed to block lysine-specific demethylase 1, an enzyme that removes chemical tags from proteins called histones and, in doing so, controls which genes are switched on or off in a cell. Most LSD1 inhibitors are being tested against cancers, particularly acute myeloid leukemia and small cell lung cancer, though researchers have also explored them for sickle cell disease, herpes virus infections, and other conditions that hinge on gene regulation gone wrong. What makes this target especially interesting is that blocking LSD1’s enzymatic activity turns out to be only part of the story; the protein also acts as a physical scaffold that holds repressive complexes together, and disrupting that scaffolding may matter just as much for therapeutic effect.
What LSD1 Actually Does in a Cell
LSD1 was the first histone demethylase ever discovered, identified in 2004 as a protein that strips methyl groups from a specific spot on histone H3, a protein that DNA wraps around inside the nucleus. That spot, known as lysine 4 on histone H3, is normally associated with active gene expression. When LSD1 removes the methyl marks there, the associated genes tend to get quieter. The reaction itself is an oxidation that generates formaldehyde as a byproduct, powered by a small helper molecule called FAD.1PubMed. Histone demethylation mediated by the nuclear amine oxidase homolog LSD1 LSD1 can also demethylate a second site, lysine 9 on the same histone, which has the opposite effect: it can activate genes. So depending on context, LSD1 can serve as either a gene silencer or a gene activator.2American Chemical Society (ACS Publications). Reversible Lysine Specific Demethylase 1 (LSD1) Inhibitors: A Promising Wrench to Impair LSD1
LSD1 does not work alone. It slots into large protein complexes, most prominently the CoREST and NuRD complexes, which are multi-protein machines that collectively silence genes.3Trends in Pharmacological Sciences. Beyond catalytic inhibition: targeting the non-enzymatic functions of LSD1 Within these assemblies, LSD1 contributes its demethylase activity but also serves a structural role, physically linking other proteins together and helping the whole complex grip chromatin. This dual function turns out to be critical for understanding how LSD1 inhibitors work and why simply blocking the enzyme’s catalytic pocket does not always capture the full therapeutic benefit.
The Scaffolding Problem
Early thinking about LSD1 inhibitors was straightforward: block the enzyme, stop it from removing methyl marks, and genes that cancer cells need silenced will reactivate. But experiments in acute myeloid leukemia cells revealed something unexpected. LSD1 inhibitors were triggering cancer cell differentiation, meaning immature leukemia cells were maturing into normal-looking blood cells. Yet the degree of differentiation did not always correlate with how thoroughly the drug shut down LSD1’s enzymatic activity. Something else was going on.
Researchers found that a key effect of LSD1 inhibitors in leukemia is breaking the physical interaction between LSD1 and a transcription factor called GFI1. GFI1 parks on enhancers near genes that drive normal blood cell maturation, and as long as LSD1 is physically attached, those genes stay off. When an inhibitor pries the two apart, the enhancers activate and the leukemia cells begin differentiating.4PubMed Central. Enhancer Activation by Pharmacologic Displacement of LSD1 from GFI1 Induces Differentiation in Acute Myeloid Leukemia A clever experiment confirmed this: when scientists fused GFI1 directly to LSD1 so no drug could separate them, the leukemia cells became resistant to the inhibitor. The physical separation was the critical event, not just the loss of demethylase activity.5Blood. Pharmacological Inhibitors of LSD1 Promote Differentiation of Myeloid Leukemia Cells through a Mechanism Independent of Histone Demethylation
This discovery reframed the drug development challenge. An ideal LSD1 inhibitor might need to do more than occupy the catalytic site; it might need to destabilize the protein-protein interactions that keep repressive complexes locked onto chromatin.6PubMed Central. Targeting the scaffolding role of LSD1 (KDM1A) poises acute myeloid leukemia cells for retinoic acid-induced differentiation
Types of LSD1 Inhibitors
Because LSD1 uses FAD the same way certain brain enzymes called monoamine oxidases (MAOs) do, the earliest LSD1 inhibitors were borrowed from psychiatry. Tranylcypromine, an old antidepressant that irreversibly binds MAO enzymes, also irreversibly binds LSD1. Many first-generation LSD1 inhibitors are chemical cousins of tranylcypromine, tweaked to prefer LSD1 over the brain’s MAO-A and MAO-B. Some derivatives achieved selectivity of over 10,000-fold for LSD1 relative to both MAO enzymes, largely eliminating the psychiatric side effects of the parent drug.7PubMed. Tying up tranylcypromine: Novel selective histone lysine specific demethylase 1 (LSD1) inhibitors
Irreversible inhibitors form a permanent covalent bond with LSD1, meaning the protein is knocked out until the cell makes a fresh copy. This can be powerful but also raises safety concerns, because you cannot dial back the effect once the drug binds. That has driven significant interest in reversible LSD1 inhibitors, which bind tightly but can eventually let go. Two reversible compounds, CC-90011 and SP-2577, have entered clinical trials.8American Chemical Society (ACS Publications). Reversible Lysine Specific Demethylase 1 (LSD1) Inhibitors: A Promising Wrench to Impair LSD1
A newer frontier involves allosteric inhibitors, compounds that bind LSD1 somewhere other than its catalytic pocket. Recent structural work using a compound called SP2509 identified two previously unknown binding sites on LSD1, both of which alter the shape of the catalytic pocket enough to block substrate access. One of these binding modes also bends a structural feature called the Tower domain, which is the region that links LSD1 to CoREST and ultimately to nucleosomes. That bending disrupts how the whole complex grips chromatin.9PubMed. Unveiling New Binding Sites and Allosteric Regulation Mechanisms of LSD1 for Novel Therapeutics The LSD1-CoREST complex itself behaves like a nanoscale clamp whose opening and closing is controlled by substrate binding, and allosteric approaches could exploit that flexibility.10PubMed Central. LSD1/CoREST is an allosteric nanoscale clamp regulated by H3-histone-tail molecular recognition
Leukemia and the Differentiation Strategy
Acute myeloid leukemia (AML) remains the cancer type where LSD1 inhibitors are most advanced. The logic is appealing: rather than killing cancer cells outright with toxic chemotherapy, you coax them into maturing into functional blood cells that eventually die on their own schedule. Several LSD1 inhibitors are being tested in combination with azacitidine, a DNA-demethylating drug already standard in AML care. A phase 2a trial of iadademstat plus azacitidine in newly diagnosed AML patients reported promising early activity and tolerable safety.11The Lancet Haematology. Iadademstat plus azacitidine in patients with newly diagnosed acute myeloid leukaemia (ALICE): an open-label, phase 2a, dose-finding study Similarly, a phase 1/2 study of seclidemstat combined with azacitidine in myelodysplastic syndromes and chronic myelomonocytic leukemia showed initial signs of activity, with two patients achieving meaningful responses in an early readout.12Blood. A Phase I/II Study of Seclidemstat, an LSD1 Inhibitor, and Azacitidine for Patients with Myelodysplastic Syndromes and Chronic Myelomonocytic Leukemia
These are still early-stage trials, and “promising activity” in a handful of patients is a long way from a proven treatment. But the biological rationale is strong, and the differentiation approach could eventually offer a gentler alternative to intensive chemotherapy for patients who cannot tolerate it.
Small Cell Lung Cancer and Neuroendocrine Tumors
Small cell lung cancer (SCLC) is one of the most aggressive solid tumors, with limited treatment options once it recurs after initial chemotherapy. SCLC cells tend to be locked in a neuroendocrine state, and LSD1 appears to help maintain that identity. Blocking LSD1 can suppress a master regulator of the neuroendocrine program called ASCL1, reducing proliferation in cell lines that depend on it. Critically, this suppression required another protein called ZFP36L1 to be intact; when ZFP36L1 was knocked out, LSD1 inhibitors lost much of their ability to reduce ASCL1 and slow growth.13Nature Communications. Regulation of neuroendocrine plasticity by the RNA-binding protein ZFP36L1
A newer compound, TAS1440, showed tumor regression in mouse models of SCLC by disrupting an interaction between LSD1 and a different neuroendocrine factor, INSM1. The drug appeared to work through both catalytic inhibition and complex disruption, reprogramming tumor-suppressive signaling pathways that the cancer had silenced.14Nature Communications. LSD1 inhibitor, TAS1440, disrupts INSM1-LSD1 complex activating tumor-suppressive pathways via transcriptional reprogramming in neuroendocrine SCLC
Not all SCLC tumors respond. One study identified a resistance mechanism in which cancer cells undergo a reversible shift into a mesenchymal-like state driven by a transcription factor called TEAD4. This epigenetic reprogramming allows the cells to sidestep LSD1 dependency entirely.15PubMed Central. Intrinsic and acquired drug resistance to LSD1 inhibitors in small cell lung cancer occurs through a TEAD4-driven transcriptional state Because this resistance is reversible, there may eventually be ways to cycle treatment or combine drugs to outmaneuver it, but for now it underscores the difficulty of treating SCLC.
Predicting which patients might respond is another active area. Work on the compound GSK2879552 showed that a pattern of low DNA methylation across dozens of genomic sites correlated with sensitivity to LSD1 inhibition in SCLC cell lines. That methylation signature also stratified primary tumor samples, suggesting it could one day serve as a biomarker to select patients likely to benefit.16Cancer Cell. Discovery of a Covalent Inhibitor of LSD1 to Treat Small Cell Lung Cancer
Ewing Sarcoma and Other Rare Cancers
Ewing sarcoma, a bone and soft-tissue cancer that primarily strikes children and young adults, is driven by a fusion protein created when two genes accidentally merge. The resulting protein, usually called EWS-FLI1, hijacks gene regulation in ways that depend on LSD1. High LSD1 expression is common in Ewing sarcoma patient samples, and LSD1 inhibition in the lab globally disrupts EWS-FLI1’s ability to drive cancer-related gene programs.17PubMed Central. Therapeutic opportunities in Ewing sarcoma: EWS-FLI inhibition via LSD1 targeting In cell culture, Ewing sarcoma cells carrying the fusion protein were roughly ten times more sensitive to the reversible LSD1 inhibitor HCI2509 than cells in which the fusion protein had been knocked down, confirming that sensitivity hinges on the fusion protein’s presence.18PubMed Central. Reversible LSD1 inhibition interferes with global EWS/ETS transcriptional activity and impedes Ewing sarcoma tumor growth This work is still preclinical, but it represents one of the clearest cases where a specific genetic lesion creates direct dependence on LSD1.
In prostate cancer, LSD1 inhibition has shown a different angle of attack. Castration-resistant prostate cancer often expresses a truncated version of the androgen receptor called AR-V7, which remains constitutively active even when testosterone is blocked. Lab work has shown that chemical inhibition of LSD1 partially dampens AR-V7’s constitutive activation, opening a potential avenue for re-sensitizing resistant tumors to hormonal therapies.19PubMed Central. LSD1 inhibition attenuates androgen receptor V7 splice variant activation in castration resistant prostate cancer models
Boosting Immunotherapy
One of the more unexpected findings about LSD1 inhibition involves the immune system. In 2018, researchers showed that knocking out LSD1 in cancer cells woke up sequences of ancient viral DNA buried in the genome, called endogenous retroviruses. These sequences produced double-stranded RNA, which the cell interpreted as a viral infection, triggering an immune alarm. The resulting interferon response made tumors more visible to T cells and increased immune cell infiltration. In mouse melanoma models that did not respond to checkpoint immunotherapy on their own, combining LSD1 loss with anti-PD-1 treatment produced significant responses.20PubMed Central. LSD1 Ablation Stimulates Anti-tumor Immunity and Enables Checkpoint Blockade
This finding is significant because the biggest limitation of checkpoint immunotherapy is that many tumors are “cold,” meaning they lack the immune infiltration needed for the drugs to work. If LSD1 inhibitors can warm up cold tumors by triggering an innate viral-mimicry response, they could dramatically expand the population of patients who benefit from immunotherapy. Clinical trials testing these combinations are ongoing, though definitive results have not yet emerged.
Dual Inhibitors and Combination Design
Because LSD1 sits in complexes alongside histone deacetylases (HDACs), another family of enzymes that silence genes, there has been substantial interest in hitting both targets simultaneously. In colorectal cancer cell lines, combining an LSD1 inhibitor with the HDAC inhibitor SAHA was more effective at reducing cell viability than either agent alone.21PubMed Central. Syntheses of LSD1/HDAC Inhibitors with Demonstrated Efficacy against Colorectal Cancer: In Vitro and In Vivo Studies Including Patient-Derived Organoids Some research groups have gone further and built bifunctional molecules, single compounds that inhibit both LSD1 and an HDAC. One such compound, JBI-097, showed efficacy as a single agent and additive or synergistic effects when combined with standard therapies or checkpoint inhibitors.22PubMed Central. Novel dual LSD1/HDAC6 inhibitor for the treatment of cancer
In diffuse intrinsic pontine glioma (DIPG), an aggressive childhood brain tumor with almost no effective treatments, combining LSD1 and HDAC inhibitors proved synergistic in cell culture. A bifunctional molecule called Corin, designed to inhibit both LSD1 and HDACs in a single compound, reduced DIPG cell viability more than either mechanism alone.23Cancer Cell. Dual Inhibition of LSD1 and HDACs Impairs DIPG Growth For a disease this desperate, even strong preclinical signals are worth noting, though the jump from cell lines to effective treatment in children remains enormous.
Side Effects and the Platelet Problem
The most consistent toxicity of LSD1 inhibitors across clinical trials is thrombocytopenia, a drop in blood platelets that can increase bleeding risk. In a phase 1/2 trial of bomedemstat in myelofibrosis, about half of participants experienced thrombocytopenia, along with taste disturbances in roughly a third and anemia in a similar proportion.24Blood Advances. The lysine-specific demethylase 1 (LSD1) inhibitor bomedemstat in myelofibrosis: results from a phase 1/2 study Preclinical toxicology studies of GSK2879552 in animals revealed thrombocytopenia, neutropenia, and changes in bone marrow and lymphoid tissue, though these effects were reversible when the drug was stopped.25PubMed Central. LSD1/KDM1A inhibitors in clinical trials: advances and prospects
The mechanism behind the platelet drop appears to involve a specific cell in bone marrow. When mice were treated with the LSD1 inhibitor INCB059872, analysis of bone marrow cells showed an accumulation of early megakaryocyte progenitors, the cells that eventually produce platelets. These progenitors got stuck in an immature, stem-like state rather than maturing normally.26PubMed Central. Nascent transcript and single-cell RNA-seq analysis defines the mechanism of action of the LSD1 inhibitor INCB059872 in myeloid leukemia This is essentially the same differentiation-blocking problem that LSD1 inhibitors exploit in leukemia cells, except here it is hitting normal blood production as an unwanted side effect. Dose management and treatment schedules are being optimized to balance efficacy against this on-target toxicity.
Beyond Cancer
Sickle cell disease may be the most advanced non-cancer application of LSD1 inhibitors. The logic is elegant: LSD1 helps silence the gene for fetal hemoglobin (HbF) after birth. Reactivating fetal hemoglobin in adults with sickle cell disease dilutes the defective hemoglobin that causes red blood cells to sickle, which is the same principle behind the drug hydroxyurea and newer gene therapies. Preclinical work with a new generation of orally bioavailable LSD1 inhibitors showed that oral dosing in a sickle cell mouse model raised fetal hemoglobin levels and reduced multiple features of disease pathology, including the blood vessel damage and organ injury characteristic of the disease.27PubMed Central. Novel, potent, and orally bioavailable LSD1 inhibitors induce fetal hemoglobin synthesis in a sickle cell disease mouse model If this translates to humans, an oral pill that boosts fetal hemoglobin and also reduces oxidative stress could be transformative for patients who lack access to gene therapy or bone marrow transplant.28PubMed Central. Potential role of LSD1 inhibitors in the treatment of sickle cell disease: a review of preclinical animal model data
Herpes simplex viruses offer another intriguing application. Both HSV-1 and HSV-2 exploit LSD1 during reactivation from latency. When the virus lurks dormant in nerve cells, its DNA is wrapped in silencing chromatin. To reactivate, the virus recruits a cellular protein called HCF-1, which in turn brings in LSD1 to strip away those repressive marks and allow viral genes to fire. Blocking LSD1 with monoamine oxidase inhibitors kept the repressive chromatin in place and blocked viral reactivation in mouse sensory neurons.29PubMed Central. Inhibition of the histone demethylase LSD1 blocks alpha-herpesvirus lytic replication and reactivation from latency A more selective LSD1 inhibitor confirmed the same effect, suppressing both active infection and reactivation from latency.30PubMed Central. A novel selective LSD1/KDM1A inhibitor epigenetically blocks herpes simplex virus lytic replication and reactivation from latency Whether this could lead to a clinical antiviral remains to be seen, but the concept of an epigenetic antiviral, one that keeps the virus locked in its dormant state rather than killing actively replicating virus, is a genuinely novel therapeutic strategy.
LSD1 in the Brain
Neurons produce their own specialized version of LSD1 through alternative splicing. This variant, called LSD1n, includes a small extra segment that changes the enzyme’s substrate preference entirely: instead of targeting histone H3, LSD1n demethylates a different histone, H4 at lysine 20. Genetic deletion of LSD1n in mice caused deficits in spatial learning and memory, revealing that this isoform controls gene programs needed for long-term memory formation.31PubMed Central. LSD1n is a H4K20 demethylase regulating memory formation via transcriptional elongation control The neuron-specific isoforms also drive neurite maturation during brain development: knocking down the neurospecific variants in cortical neurons inhibited neurite growth, while overexpressing them enhanced it.32PubMed Central. Alternative splicing of the histone demethylase LSD1/KDM1 contributes to the modulation of neurite morphogenesis in the mammalian nervous system
This neuroscience dimension raises both opportunities and cautions. On the opportunity side, understanding LSD1n’s role in memory could eventually open routes into neurodegenerative disease research. On the caution side, any LSD1 inhibitor that crosses the blood-brain barrier could theoretically interfere with neuronal LSD1 functions. Most cancer-focused LSD1 inhibitors are not specifically designed to enter the brain, but the existence of brain-specific isoforms is something drug developers need to account for in long-term safety assessments.
LSD1 and Fat Metabolism
Away from cancer and blood diseases, LSD1 plays a surprisingly important role in adipose tissue. In brown fat, the body’s heat-generating tissue, LSD1 maintains the gene programs that drive energy burning. When researchers deleted LSD1 selectively in brown fat cells, those cells shifted from an energy-burning oxidative metabolism to a sugar-dependent glycolytic one, and the brown fat gradually “whitened,” accumulating fat droplets and losing its thermogenic identity over time.33Cell Reports. Lsd1 Ablation Triggers Metabolic Reprogramming of Brown Adipose Tissue Conversely, mice engineered to overexpress LSD1 in white fat saw enhanced oxidative metabolism and were significantly more resistant to diet-induced obesity and type 2 diabetes, showing improved glucose tolerance and insulin sensitivity even on a high-fat diet.34Nature Communications. LSD1 promotes oxidative metabolism of white adipose tissue
These findings suggest that LSD1’s role in fat tissue is nuanced: you want it active in brown fat to keep energy expenditure high, and its activity in white fat can shift those cells toward a more metabolically favorable state. Whether any of this can be harnessed therapeutically for obesity or metabolic disease is unclear, but it illustrates how broadly consequential this single enzyme is across different tissue types. It also means that systemically inhibiting LSD1 could have metabolic consequences that have not yet been fully explored in clinical settings.35PubMed Central. LSD1 for the Targeted Regulation of Adipose Tissue