There is no selective PRC1 catalytic inhibitor - the entire chemistry is one 2013 tool compound and its analogues, and it hits both paralogues by design
also called PRC1 catalytic chemistry · H2A ubiquitination inhibitors · PRT4165 and successors
Several entries in this graph propose blocking the Polycomb enzyme that silences genes. It is worth knowing that no good drug for that exists: there is one laboratory tool from 2013, it hits both versions of the enzyme at once, and it was described as a way to study DNA repair rather than as a cancer drug.
The same thing, for a scientist
Every proposal in this graph that would perturb PRC1 catalysis - and several do - runs into the same wall. The available chemistry is **PRT4165** (2-pyridine-3-yl-methylene-indan-1,3-dione), reported in 2013 as an inhibitor of PRC1-mediated H2A ubiquitylation in vivo and in vitro [E1]. Three properties of it are decisive. **(1) It inhibits both RNF2 and RING1A**, which the authors describe as 'partially redundant paralogues that together account for the E3 ubiquitin ligase activity found in PRC1 complexes'. Pan-paralogue inhibition is the correct design given that redundancy, and it means the compound cannot distinguish PRC1 flavours - it hits canonical PRC1 and every variant including PRC1.1 equally. **(2) It was characterised as a DNA-damage-response tool**: the paper's subject is ubiquitin signalling at double-strand breaks, and the compound blocks DSB repair. Any cellular phenotype it produces has a genotoxic co-explanation. **(3) It is the only starting point anyone has.** A 2024 medicinal-chemistry effort designed analogues explicitly 'based on PRT4165' and reports compound 11b as a lead [E2] - still a lead, in osteosarcoma cells, with no selectivity data across PRC1 flavours. **The therapeutic consequence for this project is the opposite of the usual one**: the direction the available chemistry moves - PRC1 activity down - is the direction of the documented menin-inhibitor resistance lesion [E3].
What the graph proposes and what the chemistry can actually do
| proposal in this graph | needed chemistry | exists? |
|---|---|---|
| block the RING1B swap at MYC enhancers to test its necessity | selective RNF2 catalytic inhibitor | no - PRT4165 hits RING1A too, and both are needed for the phenotype anyway |
| turn PRC1.1 down to test its role in menin-inhibitor response | PRC1.1-flavour-selective inhibitor | no - nothing distinguishes flavours; the closest is a USP7 inhibitor, which is a complex-stability drug, not a catalytic one |
| turn PRC1.1 up to deepen menin-inhibitor silencing | PRC1.1 activator or TRIM27 inhibitor | no compound known |
| block PRC1 catalysis therapeutically | selective, tolerable inhibitor | no - and Rossi et al. show PRC1 activity is required for leukaemia maintenance, so the sign is contested before the chemistry is |
The first row is worth spelling out because it is the experiment exp-brg1-ring1b-timecourse-at-benc-under-menin-inhibition would want a pharmacological arm for. The inv(16) study used PRT4165 for exactly that purpose and obtained a partial MYC rescue 4 - which is what a tool compound with a genotoxic co-mechanism and pan-paralogue activity would be expected to give whether or not the hypothesis were true. Genetic perturbation is the better arm here, and the chemistry is not a shortcut.
The honest assessment of the target class
The brief's standing caution applies directly: a ChEMBL activity-row count measures how much medicinal chemistry has been done, not how tractable the target is. For PRC1 catalysis almost none has been done, and the reason is not that anyone has tried and failed - it is that the therapeutic hypothesis has never been clear enough to justify a programme, because PRC1 activity is tumour-suppressive in some settings and tumour-supporting in others, sometimes in the same disease.
This graph holds both signs at once. Rossi et al. show PRC1 activity and H2AK119ub are required for the leukaemogenic potential of MLL fusions in mice [E3 counter-evidence]. Zhou et al. show PRC1.1 loss confers menin-inhibitor resistance 3. And in neuroblastoma, PRC1.1 removal differentiates the tumour. A target whose inhibition helps in one paediatric cancer, harms in another, and has opposite signs depending on whether a second drug is present is not a target class in which a non-selective tool compound will settle anything.
What would change the picture. A degrader or a flavour-selective agent - one that removes PCGF1-containing PRC1.1 without touching PCGF2/PCGF4-containing canonical PRC1 - would be a genuinely new instrument, because every published perturbation in this field is either pan-PRC1 (Rossi, PRT4165) or subunit-genetic (PCGF1, BCOR knockouts) and neither is a drug. No such agent was found by this beat.
| target | PRC1 catalytic core (RNF2/RING1B and RING1/RING1A) |
|---|---|
| modality available | small-molecule tool compound only |
| adjacent pharmacology that is not catalytic | USP7 inhibitors (gene-usp7) act on complex stability rather than catalysis and are the only compounds with real leukaemia data on this axis |
| therapeutic sign is contested before the chemistry is | PRC1 activity is required for MLL-fusion leukaemogenesis (PMID 27730210) and PRC1.1 loss confers menin-inhibitor resistance (PMID 39158067), while PRC1.1 removal differentiates neuroblastoma (PMID 41855552). The same molecular action has three different therapeutic signs on record. |
| recommendation | use genetic perturbation, not PRT4165, for any PRC1 arm of an experiment in this project; treat the compound as corroborative only |
compounds
[
{
"name": "PRT4165",
"chemistry": "2-pyridine-3-yl-methylene-indan-1,3-dione",
"first_reported": 2013,
"pmid": "23902761",
"targets": "inhibits both RNF2 and RING1A in vitro; not RNF8 or RNF168",
"flavour_selectivity": "none - cannot distinguish canonical PRC1 from PRC1.1 or any other variant",
"co_mechanism": "blocks ubiquitin signalling and repair at DNA double-strand breaks; genotoxic co-explanation for any anti-proliferative effect",
"status": "laboratory tool",
"title": "A small molecule inhibitor of polycomb repressive complex 1 inhibits ubiquitin signaling at DNA double-strand breaks.",
"authors": "Ismail IH, McDonald D, Strickfaden H, et al.",
"venue": "J Biol Chem",
"year": 2013,
"title_provenance": "Title/authors/venue backfilled from PubMed esummary on 2026-09-20 because this citation stored a bare PMID with no title, which made it unscoreable by verify-citations.py. BACKFILLING IS NOT VERIFICATION: it confirms the PMID resolves to a real paper, not that this is the right paper for the claim. That still requires reading."
},
{
"name": "compound 11b",
"derivation": "designed based on PRT4165",
"first_reported": 2024,
"pmid": "38636717",
"evidence": "reduced viability and bulk H2Aub in U2OS osteosarcoma cells",
"status": "lead compound; no flavour selectivity, no DNA-damage counter-screen, no in vivo data in the abstract",
"title": "Identification of a novel histone H2A mono-ubiquitination-inhibiting cell-active small molecule.",
"authors": "Ni S, Takada Y, Ando T, et al.",
"venue": "Bioorg Med Chem Lett",
"year": 2024,
"title_provenance": "Title/authors/venue backfilled from PubMed esummary on 2026-09-20 because this citation stored a bare PMID with no title, which made it unscoreable by verify-citations.py. BACKFILLING IS NOT VERIFICATION: it confirms the PMID resolves to a real paper, not that this is the right paper for the claim. That still requires reading."
}
]graph mentions before this entry
{
"PRT4165": 0,
"XL177A": 0,
"count_method": "word-boundary regex over all 1,510 pre-existing node JSON files, 2026-09-20"
}what does not exist
[ "a PRC1.1-flavour-selective inhibitor or degrader", "a compound that RAISES PRC1.1 activity - the direction this disease would want under a menin inhibitor", "any PRC1-catalytic compound with published in vivo leukaemia data" ]
- The only available PRC1 chemistry moves the target in the direction of a documented menin-inhibitor resistance lesion, so a clinical programme built on it in this disease would be pharmacologically self-defeating.
- PRT4165's parent activity is inhibition of DNA double-strand-break repair, so it is a genotoxic agent as well as a Polycomb agent; combining it with any DNA-damaging therapy would carry an unexamined interaction.
- Pan-paralogue, pan-flavour inhibition removes PRC1 activity that normal haematopoietic progenitors require, so the therapeutic window for this target class has never been demonstrated to exist.
- A 2024 chemistry paper describes PRT4165 as a RING1A inhibitor when the original describes it as inhibiting both RING1A and RNF2. A reader relying on the later description would design a paralogue-specific experiment with a dual-paralogue compound.
- Is a PCGF1-selective degrader feasible? It would be the first instrument able to separate PRC1.1 from canonical PRC1 pharmacologically, and every published perturbation in this field is either pan-PRC1 or subunit-genetic.
- Does catalytic inhibition phenocopy subunit knockout? A catalytic inhibitor leaves a targeting-competent complex on chromatin and a knockout does not; the two could give opposite answers about menin-inhibitor resistance and nobody has compared them.
- Is there any way to raise PRC1.1 activity pharmacologically? A TRIM27 inhibitor is the only mechanistic route this beat could identify and no such compound is known.
- Do PRT4165's reported effects in leukaemia cells survive a DNA-damage counter-screen? Every cellular result with this compound carries a genotoxic alternative explanation that appears never to have been excluded.
- E1strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
PRT4165 (2-pyridine-3-yl-methylene-indan-1,3-dione) is a potent inhibitor of PRC1-mediated H2A ubiquitylation in vivo and in vitro; in vitro E3 ligase assays show it inhibits both RNF2 and RING1A - partially redundant paralogues that together account for the E3 activity of PRC1 complexes - but not RNF8 or RNF168; and it inhibits ubiquitin accumulation at DNA double-strand breaks, retention of DNA-damage-response proteins in foci, and DSB repair.
A small molecule inhibitor of polycomb repressive complex 1 inhibits ubiquitin signaling at DNA double-strand breaks. · Ismail IH, McDonald D, Strickfaden H, Xu Z, Hendzel MJ · J Biol Chem 2013What this evidence cannot show
A chemical-biology tool paper, not a drug-discovery paper: no selectivity panel across the wider ubiquitin system beyond RNF8/RNF168, no pharmacokinetics, no in vivo tolerability, and the cellular characterisation is in a DNA-damage context. Because it blocks DSB repair, any anti-proliferative effect it produces in a dividing cell has a genotoxic explanation that does not require the Polycomb mechanism at all. Potency values are not quoted here because none was read from the abstract.
Evidence that argues against this (1)
- Inhibiting PRC1 catalysis is not obviously therapeutic in this disease: genetic depletion of the same two ligases in mice shows PRC1 activity and H2AK119ub are REQUIRED for the leukaemogenic potential of MLL fusions, so the compound's on-target effect would be expected to help the leukaemia rather than harm it in that model. Rossi A et al. 2016
- E2moderate confidence 3 of 5: One strong study, or several smaller consistent ones.paper
A medicinal-chemistry effort synthesised and evaluated compounds designed based on PRT4165, described as a reported RING1A histone ubiquitin ligase inhibitor, and identified compound 11b as strongly inhibiting viability and reducing histone H2A mono-ubiquitination in human U2OS osteosarcoma cells, presenting it as a lead compound.
Identification of a novel histone H2A mono-ubiquitination-inhibiting cell-active small molecule. · Ni S, Takada Y, Ando T, Yu S, Yamashita Y, Takahashi Y, et al. · Bioorg Med Chem Lett 2024What this evidence cannot show
A lead, not a probe: one cell line, osteosarcoma, viability plus a bulk H2Aub readout, with no PRC1-flavour selectivity, no DNA-damage counter-screen and no in vivo data reported in the abstract. It is cited to establish that the field's chemical starting point in 2024 is still PRT4165, which is the point of this entry. Note also that the paper describes PRT4165 as a 'RING1A inhibitor' while the original describes it as inhibiting both RNF2 and RING1A - a single-paralogue description of a dual-paralogue compound, which is the RING1/RNF2 naming hazard recorded in gene-ring1 appearing in a chemistry abstract.
Evidence that argues against this (1)
- A compound whose parent blocks DNA double-strand-break repair and which is assayed by viability in a single line has a well-known alternative explanation for its anti-proliferative effect. Without a DNA-damage counter-screen, 'reduces H2Aub and kills cells' does not establish that the killing is through H2Aub. Ismail IH et al. 2013
- E3strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
In KMT2A-rearranged AML, loss of PRC1.1 subunits such as PCGF1 or BCOR confers menin-inhibitor resistance by epigenetic reactivation of non-canonical menin targets including MYC - so the direction in which the available chemistry moves PRC1 activity is the direction of a documented resistance lesion.
Epigenetic regulation of noncanonical menin targets modulates menin inhibitor response in acute myeloid leukemia. · Zhou X, Zhang L, Aryal S, Veasey V, Tajik A, Restelli C, et al. · Blood 2024What this evidence cannot show
Subunit knockout is not the same lesion as catalytic inhibition: a knockout removes the complex from chromatin entirely, while a catalytic inhibitor leaves a targeting-competent complex in place that simply does not ubiquitinate. Those could differ, and nobody has compared them. Myeloid models only.
Evidence that argues against this (1)
- Genetic depletion of the PRC1 catalytic subunits in mice impairs rather than supports MLL-fusion leukaemia, so PRC1 catalytic inhibition is not straightforwardly a resistance-promoting intervention either. The two results have opposite signs in overlapping systems and neither has been tested with a catalytic inhibitor in a menin-inhibitor-treated cell. Rossi A et al. 2016
- E4moderate confidence 3 of 5: One strong study, or several smaller consistent ones.paper
The one published use of this chemistry in a fusion-oncoprotein MYC-enhancer context: in inv(16) AML, MYC expression under the fusion-interface inhibitor AI-10-49 was partially rescued by RNF2 knockdown or by pharmacologic inhibition of RING1B activity with PRT4165.
CBFbeta-SMMHC Inhibition Triggers Apoptosis by Disrupting MYC Chromatin Dynamics in Acute Myeloid Leukemia · Pulikkan JA, Hegde M, Ahmad HM, Belaghzal H, Illendula A, Yu J, O'Hagan K, Ou J, Muller-Tidow C, Wolfe SA, Zhu LJ, Dekker J, Bushweller JH, Castilla LH · Cell 2018What this evidence cannot show
The rescue is explicitly partial and the compound arm is corroborative of a genetic arm rather than standing alone, which is the correct way to use a tool of this quality. Read in full from PMC6211564 on 2026-09-20. A different disease from this project's.
Evidence that argues against this (1)
- PRT4165 blocks DNA double-strand-break repair, so in a drug-treated, apoptosis-prone cell its effect on a transcript could be indirect. The genetic RNF2 knockdown arm is what carries the conclusion; the compound arm would not support it alone. Ismail IH et al. 2013
PRT4165 inhibits both RNF2 and RING1A in vitro; no flavour selectivity
the same compound hits the paralogue, by design given their redundancy
the one published use of this chemistry at a MYC enhancer; the rescue was partial and the genetic arm carries the conclusion
- confers resistance to →Revumenib (SNDX-5613, Revuforj) — first approved menin–KMT2A interface inhibitor
the only available PRC1 chemistry moves the target in the direction of the documented menin-inhibitor resistance lesion
why that experiment's PRC1 arm must be genetic rather than pharmacological
PRT4165 and XL177A each named in zero pre-existing entries