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Mechanismstrong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.Q-histone-marks

Panobinostat lowers H2B ubiquitination in MLL-rearranged ALL by suppressing RNF20/RNF40/WAC — the only clinical-stage agent that reaches this axis, and its one ALL trial was stopped for lack of efficacy

also called LBH589 and H2Bub · HDAC inhibition of the H2Bub axis

An approved HDAC-blocking drug turns out to work partly by shutting down the enzymes that put the ubiquitin tag on H2B, and it worked well against MLL-rearranged infant-type leukaemia in mice. The only trial of that drug in adult acute leukaemia was stopped early for lack of effect, which is the sobering part.

The same thing, for a scientist

Garrido Castro et al. showed panobinostat (LBH589) monotherapy has strong in vivo anti-leukaemic effects in xenograft models of MLL-rearranged ALL, extending survival and reducing disease burden, and that the activity involves **depletion of H2B ubiquitination via suppression of the RNF20/RNF40/WAC E3 ligase complex** — which they call a pivotal pathway for MLL-rearranged leukaemic maintenance — with WAC knockdown phenocopying H2Bub loss and inducing cell death [E1]. This is the only route by which a clinical-stage molecule reaches the H2Bub axis, and it is indirect: panobinostat is a pan-HDAC inhibitor and the authors themselves state it cross-inhibits multiple epigenetic pathways. The clinical record is the necessary counterweight. **NCT00723203**, a phase 2 of panobinostat in relapsed/refractory ALL and AML run by City of Hope with the NCI, enrolled 16 adults and was **terminated**, with the registry giving the reason plainly: *'Terminated early due to a lack of efficacy'* [E2]. A separate paediatric phase 1 in children with refractory haematologic malignancies, NCT01321346, enrolled 30 and completed [E3]. Neither trial selected for KMT2A rearrangement, and neither measured H2Bub.

Go deeper
What the registry says, read directly rather than through a summary tool
NCT00723203NCT01321346
TitlePanobinostat in Treating Patients With Relapsed or Refractory Acute Lymphoblastic Leukemia or Acute Myeloid LeukemiaA Study Of Panobinostat In Children With Refractory Hematologic Malignancies
Phase21
StatusTERMINATEDCOMPLETED
whyStopped"Terminated early due to a lack of efficacy"—
Enrolment (actual)1630
Ages18 years and over8–21 years
Datesstarted 2008-04, completed 2010-09started 2011-03
Lead sponsorCity of Hope Medical CenterTherapeutic Advances in Childhood Leukemia Consortium
CollaboratorNational Cancer Institute (NCI)Novartis

The whyStopped string was read from the raw ClinicalTrials.gov v2 API, per this project's standing rule, because the summarising tool does not return that field 2.

The honest reading. A 16-patient phase 2 in unselected adult relapsed/refractory acute leukaemia stopping for lack of efficacy is weak evidence about a mechanism. It says nothing specific about KMT2A-rearranged infant B-ALL, which is a different disease in a different population, and the trial predates the preclinical paper by a decade. But it is the only clinical readout that exists for the only clinical-stage agent that touches this axis, and a beat that recorded the encouraging mouse data without it would be misleading.

The mechanistic problem with using an HDAC inhibitor as an H2Bub agent

Panobinostat's effect on RNF20/RNF40/WAC is a downstream transcriptional consequence of HDAC inhibition — the complex is suppressed, not directly inhibited 1. Three things follow.

  1. No selectivity for the axis. Everything else panobinostat does comes along. The authors say so explicitly: it cross-inhibits multiple epigenetic pathways, and the H2Bub arm is one contributor of unquantified weight.
  2. No dose relationship to H2Bub. You cannot titrate H2Bub with this drug, because H2Bub depletion is several steps downstream of the target engagement you can measure.
  3. The direction may be wrong anyway. If the 'raise H2Bub' model in hyp-h2bub-direction-is-unknown-in-kmt2a is correct, then part of what panobinostat does to this axis is working against the differentiation route, and the drug's benefit in the xenografts would be coming from elsewhere. That is testable and untested.

The constructive reading is that this paper is best evidence for the importance of the axis in MLL-rearranged ALL — WAC knockdown alone killing the cells is the strong part — rather than evidence for panobinostat as the way to exploit it.

Facts and figures
agentpanobinostat (LBH589), pan-HDAC inhibitor
axis reachedRNF20/RNF40/WAC - H2B ubiquitination
mechanism of reaching ittranscriptional suppression of the ligase complex downstream of HDAC inhibition, not direct inhibition
preclinical resultin vivo efficacy in MLL-rearranged ALL xenografts; WAC knockdown phenocopies H2Bub loss and kills
targetgene-wac
clinical record
[
  {
    "nct": "NCT00723203",
    "phase": "2",
    "status": "TERMINATED",
    "why_stopped": "Terminated early due to a lack of efficacy",
    "n": 16,
    "ages": "18+",
    "population": "relapsed/refractory ALL or AML, unselected for KMT2A"
  },
  {
    "nct": "NCT01321346",
    "phase": "1",
    "status": "COMPLETED",
    "n": 30,
    "ages": "8-21",
    "population": "children with refractory haematologic malignancies"
  }
]
How this could bite us back
  • The one clinical trial of this agent in ALL was stopped for lack of efficacy. Any proposal to exploit the H2Bub axis through panobinostat has to explain that result first.
  • Panobinostat's paediatric toxicity is driven by its HDAC pharmacology, not by anything to do with H2Bub, so the therapeutic index of the axis cannot be inferred from this drug.
  • Because panobinostat lowers H2Bub, and because raising H2Bub is what permits bivalency resolution and differentiation, part of this drug's action on this axis may oppose differentiation - see hyp-h2bub-direction-is-unknown-in-kmt2a.
  • The mouse result is monotherapy in xenografts of a disease whose human treatment is already maximally intensified; xenograft survival extension has repeatedly failed to translate in this specific disease.
Still open
  1. Would restoring RNF20/RNF40/WAC expression under panobinostat rescue the leukaemia cells? That is the experiment that would tell you how much of the drug's effect runs through this axis, and the abstract does not report it.
  2. Did any patient on NCT00723203 or NCT01321346 carry a KMT2A rearrangement? Neither trial selected for it; if archived material exists, retrospective genotyping would convert two uninformative trials into a small informative series.
  3. Is there a direct RNF20/RNF40 or WAC-directed agent at any stage? This beat found none, which means panobinostat is the field's only handle on this axis and it is a poor one.
  4. Does panobinostat lower H2Bub in patient blasts at clinically achievable exposures? No trial has measured it.
Every source behind this entry (3)
  1. E1strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper

    Panobinostat (LBH589) monotherapy shows strong anti-leukaemic effects in xenograft mouse models of MLL-rearranged ALL, extending survival and reducing overall disease burden; comprehensive molecular analyses in vitro show the activity involves depletion of H2B ubiquitination via suppression of the RNF20/RNF40/WAC E3 ligase complex, described as a pivotal pathway for MLL-rearranged leukaemic maintenance; knockdown of WAC phenocopies loss of H2B ubiquitination with concomitant cell death; and panobinostat cross-inhibits multiple epigenetic pathways.

    The HDAC inhibitor panobinostat (LBH589) exerts in vivo anti-leukaemic activity against MLL-rearranged acute lymphoblastic leukaemia and involves the RNF20/RNF40/WAC-H2B ubiquitination axis. · Garrido Castro P, van Roon EHJ, Pinhancos SS, Trentin L, Schneider P, Kerstjens M, Te Kronnie G, Heidenreich O, Pieters R, Stam RW · Leukemia 2018
    PMID 28690313DOI
    What this evidence cannot show

    Abstract read in full; full text not read. Xenograft models; the specific KMT2A fusions and lines are not named in the abstract. The mechanistic chain is drug -> suppressed ligase complex -> lost H2Bub -> death, established by correlation plus a WAC knockdown phenocopy; the abstract does not report a rescue experiment restoring RNF20/40/WAC under panobinostat, which is what would establish the arm's causal weight. One author surname is rendered here without diacritics. **Resolved:** the lines are **SEM t(4;11), KOPN8 t(11;19) and REH t(12;21)**, the last serving as a non-KMT2A comparator, at **5 nM LBH589 for 24 h**. GSE78234. This is one of the few resolutions in the sweep that lands on the index fusion. Resolved 2026-09-19 by the Q18b unstated-systems sweep from the study's deposition, not from its text; bounded as a statement about the deposited data rather than about the whole study.

    Evidence that argues against this (2)
    • The only registered trial of panobinostat in acute lymphoblastic leukaemia was terminated for lack of efficacy after 16 adult patients. City of Hope Medical Center; National Cancer Institute (NCI) 2010
    • A single concentration at a single time point cannot separate an on-target chromatin effect from an early cytotoxic response, and REH is a comparator for KMT2A status but not for lineage or maturation stage, so a difference between it and SEM has more than one available explanation. NCBI Gene Expression Omnibus 2026
  2. E2established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.trial_registry

    NCT00723203, a phase 2 study of panobinostat in relapsed or refractory acute lymphoblastic leukaemia or acute myeloid leukaemia sponsored by City of Hope Medical Center with the National Cancer Institute, enrolled an actual 16 participants aged 18 years and over between April 2008 and September 2010 and has overall status TERMINATED, with the registry's whyStopped field reading 'Terminated early due to a lack of efficacy'.

    Panobinostat in Treating Patients With Relapsed or Refractory Acute Lymphoblastic Leukemia or Acute Myeloid Leukemia · City of Hope Medical Center; National Cancer Institute (NCI) · ClinicalTrials.gov 2010
    NCT00723203
    What this evidence cannot show

    Read from the raw ClinicalTrials.gov v2 API on 2026-09-19, including the whyStopped field which summarising tools omit. Sixteen unselected adult patients with relapsed/refractory acute leukaemia is a very small and very unfavourable population; this result is not evidence about KMT2A-rearranged infant B-ALL and should not be read as such. No KMT2A-rearrangement selection, no H2Bub pharmacodynamic measurement, and the trial closed eight years before the preclinical paper that motivates interest in the axis.

    Evidence that argues against this (1)
    • A registry record for a phase 2 study establishes that the trial existed and its design. It carries no result, and a completed trial without a posted or published outcome is weak support for a mechanism entry — the absence of a reported result in a registry is itself uninformative rather than negative. ClinicalTrials.gov 2026
  3. E3established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.trial_registry

    NCT01321346, a phase 1 study of panobinostat in children with refractory haematologic malignancies including childhood acute lymphoblastic and myelogenous leukaemia, Hodgkin's disease and non-Hodgkin lymphoma, run by the Therapeutic Advances in Childhood Leukemia Consortium with Novartis, enrolled an actual 30 participants aged 8 to 21 years from March 2011 and has overall status COMPLETED with no whyStopped reason recorded.

    A Study Of Panobinostat In Children With Refractory Hematologic Malignancies · Therapeutic Advances in Childhood Leukemia Consortium; Novartis · ClinicalTrials.gov 2011
    NCT01321346
    What this evidence cannot show

    Read from the raw ClinicalTrials.gov v2 API on 2026-09-19. A completed phase 1 establishes that paediatric dosing was explored; it says nothing about efficacy, and the minimum age of 8 years excludes the infant population this project is concerned with. This beat did not retrieve or read the results section or any publication from this trial.

    Evidence that argues against this (1)
    • A phase 1 study in children with refractory haematologic malignancies is a dose-finding design whose primary output is tolerability. It cannot speak to whether H2Bub depletion is the operative mechanism, and paediatric phase 1 populations are small and heavily pretreated. ClinicalTrials.gov 2026
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