As a clinician who has been treating lymphoma patients for the best part of 20 years, one of the most remarkable cases I have seen was a gentleman with relapsed and refractory angioimmunoblastic T-cell lymphoma (AITL) whose disease was ‘terminal’ having progressed through 10 lines of therapy (including intensive chemo and two different histone deacetylase inhibitors). It was 2014 and two prominent Nature Genetics papers reported on the AITL genome noting a high incidence of mutations of genes regulating DNA methylation (TET2, DNMT3A and IDH2) – a pattern more akin to myeloid disease than lymphoma. Having seen these papers presented as a side note at a dinner meeting we thought – why don’t we try the hypomethylating agent, 5’azacitidine (AZA) for our AITL patient? We did so and he promptly went into remission, was discharged from the palliative care ward and remained lymphoma free for 5 years.

I thought great … let’s do some sequencing on his lymph node biopsy, find the TET2 mutation and write up the ‘case report’. However, when we did the genomics, we couldn’t find any of the mutations we thought we were targeting with the AZA (TET2, DNMT3A and IDH2 were all wild type). We transitioned from patting ourselves on the back to scratching our heads. So much for rational repurposing of a hypomethylator. It was a story without a punchline that we didn’t publish until 2019 by which time several retrospective anecdotes supporting the efficacy of AZA in AITL had already emerged.

When you see a remarkable clinical response like this, you want to do it again in a scientifically robust way and understand why it happened. Unfortunately for us in 2015, parenteral AZA was at the end of its patent life and Celgene (who are developing CC486 – an orally available AZA formulation) were not interested in supporting an investigator-initiated trial protocol in T-cell lymphoma. Hence, along with our ‘case report’ our ‘trial concept’ was shelved.

Fast forward to 2018 and a new hypomethylator called guadecitabine was being road-tested in a suite of trials culminating in a randomised study pitching it against AZA and decitabine as front-line therapy for acute myeloid leukaemia (AML). Astex Pharmaceuticals Inc. (the company who were developing guadecitabine) were bullish about the prospect of a registration indication in AML and open to investigator-initiated proposals in other disease areas. Hence, nearly five years after our ‘exceptional responder’ we were enabled to do our phase 2 proof of concept trial in T-cell lymphoma – but with guadecitabine instead of AZA thanks to Astex.

Despite the rarity of T-cell lymphoma relative to B-cell disease, our guadecitabine trial recruited rapidly from three Australian states. The majority of patients were in dire clinical need having failed conventional and novel treatment options. As guadecitabine is considered low intensity therapy, we included patients with poor performance status who were unfit for chemotherapy. The overall response rate (ORR) was 40%, which is encouraging for a heavily pre-treated cohort treated with a single-agent intervention. We found that the main management consideration with guadecitabine was the depth of neutropenia and its attendant infection risk. This was greater than we would have anticipated based on prior experience with AZA and probably reflects the potency of guadecitabine as a hypomethylator. Although the overall progression free survival (PFS) of the study was dismal (about 3 months), those patients who responded to guadecitabine had a median duration of response of 6 months, and the overall survival of the responders was not reached at the time of study analysis (estimated median potential follow-up 22 months). Importantly, these responding patients could be bridged to subsequent lines of therapy, including allogeneic transplant which is a potentially curative intervention.

Our trial was resourced to recruit 20 patients. While this allowed exploration of both efficacy and safety signals, the sample size was underpowered to robustly identify subgroups or tumour mutations that may predict responses. Using cell free tumour DNA profiling, we were able to establish that those patients with RHOA^G17V mutations had a better PFS. This may be because RHOA^G17V mutations strongly associate with the AITL subtype of T-cell lymphoma. 

However, AITL is one at least 20 different subtypes of T-cell lymphoma and even the different subtypes can be further dissected into biologically distinct entities using molecular techniques. Interestingly, the spectrum of T-cell lymphoma is hallmarked by recurrent mutations of genes regulating epigenetic processes. To further understand what mutations may predict responses to hypomethylators in T-cell lymphoma, the Kats lab at Peter MacCallum Cancer Centre performed CRISPR/Cas9 functional genomic screening using an epigenetically focused library in T-cell lymphoma cells treated with guadecitabine. Contrary to our expectations, silencing of TET2 did not sensitize to hypomethylation. Indeed, the majority of ‘hits’ in the screen were of enzymes regulating histone rather than DNA-methylation. Of particular interest was the silencing of the histone methyltransferase SETD2, which sensitized to guadecitabine. SETD2 is recurrently mutated in rare forms of T-cell lymphoma, including monomorphic epitheliotropic intestinal T-cell lymphoma (MEITL). We were fortunate to have recruited a MEITL patient with a SETD2 mutation to the trial, and he responded to guadecitabine having relapsed and progressed after high dose chemotherapy and a stem cell transplant. Although this ‘n=1’ observation requires further validation, it provides a basis for the complementarity of CRISPR screening as a tool for identifying mutations that could predict therapeutic responses in orphan disease areas.

The results of our trial would have segued nicely into a phase 2 extension study. By comparison, currently available novel agents for T-cell lymphoma such as romidepsin and pralatrexate were approved based on ‘pivotal phase 2 studies’ demonstrating ORRs of 25% and 29% respectively. However, as guadecitabine proved only non-inferior to standard of care hypomethylators in its pivotal phase 3 AML trial, it is no longer being clinically developed. Despite earlier corporate disinterest, CC486 (oral AZA) is now being evaluated alone and as combination therapy across the spectrum of T-cell disease, with a focus on AITL. Astex Pharmaceuticals Inc. have a promising oral decitabine formulation (ASTX727, decitabine/cedazuridine; Inqovi^®) that is approved for the treatment of myelodysplasia and could ideally be cross-purposed to the T-cell lymphoma clinic. Thus, a field in desperate need of a new therapeutic backbone for clinical trials now has at least two hypomethylators to build on.

We hope that you will enjoy reading the paper and that in some small way we have advanced a difficult field and a real area of clinical need. I would like to make special mention of the efforts of our trial unit at Monash Health and Dr Robin Gasiorowski and his team at Concord for looking after the patients. The collaboration with Lev Kats’ lab at Peter Mac, and in particular Dr Emily Gruber, really helped us layer the science into the clinical story to help complete a translational body of work. Finally, I would like to thank the brave patients who participated in the trial and ‘Frank’ (pictured in remission alongside the author) – our original ‘exceptional responder’ who Tee-d off the whole thing.