Antisense Technology Produces Huntington’s Disease Breakthrough

Huntington’s Disease (HD) is one of the most prevalent severe, ultimately fatal genetic neurological diseases of our times for which impactful treatments are desperately lacking.  Based on results from a clinical study aiming to knockdown the disease-causing gene using RNaseH antisense technology, this is about to change over the next 3 years.

IONIS-HTTRx study results presented at CHDI

At the major annual HD gathering this week, lead investigator Tabrizi from the University College London presented data from a phase I-II study of antisense drug candidate IONIS-HTTRx in patients with early symptoms of the disease.

IONIS-HTTRx (aka RG6042), developed by Ionis and partnered with Roche, targets both wild-type and mutant huntingtin protein to provide all patients with the most potent and safest sequence possible.  While mutant allele-specific approaches have been discussed and are being pursued by antisense rival Wave Life Sciences, the concern that suppressing wildtype huntingtin in adults seems to me a far-fetched theoretical discussion, if not wishful thinking given that antisense is unlikely to completely eliminate wildtype huntingtin and that the drug is being given to adults.  

To wit, the concern is based on genetic mouse models where complete huntingtin knockout causes embryonic lethality. Importantly, this is not recapitulated in mouse model where knockout is delayed until adulthood.

Call me cynical, but the choice of an allele-specific approach by Wave Life Sciences has likely been primarily motivated by allowing for some follower differentiation.  Ironically, by limiting themselves to a much reduced sequence space around co-segregating single nucleotide polymorphisms, the sequences may not only lack the potency, but also the apparent safety of IONS-HTTRx.

Good safety profile maintained until highest dose tested

In this study, IONIS-HTTRx was given monthly over 3 months.  Speaking to the safety of the approach, apart from transient side effects due to the intrathecal mode of administration, no adverse events of note were reported and all 46 subjects have subsequently rolled over into the open-label extension study.

Importantly, doses as high as 120mg monthly were well tolerated at which dose the knockdown effect seems to have plateaued.  This not only bodes well for IONIS-HTTRx, but for the entire neurological disease franchise of Ionis Pharmaceuticals. 

The human safety and tolerability results are consistent with a range of small and large animal models (including dogs, pigs, non-human primates), some of which were a year or longer in duration.

Robust huntingtin lowering

It is widely thought that mutant huntingtin protein is harmful to the cell expressing it, ultimately resulting in neuronal death throughout the brain.  This can be visualized by considerable, 30%-level atrophy of the brain over the course of the disease.

Although it is not possible to assess huntingtin levels directly in brain tissue in humans from biopsies, the experience in the preclinical animal models and human data from spinal muscular atrophy drug SPINRAZA allows one to model the correlation between ASO-dependent protein changes in the cerebrospinal fluid (CSF) and the various areas of the CNS quite well.

Since SPINRAZA is based on chemistry that is highly similar to that of IONIS-HTTRx, they behave identically in terms of biodistribution for all intents and purposes.

In the two highest cohorts, 90mg and 120mg, mean 40% mutant huntingtin reductions in the CSF were observed shortly after the last dose with further reductions (50%+) seen thereafter.  Such levels are predicted to reflect 55 to up to 85% reductions in the cortex and 20-50% in the caudate.

This degree of knockdown exceeds that necessary to see changes in HD animal models, some of which with manifest disease symptoms at treatment onset.

The future is bright

The study results therefore raise hopes that  IONIS-HTTRx may halt or even reverse disease in manifest HD patients.  As exciting and medically likely with the biggest bang for the buck is the prospect that the drug candidate may prevent disease symptoms to emerge in the first place when given early to those at risk of developing the disease (~1 in 2 with a parent having HD, 200,000 in the US alone). 

The companies and clinical investigators are now busy designing a pivotal outcomes study which I would guess will ultimately involve 150-250 early-stage patients over a period of 16-24 months.  The size and extent of the trial will depend on which endpoints are chosen.  A pre-planned interim look at ~12 months may also make sense to look at brain size as a potential way to obtain accelerated approval.  Accelerated approval under the new FDA may also be possible based on functional analyses the investigators are currently conducting using data from the present study and the open-label extension with dosing well beyond 3 months..

In drug development, sometimes you can tell early on whether a drug candidate is destined to breeze through to approval. In this regard, IONIS-HTTRx certainly feels like Spinraza.