Mirna Therapeutics Brings First MicroRNA Replacement Therapy into Clinic

It has been a long wait, but 5 years following the initiation of anti-miR122 development for the treatment of HCV infection by Santaris, a second microRNA Therapeutics has now entered clinical development.  MRX34 by Mirna Therapeutics is a miR-34a replacement therapy for the treatment of liver cancer or cancers with liver involvement.  

MicroRNA replacement therapy is technically very similar to RNAi Therapeutics.  It involves adding to cells naturally occurring microRNAs to orchestrate typically a range of processes deemed to be therapeutic.  It is added to cells in the form of double-stranded RNA triggers very much alike RNAi triggers and can therefore largely utilize the same types of delivery approaches.  The delivery technology for MRX34 is the NOV340 SMARTICLE technology owned by Marina Biotech.  This liposomal delivery technology is distinct from SNALP, most notably by employing ionizable ‘amphoteric’ lipids, lipids that can take on both positive and negative charge depending on pH, and not ionizable cationic lipids, lipids that merely become positively charged at acidic pH, as in the case of SNALPs. 

An extensive literature supports miR-34a as an exciting microRNA for replacement therapy in cancer rivaling the most famous let-7.  Accordingly, miR-34 emerged as Mirna Therapeutics’ top priority based on extensive screening conducted at Ambion (now Life Technologies) from which Mirna Therapeutics was spun out.  Under transcriptional control by the guardian of the genome, p53, its pleiotropic effects range from cell cycle inhibition, to counter-acting anti-apoptotic mechanisms and to sensitizing towards chemotherapy.  The choice of liver-related cancers was thus not based on cancer biology (many cancers cancers would apply), but largely a function of where the company thinks NOV340 can deliver to.

This to me raises again the question of whether liver cancer is closer to normal liver or whether it is closer to solid tumors in general in terms of delivery.  MRX34 thus follows Alnylam’s reasonings as manifested by ALN-VSP02, but goes against what is known about blood supply differences between normal liver and liver cancer and what is practiced by the likes of Tekmira and Dicerna.

The phase I study will be a typical dose-escalation study seeking to determine the maximally tolerated dose.  Given the importance of delivery, close attention should be paid to the pharmacokinetic and biodistribution data from this trial.  These should start to become available in the first half of 2014.

The development is positive for at least two other companies.  As the provider of the delivery technology, Marina Biotech obviously stands to gain financially and otherwise from such clinical milestones.  Whether it will be sufficient to pay off its debts that have come due and consequently avert bankruptcy is an open question.  Due to the financial distress, Marina Biotech struggled to enter into deals giving it fair compensation for its technology.

Although the most direct competitor to Mirna Therapeutics, InteRNA as the other major microRNA replacement company in oncology should also benefit from the initiation of the phase I study as it helps to validate its approach.  The question will be whether among its stable of microRNA replacement candidates, there are some with as robust activities as miR-34a.   

BMS Partners with Santaris

In other recent news related to microRNA Therapeutics, Big Pharma Bristol-Myers Squibb partnered with LNA antisense company Santaris under which the Danish company collected $10M upfront.  Although the press release did not specify much the aims of the the alliance, referring to RNA Therapeutics broadly, a few factors speak in favor of microRNAs being involved.  

Most notably, BMS had terminated an phosphorothioate RNaseH antisense collaboration with ISIS Pharmaceuticals.  As Santaris in turn had terminated their PCSK9 phosphorothioate RNaseH antisense program as well, most likely due to kidney toxicity (--> phosphorothioate chemistry as also in Prosensa; van Poelgeest et al. 2013), the first suspicion that BMS sought out Santaris as a more potent PCSK9 alternative becomes less compelling.  Moreover, ISIS Pharmaceuticals is suing Santaris over US patent infringement of RNaseH technology which should hinder the ability of Santaris to enter into relationships with US companies for RNaseH antisense purposes.  Of course, the deal could also indicate that a settlement is in the making...

MicroRNA Mimicry: Not All Triggers are Created Equal

In the online version of Science, Viswanathan and colleagues report on the post-transcriptionally regulated processing of the let-7 microRNA during embryonic stem cell differentiation (Viswanathan et al.: “Selective Blockade of microRNA Processing by Lin-28.” 10.1126/science.1154040). Here, they show that the RNA-binding protein Lin-28 specifically delays the maturation of the let-7 microRNA family until later stages of differentiation. This not only provides an intriguing link between the microRNA and stem cell research areas, it also further supports the role of let-7 in cell differentiation and an additional rationalization for the use of let-7 mimicry as a cancer therapeutic.

With regards to microRNA mimicry as a therapeutic in general, this and an increasing number of other studies reporting on the post-transcriptional regulation of microRNA/small RNA function, including subcellular localization, the sorting into different small RNA effector complexes, and microRNA/small RNA maturation, however also raise the question about the critical role of the choice of the right microRNA mimic in order to achieve the desired therapeutic effect.

This may be a real problem if one assumed that in order to obtain the therapeutic benefit such a microRNA mimick would have to regulate more or less the same set of target genes as its endogenous counterpart. The introduction of a synthetic siRNA-like microRNA duplex e.g. may not sufficiently recapitulate the normal sorting mechanism if this was linked to its biogenesis. Moreover, such a synthetic small RNA would likely have to be modified for pharmacological reasons, something we know may profoundly affect, and ideally reduce the off-target spectrum of an siRNA, but in the case of a microRNA mimick may be undesirable. It is also becoming increasingly clear that even a small difference in the level of a microRNA may have profound effects on its biological output.

While this represents significant scientific challenges and calls for the use of optimal models of human disease as part of the pre-clinical validation process, it also represents IP opportunities for the increasing number of microRNA therapeutics companies. It will be interesting to see whether such companies will soon try to differentiate themselves based on the specific chemistry of microRNA mimicry or a particular gene therapy approach. More so than for RNAi Therapeutics, gene therapy may enjoy here a number of unique advantages, and it would make sense for companies like Benitec, Oxford Biomedia, or Nucleonics to consider a microRNA therapeutics program.

[see also my blog on microRNA sponges for a gene therapy approach to inhibiting microRNA function]

Journal Club: Target Mimicry as a New Way to Regulate microRNA Activity

Just how intricately RNAi-related pathways can be wired, is demonstrated by a study on the fate a nutrient-regulated microRNA in plants [Franco-Zorrilla et al. (2007). Target mimicry provides a new mechanism for regulation of microRNA activity. Nature Genetics, Advance Online Publication, doi: 10.1038/ng2079].

Unlike in animals where microRNAs recognise targets sites of incomplete base-pair complementarity to promote translational repression, microRNAs in plants largely target fully complementary sites in an mRNA. Like with perfectly paired siRNAs in animals, this leads to the destruction of the targeted transcript by endonucleolytic cleavage at the target site. This process is thought to be rapid and the small RNA is then free to recycle and target a new message. By contrast, the turnover rate for animal microRNAs is much less well understood. However, as was often the case in the relatively short history of RNAi-related research, this study in plants may offer us a clue about the kinetics of microRNA activity in animals.

Franco-Zorrilla and colleagues observe that the induction of a microRNA that is regulated based on phosphate availability and that has known target transcripts based on perfect microRNA-target site complementarities around the expected cleavage sites, is accompanied by another non-coding RNA (RNA that is not translated into proteins) that also has high base-pair complementarity to the microRNA, but with telling mismatches around the otherwise predicted cleavage site. Sure enough, this non-coding RNA is recognised by the microRNA, but not degraded. Through a series of elegant genetics, the authors demonstrate that this sufficiently diverts the microRNA, which is of low abundance to start with, so that it cannot act on their “normal” mRNA substrates any more. Hence, the non-coding RNA functions as a sink and regulates microRNA activity by tricking it to bind to itself. This strongly suggests that the turnover of the microRNA complex on incompletely based-paired targets is very slow and suppression requires a one-to-one microRNA-target site relationship.

If you have cared to read all this, I will now tell you what I think the implications are for the development of RNAi Therapeutics. When thinking about off-target effects, we have been largely concerned about the detrimental effects of suppressing unintended mRNAs, largely through microRNA-like translational suppression. However, if the plant system reflects RNA silencing kinetics in humans, then another consequence of off-targeting may be decreased on-target activity due to decreasing the pool of available siRNAs. Consequently, if it were possible to prevent such off-targeting, for example through chemical modifications and bioinformatics, then one could think about lower siRNA doses in the clinic. It is not clear whether the 2’O-methylation strategy pioneered by Dharmacon scientists to limit off-target silencing also prevents microRNA-like binding of an siRNA to incompletely based-paired RNAs, but the strategy certainly points in the right direction.