Delivery Advance Illustrates Influence of Cosmetics Skin RNAi Therapeutics

The skin has always been a target organ of considerable interest to the RNAi Therapeutics industry due to its apparent accessibility for delivery purposes plus the fact that there are various unmet needs ranging from the severe genetic disease (e.g. epidermolysis bullosa, pachyonycia congenita) to cosmetic desires.  Interestingly, it is the latter that in many ways is driving skin RNAi Therapeutics these days.

Motorized microneedle array with unprecedented silencing efficacy

    

In an important advance in the rate-limiting area of delivery, Hickerson and colleagues from TransDerm and various other collaborators recently published 80% gene silencing efficacy in a mouse model for epidermal gene expression using a motorized microneedle array borrowed from the cosmetics industry (in particular the Triple-Mby BomtechElectronics of cosmetics hot-spot South Korea).  This compares to 50% and 33% gene silencing in the same model using simple (static) microneedle arrays and intradermal needle injection, respectively, before.  Accordingly, this represents a 2.5 to 3.5-fold increase in gene silencing efficacy when considering how much of the undesired target protein you are left with!

I have to admit that I did not double-check that indeed the same siRNA sequences and self-delivering RNAi trigger modifications were used in the various studies which could have affected results.  However, since these results have all been reported by TransDerm and the goal of TransDerm was to compare delivery efficacies of various technologies, I am willing to accept the comparability claim by the authors. _­

The trick with the motorized microneedle array appears to be that following penetration of the stratum corneum barrier motion (oscillation) allows for a larger volume of drug to be deposited in the epidermis than with a static needle array.  Moreover, the depth of administration can be adjusted for optimal epidermal delivery and to make it pain free as well, unlike the original high-pressure hypodermic needle attempts by TransDerm.   With this, it should be possible to deposit low single-digit milligram of RNAi triggers to an area the size of a tip of a thumb- which is quite a bit.

A possible limitation of such microneedle arrays is that the administration itself causes microinjuries to the skin.  Therefore, you want to make sure that you do not end up making things worse, especially in applications where wound healing and restoration are the goal.  Since the technology is apparently used in the beauty industry already, it is unlikely that its application will leave insightly scars and the likes.

I look forward to seeing a technology like motorized microneedle arrays in conjunction with self-delivering RNAi trigger formats being used in the clinic.  Initially, the technology is most amenable to applications where the focus is on locally defined areas such a skin parts prone to blistering.  However, taking advantage of imaging technologies and 3-D printing, I envision a future in which the technology would also be possible to treat large areas of the skin, if not the entire body surface.  As TransDerm illustrates, combining the capabilities of existing technologies from disparate areas often enables the biggest advances.

RXI-109 for dermal anti-scarring now available under the ‘Specials’ provision in the EU

Anybody that has gone to a dermatologist knows how blurred the lines between medical and cosmetic applications have become when it comes to the skin (cosmeceutical concept).  Taking advantage of the regulatory grey zone, it is skin applications that are leading the charge in the commercialization of RNAi gene silencing in WoMan.  Following a claimed treatment for skin blemishes marketed as Britena Whitening & Anti-blotch Cream by Biomics (partnered with Benitec on HepB), it is now RXi Pharmaceuticals that has signed a distribution agreement for its dermal anti-scarring drug candidate RXI-109 with Ethicor. 

The goal of this arrangement is to drive early sales based on an exception of European drug legislation that allows for the use of experimental drugs prior to proper marketing authorization.  All it apparently takes is a judgment call by the treating physician.  I can see the point of this ‘Specials’ provision for severe, orphan diseases as a form of compassionate use when there is intriguing early clinical evidence of efficacy and safety, but for an anti-scarring treatment, mmh...you can easily see how consumers willing to take risks in the quest for beauty will make their physician give them an injection of the stuff.

But then again, when you see how much unproven, potentially harmful potions and lotions are being sold on the cosmetics market, it is hard to argue why you should make an exception with RNAi as long as care is being taken that somewhat riskier (depending on chemistry) systemic exposures remain low and yours truly does not have to pay for it via increased insurance premiums.  I guess my biggest problem with all this is that the company distributing RXI-109 calls itself ‘Ethicor’ just as I get nervous when somebody starts a sentence with ‘to be honest’ and what follows is more often than not a lie.

Update on Pachyonychia Congenita RNAi Trial

Last Friday (20 January, 2008), I was fortunate to attend a talk given by Dr. Kaspar from TransDerm about their Pachyonychia Congenita RNAi program (see 22 January, 2008 Blog). Although the talk covered essentially what is known from publications and previous presentations, there are some details that I think give interesting insights into such an orphan drug development program that underline the potential cost savings through RNAi Therapeutics.

First of all, the phase Ib volume-dose escalation trial involves just one patient with a mutation in a keratin gene that is specifically targeted by the siRNA. If there are no adverse events, then the other five patients in the world with PC known to harbor the same mutation may be given the treatment as well. Although animal genetic data would suggest that knockdown of the wild-type allele should be compensated by the expression of other keratins, the consortium apparently wanted to ensure the highest level of safety by tailoring the treatment to the specific mutation. Which platform technology other than RNAi/gene-based medicines would financially allow for the initiation of a clinical trial for such a minute patient population?

The work at TransDerm leading to the initiation of this clinical study was essentially accomplished by the three employees of this tiny company. This is quite impressive given the scientific and regulatory hurdles one has to overcome to actually enter the clinic. RNAi apparently was helpful in that regard with Dr. Kaspar noting that during his scientific life he had worked with a number of technologies, and well, his experience was a bit mixed. But with RNAi all this has changed. RNAi has exceeded all of his expectations, and judging from his enthusiasm I had every reason to believe him.

Of course, delivery remains an issue, and the talk confirmed that the current trial involves intradermal needle injection of unformulated and unmodified siRNAs. Based on staining results, a lower estimate of the cells that take up the siRNA administered this way is around 20%. This and the fact that the spread of the siRNA is limited to a small area around the injection site in my mind will limit the therapeutic potential of this treatment strategy. Although the observation that such unformulated siRNAs get taken up at all in vivo (but usually not in tissue culture) is very encouraging and should stimulate research into the relevant biological uptake pathways that could lead to advances in RNAi delivery, current conjugation or encapsulation technologies should be able to significantly increase knockdown efficiency following such intradermal injection. As I mentioned before, the company is also working on a lipid-alcohol-based “gene cream” which may also have applications for other tissues. Another promising approach the company is pursuing is the use of 1mm-spaced needle arrays which are sufficiently short so that the tips would not reach the nerves in the skin, thus rendering the treatment painless.

According to Dr. Kaspar, there are 10,000 monogenic congenital diseases, ~20% of which affect the skin. Due to potential cost savings and specificity, RNAi, the poster child of the personalized medicine revolution, is ideally suited to transform the treatment of many of such orphan disorders.

RNAi Clinical Trial Started for Genetic Skin Disorder Pachyonychia Congenita

This week saw the initiation of a clinical trial investigating the use of siRNAs for the treatment of Pachyonychia Congenita (PC). The study is sponsored by the PC Project, a public charity devoted to serving the needs of the ~500 people worldwide affected by this rare genetic disorder, and led by Sancy Leachman from the University of Utah. The siRNA itself, TD101, is the fruit of a collaboration between the International PC Project and the Californian RNAi-focused start-up TransDerm with ties to the shRNA-company SomaGenics.

PC is one of the dominant-negative epithelial fragility disorders caused by the mutation of a keratin gene. There are over 20 keratin genes in the human genome, with two different types forming heterodimers in the assembly of the keratin intermediate filaments which are important for the structural integrity of epithelia such as of the skin. A mutation in one of the dimerization partners may disrupt the organization of the filaments, which in the case of PC results in thickening of nails and skin of the palms and soles. Apart from the obvious cosmetic consequences of the disease, pain due to stress on the palms and soles is a major symptom of the disease for which no specific treatments exist.

Previous studies suggest that a 50% reduction in the mutant protein should get rid of the molecular aggregates caused by the filament assembly defect, and even the total loss of the mutant keratin should be well tolerated due to the expression of compensatory keratins. RNA- and DNA-based treatments offer the best opportunity for a specific treatment as they can address keratins directly and should be able to distinguish between mutant and wild-type genes. After considering a number of technologies, the PC Project has chosen RNAi due to its potential specificity, relative straight-forward mechanism of action which should accelerate drug development.

This is supported by two related publications from the consortia (Hickerson et al. (2007): Single-Nucleotide-Specific siRNA Targeting in a Dominant-Negative Skin Model; Smith et al. (2008): Development of Therapeutic siRNAs for Pachyonychia Congenita) which demonstrate the ability of siRNAs to specifically down-regulate the mutant keratin with an almost complete reversal of the aggregation phenotype. This is shown in both in vitro tissue culture and in vivo mouse footpad models involving the use of keratin-reporter genes.

While these studies provide proof-of-concept of RNAi for the treatment of PC, the in vivo studies were limited to the knockdown of reporter genes co-transfected with the siRNAs, rather than targeting endogenous genes in the skin epithelium. It is difficult to judge from such studies the overall delivery efficiency since cells that take up the reporter gene are likely to take up the siRNA as well, even when most of the remaining cells have not taken up any of the siRNA. (note: similar reservations apply to the recently initiated phase I RNAi studies for HBV by Nucleonics which are also heavily based on co-transfection experiments.)

The mouse models involved intradermally injected, unmodified siRNAs in a simple PBS buffer. While TransDerm is working on a topical lipid-based “gene crème”, it appears that at least for the phase Ib studies the siRNAs, targeting one of the more common mutations in PC, will be intradermally delivered by needle injection. One concern here is whether the area that such a delivery method can reach is sufficient to alleviate the symptoms of PC. Another is that using unmodified siRNAs in PBS over stabilized formulations will unnecessarily sacrifice some of the efficacy as well as necessitate more frequent treatment. Clearly, there is room for improvement, but the phase Ib studies should provide precious clinical data and inform future treatment strategies.

In addition to more sophisticated delivery methods and formulations, such strategies could also involve DNA-directed RNAi where autologous skin transplants and/or stem cells of the skin are stably corrected ex vivo, e.g. by lentiviral transduction of shRNAs, and then reapplied to the patient. This may be preferable to repeat needle injections.

TransDerm hopes that providing proof-of-concept for an RNAi Therapeutic of such a rare skin disorder with little commercial potential will be a stepping-stone for addressing much larger patient populations. This illustrates once more the value of proof-of-concept studies in general for RNAi since the ability of deliver one RNAi treatment could be rapidly expanded to delivering many more RNAi Therapeutics for the same tissue.

It will be interesting to see whether TransDerm, which by the way has opted for a classical Tuschl siRNA rather than a synthetic shRNA like SomaGenics is using, will be successful not only in the clinic, but also in their corporate strategy. In the absence of any RNAi core IP, the development of RNAi delivery technologies for the skin is probably their best shot in creating shareholder value.

PS: I wonder what happened to Sirna Therapeutics’ dermatology program which in their first development program aimed for siRNA-mediated permanent hair removal. The prospect of RNAi dermatology products is certainly exciting, especially if it were possible to develop convenient delivery methods such as topical cremes.