Skin RNAi Delivery Advance by TransDerm

Santa Cruz skin RNAi Therapeutics company TransDerm, along with collaborators from Stanford University and Thermo Fisher, recently published a paper establishing proof-of-concept for a local delivery technology that may be the first clinically relevant skin RNAi delivery approach (Lara et al., 2012).  Until now, the field of skin RNAi has been hampered either by overly painful and locally restricted delivery methods such as single-point needle injections, in vivo model systems that represent rather glorified in vitro models (e.g. co-transfections of target gene with RNAi trigger), or magic topical creams that almost work too well for me to believe.  The new data on TransDerm's dissolvable microneedles in a clinically relevant model system look promising by comparison and warrant a first clinical evaluation.  Nevertheless, for more aggressive development efforts in a number of diseases, further improvements in their knockdown potency are needed to sufficiently reduce the risk of clinical failure due to lack of efficacy.

The study by Lara and colleagues combined two prior lines of research by TransDerm: self-delivering RNAi (sdRNAi) triggers that allow for cellular delivery without nanoparticle formulation, and dissolvable microneedles.  The sdRNAi triggers had been validated in keratinocyte cell lines and skin equivalents in vitro before (Hickerson et al. 2011), although at quite high, micromolar concentrations of the oligonucleotides. Despite of the fact that born-again RXi Pharmaceuticals only in 2010 announced a collaboration with TransDerm on their particular brand of sdRNAi triggers, sd-rxRNAs, the published TransDerm research involved the Accell sdRNAi trigger design by Dharmacon (part of Thermo Fisher), most likely cholesterol or related lipophilic siRNA conjugates with additional modifications to enhance stability.  The dissolvable microneedles meanwhile had first been tested in conjunction with siRNA and plasmid delivery in transgenic mice expressing reporter genes (Gonzalez-Gonzalez et al. 2010).  Proof-of-concept for knocking down an endogenously expressed (human) gene by dissolvable microneedle-mediated sdRNAi triggers remained to be established and was only now accomplished by the Lara et al. study.

Specifically, the study targeted the CD44 gene, a gene that is more or less uniformly expressed throughout the human epidermis.  This feature allowed for the evaluation of knockdown efficiency of an endogenously expressed gene throughout the epidermal layers, not just by RNA and protein-level analyses from lysates, but also detailed immunohistochemistry.  In what I consider the best in vivo model yet, the latest research involved full-thickness human skin that was xenografted onto the backs of (immunocompromised) mice.  For skin disorders such as single-gene dominant-negative pachyonychia congenita (PC) which is the lead program at TransDerm, this may be as good an in vivo model system you can get bar clinical trials or xenograft models with skin from patients (which in this case may not be feasible technically).  In this system, repeat daily microneedle administrations achieved ~50% knockdown of the CD44 target gene.

Dissolvable microneedle delivery technology

The three major obstacles in achieving efficient skin RNAi delivery are 1) the penetration of the tough outer layer of the skin consisting of dead skin cells (stratum corneum), 2) achieving sufficient tissue distribution, and 3) achieving functional uptake into the target cells.  Moreover, the delivery ideally would avoid stimulating pain receptors in the dermal layer of the skin as normal needles would do.  The microneedle approach could potentially overcome all these obstacles.  

The technology involves an array of micron-sized needles (see illustration with penny for scale) based on a solution of ~20% polyvinyl alcohol.  Polyvinyl alcohol is an FDA-approved ingredient for medical uses, including those involving systemic exposure.  For RNAi trigger delivery, it has the added advantage that hydrophilic molecules such as nucleic acids readily diffuse into it (loading stage) and that it readily dissolves upon exposure to biological milieus, thus releasing the active ingredient.

The needles obviously function to penetrate the stratum corneum thereby addressing obstacle 1.  Because of their limited depth of penetration, however, they do not cause the pain that normal intradermal needle injection does, something which turned out to be a major drawback of the phase I study in PC that TransDerm reported on 2 years ago (Leachman et al. 2010).  By appropriately spacing the microneedles, it should also be possible to achieve a homogeneous tissue distribution (challenge No. 2).  Finally, the sdRNAi trigger design would address functional cellular uptake (challenge No. 3).

Despite the promising data, the 50% knockdown efficiency indicates that there is a need for further improvement.  This could come from a denser spacing of the needles which would appear to be a relatively simple engineering problem and more efficient sdRNAi trigger designs.

Meanwhile, TransDerm is in the process of manufacturing GMP-grade microneedle in order to go back into the clinic next year for the PC indication.  According to Roger Kaspar, scientific founder and CEO of TransDerm, the company and their various collaborators seem to believe that the current ~50% knockdown efficiency is adequate for PC based on genetic evidence from a related disease (epidermolysis bullosa simplex, aka EBS).