Tuesday, September 3, 2013

Merck Paper Reveals Interest in GalNAc-targeted RNAi Therapeutics

Merck’s efforts have to be considered to be the strongest in RNAi Therapeutics among Big Pharma.  Its RNAi Therapeutics strategy so far, however, has consisted largely of trying to replicate the most promising technologies in-house. 

The latest publication by Merck RNAi scientists on the expression pattern of the asialoglycoprotein receptor (ASGPR1; Shi et al. 2013) confirms this as ASGPR is the target receptor of the two most advanced SNALP alternatives for gene silencing in the liver: the trail-blazing DPCs by Arrowhead Research (first use of GalNAc-targeted RNAi Therapeutics in Rozema et al.2007) and more recently the GalNAc-targeted siRNA conjugates by Alnylam (what these really seem to be will be covered in an upcoming blog...so stay tuned!).

Surveying ASGPR expression levels

When developing ligand-targeted therapeutics, it is important that the corresponding receptor is present on the target cell population.  Especially when targeting cancer, it can be difficult to find receptors that are not only present in large quantities, but also throughout the cell population.  

In the case of liver cancer (hepatocellular carcinoma/HCC), this question has occupied the RNAi field ever since the finding that SNALPs work really well for knockdown in normal liver due to uptake mediated by the LDL-receptor.  But is this mechanism also present on cells in liver cancer, a cancer of high unmet need where RNAi Therapeutics could have the biggest impact in oncology near- to midterm?
The Merck scientists set out to answer essentially the same question, but instead of interrogating LDL-receptor expression, they wanted to know about ASGPR expression on HCC cells.  Using tissue microarrays, they were able to test an impressively large set of 100s of tissues, including healthy human livers, liver cancer biopsies, and biopsies for other hepatic diseases such as viral hepatitis, chronic active hepatitis and cirrhosis.

Despite some apparent limitations with the tissue microarrays (e.g. normal liver samples were often marked as false negatives despite the known very high ASGPR expression level), the results seem to confirm that liver cancers in general have a tendency towards lower ASGPR expression and that inter-sample heterogeneity is comparatively large (some liver cancer samples had much more ASGPR expression than normal liver).  

This suggests that just as in the case in breast cancer where treatment decisions are often based on receptor expression levels, ASGPR-targeted liver cancer RNAi Therapeutics should also be combined with a companion diagnostics for ASGPR. 

For those curious about the status of ASGPR expression in chronic HBV since Arrowhead’s exciting ARC520 program for this indication involves a GalNAc-targeted melittin-like peptide, relax: livers infected with HBV express ASGPR just as well as normal livers.

Home-brew versus licensing

Based on the literature and conference presentations, it is reasonable to assume that Merck is well behind Arrowhead, and even Alnylam in developing GalNAc-targeted RNAi Therapeutics.  This begs the question, as it has in the case of SNALP, why does Merck not take a license or even acquire the original?

Of course, it always takes two for a deal, but given the financial capabilities of a Merck, if it wanted access, it could get it.

The likely explanation is that Merck's strategy in replicating technologies in-house is to first identify target technologies by validating them and then to develop viable IP workaround solutions.  In fact, in the apparent absence of a broad gate-keeping patent estate around GalNAc-targeted therapeutics, ASGPR has been a recognized drug target receptors well back into the 90's, there should be relatively little restrictions on the use of GalNAc per se.

Instead, a competitive advantage is largely gained through specific know-how like how to best link the GalNAc ligand to the oligonucleotide payload and synthesize the molecules cost effectively. The latter issue came to my attention recently when access to 'proprietary process for manufacturing GalNAc conjugates' from Alnylam was mentioned as the top corporate highlight in the quarterly update provided by Regulus Therapeutics.

For the aficionados, the triantennary GalNAcs as practiced by Alnylam are much more costly to synthesize than single GalNAcs as in the case of the liver-targeted DPC versions (where high affinity through multivalency is achieved by having multiple single GalNAcs along the DPC).

This situation is not unlike other areas in the drug development industry.  Take for example antibodies where, despite the various patent battles, there have been a number of commercially viable platforms based on specific libraries or optimization methods.  Nevertheless, despite this apparent freedom-to-operate, the way by which Big Pharma ended up gaining access to monoclonal antibodies was not by way of successfully developing them in-house, but by acquiring them.

I don't expect this to be any different in RNAi Therapeutics.

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By Dirk Haussecker. All rights reserved.

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