Merck Double-Knockdown Strategy to Ameliorate Toxicity from Mtp and ApoB Inhibition

The signs are that the next RNAi Therapeutics metabolic/cardiovascular disease candidate will be a dual-targeting one.  While the initial attempts in this area were directed at specifically reducing the well-known cardiovascular risk factor LDL-cholesterol (preferred targets: ApoB and PCSK9), it has become quite tempting to exploit the rare opportunity offered by RNAi Therapeutics to target multiple gene targets with just one formulation to both broaden the therapeutic benefits in patients that typically suffer from a plethora of metabolic dysfunctions (obesity, insulin resistance, and hypercholesterolemia to name a few) and to balance the adverse effects that may result from inhibiting certain targets.

Most notable among the latter is the liver fat accumulation following ApoB knockdown.  Clinical studies with ISIS Pharmaceuticals’ antisense compound mipomersen/KYNAMRO have clearly evidenced such liver fat accumulations which were often accompanied by increases in liver enzymes, general indicators of liver toxicity.  These results further are corroborated by similar clinical observations with the small molecule lomitapide by Aegerion targeting microsomal triglyceride transfer protein (Mtp) which acts essentially at the same stage as ApoB in packaging triglycerides and cholesterol for transport out of liver cells into the circulation.  Aegerion obtained this drug candidate from BMS via UPenn as BMS did not want to further develop this compound due to these safety risks.

Both mipomersen and lomipatide have completed phase III studies and new drug applications for approval in the rare genetic disease homozygous familial hypercholesterolemia (hoFH), and in the case of mipo also for severe heterozygous FH have been submitted to the FDA and EMA.   In terms of therapeutic profile, mipomersen seems to have the edge as, being a phosphorothioate antisense compound, it preferentially accumulates in the liver.  Consequently, it does not cause the side effects resulting from the intestinal inhibition of this pathway that  have been observed with small molecule lomatipe (note: SNALP-delivered RNAi Therapeutics should have similar benefits over small molecules).  Moreover, mipomersen not only lowers LDLc, but also moderately reduces the independent cardiovascular risk factor Lp(a).   Although not a prospective primary goal of mipomersen clinical development, incidental positive findings like this one can go a long way in having regulators take a benevolent look at drug candidates.  This can be seen in the related obesity space where one of the attractive benefits of Arena Pharmaceutical's lorcaserin is that it lowers blood glucose levels.

Obviously, there should be plenty of potential gene targets involved in triglyceride synthesis and utilization/oxidation that could be exploited to concomitantly lower triglyceride content in ApoB/Mtp-inhibited livers while maintaining LDLc-lowering.

Merck Tests ApoB and Mtp Knockdown, Finds Mtp-DGAT Co-Knockdown Promising

Tep and colleagues from Merck published a paper on a study that tested whether an RNAi co-knockdown strategy could be implemented to alleviate the liver fat accumulations due to Mtp and ApoB inhibition.  To be clear, Merck did not state that they have firm intentions of developing such a co-knockdown strategy, but nevertheless noted that such a strategy would have the advantage of  not having to ‘add[ ] a novel compound on top of an approved drug’ and that dual-targeting RNAi Therapeutics candidates are already in clinical development, therefore paving the regulatory path (see ALN-VSP02, and TKM-EBOLA).

In a first step the scientists confirmed the liver fat accumulation following Mtp and ApoB siRNA knockdown.  Not only were they of similar magnitude, the effects of the two knockdowns where essentially the same in almost every other investigated regard.  Notably, there was no reduction in liver fat accumulation following prolonged siRNA treatment as one might have expected based on claims by ISIS Pharmaceuticals of liver fat normalizations with time, but widespread changes in the expression of lipid-related genes were nevertheless observed- this time consistent with claims by ISIS Pharmaceuticals.

Among the genes that were downregulated following Mtp siRNA treatment, presumably as a result of negative feedback, was DGAT2, a key enzyme in triglyceride synthesis that is also thought to represent an important regulatory node in lipid metabolism (e.g. by promoting fatty acid oxidation).  Reasoning that further reducing DGAT2 with liposomally formulated siRNAs may lead to a measurable reduction in liver fat, they then co-formulated the Mtp siRNA with a DGAT2 siRNA and injected them into mice.  Indeed, this resulted in not only the expected LDL-cholesterol reduction, but liver triglyceride increases were mitigated.  According to data not shown, it was claimed that the same beneficial effect could not be observed with an ApoB-DGAT2 siRNA combination, suggesting that Mtp may be the better target for co-knockdown strategies.

It should be added, however, that the day 14 time-point data these conclusions were based on were somewhat of an outlier as at this time the co-formulation with DGAT2 siRNA reduced the LDLc-lowering potency of Mtp knockdown.  On the other hand, the scientists report (also in data not shown) that they tested the co-inhibition strategy using DNA-directed RNAi and thus validated this conclusion.    Moreover, given the multitude of genes involved in lipid metabolism, the Merck scientists stated that the Mtp-DGAT2 co-knockdown is a proof-of-concept and that other targets besides DGAT2 are also being considered (especially Gpat1).

Given Tekmira’s interest in ApoB as a target and recent LNP work by Alnylam and their PCSK9 collaborators from UT Southwestern (Horton, Goldstein) on SCAP knockdown to alleviate hepatic steatosis and Alnylam's general interest in co-knockdown for metabolic applications, Merck will not be alone in their endeavor of finding an RNAi Therapeutic candidate that can do it all, LDLc lowering, triglyceride lowering, and more.  Given that SNALP technology would likely be used in such a clinical program, this could particularly benefit Tekmira.