Sunday, May 6, 2007
Preventing Heart Disease with RNAi Therapeutics
An RNAi Therapeutic alternative, due to the ability to target almost any gene, is particularly interesting because of well validated targets, but which have proven refractory to targeting by the existing drug classes. These genes are not directly involved in cholesterol synthesis and targeting them should be synergistic with statins. Currently the most interesting gene targets are ApoB100 and PCSK9. ApoB100 is the sole protein component of “bad” LDL cholesterol and is produced in the liver. Notably, targeting ApoB100 by RNAi in the using cholesterol-conjugated siRNAs in 2004 by Alnylam scientists was also the first demonstration of gene silencing following systemic administration of siRNAs. This study not only showed considerable reductions of ApoB mRNA and protein, but also the hoped for decrease in blood LDL cholesterol. I should add that clinical data from phase I and II trials conducted by ISIS Pharmaceuticals using antisense oligo technology further document the promise of ApoB100 as a target for hypercholesterolemia. It will be interesting to follow their further clinical progress, but I expect siRNAs to do even better, because of increased specificity and potency thus allowing for lower amounts of nucleic acids to be administered.
Two years later after the demonstration of systemic RNAi in mice, Alnylam scientists then reported even enhanced ApoB-100 silencing and improved lipid profiles in monkeys, this time using liposomal formulations originally developed by Protiva Biotherapeutics. These and similar liposomal formulations have proven to be very efficient for liver gene knockdown in general and are now being pursued by a number of companies in preparations for the first systemic RNAi clinical trials. Unfortunately, however, their promise has also led to legal haggling as to who owns the IP behind this delivery technology. Companies involved in this dispute involve Protiva, Inex Pharmaceuticals, and Sirna Therapeutics/Merck and I hope that legal issues will not do further damage to the development of this promising delivery technology. No matter who owns the commercial rights to the technology, Protiva scientists have to be credited with this major achievement.
Interestingly, despite their publication record on ApoB100, Alnylam decided to target PCSK9 for the treatment of hypercholesterolemia. Although I cannot exclude that this move is partly due to a deal with one of their collaborators in siRNA delivery, PCSK9 has a lot riding for it. In fact, they are pursuing this program in collaboration with scientists from UT Southwestern Medical Center that arguably are world-leading in the genetics of hypercholesterolemia. PCSK9 itself is a protease that degrades LDL-receptors (LDL-R). LDL uptake by the liver is important for clearing LDL in circulation and it is expected that increasing LDL-R levels by suppressing their inhibitors should lower LDL cholesterol. Indeed, data presented at this year’s Keystone Meeting support this thesis. However, it should be kept in mind that many drug development projects fail, not because the drug fails to reach its target, but because of side-effects. Side-effects are a particularly important consideration for drugs that have to be taken chronically as is often the case for hypercholesterolemia. So one of the major questions here is whether long-term downregulation of PCSK9 can have adverse consequences. Here, the genetics of PCSK9 are compelling: Naturally occurring mutations in the human population that increase PCSK9 activity have been shown to increase LDL and lead to hypercholesterolemia, while those that inactivate it lower LDL dramatically- without any obvious detrimental consequences! Of course, compensation mechanisms cannot be excluded, but this is probably as good as you can get with choosing a target based on human genetics.
In summary, due to the availability of excellent “non-druggable” targets and the ability to knockdown genes in the liver with current delivery technologies, RNAi Therapeutics are a promising strategy for treating hypercholesterolemia. Alnylam is expected to initiate phase I studies in the second half of this year, and I would not be surprised to see further studies being initiated in the near future by Protiva or Inex (mere speculation here though). The major obstacle for these trials that I see are side-effects due to the liposomal formulations, and my advice would be to carefully characterise them in animal models before committing to phase I instead of simply bowing to investor expectations.
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