Antisense Approaches — Mechanisms and Targets

An important area of our basic research is to understand the molecular mechanisms of antisense drugs. There are at least 12 known antisense mechanisms that can be exploited once an antisense drug binds to its target RNA. We have created proprietary chemical modifications to trigger many of these mechanisms for drug discovery.

An antisense mechanism is defined as the process in which an antisense drug works after it binds (hybridizes) to a target RNA forming a duplex. The formation of this duplex, or two-stranded molecule, prevents the RNA from functioning normally and from producing a protein. The majority of our late-stage antisense drugs in development bind to their target RNA and activate a cellular enzyme called RNase H. Progress in our research program is illustrated by our accomplishments in understanding RNase H. Our generation 2.2 antisense drugs are designed to increase the activity of RNase H, which should provide them with higher potency.

All of the antisense drugs in our pipeline bind to mRNAs and inhibit the production of disease-causing proteins through the RNase H mechanism described below. Recently the scientific community has discovered many new types of RNAs, including non-coding RNAs, such as microRNAs, which are involved in the regulation of protein production within the cell. Our antisense technology is uniquely suited to exploit these exciting new RNA targets, as we have done with microRNAs and Regulus Therapeutics

RNase H

The antisense mechanism that has been the main focus of our research is RNase H. This cellular enzyme is activated when antisense drugs bind to their target RNA and form a duplex. Upon activation, RNase H seeks out and destroys the target mRNA, inhibiting a cell's production of a specific protein.

We have cloned and characterized human RNase H and have effectively used that information to optimize the design of many of our antisense drugs. We will continue to advance our understanding of antisense mechanisms, including RNase H, in order to improve the pharmaceutical properties of our drugs.

In addition to our RNase H expertise, we are the leaders in understanding and exploiting all antisense mechanisms, including the RNAi mechanism.

RNAi

RNAi is an antisense mechanism that involves using small interfering RNA, or siRNA, to target an mRNA sequence. With siRNA, the cell utilizes a protein complex called RISC to destroy the mRNA, thereby preventing the production of a disease-causing protein. Isis' research led to one of the first scientific publications and key issued patents that address this mechanism.

We have a strong and growing intellectual property position in RNAi methodology and chemistries used to develop siRNA therapeutics. Our intellectual property led us to establish a strategic collaboration with Alnylam Pharmaceuticals in the development of double-stranded RNAi drugs.

We are building upon our research efforts with our partner Alnylam to include the development of single-stranded drugs that work through the RNAi pathway.

Alternative Splicing

To understand alternative splicing, it's important to understand how mRNA is created.

When mRNA is created from DNA, the entire DNA sequence is copied. This includes the sequences that encode for proteins and regions that are unnecessary for making proteins. Before mRNA can function, the regions that are unnecessary for making proteins must be deleted from the RNA strand. The natural process that removes these regions and re-forms the finished mRNA is called "splicing." Through the splicing process, the cell can create many diverse proteins from a single gene and splicing accounts for most of the diversity in proteins in the cell. In fact, of the approximately 25,000 genes in the human genome, 40% to 60% have alternative splice forms. Alternative splicing of proteins can result in the production of proteins that are involved in disease. In some cases, using antisense technology, we can direct alternate splicing to produce a protein critical for normal cellular function to correct for a genetic defect. Examples of applications of antisense modulation of splicing to treat genetic disease include, spinal muscular atrophy (SMA), thalessemia, cystic fibrosis and Duchenne’s muscular dystrophy.

We design antisense drugs to control splicing to make one protein versus another. In December 2009, we advanced the first antisense drug, ISIS-SMNRx, to enter our development pipeline that modulates splicing. ISIS-SMNRx is designed to treat the splicing disease, SMA, which is a neuromuscular disorder and the leading genetic cause of infant mortality. The discovery of ISIS-SMNRx resulted from a joint research collaboration between scientists at Isis and Cold Spring Harbor. In earlier published research, we and our collaborators at Cold Spring Harbor demonstrated the feasibility of using our antisense technology to control splicing for the treatment of SMA.

In addition to antisense mechanisms, we are also interested in the therapeutic opportunities of other RNA targets, such as microRNAs.

MicroRNA

MicroRNAs are small naturally occurring RNA molecules that are created inside cells. There are many different types of RNA that exist within the body, including mRNA. MicroRNAs are important because they appear to have critical functions in controlling processes or pathways of gene expression.

There are nearly 700 microRNAs that have been identified in the human genome, and these are believed to regulate the expression of approximately one-third of all human genes. Targeting microRNA to inhibit disease-causing pathways is an exciting development in RNA-based therapeutics.

To fully exploit the therapeutic opportunities of targeting microRNAs, we and Alnylam jointly established Regulus as a company focused on the discovery, development, and commercialization of microRNA-based therapeutics.

Learn more about Regulus