MAKE WAY FOR RNA BASED THERAPIES
The uphill battle of RNA therapeutics in the clinic continues despite extensive use in research. Recall from high school biology that RNA translates DNA code into a language ribosomes can understand in order to make proteins required by the cell.
Fighting the good fight are antisense, RNAi, and microRNA. These technologies may be tomorrow’s biotech sweetheart with their high specificity and relatively low manufacturing cost. In fact, chances are good that previously “undruggable” targets (that cannot be accessed by small or large-molecule drugs) are within arm’s reach. However, the main hurdle continues to be delivery—getting the RNA drug where it needs to be, in high enough concentrations, to be effective.
This article examines the similarities and differences between RNA therapeutics winding their way through the clinic and into the marketplace.
PUTTING THE SENSE INTO ANTISENSE
The on-again, off-again love affair with antisense therapeutics—that has been going on for over a decade—was renewed in 2013 thanks to the FDA approval of Isis Pharmaceuticals’ (Carlsbad, CA) antisense therapy Kynamro for the treatment of familial hypercholesterolemia.
Antisense drugs are short, synthetic pieces of nucleic acid whose sequence is complementary to the mRNA that codes for a disease-associated protein. When the antisense drug enters a patient’s cells, it binds to the disease-causing mRNA. This binding triggers an enzyme called RNAse H to destroy the antisense-mRNA duo (double-stranded RNA is seen as a mistake and destroyed). Without the mRNA, the disease-associated protein simply is not made—stopping malignancy in its tracks. Kynamro targets apolipoprotein B, a key component of LDL cholesterol, to lower cholesterol levels.
Over the past decade, the tumultuous love affair saw several antisense drugs fail due to lack of efficacy and drug delivery problems. By targeting the right tissues (the most promising targets are in the liver) and developing more stable formulations, Isis and others expect to see more success. Check out the antisense drugs in clinical development:
RNAI TO THE RESCUE
Like antisense, RNAi takes advantage of naturally occurring cellular pathways to target and destroy double-stranded RNA (dsRNA) to block the expression of a disease-associated protein.
Researchers introduce a double-stranded or “hairpin” shaped RNA to activate the pathway. DICER enzyme cuts up the hairpin to produce a “short interfering RNA” (siRNA). The siRNA binds to a second enzyme called RNA-induced silencing complex (RISC); the siRNA/RISC complex then attaches to a disease-associated mRNA. Now, double-stranded RISC destroys both RNA strands—siRNA and mRNA—and the disease is stopped. The RNAi therapeutics in clinical development are listed below:
MICRO RNA IN THE MIX
Antisense and RNAi are both synthesized in the lab and delivered to patients to decrease the expression of a disease-associated protein. MicroRNA (miR), on the other hand, is a type of dsRNA made by cells to regulate gene expression. Like RNAi, miR is processed by enzymes DICER and RISC into single-stranded RNA capable of binding to disease-associated mRNA with a complementary sequence. Since microRNAs only bind to one end of the mRNA, each miR is able to regulate multiple target mRNAs.
MiRs are noteworthy because their expression is significantly altered in many disease states—the patient either makes too many or too few. If the patient is not making enough of a particular miR, the therapeutic approach is to deliver so-called “miR-mimetics”— synthetic versions of the naturally occurring miR in diseases. The MicroRNA pipeline is summarized below:
REGULATING THE REGULOME
Enzymes such as RNAse H and DICER exist in our cells to regulate gene expression. Enter the regulome: the unknown world where the full complexity of these RNA-disrupting pathways is beginning to be deciphered. Stay tuned as RNA technology continues an avant-garde approach to disbanding disease—a promising story worth watching.
The landscape of RNA-based therapies, encompassing antisense, RNAi, and microRNA technologies, is a burgeoning field with the potential to revolutionize the treatment of diseases that have been traditionally difficult to target. While these therapies offer high specificity and lower manufacturing costs, challenges in effective drug delivery remain a significant hurdle. However, the FDA approval of therapies like Kynamro and ongoing clinical trials in all three RNA therapeutic categories indicate that we are on the cusp of a new medical era. As we continue exploring the regulome—the complex network of RNA-disrupting pathways—these RNA technologies may become the next frontier in effective, targeted treatments.
FREQUENTLY ASKED QUESTIONS
1. What are RNA-based therapies?
RNA-based therapies are treatments that utilize RNA molecules like antisense, RNAi, and microRNA to target and regulate specific genes associated with diseases.
2. How do antisense therapies work?
Antisense therapies use short, synthetic pieces of nucleic acid to bind to the mRNA that codes for a disease-associated protein. This binding triggers the destruction of the mRNA, effectively stopping the production of the disease-causing protein.
3. What is RNAi, and how does it differ from antisense?
RNAi, or RNA interference, also targets disease-associated mRNA but does so by introducing a “hairpin” shaped RNA that gets processed into “short interfering RNA” (siRNA). This siRNA then binds to enzymes that destroy the mRNA. Unlike antisense, RNAi uses a cellular pathway involving multiple enzymes like DICER and RISC.
4. What is microRNA?
MicroRNA (miR) is a type of double-stranded RNA produced by cells to regulate gene expression. Unlike antisense and RNAi, which are synthesized in the lab, miRs are naturally occurring and can regulate multiple target mRNAs.
5. What are the challenges facing RNA-based therapies?
The main challenge is effective drug delivery—ensuring the RNA drug reaches its target in sufficient concentrations. Stability of the RNA molecules and potential off-target effects are also concerns.
6. What is the regulome?
The regulome refers to the complex network of RNA-disrupting pathways in our cells. Understanding the regulome is crucial for advancing RNA-based therapies.
7. Are there any FDA-approved RNA-based therapies?
Yes, Kynamro, an antisense therapy for familial hypercholesterolemia, received FDA approval in 2013. Several other RNA-based therapies are in various stages of clinical trials.