of targets that antibodies can address is typically limited to extracellular protein targets. However, the majority of protein targets are located inside the cell, making them undruggable by antibodies.
Nucleic acid therapeutics represent a treatment approach differing in many important ways from small molecules and biologics. Nucleic acid therapeutics are based on the well-established scientific understanding that DNA in the nucleus of cells is converted into an intermediate molecule, called messenger RNA, or mRNA, that serves as the template for making proteins. Therapeutic gene regulation is the use of nucleic acid therapeutics to modulate the production of target proteins by changing the amount of mRNA that is converted to protein, thereby providing an approach to treating diseases at their genetic origin. Our SNAs are a type of nucleic acid therapeutic.
We believe the development timeline for nucleic acid therapeutic candidates will be shorter than that of small molecules and antibodies. Nucleic acid therapeutics can be directed against most mRNA, including the mRNA of proteins that cannot be targeted by small molecules or antibodies. Due to the detailed knowledge of mRNA sequences in humans, nucleic acid therapeutics can be engineered to be specific to a region of an mRNA sequence while interacting minimally with all other mRNA sequences. Moreover, due to the well-defined length and composition of mRNA sequences, a relatively small set of rationally designed therapeutic candidates, usually hundreds, can be synthesized and tested for activity against an mRNA target. This is in contrast to the small molecule drug development process that requires a much larger number of candidates to be screened.
Challenges in developing nucleic acid therapeutics
Significant progress has been made in the development of nucleic acid therapeutics. As of the fourth quarter of 2017, four nucleic acid therapeutics were being marketed and many were in various stages of clinical development. However, we believe there are ongoing technical challenges in the nucleic acid therapeutics field. Nucleic acids are molecules that, when administered without proper formulation, encounter a number of barriers to their bioavailability, biodistribution, and desired biological activity. These challenges have often been met by chemically modifying the oligonucleotide and by encapsulating or complexing it with a lipid or polymer carrier. Despite these advances in the delivery of oligonucleotides, the biodistribution of these molecules remains a challenge since oligonucleotides typically accumulate in the liver after subcutaneous or intravenous administration, thereby limiting their primary application to diseases of the liver. In contrast, the results of our Phase 1 clinical trial of AST-005 demonstrated the potential therapeutic application of our gene regulatory SNAs to modulate target mRNA expression after topical application to psoriatic lesions. Further, in an array of experiments, we have demonstrated that SNAs, administered locally without encapsulation or complexation, enter cells and organs. We believe the local administration of our gene regulatory SNAs will potentially enable safe and efficacious therapeutic applications to organs beyond the liver.
Overview of immuno-oncology as a therapeutic modality
In healthy individuals, the immune system fights off pathogens, such as bacteria and viruses. The immune system should also recognize cancer cells as foreign and eliminate them. However, cancers present a challenge because they have developed strategies to resist detection and clearance by the immune system. Immuno-oncology approaches help the patient’s immune system identify a cancer as foreign and stimulate a tumor-clearing immune response. One of the greatest benefits of the immuno-oncology approach is the continuous, durable anti-tumor response that can be achieved long after discontinuation of treatment.
Current immuno-oncology therapeutic approaches generally fall into three broad categories. First, there are approaches that stimulate the immune system to detect and eliminate tumors. Examples include cytokines and toll-like receptor, or TLR, agonists. Second, some therapeutics make a cancer more readily visible to the immune system. These therapeutics include checkpoint inhibitors, such as those that target CTLA4, or cytotoxic T-lymphocyte-associated protein 4, PD-1, and PD-L1, or programmed death-ligand 1. Third, there are adoptive cell transfer therapies, including dendritic cell vaccines and chimeric antigen receptor T-cells, or CAR-Ts, that direct the immune system to target a specific type of cancer.