Gene therapy has come a long way since it first emerged in the 1990s. Despite a few initial treatment successes, research slowed and then mostly halted by 2000 due to safety concerns, as several children receiving early gene therapies for one form of severe immunodeficiency syndrome developed cancers, and one teenage boy died in 1999 after receiving a gene therapy for ornithine transcarbamylase (OTC) deficiency, a serious metabolic disorder.
By 2010, however, gene therapy research was back in force as researchers improved the existing viral vectors and added genetic promoters and control elements to their payloads to better target and govern the timing and level of gene expression. In 2017, the U.S. Food and Drug Administration approved the first ex vivo gene therapy — Novartis’ Kymriah, a cell-based therapy for B-cell acute lymphoblastic leukemia (ALL). Since then, the FDA has approved several gene therapies, both for cancer and for rare diseases such as Spark Therapeutics’ Luxturna for Leber congenital amaurosis, bluebird bio’s Zyntelglo for beta thalassemia and Skysona for cerebral adrenoleukodystrophy, Novartis’ Zolgensma for spinal muscular atrophy, and CSL Behring’s Hemgenix for hemophilia B. The Alliance for Regenerative Medicine predicts that at least five additional gene therapies for rare diseases could reach the U.S. market in 2023, including treatments for sickle cell disease, Duchenne muscular dystrophy (DMD), and hemophilia A.
Despite development successes and FDA approvals, viral vectors used to deliver these products – adeno-associated virus (AAV), lentivirus, herpes, and adenovirus – have been associated with issues limiting their broad use. Each has been associated with safety risks, which in the most severe occurrences resulted in patient deaths. Also, therapies employing these vectors are complex, time-consuming, and costly to develop and manufacture. Leveraging current viral vectors has also constrained the amount of genetic information that can be delivered, thus limiting the number of diseases that may be treated using current technology For example, the maximum payload size for current AAV vectors is 4.7 kilobases (kb), whereas the size of the full-length dystrophin gene (the faulty gene in DMD) is over 11kb. Finally, many gene therapies employing viral vectors can only be dosed once, as patients may develop an immune response to the vector (or in some cases, already have natural immunity to it), which can lead to adverse reactions to the treatment.
Challenges notwithstanding, many new gene delivery strategies are under development, including both viral-based and non-viral approaches.
Improving Viral Vectors
For example, several companies have made advances that increase the size of the gene that can be delivered using viral vectors, thus potentially enabling treatment for a greater number of conditions. Splice Bio is developing a method that uses two AAV vectors to deliver genes too large for a single AAV. Partial genes are packaged with auto-processing domains known as split inteins that are capable of carrying out multi-step biochemical reactions to reconstitute the intended full-length protein in vivo. Vector Biopharma is employing a “gutless” AAV, containing only the therapeutic payload and no viral genetic information that is engineered with “adapter” ligands that direct the vector to one or more specific cellular targets. The company says its next-generation AAV vector can carry genes up to 36kb, allowing the delivery of larger genes than first-generation AAV vectors. Moreover, Vector’s gutless engineered AAVs are non-immunogenic, alleviating some safety concerns and potentially enabling re-dosing. Outside of AAV vectors, Replay has developed a herpes simplex virus (HSV) vector capable of delivering payloads of around 40kb in size and is working on a second HSV vector capable of delivering even larger genes of up to 150kb.
In parallel, other next-generation viral vectors are being developed to improve gene targeting. For example, Apertura Gene Therapy uses customized AAV capsid vectors engineered with chosen characteristics for reaching hard to treat tissues like kidney, liver, and the central nervous system. Improving targeting is also a goal of Homology Medicines, which uses a suite of 15 proprietary AAV vectors based on a unique family of AAVs found in human stem cells (AAVHSCs) that can deliver genetic material throughout the body, including across the blood-brain barrier.
Non-Viral Delivery Approaches
Other companies are seeking to avoid the limitations of viral vectors by entirely foregoing their use for gene delivery. Instead, they are pursuing non-viral technologies aimed at broadening the types of conditions that might be treated with gene therapies, as well as improving targeting specificity for greater efficacy and safety, and lowering the cost and complexity of their therapeutics manufacture and use.
For example, companies including Poseida and Beam Therapeutics are employing lipid nanoparticles (LNPs) for delivery of their gene therapies. LNPs have long been used for drug delivery and are relatively easy and affordable to manufacture. They have a larger transgene cargo capacity than standard viral approaches, enabling the potential treatment of more conditions, and are also non-immunogenic, offering improved safety and potential for redosing.
Code Biotherapeutics is developing a delivery vehicle based on synthetic DNA strands linked to form a scaffold. When combined with a targeting moiety, the scaffold can be directed to a particular tissue or cell type, bringing along a genetic payload with no size constraints. The company has demonstrated that its scaffolds can be designed to enter muscle cells to treat DMD, and can delive either the entire dystrophin gene or a gene encoding a smaller microdystrophin, both implicated in DMD. Code has also directed its scaffolds to alpha/beta cells in liver, a target for type 1 diabetes. The company says the exceptional tissue and cell-targeting specificity of its delivery scaffolds eliminates the need for high doses and minimizes both dose-related toxicity and off-target effects. Code has also shown that their DNA delivery scaffolds are non-immunogenic, potentially enabling re-dosing. In addition, manufacture of the scaffolds is simple and readily scalable, allowing bulk production of the scaffold and storage for final customization via the attachment of relevant targeting molecules and therapeutic nucleic acid-based constructs.
Xalud Therapeutics is developing a non-viral, DNA-based gene therapy platform to treat conditions caused by pathologic inflammation, such as osteoarthritis. The company uses a DNA plasmid that is taken up by targeted cells and continuously expressed, without any genome integration, and which can be re-dosed without safety concerns. The company’s first therapeutic candidate is designed to deliver IL-10, which when injected locally at sites of disease in the knee, hip or shoulder, works as an anti-inflammatory and pain reliever by reducing pro-inflammatory mediators.
Rather than pursuing a nucleic acid-based approach to delivery, Sonothera is employing ultrasound-guided microbubbles for the targeted delivery of diverse genetic medicines. Its technology employs long-used ultrasound enhancement agents and FDA-cleared ultrasound hardware, along with proprietary acoustic profiles, that allow genetic payloads to be accurately delivered through transient pores in a targeted cell’s membrane. Given the proven safety of the ultrasound agents and widespread availability of ultrasound equipment, Sonothera believes its technology could significantly lower the cost and expand the number of patients who could receive gene therapies.