In a recent review published in Gene Therapy, a group of authors explored the progress and persistent hurdles in gene therapy for inherited blood disorders, malignancies via chimeric antigen receptors (CAR)-T cells, and varied diseases treated with in vivo adeno-associated virus (AAV) vectors, addressing the transformative cures and the economic and manufacturing complexities involved.
Study: Successes and challenges in clinical gene therapy. Image Credit: metamorworks/Shutterstock.com
Further research is needed to enhance the safety, efficacy, and long-term durability of gene therapies while overcoming immunological challenges and optimizing delivery vectors for a broader range of diseases.
Overview of hematopoietic stem cell transplantation (HSCT)
Inherited blood cell diseases, affecting the production or function of blood cells, were the first to be targeted and treated using gene therapy. These diseases are caused by monogenic disorders, including hemoglobinopathies like sickle cell disease and thalassemia, inborn errors of immunity (IEI), lysosomal storage diseases, leukodystrophies, and conditions compromising hematopoietic stem cell (HSC) function.
A cure for these diseases can be achieved by transplanting normal HSCs from a healthy, matched donor, allowing the recipient's bone marrow to produce the required blood cells.
Advances in tissue typing, conditioning regimens, and supportive care have improved the outcomes of HSCT over the decades. However, the procedure's success is limited by the availability of matched donors and potential immunological complications.
Hematopoietic stem cell gene therapy (HSCGT)
HSCGT represents an evolution in treating inherited blood disorders, utilizing the patient's own HSCs modified with either an added normal gene copy or a corrected gene through editing techniques.
The process, involving the ex vivo modification of HSCs and their subsequent reinfusion, has demonstrated efficacy for an increasing number of disorders. Some therapies recently gained Food and Drug Administration (FDA) approval.
Triumphs in treating blood cell disorders
Severe combined immune deficiency (SCID) marked the first significant success in gene therapy, offering treatment where matched sibling donors are unavailable. Lentiviral vectors are safer and more effective than earlier gamma-retroviral vectors, reducing complications and improving immune reconstitution in patients.
Gene therapy has also advanced in treating hemoglobinopathies like β-thalassemia and sickle cell disease, with novel vectors and gene editing techniques mitigating disease severity and improving patient outcomes.
Challenges and opportunities in gene therapy
Gene therapy shows promise but has safety concerns, like genotoxicity and leukemia links, although new vectors are safer. Gene editing offers precision but needs careful evaluation. Making gene-modified HSCs is complex but still advantageous over allogeneic HSCT despite the risks of conditioning chemotherapy. Great effort is going into the search for safer alternatives.
Patient variability in engraftment calls for protocol refinement. High costs of HSCGT, similar to allogeneic HSCT, cover extensive medical processes, necessitating optimization for better outcomes and cost-efficiency.
Immuno-oncology: the rise of cell-based therapies
Immunotherapy has joined the ranks of chemotherapy, radiation, and surgery as a primary cancer treatment modality. It encompasses drugs, protein biologics, and now potent cell-based therapies. These therapies engineer immune cells to act against cancer, particularly through antigen-specific receptors like T cell receptors (TCRs) and CARs, redirecting T cells against tumor cells.
Redirecting T-cell specificity
TCRs leverage the natural antigen specificity of T cells. Isolation and expansion of TILs, or transgenic introduction of TCRs into non-tumor-specific T cells, have augmented anti-tumor responses.
Technologies like high-throughput screening have further refined TCR targeting. Conversely, CARs, which are synthetic constructs, can recognize antigens without Major Histocompatibility Complex (MHC) involvement and have been successfully used in T cells and other immune cells.
Advancements and FDA approvals
The FDA approval of KymriahTM for B-cell acute lymphocytic leukemia and subsequent approvals for other CAR-T therapies against Cluster of Differentiation 19 (CD19) and B-Cell Maturation Antigen (BCMA) marked a significant milestone. These treatments are now being tested as first-line therapies, expanding their impact.
Challenges in efficacy against solid tumors
While CD19 and BCMA CAR-T cells are FDA-approved, challenges remain in extending cell-based therapies to other malignancies and solid tumors. Efforts are underway to optimize CAR constructs and the biology of immune cells for broader applicability.
Enhancing T-cell potency through genetic modification
The therapeutic potential of CAR-T cells depends on precise targeting, coverage of tumor antigens, and robust expansion.
Engineering efforts focus on receptor design and genetic modifications to optimize these cells, with advancements in Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) technology aiding in the identification of genes that can be edited to improve T-cell function.
Manufacturing and clinical translation hurdles
The manufacturing of CAR-expressing cells faces constraints with viral vectors and the high cost of clinical-grade vectors. Non-viral gene delivery methods are being researched to circumvent these challenges. The personalized nature of autologous therapies and the damage from prior treatments add to the complexity of cell product manufacturing. Allogeneic therapies offer an alternative, with the potential for higher quality control and pre-manufacturing, but response durability is a concern.
Introduction to AAV therapies in gene therapy
Gene therapy harnesses the ability of viruses to infect cells and deliver genetic material. Among these, recombinant AAVs are gaining traction due to their ability to efficiently transport genes with minimal immune response and specificity to target tissues. AAVs, discovered as incidental contaminants in adenoviral preparations, are particularly suitable for gene therapy as they cause no known diseases.
The AAV cargo configuration
Gene therapy applications necessitate the replacement of AAV's native genome with a therapeutic expression cassette containing the gene of interest. This cassette, bound by two Inverted Terminal Repeats (ITRs) and inclusive of a promoter and poly A signal, allows for targeted gene expression by selecting tissue-specific promoters to mitigate undesired immune reactions.
Capsid customization
The AAV capsid determines the vector's affinity for particular tissues. By exploiting the diverse range of AAV serotypes, each with distinct receptor and co-receptor interactions, gene therapies can be tailored to target relevant tissues specifically, enhancing efficacy and minimizing off-target effects.
AAV packaging process
In the manufacturing process, essential viral replication and packaging genes are supplied externally in a cell line, such as HEK293 or sf9, with purification steps following to prepare the AAV for therapeutic use. Contract Development and Manufacturing Organizations (CDMOs) often undertake this task to produce vectors meeting Good Manufacturing Practice (GMP) standards.
Successes in AAV gene therapies
The FDA has greenlighted three AAV therapies for retinal disease, spinal muscular atrophy type I, and hemophilia B, respectively. These therapies have shown transformative results, from restoring vision to enabling movement in previously immobile children.
In Europe, a particular therapy offers a conditional solution for hemophilia A, improving patients' quality of life by significantly reducing the need for factor VIII. Another has followed with a similar approach for hemophilia B, underscoring the potential of AAV therapies in tackling complex genetic disorders.
Challenges facing AAV therapies
A major obstacle is the immune system's reaction to AAVs, which can preclude re-administration of the therapy. A significant portion of the population carries pre-existing immunity to wild-type AAV, posing a challenge to treatment effectiveness.
Safety concerns and toxicity and durability
Although generally safe, AAVs can cause adverse reactions, particularly at high doses, with liver toxicity being the most common. Other serious events like TMA or aHUS have prompted clinical holds, necessitating careful consideration of dosing and potential pre-existing antibodies.
The long-term success of AAV therapies is under scrutiny, as factors such as the lifespan of the targeted cell and immune responses may necessitate re-dosing. Strategies to enhance the longevity of these therapies are crucial for sustained patient benefit.
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