Viral Vectors: Key to Advanced Therapies

Demand for advanced (cell and gene) therapies is driving an unprecedented demand for viral vectors, whose large-scale production is roughly at the stage of monoclonal antibody manufacturing circa 1995. Since the commercialization of advanced treatments generally depends on the availability of viral vectors, significant effort is now focused on industrializing these essential tools.

Complex living systems

According to Natalia Elizalde, Business Development Director at VIVEbiotech, the manufacture of lentiviral vectors is challenging primarily because they are complex living systems with inherent characteristics playing into a wide variety of factors that could, potentially, affect the vectors’ biological function.

“As an industry, we face a common hurdle: performance of the available virus packaging cell lines; in other words, the percentage of total to functional viruses produced, compared with the total number of physical particles. Moreover, lentivectors manufactured these days are mostly vesicular stomatitis virus-pseudotyped. This surface glycoprotein is ubiquitous, so a very large number of viruses are required to infect the specific target cells to exert the desired therapeutic effect.”

These “inherent” challenges lead to a rush to develop cost-effective (or economically viable) processes with the aim of treating as many patients as possible. “This is a major point for us, and why VIVEbiotech concentrates our resources on very early development stages, for example in the design of transfer plasmids, to optimize external-internal promoter interactions, which we know strongly affect the final titer for a given lentiviral vector.”

Another area of concern involves therapy-cells, which may be refractory toward transduction, requiring very high multiplicity of infection (MOI) to achieve a therapeutically relevant number of transduced cells. “This also affects the cost-effectiveness of the therapy,” Elizalde explains. “Minimizing MOIs requires the application of transduction enhancers, many of which are commercially available.”

Novel therapeutic modalities (e.g., mRNA) and paradigm-shifting approaches to manufacturing (single-use) have disruptive effects on drugmakers. Contract manufacturers are typically the first entities to grasp the technologic nuances, to invest in those processes, establish best practices, etc. So, it is with cell and gene-based therapies, which via the patient-cell-construct-virus connection introduce an entirely new and greatly expanded dimension of complexity.

In other words, the technologies, techniques, and methods around cell-based therapies are non-core to most therapy developers, and therefore best outsourced. VIVEbiotech, a contract manufacturer specializing in viral vector technologies, naturally agrees. “From our perspective, selecting a CDMO from the outset, one that can cover all developmental stages, from concept to commercial batches, is the key to success,” Elizalde says. “These functions include a great deal of customization, for example optimizing lentiviral vector production with both titer and purity in mind, while avoiding potential regulatory hurdles, and watching both scalability and cost-effectiveness.”

At the very least, this approach avoids the costly, time-consuming process of transferring technology between sponsor and contractor, or between two or more outsourcing entities, a process that is rife with regulatory challenges.

“Biological processes must undergo continuous optimization for titer, recovery, and purity as these factors directly affect their cost-effectiveness,” Elizalde adds. “Purity is key for regulatory compliance, and is also a major contributor to cost-effectiveness, as the more pure the vector, the lower the therapeutically effective titer needs to be. You could say that continuous process optimization and de-risking is the key to a CDMO’s future success and continued relevance.”

Bottlenecks already appearing

Matthew M. Hewitt, Ph.D., Executive Director at Charles River Laboratories, agrees that roadblocks to commercializing cell and gene therapies are disparate. “In the majority of cell therapy manufacturing processes, viral vectors are used to gene-modify cells with a payload, such as a chimeric antigen receptor (CAR), targeting a specific antigen. The time and costs for manufacturing viral vectors suitable for clinical and commercial activities are high. We are already seeing bottlenecks at commercial scale.”

Hewitt is referring to comments made in July 2021 by executives at Bristol Myers Squibb, who described issues related to manufacturing scale and the availability of viral vectors for Abecma, the company’s multiple myeloma CAR-T therapy.

“BMS has publicly disclosed vector availability as a challenge to manufacturing Abecma,” Hewitt continues. “Viral vector manufacturing itself has several challenges relating to upstream and downstream workflows. The current upstream transfection method is adequate but at times insufficient for commercial uses, thereby raising costs. Additionally, depending on the process, significant quantities of viral vector products are lost during downstream processing and purification. There are solutions for these issues on the horizon, including stable producer cell lines for vector manufacturing, better downstream processing and purification workflows, as well as alternative transfection methods such as electroporation to gene-modify cells directly, eliminating the need for viral vectors.”

Rare vs. genetic diseases

According to John Maslowski, Chief Commercial Officer, Forge Biologics, the scientific challenges in commercializing cell and gene therapies depend on the therapeutic area or disease, for example prevalent genetic vs. rare diseases, and the existing depth of research for that indication. “Compared with rare diseases, prevalent genetic diseases have significantly more research and therefore more commercially available tools to aid in discovery.”

This body of research leads to “invaluable insight into the molecular mechanisms of the disease, causal genetic mutations, wild-type protein functions, as well as investment into the production of commercially available discovery tools like antibodies and ELISAs," Maslowski says. Companies in possession of these assay tools, which are required for cGMP release testing, have a significant advantage.

“Forge Biologics is a CDMO, but we also have a product development arm where we focus on gene therapy development for rare diseases. Gene therapy development for rare disease is a different ball game altogether. For most indications, there has often been limited scientific investigation into understanding the disease mechanisms and there are often no, or inadequate, commercially available resources to detect or quantify results. The path to commercializing gene therapies for rare diseases begins from well behind the starting line, due to the current lack of basic scientific research, a prospect that precludes many companies from starting at all.”

Numerous challenges related to manufacturing hinder large-scale commercialization of gene-based therapies, and they may occur at multiple levels or stages, including transfection, clarification, lysis, and the separation of empty vs. full capsids.

“At small scale,” Maslowski explains, “the challenges can easily be overcome with standard lab equipment, for example small conicals for transfection, bottle-top filters and centrifuges for clarification, rotating shaker for lysis, and cesium chloride density gradient ultracentrifugation for capsid purification. However, on scaleup of drug substances requiring high total AAV vector genomes, these processing steps require state-of-the-art technology and creativity to process successfully. For example, clarifying 1000 L of biological material cannot feasibly be done with a centrifuge and bottle-top filter, so processors turn to depth filtration, which is costly and requires skilled laboratory personnel.”