Evaluation of a Serum-free Medium for the Production of rAAV-2 using HeLa Derived Producer Cells

Development of Stable Packaging and Producer Cell Lines for the Production of AAV Vectors

Today, recombinant adeno-associated virus (rAAV) vectors represent the vector systems which are mostly used for in vivo gene therapy for the treatment of rare and less-rare diseases. Although most of the past developments have been performed by using a transfection-based method and more than half of the authorized rAAV-based treatments are based on transfection process, the tendency is towards the use of stable inducible packaging and producer cell lines because their use is much more straightforward and leads in parallel to reduction in the overall manufacturing costs. This article presents the development of HeLa cell-based packaging/producer cell lines up to their use for large-scale rAAV vector production, the more recent development of HEK293-based packaging and producer cell lines, as well as of packaging cell lines based on the use of Sf9 cells. The production features are presented in brief (where available), including vector titer, specific productivity, and full-to-empty particle ratio.

Recent advances in upstream process development for production of recombinant adeno-associated virus

Recombinant adeno-associated virus (rAAV) is rapidly emerging as the preferred delivery vehicle for gene therapies, with promising advantages in safety and efficacy. Key challenges in systemic in-vivo rAAV gene therapy applications are the gap in production capabilities versus potential market demand and complex production process. This review summarizes current available information on rAAV upstream manufacturing processes and proposed optimizations for production. The advancements in rAAV production media were reviewed with proposals to speed up the cell culture process development. Furthermore, major methods for genetic element delivery to host cells were summarized with their advantages, limitations, and future directions for optimization. In addition, culture vessel selection criteria were listed based on production cell system, scale, and development stage. Process control at the production step was also outlined with an in-depth understanding of production kinetics and quality control.

Critical challenges and advances in recombinant adeno-associated virus (rAAV) biomanufacturing

Gene therapy is a promising therapeutic approach for genetic and acquired diseases nowadays. Among DNA delivery vectors, recombinant adeno-associated virus (rAAV) is one of the most effective and safest vectors used in commercial drugs and clinical trials. However, the current yield of rAAV biomanufacturing lags behind the necessary dosages for clinical and commercial use, which embodies a concentrated reflection of low productivity of rAAV from host cells, difficult scalability of the rAAV-producing bioprocess, and high levels of impurities materialized during production. Those issues directly impact the price of gene therapy medicine in the market, limiting most patients' access to gene therapy. In this context, the current practices and several critical challenges associated with rAAV gene therapy bioprocesses are reviewed, followed by a discussion of recent advances in rAAV-mediated gene therapy and other therapeutic biological fields that could improve biomanufacturing if these advances are integrated effectively into the current systems. This review aims to provide the current state-of-the-art technology and perspectives to enhance the productivity of rAAV while reducing impurities during production of rAAV.

Scalable Production of AAV Vectors in Orbitally Shaken HEK293 Cells

Adeno-associated virus (AAV) vectors are currently among the most commonly applied for in vivo gene therapy approaches. The evaluation of vectors during clinical development requires the production of considerable amounts of highly pure and potent vectors. Here, we set up a scalable process for AAV production, using orbitally shaken bioreactors and a fully characterized suspension-adapted cell line, HEKExpress. We conducted a proof-of-concept production of AAV2/8 and AAV2/9 vectors using HEKExpress cells. Furthermore, we compared the production of AAV2/9 vectors using this suspension cell line to classical protocols based on adherent HEK293 cells to demonstrate bioequivalence in vitro and in vivo. Following upstream processing, we purified vectors via gradient centrifugation and immunoaffinity chromatography. The in vitro characterization revealed differences due to the purification method, as well as the transfection protocol and the corresponding HEK293 cell line. The purification method and cell line used also affected in vivo transduction efficiency after bilateral injection of AAV2/9 vectors expressing a GFP reporter fused with a nuclear localization signal (AAV2/9-CBA-nlsGFP) into the striatum of adult mice. These results show that AAV vectors deriving from suspension HEKExpress cells are bioequivalent and may exhibit higher potency than vectors produced with adherent HEK293 cells.

Pharmacology of Recombinant Adeno-associated Virus Production

Recombinant adeno-associated viral (rAAV) vectors have been used in more than 150 clinical trials with a good safety profile and significant clinical benefit in many genetic diseases. In addition, due to their ability to infect non-dividing and dividing cells and to serve as efficient substrate for homologous recombination, rAAVs are being used as a tool for gene-editing approaches. However, manufacturing of these vectors at high quantities and fulfilling current good manufacturing practices (GMP) is still a challenge, and several technological platforms are competing for this niche. Herein, we will describe the most commonly used upstream methods to produce rAAVs, paying particular attention to the starting materials (input) used in each platform and which related impurities can be expected in final products (output). The most commonly found impurities in rAAV stocks include defective particles (i.e., AAV capsids that do contain the therapeutic gene or are not infectious), residual proteins from host cells and helper viruses (adenovirus, herpes simplex virus, or baculoviruses), and illegitimate DNA from plasmids, cells, or helper viruses that may be encapsidated into rAAV particles. Given the role that impurities may play in immunotoxicity, this article reviews the impurities inherently associated with each manufacturing platform.

Advancements in the design and scalable production of viral gene transfer vectors

The last 10 years have seen a rapid expansion in the use of viral gene transfer vectors, with approved therapies and late stage clinical trials underway for the treatment of genetic disorders, and multiple forms of cancer, as well as prevention of infectious diseases through vaccination. With this increased interest and widespread adoption of viral vectors by clinicians and biopharmaceutical industries, there is an imperative to engineer safer and more efficacious vectors, and develop robust, scalable and cost-effective production platforms for industrialization. This review will focus on major innovations in viral vector design and production systems for three of the most widely used viral vectors: Adenovirus, Adeno-Associated Virus, and Lentivirus.

Rational plasmid design and bioprocess optimization to enhance recombinant adeno-associated virus (AAV) productivity in mammalian cells

Viral vectors used for gene and oncolytic therapy belong to the most promising biological products for future therapeutics. Clinical success of recombinant adeno-associated virus (rAAV) based therapies raises considerable demand for viral vectors, which cannot be met by current manufacturing strategies. Addressing existing bottlenecks, we improved a plasmid system termed rep/cap split packaging and designed a minimal plasmid encoding adenoviral helper function. Plasmid modifications led to a 12-fold increase in rAAV vector titers compared to the widely used pDG standard system. Evaluation of different production approaches revealed superiority of processes based on anchorage- and serum-dependent HEK293T cells, exhibiting about 15-fold higher specific and volumetric productivity compared to well-established suspension cells cultivated in serum-free medium. As for most other viral vectors, classical stirred-tank bioreactor production is thus still not capable of providing drug product of sufficient amount. We show that manufacturing strategies employing classical surface-providing culture systems can be successfully transferred to the new fully-controlled, single-use bioreactor system Integrity(TM) iCELLis(TM) . In summary, we demonstrate substantial bioprocess optimizations leading to more efficient and scalable production processes suggesting a promising way for flexible large-scale rAAV manufacturing.

Scalable downstream strategies for purification of recombinant adeno- associated virus vectors in light of the properties

Recombinant adeno-associated virus (rAAV) vector is one of the promising delivery tools for gene therapy. Currently, hundreds of clinical trials are performed but the major barrier for clinical application is the absence of any ideal large scale production technique to obtain sufficient and highly pure rAAV vector. The large scale production technique includes upstream and downstream processing. The upstream processing is a vector package step and the downstream processing is a vector purification step. For large scale downstream processing, the scientists need to recover rAAV from dozens of liters of cell lysate or medium, and a variety of purification strategies have been developed but not comprehensively compared till now. Consequently, this review will evaluate the scalable downstream purification strategies systematically, especially those based on the physicochemical properties of AAV virus, and attempt to find better scalable downstream strategies for rAAV vectors.

Generation and characterization of adeno-associated virus producer cell lines for research and preclinical vector production

Adeno-associated virus (AAV) producer cell lines represent an effective method for large-scale production of AAV vectors. We set out to evaluate and characterize the use of an abbreviated protocol to generate "masterwells" (MWs; a nonclonal cell population) as a platform for research and preclinical vector production. In this system, a single plasmid containing three components, the vector sequence, the AAV rep, and cap genes, and a selectable marker gene is stably transfected into HeLaS3 cells. Producer cell lines generating an AAV2 vector expressing a secreted form of human placental alkaline phosphatase (SEAP) have been created. Several MWs showed vector yields in the 5×10(4) to 2×10(5) DNase-resistant particles/cell range, and the productivity was stable over >60 population doublings. Integrated plasmid copy number in three high-producing MWs ranged from approximately 12 to 50; copies were arranged in a head-to-tail configuration. Upon infection with adenovirus, rep/cap copy number was amplified approximately 100-fold and high yield appeared to be dependent on the extent of amplification. Rep/cap gene expression and vector packaging both reached a peak at 48 hr postinfection. AAV2-SEAP vector was produced in 1-liter shaker culture and purified for assessment of vector quality and potency. The data showed that the majority of the capsids from the MWs contained vector DNA (≥70%) and that purified vector was free of replication-competent AAV. In vitro and in vivo analyses demonstrated that potency of the producer cell-derived vector was comparable to vector generated via the standard transfection method.

Host cells and cell banking

Gene therapy based on the use of viral vectors is entirely dependent on the use of animal cell lines, mainly of mammalian origin, but also of insect origin. As for any biotechnology product for clinical use, viral -vectors have to be produced with cells derived from an extensively characterized cell bank to maintain the appropriate standard for assuring the lowest risk for the patients to be treated. Although many different cell types and lines have been used for the production of viral vectors, HEK293 cells or their derivatives have been extensively used for production of different vector types: adenovirus, oncorectrovirus, lentivirus, and AAV vectors, because of their easy handling and the possibility to grow them adherently in serum-containing medium as well as in suspension in serum-free culture medium. Despite this, these cells are not necessarily the best for the production of a given viral vector, and there are many other cell lines with significant advantages including superior growth and/or production characteristics, which have been tested and also used for the production of clinical vector batches. This chapter presents basic -considerations concerning the characterization of cell banks, in the first part, and, in the second part, practically all cell lines (at least when public information was available) established and developed for the production of the most important viral vectors (adenoviral, oncoretroviral, lentiviral, AAV, baculovirus).

Adeno-associated viruses

Adeno-associated virus (AAV) vectors have evolved over the past decade as a particularly useful gene -vector for in vivo applications. In contrast to oncoretro- and lentiviral vectors, this vector stays essentially episomal after gene transfer, making it safer because of the absence of insertional mutagenesis. AAV's non-pathogenicity is a further advantage. For decades, this vector could only be produced at a small scale for research purposes and, eventually, used at very small doses for clinical studies, because only transfection methods were available, which have limited scalability. However, since the development of scalable production methods, this bottleneck is resolved and, from a technical point of view, large quantities of AAV vectors can be produced, opening the possibility of using AAV vectors for whole body treatments in gene therapy trials. This chapter presents the basic principles of small- and large-scale production procedures as well as detailed procedure of small-scale production, purification, and analytical protocols for AAV vectors. In Chapter 10, the reader will find a large-scale production method based on the use of the insect cell/baculovirus system.

Engineering of baculovirus vectors for the manufacture of virion-free biopharmaceuticals

A novel baculovirus-based protein expression strategy was developed to produce recombinant proteins in insect cells without contaminating baculovirus virions. This novel strategy greatly simplifies the downstream processing of biopharmaceuticals produced in insect cells. The formation of these virions is prevented by deletion of a baculovirus gene essential for virion formation. The deletion is trans-complemented in a transgenic insect cell line in which the baculovirus seed stock is produced. The Autographa californica multicapsid nucleopolyhedrovirus vp80 gene was selected for this purpose, as absence of VP80 prevented the formation of budded virus as well as occlusion-derived virus, while foreign gene expression was not affected. Sf9 insect cells were engineered to functionally complement the vp80 deletion in the expression vector virus during seed stock production. The trans-complemented vp80-deletion baculovirus seed produced an amount of recombinant protein similar to that produced with conventional baculovirus vectors but without contaminating virions. This novel expression method obviates the need to purify the virions away from the biopharmaceuticals.

Cell culture processes for the production of viral vectors for gene therapy purposes

Gene therapy is a promising technology for the treatment of several acquired and inherited diseases. However, for gene therapy to be a commercial and clinical success, scalable cell culture processes must be in place to produce the required amount of viral vectors to meet market demand. Each type of vector has its own distinct characteristics and consequently its own challenges for production. This article reviews the current technology that has been developed for the efficient, large-scale manufacture of retrovirus, lentivirus, adenovirus, adeno-associated virus and herpes simplex virus vectors.