A Review for Leveraging Historical Technologies in New Ways
Innovations designed to achieve better control of delivery vectors are needed to substantially enhance the applicability of gene therapy beyond adoptive cell therapy for cancer treatment and in vivo therapies for inherited genetic disorders. ViroCell Biologics was founded with the goal of developing solutions in a targeted manner to enable better control, improved responses, and wider applicability of gene therapies. Projects have included identification of small-molecule additives that boost titers, development of cell lines that afford improved transient transfection performance and stable producer cell lines that dramatically increase manufacturing efficiencies. Ligand-modified lentiviral vectors (LVVs) enable improved purification and concentration processes and greater cell/tissue targeting. Aspirational work focuses on development of non-integrating LVVs that have the potential to dramatically expand the diseases for which gene and adoptive cell therapies can be developed.
Building Better Control into Viral Vectors with Revolutionary Precision
There is growing recognition across the biopharmaceutical industry that expanding the potential of gene therapies will require far greater control over the behavior of viral vectors. While current applications, such as CAR-T cell therapy and rare monogenic disease treatment, have validated the modality, these successes have been enabled by tightly defined use cases and highly selective clinical settings. Moving beyond these boundaries to broader indications — including autoimmune diseases, cardiovascular conditions, and in vivo CAR-T therapies — demands a new generation of vectors with built-in precision and predictability.
ViroCell Biologics was established in 2021 to address this challenge head-on. Drawing on more than two decades of gene therapy development expertise at King’s College London and King’s College Hospital, the company was founded to address the critical unmet needs in gene delivery — particularly the need for fine-tuned control systems that allow for robust expression where needed and silence where not.
The company is developing programmable lentiviral vectors designed to behave intelligently/intuitively within biological systems. These include highly selective expression systems driven by combinations of tissue-specific promoters, enhancers, microRNAs, and other regulatory elements that enable tight post-transcriptional and post-translational control. By adjusting these molecular "dials," ViroCell is building vectors that can upregulate or downregulate genes in response to cell-specific signals, helping to ensure expression occurs only where biologically appropriate.
This approach holds enormous promise not only for increasing therapeutic efficacy, but also for minimizing off-target effects and mitigating risks such as toxicity or immune activation. The goal is not simply to engineer a gene delivery system, but to design environmentally responsive, self-regulating genetic tools that act with the same precision as a biologic therapy — with fewer unintended consequences.
Rather than introducing wholly new components, ViroCell’s innovation lies in how it recomposes known, well-understood biological elements into configurations that deliver entirely new capabilities. It is this reimagining of vector design — grounded in the familiar but applied in novel, integrated ways — that enables ViroCell to reduce development risk while opening new therapeutic frontiers.
Rethinking Scale, Risk, and Readiness in Gene Therapy Development
Innovation at contract development and manufacturing organizations (CDMOs) is typically focused on improving scalability and streamlining technology transfer. ViroCell invests deeply in these core capabilities — developing optimized upstream and downstream manufacturing solutions — but also pursues innovation that extends well beyond platform standardization or incremental improvements in yield and purity.
For example, in transient transfection–based production of viral vectors, ViroCell has demonstrated that the addition of select small molecules can yield a fourfold increase in vector titer. This effectively multiplies manufacturing capacity without scaling from a 50-liter to a 200-liter bioreactor, significantly reducing the cost of goods. Similarly, the company’s new HEK293 cell lines have been engineered to tolerate five- to 10-fold greater cell densities before reaching viability limits, further boosting productivity without changes to vessel size or hardware.
Another initiative involves the development of serum-free suspension-adapted HEK293 cell lines capable of producing both lentiviral (LV) and adeno-associated viral (AAV) vectors. While adherent HEK293 cells are often used during early-phase development, suspension processes are far more efficient at scale. ViroCell’s proprietary suspension HEK293 line supports high-titer vector production, as demonstrated by strong yields using transfection with both green fluorescent protein (GFP) and chimeric antigen receptor–GFP (CAR-GFP) plasmids. These cells are also being used in ongoing studies evaluating the role of small molecule additives in further enhancing titers.
To move beyond the limitations of transient transfection altogether, ViroCell is developing stable packaging and producer cell lines — a technically challenging endeavor owing to the cytotoxicity of certain vector components, like vesicular stomatitis virus G (VSV-G). One notable solution involves controlled, reversible expression of VSV-G using an inhibitory small molecule. In this system, VSV-G is fused to a responsive sequence, and in the presence of the inhibitor, its translocation is suppressed. After culture expansion, the inhibitory molecule is removed to induce vector release. This enables dose-dependent expression control and opens the door to long-term, high-yield LVV production in suspension cultures. Ongoing experiments are evaluating integration efficiency, small molecule dose responsiveness, yield, and genomic stability.
For in vivo applications, insertional mutagenesis remains a well-known concern with integrating LVVs. While integration can offer the benefit of long-term expression, it also raises safety questions — particularly in pediatric and large-population indications. In addition, LVVs can carry a greater payload, over twice the capacity offered by AAVs. Unlike AAVs, lentiviral vectors are not typically associated with liver toxicity, which is an important consideration given the vast quantities of, especially AAV required due to inefficient targeting, making them attractive alternatives if integration risks can be mitigated.
To address these limitations, ViroCell employs a multipronged strategy to reduce risk while retaining efficacy. This includes the use of non-integrating lentiviral vectors that remain episomally maintained, as well as constructs that target “safe harbor” genomic sites when stable expression is needed. Vectors may also include built-in safety features, such as CRE-inducible suicide genes, designed to trigger apoptosis in the event of uncontrolled proliferation or mutation.
Targeting remains a key challenge for in vivo gene delivery. AAV platforms have gained popularity in part owing to the diversity of available serotypes that allow for tissue-specific delivery. While pseudotyped RNA viruses offer promise in this domain, their use has remained largely experimental and difficult to translate into GMP-compliant platforms. ViroCell is actively working to expand the toolkit of usable pseudotypes — an effort aimed at making clinically viable lentiviral targeting a reality.
The potential for gene therapy to address common diseases — from inflammatory conditions to cardiovascular disorders — is enormous. But it will only be realized by taking the necessary first steps to improve the safety, precision, and manufacturability of vectors. ViroCell is committed to leading that effort.
At the same time, the company recognizes the pressures many clients face to be first in the clinic. For programs on an accelerated timeline, ViroCell offers a plug-and-play platform capable of manufacturing QP-released vectors in six months or less. For clients with more flexibility, the company can optimize vector design, evaluate multiple permutations, and select the best construct for GMP manufacturing. Whether the priority is speed or sophistication, ViroCell’s platform is built to deliver.
Targeted by Design: Enhancing Selectivity in Viral Vector Delivery
For gene therapy to realize its full potential, viral vectors must evolve to enable highly selective delivery to diseased tissues, minimizing off-target effects while maximizing therapeutic efficacy. This is especially important in the move from ex vivo therapies to in vivo gene delivery, where the vector must navigate the body, identify its intended cell type, and deliver its payload with precision.
ViroCell is advancing two complementary strategies to achieve this goal: ligand-directed targeting and tissue-specific promoter design.
In one approach, viral vectors — both LV and AAV — can be chemically modified using click chemistry to attach ligands that guide the virus to specific tissues. This form of surface engineering allows the viral envelope to recognize and bind selectively to target cell receptors, enabling localized transduction while minimizing uptake by off-target tissues.
This method has several compelling use cases. In in vivo CAR-T cell therapies, the viral vector is delivered systemically — often via intravenous injection — but must only infect a specific subset of T cells or another target cell population. Ligand-directed targeting ensures that only the intended cells are transduced, reducing the risk of unintended gene modification and improving safety.
Beyond targeting, ligand functionalization also improves downstream processing. Modified vectors can be more easily captured, purified, and concentrated, an important advantage when therapies must be administered in ultra-low volumes. This is particularly critical for intracranial injections, inner ear delivery, or ocular gene therapies, where only tens of microliters may be deliverable. In such cases, every microliter must carry high potency, and the vector must meet stringent purity thresholds. Ligand-mediated purification can enhance recovery and purity, making these therapies more feasible.
Together, ligand engineering and tissue-specific transcriptional control represent powerful tools for expanding the therapeutic utility of lentiviral and retroviral platforms. ViroCell’s ongoing work in this space is designed not only to broaden the range of treatable diseases, but also to make these next-generation therapies safer and more effective.
Growing Potential of Nonintegrating LV Vectors
It may come as a surprise to some, but non-integrating LVVs have been under investigation for more than two decades. Studies have shown that non-integrating lentiviral vectors, transiently maintained episomally — outside the host genome — can be manufactured at titers comparable to their integrating counterparts. These vectors hold considerable promise for safer in vivo gene therapies, but realizing their full potential requires overcoming several biological and technical hurdles.
LVVs already offer one distinct advantage over AAV vectors: payload capacity. Lentiviral vectors can carry transgenes up to ~9 kb — more than twice the ~4 kb limit of AAV — making them better suited for complex or multi-gene payloads. Non-integrating LVVs add a further advantage: they virtually eliminate the risk of insertional mutagenesis, a major safety concern in in vivo applications. However, because episomal vectors do not integrate into the host genome, their expression is not heritably maintained through cell division, leading to dilution of therapeutic benefit in dividing cells, much like AAV.
What is needed is a way to stably maintain episomal transgenes across cell generations without integration. In nature, Epstein–Barr virus (EBV) achieves this in B cells using the EBNA1 protein and oriP sequence to tether its genome to the host’s during mitosis. These sequences can be used in plasmid systems to achieve episomal maintenance, but primarily in B cells, limiting broader applicability. This is why EBV persists in B cell malignancies and nasopharyngeal carcinoma but not elsewhere.
ViroCell is actively working to extend this mechanism to a broader range of human cell types. Through a combination of rational vector engineering, in vitro evolution, and machine learning–guided design, the team is identifying new regulatory elements and sequence motifs that promote episomal persistence beyond the B cell lineage. These efforts aim to develop non-integrating LVVs that can deliver durable expression in vivo without compromising genomic integrity.
This is an area where scientific collaboration is both welcomed and encouraged. ViroCell invites partners across academia and industry who are equally excited about expanding the scope of episomal vector technology to join in advancing safer, smarter gene therapy solutions for a wider range of diseases.
The Case for a Lentiviral Renaissance
Over the past two decades, the field of gene therapy has largely been shaped by the three viral platforms: AAVs, lentiviral vectors LVVs, and large DNA viruses, such as oncolytic viruses. Each has made important contributions, with AAVs dominating in vivo applications and LVVs serving as the backbone for ex vivo therapies like CAR-T.
At ViroCell, our work has always been vector-agnostic. Whether the goal is to increase vector titers, improve purity, or enhance tissue specificity, we focus on solving technical challenges that apply across platforms. This includes innovations in ligand-directed targeting, stable cell line development, and vector design, all of which can benefit LVV, AAV, and even non-viral systems alike.
Looking ahead, however, LVVs may be poised for broader in vivo use, especially if current limitations can be addressed. ViroCell is actively developing non-integrating LVVs capable of stable episomal maintenance, which could combine the safety of AAVs with the greater payload capacity, reduced hepatotoxicity, and targeting flexibility that LVVs uniquely offer. Several published studies have already demonstrated the feasibility of ligand-directed targeting in lentiviral systems, highlighting their potential for precision delivery.
Non-viral alternatives — such as lipid nanoparticles (LNPs) delivering episomally maintained nucleic acids — also hold promise, particularly for RNA therapies that undergo reverse transcription to DNA in target cells. However, these platforms remain constrained by delivery efficiency, expression duration, and tissue targeting precision.
In contrast, lentiviral vectors offer a more mature, customizable toolkit that, with the right engineering, could overcome many of the limitations faced by other systems. ViroCell’s vision is to help bring LVVs back to the forefront — not by displacing other modalities but by ensuring the field has a wider spectrum of tools to draw from, each fit for purpose and optimized for safety, scale, and clinical impact.