While in vivo CAR-T therapies eliminate complex ex vivo manufacturing, they introduce a new critical variable: the patient's own immune system. The therapy's efficacy relies on modifying T-cells within the body, but each patient's immune status is different, especially after prior treatments. This makes optimizing and standardizing the dose a significant challenge compared to engineered cell therapies.

While redosing may be an option for RNA-based in vivo CAR-Ts, viral vector-based platforms face a significant challenge. The potential for an immune response against the vector, a well-known issue in AAV gene therapy, could prevent subsequent doses and limit the long-term treatment strategy for these therapies.

An investigational in vivo CAR-T therapy uses viral particles infused directly into the patient to convert their T-cells into CAR-T cells. This approach eliminates the complex steps of apheresis, lymphodepletion, and ex vivo manufacturing, effectively creating an off-the-shelf product that becomes an autologous treatment inside the body.

Regenexx's gene therapy program was halted after its AAV vector integrated into a patient's genome, likely causing a tumor. While AAVs are designed to avoid this, the event puts a spotlight on the known, but rare, risk of insertional mutagenesis for the entire AAV field.

Developing CAR T-cell therapies for solid tumors is difficult because many tumor-associated antigens are also expressed on normal tissues. This creates a significant risk of "on-target, off-tumor" effects, causing severe toxicity. Mitigating this risk, for instance with engineered "kill switches," is as crucial as preserving the therapy's efficacy.

Despite exciting early efficacy data for in vivo CAR-T therapies, the modality's future hinges on the critical unanswered question of durability. How long the therapeutic effects last, for which there is little data, will ultimately determine its clinical viability and applications in cancer versus autoimmune diseases.

Many current gene therapies require a complex "ex vivo" process: removing cells, reprogramming them in a lab, and reinfusing them. The true breakthrough is developing "in vivo" treatments administered via a simple infusion that autonomously target the correct cells within the body.

Voyager CEO Al Sandrock explains their AAV capsids are engineered to be so potent at crossing the blood-brain barrier that doses can be an order of magnitude lower than standard. Crucially, the capsids are also designed to *avoid* the liver, directly addressing the toxicity issues that have plagued the field.

Create Medicines chose LNP-delivered RNA for its in vivo platform to give physicians control. Unlike permanent lentiviral approaches, repeatable dosing allows for adapting to tumor antigen escape and managing durability and safety over time. This flexibility is a core strategic advantage for complex diseases like solid tumors.