Why In Vivo CAR-T Editing Faces Major Hurdles: Delivery, Multiplexing, and the Real-World Gap

The possibility of engineering a patient’s immune cells directly inside their body—with just an infusion instead of a complex, weeks-long manufacturing process—represents a huge leap for cell therapy. Scientists are charging ahead with new tools like lipid nanoparticles (LNPs), viral vectors, and synthetic nanocarriers that promise in vivo CAR-T generation. But the reality today is more complicated.

Let’s walk through where the field stands, why in vivo editing still lags behind ex vivo approaches (even with the newest delivery systems), and why the biggest technical obstacles aren’t just about getting a gene into a cell.

The Main Delivery Platforms: Viral Vectors, LNPs, and Beyond

The vision for in vivo CAR-T hinges on technologies that can safely deliver genetic cargo directly to specific immune cells inside the patient:

Viral Vectors: These use modified viruses (typically lentiviruses, retroviruses, or adeno-associated viruses) to deliver the CAR gene to T cells inside the body. High efficiency is possible, but delivery specificity, safety and immunogenicity remain concerns.¹
Lipid Nanoparticles (LNPs): Fat-based nanoscale carriers that package mRNA (or DNA) and can be engineered with targeting ligands to reach specific immune subsets. LNPs have become a leading vehicle for in vivo CAR editing because they are relatively safe and flexible.²
Polymer and Hybrid Carriers: Synthetic materials and polymer nanocarriers are being trialed for more precise or combinatorial delivery, but most remain preclinical.³

What Actually Happens In Vivo?

LNP and Nonviral Delivery

In mouse experiments, LNPs modified with anti-CD3, CD4, or CD5 antibodies reliably deliver mRNA to T cells. Billingsley et al. showed these LNP-CAR-Ts can deplete B cells and even shrink tumors when enough healthy T cells are present².
Tilsed et al. found that pre-conditioning with IL-7 boosts the efficiency and persistence of CAR expression delivered by LNPs, but again—healthy T cell numbers are required for it to work⁴.
Polymer-based nanocarriers can transiently reprogram T cells to attack tumors comparably to ex vivo T cells, but only when the starting immune system isn’t exhausted by prior disease or therapy³.
CD4-homing LNPs and other antibody-targeted systems have shown similar results in preclinical models⁵. However, the efficacy studies against refractory conditions such as solid tumors remain to be seen even in the pre-clinical models.

Viral Vector Delivery

Viral vectors remain highly efficient for delivering CAR constructs to T cells in animals and sometimes in humans. However, concerns linger around immune responses against the virus, off-targeting, difficulty with repeat dosing, and—in the case of integrating vectors—potential for insertional mutagenesis¹.

Hybrid/Novel Platforms

Some studies now explore hybrid or alternative nanoparticles (including cardiolipin-mimicking LNPs and biopolymer blends), aiming for even greater targeting or payload capacity. Most are still at the preclinical stage, with success in healthy models but limited patient data so far⁶. Furthermore, it also remains to be demonstrated whether selectivity in payload delivery through these approaches- which rely on the biophysical parameters of the LNP rather than affinity, as in case of conventional targeting strategies- is restricted to T cells without off-target expression in the non-immune components of the spleen.

The Clinical and Practical Reality: Why Isn’t This Enough?

On paper, these platforms are promising. In the real world—especially in patients with cancer—several stubborn limitations keep in vivo approaches trailing behind their ex vivo counterparts:

1. Patient T Cells Are Often Too Depleted or Exhausted

Most cancer patients eligible for CAR-T therapies have small, dysfunctional pools of T cells left after years of chemo, radiation, or immune-suppressing disease. In vivo delivery can only edit what’s available, so the resulting CAR-T population may be small, stressed, and short-lived⁴.

2. Durability and Expansion Still Lag Behind

LNP approaches most commonly deliver mRNA, which produces only short-term CAR expression (days to <2 weeks) unless dosing is repeated; and this yields about the same tumor control as a single dose of ex vivo-modified CAR-T cells³. Even viral vectors—while sometimes integrating into DNA—are not guaranteed to produce persistent, highly-expanding T cells unless the starting population is healthy⁷.

3. Lack of Manufacturing Control

Ex vivo methods allow scientists to select, expand, and QC the healthiest T cells, maximizing product quality. In vivo, everything relies on the patient’s own biology, and hence left to chance.

4. Multiplexed Editing: The Largest Barrier

Here’s a critical—and underappreciated—gap: Many advanced CAR-T therapies require “multiplexing” more than one genetic change at a time. For instance, disabling checkpoint receptors like PD-1, or inserting multiple transgenes for combinatorial targeting or safety. Ex vivo editing can use CRISPR/Cas9 and other sophisticated platforms to do all these in parallel, and then select the “edited” cells for therapy.

In vivo, robust multiplexed gene editing is currently not feasible: delivery methods (LNPs, viral, or hybrid) struggle to efficiently introduce, synchronize, and express multiple genetic changes in the same cell without creating toxicities, off-target effects, or simply low efficiency¹. That’s a huge problem for real-world cancer treatment, where advanced protocols (like dual-targeting, allo-evasion, resistance to TME-induced immune exhaustion and safety switches) are rapidly becoming standard of care.

In Vivo Editing in Other Cell Types: Lessons and Limits

Hematopoietic Stem Cells (HSCs): LNPs have successfully delivered editing tools to HSCs (like CD117+ stem cells), raising hope for in vivo cures of blood diseases⁸. But, editing rates are lower than with ex vivo approaches, with some studies reporting an in vivo transfection efficiency of CAR in T cells at just 8%⁹. Furthermore, similar delivery/fitness barriers exist in patients preventing optimal tumor control despite the presence of in vivo generated CAR-T cells in cancer patients¹⁰.
Hepatocytes: LNPs are already licensed for in vivo CRISPR-based editing of liver cells (for example, treatment of transthyretin amyloidosis). But liver cells are much more accessible than immune cells—making translation to the hematopoietic or lymphoid system much less straightforward¹¹.
Other Immune Cells: There is some early research on in vivo editing of NK, B cells, and myeloid cells, but the same hurdles—especially around efficiency, safety, and multiplexing—persist.

Takeaways

LNPs, viral vectors, and hybrid systems are transforming in vivo CAR-T delivery and opening up new frontiers in cell therapy.
In experimental animal systems with robust, unstressed immune cells, they can approach ex vivo efficacy for simple, single-gene edits against non-refractory diseases such as auto-immune diseases, certain infections, cardiac injury and some hematological cancers.
In actual cancer patients—where cell health, “multiplex” modifications, and scalability are most critical—in vivo editing currently falls short on efficacy, durability, and complexity. This limitation becomes even more dire in refractory solid tumor cases, where just CAR-expression in T or NK or macrophages is far from being an effective strategy.
Until multiplexed, controllable in vivo editing becomes routine (and delivery tech can work in stressed, low-T cell contexts), ex vivo cell engineering will remain the dominant path for advanced CAR-T therapies.

Immumem’s Solution: Ex Vivo Multiplexed T and NK Cells for Solid Tumors

At Immumem Therapeutics, we’re building on the lessons from in vivo delivery and using what works best—robust ex vivo editing, quality control, and cell expansion. Our platform leverages ex vivo gene editing to engineer both T and NK cells with multiplexed modifications, not just adding a CAR but also tailoring cells to overcome the solid tumor microenvironment.

With ex vivo methods, we can reliably insert CARs, knock out or knock down multiple inhibitory pathways, optimize cytokine signaling etc.—all within a well-controlled GMP process. Because we start with healthy donor cells, expand them in optimal conditions, and confirm the edits by high-throughput QC before infusion, our therapies can deliver powerful, “fit” effector cells that persist and function in people with advanced, heavily pretreated cancers.

For solid tumor patients—where tumor immune evasion and T cell exhaustion are especially problematic—these multiplexed, precision-edited T and NK cells are poised to finally bring the level of control, potency, and durability that in vivo methods haven’t yet achieved. As next-wave delivery science matures, we’re committed to using the best of both worlds—but for now, the Immumem ex vivo platform is uniquely equipped to meet this urgent clinical need.