The concept of Chimeric antigen receptor T cell (CAR-T) therapy was first proposed in the late 1980s, and CAR design has continuously evolved in both structure and function.
This form of therapy has already benefitted tens of thousands of cancer patients, and the global CAR-T therapy market was valued at $5.76 billion in 2025. That figure is expected to reach $7.51 billion in 2026, and is poised to soar to $81.45 billion by 2035. To date, more than ten CAR-T products, primarily targeting CD19 or BCMA, have been approved worldwide.
Researchers are now developing in vivo CAR-T therapies to complement and improve the conventional ex vivo treatments, and this new approach holds much promise. However, researchers must understand and overcome the bioanalytical challenges faced during in vivo CAR-T development if they are to fulfill their potential.
CAR-T Technology Explained
CAR-T therapy endows T cells with new antigen-recognition capabilities by engineering them to express artificially designed chimeric antigen receptors (CARs).
Conventional CAR-T cells typically recognize specific cell-surface target antigens and kill cells that express them. In cancer therapy, CAR-T targets typically include tumor-associated antigens (TAA) and some tumor-specific antigens (TSA) that are expressed on cell surfaces. However, conventional CAR-T cannot directly recognize TSAs or neoantigens that originate inside cells and are presented as peptide-MHC complexes.
When a CAR binds to its target antigen, it activates intracellular signaling pathways within the T cell. This drives T cell activation, expansion, and cytotoxic effector functions. In conventional CAR-T therapy, T cells are genetically modified and expanded outside the patient’s body before reinfusion. But in vivo CAR-T therapy takes an alternative approach: the CAR-encoding genetic material is delivered directly to T cells inside the patient, which means the cells are converted into CAR T cells in situ. This approach offers many potential advantages.
The Benefits of an In Vivo Approach
Conventional ex vivo CAR-T therapy faces major bottlenecks that threaten its commercialization and large-scale clinical adoption. The ex vivo model is a complex procedure involving autologous peripheral blood mononuclear cell (PBMC) collection from the patient, isolation and enrichment of T cells, T-cell activation, transduction with the CAR gene, ex vivo expansion, cryopreservation, transport of CAR-T cells, and finally reinfusion into the patient.
The vein-to-vein time for the process can take 3-4 weeks, making timely treatment difficult for patients with rapidly progressing late-stage conditions. The costs are also high due to the personalized “one patient-one batch” manufacturing model, which can severely limit access. Additionally, variability in patients’ own T-cell quality can lead to poor product consistency, and T-cell exhaustion during ex vivo expansion can weaken durable antitumor activity after reinfusion.
In vivo CAR-T can address many of these limitations. It uses targeted delivery systems such as engineered viral vectors or lipid nanoparticles (LNPs) to deliver CAR-encoding DNA/RNA fragments directly into the T cells of patients. This approach brings several benefits:
- Shorter treatment preparation time
- Reduced manufacturing complexity
- Improved potential of standardization and scale-up
The Key Challenges (and Solutions) of In Vivo CAR-T Therapy Development
In vivo CAR-T therapy combines characteristics of gene delivery and cell-based therapy, creating a distinct set of bioanalytical and regulatory considerations. Bioanalytical strategies therefore need to consider both the gene-delivery component and the resulting CAR-T cell population, with specific requirements depending on the delivery modality and product design.
A single platform is insufficient to comprehensively characterize the in vivo mechanism of in vivo CAR-T. A cross-platform, integrated analytical strategy is required, including LC-MS/MS, PCR, flow cytometry (FACS), ligand-binding assays (ELISA/MSD/Luminex), and ELISpot. Each of these platforms presents challenges that can be overcome with the right expertise and experience.
LNP Lipid Component Pharmacokinetic Analysis (LC-MS/MS)
In LNP-mediated in vivo CAR-T studies, LC-MS/MS can be used to quantify formulation-defining components such as ionizable lipids and PEG-lipids.
- Challenges: Interference from endogenous lipids in biological matrices and achieving accurate quantification when stable-isotope-labeled internal standards are unavailable.
- Solutions: Sample preparation should be optimized based on the sensitivity and physicochemical properties of each lipid and matrix type. Protein precipitation, liquid-liquid extraction, solid-phase extraction, or combined approaches can improve cleanup and reduce matrix effects.
CAR-encoding and Vector-related Nucleic Acid Analysis (qPCR/ddPCR)
PCR-based platforms dominate PK analysis of in vivo CAR-T, regardless of the delivery vehicle used.
- Challenge: In LNP-based delivery, CAR mRNA is highly labile and can be degraded by environmental RNases.
- Solution: For LNP-based delivery, use specialized RNA-protective blood collection tubes or RNA stabilizers at the point of sample collection.
CAR-Positive T cell Quantification and Cell Kinetic Analysis (Flow Cytometry)
Flow cytometry can quantify the absolute number of CAR-positive T cells per unit volume of whole blood at serial time points, enabling characterization of the in vivo expansion-contraction kinetics of CAR-T cells and supporting the assessment of efficacy and toxicity.
- Challenge: Developing anti-CAR reagents with high affinity and resistance to interference.
- Solution: Use specific anti-CAR reagents, such as anti-F(ab′)2 antibodies or recombinant target antigens, in combination with T-cell markers including CD3, CD4, and CD8.
Humoral Immunogenicity Analysis (ELISA/MSD)
Both the delivery system and the CAR-expressed product must be evaluated for humoral immunogenicity in in vivo CAR-T studies.
- Challenges: Pre-existing anti-PEG antibodies may already be present, making it difficult to distinguish them from treatment-induced or treatment-boosted responses. Soluble antigen, residual vector components, or other substances may interfere with anti-drug antibody detection.
- Solutions: Conduct baseline, pre-dose testing using ELISA or MSD, and use pretreatment methods such as acid dissociation (AD) or solid-phase extraction with acid dissociation (SPEAD) to reduce interference.
Cell Immunogenicity and Functionality Analysis (ELISPOT /Flow Cytometry)
ELISpot is a powerful tool for assessing cellular immunogenicity.
- Challenge: In Vivo CAR-T may also induce a cellular immune response. When there is abnormal drug persistence or efficacy, for example, unusual drug clearance, the cell immunogenicity against the drug needs to be investigated.
- Solution: ELISpot is a useful assay for evaluating cell immunogenicity. Flow cytometry can be applied to identify immunocell subsets, which is important for cell immunogenicity and drug efficacy assessment.
A Final Word on In Vivo CAR-T Therapies
Ex vivo CAR-T remains the mainstream therapy, and its development focuses on improving manufacturing efficiency and safety, as well as expanding indications. But while that development continues, in vivo CAR-T offers a promising new direction, enabling rapid, lower-cost therapies.
However, this type of therapy still faces hurdles, including limited delivery efficiency, cell specificity, off-target transduction, immune-related toxicities, and long-term safety concerns. While these challenges are primarily addressed through advances in vector design and therapeutic development, they also create distinct bioanalytical needs. Multidimensional bioanalytical strategies are therefore essential to characterize in vivo transduction, cellular kinetics and persistence, biodistribution, immunogenicity, and pharmacodynamic activity throughout development.
