Engineering Transient In Vivo CAR-T: Extending Expression, Expanding Access, And Rethinking Immune Reset

A Q&A with Michael Lam, Ph.D., vice president, strategy and scientific external affairs, Sail Biomedicines

Michael Soloway, Advancing RNA’s acquisition editor, recently spoke with Michael Lam, Ph.D., to discuss his company’s approach to in vivo CAR-T — centered on its “endless” RNA construct and targeted nanoparticle delivery system. Unlike conventional linear mRNA, this circularized RNA format is designed to extend protein expression while maintaining transience, a balance that may be critical for achieving deep B-cell depletion without the long-term risks associated with permanent cell modification.

Mechanism & Platform Design

ARNA: How does your “endless” RNA differ from conventional mRNA in terms of structure, stability, and duration of CAR expression?

ML: Endless RNA is fundamentally different from linear mRNA in that it has no free 5′ or 3′ ends, which makes it less susceptible to exonuclease degradation. That structural difference matters because it can support a longer productive expression window, rather than the early burst-and-drop profile that is typical of linear mRNA. In this setting, that translates into more durable, still transient CAR expression, which is important for dose efficiency and for generating enough pharmacology to drive a potentially curative immune reset.

ARNA: What design features allow you to extend CAR signaling without increasing innate immune activation or toxicity?

ML: Our approach is not about forcing more expression at any cost. It is about combining endless RNA with targeted nanoparticles so we can extend useful expression, keep it transient, and put more of the dose into the right cells rather than dosing higher and running into toxicity issues. We also design the delivery system to reduce hepatocyte expression, improve endosomal escape in T cells, and avoid T-cell activation in the absence of CAR engagement, all of which support a wider therapeutic index.

In Vivo CAR-T Biology

ARNA: What are the biggest biological challenges in generating functional CAR-T-like activity in vivo versus ex vivo approaches?

ML: The challenge is not simply getting high CAR expression. The real challenge is generating enough CAR expression, in enough T cells, for long enough, and in the right physiological compartments to re-create the depth of B-cell depletion that has mattered clinically in autoimmune disease. Doing that in vivo, without lymphodepletion, ex vivo manipulation, or permanent genomic integration, requires tight control of pharmacology, delivery, tissue access, and safety all at once.

ARNA: Why is it important to target both CD4 and CD8 T cells, and how does that impact outcomes in autoimmune disease?

ML: For us, pan-T-cell targeting matters because this is a transient approach. If you want deep and efficient B-cell depletion from a short dosing cycle, broadening the effector pool is important. CD4 T cells are not just bystanders; they can contribute meaningfully to killing, and because they are more abundant than CD8 T cells, targeting both populations can increase overall killing capacity and help drive the tissue-level depletion required for immune reset.

Delivery & Targeting

ARNA: How are your nanoparticles engineered to selectively deliver RNA to T cells while minimizing off-target effects?

ML: The nanoparticles are built around selective targeting ligands that bind CD4 and CD8 T cell subsets and materially improve uptake and internalization compared with untargeted particles. That targeting layer is combined with our proprietary lipid composition optimized for systemic circulation, endosomal escape, and reduced hepatocyte expression. The result is a delivery system intended to put more of the payload into T cells, while limiting wasted exposure elsewhere.

ARNA: What enables effective delivery into lymphoid tissues like lymph nodes and bone marrow?

ML: A key part of the answer is that we are not relying on passive distribution alone. We are efficiently delivering into T cells, including memory subsets with tissue-migrating potential, and we see that translated into delivery and activity in blood and lymphoid tissues across preclinical models, including nonhuman primates. That is important because autoimmune biology is not confined to peripheral blood, and neither can the therapeutic effect be.

Tissue-Level Efficacy

ARNA: Why is deep B-cell depletion in lymph nodes and bone marrow critical for treating autoimmune disease?

ML: Because partial depletion in blood is not the same thing as resetting disease biology. If autoreactive B-cell populations persist in lymphoid tissues, or if progenitor populations in the bone marrow are not adequately addressed, it becomes much harder to achieve the depth of depletion associated with durable remission. That is why we focus on deep depletion not only in circulation but also in lymph nodes and bone marrow progenitors.

ARNA: How does your approach overcome the limitations of therapies that primarily act in peripheral blood?

ML: The goal is to move beyond a blood-only readout and generate the kind of systemic biology that matters in autoimmune disease. In our preclinical models, we have seen deep depletion across blood, spleen, lymph nodes, and bone marrow, followed by repopulation with a largely immature B-cell phenotype. That is a very different biological picture from therapies that reduce circulating B cells but leave tissue reservoirs and progenitor compartments largely intact.

Pharmacology & Control

ARNA: What have you observed about the kinetics of CAR expression, and what duration appears necessary for therapeutic effect?

ML: What matters is not permanent CAR expression. What matters is achieving enough expression over a short, defined window to activate T cells, drive expansion, and produce deep depletion. Our work suggests that a several-day expression window, paired with a short, fractionated dosing cycle, is sufficient to generate the pharmacology needed for effect while preserving the transient nature of the approach.

ARNA: How precisely can CAR activity be tuned through dosing, and what defines the optimal therapeutic window?

ML: This is a highly tunable system because the exposure profile is shaped by both the RNA and the delivery vehicle. We can tune translation characteristics through the endless RNA design and tune cell access and dose efficiency through the targeted nanoparticle. In our view, the optimal window is the one that produces deep depletion and immune reset from a short treatment cycle, while keeping CAR expression transient and maintaining a wide therapeutic index.

Safety & Transience

ARNA: How does transient CAR expression change the safety profile compared to traditional CAR-T therapies?

ML: Transient expression changes the equation in an important way. We are aiming for the biologic effect of deep depletion without permanent engineered cell populations, genomic integration, lymphodepleting conditioning, or the long tail of exposure associated with conventional autologous CAR-T. Said differently, the intent is to separate durable therapeutic effect from durable therapeutic presence.

ARNA: What are the key risks you are monitoring, particularly around cytokine release and repeat dosing?

ML: The key questions are the ones you would expect for an in vivo immune cell-targeting approach: cytokine release, infusion-related effects, off-target expression, liver exposure, and repeat-dose tolerability. Preclinically, the profile is intended to reduce those risks through transient expression, targeted delivery, and a lower effective dose, but these remain critical areas to monitor closely in translation. For repeat dosing in particular, the field still has work to do, and we view that as an important area for continued optimization.

Immune Reset & Clinical Translation

ARNA: What does “resetting” the immune system look like mechanistically in your preclinical models?

ML: Mechanistically, immune reset starts with deep depletion of CD19-positive B cells across circulation, lymphoid tissues, and bone marrow progenitors during a short cycle of transient CAR expression. What follows is just as important: B-cell aplasia (depletion) persists, then repopulation begins from an immature state rather than simply restoring the same mature compartment that was there before. That pattern is what makes this more than a depletion story; it is an immune reset story.

ARNA: What biomarkers will be most important in early clinical studies to demonstrate efficacy and durability?

ML: At a platform level, the clearest early readouts are depth of circulating B-cell depletion, duration of aplasia, the timing of repopulation, and the maturity state of repopulating B cells. Those are the most direct translational links to the biology we think matters. On top of that, each indication will need its own disease-relevant markers, but the foundational proof points are depth, durability, and evidence that repopulation is occurring from a reset B-cell compartment rather than a persistently dysregulated one.

Platform Potential

ARNA: Beyond CAR-T, how broadly can your RNA and delivery platform be applied across other immune-mediated diseases?

ML: We see this as broader than a single CAR or a single disease. The combination of durable but controllable RNA expression and targeted extrahepatic delivery should be relevant anywhere immune cell-directed pharmacology is the bottleneck. In autoimmune disease specifically, the same underlying approach has the potential to extend across multiple B cell-driven settings, and longer term the platform should support a broader set of immune-directed payloads as well.

About The Expert:

Michael Lam, Ph.D., is vice president, strategy and scientific external affairs at Sail Biomedicines. Lam is a strategic and scientific leader who shapes Sail’s R&D narrative, partnership strategy, and external positioning — bridging science and business to drive innovation and value creation across the company’s genetic medicine programs. A cancer geneticist by training, Lam brings over 16 years of experience across CRISPR-Cas9 and base editing, RNA therapeutics, small molecules, and cell therapy modalities. Before joining Sail, he held senior leadership roles at Intellia Therapeutics, where he led the ex vivo cell therapy portfolio, was key to IND-enabling activities for NTLA-5001 and delivered the company’s first allogeneic CAR-T development candidate. Earlier in his career at Merck Research Laboratories, Lam led the mutant IDH1 small molecule program and served as translational science lead for the MK-2206 and MK-8628 clinical teams. An accomplished translational scientist and drug hunter, Mike has authored multiple Nature and Cell publications, co-invented patents on gene-edited cell therapies, and is recognized for translating complex science into impactful partnerships and R&D programs. He earned his Ph.D. in cell biology from Baylor College of Medicine and completed postdoctoral training at the Whitehead Institute for Biomedical Research (MIT) as an American Cancer Society Fellow.