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.

The hype around in-vivo CAR-T overlooks that it's a more complex form of gene therapy. It relies on the same delivery vectors (AAVs, LNPs) that have caused patient deaths and off-target effects like liver concentration in simpler gene therapies, suggesting a difficult path to success.

The success of early CAR-T cell therapies was partly luck. Future therapies face a high bar, as an ideal target must meet three criteria: 1) be abundant on cancer cells, 2) be indispensable for the cancer's survival, and 3) be dispensable for the patient's healthy tissues to avoid lethal toxicity.

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.

T-cell engagers (TCEs) are likely to be safer in autoimmune conditions than in cancer. Autoimmune patients have a relatively normal B-cell count, unlike the massive proliferation in hematologic cancers. This lower target cell burden naturally limits the scale of T-cell activation and inflammatory toxicity.

A leading hypothesis for the fatal toxicities in Novartis's autoimmune CAR T trial is its "T-Charge" rapid manufacturing platform. By minimizing ex vivo manipulation, the process yields more "naive" and potent T-cells, which may also be more prone to triggering severe inflammatory cascades.

CAR-T therapy involves a significant but finite period of acute toxicity (CRS, ICANS) that resolves within about a month. In contrast, bispecific antibodies can cause persistent, low-grade immune activation symptoms like fatigue and malaise that last for the entire duration of the continuous treatment.

Published data from top cancer centers indicates a real-world treatment-related mortality (TRM) rate of 10% for Siltacel CAR T therapy. This figure is higher than reported in pivotal trials and underscores the significant risks of managing these patients outside of a controlled study.