1. A lack of ideal solid cancer target antigens
The current CAR-T market is driven largely by CAR-T cells targeted at blood cancers and at very few antigens (the main one being CD19). CD19 is the target antigen for the only two FDA-approved CAR-T therapies (Kymriah and Yescarta, both approved for use in in the US 2017 and Kymriah approved for use in Australia in December 2018). However, CD19 is only present on B cells and no other cells, so a CAR-T expressing a CD19 CAR is only useful against malignancies of B cells.
In solid cancer clinical trials, there has been little positive data for most of the antigens studied. Further, the vast majority of antigens targeted in clinical trials so far have not been specific for solid cancer cells, but are over-expressed on tumour cells but also still present on healthy cells. Significant on-target off-tumour effects have been recorded where the CAR-Ts have attacked both diseased and healthy cells.