Synthetic D-domain TAG epitope (Alpha-fetoprotein-derived) (TAG)

The Synthetic D-domain TAG epitope is a proprietary molecular component of the ARC-SparX platform, a modular and controllable chimeric antigen receptor (CAR) T-cell technology developed by Arcellx. This epitope is a synthetic fragment derived from human alpha-fetoprotein (AFP), specifically engineered to be occluded or hidden in the native, endogenous AFP protein to prevent off-target interactions with circulating proteins. In this therapeutic system, the TAG epitope is incorporated into soluble adapter molecules known as SparX (Soluble Protein Antigen-Receptor X-linkers). The corresponding ARC-T cells (Antigen Receptor Complex T cells) express a synthetic receptor featuring a D-domain—a small, stable, 73-amino acid three-alpha-helical protein scaffold—that specifically recognizes and binds to this TAG epitope with high affinity. In a clinical setting, SparX proteins serve as molecular bridges: one end of the SparX protein binds a tumor-associated antigen (such as BCMA in multiple myeloma or CD123 in AML), while the other end (the TAG) is recognized by the ARC-T cell. This interaction forms a stable ternary complex (ARC-T + SparX + Tumor Cell) that triggers T-cell signaling, proliferation, and the cytolytic destruction of the target tumor cell. This modular design allows for precise control over T-cell activity through SparX dosing regimens and enables the redirection of T cells to different antigens by switching the SparX adapter without the need for further genetic modification of the patient's T cells. The platform is currently being evaluated in clinical trials for various hematologic malignancies, demonstrating potent anti-tumor activity and a manageable safety profile.

Other names

AFP-derived TAGSparX TAGUniversal TAGAlpha-fetoprotein fragment epitopeSynthetic D-domain binder target

The ARC-T cell expresses a chimeric antigen receptor (CAR) that utilizes a synthetic D-domain (a small, three-alpha-helical protein scaffold) to specifically recognize a TAG epitope derived from human alpha-fetoprotein (AFP). This TAG epitope is incorporated into soluble adapter proteins called SparX (Soluble Protein Antigen-Receptor X-linkers). When the SparX protein binds to a tumor-associated antigen (e.g., BCMA), the ARC-T cell's D-domain receptor binds the TAG on the SparX protein, forming a ternary complex that triggers T-cell activation and targeted tumor cell lysis.