Antibody-Drug Conjugates Show Promise in Development of Targeted Therapy for T-Cell Lymphomas

In general, peripheral T-cell lymphomas (PTCLs) are associated with a poor prognosis and limited therapeutic options for patients with refractory or relapsed disease.^1,2  In contrast to the rapid growth in targeted therapy directed against B-lineage antigens seen in the past 20 years, progress in targeted therapy in PTCL has been slower. The reasons for this are varied.^3,4  A major barrier is that a pan-T cell antigen approach damages both healthy and neoplastic T cells, resulting in severe immunosuppression.

The TRBC allele presents a possible mechanism for targeting neoplastic T cells while sparing normal T cells. During somatic recombination, alpha-beta T cells show mutually exclusive expression of one of two TRBC alleles. Therefore, a normal population of T cells will express either TRBC domain 1 or 2 (TRBC1 or TRBC2) as part of the T-cell receptor in an approximately equal ratio, while a neoplastic population of T cells will express only one or the other. In 2017, Paul M. Maciocia, PhD, and colleagues published their identification of JOVI-1, a monoclonal antibody to TRBC1.⁵  Five years later, Kate Cwynarski, PhD, and colleagues presented the preliminary results from an ongoing phase I/II clinical trial testing the safety and efficacy of anti-TRBC1 CAR T cells (NCT03590574).⁶  The treatment was well-tolerated, with no dose-limiting toxicities. In the parallel specialty of hematopathology, identification of the JOVI-1 antibody led to its incorporation into diagnostic flow cytometry assays.^7-10  Its use may serve as a rapid screen to allow for more judicious use of T-cell receptor (TCR) gene rearrangement studies. It shows exciting potential to help identify T-cell lymphomas with minimal T-cell receptor aberrancy, confidently exclude a clonal T-cell process, and help assess peripheral blood involvement by mycosis fungoides.^7,11,12  The technical aspects and interpretation of flow cytometric TRBC1 expression is easier than flow cytometry assessment of TCR beta-chain variable region expression, a technique available for the past 20 years but with limited adoption.¹³ 

In 2024, Tushar D. Nichakawade and colleagues provided evidence, by way of a series of in vitro experiments, of normal T cells killing anti-TRBC1 CAR T cells.¹⁴  This so-called “fratricide” may explain the lack of CAR-T expansion subsequently reported by Dr. Cwynarski’s research team.⁶  The remainder and bulk of Dr. Nichikawade’s paper describes the development and testing of anti-TRBC1 antibody-drug conjugates (ADCs), which use monoclonal antibodies to deliver other compounds or proteins to cells expressing certain cell surface markers. After proving that anti-TRBC1 antibodies are trafficked into lysosomes and could therefore be used for drug delivery, the ADCs of five drugs fatal to T-cell cancer cell lines were tested, with the most potent of these (the anti-TRBC1 SG3249) evaluated further, including its specificity against TRBC1+ over TRBC2+ cells. In vitro testing in cell culture showed that the presence of normal T cells did not affect the toxicity of the ADCs and confirmed minimal toxicity of the ADCs on the TRBC2+ normal T-cell subset. In mouse models with TRBC1+ T-cell cancer (Jurkat) cells, mice treated with SG3249 showed tumor regression and prolonged survival. Human studies using this ADC have not yet been performed.

Also in 2024, Mathieu Ferrari, PhD, and colleagues reported on the development of a monoclonal antibody against TRBC2.¹⁵  The anti-TRBC2 antibody was developed via strategic mutations on complementarity-determining regions of the JOVI-1 antibody, resulting in switched specificity to TRBC2. Anti-TRBC2 monoclonal antibodies allow for dual-flow cytometric evaluation of both TRBC1 and TRBC2, which can help diagnostically with a small subset of cases in which interpretation of TRBC1 alone can be challenging.¹⁶ TRBC1 is already incorporated into flow cytometry panels in many clinical flow cytometry laboratories, and we will likely start to see more laboratories adopt TRBC2 evaluation. (The first commercially available anti-TRBC2 conjugated antibody for diagnostic flow cytometry became available in January 2024). TRBC2-directed CAR T cells or other TRBC2-targeted therapies have yet to be evaluated clinically.

This is an exciting transition point from the benchwork generation of specific antibodies against TRBC1 and TRBC2 to their use in diagnostic clinical flow cytometry laboratories and early advances into anti-TRBC targeted therapies in T-cell lymphoma. While there is still much to learn going forward, it is inspiring to look back and see how far the field has come.