Late Polyclonal P-BCMA-101 CAR-T Cell Re-Expansion and Rapid Complete Response in a Patient with Relapsed Multiple Myeloma Treated with One Cycle of Talquetamab, More Than 3 Years after CAR-T Infusion

BCMA directed CAR-T therapy has revolutionized the treatment of relapsed refractory multiple myeloma (RRMM) and patients who receive CAR-T can achieve deep and durable remissions. However, relapse appears uniform, possibly due to limited CAR persistence as detection of CAR (+) cells wanes rapidly over the first 6 months and significant re-expansion of CAR-Ts has not been previously demonstrated. More recently, reports of malignant transformation of the infused CAR-T cells have led to boxed warnings regarding the risk of T-cell lymphoma following CAR-T therapy.

Here, we report a case of a 57-year-old female with RRMM who initially received P-BCMA-101, a T_SCM rich autologous CAR-T manufactured using the piggyBac DNA delivery system, on 8/24/20 in combination with Rituximab (375 mg/m² on days -12, -5 and then every 8 weeks for 10 cycles) and LD (cyclophosphamide 300 mg/m²/day and fludarabine 30 mg/m²/day on days -5, -4 and -3). Following CAR infusion, the patient had no CRS or ICANS and a maximum absolute lymphocyte count of 1.2 x10⁹ cells/L. She achieved a stringent complete response and remained in remission for approximately 31 months at which time she had rising lambda light chains. She was treated with Daratumumab, Pomalidomide and dexamethasone for ~7 months with stable disease and then Selinexor and dexamethasone for 2 months which was poorly tolerated.

On 11/29/23, more than 3 years since prior P-BCMA-101 CAR therapy, the patient initiated talquetamab, a GPRC5D/CD3 targeted TCE. She received 3 step-up and 1 full-dose of talquetamab (0.01 mg, 0.06 mg, 0.4 mg and 0.8 mg/kg on days 1, 3, 5, 7). Prior to talquetamab, the WBC was 3.7 X 10⁹ cells/L and the lymphocyte count (ALC) was 0.78 X 10⁹ cells/L. On day 7 the patient’s WBC count was 7.1 X 10⁹ cells/L and ALC was 4.1 X 10⁹ cells/ L. On day 8, the WBC increased to 40.6 X10⁹/L and ALC to 34.96 X 10⁹ cells/L. no further talquetamab was administered. The WBC and ALC reached a peak of 91.9 X 10⁹ and 88 X 10⁹ cells/L, respectively on day 14 following talquetamab. Flow cytometry on peripheral blood and marrow aspirate samples revealed a T-cell population that expressed TCR αβ, CD2, CD3, CD4, CD5 and variable CD57. The cells did not express CD7, CD8, CD10 or CD56. These findings raised the concern for a mature T-cell lymphoproliferative disorder.

The sample underwent PCR analysis for TRB and TRG which likewise demonstrated clonality. Further analysis ensued including quantitative polymerase chain reaction (qPCR) of peripheral blood to detect the presence of the CAR-T transcript. These studies revealed 678,120 copies/µg DNA of the CAR transgene confirming that the CAR T cells accounted for most circulating WBC/T cells. Integration site analysis was performed using LM-PCR and Illumina NGS on 2 samples, drawn from the patient on 21-Mar-2023 and 14-Dec-2023), and each with 6 replicates. Over 12,500 CAR transgene insertion sites total were detected, with 8,612 sites detected in the March sample and 5,092 sites detected in the December samples, and 1,069 sites shared in both samples. The 4 most abundant insertions mapped to introns or proximal promoters of 4 genes. Cancer gene census database queries of these genes revealed that none of these 4 genes as a Tier 1/2 cancer census gene, a cancer curated gene, or a cancer hallmark gene in the Catalog of Somatic Mutations in Cancer (COSMIC) database. The diversity of CAR transgene insertion sites discovered was consistent with a polyclonal process. Critically, standard methods for determining clonality in clinical practice – flow cytometry and TCR molecular assays – would have inappropriately led to the conclusion that the proliferation was neoplastic.

The WBC and ALC decreased spontaneously, measuring 15.1 X 10⁹ and 12.29 x 10⁹ cells/L, respectively, on day 50 following talquetamab. The patient rapidly achieved undetectable serum free light chains and continues to be complete remission >6 months from last dose of talquetamab. It is tempting to speculate that the strong anti-myeloma response seen after one therapeutic dose of talquetamab was augmented by the second wave of CAR-T proliferation. This is the first such report of a transposon manufactured T_SCM rich CAR-T showing late and dramatic re-expansion following exposure to a TCE. This case also demonstrates that T-cell proliferations following CAR-T therapy may not always be malignant and thorough molecular analysis beyond what is the clinical standard is necessary to rule out benign processes.