Fixed-Dose CAR T Cells Followed by Escalating UB-TT170 Increases Cytokine Levels in R/R Osteosarcoma

Administration of fixed-dose autologous anti-FL(FITC-E2) CAR T cells followed by escalating doses of the small molecule “adapter” UB-TT170 led to significant changes in cytokine levels; marked increases in post-infusion levels compared with pre-infusion levels; and a significant increase in levels of the cytokines interleukin-6 (IL-6), interferon gamma, and IL-10 in patients with relapsed/refractory osteosarcoma, according to data from the phase 1 ENLIGHTen-01 trial (NCT05312411), which were shared in a poster presentation at the 2024 SITC Annual Meeting.

Since the trial’s opening in July 2022, 10 patients have been enrolled, and 4 have been treated. Two patients, who were treated on dose regimen 1 (DR1), experienced rapid CAR T-cell expansion after a single dose of UB-TT170, demonstrating the potent biologic activity of the agent but precluding further dosing. Notably, following some dose-limiting toxicities (DLTs) on DR1, 2 patients were then treated on DR2 and did not experience DLTs but still experienced robust UB-TT170-dependent expansion and engraftment of CAR T cells.

Because UB-TT170 penetrates tumors and is retained for long periods of time due to high affinity for folate receptors, and unbound UB-TT170 is simultaneously rapidly cleared from the blood, investigators hypothesized that administration of a fixed dose of autologous anti-FL(FITC-E2) CAR T cells followed by escalating doses of UB-TT170 will deliver personalized immunotherapy to patients with high-risk osteosarcoma. The small molecule “adapter”—consisting of folate conjugated to fluorescein, a fluorescent dye used for labeling biologic specimens—offers an intriguing safety mechanism since withholding UB-TT170 dosing and administration of free fluorescein or folic acid can reverse CAR T-cell cytotoxicity.

Eligible patients for this investigation included adolescents and young adults (15-30 years of age) with relapsed/refractory osteosarcoma, measurable disease, and adequate organ function. Central nervous system metastasis was permitted so long as it was not symptomatic nor requiring medical intervention.

This phase 1 study used a 3 + 3 design for dose escalation, and the experimental therapy consisted of anti-FL(FITC-E2) second-generation CAR(4-1BB:zeta) plus intrapatient escalating doses of UB-TT170. The primary objectives of the study were the assessment of safety and tolerability, as well as the identification of a recommended dose-escalation sequence of UB-TT170 to move forward in clinical development.

Patients who were evaluated or enrolled included:

• Subject S001, who was enrolled on DR1 at age 21 with recurrent/progressive disease. The site of disease at the time of CAR T-cell infusion was pulmonary/pleural.
• Subject S002, who was not dosed at age 19 and had recurrent/progressive disease, with no specific site of disease listed.
• Subject S003, who has enrollment pending at age 18 with recurrent/progressive disease and no specific disease site listed.
• Subject S004, who was not dosed at age 15 and had recurrent/progressive disease and no site of disease provided.
• Subject S005, who was not dosed at age 15 and presented with refractory disease in no listed disease site.
• Subject S006, who was enrolled on DR2 at age 19 with recurrent/progressive disease. The disease site was pulmonary.
• Subject S007, who was enrolled on DR1 at age 27 with recurrent/progressive disease located in the pulmonary/pleural regions.
• Subject S008, who was not dosed at age 22 and had recurrent/progressive disease and no specific site of disease listed.
• Subject S009, who has enrollment pending at age 26 with recurrent/progressive diseaseand no specific disease site provided.
• Subject S010, who enrolled on DR2 at age 25 with recurrent/progressive disease located in the pulmonary/pleural regions.

DR1 was administered to patients S001 and S007. These patients received lymphodepletion with fludarabine/cyclophosphamide on days –6 to –3 followed by CAR T-cell infusion of 1.0 × 106 cells/kg on day 0. Patients then received TT170 dose escalation, starting with 1% of the full UB-TT170 dose on day 4 and followed by escalation to 10% of the full UB-TT170 dose on day 7 and escalation to 100% of the full UB-TT170 dose on day 11. Subsequently, these patients received a stable UB-TT170 dose at the maximum tolerated dose (MTD), as determined during the dose-escalation phase, with 1%, 10%, or 100% of the full UB-TT170 dose on days 18 and 25. Disease restaging occurred on days 26 to 32.

Notably, a full dose of UB-TT170 was defined as 3.1 × 10­–2 mg/kg; UB-TT170 doses were characterized as a percent of the full dose (1%, 10%, or 100% of 3.1 × 10–2 mg/kg).

DR2 was administered to patients S006 and S010. On days –6 to –3, lymphodepletion with fludarabine/cyclophosphamide occurred, followed by CAR T-cell therapy dosing at 0.5 × 106 cells/kg; on day 0. On day 4, these patients received 0.1% of the full UB-TT170 dose; on day 7, this dose was escalated to 0.25% of the full UB-TT170 dose, which was then escalated to 0.5% of the full UB-TT170 dose on day 11. On days 18 and 25, these patients received the stable UB-TT170 dose at the MTD as determined during the dose-escalation phase, with doses at 0.1%, 0.25%, or 0.5% of the full UB-TT170 dose. On days 26 to 32, disease restaging took place.

Regarding the safety profile of the investigational approach, patients who received DR1 with 1 infusion had DLTs including grade 3 immune effector cell–associated hemophagocytic lymphohistiocytosis–like syndrome (IEC-HS), decreased fibrinogen levels, anorexia, dyspnea, hypoxia, and hypertriglyceridemia, as well as grade 2 pancreatitis. Other grade 3 toxicities included cytokine release syndrome (CRS) and headache. Other grade 2 adverse effects (AEs) included immune effector cell­–associated neurotoxicity syndrome (ICANS) and pleural effusion.

In patient S006, who received DR2 with 4 infusions, AEs included grade 3 CRS, hypoxia, and headache; grade 2 IEC-HS; and grade 1 ICANS. In patient S010, who received DR2 with 12 infusions, AEs included grade 2 IEC-HS, disseminated intravascular coagulation, CRS, and headache.

These AEs were managed with multi-agent immunomodulation with a duration of 2 to 8 weeks and 2 to 7 days for DR1 and DR2, respectively.

“The trial is now closed to further accrual with review of translational and clinical data, including pharmacokinetic data, to inform future dosing strategy,” study authors concluded in a poster presentation of the data.

Reference

Albert K, Pinto N, Taylor M, et al. Early experience on ENLIGHTen-01: A phase 1 study of FITC-E2 CAR T-cells with CAR adaptor molecule UB-TT170 for relapsed/refractory osteosarcoma. Presented at: 2024 SITC Annual Meeting; November 6-10, 2024; Houston, TX. Abstract 233.