Predicting T-ALL response to pre-TCR and IL-7R coinhibition Free

In this issue of Blood, Courtois et al present compelling preclinical data in support of a therapeutic role for coinhibition of the interleukin-7 receptor (IL-7R) and pre-T-cell receptor (TCR) pathways for a subset of T-lymphoblastic leukemia (T-ALL) and identify surface expression of the pre-TCR α chain (pTα) as a biomarker predictive of pre-TCR-directed treatment response.¹ 

Novel precision-medicine approaches will be crucial for improving T-ALL outcomes, as there remains a paucity of effective salvage options for relapsed/refractory (R/R) T-ALL. In their efforts to improve directed therapy, Courtois et al applied a patient-derived xenograft (PDX) model to show that high surface expression of pTα in T-ALL with the cortical developmental phenotype (coT-ALL) serves as a biomarker for LCK activation, which, in turn, begets tyrosine kinase inhibitor (TKI) sensitivity. LCK encodes lymphocyte-specific protein tyrosine kinase (Lck), which, when activated via the pre-TCR complex, promotes TCR signaling. Expanding upon their prior work in which they identified IL-7R expression as a predictor of T-ALL sensitivity to JAK inhibition,² Courtois et al here highlight that coinhibition of both pathways, pre-TCR (via TKI) and IL-7R (via JAK inhibition), provides synergistic activity in vitro (pre-TCR⁺ IL-7R⁺ patient-derived T-ALL), ex vivo (PDX), and in vivo, and may have relevance as a therapeutic strategy.

Courtois et al first screened a public human RNA sequencing (RNA-seq) dataset to identify signs of pre-TCR activation across thymocyte subtypes; they validated the pre-TCR-related expression pattern they discovered within their own bulk RNA-seq dataset from 146 adult patients with T-ALL, revealing enrichment of pre-TCR-specific features (namely PTCRA, which encodes pre–T-cell antigen receptor α [pTα]) in coT-ALL. They then applied anti-pTα surface staining to 50 adult T-ALL PDXs representing 3 phenotypes (immature, cortical, and mature) to reveal peak pTα expression in coT-ALL. Next, they demonstrated peak pLCK expression (as an indicator of pre-TCR activity) in coT-ALL in correlation with spTα, such that spTα provides a “surrogate” of LCK activation. They tested ex vivo sensitivity to dasatinib in this same PDX series to reveal that sensitive samples were those with a pre-αβ (sCD3⁻, cTCRβ⁺ coT-ALL) or mature-TCR phenotype. These authors had previously shown nonphysiologic coexpression of the IL-7R pathway in coT-ALL,² begetting the hypothesis that IL-7R signaling “rescues” pre-TCR⁺ coT-ALL from LCK inhibition. Indeed, they here show in 5 pre-TCR⁺ PDXs that ectopic IL-7R expression mitigates the cytotoxic effect of dasatinib. Finally, using 11 PDXs, they show that ex vivo pre-TCR (dasatinib) and IL-7R (ruxolitinib) coinhibition produces synergistic T-ALL cytotoxicity, an observation they recapitulate in vivo via NSG mice injected with pre-TCR⁺IL-7R⁺ T-ALL assigned to 4 treatment arms: control, dasatinib only, ruxolitinib only, and coinhibition. Finally, in a sophisticated attempt to validate the potential “real-life” role of dual pre-TCR/IL-7R inhibition, Courtois et al performed ex vivo testing of dasatinib and tofacitinib combination therapy in 28 patient-derived R/R T-ALL samples, 5 of which had a cortical cTCRβ⁺/pre-αβ/CD127⁺(IL-7Rα) phenotype: 4 of 5 showed high spTα expression and dasatinib/tofacitinib synergism.

T-ALL has been historically classified into immature, cortical, and mature subtypes according to its state of intrathymic differentiation arrest. At the cortical T-cell stage, LCK stimulates downstream thymocyte proliferation and differentiation via activated pre-TCR signaling. Developmental arrest and pre-TCR activation thus beget sensitivity to LCK inhibition. Dasatinib, which inhibits phosphorylation and activation of Lck, stifling TCR-pathway activation, is under keen investigation in T-ALL: Gocho et al showed ex vivo dasatinib sensitivity in 44.4% of subjects with childhood T-ALL and 16.7% of adult subjects with T-ALL, which was directly linked to pre-TCR/LCK activation,³ and Yoshimura et al established that foundational exposure-to-response pharmacologic data for TKI therapy in LCK-activated T-ALL⁴ could inform human trials like the current phase 2 investigation of dasatinib as a potential means of improving end of induction minimal residual disease negativity in children and adolescents with T-ALL (clinicaltrials.gov NCT06390319). However, the transient responsiveness to TKI monotherapy observed in some studies engendered enthusiasm for a combination approach. For example, Laukkanen et al’s ex vivo (zebrafish) high-throughput screening and preclinical T-ALL PDX validation revealed synergism between dasatinib/LCK inhibition and mTORC1 inhibition in T-ALL.⁵ The IL-7R-pathway was a more optimal target for Courtois et al to explore for this purpose, given their established model for IL-7R pathway inhibition,² the foundational role of this hematopoietic growth factor in promoting T lymphoid leukemogenesis,⁶ and the relationship between IL-7R signaling activation and numerous other targetable downstream pathways, including JAK-STAT.⁷ 

In this report, Courtois et al expand upon existing data to introduce pTα expression status as a novel biomarker for predicting which patients might most benefit from LCK inhibition, and they highlight a potential benefit of cotargeting LCK and JAK in coT-ALL (see figure). Clinical assessment of LCK activation (as a marker of dependence on pre-TCR signaling and therapeutic vulnerability) has been limited by a lack of validated markers, but Courtois et al have identified a reliable spTα antibody worthy of prospective evaluation. Of note, the data focus on the cortical T-ALL phenotype (with its intrinsic link to pre-TCR expression), which could limit generalizability, as coT-ALL represents only a subset of T-ALL and does not comprise the historically most “unfavorable” developmental subgroup, early T-cell precursor-ALL. However, Courtois et al also highlight a subset of mature T-ALL with aberrant high pTα expression, which may suggest broader clinical application, pending further study. Fortunately, numerous TKI options have already been clinically implemented with favorable safety profiles for other leukemia subtypes, namely Ph⁺ B-ALL, and JAK pathway inhibitors are likewise readily available. The challenge will be in selecting which of many evolving combinatorial approaches to focus on in the trial setting. Perhaps tailored investigations aligned to developmental state-specific features will hone trial eligibility according to each patient’s individual predicted response. In their supposition of pTα as a pre-TCR-specific biomarker predictive of dasatinib response, Courtois et al are off to a good start in informing the future of such precision medicine initiatives for T-ALL.

In summary, Courtois at al have once again identified a novel synergistic role for IL-7R-pathway inhibition in combination with a directed approach already under investigation for T-ALL, pre-TCR/LCK inhibition. Their findings pose potential benefit to patients with R/R T-ALL and are worthy of validation in the clinical setting.

Conflict-of-interest disclosure: C.F. declares no competing financial interests.