A newcomer in allosteric therapy for Ph⁺ leukemias Free

In this issue of Blood, Shi et al¹ report the biologic preclinical characterization of TGRX-678, a new allosteric BCR::ABL1 inhibitor to specifically target the myristoyl-binding pocket of the oncoprotein. The ultimate goal of this study was to develop a drug that inactivates gatekeeper and compound mutations in relapsed or refractory chronic myeloid leukemia (CML) or Philadelphia-positive acute lymphoblastic leukemia (Ph⁺ ALL).

The term allosteric inhibition was introduced in the 1960s by François Jacob and Jacques Monod to describe a mechanism of feedback inhibition in which regulatory ligands bind to enzymatic sites that are distinct from the substrate-binding site.² The concept further expanded over the years and allosteric proteins were recognized as key components of physiological regulatory systems and contributors to various pathogenic processes. The discovery of allostery has enabled the subsequent identification and rational design of allosteric drugs, which have proven effective in treating a wide variety of diseases.³ 

The first insights into the regulation of the tyrosine-kinase ABL1 through an allosteric mechanism emerged in the early 2000s. This regulation involves a myristoyl-binding pocket at the C-terminus of the kinase domain (KD) and the myristoylation of the N-terminus of the 1b isoform of ABL1, which triggers a closed, autoinhibited conformation.⁴ This discovery opened the door for therapeutic allosteric inhibition of BCR::ABL1. At the same time, the development and approval of adenosine triphosphate (ATP) competitors at the ATP binding pocket of the KD of the oncoprotein BCR::ABL1 (tyrosine kinase inhibitors [TKIs]) provided the key proof-of-concept evidence that specifically targeting the oncogenic driver of CML provided dramatic therapeutic efficacy.⁵ Nevertheless, 25 years after the advent of imatinib and subsequent generations of more potent ATP-competitive inhibitors, significant therapeutic gaps and unmet needs remain. These include leukemic stem cell neutralization, resistance, and iatrogenicity.

The first allosteric inhibitors of ABL1 identified, GNF-2 and GNF-5, were deemed inadequate to progress toward clinical development.⁶ In contrast, asciminib demonstrated a remarkable capacity to overcome mutations that confer resistance to ATP-competitive TKIs in vitro and in murine models, as well as a high degree of selectivity.⁷ The therapeutic potential of asciminib was validated in clinical trials in which this allosteric inhibitor demonstrated stronger efficacy and better tolerance than first- and second-generation ATP-competitive TKIs.^8-10 It is the first US Food and Drug Administration–approved allosteric TKI authorized for use in late-line, T315I-mutated, and newly diagnosed CML.

In the work reported in this issue, Shi et al found that TGRX-678 exhibited potent inhibitory activity against both native and T315I-mutated BCR::ABL1 and showed high selectivity in in vitro kinase and cell-proliferation assays, as expected. In Ba/F3 mutant cells that expressed BCR::ABL1T315I, TGRX-678 exhibited strong antiproliferative activity, although it was not as potent as the third-generation ATP-competitive TKIs, ponatinib or olverembatinib. As has been observed previously with asciminib, mutations within or near the myristoyl pocket or mutations that destabilized the SH3/SH2/KD interface, like those in the SH3 domain, impaired or abolished its activity.

So, what makes TGRX-678 different from asciminib? When combined with ponatinib, TGRX-678 resensitized cells that harbored the ponatinib-resistant T315M mutant. Moreover, TGRX-678 and ponatinib showed greater cooperative efficacy against compound gatekeeper/P-loop mutants, such as T315I/Y253H and T315I/E255V, than the combination of asciminib and ponatinib. Furthermore, TGRX-678 did not seem to be affected by efflux transporters, a known mechanism of resistance to asciminib.

TGRX-678 demonstrated efficacy in inducing tumor regression across various native and T315I-mutated CML and Ph⁺ ALL xenograft models in vivo. TGRX-678, but not asciminib, was able to penetrate the cerebrospinal fluid at potentially therapeutic concentrations and prolonged animal survival in central nervous system tumor models more effectively than ponatinib, likely because of its better tolerability profile. In animal models, Shi et al observed a distinct pharmacokinetic profile for TGRX-678 when compared with asciminib with no apparent food effect; however, this remains to be confirmed in human studies.

Overall, although Shi et al did not provide molecular insights into why TGRX-678 differs from asciminib, the observed differences in efficacy, pharmacologic behavior, and cooperation with ponatinib suggest distinct structural or binding characteristics and may pave the way for new therapeutic indications in both CML and Ph⁺ ALL. The question of whether TGRX-678 will demonstrate effectiveness and safety in humans remains an open question, and the drug is currently undergoing development in the context of a phase 1 trial in China (ClinicalTrials.gov identifier: NCT05434312). Looking ahead, we are undoubtedly entering a novel therapeutic era in CML and Ph⁺ ALL, marked by a growing interest in allosteric inhibitors. These agents, whether used as monotherapies or in combination with ATP-competitive TKIs, degraders, or other innovative compounds, hold great promise. This evolving landscape may ultimately lead to enhanced safety, improved quality of life, and eventually a cure for Ph⁺ leukemias.

Conflict-of-interest disclosure: D.R. reports receiving honoraria from Ascentage Pharma, Enliven Therapeutics, Incyte, Novartis Pharma, and Terns Pharma.