Abstract: The MYC oncogene is frequently deregulated in human malignancies and plays a critical role in tumor progression and immune evasion. Historically considered "undruggable" due to its intrinsically disordered structure and lack of deep hydrophobic pockets, recent pharmacological advancements have successfully identified small-molecule inhibitors capable of targeting MYC. This review focuses on MYCi975, a prominent MYC-MAX low molecular weight antagonist, and its therapeutic potential in Triple-Negative Breast Cancer (TNBC). MYCi975 disrupts MYC-MAX heterodimerization, promotes MYC degradation via T58 phosphorylation, and modulates the tumor microenvironment by enhancing CD8+ T cell infiltration. Preclinical models demonstrate that MYCi975 exhibits excellent pharmacokinetic properties, minimal toxicity, and synergistic growth inhibition when combined with immunotherapies (such as atezolizumab) or chemotherapies (such as doxorubicin and paclitaxel). These findings highlight MYCi975 as a highly promising targeted therapy for TNBC, paving the way for its upcoming clinical evaluation.
1. Introduction
The MYC family of genes, particularly C-MYC, represents one of the most frequently altered oncogenes in human cancer, with deregulation occurring in approximately 70% of all malignancies [1]. MYC drives tumorigenesis through both tumor-cell intrinsic mechanisms—such as promoting cell cycle progression, altering metabolism, and blocking DNA repair—and extrinsic mechanisms, notably by orchestrating immune evasion within the tumor microenvironment [1]. Despite its central role in cancer, MYC has historically been deemed "undruggable." This classification stemmed from its nature as an intrinsically disordered protein lacking deep, stable hydrophobic pockets for small-molecule binding, its predominant nuclear localization, and the absence of targetable enzymatic activity [1].
Recently, these historical barriers have been overcome with the development of novel anti-MYC compounds. Among the most widely investigated experimental classes are the MYC-MAX low molecular weight antagonists. Within this class, MYCi975 has emerged as a highly promising candidate [1]. Preclinical studies have demonstrated its efficacy across various cancer models, including prostate cancer, head and neck cancer, leukemia, and notably, Triple-Negative Breast Cancer (TNBC), making it a focal point for next-generation targeted oncology [1].
2. Pharmacological Activity
MYCi975 exhibits excellent pharmacokinetic properties across multiple modes of delivery, including intravenous, intraperitoneal, and per-oral routes. Depending on the administered dose, its half-life ranges between 7 and 12 hours [1]. In vivo, MYCi975 effectively inhibits cancer cell proliferation and modulates the immune cell composition of the tumor microenvironment. In mouse models of TNBC, administration of MYCi975 specifically altered the tumor microenvironment by increasing the infiltration of anti-tumor CD8+ T cells [1].
A significant pharmacological advantage of MYCi975 is its ability to synergize with existing cancer therapies. In TNBC models, combined treatment with MYCi975 and the anti-PD-L1 immunotherapy antibody atezolizumab suppressed tumor growth more strongly than either therapy alone. This synergistic effect is attributed to the combination of MYCi975's tumor-intrinsic growth inhibition and its extrinsic immune-enhancing effects [1]. Furthermore, co-treatment of MYCi975 with standard chemotherapeutic agents, such as doxorubicin or paclitaxel, resulted in enhanced growth inhibition in breast cancer cells [1].
Importantly, MYCi975 demonstrates a highly favorable safety profile. In animal models, it exhibited little to no toxicity, with no adverse effects on blood biochemistry, kidney function, or liver function tests. Gross and histological examinations of multiple organs (including the brain, heart, lung, liver, kidney, and spleen) showed no evidence of toxicity even at doses up to 1000 mg/kg, which is considerably higher than the therapeutic dose [1].
3. Molecular Mechanism of Action
The primary mechanism of action of MYCi975 involves the direct antagonism of the MYC-MAX protein complex. MYC relies on heterodimerization with its obligate partner, MAX, to bind to DNA and regulate gene transcription. MYCi975 physically blocks this MYC-MAX heterodimerization [1].
Beyond simply preventing DNA binding, the disruption of the MYC-MAX complex by MYCi975 leads to increased phosphorylation of the tyrosine 58 (T58) residue on the MYC protein. This specific phosphorylation event marks MYC for degradation via the 26S proteasome system, effectively depleting the oncogenic protein from the cell [1].
Consequently, MYCi975 alters the chromatin binding of MYC and profoundly modulates downstream gene expression. Treatment results in the decreased expression of genes involved in cell cycle progression, chromosome organization, DNA repair, and DNA replication. Conversely, genes involved in signal transduction pathways are upregulated, while genes implicated in core transcriptional activities and RNA biogenesis remain largely unaffected [1].
4. Structure-Activity Relationship (SAR)
MYCi975 is a small-molecule inhibitor with the chemical structure 4'-chloro-6-((4-chlorobenzyl)oxy)-3-(1-methyl-3-(trifluoromethyl)-1H-pyrazol-5-yl)-3'-(trifluoromethyl)-[1,1'-biphenyl]-2-ol [1]. Its structural design allows it to specifically target and bind to MYC at the amino acid sequence 366-381, which is located within the helix-loop-helix domain of the protein [1].
This specific binding sequence (residues 366-381) is highly enriched in hydrophobic amino acids and is postulated to exist in a transient secondary structure. By interacting with this hydrophobic-rich, transient structural region, MYCi975 successfully overcomes the traditional lack of deep binding pockets in the MYC protein, effectively preventing the structural alignment required for MAX binding [1].
5. Current Limitations
While MYCi975 has shown remarkable preclinical success, there are limitations and concerns regarding the long-term pharmacological targeting of MYC. Because MYC is involved in several normal cellular activities, avoiding severe toxicity during prolonged inhibition remains a theoretical challenge. Although short-term data shows minimal toxicity, long-term treatment data is currently lacking [1]. Caution is warranted, as recent studies have indicated that near-complete, body-wide inactivation of MYC in mice can lead to premature aging and the deterioration of several age-related body functions [1]. Additionally, the precise specificity of indirect-acting inhibitors and the potential for acquired resistance mechanisms require further investigation [1].
6. Future Perspectives
The preclinical success of MYCi975 has set the stage for clinical translation, with MYCi975 or an optimized version of the molecule expected to commence evaluation in clinical trials soon [1]. Future research must focus on identifying and validating biomarkers that can accurately predict patient response to MYCi975 and other anti-MYC compounds [1].
Furthermore, establishing whether the anti-cancer efficacy of MYCi975 can be consistently improved through combination therapies—such as with immune checkpoint inhibitors (e.g., atezolizumab) or traditional chemotherapies (e.g., paclitaxel and doxorubicin)—will be crucial for its application in TNBC [1]. Finally, the integration of artificial intelligence tools, such as AlphaFold and PocketMiner, may help identify previously unrecognized cryptic pockets within the MYC structure, potentially guiding the design of next-generation inhibitors with even greater potency and specificity [1].
