Lung cancer remains the leading cause of cancer-related mortality worldwide, with non-small cell lung cancer (NSCLC) accounting for approximately 85% of all cases [1]. While the advent of molecularly targeted therapies and immune checkpoint inhibitors has revolutionized the treatment landscape for advanced NSCLC, formidable challenges persist. These include intrinsic and acquired resistance to existing drugs, significant tumor heterogeneity, and metastatic dissemination, which collectively contribute to therapeutic failure and poor patient survival [2], [3]. Consequently, the identification of novel druggable targets and the development of corresponding therapeutic agents are urgently needed to overcome these limitations and improve clinical outcomes [4].

The mitogen-activated protein kinase (MAPK) signaling pathways are central regulators of crucial cellular processes such as proliferation, differentiation, survival, and apoptosis. Dysregulation of these pathways is a hallmark of numerous human cancers [5]. Within the canonical MAPK cascades, such as the RAS-RAF-MEK-ERK1/2 pathway, several inhibitors have been successfully developed and deployed in the clinic (e.g., BRAF and MEK inhibitors). However, extracellular signal-regulated kinase 5 (ERK5), also known as big MAPK1 (BMK1), represents a distinct and relatively underexplored member of the MAPK family [6]. Unlike ERK1/2, ERK5 possesses a unique C-terminal domain that contains a nuclear localization signal and a potent transcriptional activation function. Upon stimulation by growth factors, cellular stress, or oncogenic signals, ERK5 is phosphorylated and activated by its upstream kinase, MEK5. Activated ERK5 translocates to the nucleus, where it phosphorylates and modulates the activity of transcription factors, most notably members of the myocyte enhancer factor 2 (MEF2) family, thereby driving the expression of genes promoting tumor cell proliferation, survival, invasion, angiogenesis, and therapy resistance [7], [8].

Substantial preclinical evidence indicates that the ERK5 signaling pathway plays a critical role in the tumorigenesis of NSCLC. Studies have revealed that ERK5 is frequently overexpressed in NSCLC tissue samples, and its expression level is positively correlated with poor patient prognosis [9], [10]. Inhibition of ERK5 signaling can effectively suppress the proliferation and colony-forming ability of NSCLC cells in vitro, while inducing cell cycle arrest and apoptosis [11], [12].

Given its validated role in oncogenesis, the development of potent and selective small-molecule inhibitors of ERK5 has emerged as a promising frontier in medicinal chemistry and cancer therapeutics. First-generation tool compounds, such as BIX02189 and XMD8–92 (Fig. 1), provided crucial proof-of-concept by demonstrating that pharmacological inhibition of ERK5 recapitulates the anti-tumor effects observed with genetic ablation [13], [14]. Recent years have witnessed the rational design of next-generation inhibitors with novel chemotypes and improved profiles. Compounds such as BAY-885 and compound 5 (Fig. 1) have demonstrated excellent potency and enhanced kinome selectivity, along with robust anti-tumor efficacy in preclinical models, both as monotherapies and in combination regimens [15], [16]. These advancements not only reinforce ERK5 as a tractable therapeutic target but also provide invaluable structural insights and structure-activity relationship (SAR) knowledge for further inhibitor optimization. Despite this progress, most ERK5 inhibitors in the preclinical stage still face challenges. These inhibitors often suffered from suboptimal selectivity profiles (e.g., significant off-target activity against BRD4 or other kinases), unfavorable pharmacokinetic (PK) properties, or unacceptable toxicity in vivo, limiting their clinical translation [17]. Therefore, the discovery of novel chemotypes exhibiting high potency, exceptional selectivity, and favorable PK properties remains a significant and compelling objective in the field.

In our previous research, we designed and synthesized a series of 7-azaindole derivatives as novel ERK5 inhibitors and officially demonstrated the anti-lung cancer potential of such compounds. The structure-activity relationship study confirmed that the double bond in 1,2,3,6-tetrahydropyridine connected to the C3 position of the 7-azaindole nucleus is an important functional group for exerting anti-tumor activity [18]. The 5,7-diazaindole scaffold has attracted considerable interest due to its distinctive physicochemical and pharmacological properties and is widely recognized as a privileged structure in medicinal chemistry and drug discovery programs, where it is frequently employed as a bioisostere of indole and purine moieties. Owing to its ability to interact with enzymes and receptors with high binding affinity, 5,7-diazaindole has been particularly utilized as a core template in the development of targeted inhibitors with anticancer activity, such as Ribociclib, Capivasertib, and Trilaciclib (Fig. 2) [19], [20], [21]. As an important class of nitrogen-containing heterocycles, the 1,2,3,6-tetrahydropyridine ring is also a common functional motif in drug design, capable of forming critical interactions with biological targets and enhancing compound potency. Several anticancer agents incorporating the 1,2,3,6-tetrahydropyridine ring have advanced into Phase I clinical trials, including the ALK inhibitor APG-2449 and the ERα inhibitor EH2 (Fig. 2) [22], [23].

The latest research confirms that the aminopyrimidine scaffold can act as an effective fragment to interact with Met140 in the ERK5 hinge region (PDB code: 4B99) [24]. Driven by successful previous research and rational drug design, we synthesized a series of structurally related 7-H-pyrrole [2,3-d] pyrimidine (7-deazapurine) derivatives (Fig. 3). By studying the structure-activity relationship of the system and introducing specific substituents to optimize the interaction with the ERK5 ATP binding pocket, the aim is to obtain lead compounds with good druglike properties. Subsequently, we screened all synthesized compounds for in vitro anti lung cancer activity and conducted in-depth functional studies on the optimal compound, including its effects on proliferation, apoptosis, cell cycle, and downstream signaling pathways in NSCLC cell lines. Finally, we evaluated the in vivo anti lung cancer efficacy and preliminary safety of the lead compound in a mouse model. This study not only reports a promising class of lead compounds for anti-lung cancer, but also provides important chemical and biological basis for the development of drugs targeting the ERK5 target in the future.