Tyrosine kinases are enzymes that catalyze the transfer of phosphate groups from ATP to tyrosine residues on proteins within cells. It plays a crucial role in regulating the “on” and “off” states of cellular signaling pathways and is involved in various physiological and pathological processes. RET is a receptor tyrosine kinase that encodes a single-channel membrane protein belonging to the GDNF (glial cell line-derived neurotrophic factor) family, located on human chromosome 10 [1]. It plays a crucial role in intracellular signal transduction and regulates various biological processes, participating in numerous physiological and developmental functions.

The role of RET in the individual development within the central nervous system, enteric nervous system, and kidneys is well-documented in the field of biology [2]. In 1985, Takahashi et al. first reported the oncogenic activity of RET. The primary mechanisms for aberrant RET signaling activation include overexpression, point mutations, and fusions with other proteins [3]. When RET becomes abnormal, it can affect various organ systems and frequently leads to tumorigenesis; thus, it has emerged as a significant therapeutic target for various cancers [2,[4], [5], [6], [7]].The overexpression of RET is observed in various malignant tumors, including breast cancer, lung cancer, colorectal cancer, melanoma, glioma, endometrial carcinoma, head and neck cancers, neuroendocrine cancers, and pancreatic cancer [8,9]. Twenty-two point mutations (such as V804L/M, A883F, S891A, and M918T) have been observed in the RET kinase domain. Research has demonstrated a significant association between RET mutations and the occurrence of several tumors: pheochromocytoma [10]; epithelial ovarian carcinoma(EOC) [11]; osteosarcoma [12,13]; glioblastoma multiforme(GBM) [14]; neuroblastoma [15,16]; large cell neuroendocrine carcinoma (LCNEC) [17]; as well as multiple endocrine neoplasia type 1/2 A/MEN2B and familial medullary thyroid carcinoma (FMTC) [[18], [19], [20]]. In addition, RET fusions have also been detected in various tumor cells, including those from patients with papillary thyroid carcinoma (PTC), non-small cell lung cancer, spitzoid tumors, and metastatic colorectal cancer (mCRC) [[21], [22], [23], [24], [25], [26], [27], [28], [29]].

Given that RET is an important therapeutic target for various refractory tumors, multiple kinase inhibitors (MKIs) have been employed in the treatment of RET-driven cancers. Since 2001, when the first small molecule protein kinase inhibitor imatinib was approved by the FDA for the treatment of chronic myeloid leukemia (CML) [30], the development of small molecule inhibitors targeting protein kinases has become a focal point for research institutions and pharmaceutical companies. Cabozantinib(1) [[31], [32], [33]], Vandetanib(2) [34], Sorafenib(3) [35,36], Lenvatinib(4) [37,38], regorafenib [[39], [40], [41]], ponatinib [42], and Apatinib [43] have all been approved for use in patients with RET-positive cancers mentioned above (Fig. 1). Thousands of cancer patients with RET abnormalities have benefited from treatment with these multi-kinase inhibitors. However, due to the nonspecific inhibition of dozens of kinases by these compounds and their lack of specificity [44], particularly the significant inhibitory effect on VEGFR2 (KDR), the therapeutic index of MKIs is relatively narrow [[45], [46], [47]]. This results in a lower tolerable dose, which can lead to adverse reactions such as thyroid dysfunction, hypertension, rash, and diarrhea. During MKI treatment, many patients have been observed to require dose reductions or even discontinuation of therapy due to an inability to tolerate these side effects [31,[48], [49], [50], [51]]. In addition, these multi-enzyme inhibitors exhibit no activity against the common RET-V804L/M/E mutations found in various cancers, which limits the application of these multi-target drugs in the treatment of RET-related tumors [31,32,34,36,37,49,52]. Alarmingly, beyond these prevalent adverse reactions, some patients have also experienced significant cardiovascular toxicity due to off-target effects [53,54], severe skin toxicity [55], and even serious bacterial infections following treatment [56]. Additionally, notable psychiatric side effects have been reported [57]; there are cases involving corneal perforation post-treatment [58]. Furthermore, life-threatening events such as heart attacks, myocardial infarctions, and strokes have occurred as well [59]. These drug-related side effects pose significant challenges for clinical applications. Moreover, preclinical studies and recent clinical reports indicate that gatekeeper mutations (V804L and V804M) within RET confer resistance to MKIs [[60], [61], [62]]. Therefore, there is an urgent need to develop selective RET inhibitors targeting these gatekeeper mutations.

In recent years, significant progress has been made in the development of novel selective RET inhibitors targeting RET fusion and mutation. Among them, Selpercatinib, as the first selective RET inhibitor approved by the US Food and Drug Administration (FDA), effectively inhibits the activity of dimeric RET fusion proteins and blocks the function of oncogenic point mutants by specifically binding to the active site of RET kinase, thereby significantly inhibiting downstream signaling pathways related to tumor proliferation and survival [57,62,63]. This drug has shown promising clinical prospects in the treatment of malignant tumors such as large cell neuroendocrine carcinoma (LCNEC) due to its excellent blood-brain barrier penetration ability [17]. At the same time, Pralsetinib, as another selective RET kinase inhibitor, exhibits higher targeting specificity and safety in cancer models compared to early multi kinase inhibitors, especially with significant inhibitory effects on gatekeeper mutations (such as RET-V804L/M) and fusion mutations [2,62,[64], [65], [66], [67], [68]].

However, recent studies [69] have revealed potential limitations of these inhibitors: Selpercatinib and Pralsetinib not only primarily target RET, but also exhibit significant cross inhibitory activity against kinases such as FLT3 (IC50 of 0.40 nmol/L and 3.3 nmol/L, respectively), FGFR1–3 (IC50 of 16–50 nmol/L), and TrkC. In addition, clinical observations have found that Selpercatinib and Pralsetinib still maintain nanomolar binding affinity for targets such as VEGFR2 (such as Pralsetinib's Ki ≈ 16 nM), and are ineffective against solvent front region mutations (such as G810C/S/R) and specific resistance mutations (such as L730V/I, V738). These findings suggest that current RET inhibitors still face challenges such as target selectivity and resistance.

Based on the above research progress, future development of RET inhibitors should focus on the following directions: (1) optimizing molecular structure to overcome acquired resistance mutations. (2) Improve targeting specificity to reduce off target effects. (3) Improve drug safety characteristics. Through innovative drug design strategies, the new generation of RET inhibitors is expected to provide more precise and lasting clinical benefits for patients with malignant tumors.