Protein tyrosine phosphatase SHP2, encoded by the PTPN11 gene, represents a nonreceptor PTP containing two tandem N-terminal SH2 domains (N-SH2 and C-SH2), a classic catalytic protein tyrosine phosphatase (PTP) domain, and a C-terminal tail containing several phosphorylation sites [1,2]. It serves as a critical modulator in the regulation of diverse cellular signaling cascades, including PI3K/AKT, RAS/RAF/MEK/ERK, and RAS/MAPK [[3], [4], [5]].Besides, SHP2 is also reported to have synergy with PD-1 in the immunosuppression of the PD-1/PD-L1 signaling pathway, suggesting possible application of SHP2 inhibition in immune-oncology [[6], [7], [8], [9], [10]]. This ubiquitously expressed enzyme exerts as positive functions in diseases including Noonan syndrome (NS), LEOPARD syndrome (LS), juvenile myelomonocytic leukemia (JMML), B-cell acute lymphoblastic leukemia (B-ALL), and various solid tumors [[11], [12], [13]]. The clear connection between dysregulates of SHP2 and these human diseases qualifies SHP2 as highly relevant biological target for the development of anticancer therapeutics [14].

During the past two decades, several small molecule active site inhibitors have been described, but none have progressed to clinic application due to the highly conserved and positively charged nature of the active site pocket shared by all PTPs family members [15,16]. Until 2016, Novartis reported the first allosteric inhibitor SHP099 (SHP2WT IC50 = 0.071 μM) with good selectivity and oral bioavailability [17,18]. Since that time, a few allosteric inhibitors of SHP2, including TNO155, RMC-4630, and JAB-3068, have been progressed into clinical trials for the treatment of numerous cancers [19,20]. However, it is noteworthy that no successful marketing of SHP2 inhibitors has been reported due to the potential toxic-side effects [16]. Currently, conventional anticancer drugs suffer numerous limitations, including nonspecific tissue distribution, toxic side effects, and poor biocompatibility. Meanwhile, the biological fluorescence imaging technique has been widely applied to visual biological process in chemical biological research and medical diagnosis owing to its non-invasive detection, high spatiotemporal resolution, continuous real-time monitoring and easy operation. [[21], [22], [23], [24], [25]]. Various fluorescent molecules have been used as selective bioimaging tools for metal/cation/anion ions, enzymes, biological organ/tissues and so on [[26], [27], [28], [29], [30], [31]]. Therefore, the fluorescent molecules have the potential for promoting novel SHP2 inhibitors development.

In our previous study, we have developed molecule A with fluorescence characteristics and biological activity for SHP1, but the inhibitory activity and fluorescence performance are needed to be improved [32]. On the other hand, thiadiazole compound B showed better SHP2 inhibitory activity with an IC50 of 2.11 ± 0.99 μM [33](Fig. 1). Encouraged by these research experience, imidazo[2,1-b][1,3,4]thiadiazole scaffold caught our attention. The imidazo[2,1-b][1,3,4]thiadiazole scaffold stands for a vital class of pharmacophores in medicinal chemistry due to their myriad bioactivities, such as antibacterial, antimalarial, anti-inflammatory, anti-depressive, antiviral, and anticancer activities [[34], [35], [36], [37], [38], [39], [40]]. Although its biological activity has been extensively studied, but few studies on optical properties have been reported. In 2019, Viprabha Kakekochi's team designed and synthesized a new class of thiophene and imidazo[2,1-b][1,3,4]thiadiazole based conjugated azomethines, studied their photophysical and electrochemical properties by experiments and theoretical calculations [41]. Therefore, the fluorescence properties of some reported imidazo[2,1-b][1,3,4]thiadiazole derivatives as bio-active compounds (shown in Table 1) were evaluated by quantum chemical calculation. As listed in Table 1, these imidazo[2,1-b][1,3,4]thiadiazole derivatives (I-IV) as bio-active compounds exhibited potential fluorescence signal at the wavelength of 428–483 nm [37,[42], [43], [44]].

To further explore novel molecular frameworks with inhibitory activities against SHP2 and excellent fluorescence characteristics, based on previous research experience [[45], [46], [47]], we designed and synthesized a series of imidazo[2,1-b][1,3,4]thiadiazole derivatives, explored their inhibitory activities against SHP2 and photophysical performances (Fig. 1). With the help of quantum chemical calculation, the molecular planarity of designed compounds 4a, 4–1 and 4q was evaluated. From Fig. S1, imidazo[2,1-b][1,3,4]thiadiazole skeleton exhibited medium rigidity (4a, MPP = 0.186, SDP = 0.850) and the introduction of conjugated structures increased molecular planarity (4–1, MPP = 0.029, SDP = 0.107), implying the potential better photophysical characteristics of 6-diaryl-imidazo[2,1-b][1,3,4]thiadiazole skeleton. After systematic investigation, several imidazo[2,1-b][1,3,4]thiadiazole derivatives exhibited potent inhibitory activities against SHP2. The representative compound 4q showed good potency against SHP2 with IC50 of 2.89 ± 1.60 μM and biological assays results confirmed its effectiveness in blocking SHP2-mediated p-ERK signaling pathway and demonstrated the ability to inhibit the proliferation of MV4–11 cells in vitro with IC50 of 7.90 ± 0.75 μM. Moreover, compound 4q showed blue/green fluorescence imaging in HeLa cells and zebrafish. (Scheme 1).