Design and Synthesis of Cyclolipopeptide Mimics of Dysoxylactam A and Evaluation of the Reversing Potencies against P-Glycoprotein-Mediated Multidrug Resistance.

Inspired by the structure of dysoxylactam A (DLA) that has been demonstrated to reverse P-glycoprotein (P-gp)-mediated multidrug resistance (MDR) effectively, 61 structurally simplified cyclolipopeptides were thus designed and synthesized via an effective method, and their reversing P-gp-mediated MDR potentials were evaluated, which provided a series of more potent analogues and allowed us to explore their structure-activity relationship (SAR). Among them, a well-simplified compound, 56, with only two chiral centers that all derived from amino acids dramatically reversed drug resistance in KBV200 cells at 10 µM in combination with vinorelbine (VNR), paclitaxel (PTX), and adriamycin (ADR), respectively, which is more promising than DLA. The mechanism study showed that 56 reversed the MDR of tumor cells by inhibiting the transport function of P-gp rather than reducing its expression. Notably, compound 56 effectively restored the sensitivity of MDR tumors to VNR in vivo at a dosage without obvious toxicity.

SIBP-03, a novel anti-HER3 antibody, exerts antitumor effects and synergizes with EGFR- and HER2-targeted drugs.

HER3 (human epidermal growth factor receptor 3) acts through heterodimerization with EGFR (epidermal growth factor receptor) or HER2 to play an essential role in activating phosphoinositide 3-kinase (PI3K) and AKT signaling-a crucial pathway that promotes tumor cell survival. HER3 is a promising target for cancer therapy, and several HER3-directed antibodies have already entered into clinical trials. In this study we characterized a novel anti-HER3 monoclonal antibody, SIBP-03. SIBP-03 (0.01-10 µg/mL) specifically and concentration-dependently blocked both neuregulin (NRG)-dependent and -independent HER3 activation, attenuated HER3-mediated downstream signaling and inhibited cell proliferation. This antitumor activity was dependent, at least in part, on SIBP-03-induced, cell-mediated cytotoxicity and cellular phagocytosis. Importantly, SIBP-03 enhanced the antitumor activity of EGFR- or HER2-targeted drugs (cetuximab or trastuzumab) in vitro and in vivo. The mechanisms underlying this synergy involve increased inhibition of HER3-mediated downstream signaling. Collectively, these results demonstrated that SIBP-03, which is currently undergoing a Phase I clinical trial in China, may offer a new treatment option for patients with cancers harboring activated HER3, particularly as part of a combinational therapeutic strategy.

Preclinical Evaluation of 9MW2821, a Site-Specific Monomethyl Auristatin E-based Antibody-Drug Conjugate for Treatment of Nectin-4-expressing Cancers.

Overexpression of nectin cell adhesion protein 4 correlates with cancer progression and poor prognosis in many human malignancies. Enfortumab vedotin (EV) is the first nectin-4-targeting antibody-drug conjugate (ADC) approved by the FDA for the treatment of urothelial cancer. However, inadequate efficacy has limited progress in the treatment of other solid tumors with EV. Furthermore, ocular, pulmonary, and hematologic toxic side effects are common in nectin-4-targeted therapy, which frequently results in dose reduction and/or treatment termination. Thus, we designed a second generation nectin-4-specific drug, 9MW2821, based on interchain-disulfide drug conjugate technology. This novel drug contained a site specifically conjugated humanized antibody and the cytotoxic moiety monomethyl auristatin E. The homogenous drug-antibody ratio and novel linker chemistry of 9MW2821 increased the stability of conjugate in the systemic circulation, enabling highly efficient drug delivery and avoiding off-target toxicity. In preclinical evaluation, 9MW2821 exhibited nectin-4-specific cell binding, efficient internalization, bystander killing, and equivalent or superior antitumor activity compared with EV in both cell line-derived xenograft and patient-derived xenograft (PDX) models. In addition, 9MW2821 demonstrated a favorable safety profile; the highest nonseverely toxic dose in monkey toxicologic studies was 6 mg/kg, with milder adverse events compared with EV. Overall, 9MW2821 is a nectin-4-directed, investigational ADC based on innovative technology that endowed the drug with compelling preclinical antitumor activity and a favorable therapeutic index. The 9MW2821 ADC is being investigated in a phase I/II clinical trial (NCT05216965 and NCT05773937) in patients with advanced solid tumors.

Carbohydrate Strengthens the Immunotherapeutic Effect of Small-Molecule PD-L1 Inhibitors.

PD-1/PD-L1 checkpoint blockade has demonstrated great success in cancer immunotherapy. Small-molecule PD-L1 inhibitors also attract significant research interests but remain challenging in the efficacy and safety. Carbohydrate moiety and carbohydrate-binding proteins (lectins) play important roles in immune modulation including antigen recognition and presenting. Herein, we reported a novel strategy to strengthen the immunotherapeutic effect of small-molecule PD-L1 inhibitors by introducing sugar motifs, which may utilize the carbohydrate-mediated immune enhancement for cancer treatment. The data revealed that glycoside compounds containing mannose or N-acetylglucosamine exhibited the best results in IFN-γ secretion. Moreover, compared to the nonglycosylated compounds, glycosides C3 and C15 demonstrated significant lower cytotoxicity and effective in vivo antitumor potency in the CT26 and melanoma B16-F10 tumor models with good tolerance. Notably, tumor-infiltrating lymphocyte (TIL) analysis validated increased CD3+, CD4+, CD8+, and granzyme B+ T cells after glycoside treatments. This work presents a new concept to improve the immunotherapy.

Modular Biomimetic Strategy Enabled Discovery of Simplified Pseudo-Natural Macrocyclic P-Glycoprotein Inhibitors Capable of Overcoming Multidrug Resistance.

Natural macrocycles have shown impressive activity to overcome P-glycoprotein (P-gp)-mediated multidrug resistance (MDR). However, the total synthesis and structural modification of natural macrocycles are challenging, which would hamper the deeper investigations of their structure-activity relationship (SAR) and drug likeness. Herein, we describe a modular biomimetic strategy to expeditiously achieve a new class of macrocycles featuring polysubstituted 1,3-diene, which efficiently inhibited P-gp and reversed MDR in cancer cells. The SAR analysis revealed that the size and linker of the macrocycles are important structural characteristics to restore activity. Particularly, 32 containing a naphthyl group and (d)-Phe moiety has higher potency with an excellent reversal fold than verapamil at a concentration of 5 µM, which induces conformational change of P-gp and inhibits its function instead of altering P-gp expression. Furthermore, 23 and 32 were identified to be attractive leads, which possess a good pharmacokinetic profile and antitumor activity in a KBV200 xenograft mouse model.

Combined inhibition of PARP and ATR synergistically potentiates the antitumor activity of HER2-targeting antibody-drug conjugate in HER2-positive cancers.

The therapeutic management of various HER2-positive malignancies involves the use of HER2-targeted antibody-drug conjugates (ADCs). The primary mechanism of action of ADCs is the release of cytotoxic chemicals, which leads to single- or double-strand DNA breaks and cell death. Since both endogenous and exogenous sources of DNA damage are unavoidable, cells have evolved DNA damage-repair mechanisms. Therefore, combining inhibitors of DNA damage repair and HER2-targeted ADCs may be a practical strategy for treating HER2-positive cancers. Effects of the HER2-targeted ADC, DS-8201, in combination with PARPi (AZD2281), a DNA damage repair inhibitor that targets poly(ADP-ribose) polymerase, and ATRi (BAY1895344), which inhibits the serine/threonine kinase ATR, were determined by assessing cell-growth inhibition, apoptosis and cell-cycle arrest, as well as using in vivo pharmacodynamic studies. Combined use of AZD2281 and BAY1895344 synergistically potentiated the inhibitory effects of DS-8201 on the growth of HER2-positive cancer cells, inducing DNA damage and apoptosis, but had no effect on HER2-negative MDA-MB-231 breast cancer cells. Our data demonstrate that DS-8201 and DNA damage repair inhibitors together have synergistic anticancer effects in NCI-N87 xenograft models, effects that may reflect upregulation of γ-H2AX protein in tumor tissues. Collectively, our results indicate that the combination of DS-8201, BAY1895344, and AZD2281 exerts significant synergistic antitumor activity, suggesting that DNA damage-repair inhibitors in combination with HER2-targeted ADCs is a potential approach for treating HER2-positive malignancies, offering a promising strategy for future clinical applications.

Trastuzumab in combination with PEGylated interferon-α1b exerts synergistic antitumor activity through enhanced inhibition of HER2 downstream signaling and antibody-dependent cellular cytotoxicity.

The anti-HER2 monoclonal antibody trastuzumab is the mainstay of treatment for HER2-positive breast and gastric cancer, and its combination with multiple chemotherapeutic agents has represented an effective and rational strategy in the clinic. In this study, we report that trastuzumab in combination with PEGylated interferon-α1b (IFN-α1b), a polyethylene glycol (PEG)-conjugated form of a subtype of interferon alpha (IFN-α), synergistically inhibited the proliferation of HER2-positive cells, including BT-474 and SK-BR-3 breast cancer cells and NCI-N87 gastric cancer cells, and also induced their apoptosis, but had no effect on HER2-negative MDA-MB-231 breast cancer cells. Trastuzumab inhibited phosphorylation of HER2, AKT and ERK, an effect that was enhanced by PEGylated IFN-α1b, likely owing to PEGylated IFN-α1b-mediated downregulation of HER2 through the lysosomal degradation pathway. Moreover, PEGylated IFN-α1b significantly enhanced trastuzumab-mediated antibody-dependent cellular cytotoxicity (ADCC) in HER2-positive cells. Importantly, trastuzumab combined with PEGylated IFN-α1b exhibited significant synergistic antitumor activity in HER2-positive BT-474 xenografts, an effect that was associated with enhanced inhibition of HER2 expression and AKT and ERK phosphorylation. Strikingly, depletion of natural killer cells with anti-Asialo GM1 antibody abrogated the synergistic antitumor activity, indicating that augmented ADCC is essential for this synergy. Taken together, our findings indicate that both enhanced inhibition of HER2 downstream signaling and augmented ADCC contribute to the synergistic antitumor activity of trastuzumab with PEGylated IFN-α1b, and imply that combining trastuzumab with PEGylated IFN-α1b could be a promising strategy for HER2-positive cancers.

TSL-1502, a glucuronide prodrug of a poly (ADP-ribose) polymerase (PARP) inhibitor, exhibits potent anti-tumor activity in preclinical models.

Poly (ADP-ribose) polymerase (PARP) enzymes play an important role in the cellular response to DNA damage and the inhibition of PARP causes synthetic lethality in homologous recombination (HR)-deficient cancer. Multiple PARP inhibitors have been developed and have shown remarkable clinical benefits. However, treatment-related toxicities, especially the hematologic toxicities, are common and restrict the clinical applications of PARP inhibitors. In this study, we designed the first glucuronide prodrug of PARP inhibitor, TSL-1502, based on a novel and highly potent PARP inhibitor TSL-1502M. TSL-1502M exhibited promising inhibitory activity on PARP1/2, significantly induced DNA double strand breaks, G2/M arrest and apoptosis in HR-deficient cells, selectively inhibited the proliferation of HR-deficient cancer cells and sensitized both HR-deficient and HR-proficient cancer cells to conventional chemotherapy. Notably, TSL-1502M was superior to olaparib, the first-in-class PARP inhibitor, in all these processes. TSL-1502 had no inhibitory effects on PARP1/2 itself, but could selectively liberate the active drug TSL-1502M in tumor after administration in nude mice. Moreover, TSL-1502 elicited significant more potent inhibitory effects than olaparib in HR-deficient tumors, and sensitized chemotherapy in both HR-deficient and HR-proficient tumors. No severe toxicities were caused by TSL-1502 in this study. Based on the encouraging preclinical antitumor activity and the selective decomposition characteristic of TSL-1502, a clinical phase I study was initiated in China, and an Investigational New Drug (IND) was granted by the US FDA. TSL-1502 could represent a new potential therapeutic choice of PARP inhibitors.

Third-generation EGFR inhibitor HS-10296 in combination with famitinib, a multi-targeted tyrosine kinase inhibitor, exerts synergistic antitumor effects through enhanced inhibition of downstream signaling in EGFR-mutant non-small cell lung cancer cells.

BACKGROUND: As a highly heterogeneous disease, lung cancer has a multitude of cellular components and patterns of gene expression which are not dependent on a single mutation or signaling pathway. Thus, using combined drugs to treat lung cancer may be a practical strategy. METHODS: The combined antitumor effects of HS-10296, a third-generation EGFR inhibitor targeting EGFR T790M mutation, with the multitargeted tyrosine kinase inhibitor (TKI) famitinib in non-small cell lung cancer (NSCLC) were evaluated by in vitro methods such as cell proliferation, apoptosis, angiogenesis assays, and in vivo animal efficacy studies. RESULTS: Famitinib strengthened the effects of HS-10296 on inhibiting proliferation and inducing apoptosis of NSCLC cells, possibly by synergistic inhibition of AKT and ERK phosphorylation. Meanwhile, HS-10296 significantly potentiated the effects of famitinib on inhibiting the proliferation and migration of HUVEC, which may be through synergistic inhibition of ERK phosphorylation in HUVEC, suggesting that HS-10296 may improve the inhibition of angiogenesis by famitinib. Moreover, combination of HS-10296 and famitinib exerted synergistic antitumor activity in NCI-H1975 and PC-9 xenograft models, and this effect may be accomplished by synergistic inhibition of phosphorylation of AKT and ERK and tumor angiogenesis in tumor tissues. CONCLUSIONS: Collectively, our results indicate that HS-10296 and famitinib exhibit significant synergistic antitumor activity, suggesting that the third-generation EGFR inhibitor combined with VEGFR inhibitor provides a promising strategy in the treatment of EGFR-mutant NSCLC.

DCZ5248, a novel dual inhibitor of Hsp90 and autophagy, exerts antitumor activity against colon cancer.

Hsp90 is a potential therapeutic target for tumor, as it maintains the stability of a variety of proteins related to tumor development and progression. Autophagy is a self-degradation process to maintain cellular homeostasis and autophagy inhibitors can suppress tumor growth. In this study, we identified DCZ5248, a triazine derivative, was a dual inhibitor of both Hsp90 and late-autophagy with potent antitumor activity against colon cancer cells in vitro and in vivo. We showed that DCZ5248 (0.1-10 µM) induced dose-dependent degradation of Hsp90 client proteins (AKT, CDK4, CDK6 and RAF-1) in HCT 116 colon cancer cells through a proteasome-dependent pathway. Meanwhile, DCZ5248 (0.3 µM) induced cytoplasmic vacuole formation, LC3 II conversion, p62 protein upregulation, and inhibited autophagy at the late stage in the colon cancer cell lines tested. We further revealed that the inhibition of autophagy was achieved by impairing lysosomal functions through induction of lysosomal acidification and attenuation of lysosomal cathepsin activity. The modulation of autophagy by DCZ5248 was independent of Hsp90 inhibition as the autophagy inhibition was not blocked by Hsp90 knockdown. Importantly, inhibition of both Hsp90 function and autophagy by DCZ5248 induced G1-phase cell cycle arrest, apoptosis, and exerted potent antitumor activity against colon cancer cells both in vitro and in vivo. These findings demonstrate that DCZ5248 is a novel dual inhibitor of Hsp90 and autophagy with potential for colon cancer therapy.

YES1 amplification confers trastuzumab-emtansine (T-DM1) resistance in HER2-positive cancer.

BACKGROUND: Trastuzumab-emtansine (T-DM1), one of the most potent HER2-targeted drugs, shows impressive efficacy in patients with HER2-positive breast cancers. However, resistance inevitably occurs and becomes a critical clinical problem. METHODS: We modelled the development of acquired resistance by exposing HER2-positive cells to escalating concentrations of T-DM1. Signalling pathways activation was detected by western blotting, gene expression was analysed by qRT-PCR and gene copy numbers were determined by qPCR. The role of Yes on resistance was confirmed by siRNA-mediated knockdown and stable transfection-mediated overexpression. The in vivo effects were tested in xenograft model. RESULTS: We found that Yes is overexpressed in T-DM1-resistant cells owing to amplification of chromosome region 18p11.32, where the YES1 gene resides. Yes activated multiple proliferation-related signalling pathways, including EGFR, PI3K and MAPK, and led to cross-resistance to all types of HER2-targeted drugs, including antibody-drug conjugate, antibody and small molecule inhibitor. The outcome of this cross-resistance may be a clinically incurable condition. Importantly, we found that inhibiting Yes with dasatinib sensitised resistant cells in vitro and in vivo. CONCLUSIONS: Our study revealed that YES1 amplification conferred resistance to HER2-targeted drugs and suggested the potential application of the strategy of combining HER2 and Yes inhibition in the clinic.

A modular biomimetic strategy for the synthesis of macrolide P-glycoprotein inhibitors via Rh-catalyzed C-H activation.

One of the key challenges to overcome multidrug resistance (MDR) in cancer is the development of more effective and general strategies to discover bioactive scaffolds. Inspired by natural products, we describe a strategy to achieve this goal by modular biomimetic synthesis of scaffolds of (Z)-allylic-supported macrolides. Herein, an Rh(III)-catalyzed native carboxylic acid-directed and solvent-free C-H activation allylation with high stereoselectivity and chemoselectivity is achieved. The generated poly-substituted allylic alcohol as a multifunctional and biomimetic building block is crucial for the synthesis of (Z)-allylic-supported macrolides. Moreover, the unique allylic-supported macrolides significantly potentiate the sensitivity of tumor cells to cytotoxic agents such as vinorelbine and doxetaxel by reversing p170-glycoprotein-mediated MDR. Our findings will inspire the evolution of synthetic chemistry and open avenues for expedient and diversified synthesis of bioactive macrocyclic molecules.

Pd-Catalyzed Decarboxylative Olefination: Stereoselective Synthesis of Polysubstituted Butadienes and Macrocyclic P-glycoprotein Inhibitors.

The efficient and stereoselective synthesis of polysubstituted butadienes, especially the multifunctional butadienes, represents a great challenge in organic synthesis. Herein, we wish to report a distinctive Pd(0) carbene-initiated decarboxylative olefination approach that enables the direct coupling of diazo esters with vinylethylene carbonates (VECs), vinyl oxazolidinones, or vinyl benzoxazinones to afford alcohol-, amine-, or aniline-containing 1,3-dienes in moderate to high yields and with excellent stereoselectivity. This protocol features operational simplicity, mild reaction conditions, a broad substrate scope, and gram-scalability. Notably, a structurally unique allylic Pd(II) intermediate was isolated and characterized. DFT calculation and control experiments demonstrated that a rare Pd(0) carbene intermediate could be involved in this reaction. Moreover, the polysubstituted butadienes as novel building blocks were unprecedentedly assembled into macrocycles, which efficiently inhibited the P-glycoprotein and dramatically reversed multidrug resistance in cancer cells by 190-fold.

SHR-A1403, a novel c-mesenchymal-epithelial transition factor (c-Met) antibody-drug conjugate, overcomes AZD9291 resistance in non-small cell lung cancer cells overexpressing c-Met.

Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKI) have been used as the first-line treatment of non-small cell lung cancers (NSCLC) harboring EGFR-activating mutations, but acquired resistance is ubiquitous and needs to be solved urgently. Here, we introduce an effective approach for overcoming resistance to the EGFR-TKI, AZD9291, in NSCLC cells using SHR-A1403, a novel c-mesenchymal-epithelial transition factor (c-Met)-targeting antibody-drug conjugate (ADC) consisting of an anti-c-Met monoclonal antibody (c-Met mAb) conjugated to a microtubule inhibitor. Resistant cells were established by exposing HCC827 to increasing concentrations of EGFR-TKI. c-Met was found to be overexpressed in most resistant cells. AZD9291 resistance was partially restored by combination of AZD9291 and crizotinib only in resistant cells overexpressing phospho-c-Met, which synergistically inhibited c-Met-mediated phosphorylation of the downstream targets ERK1/2 and AKT. In resistant cells overexpressing c-Met, neither crizotinib nor c-Met mAb was able to overcome AZD9291 resistance. In contrast, SHR-A1403 strongly inhibited proliferation of AZD9291-resistant HCC827 overexpressing c-Met, regardless of the levels of c-Met phosphorylation. SHR-A1403 bound to resistant cells overexpressing c-Met was internalized into cells and released associated microtubule inhibitor, resulting in cell-killing activity that was dependent on c-Met expression levels only, irrespective of the involvement of c-Met or EGFR signaling in AZD9291 resistance. Consistent with its activity in vitro, SHR-A1403 significantly inhibited the growth of AZD9291-resistant HCC827 tumors and caused tumor regression in vivo. Thus, our findings show that SHR-A1403 efficiently overcomes AZD9291 resistance in cells overexpressing c-Met, and further indicate that c-Met expression level is a biomarker predictive of SHR-A1403 efficacy.

Pharmacological characterization of TQ05310, a potent inhibitor of isocitrate dehydrogenase 2 R140Q and R172K mutants.

Isocitrate dehydrogenase 2 (IDH2), an important mitochondrial metabolic enzyme involved in the tricarboxylic acid cycle, is mutated in a variety of cancers. AG-221, an inhibitor primarily targeting the IDH2-R140Q mutant, has shown remarkable clinical benefits in the treatment of relapsed or refractory acute myeloid leukemia patients. However, AG-221 has weak inhibitory activity toward IDH2-R172K, a mutant form of IDH2 with more severe clinical manifestations. Herein, we report TQ05310 as the first mutant IDH2 inhibitor that potently targets both IDH2-R140Q and IDH2-R172K mutants. TQ05310 inhibited mutant IDH2 enzymatic activity, suppressed (R)-2-hydroxyglutarate (2-HG) production and induced differentiation in cells expressing IDH2-R140Q and IDH2-R172K, but not in cells expressing wild-type IDH1/2 or mutant IDH1. TQ05310 bound to both IDH2-R140Q and IDH2-R172K, with Q316 being the critical residue mediating the binding of TQ05310 with IDH2-R140Q, but not with IDH2-R172K. TQ05310 also had favorable pharmacokinetic characteristics and profoundly inhibited 2-HG production in a tumor xenografts model. The results of the current study establish a solid foundation for further clinical investigation of TQ05310, and provide new insight into the development of novel mutant IDH2 inhibitors.

Design, Synthesis and Pharmacological Evaluation of Novel Hsp90N-terminal Inhibitors Without Induction of Heat Shock Response.

Heat shock protein 90 (Hsp90) is a potential oncogenic target. However, Hsp90 inhibitors in clinical trial induce heat shock response, resulting in drug resistance and inefficiency. In this study, we designed and synthesized a series of novel triazine derivatives (A1-26, B1-13, C1-23) as Hsp90 inhibitors. Compound A14 directly bound to Hsp90 in a different manner from traditional Hsp90 inhibitors, and degraded client proteins, but did not induce the concomitant activation of Hsp72. Importantly, A14 exhibited the most potent anti-proliferation ability by inducing autophagy, with the IC50 values of 0.1 µM and 0.4 µM in A549 and SK-BR-3 cell lines, respectively. The in  vivo study demonstrated that A14 could induce autophagy and degrade Hsp90 client proteins in tumor tissues, and exhibit anti-tumor activity in A549 lung cancer xenografts. Therefore, the compound A14 with potent antitumor activity and unique pharmacological characteristics is a novel Hsp90 inhibitor for developing anticancer agent without heat shock response.

Dysoxylactam A: A Macrocyclolipopeptide Reverses P-Glycoprotein-Mediated Multidrug Resistance in Cancer Cells.

A 17-membered macrocyclolipopeptide, named dysoxylactam A (1) comprising an unprecedented branched C19 fatty acid and an l-valine, was isolated from the plants of Dysoxylum hongkongense. The challenging relative configuration of 1 was established by means of residual dipolar coupling-based NMR analysis. The absolute configuration of 1 was determined by single-crystal X-ray diffraction on its p-bromobenzoate derivative (2). Compound 1 dramatically reversed multidrug resistance in cancer cells with the fold-reversals ranging from 28.4 to 1039.7 at the noncytotoxic concentration of 10 µM. The mode-of-action study of 1 revealed that it inhibited the function of P-glycoprotein (P-gp), a key mediator in multidrug resistance.

Preclinical characterization of SHR6390, a novel CDK 4/6 inhibitor, in vitro and in human tumor xenograft models.

Inhibition of the cyclin-dependent kinase (CDK) 4/6-retinoblastoma (RB) pathway is an effective therapeutic strategy against cancer. Here, we performed a preclinical investigation of the antitumor activity of SHR6390, a novel CDK4/6 inhibitor. SHR6390 exhibited potent antiproliferative activity against a wide range of human RB-positive tumor cells in vitro, and exclusively induced G1 arrest as well as cellular senescence, with a concomitant reduction in the levels of Ser780-phosphorylated RB protein. Compared with the well-known CDK4/6 inhibitor palbociclib, orally administered SHR6390 led to equivalent or improved tumor efficacy against a panel of carcinoma xenografts, and produced marked tumor regression in some models, in association with sustained target inhibition in tumor tissues. Furthermore, SHR6390 overcame resistance to endocrine therapy and HER2-targeting antibody in ER-positive and HER2-positive breast cancer, respectively. Moreover, SHR6390 combined with endocrine therapy exerted remarkable synergistic antitumor activity in ER-positive breast cancer. Taken together, our findings indicate that SHR6390 is a novel CDK4/6 inhibitor with favorable pharmaceutical properties for use as an anticancer agent.