Strategies to overcome resistance to targeted protein kinase inhibitors in the treatment of cancer

Expression and clinical significance of U2AF homology motif kinase 1 in oral squamous cell carcinoma

OBJECTIVE

U2AF homology motif kinase 1 (UHMK1) is a newly discovered molecule that may have multiple functions. Recent studies have revealed that UHMK1 had aberrant expression in many tumors and was associated with tumor progression. However, UHMK1 was rarely reported in oral squamous cell carcinoma (OSCC).

STUDY DESIGN

In this study, Western blot, quantitative real-time polymerase chain reaction (PCR), and immunohistochemistry were used to detect the expression of UHMK1 in OSCC and peritumoral non-neoplastic tissues. Then, its relationship with clinicopathologic parameters was analyzed. The Kaplan-Meier method and Cox regression model were used to analyze the effects of UHMK1 expression on the prognosis and survival of OSCC patients.

RESULTS

Our results showed that UHMK1 had higher expression in OSCC tissues compared with in peritumoral non-neoplastic tissues, and its high expression was associated with high TNM stage and lymph node metastasis. High UHMK1 expression was related to short overall and disease-free survival times. Moreover, UHMK1 expression was identified as an independent prognostic factor that influences overall and disease-free survival of OSCC patients.

CONCLUSIONS

High expression of UHMK1 is associated with the poor prognosis of patients, and it can be used as a potential prognostic molecule for OSCC.

A close-up shot of protein-protein docking, from experiment to theory and reverse with the PROTAC performers

PROTACs (Proteolysis Targeting Chimeras), heterobifunctional molecules, exhibit selectivity in degrading target proteins through E3 ubiquitin ligases. Designing effective PROTACs requires a deep understanding of the intricate binding interactions in the ternary complex (POI/PROTAC/E3 ligase), crucial for efficient target protein degradation. To address this challenge, we introduce a novel computational virtual screening method that considers essential amino acid interactions between the protein of interest and the chosen E3 ligase. This approach enhances accuracy and reliability, facilitating the strategic development of potent PROTACs. Utilizing a crystallized model of the VHL:PROTAC:SMARCA2BD ternary complex (PDB: 7Z6L), we assessed the effectiveness of our method. Our study reveals that increasing the number of essential restraints between the two proteins reduces the generated docking poses, leading to closer alignment with the experimental ternary complex. Specifically, utilizing three restraints showed the closest resemblance to the published complex, highlighting crucial interactions such as an H-bond between A:Gln 89 and B:Asn 67, along with two hydrophobic interactions: A:Gly 22 with B:Arg 69 and A:Glu 37 with B:Pro 99. This resulted in a significant decrease in the mean RMSD value from 31.8 and 31.0 Å to 24.4 Å, respectively. This underscores the importance of incorporating multiple essential restraints to enhance docking accuracy. Building on this progress, we introduce a systematic approach to design potential PROTACs between the Estrogen receptor and the E3 ligase, utilizing bridging intermediates with 4, 6, or 7 carbon atoms. By providing a more accurate and efficient means of identifying optimal PROTAC candidates, this approach has the potential to accelerate the development of targeted therapies and reduce the time and costs associated with drug discovery.

Glutathione-responsive PROTAC for targeted degradation of ERα in breast cancer cells

ERα (estrogen receptor-α)-targeting PROTACs (PROteolysis TArgeting Chimeras) have emerged as a novel and promising modality for breast cancer therapeutics. However, ERα PROTACs-induced degradation in normal tissues raises concerns about potential off-tissue toxicity. Tumor microenvironment-responsive strategy provides potential for specific control of the PROTAC's on-target degradation activity. The glutathione (GSH) level has been reported to be significantly increased in tumor cells. Here, we designed a GSH-responsive ERα PROTAC, which is generated by conjugating an o-nitrobenzenesulfonyl group to the hydroxyl group of VHL-based ERα PROTAC through a nucleophilic substitution reaction. The o-nitrobenzenesulfonyl group as a protecting group blocks the bioactivity of ERα PROTAC (ER-P1), and that can be specifically recognized and removed by highly abundant GSH in cancer cells. Consequently, the GSH-responsive ERα PROTAC (GSH-ER-P1) exhibits significantly enhanced degradation of ERα in cancer cells compared to that in normal cells, leading to a remarkable inhibition of breast cancer cell proliferation and less toxic effects on normal cells. This study provides a potentially valuable strategy for breast cancer treatment using tumor microenvironment-responsive PROTACs.

An overview on Estrogen receptors signaling and its ligands in breast cancer

Estrogen governs the regulations of various pathological and physiological actions throughout the body in both males and females. Generally, 17β-estradiol an endogenous estrogen is responsible for different health problems in pre and postmenopausal women. The major activities of endogenous estrogen are executed by nuclear estrogen receptors (ERs) ERα and ERβ while non-genomic cytoplasmic pathways also govern cell growth and apoptosis. Estrogen accomplished a fundamental role in the formation and progression of breast cancer. In this review, we have hyphenated different studies regarding ERs and a thorough and detailed study of estrogen receptors is presented. This review highlights different aspects of estrogens ranging from receptor types, their isoforms, structures, signaling pathways of ERα, ERβ and GPER along with their crystal structures, pathological roles of ER, ER ligands, and therapeutic strategies to overcome the resistance.

PROTACs: Promising Approaches for Epigenetic Strategies to Overcome Drug Resistance

Epigenetic modulation of gene expression is essential for tissue-specific development and maintenance in mammalian cells. Disruption of epigenetic processes, and the subsequent alteration of gene functions, can result in inappropriate activation or inhibition of various cellular signaling pathways, leading to cancer. Recent advancements in the understanding of the role of epigenetics in cancer initiation and progression have uncovered functions for DNA methylation, histone modifications, nucleosome positioning, and non-coding RNAs. Epigenetic therapies have shown some promise for hematological malignancies, and a wide range of epigenetic-based drugs are undergoing clinical trials. However, in a dynamic survival strategy, cancer cells exploit their heterogeneous population which frequently results in the rapid acquisition of therapy resistance. Here, we describe novel approaches in drug discovery targeting the epigenome, highlighting recent advances the selective degradation of target proteins using Proteolysis Targeting Chimera (PROTAC) to address drug resistance.

Polo-like Kinase 1 as an emerging drug target: structure, function and therapeutic implications

Polo-like kinase 1 (PLK1) is a conserved mitotic serine-threonine protein kinase, functions as a regulatory protein, and is involved in the progression of the mitotic cycle. It plays important roles in the regulation of cell division, maintenance of genome stability, in spindle assembly, mitosis, and DNA-damage response. PLK1 is consist of a N-terminal serine-threonine kinase domain, and a C-terminal Polo-box domain (regulatory site). The expression of PLK1 is controlled by transcription repressor in the G1 stage and transcription activators in the G2 stage of the cell cycle. Overexpression of PLK1 results in undermining of checkpoints causes excessive cellular division resulting in abnormal cell growth, leading to the development of cancer. Blocking the expression of PLK1 by an antibody, RNA interference, or kinase inhibitors, causes a subsequent reduction in the proliferation of tumour cells and induction of apoptosis in tumour cells without affecting the healthy cells, suggesting an attractive target for drug development. In this review, we discuss detailed information on expression, gene and protein structures, role in different diseases, and progress in the design and development of PLK1 inhibitors. We have performed an in-depth analysis of the PLK1 inhibitors and their therapeutic implications with special focus to the cancer therapeutics.

Targeting estrogen receptor α for degradation with PROTACs: A promising approach to overcome endocrine resistance

Estrogen receptor alfa (ERα) is expressed in approximate 70% of breast cancer (BC) which is the most common malignancy in women worldwide. To date, the foremost intervention in the treatment of ER positive (ER+) BC is still the endocrine therapy. However, resistance to endocrine therapies remains a major hurdle in the long-term management of ER + BC. Although the mechanisms underlying endocrine resistance are complex, cumulative evidence revealed that ERα still plays a critical role in driving BC tumor cells to grow in resistance state. Fulvestrant, a selective estrogen receptor degrader (SERD), has moved to first line therapy for metastatic ER + BC, suggesting that removing ERα would be a useful strategy to overcome endocrine resistance. Proteolysis-Targeting Chimera (PROTAC) technology, an emerging paradigm for protein degradation, has the potential to eliminate both wild type and mutant ERα in breast cancer cells. Excitingly, ARV-471, an ERα-targeted PROTAC developed by Arvinas, has been in phase 1 clinical trials. In this review, we will summarize recent progress of ER-targeting PROTACs from publications and patents along with their therapeutic opportunities for the treatment of endocrine-resistant BC.

PROTACs: great opportunities for academia and industry

Although many kinds of therapies are applied in the clinic, drug-resistance is a major and unavoidable problem. Another disturbing statistic is the limited number of drug targets, which are presently only 20-25% of all protein targets that are currently being studied. Moreover, the focus of current explorations of targets are their enzymatic functions, which ignores the functions from their scaffold moiety. As a promising and appealing technology, PROteolysis TArgeting Chimeras (PROTACs) have attracted great attention both from academia and industry for finding available approaches to solve the above problems. PROTACs regulate protein function by degrading target proteins instead of inhibiting them, providing more sensitivity to drug-resistant targets and a greater chance to affect the nonenzymatic functions. PROTACs have been proven to show better selectivity compared to classic inhibitors. PROTACs can be described as a chemical knockdown approach with rapidity and reversibility, which presents new and different biology compared to other gene editing tools by avoiding misinterpretations that arise from potential genetic compensation and/or spontaneous mutations. PRTOACs have been widely explored throughout the world and have outperformed not only in cancer diseases, but also in immune disorders, viral infections and neurodegenerative diseases. Although PROTACs present a very promising and powerful approach for crossing the hurdles of present drug discovery and tool development in biology, more efforts are needed to gain to get deeper insight into the efficacy and safety of PROTACs in the clinic. More target binders and more E3 ligases applicable for developing PROTACs are waiting for exploration.

PROTACs as Potential Therapeutic Agents for Cancer Drug Resistance

Cancer drug resistance has become the major problem facing current clinical treatment via different kinds of therapies. Proteolysis targeting chimeras (PROTACs) as a novel and powerful strategy have attracted a great deal of attention both from academia and from industry for their sensitivity to drug-resistant targets relying on their unique characteristics compared to those of traditional inhibitors. PROTACs exert their function by degrading the target protein instead of inhibiting targets. Thus, different kinds of resistance could be conquered by PROTACs such as target mutation or overexpression. Various resistant targets have been overcome by PROTACs, including AR, ER, BTK, BET, and BCR-ABL. Though PROTACs have achieved some significant advances in combating drug resistance, more cases are needed to prove the efficiency of PROTACs in addressing the hurdle of resistance in the near future.

Serine/threonine kinase 32C is overexpressed in bladder cancer and contributes to tumor progression

Tumor markers of bladder cancer (BC) have been investigated for many years, but the clinical treatment based on these biomarkers is still unsatisfactory. STK32C, a member of the serine/threonine protein kinase of AGC superfamily, was first found to be highly expressed in brain tissues; however, the role of STK32C in malignant disease has not been determined. Data from TCGA database showed that the STK32C gene is overexpressed in BC and a number of other human tumors. In the current study, immunohistochemistry revealed that high expression of STK32C protein in tumor tissues was significantly associated with poor clinico pathologic features and a short relapse-free survival (RFS) in patients with BC. Slicing of STK32C inhibited tumor cell proliferation, migration and invasion in vitro. In vivo animal experiments demonstrated that knocking-down of STK32C restricted the growth of tumor cells in mice. Finally, microarray analysis revealed that silencing of STK32C inhibited the activity of the HMGB1 pathway and regulated the expression of key genes in this pathway. In conclusion, our study showed novel promoting roles for STK32C in human tumors, which may provide a new therapeutic target for the patients with BC.

PLK1, A Potential Target for Cancer Therapy

Polo-like kinase 1 (PLK1) plays an important role in the initiation, maintenance, and completion of mitosis. Dysfunction of PLK1 may promote cancerous transformation and drive its progression. PLK1 overexpression has been found in a variety of human cancers and was associated with poor prognoses in cancers. Many studies have showed that inhibition of PLK1 could lead to death of cancer cells by interfering with multiple stages of mitosis. Thus, PLK1 is expected to be a potential target for cancer therapy. In this article, we examined PLK1’s structural characteristics, its regulatory roles in cell mitosis, PLK1 expression, and its association with survival prognoses of cancer patients in a wide variety of cancer types, PLK1 interaction networks, and PLK1 inhibitors under investigation. Finally, we discussed the key issues in the development of PLK1-targeted cancer therapy.

The chemical class of quinazoline compounds provides a core structure for the design of anticytomegaloviral kinase inhibitors

HCMV is a member of the family Herpesviridae and represents a worldwide distributed pathogen with seropositivity rates in the adult population ranging between 40% and 90%. Notably, HCMV infection is a serious, sometimes life-threatening medical problem for newborns and immunosuppressed individuals, including transplant recipients and patients under antitumoral chemotherapy. Current standard therapy with valganciclovir has the disadvantage of inducing drug-resistant virus mutants and toxicity-related side effects. Our analysis stresses the earlier finding that kinase inhibitors of the quinazoline class exert an antiviral response by targeting the viral protein kinase pUL97 without inducing resistance. Therefore, quinazolines have been used as a core structure to gain insight in the mode of inhibitor-kinase interaction. Here, we demonstrate that (i) the novel quinazolines Vi7392 and Vi7453 are highly active against HCMV laboratory and clinically relevant strains including maribavir- and ganciclovir-resistant variants, (ii) antiviral activity is not cell-type specific and was also detected in a placental explant tissue model using a genetically intact HCMV strain (iii) the viral kinase pUL97 represents a target of the anticytomegaloviral activity of these compounds, (iv) induction of pUL97-conferring drug resistance was not detectable under single-step selection, thus differed from the induction of ganciclovir resistance, and (v) pUL97 drug docking simulations enabled detailed insights into specific drug-target binding properties providing a promising basis for the design of optimized kinase inhibitors. These novel findings may open new prospects for the future medical use of quinazoline drug candidates and the use of drug-target dynamic simulations for rational design of antivirals.

Specific delivery of kinase inhibitors in nonmalignant and malignant diseases

INTRODUCTION

Kinase inhibitors have been hailed as a breakthrough in the treatment of cancer. Extensive research is now being devoted to the development of kinase inhibitors as a treatment for many nonmalignant diseases. However, the use of kinase inhibitors in both malignant and nonmalignant diseases is also associated with side effects and the development of resistance. It may be worthwhile to explore whether cell-specific delivery of kinase inhibitors improves therapeutic efficacy and reduces side effects.

AREAS COVERED

This review aims to provide an overview of the preclinical studies performed to examine the specific targeting of kinase inhibitors in vitro and in vivo. It gives an introduction to kinase signaling pathways induced during disease, along with the possible problems associated with their inhibition. It also discusses the studies on specific delivery and shows that altering the specificity of kinase inhibitors by targeting methods improves their effectivity and safety.

EXPERT OPINION

Compared with the delivery of cytotoxic compounds, the specific delivery of kinase inhibitors has not yet been studied extensively. The studies discussed in this review provide an insight into methods used to target kinase inhibitors to different organs. The targeting of different kinase inhibitors has improved their therapeutic possibilities, but many questions still remain to be studied.

Mechanisms of drug resistance in kinases

INTRODUCTION

because of their important roles in disease and excellent 'druggability', kinases have become the second largest drug target family. The great success of the BCR-ABL inhibitor imatinib in treating chronic myelogenous leukemia illustrates the high potential of kinase inhibitor (KI) therapeutics, but also unveils a major limitation: the development of drug resistance. This is a significant concern as KIs reach large patient populations for an expanding array of indications.

AREAS COVERED

we provide an up-to-date understanding of the mechanisms through which KIs function and through which cells can become KI-resistant. We review current and future approaches to overcome KI resistance, focusing on currently approved KIs and KIs in clinical trials. We then discuss approaches to improve KI efficacy and overcome drug resistance and novel approaches to develop less drug resistance-prone KI therapeutics.

EXPERT OPINION

although drug resistance is a concern for current KI therapeutics, recent progress in our understanding of the underlying mechanisms and promising technological advances may overcome this limitation and provide powerful new therapeutics.

Elimination of human lung cancer stem cells through targeting of the stem cell factor-c-kit autocrine signaling loop

Cancer stem cells (CSC) are thought to be responsible for tumor initiation and tumor regeneration after chemotherapy. Previously, we showed that chemotherapy of non-small cell lung cancer (NSCLC) cells lines can select for outgrowth of highly tumorigenic and metastatic CSCs. The high malignancy of lung CSCs was associated with an efficient cytokine network. In this study, we provide evidence that blocking stem cell factor (SCF)-c-kit signaling is sufficient to inhibit CSC proliferation and survival promoted by chemotherapy. CSCs were isolated from NSCLC cell lines as tumor spheres under CSC-selective conditions and their stem properties were confirmed. In contrast to other tumor cells, CSCs expressed c-kit receptors and produced SCF. Proliferation of CSCs was inhibited by SCF-neutralizing antibodies or by imatinib (Gleevec), an inhibitor of c-kit. Although cisplatin treatment eliminated the majority of tumor cells, it did not eliminate CSCs, whereas imatinib or anti-SCF antibody destroyed CSCs. Significantly, combining cisplatin with imatinib or anti-SCF antibody prevented the growth of both tumor cell subpopulations. Our findings reveal an important role for the SCF-c-kit signaling axis in self-renewal and proliferation of lung CSCs, and they suggest that SCF-c-kit signaling blockade could improve the antitumor efficacy of chemotherapy of human NSCLC.

New anti-cancer role for PDK1 inhibitors: preventing resistance to tamoxifen

Tamoxifen is one of the most prescribed anti-breast-cancer drugs, but tumours becoming resistant hinder its efficacy in the clinic. There is therefore great interest in developing strategies to reduce resistance and sensitize breast cancer cells to tamoxifen. A groundbreaking study by Iorns et al. published in this issue of the Biochemical Journal suggests that a signal transduction pathway controlled by PDK1 (phosphoinositide-dependent kinase 1) plays a crucial role in regulating the sensitivity of breast cancer cells to tamoxifen. The implications of this study are that PDK1 or PI3K (phosphoinositide 3-kinase), Akt (also known as protein kinase B), S6K (S6 kinase) and mTOR (mammalian target of rapamycin) inhibitors, already being developed for cancer therapy, are likely to have additional utility in sensitizing breast tumours to tamoxifen. In this commentary we also discuss the possibility that inhibiting the PDK1 pathway may help overcome acquired resistance to other anti-cancer treatments.

Protein kinase inhibitors of the quinazoline class exert anti-cytomegaloviral activity in vitro and in vivo

Cytomegalovirus infection is associated with severe disease in immunocompromised individuals. Current antiviral therapy faces several limitations. In a search of novel drug candidates, we describe here the anti-cytomegaloviral properties of two compounds of the chemical class of quinazolines, gefitinib (Iressa) and Ax7396 (RGB-315389). Both compounds showed strong inhibitory effects in vitro against human and animal cytomegaloviruses with IC(50)s in a low micromolar range. Cytotoxicity did not occur at these effective concentrations. The antiviral mode of action was based on the inhibition of protein kinase activity, mainly directed to a viral target kinase (UL97/M97) in addition to cellular target candidates. This was demonstrated by a high sensitivity of the respective protein kinases in vitro and by infection experiments with viral mutants carrying genomic alterations in the ORF UL97/M97 modulating viral drug sensitivity. In a guinea pig model, gefitinib showed inhibition of cytomegaloviral loads in blood and lung tissue. Importantly, the rate of mortality of infected animals was reduced by gefitinib treatment. In contrast to the in vitro data, Ax7396 showed no significant antiviral activity in a mouse model. Further in vivo analyses have to assess the potential use of gefitinib in the treatment of cytomegalovirus disease.

Inhibition of Bcr-Abl Phosphorylation and Induction of Apoptosis by Pyrazolo[3,4-d]pyrimidines in Human Leukemia Cells

A series of pyrazolo[3,4-d]pyrimidines, previously found to be Src inhibitors, was tested for their ability to inhibit proliferation of three Bcr-Abl-positive human leukemia cell lines (K-562, KU-812, and MEG-01), on the basis of the experimental evidence that various Src inhibitors are also active against Bcr-Abl kinase (the so called dual Src/Abl inhibitors). They reduce Bcr-Abl tyrosine phosphorylation and promote apoptosis of the Bcr-Abl-expressing cells. A cell-free enzymatic assay on isolated c-Abl confirmed that such compounds directly inhibit Abl activity. Finally, molecular modeling simulations were also performed to hypothesize the binding mode of the compounds into the Abl binding site.