Purvalanol A, olomoucine II and roscovitine inhibit ABCB1 transporter and synergistically potentiate cytotoxic effects of daunorubicin in vitro

An insight into the risk factors of brain tumors and their therapeutic interventions

Brain tumors are an abnormal growth of cells in the brain, also known as multifactorial groups of neoplasm. Incidence rates of brain tumors increase rapidly, and it has become a leading cause of tumor related deaths globally. Several factors have potential risks for intracranial neoplasm. To date, the International Agency for Research on Cancer has classified the ionizing radiation and the N-nitroso compounds as established carcinogens and probable carcinogens respectively. Diagnosis of brain tumors is based on histopathology and suitable imaging techniques. Labeled amino acids and fluorodeoxyglucose with or without contrast-enhanced MRI are used for the evaluation of tumor traces. T2-weighted MRI is an advanced diagnostic implementation, used for the detection of low-grade gliomas. Treatment decisions are based on tumor size, location, type, patient's age and health status. Conventional therapeutic approaches for tumor treatment are surgery, radiotherapy and chemotherapy. While the novel strategies may include targeted therapy, electric field treatments and vaccine therapy. Inhibition of cyclin-dependent kinase inhibitors is an attractive tumor mitigation strategy for advanced-stage cancers; in the future, it may prove to be a useful targeted therapy. The blood-brain barrier poses a major hurdle in the transport of therapeutics towards brain tissues. Moreover, nanomedicine has gained a vital role in cancer therapy. Nano drug delivery system such as liposomal drug delivery has been widely used in the cancer treatment. Liposome encapsulated drugs have improved therapeutic efficacy than free drugs. Numerous treatment therapies for brain tumors are in advanced clinical research.

Scaffold fragmentation and substructure hopping reveal potential, robustness, and limits of computer-aided pattern analysis (C@PA)

Computer-aided pattern analysis (C@PA) was recently presented as a powerful tool to predict multitarget ABC transporter inhibitors. The backbone of this computational methodology was the statistical analysis of frequently occurring molecular features amongst a fixed set of reported small-molecules that had been evaluated toward ABCB1, ABCC1, and ABCG2. As a result, negative and positive patterns were elucidated, and secondary positive substructures could be suggested that complemented the multitarget fingerprints. Elevating C@PA to a non-statistical and exploratory level, the concluded secondary positive patterns were extended with potential positive substructures to improve C@PA's prediction capabilities and to explore its robustness. A small-set compound library of known ABCC1 inhibitors with a known hit rate for triple ABCB1, ABCC1, and ABCG2 inhibition was taken to virtually screen for the extended positive patterns. In total, 846 potential broad-spectrum ABCB1, ABCC1, and ABCG2 inhibitors resulted, from which 10 have been purchased and biologically evaluated. Our approach revealed 4 novel multitarget ABCB1, ABCC1, and ABCG2 inhibitors with a biological hit rate of 40%, but with a slightly lower inhibitory power than derived from the original C@PA. This is the very first report about discovering novel broad-spectrum inhibitors against the most prominent ABC transporters by improving C@PA.

A rapid and sensitive bioanalytical LC-MS/MS method for the quantitation of a novel CDK5 inhibitor 20-223 (CP668863) in plasma: Application to in vitro metabolism and plasma protein-binding studies

A rapid, selective, and sensitive liquid chromatography coupled with tandem mass spectrometry (MS/MS) method was developed and validated for the quantitation of the novel CDK5 inhibitor '20-223' in mouse plasma. Separation of analytes was achieved by a reverse-phase ACE Excel C18 column (1.7 μm, 100 × 2.1 mm) with gradient elution using 0.1% formic acid (FA) in methanol and 0.1% FA as the mobile phase. Analytes were monitored by MS/MS with an electrospray ionization source in the positive multiple reaction monitoring mode. The MS/MS response was linear over the concentration range 0.2-500 ng/mL for 20-223. The within- and between-batch precision were within the acceptable limits as per Food and Drug Administration guidelines. The validated method was successfully applied to plasma protein binding and in vitro metabolism studies. Compound 20-223 was highly bound to mouse plasma proteins (>98% bound). Utilizing mouse S9 fractions, in vitro intrinsic clearance (CLint ) was 24.68 ± 0.99 μL/min/mg protein. A total of 12 phase I and II metabolites were identified with hydroxylation found to be the major metabolic pathway. The validate method required a low sample volume, was linear from 0.2 to 500 ng/mL, and had acceptable accuracy and precision.

Phytochemicals: Potential Lead Molecules for MDR Reversal

Multidrug resistance (MDR) is one of the main impediments in the treatment of cancers. MDR cancer cells are resistant to multiple anticancer drugs. One of the major mechanisms of MDR is the efflux of anticancer drugs by ABC transporters. Increased activity and overexpression of these transporters are important causes of drug efflux and, therefore, resistance to cancer chemotherapy. Overcoming MDR is a fundamental prerequisite for developing an efficient treatment of cancer. To date, various types of ABC transporter inhibitors have been employed but no effective anticancer drug is available at present, which can completely overcome MDR. Phytochemicals can reverse MDR in cancer cells via affecting the expression or activity of ABC transporters, and also through exerting synergistic interactions with anticancer drugs by addressing additional molecular targets. We have listed numerous phytochemicals which can affect the expression and activity of ABC transporters in MDR cancer cell lines. Phytochemicals in the groups of flavonoids, alkaloids, terpenes, carotenoids, stilbenoids, lignans, polyketides, and curcuminoids have been examined for MDR-reversing activity. The use of MDR-reversing phytochemicals with low toxicity to human in combination with effective anticancer agents may result in successful treatment of chemotherapy-resistant cancer. In this review, we summarize and discuss published evidence for natural products with MDR modulation abilities.

Pyrimidine: A promising scaffold for optimization to develop the inhibitors of ABC transporters

The multidrug resistance (MDR) phenomenon in cancer cells is the major obstacle leading to failure of chemotherapy accompanied by the feature of intractable and recurrence of cancers. As significant contributors that cause MDR, ABC superfamily proteins can transport the chemotherapeutic drugs out of the tumor cells by the energy of adenosine triphosphate (ATP) hydrolysis, thereby reducing their intracellular accumulation. The ABC transports like ABCB1, ABCC1 and ABCG2 have been extensively studied to develop modulators for overcoming MDR. To date, no reversal agents have been successfully marketed for clinical application, and little information about the ABC proteins bound to specific inhibitors is known, which make the design of MDR inhibitors with potency, selectivity and low toxicity a major challenge. In recent years, it has been increasingly recognized that pyrimidine-based derivatives have the potential for reversing ABC-mediated MDR. In this review, we summarized the pyrimidine-based inhibitors of ABC transporters, and mainly focused on their structure optimizations, development strategies and structure-activity relationship studies in hope of providing a reference for medicinal chemists to develop new modulators of MDR with highly potency and fewer side effects.

Ensartinib (X-396) Effectively Modulates Pharmacokinetic Resistance Mediated by ABCB1 and ABCG2 Drug Efflux Transporters and CYP3A4 Biotransformation Enzyme

Ensartinib (X-396) is a promising tyrosine kinase inhibitor currently undergoing advanced clinical evaluation for the treatment of non-small cell lung cancer. In this work, we investigate possible interactions of this promising drug candidate with ATP-binding cassette (ABC) drug efflux transporters and cytochrome P450 biotransformation enzymes (CYPs), which play major roles in multidrug resistance (MDR) and pharmacokinetic drug-drug interactions (DDIs). Accumulation studies showed that ensartinib is a potent inhibitor of ABCB1 and ABCG2 transporters. Additionally, incubation experiments with recombinant CYPs showed that ensartinib significantly inhibits CYP3A4 and CYP2C9. Subsequent molecular docking studies confirmed these findings. Drug combination experiments demonstrated that ensartinib synergistically potentiates the antiproliferative effects of daunorubicin, mitoxantrone, and docetaxel in ABCB1, ABCG2, and CYP3A4-overexpressing cellular models, respectively. Advantageously, ensartinib’s antitumor efficiency was not compromised by the presence of MDR-associated ABC transporters, although it acted as a substrate of ABCB1 in Madin-Darby Canine Kidney II (MDCKII) monolayer transport assays. Finally, we demonstrated that ensartinib had no significant effect on the mRNA-level expression of examined transporters and enzymes in physiological and lung tumor cellular models. In conclusion, ensartinib may perpetrate clinically relevant pharmacokinetic DDIs and modulate ABCB1-, ABCG2-, and CYP3A4-mediated MDR. The in vitro findings presented here will provide a valuable foundation for future in vivo investigations.

Natural products as multidrug resistance modulators in cancer

Cancer is a prominent cause of death globally. Currently, many drugs that are in clinical practice are having a high prevalence of side effect and multidrug resistance. Risk of tumors acquiring resistance to chemotherapy (multidrug resistance) remains a significant hurdle to the successful treatment of various types of cancer. Membrane-embedded drug transporters, generally overexpressed in cancer, are the leading cause among multiple mechanisms of multidrug resistance (MDR). P-glycoprotein (P-gp) also MDR1/ABCB1, multidrug resistance associated protein 1 (MRP1/ABCC1), MRP2 and breast cancer resistance protein (BCRP/ABCG2) are considered to be a prime factor for induction of MDR. To date, several chemical substances have been tested in a number of clinical trials for their MDR modulatory activity which are not having devoid of any side effects that necessitates to find newer and safer way to tackle the current problem of multidrug resistance in cancer. The present study systematically discusses the various classes of natural products i.e flavonoids, alkaloids, terpenoids, coumarins (from plants, marine, and microorganisms) as potential MDR modulators and/or as a source of promising lead compounds. Recently a bisbenzyl isoquinoline alkaloid namely tetrandrine, isolated from Chinese herb Stephania tetrandra (Han-Fang-Chi) is in clinical trials for its MDR reversal activity.

The survivin molecule as a double-edged sword in cellular physiologic and pathologic conditions and its role as a potential biomarker and therapeutic target in cancer

Survivin is a member of the family of apoptosis inhibitory proteins with increased expression level in most cancerous tissues. Evidence shows that survivin plays regulatory roles in proliferation or survival of normal adult cells, principally vascular endothelial cells, T lymphocytes, primitive hematopoietic cells, and polymorphonuclear neutrophils. Survivin antiapoptotic role is, directly and indirectly, related to caspase proteins and shows its role in cell division through the chromosomal passenger complex. Survivin contains many genetic polymorphisms that the role of some variations has been proven in several cancers. The -31G/C polymorphism is one of the most important survivin mutations which is located in the promoter region on a CDE/CHR motif. This polymorphism can upregulate the survivin messenger RNA. In addition, its allele C can increase the risk of cancers in 1.27-fold than allele G. Considering the fundamental role of survivin in different cancers, this protein could be considered as a new therapeutic target in cancer treatment. For this purpose, various strategies have been designed including the prevention of survivin expression through inhibition of mRNA translation using antagonistic molecules, inhibition of survivin gene function through small inhibitory molecules, gene therapy, and immunotherapy. In this study, we describe the structure, played roles in physiological and pathological states and genetic polymorphisms of survivin. Finally, the role of survivin as a potential target in cancer therapy given challenges ahead has been discussed.

Cyclin-dependent kinase inhibitors AZD5438 and R547 show potential for enhancing efficacy of daunorubicin-based anticancer therapy: Interaction with carbonyl-reducing enzymes and ABC transporters

Daunorubicin (DAUN) has served as an anticancer drug in chemotherapy regimens for decades and is still irreplaceable in treatment of acute leukemias. The therapeutic outcome of DAUN-based therapy is compromised by its cardiotoxicity and emergence of drug resistance. This phenomenon is often caused by pharmacokinetic mechanisms such as efflux of DAUN from cancer cells through ATP-binding cassette (ABC) transporters and its conversion to less cytostatic but more cardiotoxic daunorubicinol (DAUN-OL) by carbonyl reducing enzymes (CREs). Here we aimed to investigate, whether two cyclin-dependent kinase inhibitors, AZD5438 and R547, can interact with these pharmacokinetic mechanisms and reverse DAUN resistance. Using accumulation assays, we revealed AZD5438 as potent inhibitor of ABCC1 showing also weaker inhibitory effect to ABCB1 and ABCG2. Combination index analysis, however, shown that inhibition of ABCC1 does not significantly contribute to synergism between AZD5438 and DAUN in MDCKII-ABCC1 cells, suggesting predominant role of other mechanism. Using pure recombinant enzymes, we found both tested drugs to inhibit CREs with aldo-keto reductase 1C3 (AKR1C3). This interaction was further confirmed in transfected HCT-116 cells. Moreover, these cells were sensitized to DAUN by both compounds as Chou-Talalay combination index analysis showed synergism in AKR1C3 transfected HCT-116, but not in empty vector transfected control cell line. In conclusion, we propose AZD5438 and R547 as modulators of DAUN resistance that can prevent AKR1C3-mediated DAUN biotransformation to DAUN-OL. This interaction could be beneficially exploited to prevent failure of DAUN-based therapy as well as the undesirable cardiotoxic effect of DAUN-OL.

Cyclin-dependent kinase inhibitors, roscovitine and purvalanol, induce apoptosis and autophagy related to unfolded protein response in HeLa cervical cancer cells

Roscovitine (Rosc) and purvalanol (Pur) are competitive inhibitors of cyclin-dependent kinases (CDKs) by targeting their ATP-binding pockets. Both drugs are shown to be effective to decrease cell viability and dysregulate the ratio of pro- and anti-apoptotic Bcl-2 family members, which finally led to apoptotic cell death in different cancer cell lines in vitro. It was well established that Bcl-2 family members have distinct roles in the regulation of other cellular processes such as endoplasmic reticulum (ER) stress. The induction of ER stress has been shown to play critical role in cell death/survival decision via autophagy or apoptosis. In this study, our aim was to investigate the molecular targets of CDK inhibitors on ER stress mechanism related to distinct cell death types in time-dependent manner in HeLa cervical cancer cells. Our results showed that Rosc and Pur decreased the cell viability, cell growth and colony formation, induced ER stress-mediated autophagy or apoptosis in time-dependent manner. Thus, we conclude that exposure of cells to CDK inhibitors induces unfolded protein response and ER stress leading to autophagy and apoptosis processes in HeLa cervical cancer cells.

ABCB1 as predominant resistance mechanism in cells with acquired SNS-032 resistance

The CDK inhibitor SNS-032 had previously exerted promising anti-neuroblastoma activity via CDK7 and 9 inhibition. ABCB1 expression was identified as major determinant of SNS-032 resistance. Here, we investigated the role of ABCB1 in acquired SNS-032 resistance. In contrast to ABCB1-expressing UKF-NB-3 sub-lines resistant to other ABCB1 substrates, SNS-032-adapted UKF-NB-3 (UKF-NB-3^rSNS- 032^300nM) cells remained sensitive to the non-ABCB1 substrate cisplatin and were completely re-sensitized to cytotoxic ABCB1 substrates by ABCB1 inhibition. Moreover, UKF-NB-3rSNS-032^300nM cells remained similarly sensitive to CDK7 and 9 inhibition as UKF-NB-3 cells. In contrast, SHEPrSNS-032^2000nM, the SNS-032-resistant sub-line of the neuroblastoma cell line SHEP, displayed low level SNS-032 resistance also when ABCB1 was inhibited. This discrepancy may be explained by the higher SNS-032 concentrations that were used to establish SHEPrSNS-032^2000nM cells, since SHEP cells intrinsically express ABCB1 and are less sensitive to SNS-032 (IC₅₀ 912 nM) than UKF-NB-3 cells (IC₅₀ 153 nM). In conclusion, we show that ABCB1 expression represents the primary (sometimes exclusive) resistance mechanism in neuroblastoma cells with acquired resistance to SNS-032. Thus, ABCB1 inhibitors may increase the SNS-032 efficacy in ABCB1-expressing cells and prolong or avoid resistance formation.

MDR1 and BCRP Transporter-Mediated Drug-Drug Interaction between Rilpivirine and Abacavir and Effect on Intestinal Absorption

Rilpivirine (TMC278) is a highly potent nonnucleoside reverse transcriptase inhibitor (NNRTI) representing an effective component of combination antiretroviral therapy (cART) in the treatment of HIV-positive patients. Many antiretroviral drugs commonly used in cART are substrates of ATP-binding cassette (ABC) and/or solute carrier (SLC) drug transporters and, therefore, are prone to pharmacokinetic drug-drug interactions (DDIs). The aim of our study was to evaluate rilpivirine interactions with abacavir and lamivudine on selected ABC and SLC transporters in vitro and assess its importance for pharmacokinetics in vivo Using accumulation assays in MDCK cells overexpressing selected ABC or SLC drug transporters, we revealed rilpivirine as a potent inhibitor of MDR1 and BCRP, but not MRP2, OCT1, OCT2, or MATE1. Subsequent transport experiments across monolayers of MDCKII-MDR1, MDCKII-BCRP, and Caco-2 cells demonstrated that rilpivirine inhibits MDR1- and BCRP-mediated efflux of abacavir and increases its transmembrane transport. In vivo experiments in male Wistar rats confirmed inhibition of MDR1/BCRP in the small intestine, leading to a significant increase in oral bioavailability of abacavir. In conclusion, rilpivirine inhibits MDR1 and BCRP transporters and may affect pharmacokinetic behavior of concomitantly administered substrates of these transporters, such as abacavir.

Natural alkaloids as P-gp inhibitors for multidrug resistance reversal in cancer

The biggest challenge associated with cancer chemotherapy is the development of cross multi-drug resistance to almost all anti-cancer agents upon chronic treatment. The major contributing factor for this resistance is efflux of the drugs by the p-glycoprotein pump. Over the years, inhibitors of this pump have been discovered to administer them in combination with chemotherapeutic agents. The clinical failure of first and second generation P-gp inhibitors (such as verapamil and cyclosporine analogs) has led to the discovery of third generation potent P-gp inhibitors (tariquidar, zosuquidar, laniquidar). Most of these inhibitors are nitrogenous compounds and recently a natural alkaloid CBT-01^® (tetrandrine) has advanced to the clinical phase. CBT-01 demonstrated positive results in Phase-I study in combination with paclitaxel, which warranted conducting it's Phase II/III trial. Apart from this, there exist a large number of natural alkaloids possessing potent inhibition of P-gp efflux pump and other related pumps responsible for the development of resistance. Despite the extensive contribution of alkaloids in this area, has never been reviewed. The present review provides a comprehensive account on natural alkaloids possessing P-gp inhibition activity and their potential for multidrug resistance reversal in cancer.

MicroRNA-143 replenishment re-sensitizes colorectal cancer cells harboring mutant, but not wild-type, KRAS to paclitaxel treatment

Colorectal cancer (CRC) global incidence is one of the highest among cancers. The KRAS gene has been shown as a robust biomarker for poor prognosis and drug resistance. MicroRNA-143 (miR-143) and let-7 are families of tumor suppressor microRNAs that are often downregulated in CRC, especially with coexistent KRAS mutations. In order to evaluate if miR-143 and/or let-7b replenishment would re-sensitize CRC cells to paclitaxel treatment, we investigated in effect of miR-143 and let-7b replenishments on sensitivity to paclitaxel treatment in KRAS mutant LoVo and wild-type SW48 CRC cell lines. Our results showed that miR-143, but not let-7b, increased sensitization of KRAS mutant tumor cells to paclitaxel. Furthermore, transfection of miR-143, but not let-7b, mimic negatively regulated the expression of mutant but not wild-type KRAS. Combination of miR-143 mimic and paclitaxel induced the onset of apoptosis, and reverted in vitro metastatic properties (migration and invasion) in KRAS mutant tumor cells. MiR-143 thus can be used as a chemosensitizer for the treatment of KRAS mutant tumors and warrants further investigations in in vitro and pre-clinical in vivo models.

Placental passage of olomoucine II, but not purvalanol A, is affected by p-glycoprotein (ABCB1), breast cancer resistance protein (ABCG2) and multidrug resistance-associated proteins (ABCCs)

1. Purine cyclin-dependent kinase inhibitors have recently been recognised as promising candidates for the treatment of various cancers. While pharmacodynamic properties of these compounds are relatively well understood, their pharmacokinetics including possible interactions with placental transport systems have not been characterised to date. 2. In this study, we investigated transplacental passage of olomoucine II and purvalanol A in rat focusing on possible role of p-glycoprotein (ABCB1), breast cancer resistance protein (ABCG2) and/or multidrug resistance-associated proteins (ABCCs). Employing the in situ method of dually perfused rat term placenta, we demonstrate transplacental passage of both olomoucine II and purvalanol A against the concentration gradient in foetus-to-mother direction. Using several ATP-binding cassette (ABC) drug transporter inhibitors, we confirm the participation of ABCB1, ABCG2 and ABCCs transporters in the placental passage of olomoucine II, but not purvalanol A. 3. Transplacental passage of olomoucine II and purvalanol A from mother to foetus is significantly reduced by active transporters, restricting thereby foetal exposure and providing protection against harmful effects of these xenobiotics. Importantly, we demonstrate that in spite of their considerable structural similarity, the two molecules utilise distinct placental transport systems. These facts should be kept in mind when introducing these prospective anticancer candidates and/or their analogues into the clinical area.

Dinaciclib, a cyclin-dependent kinase inhibitor, is a substrate of human ABCB1 and ABCG2 and an inhibitor of human ABCC1 in vitro

Dinaciclib is a novel cyclin-dependent kinase inhibitor (CDKI) with significant activity against various cancers in vitro and in vivo. ABC efflux transporters play an important role in drug disposition and are responsible for multidrug resistance in cancer cells. Inhibitors and substrates of these transporters may participate in pharmacokinetic drug-drug interactions (DDIs) that alter drug disposition during pharmacotherapy. To assess such risks associated with dinaciclib we evaluated its possible effects on efflux activities of ABCB1, ABCC1 and ABCG2 transporters in vitro. Monolayer transport, XTT cell proliferation, ATPase and intracellular accumulation assays were employed. Here, we show that the transport ratio of dinaciclib was far higher across monolayers of MDCKII-ABCB1 and MDCKII-ABCG2 cells than across MDCKII parental cell layers, demonstrating that dinaciclib is a substrate of ABCB1 and ABCG2. In addition, overexpression of ABCB1, ABCG2 and ABCC1 conferred resistance to dinaciclib in MDCKII cells. In ATPase assays, dinaciclib decreased stimulated ATPase activity of ABCB1, ABCG2 and ABCC1, confirming it has interactive potential toward all three transporters. Moreover, dinaciclib significantly inhibited ABCC1-mediated efflux of daunorubicin (EC50=18 μM). The inhibition of ABCC1 further led to a synergistic effect of dinaciclib in both MDCKII-ABCC1 and human cancer T47D cells, when applied in combination with anticancer drugs. Taken together, our results suggest that ABC transporters can substantially affect dinaciclib transport across cellular membranes, leading to DDIs. The DDIs of dinaciclib with ABCC1 substrate chemotherapeutics might be exploited in novel cancer therapies.

Interactions of cyclin-dependent kinase inhibitors AT-7519, flavopiridol and SNS-032 with ABCB1, ABCG2 and ABCC1 transporters and their potential to overcome multidrug resistance in vitro

PURPOSE

ATP-binding cassette (ABC) transporters play an important role in multidrug resistance (MDR) toward anticancer drugs. Here, we evaluated interactions of cyclin-dependent kinase inhibitors (CDKi) AT-7519, flavopiridol and SNS-032 with the following ABC transporters in vitro: P-glycoprotein (ABCB1), breast cancer resistance protein (ABCG2) and multidrug resistance-associated protein 1 (ABCC1).

METHODS

Inhibitory potency of studied CDKi to the transporters was evaluated by accumulation assays using fluorescent substrates and MDCKII cells overexpressing human ABCB1, ABCG2 or ABCC1. Resistance of transporter-expressing cells to the CDKi was evaluated by XTT proliferation assay. Observed interactions of CDKi were verified by ATPase assay in ABC transporter-expressing Sf9 membrane vesicles. Combination index analysis was additionally performed in ABC transporter-expressing cancer cell lines, HepG2 and T47D.

RESULTS

Flavopiridol showed a significant inhibitory potency toward ABCG2 and ABCC1. SNS-032 also decreased ABCG2-mediated efflux, while AT-7519 failed to inhibit ABCB1, ABCG2 or ABCC1. Both flavopiridol and SNS-032 showed synergistic antiproliferative effects in combination with relevant ABC transporter substrates such as daunorubicin and topotecan in cancer cells. ABCB1 was found to confer significant resistance to AT-7519 and SNS-032, but not to flavopiridol. In contrast, ABCG2 and ABCC1 conferred resistance to flavopiridol, but not to AT-7519 and SNS-032.

CONCLUSION

Our data provide detailed information on interactions of flavopiridol, SNS-032 and AT-7519 with ABC transporters, which may help elucidate the pharmacokinetic behavior and toxicity of these compounds. Moreover, we show the ability of flavopiridol and SNS-032, but not AT-7519, to overcome ABC transporter-mediated MDR.

Let-7 Sensitizes KRAS Mutant Tumor Cells to Chemotherapy

KRAS is the most commonly mutated oncogene in human cancers and is associated with poor prognosis and drug resistance. Let-7 is a family of tumor suppressor microRNAs that are frequently suppressed in solid tumors, where KRAS mutations are highly prevalent. In this study, we investigated the potential use of let-7 as a chemosensitizer. We found that let-7b repletion selectively sensitized KRAS mutant tumor cells to the cytotoxicity of paclitaxel and gemcitabine. Transfection of let-7b mimic downregulated the expression of mutant but not wild-type KRAS. Combination of let-7b mimic with paclitaxel or gemcitabine diminished MEK/ERK and PI3K/AKT signaling concurrently, triggered the onset of apoptosis, and reverted the epithelial-mesenchymal transition in KRAS mutant tumor cells. In addition, let-7b repletion downregulated the expression of β-tubulin III and ribonucleotide reductase subunit M2, two proteins known to mediate tumor resistance to paclitaxel and gemcitabine, respectively. Let-7 may represent a new class of chemosensitizer for the treatment of KRAS mutant tumors.

Recent Advances on Small-Molecule Survivin Inhibitors

Survivin, a member of the inhibitor of apoptosisproteins family, is highly expressed in most human neoplasms, but its expression is very low or undetectable in terminally differentiated normal tissues. Survivin has been shown to inhibit cancer cell apoptosis and promote cell proliferation. The overexpression of survivin closely correlates with tumor progression and drug resistance. Because of its key role in tumor formation and maintenance, survivin is considered as an ideal target for anticancer treatment. However, the development of small-molecule survivin inhibitors has been challenging due to the requirement to disrupt the protein-protein interactions. Currently only a limited number of survivin inhibitors have been developed in recent years, and most of these inhibitors reduce survivin levels by interacting with other biomolecules instead of directly interacting with survivin protein. Despite these challenges, developing potent and selective small-molecule survivin inhibitors will be important in both basic science to better understand survivin biology and in translational research to develop potentially more effective, broad-spectrum anticancer agents. In this review, the functions of survivin and its role in cancer are summarized. Recent developments, challenges, and future direction of small-molecule survivin inhibitors are also discussed in detail.