Inhibition of cancer cell growth by cyclin dependent kinase 4 inhibitors synthesized based on the structure of fascaplysin

The Potential Strategies for Overcoming Multidrug Resistance and Reducing Side Effects of Monomer Tubulin Inhibitors for Cancer Therapy

BACKGROUND

Tubulin is an essential target in tumor therapy, and this is attributed to its ability to target MT dynamics and interfere with critical cellular functions, including mitosis, cell signaling, and intracellular trafficking. Several tubulin inhibitors have been approved for clinical application. However, the shortcomings, such as drug resistance and toxic side effects, limit its clinical application. Compared with single-target drugs, multi-target drugs can effectively improve efficacy to reduce side effects and overcome the development of drug resistance. Tubulin protein degraders do not require high concentrations and can be recycled. After degradation, the protein needs to be resynthesized to regain function, which significantly delays the development of drug resistance.

METHODS

Using SciFinder® as a tool, the publications about tubulin-based dual-target inhibitors and tubulin degraders were surveyed with an exclusion of those published as patents.

RESULTS

This study presents the research progress of tubulin-based dual-target inhibitors and tubulin degraders as antitumor agents to provide a reference for developing and applying more efficient drugs for cancer therapy.

CONCLUSION

The multi-target inhibitors and protein degraders have shown a development prospect to overcome multidrug resistance and reduce side effects in the treatment of tumors. Currently, the design of dual-target inhibitors for tubulin needs to be further optimized, and it is worth further clarifying the detailed mechanism of protein degradation.

A New Mild Method for Synthesis of Marine Alkaloid Fascaplysin and Its Therapeutically Promising Derivatives

Fascaplysin is a marine alkaloid which is considered to be a lead drug candidate due to its diverse and potent biological activity. As an anticancer agent, fascaplysin holds a great potential due to the multiple targets affected by this alkaloid in cancer cells, including inhibition of cyclin-dependent kinase 4 (CDK4) and induction of intrinsic apoptosis. At the same time, the studies on structural optimization are hampered by its rather high toxicity, mainly caused by DNA intercalation. In addition, the number of methods for the syntheses of its derivatives is limited. In the current study, we report a new two-step method of synthesis of fascaplysin derivatives based on low temperature UV quaternization for the synthesis of thermolabile 9-benzyloxyfascaplysin and 6-tert-butylfascaplysin. 9-Benzyloxyfascaplysin was used as the starting compound to obtain 9-hydroxyfascaplysin. However, the latter was found to be chemically highly unstable. 6-tert-Butylfascaplysin revealed a significant decrease in DNA intercalation when compared to fascaplysin, while cytotoxicity was only slightly reduced. Therefore, the impact of DNA intercalation for the cytotoxic effects of fascaplysin and its derivatives needs to be questioned.

Marine-Derived Lead Fascaplysin: Pharmacological Activity, Total Synthesis, and Structural Modification

Fascaplysin is a planar structure pentacyclic alkaloid isolated from sponges, which can effectively induce the apoptosis of cancer cells. In addition, fascaplysin has diverse biological activities, such as antibacterial, anti-tumor, anti-plasmodium, etc. Unfortunately, the planar structure of fascaplysin can be inserted into DNA and such interaction also limits the further application of fascaplysin, necessitating its structural modification. In this review, the biological activity, total synthesis and structural modification of fascaplysin will be summarized, which will provide useful information for pharmaceutical researchers interested in the exploration of marine alkaloids and for the betterment of fascaplysin in particular.

Cink4T, a quinazolinone-based dual inhibitor of Cdk4 and tubulin polymerization, identified via ligand-based virtual screening, for efficient anticancer therapy

Inhibition of cyclin dependent kinase 4 (Cdk4) prevents cancer cells from entering the early G₀/G₁ phase of the cell division cycle whereas inhibiting tubulin polymerization blocks cancer cells' ability to undergo mitosis (M) late in the cell cycle. We had reported earlier that two non-planar and relatively non-toxic fascaplysin derivatives, an indole and a tryptoline, inhibit Cdk4 with IC₅₀ values of 6.2 and 10 μM, respectively. Serendipitously, we had also found that they inhibited tubulin polymerization. The molecules were efficacious in mouse tumor models. We have now identified Cink4T in a 59-compound quinazolinone library, designed on the basis of ligand-based virtual screening, as a compound that inhibits Cdk4 and tubulin. Its IC₅₀ value for Cdk4 inhibition is 0.47 μM and >50 μM for inhibition of Cdk1, Cdk2, Cdk6, Cdk9. Cink4T inhibits tubulin polymerization with an IC₅₀ of 0.6 μM. Molecular modelling studies on Cink4T with Cdk4 and tubulin crystal structures lend support to these observations. Cancer cell cycle analyses confirm that Cink4T blocks cells at both G₀/G₁ and M phases as it should if it were to inhibit both Cdk4 and tubulin polymerization. Our results show, for the very first time, that virtual screening can be used to design novel inhibitors that can potently block two crucial phases of the cell division cycle.

Fascaplysin Exerts Anti-Cancer Effects through the Downregulation of Survivin and HIF-1α and Inhibition of VEGFR2 and TRKA

Fascaplysin has been reported to exert anti-cancer effects by inhibiting cyclin-dependent kinase 4 (CDK4); however, the precise mode of action by which fascaplysin suppresses tumor growth is not clear. Here, we found that fascaplysin has stronger anti-cancer effects than other CDK4 inhibitors, including PD0332991 and LY2835219, on lung cancer cells that are wild-type or null for retinoblastoma (RB), indicating that unknown target molecules might be involved in the inhibition of tumor growth by fascaplysin. Fascaplysin treatment significantly decreased tumor angiogenesis and increased cleaved-caspase-3 in xenografted tumor tissues. In addition, survivin and HIF-1α were downregulated in vitro and in vivo by suppressing 4EBP1-p70S6K1 axis-mediated de novo protein synthesis. Kinase screening assays and drug-protein docking simulation studies demonstrated that fascaplysin strongly inhibited vascular endothelial growth factor receptor 2 (VEGFR2) and tropomyosin-related kinase A (TRKA) via DFG-out non-competitive inhibition. Overall, these results suggest that fascaplysin inhibits TRKA and VEGFR2 and downregulates survivin and HIF-1α, resulting in suppression of tumor growth. Fascaplysin, therefore, represents a potential therapeutic approach for the treatment of multiple types of solid cancer.

Antitumour potential of BPT: a dual inhibitor of cdk4 and tubulin polymerization

The marine natural product fascaplysin (1) is a potent Cdk4 (cyclin-dependent kinase 4)-specific inhibitor, but is toxic to all cell types possibly because of its DNA-intercalating properties. Through the design and synthesis of numerous fascaplysin analogues, we intended to identify inhibitors of cancer cell growth with good therapeutic window with respect to normal cells. Among various non-planar tryptoline analogues prepared, N-(biphenyl-2-yl) tryptoline (BPT, 6) was identified as a potent inhibitor of cancer cell growth and free from DNA-binding properties owing to its non-planar structure. This compound was tested in over 60 protein kinase assays. It displayed inhibition of Cdk4-cyclin D1 enzyme in vitro far more potently than many other kinases including Cdk family members. Although it blocks growth of cancer cells deficient in the mitotic-spindle checkpoint at the G0/G1 phase of the cell cycle, the block occurs primarily at the G2/M phase. BPT inhibits tubulin polymerization in vitro and acts as an enhancer of tubulin depolymerization of paclitaxel-stabilized tubulin in live cells. Western blot analyses indicated that, in p53-positive cells, BPT upregulates the expression of p53, p21 and p27 proteins, whereas it downregulates the expression of cyclin B1 and Cdk1. BPT selectively kills SV40-transformed mouse embryonic hepatic cells and human fibroblasts rather than untransformed cells. BPT inhibited the growth of several human cancer cells with an IC50<1 μM. The pharmacokinetic study in BALB/c mice indicated good plasma exposure after intravenous administration. It was found to be efficacious at 1/10th the maximum-tolerated dose (1000 mg/kg) against human tumours derived from HCT-116 (colon) and NCI-H460 (lung) cells in SCID (severe-combined immunodeficient) mice models. BPT is a relatively better anticancer agent than fascaplysin with an unusual ability to block two overlapping yet crucial phases of the cell cycle, mitosis and G0/G1. Its ability to effectively halt tumour growth in human tumour-bearing mice would suggest that BPT has the potential to be a candidate for further clinical development.

Tetrahydro-β-carboline derivatives targeting fatty acid amide hydrolase (FAAH) and transient receptor potential (TRP) channels

A series of twenty-five derivatives of tetrahydro-β-carbolines 1-3 was synthesized and assayed on FAAH and TRPV1 and TRPA1 channels. Four carbamates, that is, 5a,c,e, and 9b inhibited FAAH with significant potency and interacted also effectively with TRPV1 and TRPA1 nociceptive receptors, while ureas 7b,d,f, and 8a,b were endowed with specific submicromolar TRPV1 modulating activities.

Fascaplysin as a specific inhibitor for CDK4: insights from molecular modelling

Cyclin-dependent kinases (CDKs) play a key role in the cell cycle and are important anti-cancer drug targets. The natural product fascaplysin inhibits CDK4 with surprising selectivity (IC(50) = 0.4 µM) compared to the close homolog CDK2 (IC(50) = 500 µM). Free energy calculations of the positively charged fascaplysin and an uncharged iso-electronic derivative in the CDK2 and CDK4 inhibitor complexes indicate that the positive charge of fascaplysin is crucial for selectivity. This finding will guide further improvements in the design of fascaplysin-based selective inhibitors for CDK4.

Cyclin-dependent kinases 4 and 6 control tumor progression and direct glucose oxidation in the pentose cycle

Cyclin-dependent kinases CDK4 and CDK6 are essential for the control of the cell cycle through the G(1) phase. Aberrant expression of CDK4 and CDK6 is a hallmark of cancer, which would suggest that CDK4 and CDK6 are attractive targets for cancer therapy. Herein, we report that calcein AM (the calcein acetoxymethyl-ester) is a potent specific inhibitor of CDK4 and CDK6 in HCT116 human colon adenocarcinoma cells, inhibiting retinoblastoma protein (pRb) phosphorylation and inducing cell cycle arrest in the G(1) phase. The metabolic effects of calcein AM on HCT116 cells were also evaluated and the flux between the oxidative and non-oxidative branches of the pentose phosphate pathway was significantly altered. To elucidate whether these metabolic changes were due to the inhibition of CDK4 and CDK6, we also characterized the metabolic profile of a CDK4, CDK6 and CDK2 triple knockout of mouse embryonic fibroblasts. The results show that the metabolic profile associated with the depletion of CDK4, CDK6 and CDK2 coincides with the metabolic changes induced by calcein AM on HCT116 cells, thus confirming that the inhibition of CDK4 and CDK6 disrupts the balance between the oxidative and non-oxidative branches of the pentose phosphate pathway. Taken together, these results indicate that low doses of calcein can halt cell division and kill tumor cells. Thus, selective inhibition of CDK4 and CDK6 may be of greater pharmacological interest, since inhibitors of these kinases affect both cell cycle progression and the robust metabolic profile of tumors.

Fascaplysin exert anti-tumor effects through apoptotic and anti-angiogenesis pathways in sarcoma mice model

Previous studies indicated that fascaplysin derived from marine sponge can induce tumor cell death by apoptosis and possesses anti-angiogenesis activity. In order to verify these two effects in animal model and to identify action mechanisms, we established a sarcoma mice model, and treated mice with fascaplysin for 10 days. The tumor tissues were examined morphologically and immunohistochemically. The differential gene expression was also investigated by mRNA array. Fascaplysin treatment resulted in a significant suppression of tumor growth. Typical apoptotic phenomena were observed by transmission electron microscope and histological detection. Tissue sections were stained with monoclonal antibody directed to proliferating cell nuclear antigen (PCNA) and CD31. The decreased PCNA and CD31 antigen staining indicate the reduction of tumor cell proliferation and tumor vasculature property of fascaplysin in vivo. Microarrays were used to examine the gene expression profiles of tumors on CapitalBio mouse genome oligo array. The regulated genes analyzed from the expression level showed overlapping gene ontology (GO) categories and pathway mapping. Our findings indicate that cell cycle arrest, apoptosis, regulation of actin cytoskeleton, and cell adhesion all play important roles in the onset of fascaplysin. Detailed analysis by real time PCR of key genes confirmed the experimental results of microarrays. From these findings, it can be considered that fascaplysin can inhibit the growth of S180 cell implanted tumor, and the action mechanisms may involve in apoptosis, anti-angiogenesis, or cell cycle arrest.

CDK-mediated regulation of cell functions via c-Jun phosphorylation and AP-1 activation

Cyclin-dependent kinases (CDKs) and their targets have been primarily associated with regulation of cell-cycle progression. Here we identify c-Jun, a transcription factor involved in the regulation of a broad spectrum of cellular functions, as a newly recognized CDK substrate. Using immune cells from mouse and human, and several complementary in vitro and in vivo approaches including dominant negative protein expression, pharmacologic inhibitors, kinase assays and CDK4 deficient cells, we demonstrate the ability of CDK4 to phosphorylate c-Jun. Additionally, the activity of AP-1, a ubiquitous transcription factor containing phosphorylated c-Jun as a subunit, was inhibited by abrogating CDK4. Surprisingly, the regulation of c-Jun phosphorylation by CDK4 occurred in non-dividing cells, indicating that this pathway is utilized for cell functions that are independent of proliferation. Our studies identify a new substrate for CDK4 and suggest a mechanism by which CDKs can regulate multiple cellular activation functions, not all of which are directly associated with cell cycle progression. These findings point to additional roles of CDKs in cell signaling and reveal potential implications for therapeutic manipulations of this kinase pathway.

Cyclin degradation for cancer therapy and chemoprevention

Cancer is characterized by uncontrolled cell division resulting from multiple mutagenic events. Cancer chemoprevention strategies aim to inhibit or reverse these events using natural or synthetic pharmacologic agents. Ideally, this restores normal growth control mechanisms. Diverse classes of compounds have been identified with chemopreventive activity. What unites many of them is an ability to inhibit the cell cycle by specifically modulating key components. This delays division long enough for cells to respond to mutagenic damage. In some cases, damage is repaired and in others cellular damage is sufficient to trigger apoptosis. It is now known that pathways responsible for targeting G1 cyclins for proteasomal degradation can be engaged pharmacologically. Emergence of induced cyclin degradation as a target for cancer therapy and chemoprevention in pre-clinical models is discussed in this article. Evidence for cyclin D1 as a molecular pharmacologic target and biological marker for clinical response is based on experience of proof of principle trials.