Design, synthesis and anticancer activity of N-aryl indolylsulfoximines: Identification of potent and selective anticancer agents

A facile and efficient approach utilizing copper-mediated cross-coupling reaction of N-boc-3-indolylsulfoximines with aryl iodides was developed to synthesize a diverse range of N-arylated indolylsulfoximines 11a-m in excellent yields (up to 91%). The key precursors, free NH sulfoximines 9 were readily prepared by the treatment of N-boc-3-methylthioindoles 8 with a combination of IBD and ammonium carbamate. Under similar conditions NH-free indolylsulfoximine 9a was successfully prepared in gram-scale quantities. The reaction is highly chemoselective and tolerant of a wide range of functional groups. The process is environmentally friendly and is amenable to scale-up. Among the prepared N-arylated indolylsulfoximines 11a-m, compounds 11i-j (2.68-2.76 μM), 11f-g (1.9-3.7 μM) and 11k (1.28 μM) showed potent and selective cytotoxicity against 22Rv1, C4-2 and MCF7 cells, respectively. Indolylsulfoximine derivative 11l displayed a broad spectrum of activity (1.7-8.2 μM) against the tested cancer cell lines. These compounds were found to be non-cytotoxic to normal HEK293 cells, indicating their potential selectivity for cancer cells. We analysed the impact of 11l on various cellular assays to uncover its mechanism of action. Cellular assay shows that 11l increases the endogenous level of ROS, leading to the increased level of p-53 and c-jun inducing apoptosis. 11l also induced mitochondrial dysfunction, further promoting apoptotic pathways. Besides, 11l also restricts cell invasiveness, indicating that it could serve as an effective anti-metastatic agent. As oxidative stress severe F actin causing tubulin depolymerization, we examined the impact of 11l on tubulin dynamics. Accordingly, 11l treatment decreased the levels of polymerized tubulin in 22Rv1 and C4-2 cells. Although future studies are needed to determine their exact molecular target(s), our data shows that N-aryl indolylsulfoximines could serve as effective anti-cancer agents.

Simple monocyclic pyrimidine analogs as microtubule targeting agents binding to the colchicine site

Complex natural products that bind to tubulin/microtubules come under the broad category of microtubule binding agents. The design of simplified analogs of previously reported bicyclic, microtubule depolymerizer, pyrrolo[2,3-d]pyrimidine, provided valuable structure-activity relationship data and led to the identification of novel monocyclic pyrimidine analogs of which 12 was 47-fold more potent (EC50 123 nM) for cellular microtubule depolymerization activity and 7.5-fold more potent (IC50 24.4 nM) at inhibiting the growth of MDA-MB-435 cancer cells, suggesting significantly better binding of the target within the colchicine site of tubulin compared to lead compound 1. This compound and others of this series of monocyclic pyrimidine analogs were able to overcome multidrug resistance due to the expression of the βIII-isotype of tubulin and P-glycoprotein. In vivo evaluation of the most potent analog 12 in an MDA-MB-435 xenograft mouse model indicated, along with paclitaxel, that both compounds showed a trend towards lower tumor volume however neither compound showed significant antitumor activity in the trial. To our knowledge these are the first examples of simple substituted monocyclic pyrimidines as colchicine site binding antitubulin compounds with potent antitumor activity.

Structural Perspective of Benzophenones Targeting Tubulin as Anticancer Agents

Cancer is the leading cause of death and the most significant determinant of life expectancy in almost every country in this twenty-first century. According to the World Health Organization (WHO), cancer is responsible for the leading cause of death globally. Benzophenone derivatives are found in a variety of naturally occurring compounds which are known to be pharmacologically efficacious against a variety of diseases, including cancer. Microtubules are thought to be a good target for cancer chemotherapies. Microtubule polymerization and depolymerization are induced by a variety of natural, synthetic, and semisynthetic chemicals having a benzophenone nucleus, affecting tubulin dynamics. Several medications that affect microtubule dynamics are in various stages of clinical trials, including Combretastatins (phase II), Vincristine (clinically approved), Paclitaxel (in clinical usage), and epothilone (phase III), and only a few have been patented. Benzophenone derivatives target the colchicine binding site of microtubules, damage them and cause cell cycle arrest in the G2-M phase. Belonging to this class of molecules, phenstatin, a potent inhibitor of tubulin polymerization, has shown strongly inhibit cancer cell growth and arrest the G2/M phase of the cell cycle by targeting the colchicine binding site of microtubules. In the present manuscript, we described the benzophenone as tubulin polymerization inhibitors, their Structure-Activity Relationships (SARs) and molecular docking studies that reveal its binding affinity with the colchicine binding site.

Combination of microtubule targeting agents with other antineoplastics for cancer treatment

Microtubule targeting agents (MTAs) have attracted extensive attention for cancer treatment. However, their clinical efficacies are limited by intolerable toxicities, inadequate efficacy and acquired multidrug resistance. The combination of MTAs with other antineoplastics has become an efficient strategy to lower the toxicities, overcome resistance and improve the efficacies for cancer treatment. In this article, we review the combinations of MTAs with some other anticancer drugs, such as cytotoxic agents, kinases inhibitors, histone deacetylase inhibitors, immune checkpoints inhibitors, to overcome these obstacles. We strongly believe that this review will provide helpful information for combination therapy based on MTAs.

Indazole-based microtubule-targeting agents as potential candidates for anticancer drugs discovery

Tremendous research is focused on developing novel drug candidates targeting microtubules to inhibit their function in several cellular processes, including cell division. In this regard, several indazole derivatives were sought to target the colchicine binding site on the β-tubulin, a crucial protein required to form microtubules, to develop microtubule targeting agents. Even though there are several reviews on the indazole-based compounds, none of them focused on using indazole scaffold to develop microtubule targeting agents. Therefore, this review aims to present the advances in research on compounds containing indazole scaffolds as microtubule targeting agents based on the articles published in the last two decades. Among the articles reviewed, we found that compounds 6 and 7 showed the lowest IC₅₀ values of 0.6 ∼ 0.9 nM in the cell line studies, making them the strongest indazole derivatives that target microtubules. The compounds 30, 31, 37 (IC₅₀ = ∼ 1 nM) and compounds 8, 38 (IC₅₀ = ∼ 2 nM) have proved to be potent microtubule inhibitors. The compounds 18, 31, 44, 45 also showed strong anticancer activity (IC₅₀ = ∼ 8 nM). It is important to notice that except for compounds 9, 12, 13, 15, and SRF, the top activity compounds including 6, 7, 8, 10, 11, 30, 31, 37, 44, and 45 contain 3,4,5‑trimethoxyphenyl substitution similar to that of colchicine. Therefore, it appears that the 3,4,5‑trimethoxyphenyl substituent on the indazole scaffold is crucial for targeting CBS.

Structure modification and biological evaluation of indole-chalcone derivatives as anti-tumor agents through dual targeting tubulin and TrxR

Microtubule target agents (MTAs) are widely-used clinical anti-cancer drugs for decades, but the acquired drug resistance severely restricted their application. Thioredoxin reductases (TrxR) was reported to be overexpressed in most tumors and closely related to high risk of cancer recurrence and drug resistance, making it a potential target for anticancer drug discovery. Multi-target-directed ligands (MTDLs) by a single molecule provide a logical and alternative approach to drug combinations. In this work, based on the structure-activity relationships obtained in our previous study, some structure modifications were performed. On one hand, the retained skeleton structure of MTAs endowed its tubulin polymerization inhibition activity, on the other hand, the selenium-containing structure and α,β-unsaturated ketone moiety endowed the TrxR inhibition activity. As results, the newly obtained compounds exhibited superior anti-proliferative activities towards various human cancer cells and drug-resistance cells, and displayed high selectivity towards various human normal cells. The mechanism study revealed that the dual effect of cell cycle arrest triggered by targeting tubulin and the abnormal accumulation of ROS caused by TrxR inhibition eventually lead to cell apoptosis. Notably, compared with the MTA agents CA-4P, and the TrxR inhibitor Ethaselen, the optimized compound 14c, which served as dual-targeting inhibitor of tubulin and TrxR, exerted greatly improved in vivo anti-tumor activity. In summary, 14c deserved further consideration for cancer therapy.

A comprehensive assessment of a new series of 5',6'-difluorobenzotriazole-acrylonitrile derivatives as microtubule targeting agents (MTAs)

Microtubules (MTs) are the principal target for drugs acting against mitosis. These compounds, called microtubule targeting agents (MTAs), cause a mitotic arrest during G2/M phase, subsequently inducing cell apoptosis. MTAs could be classified in two groups: microtubule stabilising agents (MSAs) and microtubule destabilising agents (MDAs). In this paper we present a new series of (E) (Z)-2-(5,6-difluoro-(1H)2H-benzo[d] [1,2,3]triazol-1(2)-yl)-3-(R)acrylonitrile (9a-j, 10e, 11a,b) and (E)-2-(1H-benzo[d] [1,2,3]triazol-1-yl)-3-(R)acrylonitrile derivatives (13d,j), which were recognised to act as MTAs agents. They were rationally designed, synthesised, characterised and subjected to different biological assessments. Computational docking was carried out in order to investigate the potential binding to the colchicine-binding site on tubulin. From this first prediction, the di-fluoro substitution seemed to be beneficial for the binding affinity with tubulin. The new fluorine derivatives, here presented, showed an improved antiproliferative activity when compared to the previously reported compounds. The biological evaluation included a preliminary antiproliferative screening on NCI60 cancer cells panel (1-10 μM). Compound 9a was selected as lead compound of the new series of derivatives. The in vitro XTT assay, flow cytometry analysis and immunostaining performed on HeLa cells treated with 9a showed a considerable antiproliferative effect, (IC50 = 3.2 μM), an increased number of cells in G2/M-phase, followed by an enhancement in cell division defects. Moreover, β-tubulin staining confirmed 9a as a MDA triggering tubulin disassembly, whereas colchicine-9a competition assay suggested that compound 9a compete with colchicine for the binding site on tubulin. Then, the co-administration of compound 9a and an extrusion pump inhibitor (EPI) was investigated: the association resulted beneficial for the antiproliferative activity and compound 9a showed to be client of extrusion pumps. Finally, structural superimposition of different colchicine binding site inhibitors (CBIs) in clinical trial and our MDA, provided an additional confirmation of the targeting to the predicted binding site. Physicochemical, pharmacokinetic and druglikeness predictions were also conducted and all the newly synthesised derivatives showed to be drug-like molecules.

MIL-1, a novel antitumor agent derived from natural product millepachine, acts as tubulin polymerization inhibitor for the treatment of hepatocellular carcinoma

Natural products are a large source of clinically effective antitumor drugs. Millepachine, a natural product derived from leguminous plants, was reported to display antitumor activity. In this study, the novel compound, (1H-indol-5-yl) (5-methoxy-2,2-dimethyl-2H-chromen-8-yl)methanone (MIL-1), was designed and synthesized by fusing millepachine and indole rings. MIL-1 exerted much better antitumor activity than millepachine, manifesting as a 24- to 201-fold increase in vitro cytotoxicity and a 2.4-fold increase in in vivo antitumor activity in hepatocellular cell lines-derived models. The immunofluorescence and HPLC detection revealed that MIL-1 was a potent microtubule targeting agent by interfering with the equilibrium of tubulin-microtubule dynamics and irreversibly binding to tubulin. MIL-1 displayed remarkable antitumor activity with an IC50 of 31-207 nM towards various human cancer cell lines derived from various organs and tissues, and it exerted no evidence of toxicity against normal cells. Mechanistic studies showed that MIL-1 arrested the cell cycle at G2/M phase and induced apoptosis by activating caspase-3 activity and reactive oxygen species (ROS) accumulation. Moreover, the superior antitumor effect of MIL-1 is worthy of further detailed study for the treatment of hepatocellular carcinoma (HCC).

Potential of substituted quinazolines to interact with multiple targets in the treatment of cancer

The efficacy of quinazoline-based antiglioma agents has been attributed to their effects on microtubule dynamics.1,2 The design, synthesis and biological evaluation of quinazolines as potent inhibitors of multiple intracellular targets, including microtubules and multiple RTKs, is described. In addition to the known ability of quinazolines 1 and 2 to cause microtubule depolymerization, they were found to be low nanomolar inhibitors of EGFR, VEGFR-2 and PDGFR-β. Low nanomolar inhibition of EGFR was observed for 1-3 and 9-10. Compounds 1 and 4 inhibited VEGFR-2 kinase with activity better than or equal to that of sunitinib. In addition, compounds 1 and 2 had similar potency to sunitinib in the CAM angiogenesis assay. Multitarget activities of compounds in the present study demonstrates that the quinazolines can affect multiple pathways and could lead to these agents having antitumor potential caused by their activity against multiple targets.

Novel pyrazolo[4,3-d]pyrimidine microtubule targeting agents (MTAs): Synthesis, structure-activity relationship, in vitro and in vivo evaluation as antitumor agents

The design, synthesis, and biological evaluation of a series novel N1‑methyl pyrazolo[4,3-d]pyrimidines as inhibitors of tubulin polymerization and colchicine binding were described here. Synthesis of target compounds involved alkylation of the pyrazolo scaffold, which afforded two regioisomers. These were separated, characterized and identified with ¹H NMR and NOESY spectroscopy. All compounds, except 10, inhibited [³H]colchicine binding to tubulin, and the potent inhibition was similar to that obtained with CA-4. Compounds 9 and 11-13 strongly inhibited the polymerization of tubulin, with IC₅₀ values of 0.45, 0.42, 0.49 and 0.42 μM, respectively. Compounds 14-16 inhibited the polymerization of tubulin with IC_50s near ∼1 μM. Compounds 9, 12, 13 and 16 inhibited MCF-7 breast cancer cell lines and circumvented βIII-tubulin mediated cancer cell resistance to taxanes and other MTAs, and compounds 9-17 circumvented Pgp-mediated drug resistance. In the standard NCI testing protocol, compound 9 exhibited excellent potency with low to sub nanomolar GI₅₀ values (≤10 nM) against most tumor cell lines, including several multidrug resistant phenotypes. Compound 9 was significantly (P < 0.0001) better than paclitaxel at reducing MCF-7 TUBB3 (βIII-tubulin overexpressing) tumors in a mouse xenograft model. Collectively, these studies support the further preclinical development of the pyrazolo[4,3-d]pyrimidine scaffold as a new generation of tubulin inhibitors and 9 as an anticancer agent with advantages over paclitaxel.

Mitotic checkpoint defects: en route to cancer and drug resistance

Loss of mitosis regulation is a common feature of malignant cells that leads to aberrant cell division with inaccurate chromosome segregation. The mitotic checkpoint is responsible for faithful transmission of genetic material to the progeny. Defects in this checkpoint, such as mutations and changes in gene expression, lead to abnormal chromosome content or aneuploidy that may facilitate cancer development. Furthermore, a defective checkpoint response is indicated in the development of drug resistance to microtubule poisons that are used in treatment of various blood and solid cancers for several decades. Mitotic slippage and senescence are important cell fates that occur even with an active mitotic checkpoint and are held responsible for the resistance. However, contradictory findings in both the scenarios of carcinogenesis and drug resistance have aroused questions on whether mitotic checkpoint defects are truly responsible for these dismal outcomes. Here, we discuss the possible contribution of the faulty checkpoint signaling in cancer development and drug resistance, followed by the latest research on this pathway for better outcomes in cancer treatment.

The 3-D conformational shape of N-naphthyl-cyclopenta[d]pyrimidines affects their potency as microtubule targeting agents and their antitumor activity

A series of methoxy naphthyl substituted cyclopenta[d]pyrimidine compounds, 4-10, were designed and synthesized to study the influence of the 3-D conformation on microtubule depolymerizing and antiproliferative activities. NOESY studies with the N,2-dimethyl-N-(6'-methoxynaphthyl-1'-amino)-cyclopenta[d]pyrimidin-4-amine (4) showed hindered rotation of the naphthyl ring around the cyclopenta[d]pyrimidine scaffold. In contrast, NOESY studies with N,2-dimethyl-N-(5'-methoxynaphthyl-2'-amino)-cyclopenta[d]pyrimidin-4-amine (5) showed free rotation of the naphthyl ring around the cyclopenta[d]pyrimidine scaffold. The rotational flexibility and conformational dissimilarity between 4 and 5 led to a significant difference in biological activities. Compound 4 is inactive while 5 is the most potent in this series with potent microtubule depolymerizing effects and low nanomolar IC50 values in vitro against a variety of cancer cell lines. The ability of 5 to inhibit tumor growth in vivo was investigated in a U251 glioma xenograft model. The results show that 5 had better antitumor effects than the positive control temozolomide and have identified 5 as a potential preclinical candidate for further studies. The influence of conformation on the microtubule depolymerizing and antitumor activity forms the basis for the development of conformation-activity relationships for the cyclopenta[d]pyrimidine class of microtubule targeting agents.

AQ-4, a deuterium-containing molecule, acts as a microtubule-targeting agent for cancer treatment

The important physiological function of microtubules makes them an indispensable and clinically effective target of anti-tumor agents. Herein, we sought to design, synthesize, and evaluate a novel 4-anilinoquinazoline derivative and identify its anti-tumor activity in vitro and in vivo. The novel compound, N-(4-methoxyphenyl)-N-methyl-2-(methyl-d₃)quinazolin-4-amine (AQ-4), was identified as a representative scaffold and potent microtubule-targeting agent. As a promising antimitotic agent, AQ-4 displayed remarkable anti-tumor activity with an average IC₅₀ value of 19 nM across a panel of 14 human cancer cell lines. AQ-4 also exhibited nearly identical potent activities against drug-resistant cells, with no evidence of toxicity towards normal cells. A further target verification study revealed that AQ-4 targets the tubulin-microtubule system by significantly inhibiting tubulin polymerization and disrupting the intracellular microtubule spindle dynamics. According to the results of mechanism study, AQ-4 induced cell cycle arrest in the G₂/M phase, promoting evident apoptosis and a collapses of mitochondrial membrane potential. The superior anti-tumor effect of AQ-4 in vivo suggests that it should be further investigated to validate its use for cancer therapy.

Tubulin Maytansine Site Binding Ligands and their Applications as MTAs and ADCs for Cancer Therapy

BACKGROUND

Microtubule Targeting Agents (MTAs) represent the most successful anticancer drugs for cancer chemotherapy. Through interfering with the tubulin polymerization and depolymerization dynamics, MTAs influence intracellular transport and cell signal pathways, inhibit cell mitosis and cell proliferation, and induce cell apoptosis and death. The tubulin maytansine site binding agents are natural or nature-derived products that represent one type of the MTAs that inhibit tubulin polymerization and exhibit potent antitumor activity both in vitro and in vivo. They are used as Antibody-Drug Conjugates (ADCs) in cancer chemotherapy.

METHODS

Using SciFinder® as a tool, the publications about maytansine, its derivatives, maytansine binding site, maytansine site binding agents and their applications as MTAs for cancer therapy were surveyed with an exclusion on those published as patents. The latest progresses in clinical trials were obtained from the clinical trial web.

RESULTS

This article presents an introduction about MTAs, maytansine, maytansine binding site and its ligands, the applications of these ligands as MTAs and ADCs in cancer therapy.

CONCLUSION

The maytansine site binding agents are powerful MTAs for cancer chemotherapy. The maytansine site ligands-based ADCs are used in clinic or under clinical trials as cancer targeted therapy to improve their selectivity and to reduce their side effects. Further improvements in the delivery efficiency of the ADCs will benefit the patients in cancer targeted therapy.

C12, a combretastatin-A4 analog, exerts anticancer activity by targeting microtubules

Combretastatin A4 and its analogs are undergoing various clinical trials for the treatment of different cancers. This study illustrated the molecular mechanism and antitumor activity of C12, (5-Quinolin-3-yl and 4-(3,4,5-trimethoxyphenyl) substituted imidazol-2-amine), a synthetic analog of CA-4. C12 reduced the tumor volume of MCF-7 xenograft in NOD-SCID mice without affecting the bodyweight of the mice. Further, C12 inhibited the proliferation of several types of cancer cells more efficiently than their noncancerous counterparts. Using GFP-EB1 imaging, the effects of C12 on the interphase microtubule dynamics were determined in live HeLa cells. C12 (10 nM, half-maximal proliferation inhibitory concentration) reduced the growth rate of microtubules by 52% and increased the pause time of microtubules by 68%. In addition, fluorescence recovery after photobleaching analysis demonstrated that 10 nM C12 strongly suppressed spindle microtubule dynamics in HeLa cells. C12 treatment reduced the interpolar distance between the two spindle poles, increased the chromosome congression index, inhibited chromosome movement, and increased the level of mitotic checkpoint complex proteins BubR1 and Mad2. The evidence presented here indicated that C12 could induce different modes of cell death, depending on the extent of microtubule depolymerization. Since C12 targets both the mitotic and non-mitotic cells and showed a stronger activity against cancerous cells than non-cancerous cells, it may have an advantage in cancer chemotherapy. The results significantly enhance our understanding of the antitumor mechanism of the microtubule-depolymerizing agents.

CKT0353, a novel microtubule targeting agent, overcomes paclitaxel induced resistance in cancer cells

Microtubule targeting agents (MTAs) are extensively used in cancer treatment and many have achieved substantial clinical success. In recent years, targeting microtubules to inhibit cell division has become a widespread pharmaceutical approach for treatment of various cancer types. Nevertheless, the development of multidrug resistance (MDR) in cancer remains a major obstacle for successful application of these agents. Herein, we provided the evidence that CKT0353, α-branched α,β-unsaturated ketone, possesses the capacity to successfully evade the MDR phenotype as an MTA. CKT0353 induced G₂/M phase arrest, delayed cell division via spindle assembly checkpoint activation, disrupted the mitotic spindle formation and depolymerized microtubules in human breast, cervix, and colorectal carcinoma cells. Molecular docking analysis revealed that CKT0353 binds at the nocodazole binding domain of β-tubulin. Furthermore, CKT0353 triggered apoptosis via caspase-dependent mechanism. In addition, P-glycoprotein overexpressing colorectal carcinoma cells showed higher sensitivity to this agent when compared to their sensitive counterpart, demonstrating the ability of CKT0353 to overcome this classic MDR mechanism involved in resistance to various MTAs. Taken together, these findings suggest that CKT0353 is an excellent candidate for further optimization as a therapeutic agent against tumors with MDR phenotype.

Revisiting microtubule targeting agents: α-Tubulin and the pironetin binding site as unexplored targets for cancer therapeutics

Molecules that bind to tubulin and disrupt tubulin dynamics are known as microtubule targeting agents. Treatment with a microtubule targeting agent leads to cell cycle arrest followed by apoptosis. Tubulin inhibitors have been highly effective in the clinical treatment of a variety of tumors and are being investigated for treatment of several other diseases. Currently, all FDA approved microtubule inhibitors bind to β-tubulin. Given the overall success of tubulin-binding agents in anticancer chemotherapy, α-tubulin is an attractive and unexplored target. Herein, we will discuss pironetin, the only compound known to bind α-tubulin, with particular focus on the known biological properties, the total syntheses, exploration of its structure-activity relationship, and future directions.

Sterically induced conformational restriction: Discovery and preclinical evaluation of novel pyrrolo[3,2-d]pyrimidines as microtubule targeting agents

The discovery, synthesis and biological evaluations of a series of nine N5-substituted-pyrrolo[3,2-d]pyrimidin-4-amines are reported. Novel compounds with microtubule depolymerizing activity were identified. Some of these compounds also circumvent clinically relevant drug resistance mechanisms (expression of P-glycoprotein and βIII tubulin). Compounds 4, 5, and 8-13 were one to two-digit nanomolar (IC₅₀) inhibitors of cancer cells in culture. Contrary to recent reports (Banerjee et al. J. Med. Chem.2018, 61, 1704-1718), the conformation of the most active compounds determined by ¹H NMR and molecular modeling are similar to that reported previously and in keeping with recently reported X-ray crystal structures. Compound 11, freely water soluble as the HCl salt, afforded statistically significant inhibition of tumor growth in three xenograft models [MDA-MB-435, MDA-MB-231 and NCI/ADR-RES] compared with controls. Compound 11 did not display indications of animal toxicity and is currently slated for further preclinical development.

Spastin regulates VAMP7-containing vesicles trafficking in cortical neurons

Alteration of axonal transport has emerged as a common precipitating factor in several neurodegenerative disorders including Human Spastic Paraplegia (HSP). Mutations of the SPAST (SPG4) gene coding for the spastin protein account for 40% of all autosomal dominant uncomplicated HSP. By cleaving microtubules, spastin regulates several cellular processes depending on microtubule dynamics including intracellular membrane trafficking. Axonal transport is fundamental for the viability of motor neurons which often have very long axons and thus require efficient communication between the cell body and its periphery. Here we found that the anterograde velocity of VAMP7 vesicles, but not that of VAMP2, two vesicular-SNARE proteins implicated in neuronal development, is enhanced in SPG4-KO neurons. We showed that this effect is associated with a slight increase of the level of acetylated tubulin in SPG4-KO neurons and correlates with an enhanced activity of kinesin-1 motors. Interestingly, we demonstrated that an artificial increase of acetylated tubulin by drugs reproduces the effect of Spastin KO on VAMP7 axonal dynamics but also increased its retrograde velocity. Finally, we investigated the effect of microtubule targeting agents which rescue axonal swellings, on VAMP7 and microtubule dynamics. Our results suggest that microtubule stabilizing agents, such as taxol, may prevent the morphological defects observed in SPG4-KO neurons not simply by restoring the altered anterograde transport to basal levels but rather by increasing the retrograde velocity of axonal cargoes.

Discovery and preclinical evaluation of 7-benzyl-N-(substituted)-pyrrolo[3,2-d]pyrimidin-4-amines as single agents with microtubule targeting effects along with triple-acting angiokinase inhibition as antitumor agents

The utility of cytostatic antiangiogenic agents (AA) in cancer chemotherapy lies in their combination with cytotoxic chemotherapeutic agents. Clinical combinations of AA with microtubule targeting agents (MTAs) have been particularly successful. The discovery, synthesis and biological evaluations of a series of 7-benzyl-N-substituted-pyrrolo[3,2-d]pyrimidin-4-amines are reported. Novel compounds which inhibit proangiogenic receptor tyrosine kinases (RTKs) including vascular endothelial growth factor receptor-2 (VEGFR-2), platelet-derived growth factor receptor-β (PDGFR-β) and epidermal growth factor receptor (EGFR), along with microtubule targeting in single molecules are described. These compounds also inhibited blood vessel formation in the chicken chorioallantoic membrane (CAM) assay, and some potently inhibited tubulin assembly (with activity comparable to that of combretastatin A-4 (CA)). In addition, some of the analogs circumvent the most clinically relevant tumor resistance mechanisms (P-glycoprotein and β-III tubulin expression) to microtubule targeting agents (MTA). These MTAs bind at the colchicine site on tubulin. Two analogs displayed two to three digit nanomolar GI50 values across the entire NCI 60 tumor cell panel and one of these, compound 7, freely water soluble as its HCl salt, afforded excellent in vivo antitumor activity against an orthotopic triple negative 4T1 breast cancer model and was superior to doxorubicin.

Back to the tubule: microtubule dynamics in Parkinson's disease

Cytoskeletal homeostasis is essential for the development, survival and maintenance of an efficient nervous system. Microtubules are highly dynamic polymers important for neuronal growth, morphology, migration and polarity. In cooperation with several classes of binding proteins, microtubules regulate long-distance intracellular cargo trafficking along axons and dendrites. The importance of a delicate interplay between cytoskeletal components is reflected in several human neurodegenerative disorders linked to abnormal microtubule dynamics, including Parkinson’s disease (PD). Mounting evidence now suggests PD pathogenesis might be underlined by early cytoskeletal dysfunction. Advances in genetics have identified PD-associated mutations and variants in genes encoding various proteins affecting microtubule function including the microtubule-associated protein tau. In this review, we highlight the role of microtubules, their major posttranslational modifications and microtubule associated proteins in neuronal function. We then present key evidence on the contribution of microtubule dysfunction to PD. Finally, we discuss how regulation of microtubule dynamics with microtubule-targeting agents and deacetylase inhibitors represents a promising strategy for innovative therapeutic development.

βI-tubulin mutations in the laulimalide/peloruside binding site mediate drug sensitivity by altering drug-tubulin interactions and microtubule stability

Peloruside A (PLA) and laulimalide (LAU) are potent microtubule-stabilizing natural products that are effective against a broad spectrum of cancer cells. The interactions of PLA and LAU with tubulin have attracted a great deal of attention, mainly because they bind to β-tubulin at a site that is different from the classical taxoid site. Multiple βI-tubulin amino acid residues have been predicted by computer modelling studies and more recently by protein crystallography to participate in the binding of PLA and LAU to tubulin. The relevance of these residues in determining cellular sensitivity to the compounds, however, remains largely uncertain. To determine the role of four binding site residues, Q291, D295, V333, and N337 on PLA and LAU activity, we introduced single mutations to these sites by site-directed mutagenesis and transfected each mutant tubulin separately into HEK and/or HeLa cells. We found that a Q291M βI-tubulin mutation increased sensitivity of the cells to PLA, but not to LAU, paclitaxel (PTX), or vinblastine (VBL). In contrast, V333W and N337L mutations led to less stable microtubules, with the V333W causing resistance to PLA and PTX, but not LAU, and the N337L causing resistance to PLA, LAU, and PTX. Moreover, cells expressing either W333 or L337 were hypersensitive to the microtubule-destabilizing agent, VBL. The D295I mutation conferred resistance to both PLA and LAU without affecting microtubule stability or sensitivity to PTX or ixabepilone (IXB). This study identifies the first mammalian βI-tubulin mutation that specifically increases sensitivity to PLA, and reports mutations at PLA and LAU binding site residues that can either reduce microtubule stability or impair drug-tubulin binding, conferring resistance to these microtubule-stabilizing agents. This information provides insights on β-tubulin residues important for maintaining microtubule structural integrity and for sensitivity to microtubule-targeting agents, and suggests novel directions for rational structure-based design of new and more potent agents for cancer treatment that target the LAU/PLA site.

Re-expression of LKB1 in LKB1-mutant EKVX cells leads to resistance to paclitaxel through the up-regulation of MDR1 expression

OBJECTIVES

The tumor suppressor LKB1 has recently been shown to be involved in the regulation of microtubule dynamics, thus cancer cells with inactivated LKB1 may have developed a means to overcome dysregulated microtubule functions, making them intrinsically resistant to microtubule targeting agents. Here, we generated isogenic LKB1-wild type and mutant non-small cell lung cancer (NSCLC) cell lines to evaluate the role of LKB1 in paclitaxel resistance.

MATERIALS AND METHODS

SRB, flow cytometry and immunoblotting were used to assess cell proliferation and apoptosis in NSCLC cell lines after paclitaxel treatment. Expression of LKB1 was restored in LKB1-null cells by retrovirus infection and was reduced in LKB1-wild type cells by shRNA knock down.

RESULTS AND CONCLUSION

The restoration of LKB1 in LKB1-null cells failed to promote paclitaxel-induced apoptosis in both p53-wild type and p53-mutant backgrounds, indicating that LKB1 was not required for paclitaxel-induced apoptosis. Interestingly, the re-establishment of LKB1 expression led to the up-regulation of class III beta-tubulin and MDR1 in EKVX cells. The up-regulation of MDR1 protein and transcripts in EKVX cells was specifically associated with the expression of wild-type LKB1 and mainly responsible for the increased cellular resistance to paclitaxel. However, the presence of LKB1 protein was not required to maintain this increased MDR1 expression even though there was no genetic amplification or promoter de-methylation of the ABCB1 locus in EKVX-LKB1-WT cells. These data suggest that LKB1 does not promote paclitaxel-induced apoptosis in most NSCLC cell lines. In contrast, in some NSCLC, the presence of LKB1 may facilitate increases in either MDR1 or class III beta-tubulin expression which can lead to paclitaxel resistance.

The design and discovery of water soluble 4-substituted-2,6-dimethylfuro[2,3-d]pyrimidines as multitargeted receptor tyrosine kinase inhibitors and microtubule targeting antitumor agents

The design, synthesis and biological evaluations of fourteen 4-substituted 2,6-dimethylfuro[2,3-d]pyrimidines are reported. Four compounds (11-13, 15) inhibit vascular endothelial growth factor receptor-2 (VEGFR-2), platelet-derived growth factor receptor β (PDGFR-β), and target tubulin leading to cytotoxicity. Compound 11 has nanomolar potency, comparable to sunitinib and semaxinib, against tumor cell lines overexpressing VEGFR-2 and PDGFR-β. Further, 11 binds at the colchicine site on tubulin, depolymerizes cellular microtubules and inhibits purified tubulin assembly and overcomes both βIII-tubulin and P-glycoprotein-mediated drug resistance, and initiates mitotic arrest leading to apoptosis. In vivo, its HCl salt, 21, reduced tumor size and vascularity in xenograft and allograft murine models and was superior to docetaxel and sunitinib, without overt toxicity. Thus 21 affords potential combination chemotherapy in a single agent.