Unraveling the molecular mechanism of BNC105, a phase II clinical trial vascular disrupting agent, provides insights into drug design.

Microtubules are made up of tubulin protein and play a very important part in numerous cellular events of eukaryotic cells, which is why they are seen as attractive targets for tumor chemotherapy. BNC105, a known vascular targeting agent, has entered in phase II clinical trials. It has previously been confirmed that BNC105 is an effective microtubule targeting agent for various cancers. BNC105 exhibits selectivity for tumor cells, elicits vascular disrupting effects, and inhibits tumor growth. However, the molecular mechanism of BNC105 is still elusive. Herein, the crystal structure of BNC105 in complex with tubulin protein is revealed, demonstrating the its interaction with the colchicine binding site. In order to thoroughly evaluate its molecular mechanism from a structural-activity-relationship standpoint, the binding mode of tubulin to BNC-105 is compared with colchicine, CA-4 and other BNC-105 derivatives. Our study not only confirms the detailed interactions of the BNC105-tubulin complex, but also offer substantial structural foundation for the design and development of novel benzo[b]furan derivatives as microtubule targeting agents.

Current advances of tubulin inhibitors as dual acting small molecules for cancer therapy.

Microtubule (MT)-targeting agents are highly successful drugs as chemotherapeutic agents, and this is attributed to their ability to target MT dynamics and interfere with critical cellular functions, including, mitosis, cell signaling, intracellular trafficking, and angiogenesis. Because MT dynamics vary in the different stages of the cell cycle, these drugs tend to be the most effective against mitotic cells. While this class of drug has proven to be effective against many cancer types, significant hurdles still exist and include overcoming aspects such as dose limited toxicities and the development of resistance. Newer generations of developed drugs attack these problems and alternative approaches such as the development of dual tubulin and kinase inhibitors are being investigated. This approach offers the potential to show increased efficacy and lower toxicities. This review covers different categories of MT-targeting agents, recent advances in dual inhibitors, and current challenges for this drug target.

Colchicine Binding Site Agent DJ95 Overcomes Drug Resistance and Exhibits Antitumor Efficacy.

Interfering with microtubule dynamics is a well-established strategy in cancer treatment; however, many microtubule-targeting agents are associated with drug resistance and adverse effects. Substantial evidence points to ATP-binding cassette (ABC) transporters as critical players in the development of resistance. Herein, we demonstrate the efficacy of DJ95 (2-(1H-indol-6-yl)-4-(3,4,5-trimethoxyphenyl)-1H-imidazo[4,5-c]pyridine), a novel tubulin inhibitor, in a variety of cancer cell lines, including malignant melanomas, drug-selected resistant cell lines, specific ABC transporter-overexpressing cell lines, and the National Cancer Institute 60 cell line panel. DJ95 treatment inhibited cancer cell migration, caused morphologic changes to the microtubule network foundation, and severely disrupted mitotic spindle formation of mitotic cells. The high-resolution crystal structure of DJ95 in complex with tubulin protein and the detailed molecular interactions confirmed its direct binding to the colchicine site. In vitro pharmacological screening of DJ95 using SafetyScreen44 (Eurofins Cerep-Panlabs) revealed no significant off-target interactions, and pharmacokinetic analysis showed that DJ95 was maintained at therapeutically relevant plasma concentrations for up to 24 hours in mice. In an A375 xenograft model in nude mice, DJ95 inhibited tumor growth and disrupted tumor vasculature in xenograft tumors. These results demonstrate that DJ95 is potent against a variety of cell lines, demonstrated greater potency to ABC transporter-overexpressing cell lines than existing tubulin inhibitors, directly targets the colchicine binding domain, exhibits significant antitumor efficacy, and demonstrates vascular-disrupting properties. Collectively, these data suggest that DJ95 has great potential as a cancer therapeutic, particularly for multidrug resistance phenotypes, and warrants further development. SIGNIFICANCE STATEMENT: Paclitaxel is a widely used tubulin inhibitor for cancer therapy, but its clinical efficacy is often limited by the development of multidrug resistance. In this study, we reported the preclinical characterization of a new tubulin inhibitor DJ95, and demonstrated its abilities to overcome paclitaxel resistance, disrupt tumor vasculature, and exhibit significant antitumor efficacy.

Tubulin Inhibitor-Based Antibody-Drug Conjugates for Cancer Therapy.

Antibody-drug conjugates (ADCs) are a class of highly potent biopharmaceutical drugs generated by conjugating cytotoxic drugs with specific monoclonal antibodies through appropriate linkers. Specific antibodies used to guide potent warheads to tumor tissues can effectively reduce undesired side effects of the cytotoxic drugs. An in-depth understanding of antibodies, linkers, conjugation strategies, cytotoxic drugs, and their molecular targets has led to the successful development of several approved ADCs. These ADCs are powerful therapeutics for cancer treatment, enabling wider therapeutic windows, improved pharmacokinetic/pharmacodynamic properties, and enhanced efficacy. Since tubulin inhibitors are one of the most successful cytotoxic drugs in the ADC armamentarium, this review focuses on the progress in tubulin inhibitor-based ADCs, as well as lessons learned from the unsuccessful ADCs containing tubulin inhibitors. This review should be helpful to facilitate future development of new generations of tubulin inhibitor-based ADCs for cancer therapy.

X-ray Crystallography-Guided Design, Antitumor Efficacy, and QSAR Analysis of Metabolically Stable Cyclopenta-Pyrimidinyl Dihydroquinoxalinone as a Potent Tubulin Polymerization Inhibitor.

Small molecules that interact with the colchicine binding site in tubulin have demonstrated therapeutic efficacy in treating cancers. We report the design, syntheses, and antitumor efficacies of new analogues of pyridopyrimidine and hydroquinoxalinone compounds with improved drug-like characteristics. Eight analogues, 5j, 5k, 5l, 5m, 5n, 5r, 5t, and 5u, showed significant improvement in metabolic stability and demonstrated strong antiproliferative potency in a panel of human cancer cell lines, including melanoma, lung cancer, and breast cancer. We report crystal structures of tubulin in complex with five representative compounds, 5j, 5k, 5l, 5m, and 5t, providing direct confirmation for their binding to the colchicine site in tubulin. A quantitative structure-activity relationship analysis of the synthesized analogues showed strong ability to predict potency. In vivo, 5m (4 mg/kg) and 5t (5 mg/kg) significantly inhibited tumor growth as well as melanoma spontaneous metastasis into the lung and liver against a highly paclitaxel-resistant A375/TxR xenograft model.

X-ray Crystal Structure Guided Discovery and Antitumor Efficacy of Dihydroquinoxalinone as Potent Tubulin Polymerization Inhibitors.

Because of its multifaceted role in cellular functions, tubulin is a validated and productive drug target for cancer therapy. While many tubulin inhibitors demonstrate clinical efficacy, they are often limited by the development of multidrug resistance. Therefore, implementation of tubulin inhibitors that can overcome resistance could provide significant therapeutic benefits. To optimize our previously reported tubulin inhibitor, 4a, we designed and synthesized two new analogues, SB202 and SB204, based on the crystal structure of 4a in complex with tubulin protein. SB202 and SB204 achieved enhanced binding at the colchicine site in tubulin and also showed improved metabolic stability and antiproliferative potency in vitro. Functional studies confirmed that SB202 and SB204 inhibit tubulin polymerization, arrest cells in the G2/M phase of the cell cycle, interfere with cancer cell migration and proliferation, and enhance apoptotic cascades. When evaluated in vivo, SB202 exhibited antitumor and vascular disrupting action against paclitaxel-resistant mouse xenograft models, strongly suggesting the potential of this scaffold to overcome multidrug resistance for cancer therapy.

Structure-Guided Design, Synthesis, and Biological Evaluation of (2-(1H-Indol-3-yl)-1H-imidazol-4-yl)(3,4,5-trimethoxyphenyl) Methanone (ABI-231) Analogues Targeting the Colchicine Binding Site in Tubulin.

ABI-231 is a potent, orally bioavailable tubulin inhibitor that interacts with the colchicine binding site and is currently undergoing clinical trials for prostate cancer. Guided by the crystal structure of ABI-231 in complex with tubulin, we performed structure-activity relationship studies around the 3-indole moiety that led to the discovery of several potent ABI-231 analogues, most notably 10ab and 10bb. The crystal structures of 10ab and 10bb in complex with tubulin confirmed their improved molecular interactions to the colchicine site. In vitro, biological studies showed that new ABI-231 analogues disrupt tubulin polymerization, promote microtubule fragmentation, and inhibit cancer cell migration. In vivo, analogue 10bb not only significantly inhibits primary tumor growth and decreases tumor metastasis in melanoma xenograft models but also shows a significant ability to overcome paclitaxel resistance in a taxane-resistant PC-3/TxR model. In addition, pharmacological screening suggested that 10bb has a low risk of potential off-target function.

Synthesis and biological evaluation of indole-based UC-112 analogs as potent and selective survivin inhibitors.

The anti-apoptotic protein survivin is highly expressed in cancer cells but has a very low expression in fully differentiated adult cells. Overexpression of survivin is positively correlated with cancer cell resistance to chemotherapy and radiotherapy, cancer cell metastasis, and poor patient prognosis. Therefore, selective targeting survivin represents an attractive strategy for the development of anticancer therapeutics. Herein, we reported the extensive structural modification of our recently discovered selective survivin inhibitor UC-112 and the synthesis of thirty-three new analogs. The structure-activity relationship (SAR) study indicated that replacement of the benzyloxy moeity in UC-112 with an indole moiety was preferred to other moieties. Among these UC-112 analogs, 10f, 10h, 10k, 10n showed the most potent antiproliferative activities. Interestingly, they were more potent against the P-glycoprotein overexpressing cancer cell lines compared with the parental cancer cell lines. Mechanistic studies confirmed that new analogs maintained their unique selectivity against survivin among the IAP family members. In vivo study using 10f in a human A375 melanoma xenograft model revealed that it effectively inhibited melanoma tumor growth without observable acute toxicity. Collectively, this study strongly supports the further preclinical development of selective survivin inhibitors based on the UC-112 scaffold.

Structural Modification of the 3,4,5-Trimethoxyphenyl Moiety in the Tubulin Inhibitor VERU-111 Leads to Improved Antiproliferative Activities.

Colchicine binding site inhibitors (CBSIs) hold great potential in developing new generations of antimitotic drugs. Unlike existing tubulin inhibitors such as paclitaxel, they are generally much less susceptible to resistance caused by the overexpression of drug efflux pumps. The 3,4,5-trimethoxyphenyl (TMP) moiety is a critical component present in many CBSIs, playing an important role in maintaining suitable molecular conformations of CBSIs and contributing to their high binding affinities to tubulin. Previously reported modifications to the TMP moiety in a variety of scaffolds of CBSIs have usually resulted in reduced antiproliferative potency. We previously reported a potent CBSI, VERU-111, that also contains the TMP moiety. Herein, we report the discovery of a VERU-111 analogue 13f that is significantly more potent than VERU-111. The X-ray crystal structure of 13f in complex with tubulin confirms its direct binding to the colchicine site. In addition, 13f exhibited a strong inhibitory effect on tumor growth in vivo.

Heterocyclic-Fused Pyrimidines as Novel Tubulin Polymerization Inhibitors Targeting the Colchicine Binding Site: Structural Basis and Antitumor Efficacy.

We report the design, synthesis, and biological evaluation of heterocyclic-fused pyrimidines as tubulin polymerization inhibitors targeting the colchicine binding site with significantly improved therapeutic index. Additionally, for the first time, we report high-resolution X-ray crystal structures for the best compounds in this scaffold, 4a, 4b, 6a, and 8b. These structures not only confirm their direct binding to the colchicine site in tubulin and reveal their detailed molecular interactions but also contrast the previously published proposed binding mode. Compounds 4a and 6a significantly inhibited tumor growth in an A375 melanoma xenograft model and were accompanied by elevated levels of apoptosis and disruption of tumor vasculature. Finally, we demonstrated that compound 4a significantly overcame clinically relevant multidrug resistance in a paclitaxel resistant PC-3/TxR prostate cancer xenograft model. Collectively, these studies provide preclinical and structural proof of concept to support the continued development of this scaffold as a new generation of tubulin inhibitors.

A Potent, Metabolically Stable Tubulin Inhibitor Targets the Colchicine Binding Site and Overcomes Taxane Resistance.

Antimitotics that target tubulin are among the most useful chemotherapeutic drugs, but their clinical activity is often limited by the development of multidrug resistance. We recently discovered the novel small-molecule DJ101 as a potent and metabolically stable tubulin inhibitor that can circumvent the drug efflux pumps responsible for multidrug resistance of existing tubulin inhibitors. In this study, we determined the mechanism of action of this drug. The basis for its activity was illuminated by solving the crystal structure of DJ101 in complex with tubulin at a resolution of 2.8Å. Investigations of the potency of DJ101 in a panel of human metastatic melanoma cell lines harboring major clinically relevant mutations defined IC50 values of 7-10 nmol/L. In cells, DJ101 disrupted microtubule networks, suppressed anchorage-dependent melanoma colony formation, and impaired cancer cell migration. In melanoma-bearing mice, DJ101 administration inhibited tumor growth and reduced lung metastasis in the absence of observable toxicity. DJ101 also completely inhibited tumor growth in a paclitaxel-resistant xenograft mouse model of human prostate cancer (PC-3/TxR), where paclitaxel was minimally effective. Our findings offer preclinical proof of concept for the continued development of DJ101 as a next-generation tubulin inhibitor for cancer therapy.Significance: These findings offer preclinical proof of concept for the continued development of DJ101 as a next-generation antitubulin drug for cancer therapy. Cancer Res; 78(1); 265-77. ©2017 AACR.