Synthesis and biological evaluation of new tetrahydro-β-carbolines as inhibitors of the mitotic kinesin Eg5

Kinesin motor proteins as targets for cancer therapy

The process of mitosis is a validated point of intervention in cancer therapy and a variety of anti-mitotic drugs are successfully being used in the clinic. To date, all approved antimitotics target the spindle microtubules, thus interfering with spindle dynamics, leading to mitotic arrest and apoptosis. While effective, these drugs are also associated with a variety of side effects, including neurotoxicity. In recent years, mitotic kinesins have attracted significant attention in the search for novel, alternative mitotic drug targets. Due to their specific function in mitosis, targeting these proteins creates an opportunity for the development of more selective antimitotics with an improved side effect profile. In addition, kinesin inhibitors may overcome resistance to microtubule targeting drugs. Drug discovery efforts in this area have initially focused on the plus-end directed kinesin spindle protein (KSP) and a variety of compounds are currently undergoing clinical testing.

Assessing compound binding to the Eg5 motor domain using a thermal shift assay

Eg5 is a kinesin whose inhibition leads to cycle arrest during mitosis, making it a potential therapeutic target in cancers. Circular dichroism and isothermal titration calorimetry of our pyrrolotriazine-4-one series of inhibitors with Eg5 motor domain revealed enhanced binding in the presence of adenosine 5'-diphosphate (ADP). Using this information, we studied the interaction of this series with ADP-Eg5 complexes using a thermal shift assay. We measured up to a 7 degrees C increase in the thermal melting (T(m)) of Eg5 for an inhibitor that produced IC(50) values of 60 and 130 nM in microtubule-dependent adenosine triphosphatase (ATPase) and cell-based cytotoxicity assays, respectively. In general, the inhibitor potency of the pyrrolotriazine-4-one series in in vitro biological assays correlated with the magnitude of the thermal stability enhancement of ADP-Eg5. The thermal shift assay also confirmed direct binding of Eg5 inhibitors identified in a high-throughput screen and demonstrated that the thermal shift assay is applicable to a range of chemotypes and can be useful in evaluating both potent (nM) and relatively weakly binding (microM) leads. Overall, the thermal shift assay was found to be an excellent biophysical method for evaluating direct binding of a large number of compounds to Eg5, and it complemented the catalytic assay screens by providing an alternative determination of inhibitor potency.

Structure-activity relationship of S-trityl-L-cysteine analogues as inhibitors of the human mitotic kinesin Eg5

The human kinesin Eg5 is a potential drug target for cancer chemotherapy. Eg5 specific inhibitors cause cells to block in mitosis with a characteristic monoastral spindle phenotype. Prolonged metaphase block eventually leads to apoptotic cell death. S-trityl-L-cysteine (STLC) is a tight-binding inhibitor of Eg5 that prevents mitotic progression. It has proven antitumor activity as shown in the NCI 60 tumor cell line screen. It is of considerable interest to define the minimum chemical structure that is essential for Eg5 inhibition and to develop more potent STLC analogues. An initial structure-activity relationship study on a series of STLC analogues reveals the minimal skeleton necessary for Eg5 inhibition as well as indications of how to obtain more potent analogues. The most effective compounds investigated with substitutions at the para-position of one phenyl ring have an estimated K i (app) of 100 nM in vitro and induce mitotic arrest with an EC 50 of 200 nM.

Carbamazepine induces mitotic arrest in mammalian Vero cells

We reported recently that the anticonvulsant drug carbamazepine, at supratherapeutic concentrations, exerts antiproliferative effects in mammalian Vero cells, but the underlying mechanism has not been elucidated. This motivates us to examine rigorously whether growth arrest was associated with structural changes in cellular organization during mitosis. In the present work, we found that exposure of the cells to carbamazepine led to an increase in mitotic index, mainly due to the sustained block at the metaphase/anaphase boundary, with the consequent inhibition of cell proliferation. Indirect immunofluorescence, using antibodies directed against spindle apparatus proteins, revealed that mitotic arrest was associated with formation of monopolar spindles, caused by impairment of centrosome separation. The final consequence of the spindle defects induced by carbamazepine, depended on the duration of cell cycle arrest. Following the time course of accumulation of metaphase and apoptotic cells during carbamazepine treatments, we observed a causative relationship between mitotic arrest and induction of cell death. Conversely, cells released from the block of metaphase by removal of the drug, continued to progress through mitosis and resume normal proliferation. Our results show that carbamazepine shares a common antiproliferative mechanism with spindle-targeted drugs and contribute to a better understanding of the cytostatic activity previously described in Vero cells. Additional studies are in progress to extend these initial findings that define a novel mode of action of carbamazepine in cultured mammalian cells.

Discovery and biochemical characterization of selective ATP competitive inhibitors of the human mitotic kinesin KSP

The kinesin spindle protein (KSP, also known as Eg5) is essential for the proper separation of spindle poles during mitosis, and inhibition results in mitotic arrest and the formation of characteristic monoaster spindles. Several distinct classes of KSP inhibitors have been described previously in the public and patent literature. However, most appear to share a common induced-fit allosteric binding site, suggesting a common mechanism of inhibition. In a high-throughput screen for inhibitors of KSP, a novel class of thiazole-containing inhibitors was identified. Unlike the previously described allosteric KSP inhibitors, the thiazoles described here show ATP competitive kinetic behavior, consistent with binding within the nucleotide binding pocket. Although they bind to a pocket that is highly conserved across kinesins, these molecules exhibit significant selectivity for KSP over other kinesins and other ATP-utilizing enzymes. Several of these compounds are active in cells and produce a phenotype similar to that observed with previously published allosteric inhibitors of KSP.

Novel ATP-competitive kinesin spindle protein inhibitors

Kinesin spindle protein (KSP), an ATPase responsible for spindle pole separation during mitosis that is present only in proliferating cells, has become a novel and attractive anticancer target with potential for reduced side effects compared to currently available therapies. We report herein the discovery of the first known ATP-competitive inhibitors of KSP, which display a unique activity profile as compared to the known loop 5 (L5) allosteric KSP inhibitors that are currently under clinical evaluation. Optimization of this series led to the identification of biphenyl sulfamide 20, a potent KSP inhibitor with in vitro antiproliferative activity against human cells with either wild-type KSP (HCT116) or mutant KSP (HCT116 D130V). In a murine xenograft model with HCT116 D130V tumors, 20 showed significant antitumor activity following intraperitoneal dosing, providing in vivo proof-of-principle of the efficacy of an ATP-competitive KSP inhibitor versus tumors that are resistant to the other known KSP inhibitors.

Docking studies on kinesin spindle protein inhibitors: an important cooperative ‘minor binding pocket’ which increases the binding affinity significantly

Fifteen KSP inhibitors were docked into the receptor and the binding mode was analyzed for the first time. It was considered that in addition to the main binding pocket all the inhibitors merged in, there exists a cooperative minor binding pocket, which could be explored for significantly increased binding affinity. In addition, a good linear relationship between the biological activities and the lowest binding free energies has also been found. This may help in predicting the binding affinity of newly designed KSP inhibitors.

Synthesis and SAR of thiophene containing kinesin spindle protein (KSP) inhibitors

We have identified and synthesized a series of thiophene containing inhibitors of kinesin spindle protein. SAR studies led to the synthesis of 33, which was co-crystallized with KSP and determined to bind to an allosteric pocket previously described for other known KSP inhibitors.

Actin- and microtubule-targeting bioprobes: their binding sites and inhibitory mechanisms

Actin filaments and microtubules play important biological functions in mammalian cells, such as mitosis, cytokinesis, cell signaling, intracellular transport, and cell motility. Therefore, small molecules that interact with these cytoskeletons are expected to be useful not only as antitumor agents, but also as tools for understanding a wide variety of the cellular functions of cytoskeletons. A large number of compounds have been reported as anti-microtubule or anti-actin agents, but only a few compounds have been clarified as to their binding sites on target molecules and their inhibition mechanisms. Here, I describe our recent research into anti-actin and anti-microtubule natural products. Some inhibitors contain active moieties, such as alpha,beta-unsaturated delta-lactone or allely epoxide, in their structure, and covalently bind to their target molecules. Furthermore, some compounds show new inhibition mechanisms by binding on novel sites in target molecules.

Targeted anti-mitotic therapies: can we improve on tubulin agents?

The advent of molecularly targeted drug discovery has facilitated the identification of a new generation of anti-mitotic therapies that target proteins with specific functions in mitosis. The exquisite selectivity for mitosis and the distinct ways in which these new agents interfere with mitosis provides the potential to not only overcome certain limitations of current tubulin-targeted anti-mitotic drugs, but to expand the scope of clinical efficacy that those drugs have established. The development of these new anti-mitotic drugs as targeted therapies faces significant challenges; nevertheless, these potential therapies also serve as unique tools to dissect the molecular mechanisms of the mitotic-checkpoint response.

Pathway of ATP hydrolysis by monomeric kinesin Eg5

Kinesin-5 family members including human Eg5/KSP contribute to the plus-end-directed force necessary for the assembly and maintenance of the bipolar mitotic spindle. We have used monomeric Eg5-367 in the nucleotide-free state to evaluate the role of microtubules at each step in the ATPase cycle. The pre-steady-state kinetic results show that the microtubule-Eg5 complex binds MgATP tightly, followed by rapid ATP hydrolysis with a subsequent slow step that limits steady-state turnover. We show that microtubules accelerate the kinetics of each step in the ATPase pathway, suggesting that microtubules amplify the nucleotide-dependent structural transitions required for force generation. The experimentally determined rate constants for phosphate product release and Eg5 detachment from the microtubule were similar, suggesting that these two steps are coupled with one occurring at the slow rate after ATP hydrolysis followed by the second step occurring more rapidly. The rate of this slow step correlates well with the steady-state k(cat), indicative that it is the rate-limiting step of the mechanism.

Dimeric Eg5 maintains processivity through alternating-site catalysis with rate-limiting ATP hydrolysis

Eg5/KSP is a homotetrameric, Kinesin-5 family member whose ability to cross-link microtubules has associated it with mitotic spindle assembly and dynamics for chromosome segregation. Transient-state kinetic methodologies have been used to dissect the mechanochemical cycle of a dimeric motor, Eg5-513, to better understand the cooperative interactions that modulate processive stepping. Microtubule association, ADP release, and ATP binding are all fast steps in the pathway. However, the acid-quench analysis of the kinetics of ATP hydrolysis with substrate in excess of motor was unable to resolve a burst of product formation during the first turnover event. In addition, the kinetics of P(i) release and ATP-promoted microtubule-Eg5 dissociation were observed to be no faster than the rate of ATP hydrolysis. In combination the data suggest that dimeric Eg5 is the first kinesin motor identified to have a rate-limiting ATP hydrolysis step. Furthermore, several lines of evidence implicate alternating-site catalysis as the molecular mechanism underlying dimeric Eg5 processivity. Both mantATP binding and mantADP release transients are biphasic. Analysis of ATP hydrolysis through single turnover assays indicates a surprising substrate concentration dependence, where the observed rate is reduced by half when substrate concentration is sufficiently high to require both motor domains of the dimer to participate in the reaction.

Novel Therapeutic Developments Other Than EGFR and VEGF Inhibition in Colorectal Cancer

Developments that may improve existing cytotoxic therapy for colorectal cancer (CRC) include alternatives to 5-fluorouracil (5-FU) such as the liposomal Thymidylate Synthase inhibitor OSI-7904L and the multitargeted antifolate pemetrexed. Studies have explored means of reformulating irinotecan, modulating its pharmacokinetics, and enhancing its activity by maximizing DNA damage through poly(ADP-ribose) polymerase inhibition. Cell cycle inhibitors may offer an alternative to combination with 5-FU. However, as standard regimens become more complex, so do the clinical trials needed to develop new agents, and the path to registration becomes ever more tortuous. It is therefore likely that several drugs with promise in CRC will not be developed for this indication.

Synthesis and SAR of pyrrolotriazine-4-one based Eg5 inhibitors

Synthesis and SAR of substituted pyrrolotriazine-4-one analogues as Eg5 inhibitors are described. Many of these analogues displayed potent inhibitory activities in the Eg5 ATPase and A2780 cell proliferation assays. In addition, pyrrolotriazine-4-one analogue 26 demonstrated in vivo efficacy in an iv P388 murine leukemia model. Both NMR and X-ray crystallographic studies revealed that these analogues bind to an allosteric site on the Eg5 protein.

Inhibitors of human mitotic kinesin Eg5: Characterization of the 4-phenyl-tetrahydroisoquinoline lead series

In a high-throughput screening effort, a series of tetrahydroisoquinolines was identified as modest inhibitors of human Eg5. A medicinal chemistry optimization effort led to the identification of R-4-(3-hydroxyphenyl)-N,N-7,8-tetramethyl-3,4-dihydroisoquinoline-2(1H)-carboxamide (32a) as a potent inhibitor of human Eg5 (ATPase IC50 104 nM) with good anti-proliferative activity in A2780 cells (IC50 234 nM).