Kinesin spindle protein (KSP), known as Hs Eg5, a member of the kinesin-5 family, plays an important role in the formation and maintenance of the bipolar spindle. We previously reported S-trityl-L-cysteine derivatives as selective KSP inhibitors. Here, we report further optimizations using docking modeling in the L5 allosteric binding site, which led to the discovery of several high affinity derivatives with two fused phenyl rings in the trityl group giving low nanomolar range KSP ATPase inhibition. The representative derivatives potently inhibited cell growth of HCT116 cells in correlation with KSP inhibitory activities and significantly suppressed tumor growth in the xenograft model in vivo. KEYWORDS: Kinesin spindle protein, L-cysteine derivative, molecular modeling, differential scanning fluorimetry, HCT-116 xenograft model A ntimitotic agents such as microtubule stabilizers (e.g., taxanes) and destabilizers (e.g., vinca alkaloids) have been clinically validated in the treatment of multiple cancers. 1 Eribulin, which is derived from the natural product haricondolyn B, has recently been approved as a microtubule inhibitor with a new mode of action for the treatment of metastatic breast cancer.2 Microtubules play important roles throughout the cell cycle of cancer cells; however, the disruption of microtubule dynamics produces undesirable side effects, such as neurotoxicity and peripheral neuropathy, which are related to the central role tubulin plays in cell signaling and vesicular transport.3 Acquired drug resistance is another obstacle that can arise following long-term use of these agents. 4 To overcome these limitations, novel anticancer agents that do not directly act on microtubules are being developed.5 Kinesin spindle protein (KSP), known as Hs Eg5, a member of the kinesin-5 family, plays an important role in the formation and maintenance of the bipolar spindle.6 Inhibition of KSP leads to cell cycle arrest during mitosis and results in cells with monopolar spindles (so-called monoasters), followed by apoptosis.7 As KSP is absent in postmitotic neurons and is likely to act only in dividing cells, its inhibitors might provide better specificity than microtubule inhibitors in the treatment of human malignancies.8 A large number of KSP inhibitors with various chemical scaffolds have been reported and some of them, including ispinesib (1) and filanesib (known as ARRY-520, 2), have been entered into clinical trials (Figure 1).
9−13Although the clinical efficacy of these inhibitors has been limited to date, better results have recently been obtained in the treatment of hematological malignancies with thiadiazole-based KSP inhibitors, such as filanesib. Currently, filanesib is undergoing clinical evaluation in combination with proteasome inhibitors such as bortezomib or carfilzomib.14 Previously, we reported structure−activity relationship studies (SARs) of S-trityl-L-cysteine (STLC, 3) derivatives as KSP inhibitors (Figure 1). 15 Several STLC derivatives, such as compounds 3a and 3b with a single ...
