Ispinesib (SB-715992)

Ispinesib is a potent, allosteric, reversible, and specific inhibitor of KSP, which changes the binding property of KSP to microtubules and disturbs its movement by inhibiting ADP release without altering the release of the KSP-ADP complex from the microtubule. ^[1] Ispinesib shows potent cytotoxic activity in a panel of tumor cell lines, including Colo205, Colo201, HT-29, M5076, Madison-109, and MX-1, with IC50 of 1.2 nM to 9.5 nM. ^[2] In PC-3 prostate cancer cells, Ispinesib (15 nM and 30 nM) blocks cell proliferation and induces apoptosis by regulating the expression levels of genes that controls apoptosis, cell proliferation, cell cycle, and cell signaling, such as EGFR, p27, p15, and IL-11. ^[3] In a panel of 53 breast cell lines, Ispinesib (7.4 nM–600 nM) demonstrates broad inhibitory activity. In BT-474 and MDA-MB-468 cells, Ispinesib (150 nM) induces apoptosis, as revealed by a higher proportion of apoptotic cells, lower antiapoptotic Bcl-X_L level, and higher proapoptotic Bax and Bid levels. ^[4]

Ispinesib (4.5 mg/kg–15 mg/kg) exhibits inhibitory effects against Colo205, Colo201, HT-29, but not MX-1 cells, in mouse xenograft models. SB-715992 (6 mg/kg–10 mg/kg ) also inhibits murine solid tumors, including Madison 109 lung carcinoma, M5076 sarcoma, as well as L1210 and P388 leukemias. ^[2] In mice xenograft models of breast cancer cells MCF-7, HCC1954, MDA-MB-468, and KPL4, Ispinesib (8 mg/kg–10 mg/kg) inhibits tumor growth. ^[4]

• Steady-State Kinetic Analysis of Human KSP ATPase Activity and Inhibition by Ispinesib

  Kinesin specificity analysis is carried out using a pyruvate kinase-lactate dehydrogenase detection system that couples the production of ADP to oxidation of NADH. Absorbance changes are monitored at 340 nm. Steady-state studies using nanomolar concentrations of KSP are performed using a sensitive fluorescence-based assay utilizing a pyruvate kinase, pyruvate oxidase, and horseradish peroxidase (HRP) coupled detection system that couples the generation of ADP to oxidation of Amplex Red to fluorescent resorufin. Generation of resorufin is monitored by fluorescence (λ_excitation = 520 nm and λ_emission = 580 nm). Steady-state biochemical experiments are performed in PEM25 buffer [25 mM Pipes-K⁺ (pH 6.8), 2 mM MgCl₂, 1 mM EGTA] for experiments involving microtubules. The IC50 for steady-state inhibition is determined at 500 µM ATP, 5 µM Microtubules, and 1 nM KSP in PEM25 buffer. K_{i app} (apparent inhibitor dissociation constant) values of Ispinesib are extracted from the dose-response curves, with explicit correction for enzyme concentration by using the Morrison equation.Inhibitor modality (e.g., competitive, noncompetitive, uncompetitive, or mixed) under steady-state conditions is determined by measuring the effect of inhibitor concentration on initial velocity as a function of substrate concentrations. Data are fit using equations in GraFit to velocity equations for the various modes of inhibition.

• Cell lines

  Breast cancer cells, including MCF-7, HCC1954, MDA-MB-468, and KPL4

• Concentrations

  0.085 nM–33 µM

• Incubation Time

  72 hours

• Method

  Cells are plated in log phase of growth in 96-well plates and treated with Ispinesib for 72 hours. Then, cell growth is measured using CellTiter-Glo, and luminescence is detected using BioTek FLx800. Data are analyzed and the IC50 value, defined as the drug concentration that results in 50% growth inhibition relative to control, is calculated.

• Animal Models

  Nude (nu/nu) mice models of MCF7, KPL4, and HCC1954 cells; severe combined immunodeficient (SCID) mice model of MDA-MB-468 cells;

• Dosages

  10 mg/kg for nude mice or 8 mg/kg for SCID mice

• Administration

  Intraperitoneal injection on a q4d× schedule (3 doses, every 4 day