Kinesin Spindle Protein (KSP) Inhibitors. 9. Discovery of (2 S )-4-(2,5-Difluorophenyl)- N -[(3 R ,4 S )-3-fluoro-1-methylpiperidin-4-yl]-2-(hydroxymethyl)- N -methyl-2-phenyl-2,5-dihydro-1 H -pyrrole-1-carboxamide (MK-0731) for the Treatment of Taxane-Refractory Cancer