PF-4708671 is a cell-permeable inhibitor of p70 ribosomal S6 kinase (S6K1 isoform) .In cell-free assays, PF-4708671(PF4708671) is potent for S6K1(Ki50=20 nM, IC50=160 nM).

S6K1:20 nM (Ki), S6K2:65 μM, p70 S6K1:160 nM

Affinity determination: Purified activated FAK kinase domain (amino acids 410–689) is reacted with 50 μM ATP, and 10 μg/well of a random peptide polymer of Glu and Tyr (molar ratio of 4:1), poly(Glu/Tyr) in kinase buffer (50 mM HEPES, pH 7.5, 125 mM NaCl, 48 mM MgCl2) for 15 min. Phosphorylation of poly(Glu/Tyr) is challenged with serially diluted compounds at 1/2-Log concentrations starting at a top concentration of 1 μM. Each concentration is run in triplicate. Phosphorylation of poly(Glu/Tyr) is detected with a general anti-phospho-tyrosine (PY20) antibody, followed by horseradish peroxidase-conjugated goat anti-mouse IgG antibody. The standard horseradish peroxidase substrate 3, 3', 5, 5'-tetramethylbenzidine is added, and Optical Density readings at 450 nm are obtained following the addition of stop solution (2 M H2SO4). The IC50 values are determined using the Hill slope model.