PP58 can exhibit some degree of selectivity at low nanomolar concentrations in vivo^[1].

PP58 effectively targets Src kinase, demonstrating inhibition with an IC50 value in the subnanomolar range during assays. At elevated concentrations of Src protein, PP58 acts as a titration agent. Specific depletion of Src from total lysate is achieved through the PP58 matrix, as shown by immunoblotting using targeted antiserum. This depletion effect is hindered when a free inhibitor is present, preventing binding to PP58 beads. Moreover, PP58 beads uniquely retain the ectopically expressed FGFR1 receptor tyrosine kinase. This indicates the PP58 matrix's potential as an innovative affinity tool for isolating cellular targets of the pyrido[2,3-d].pyrimidine inhibitor. Through PP58 affinity chromatography, protein kinases from various groups and families have been identified, demonstrating that the inhibitor does not exclusively select for phylogenetically related kinases within the human kinome. PP58 shows Ki values of 3.8±1.9 nM for p38α and 0.32±0.04 μM for JNK2. Additionally, it acts as an effective purification agent for protein kinases that do not share the pyrido[2,3-d].pyrimidine structural motif, exhibiting lower affinities for PP58. The inhibitor dose-dependently blocks anisomycin-activated p38 with an IC50 under 10 nM and strongly inhibits LPS-induced TNF-α production at a cellular IC50 of approximately 3 nM^[1].

A T341M mutation significantly reduces PP58 sensitivity, increasing the cellular IC50 to over 1000 times the initial value of approximately 10 nM. While PP58 at low nanomolar concentrations efficiently inhibits wild-type FGFR1 activity, the FGFR1-V561M mutation results in substantial resistance. The IC50 value of PP58 for inhibiting CSK activity is around 100 nM^[2].