Indeed, recently it has been demonstrated that OCT-1 mediates cellular influx of imatinib, and that transporter activity correlates with efficacy. On the other hand, it has been shown that OCT-1 has less impact on cellular influx of dasatinib and nilotinib. Therefore, we believe that additional drug-transporter proteins contribute to TH-302 intracellular accumulation of TKIs. However, the data presented here is consistent with a model where intracellular accumulation and Life Science Reagents retention of TKIs in vivo also translates into significantly higher intracellular TKI concentrations as compared to the extracellular medium. It is conceivable that in the setting of high-dose pulse therapy this may then result in prolonged intracellular TKI exposure significantly exceeding plasma halflife of a given TKI. In conclusion, we show that dramatic intracellular TKI accumulation and retention result in prolonged target inhibition which appears to be the sole underlying molecular mechanism in HD-TKI pulse-exposure mediated induction of apoptosis in vitro. Moreover, the data illustrate that potent but transient kinase inhibition per se is not sufficient to irreversibly commit oncogene transformed cells to apoptosis. As we have observed intracellular TKI accumulation and retention in other oncogenic kinase models such as FLT3-ITD and JAK2V617F, the mechanism described here may indicate a general pharmacokinetic feature of TKIs. However, this point clearly requires further investigation. Based on our data presented here, monitoring both, plasma and intracellular drug levels of imatinib and dasatinib in vivo will provide pharmacokinetic data which may prove useful to optimize dosing schedules in upcoming clinical trials. We speculate that either the design of inhibitors that accumulate and are retained in target cells or, alternatively, co-administration of drugs which result in intracellular enrichment of specific TKIs may improve TKI therapy in the future. PLP is a reactive compound because the 49-aldehyde forms aldimines with a-amines of amino acids and other compounds containing amino groups, the e-amino group of lysine residues on non-B6 proteins, and thiazolidine adducts with sulfhydryl groups like cysteine. PL, which also contains the 49-aldehyde moiety is much less reactive than PLP, since its aldehyde group, in aqueous solution and at neutral pH, exists mostly in the hydrated form. This reactivity of PLP poses two problems for the cell. First, to keep the cellular level of free PLP low so that it does not react with other nucleophiles, especially non B6-enzymes, and second, to supply enough PLP for the dozens of newly synthesized apo-B6 enzymes to form catalytically active holo-B6 enzymes. Some have proposed that the level of PLP is regulated and kept low by being an effective feedback inhibitor of both PL kinase and PNP oxidase. We report here on properties of E. coli PL kinase showing that PLP serves as a slow tight binding inhibitor of the enzyme. The structure of PL kinase has been determined from several sources. In E. coli there are two PL kinases referred to as PL kinase1 and PL kinase2. The activity of PL kinase2 is very low and there is a question if its function in the cell is to convert PL to PLP or if it is the enzyme for another unknown reaction. We have determined the structure and properties of both E. coli enzymes. This study reports on the properties of only E. coli PL kinase1 and we refer to it as ePL kinase. During our kinetic measurements, we observed that the enzyme rapidly loses activity as it catalyzes the conversion of PL and ATP into PLP and ADP. The reason for this inhibition is the subject of this report.