These five FLT3 inhibitors—lestaurtinib, midostaurin, sorafenib, KW-2449, and AC220 are currently being evaluated in clinical trials. The first three of these (lestaurtinib, midostaurin, and sorafenib) are older multitargeted compounds that happened to also inhibit FLT3 [19
]. These moved relatively quickly into clinical trials [14
]. The latter two (KW-2449 and AC220) were more recently designed and more specifically developed as FLT3 inhibitors [24
]. Using the PIA assay on plasma samples from patients treated with these five inhibitors reveals the striking differences between in vitro activity and in vivo activity. These differences are a reflection of the effects of both plasma protein binding and drug metabolism. The IC50
values for inhibition of mutant FLT3 in culture medium and plasma, as well as an estimate of plasma half-life, are summarized in . While the IC50
values for each of these drugs in cell culture medium are quite similar, the potencies in plasma differ by orders of magnitude. Here we see that the indolocarbazole inhibitors (midostaurin and lestaurtinib) are 40-50-fold less potent in plasma than the newer compound AC220. Furthermore, the plasma half-life of these agents as determined from phase 1 pharmacokinetic studies are likewise highly variable [26
]. Given that most cytotoxic effects of FLT3 inhibition are achieved in vitro with continued suppression of FLT3 over days, a compound with a short in vivo half-life is at a serious disadvantage.
FLT3 inhibitors, with comparison of IC50 estimates for inhibition of FLT3/ITD autophosphorylation in cell culture medium versus plasma.
Selectivity towards the target kinase is another parameter to be considered in this comparison of inhibitors. Four of the five FLT3 inhibitors listed in have been evaluated for selectivity by a variety of in vitro kinase assays using panels of kinases. The conclusion that can be drawn from these studies is that lestaurtinib and midostaurin are highly promiscuous kinase inhibitors, while AC220 appears to be the most selective [13
]. Sorafenib (and probably KW-2449) is somewhere in between. The general pattern that emerges, then, is that the early FLT3 inhibitors were less selective and less potent in vivo (but immediately available for study), while the newer generation compounds were more potent and more directly targeted at FLT3.
With this information about each of these inhibitors, the emerging results from clinical studies can be better understood.
This indolocarbazole derivative was initially developed as an inhibitor of TrkA, then subsequently characterized as a FLT3 inhibitor [19
]. Two separate trials of lestaurtinib as a single agent yielded evidence of modest clinical activity, although no actual remissions were achieved in any FLT3 mutant patients [14
]. In the company-sponsored trial Cephalon 204 and in the United Kingdom’s Medical Research Council trial AML15, relapsed and newly diagnosed AML patients, respectively, were randomized to receive chemotherapy followed by lestaurtinib or chemotherapy alone [30
]. All patients in these trials had FLT3
ITD mutations. The newly diagnosed patients, those in AML15, had much more effective FLT3 inhibition than the relapsed patients, who were part of the 204 trial. Preliminary reports from both trials were encouraging, but the final results from the Cephalon 204 trial, reported at the American Society of Hematology meeting in December 2009, were disappointing. FLT3 inhibition in vivo correlated with remission rate, but treatment with lestaurtinib did not lead to any improvement in overall survival. Lestaurtinib’s complex pharmacokinetics and overall lack of in vivo potency appear to be major obstacles to this drug’s being of any utility for this disease.
Midostaurin is the other major indolocarbazole derivative currently being investigated as a FLT3 inhibitor. Like lestaurtinib, this drug was originally developed for use against a different target (protein kinase C) and was found to have activity against FLT3 in vitro [20
]. In vivo, as monotherapy, the drug was found to be reasonably potent at a dose of 75 mg administered three times a day [13
]. In the ongoing RATIFY trial, however, in which the drug is being administered following chemotherapy, the dose is 50 mg twice a day [32
]. It remains to be seen how effectively FLT3 will be inhibited in vivo in this context. As with lestaurtinib, complicated pharmacokinetics and off-target effects from its relative lack of selectivity may ultimately limit midostaurin’s utility.
KW-2449 is a novel compound with potent activity against FLT3 and, curiously, the T315I variant of BCR-ABL [24
]. The compound was tested in a phase 1 trial in relapsed or refractory AML patients [16
]. While the drug was confirmed to be a potent inhibitor of FLT3 in vivo, a different type of pharmacokinetic problem surfaced. KW-2449 proved to have a very short half-life in vivo. The limited clinical activity of a compound that could only inhibit FLT3 for a few hours a day quickly became evident, and development of KW-2449 as a FLT3 inhibitor was discontinued. Nonetheless, KW-2449 serves as a useful illustration of the importance of sustained FLT3 inhibition for clinical benefit.
Sorafenib was first developed as an inhibitor of raf kinase [21
]. In clinical trials of solid tumor patients, significant activity was observed in renal cell carcinoma and hepatocellular carcinoma [33
]. The exact target remains unclear, although inhibition of the vascular endothelial growth factor receptors (VEGFR) remains a distinct possibility. When administered as monotherapy, sorafenib seems to be much more effective than either lestaurtinib or midostaurin at inhibiting FLT3 in vivo [18
]. When sorafenib is metabolized by the liver, an N-oxide metabolite of sorafenib is produced. This metabolite is a more potent FLT3 inhibitor than the parent compound; in plasma, the IC50
of sorafenib is 308 nM, while the IC50
of sorafenib N-oxide is 21 nM [17
]. The combination of parent and metabolite brings the in vivo IC50
below 300 nM. Furthermore, the drug has a relatively long half-life in vivo. This combination of in vivo potency and half-life makes it a more effective FLT3 inhibitor than either KW-2449 or the indolocarbazoles. Possibly in confirmation of this, monotherapy of FLT3/ITD AML with sorafenib can induce remissions, albeit in somewhat sporadic fashion [35
]. When the agent was combined with chemotherapy, it was well-tolerated, but of unclear efficacy.[18
] It has not yet been tested in a randomized trial, but several such trials are in the planning stages.
The newest FLT3 inhibitor to arrive on the scene is AC220. This drug is actually the first agent specifically designed with the intent of targeting FLT3 [25
]. Preliminary in vitro studies suggest it is the most potent and selective FLT3 inhibitor identified to date, and the phase 1 trial, intriguingly, yielded a number of complete remissions. AC220 monotherapy, even at very low doses, is effective in completely inhibiting both mutant and wild-type FLT3 [37
]. Furthermore, FLT3 inhibition continues for more than a day after AC220 is administered, suggesting that it has a half-life longer than a day [38
]. A multicenter phase 2 trial of AC220 monotherapy in FLT3/ITD AML patients is currently accruing, and combination trials of chemotherapy and AC220 are in the planning stages.