It has recently been reported that HDACIs induce NF _B through oxidative injury and the atypical ATM/NEMO/SUMOylation DNA damage related pathway. Regardless of the mechanism of initial induction, HDACI mediated NF _B activation, although more sustained than that triggered by TNF_, is eventually reversed, i. e., declining to basal levels after 20 h exposure. Notably, after the NF _B response was abrogated, in sharp contrast to untreated cells, fludarabine failed to trigger NF _B activation, followed by JNK activation. This suggests that events responsible for termination of HDACI mediated NF _B activation prevent NF _B induction by fludarabine, leading to enhanced lethality. Studies defining the mechanism for termination of the HDACI induced NF _B response are underway.
The present findings demonstrate a significant functional role for JNK activation in potentiation of fludarabine lethality by LBH 589. Previous studies highlighted the importance of JNK activation in cytokine mediated transformed cell death, and its reversal by NF PARP Inhibitors _B activation. Analogously, NF _B inactivation, i. e., by IKK or proteasome inhibitors, increase HDACI lethality toward human leukemia cells through a JNK dependent mechanism. The findings that abrogation of NF _B activation by HDACI pre treatment substantially enhanced fludarabine mediated JNK activation, and that pharmacological or genetic interruption of JNK significantly attenuated LBH 598/fludarabine mediated lethality, suggest that similar events occur in leukemia cells exposed to fludarabine. The mechanism by which NF _B prevents JNK activation may vary in different cell types.
For example, in murine colon cells, this stems from reduction in ROS mediated inactivation of MAP kinase phosphatases that dephosphorylate JNK. Alternatively, in MEF cells exposed to TNF_, this process p53 Signaling Pathway involves the NF _B dependent protein XIAP. In the present studies, ectopic expression of XIAP substantially attenuated LBH 589/fludarabine mediated JNK activation, and significantly reduced lethality, suggesting a similar mechanism. Given evidence that attenuation of HDACI mediated NF _B activation, i. e., by either IKK or proteasome inhibitors also diminishes XIAP expression, it is possible that the failure of fludarabine to trigger NF _B activation in LBH 589 pretreated cells induces XIAP downregulation, leading to enhanced JNK activation and lethality.
In addition, XIAP has been implicated in NF _B activation, raising the possibility that XIAP down regulation may also contribute to suppression of NF _B signaling. Finally, XIAP down regulation, accompanied by cleavage, was most marked in cells exposed to both LBH 589 and fludarabine, raising the possibility that PLK the latter phenomenon may amplify the regimens lethality. Results of in vivo studies demonstrated a striking increase in activity for the LBH 589/fludarabine regimen. Notably, administration of LBH 589 alone had minimal effects, whereas fludarabine alone significantly suppressed tumor growth during the administration interval. However, discontinuation of fludarabine resulted in a rapid re growth of tumor within 7?C10 days.
In striking contrast, sequential administration of LBH 589 followed by fludarabine resulted in the virtual disappearance of tumors within 1 week. Significantly, after discontinuation of treatment, tumor re growth did not occur by over the ensuing 50 day observation period, indicating profound RAF Signaling Pathway in vivo effects on tumor cell survival. Interestingly, an identical regimen substantially inhibited the re growth of XIAP overexpressing tumors in 50% of inoculated mice, and eliminated detectable tumors in the remaining 50%. Consistent with the observation that ectopic expression of XIAP only partially protected cells from the LBH 598/fludarabine regimen in vitro. Collectively, these findings indicate that potentiation of fludarabine lethality by HDACIs is not restricted to the in vitro setting, but also occurs in vivo.
In summary, the present findings suggest a hierarchy of signaling events that VEGF may contribute to HDACI mediated potentiation of fludarabine lethality. Specifically, they suggest that in HDACI pretreated leukemia cells, attenuation of fludarabinemediated NF _B activation through a yet to be defined mechanism diminishes expression of anti apoptotic NF _B dependent proteins, including XIAP. Down regulation of the latter releases inhibitory effects on JNK activation, which in turn plays a significant functional role in cell death. Notably, potentiation of fludarabine lethality by LBH 589 occurs in at least some primary AML samples, and combined treatment exhibited substantial in vivo efficacy. However, in view of the small sample size, a larger number of primary blast specimens will clearly be necessary to validate the present observations.
Nevertheless, given evidence of the activity of fludarabine containing regimens in the treatment of refractory AML, incorporating HDACIs into such approaches, if tolerable, warrants consideration. Based on the present findings, post treatment levels of XIAP and phospho JNK in leukemic blasts represent plausible candidate pharmacodynamic determinants of regimen activity. INCREASING AGE HAS BEEN HISTORIcally implicated in higher mortality after high dose allogeneic hematopoietic cell transplantation for patients with hematologic malignancies. Such transplants are preceded by intense, cytotoxic conditioning regimens aimed at reducing tumor burden. The risk of organ toxicities has limited the use of high dose regimens to younger patients in good medical condition. Therefore, age cutoffs of 55 to 60 years have been in place for decades for high dose HCT. This excluded the vast majority of patients from allogeneic HCT, given that median ages of patients at diagnoses of most hematologic malignancies range from 65 to 70 years. To circumvent this limitation, a nonmyeloablative conditioning regimen for allogeneicHCTwas developed.