Tagraxofusp in Blastic Plasmacytoid Dendritic-Cell Neoplasm.

BACKGROUND: Blastic plasmacytoid dendritic-cell neoplasm (BPDCN) is an aggressive hematologic cancer that is caused by transformed plasmacytoid dendritic cells that overexpress interleukin-3 receptor subunit alpha (IL3RA or CD123). Tagraxofusp (SL-401) is a CD123-directed cytotoxin consisting of human interleukin-3 fused to truncated diphtheria toxin. METHODS: In this open-label, multicohort study, we assigned 47 patients with untreated or relapsed BPDCN to receive an intravenous infusion of tagraxofusp at a dose of 7 µg or 12 µg per kilogram of body weight on days 1 to 5 of each 21-day cycle. Treatment continued until disease progression or unacceptable toxic effects. The primary outcome was the combined rate of complete response and clinical complete response among patients who had not received previous treatment for BPDCN. A secondary outcome was the duration of response. RESULTS: Of the 47 patients, 32 were receiving tagraxofusp as first-line treatment and 15 had received previous treatment. The median age of the patients was 70 years (range, 22 to 84). Among the 29 previously untreated patients who received tagraxofusp at a dose of 12 µg per kilogram, the primary outcome occurred in 21 (72%), and the overall response rate was 90%; of these patients, 45% went on to undergo stem-cell transplantation. Survival rates at 18 and 24 months were 59% and 52%, respectively. Among the 15 previously treated patients, the response rate was 67%, and the median overall survival was 8.5 months. The most common adverse events were increased levels of alanine aminotransferase (64%) and aspartate aminotransferase (60%), hypoalbuminemia (55%), peripheral edema (51%), and thrombocytopenia (49%). Capillary leak syndrome was reported in 19% of the patients and was associated with one death in each of the dose subgroups. CONCLUSIONS: In adult patients with untreated or relapsed BPDCN, the use of tagraxofusp led to clinical responses. Serious adverse events included capillary leak syndrome; hepatic dysfunction and thrombocytopenia were common. (Funded by Stemline Therapeutics and the Leukemia and Lymphoma Society Therapy Acceleration Program; ClinicalTrials.gov number, NCT02113982.).

Clinical Remission in Severe Asthma: A Pooled Post Hoc Analysis of the Patient Journey with Benralizumab.

INTRODUCTION: Consensus definitions for clinical remission and super-response were recently established for severe asthma. Benralizumab is an interleukin-5 (IL-5) receptor α-directed monoclonal antibody for severe, uncontrolled asthma; efficacy and safety were demonstrated in previous pivotal phase 3 trials (SIROCCO, CALIMA, ZONDA). This analysis applied a composite remission definition to characterize individual responses to benralizumab after 6 and 12 months. METHODS: In previous phase 3 studies, eligible patients were those with severe, uncontrolled asthma receiving medium- or high-dosage inhaled corticosteroids plus long-acting ß2-agonists. This post hoc analysis included patients randomized to the approved benralizumab dose and not receiving oral corticosteroids (OCS) at baseline (SIROCCO/CALIMA) or OCS ≤ 12.5 mg per day (ZONDA). Individual remission components were zero exacerbations; zero OCS use; Asthma Control Questionnaire-6 (ACQ-6) score < 1.5 or ≤ 0.75; and pre-bronchodilator forced expiratory volume in 1 s (FEV1) increase ≥ 100 mL; clinical remission incorporated zero exacerbations, zero OCS use, ACQ-6 score ≤ 0.75, and pre-bronchodilator FEV1 increase ≥ 100 mL after 6 or 12 months. RESULTS: Overall, 609 patients (N = 301 and N = 308) and 586 patients (N = 293 and N = 293) receiving benralizumab in SIROCCO and CALIMA were included at 6 and 12 months, respectively; 40 ZONDA patients were included after 6 months. In SIROCCO/CALIMA, similar to 6-month findings, approx. 83% and approx. 49% receiving benralizumab, and 77% and 37% on placebo achieved ≥ 2 and ≥ 3 remission components after 12 months; 14.5% (85/586) on benralizumab and 7.7% (48/620) on placebo achieved clinical remission at 12 months. Among ZONDA patients, 75% and approx. 48% on benralizumab and 35% and 20% on placebo achieved ≥ 2 and ≥ 3 remission components at 6 months, respectively; 22.5% (9/40) on benralizumab and 7.5% on placebo achieved clinical remission. CONCLUSIONS: This analysis demonstrates clinical remission is achievable by targeting the underlying drivers of inflammation. Precision medicines can help shift treatment paradigms toward treat-to-target, with clinical remission as the ultimate therapeutic goal in severe asthma. CLINICAL TRIAL REGISTRATION: SIROCCO (NCT01928771); CALIMA (NCT01914757); ZONDA (NCT02075255).

Widely accepted definitions for disease remission are already established for the treatment of rheumatoid arthritis, ulcerative colitis, and cancer, among others. Two separate expert groups recently collaborated to discuss clinical remission/super-response to treatment in patients with severe asthma. Both groups developed separate, yet similar ways to determine whether a patient should be considered "in remission." In this study, we used the results from three previous trials (SIROCCO, CALIMA, and ZONDA) that were conducted to assess a therapy called benralizumab in patients with severe asthma to identify patients who met some or all of the criteria for disease remission in severe asthma. These criteria included zero asthma exacerbations; zero oral steroid (OCS) use; asthma control score; and improvement in lung function. Across all three trials, about three quarters of the patients achieved two or more remission components and about half achieved three or more remission components after 6 months of treatment; furthermore, these rates were generally similar to the numbers of patients who achieved two or more components and three or more components of remission after 12 months of treatment. Overall, 15­23% of patients achieved clinical remission in 6 months, and approximately 15% achieved remission within 12 months. The results show that biologic therapies like benralizumab help improve the symptoms of severe asthma and allow patients to achieve disease remission.

Long-Term Benefits of Tagraxofusp for Patients With Blastic Plasmacytoid Dendritic Cell Neoplasm.

Clinical trials frequently include multiple end points that mature at different times. The initial report, typically based on the primary end point, may be published when key planned co-primary or secondary analyses are not yet available. Clinical Trial Updates provide an opportunity to disseminate additional results from studies, published in JCO or elsewhere, for which the primary end point has already been reported.Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is an aggressive myeloid malignancy. We report long-term results, including data from the continued access phase, of the largest prospective BPDCN trial evaluating the CD123-targeted therapy tagraxofusp (TAG) in adults with treatment-naive and relapsed/refractory BPDCN. The primary outcome was complete response (CR) + clinical CR (CRc: CR with residual skin abnormality not indicative of active disease). Eighty-four (65 treatment-naive and 19 relapsed/refractory) of 89 patients received TAG 12 µg/kg once daily; the median follow-up was 34.0 months. For treatment-naive patients, the overall response rate was 75%; 57% achieved CR + CRc. The median time to remission was 39 (range, 14-131) days, and the median CR + CRc duration was 24.9 (95% CI, 3.8 to not reached) months. Nineteen patients (51%) with CR + CRc were bridged to stem-cell transplant, with a median CR + CRc duration of 22.2 (range, 1.5-57.4) months. Most common adverse events were increased alanine (64%) or aspartate (60%) aminotransferase and hypoalbuminemia (51%); most occurred in cycle 1 and were transient. Capillary leak syndrome occurred in 21% of patients (grade ≥ 3: 7%). In first-line patients with BPDCN, TAG monotherapy resulted in high and durable responses, allowing many to bridge to stem-cell transplant. TAG was generally well-tolerated with a predictable and manageable safety profile.

Ubiquitination, phosphorylation and acetylation: the molecular basis for p53 regulation.

The p53 tumor suppressor exerts anti-proliferative effects, including growth arrest, apoptosis and cell senescence, in response to various types of stress. Tight regulation of p53 activation is imperative for preventing tumorigenesis and maintaining normal cell growth; p53 stabilization and transcriptional activation are crucial early events in a cell's battle against genotoxic stress. Ubiquitination, phosphorylation and acetylation are post-translational modifications to p53 that affect its overall appearance and activity. Recent findings suggest that these modifications have a profound affect on p53 stability and function. Defining the precise roles of these modifications in p53 function may show not only that they are markers of the stress response but also that they serve as the focal point in the regulation of p53.

DNA methyltransferase inhibition overcomes diphthamide pathway deficiencies underlying CD123-targeted treatment resistance.

The interleukin-3 receptor α subunit, CD123, is expressed in many hematologic malignancies including acute myeloid leukemia (AML) and blastic plasmacytoid dendritic cell neoplasm (BPDCN). Tagraxofusp (SL-401) is a CD123-targeted therapy consisting of interleukin-3 fused to a truncated diphtheria toxin payload. Factors influencing response to tagraxofusp other than CD123 expression are largely unknown. We interrogated tagraxofusp resistance in patients and experimental models and found that it was not associated with CD123 loss. Rather, resistant AML and BPDCN cells frequently acquired deficiencies in the diphthamide synthesis pathway, impairing tagraxofusp's ability to ADP-ribosylate cellular targets. Expression of DPH1, encoding a diphthamide pathway enzyme, was reduced by DNA CpG methylation in resistant cells. Treatment with the DNA methyltransferase inhibitor azacitidine restored DPH1 expression and tagraxofusp sensitivity. We also developed a drug-dependent ADP-ribosylation assay in primary cells that correlated with tagraxofusp activity and may represent an additional novel biomarker. As predicted by these results and our observation that resistance also increased mitochondrial apoptotic priming, we found that the combination of tagraxofusp and azacitidine was effective in patient-derived xenografts treated in vivo. These data have important implications for clinical use of tagraxofusp and led to a phase 1 study combining tagraxofusp and azacitidine in myeloid malignancies.

Monoubiquitination: the signal for p53 nuclear export?

The ubiquitin-proteasome pathway has become an increasingly important regulatory mechanism for protein function. Countless proteins are degraded by the addition of polymeric ubiquitin chains, but more recently, monoubiquitination has emerged as a mechanism for regulatory functions other than proteasomal degradation. Monoubiquitination acts as a signal in nuclear export for the tumor suppressor protein p53. Different levels of Mdm2 are capable of inducing both mono- and polyubiquitination in a dosage dependent manner, thus determining p53's fate. Our findings demonstrate monoubiquitin-mediated protein trafficking can be expanded to nuclear-cytoplasmic shuttling, and also imply similar scenarios may apply to other cellular factors.

p53 regulation by ubiquitin.

The ubiquitination pathway is a highly dynamic and coordinated process that regulates degradation as well as numerous processes of proteins within a cell. The p53 tumor suppressor and several factors in the pathway are regulated by ubiquitin as well as ubiquitin-like proteins. These modifications are critical for the function of p53 and control both the degradation of the protein as well as localization and activity. Importantly, more recent studies have identified deubiquitination enzymes that can specifically remove ubiquitin moieties from p53 or other factors in the pathway, and the reversible nature of this process adds yet another layer of regulatory control of p53. This review highlights the recent advances in our knowledge of ubiquitin and the p53 pathway.

The impact of acetylation and deacetylation on the p53 pathway.

The p53 tumor suppressor is a sequence-specific transcription factor that undergoes an abundance of post-translational modifications for its regulation and activation. Acetylation of p53 is an important reversible enzymatic process that occurs in response to DNA damage and genotoxic stress and is indispensible for p53 transcriptional activity. p53 was the first non-histone protein shown to be acetylated by histone acetyl transferases, and a number of more recent in vivo models have underscored the importance of this type of modification for p53 activity. Here, we review the current knowledge and recent findings of p53 acetylation and deacetylation and discuss the implications of these processes for the p53 pathway.

New insights into p53 activation.

The tumor suppressor p53 is a multifunctional, highly regulated, and promoter-specific transcriptional factor that is uniquely sensitive to DNA damage and cellular stress signaling. The mechanisms by which p53 directs a damaged cell down either a cell growth arrest or an apoptotic pathway remain poorly understood. Evidence suggests that the in vivo functions of p53 seem to balance the cell-fate choice with the type and severity of damage that occurs. The concept of antirepression, or inhibition of factors that normally keep p53 at bay, may help explain the physiological mechanisms for p53 activation. These factors also provide novel chemotherapeutic targets for the reactivation of p53 in tumors harboring a wild-type copy of the gene.

How does SIRT1 affect metabolism, senescence and cancer?

SIRT1 is a multifaceted, NAD(+)-dependent protein deacetylase that is involved in a wide variety of cellular processes from cancer to ageing. The function of SIRT1 in cancer is complex: SIRT1 has been shown to have oncogenic properties by downregulating p53 activity, but recent studies indicate that SIRT1 acts as a tumour suppressor in a mutated p53 background, raising intriguing questions regarding its mechanism of action. Here we discuss the current understanding of how SIRT1 functions in light of recent discoveries and propose that the net outcome of the seemingly opposite oncogenic and tumour-suppressive effects of SIRT1 depends on the status of p53.

p53 Activation: a case against Sir.

The p53 tumor suppressor is a critical transcription factor for controlling cell growth and apoptosis during times of cellular stress. In this issue of Cancer Cell, Lain et al. have used a p53-responsive reporter gene as the readout for screening small-molecule activators of p53 that could potentially reduce tumor growth. Using this approach, tenovin-6 was identified as a potent SIRT1 and SIRT2 inhibitor that indirectly activated p53 at single-digit micromolar concentrations. The identification of a specific sirtuin inhibitor has broad implications in understanding sirtuin-p53 signaling and the development of novel chemotherapeutics.

Mechanistic studies of MDM2-mediated ubiquitination in p53 regulation.

As a central regulator for cell cycle arrest, apoptosis, and cellular senescence, p53 requires multiple layers of regulatory control to ensure correct temporal and spatial functions. It is well accepted that Mdm2-mediated ubiquitination plays a crucial role in p53 regulation. In addition to proteasome-mediated degradation, ubiquitination of p53 by Mdm2 acts a key signal for its nuclear export. Nuclear export has previously been thought to require the disassociation of the p53 tetramer and exposure of the intrinsic nuclear export signal. To elucidate the molecular mechanism of degradation-independent repression on p53 by Mdm2, we have developed a two-step approach to purify ubiquitinated forms of p53 induced by Mdm2 from human cells. Surprisingly, however, we found that ubiquitination has no effect on the tetramerization/oligomerization of p53, arguing against this seemingly well accepted model. Moreover, nuclear export of p53 alone is not sufficient to completely abolish p53 activity. Ubiquitination-mediated repression of p53 by Mdm2 acts at least, in part, through inhibiting the sequence-specific DNA binding activity. Thus, our results have important implications regarding the mechanisms by which Mdm2 acts on p53.

p53 ubiquitination: Mdm2 and beyond.

Although early studies have suggested that the oncoprotein Mdm2 is the primary E3 ubiquitin ligase for the p53 tumor suppressor, an increasing amount of data suggests that p53 ubiquitination and degradation are more complex than once thought. The discoveries of MdmX, HAUSP, ARF, COP1, Pirh2, and ARF-BP1 continue to uncover the multiple facets of this pathway. There is no question that Mdm2 plays a pivotal role in downregulating p53 activities in numerous cellular settings. Nevertheless, growing evidence challenges the conventional view that Mdm2 is essential for p53 turnover.

Dynamics in the p53-Mdm2 ubiquitination pathway.

The tumor suppressor p53 is highly regulated under various states of cellular stress. p53 stability is predominantly regulated through the ubiquitin-proteasomal pathway by the E3 ligase Mdm2. p53 ubiquitination is a dynamic process with Mdm2 capable of catalyzing both mono- and polyubiquitination. Additionally, deubiquitination is an important step occurring in p53 and Mdm2 stabilities. Factors such as HAUSP, p14(ARF), and MdmX play important regulatory roles in p53 ubiquitination/deubiquitination and their interplay with Mdm2 and p53 compound layers of complexity for regulating this important pathway.

A dynamic role of HAUSP in the p53-Mdm2 pathway.

Our previous study showed that ubiquitination of p53 is reversible and that the ubiquitin hydrolase HAUSP can stabilize p53 by deubiquitination. Here, we found that partial reduction of endogenous HAUSP levels by RNAi indeed destabilizes endogenous p53; surprisingly, however, nearly complete ablation of HAUSP stabilizes and activates p53. We further show that this phenomenon occurs because HAUSP stabilizes Mdm2 in a p53-independent manner, providing an interesting feedback loop in p53 regulation. Notably, HAUSP is required for Mdm2 stability in normal cells; in HAUSP-ablated cells, self-ubiquitinated-Mdm2 becomes extremely unstable, leading to indirect p53 activation. Furthermore, this feedback regulation is specific to Mdm2; in HeLa cells, where p53 is preferentially degraded by viral E6-dependent ubiquitination, depletion of HAUSP fails to activate p53. This study provides an example of an ubiquitin ligase (Mdm2) that is directly regulated by a deubiquitinase (HAUSP) and also reveals a dynamic role of HAUSP in the p53-Mdm2 pathway.

Acetylation of p53 inhibits its ubiquitination by Mdm2.

In response to DNA damage, the activity of the p53 tumor suppressor is modulated by protein stabilization and post-translational modifications including acetylation. Interestingly, both acetylation and ubiquitination can modify the same lysine residues at the C terminus of p53, implicating a role of acetylation in the regulation of p53 stability. However, the direct effect of acetylation on Mdm2-mediated ubiquitination of p53 is still lacking because of technical difficulties. Here, we have developed a method to obtain pure acetylated p53 proteins from cells, and by using an in vitro purified system, we provide the direct evidence that acetylation of the C-terminal domain is sufficient to abrogate its ubiquitination by Mdm2. Importantly, even in the absence of DNA damage, acetylation of the p53 protein is capable of reducing the ubiquitination levels and extending its half-life in vivo. Moreover, we also show that acetylation of p53 can affect its ubiquitination through other mechanisms in addition to the site competition. This study has significant implications regarding a general mechanism by which protein acetylation modulates ubiquitination-dependent proteasome proteolysis.

Mono- versus polyubiquitination: differential control of p53 fate by Mdm2.

Although Mdm2-mediated ubiquitination is essential for both degradation and nuclear export of p53, the molecular basis for the differential effects of Mdm2 remains unknown. Here we show that low levels of Mdm2 activity induce monoubiquitination and nuclear export of p53, whereas high levels promote p53's polyubiquitination and nuclear degradation. A p53-ubiquitin fusion protein that mimics monoubiquitinated p53 was found to accumulate in the cytoplasm in an Mdm2-independent manner, indicating that monoubiquitination is critical for p53 trafficking. These results clarify the nature of ubiquitination-mediated p53 regulation and suggest that distinct mechanisms regulate p53 function in accordance with the levels of Mdm2 activity.