Acute-myeloid-leukemia-targeted toxins kill tumor cells by cell-type-specific mechanisms and synergize with TRAIL to allow manipulation of the extent and mechanism of tumor cell death

Antineoplastic effect of a diphtheria toxin-based nanoparticle targeting acute myeloid leukemia cells overexpressing CXCR4

Nanomedicine has opened an opportunity to improve current clinical practice by enhancing the selectivity in the delivery of antitumor drugs to specific cancer cells. These new strategies are able to bypass toxicity on normal cells increasing the effectiveness of current anticancer treatments. In acute myeloid leukemia (AML) current chemotherapy treatments generate a relevant toxic impact in normal cells and severe side effects or even patient death. In this study, we have designed a self-assembling protein nanoparticle, T22-DITOX-H6, which incorporates a ligand (T22) targeting CXCR4-overexpressing (CXCR4⁺) cells, and a potent cytotoxic diphtheria toxin domain. CXCR4 is overexpressed in AML leukemic cells and associates with poor prognosis, being, therefore, a relevant clinical target. We demonstrate here that T22-DITOX-H6 induces apoptosis in CXCR4⁺ leukemic cells through CXCR4-dependent internalization. In addition, repeated T22-DITOX-H6 treatment (10 μg/dose per 10 doses, intravenously injected) in a disseminated AML mouse model (NSG mice intravenously injected with THP-1-Luci cells, n = 10 per group) potently blocks the dissemination of AML cells in bone marrow, spleen and liver of treated mice, without inducing toxicity in healthy tissues. In conclusion, our strategy of selectively ablating CXCR4 positive leukemic cells by administering the T22-DITOX-H6 nanoparticle could be a promising treatment, especially in patients undergoing AML relapse after chemotherapy, in which leukemic cells overexpress CXCR4.

Activity of SL-401, a targeted therapy directed to interleukin-3 receptor, in blastic plasmacytoid dendritic cell neoplasm patients

This is the first prospective study of treatment of patients with blastic plasmacytoid dendritic cell neoplasm (BPDCN), an aggressive hematologic malignancy derived from plasmacytoid dendritic cells that typically involves the skin and rapidly progresses to a leukemia phase. Despite being initially responsive to intensive combination chemotherapy, most patients relapse and succumb to their disease. Because BPDCN blasts overexpress the interleukin-3 receptor (IL3R), the activity of SL-401, diptheria toxin (DT)388IL3 composed of the catalytic and translocation domains of DT fused to IL3, was evaluated in BPDCN patients in a phase 1-2 study. Eleven patients were treated with a single course of SL-401 at 12.5 μg/kg intravenously over 15 minutes daily for up to 5 doses; 3 patients who had initial responses to SL-401 received a second course in relapse. The most common adverse events including fever, chills, hypotension, edema, hypoalbuminemia, thrombocytopenia, and transaminasemia were transient. Seven of 9 evaluable (78%) BPDCN patients had major responses including 5 complete responses and 2 partial responses after a single course of SL-401. The median duration of responses was 5 months (range, 1-20+ months). Further studies of SL-401 in BPDCN including those involving multiple sequential courses, alternate schedules, and combinations with other therapeutics are warranted. This trial is registered at clinicaltrials.gov as #NCT00397579.

EGFR-targeted diphtheria toxin stimulates TRAIL killing of glioblastoma cells by depleting anti-apoptotic proteins

Current treatments for Glioblastoma multiforme (GBM) involve surgery, radiotherapy, and cytotoxic chemotherapy; however, these treatments are not effective and there is an urgent need for better treatments. We investigated GBM cell killing by a novel drug combination involving DT-EGF, an Epidermal Growth Factor Receptor-targeted bacterial toxin, and Tumor Necrosis Factor-Related Apoptosis Inducing Ligand (TRAIL) or antibodies that activate the TRAIL receptors DR4 and DR5. DT-EGF kills GBM cells by a non apoptotic mechanism whereas TRAIL kills by inducing apoptosis. GBM cells treated with DT-EGF and TRAIL were killed in a synergistic fashion in vitro and the combination was more effective than either treatment alone in vivo. Tumor cell death with the combination occurred by caspase activation and apoptosis due to DT-EGF positively regulating TRAIL killing by depleting FLIP, a selective inhibitor of TRAIL receptor-induced apoptosis. These data provide a mechanism-based rationale for combining targeted toxins and TRAIL receptor agonists to treat GBM.

Regulation of HMGB1 release by autophagy

The characteristics of tumor cell killing by an anticancer agent can determine the long-term effectiveness of the treatment. For example, if dying tumor cells release the immune modulator HMGB1 after treatment with anticancer drugs, they can activate a tumor-specific immune response that boosts the effectiveness of the initial treatment. Recent work from our group examined the mechanism of action of a targeted toxin called DT-EGF that selectively kills Epidermal Growth Factor Receptor-expressing tumor cells. We found that DT-EGF kills glioblastoma cells by a caspase-independent mechanism that involves high levels of autophagy, which inhibits cell death by blocking apoptosis. In contrast, DT-EGF kills epithelial tumor cells by caspase-dependent apoptosis and in these cells autophagy is not induced. These differences allowed us to discover that the different death mechanisms were associated with differences in the release of HMGB1 and that autophagy induction is required and sufficient to cause release of HMGB1 from the dying cells. These data identify a new function for autophagy during cell death and open up the possibility of manipulating autophagy during cancer treatment as a way to influence the immunogenicity of dying tumor cells.

Acute myeloid leukemia-targeted toxin activates both apoptotic and necroptotic death mechanisms

Background

Acute myelogenous leukemia (AML) is the second most common leukemia with approximately 13,410 new cases and 8,990 deaths annually in the United States. A novel fusion toxin treatment, diphtheria toxin GM-CSF (DT-GMCSF) has been shown to selectively eliminate leukemic repopulating cells that are critical for the formation of AML. We previously showed that DT-GMCSF treatment of U937 cells, an AML cell line, causes activation of caspases and the induction of apoptosis.

Methods and Findings

In this study we further investigate the mechanisms of cell death induced by DT-GMCSF and show that, in addition to the activation of caspase-dependent apoptosis, DT-GMCSF also kills AML cells by simultaneously activating caspase-independent necroptosis. These mechanisms depend on the ability of the targeted toxin to inhibit protein synthesis, and are not affected by the receptor that is targeted or the mechanism through which protein synthesis is blocked.

Conclusions

We conclude that fusion toxin proteins may be effective for treating AML cells whether or not they are defective in apoptosis.

TRAIL receptor-targeted therapeutics: resistance mechanisms and strategies to avoid them

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) receptors are attractive therapeutic targets in cancer because agents that activate these receptors directly induce tumor cell apoptosis and have low toxicity to normal tissues. Consequently, several different drugs that target these receptors (recombinant TRAIL and various agonistic antibodies that activate one of the two TRAIL receptors) have been developed and are being tested in human clinical trials. However, in vitro and in vivo data suggest that resistance to these agents may limit their clinical effectiveness. In this review, we discuss recent findings about some of the ways these resistance mechanisms arise, potential biomarkers to identify TRAIL resistance in patients (Six1, GALNT14, XIAP, certain microRNAs) and potential ways to circumvent resistance and resensitize tumors.