Chaetocin, a thiodioxopiperazine natural product previously unreported to have anticancer effects, was found to have potent antimyeloma activity in IL-6-dependent and -independent myeloma cell lines in freshly collected sorted and unsorted patient CD138(+) myeloma cells and in vivo. Chaetocin largely spares matched normal CD138(-) patient bone marrow leukocytes, normal B cells, and neoplastic B-CLL (chronic lymphocytic leukemia) cells, indicating a high degree of selectivity even in closely lineage-related B cells. Furthermore, chaetocin displays superior ex vivo antimyeloma activity and selectivity than doxorubicin and dexamethasone, and dexamethasone- or doxorubicin-resistant myeloma cell lines are largely non-cross-resistant to chaetocin. Mechanistically, chaetocin is dramatically accumulated in cancer cells via a process inhibited by glutathione and requiring intact/unreduced disulfides for uptake. Once inside the cell, its anticancer activity appears mediated primarily through the imposition of oxidative stress and consequent apoptosis induction. Moreover, the selective antimyeloma effects of chaetocin appear not to reflect differential intracellular accumulation of chaetocin but, instead, heightened sensitivity of myeloma cells to the cytotoxic effects of imposed oxidative stress. Considered collectively, chaetocin appears to represent a promising agent for further study as a potential antimyeloma therapeutic.

Despite preclinical in vivo activity in ATC, pazopanib has minimal single-agent clinical activity in advanced ATC.

We recently reported that the antineoplastic thiodioxopiperazine natural product chaetocin potently induces cellular oxidative stress, thus selectively killing cancer cells. In pursuit of underlying molecular mechanisms, we now report that chaetocin is a competitive and selective substrate for the oxidative stress mitigation enzyme thioredoxin reductase-1 (TrxR1) with lower K m than the TrxR1 native substrate thioredoxin (Trx; chaetocin K m ¼ 4.6 ± 0.6 mM, Trx K m ¼ 104.7 ± 26 mM), thereby attenuating reduction of the critical downstream ROS remediation substrate Trx at achieved intracellular concentrations. Consistent with a role for TrxR1 targeting in the anticancer effects of chaetocin, overexpression of the TrxR1 downstream effector Trx in HeLa cells conferred resistance to chaetocin-induced, but not to doxorubicin-induced, cytotoxicity. As the TrxR=Trx pathway is of central importance in limiting cellular reactive oxygen species (ROS)-and as chaetocin exerts its selective anticancer effects via ROS imposition-the inhibition of TrxR1 by chaetocin has potential to explain its selective anticancer effects. These observations have important implications not just with regard to the mechanism of action and clinical development of chaetocin and related thiodioxopiperazines, but also with regard to the utility of molecular targets within the thioredoxin reductase=thioredoxin pathway in the development of novel candidate antineoplastic agents.

Pazopanib has promising clinical activity in metastatic MTC with overall manageable toxicities.

Purpose: Flavopiridol, a cyclin-dependent kinase inhibitor, transcription inhibitor, and DNA-interacting agent, was combined with cisplatin or carboplatin to establish toxicities, evaluate pharmacokinetics, and examine its effects on patient cancers and levels of selected polypeptides in patient peripheral blood mononuclear cells (PBMC). Experimental Design: Therapy was given every 3 weeks. Stage I: cisplatin was fixed at 30 mg/m 2 with escalating flavopiridol. Stage II: flavopiridol was fixed at the stage I maximum tolerated dose (MTD) with escalation of cisplatin. Stage III: flavopiridol was fixed at the stage I MTD with escalation of carboplatin. Results: Thirty-nine patients were treated with 136 cycles of chemotherapy. Neutropenia was seen in only 11% of patients. Grade 3 flavopiridol/CDDP toxicities were nausea (30%), vomiting (19%), diarrhea (15%), dehydration (15%), and neutropenia (10%). Flavopiridol combined with carboplatin resulted in unexpectedly high toxicities and one treatment-related death. Stable disease (>3 months) was seen in 34% of treated patients, but there were no objective responses. The stage II MTD was 60 mg/m 2 cisplatin and 100 mg/m 2 /24 hours flavopiridol. As given, CDDP did not alter flavopiridol pharmacokinetics. Flavopiridol induced increased p53 and pSTAT3 levels in patient PBMCs but had no effects on cyclin D1, phosphoRNA polymerase II, or Mcl-1. Conclusions: Flavopiridol and cisplatin can be safely combined in the treatment of cancer patients. Unexpected toxicity in flavopiridol/carboplatin-treated patients attenuates enthusiasm for this alternative combination. Analysis of polypeptide levels in patient PBMCs suggests that flavopiridol may be affecting some, but not all, of its known in vitro molecular targets in vivo.

Anaplastic thyroid cancer (ATC) has perhaps the worst prognosis of any cancer, with a median survival of only about 5 months regardless of stage. Pazopanib monotherapy has promising clinical activity in differentiated thyroid cancers (generally attributed to vascular endothelial growth factor receptor inhibition), yet has less effective single-agent activity in ATC. We now report that combining pazopanib with microtubule inhibitors such as paclitaxel produced heightened and synergistic antitumor effects in ATC cells and xenografts that were associated with potentiated mitotic catastrophe. We hypothesized that combined effects may reflect enhanced paclitaxel-induced cytotoxicity mediated by cell cycle regulatory kinase inhibition by pazopanib. Indeed, pazopanib potently inhibited aurora A, with pazopanib/paclitaxel synergy recapitulated by aurora A short hairpin RNA knockdown or by specific aurora A pharmacological inhibition. Pazopanib/paclitaxel synergy was reversed by aurora A knockdown. Moreover, aurora A (but not B or C) message and protein levels were significantly increased in patient ATCs, and durable benefit resulted from pilot clinical translation of pazopanib/paclitaxel therapy in a patient with metastatic ATC. Collectively, these results suggest that the pazopanib/paclitaxel combination is a promising candidate therapeutic approach in ATC and that aurora A may represent a potentially viable therapeutic molecular target in ATC.

Up-regulated signal transducers and activators of transcription (STAT) -mediated signaling is believed to contribute to the pathogenesis of a variety of solid and hematologic cancers. Consequently, inhibition of STATmediated signaling has recently been proposed as a potential new therapeutic approach to the treatment of cancers. Having shown previously that the pan -cyclindependent kinase inhibitor flavopiridol binds to DNA and seems to kill cancer cells via that process in some circumstances, we evaluated the hypothesis that flavopiridol might consequently disrupt STAT3/DNA interactions, attenuate STAT3-directed transcription, and down-regulate STAT3 downstream polypeptides, including the antiapoptotic polypeptide Mcl-1. SDS-PAGE/immunoblotting and reverse transcription-PCR were used to assess RNA and polypeptide levels, respectively. DNA cellulose affinity chromatography and a nuclear elution assay were used to evaluate the ability of flavopiridol to disrupt STAT3/DNA interactions. A STAT3 luciferase reporter assay was used to examine the ability of flavopiridol to attenuate STAT3-directed transcription. Colony-forming assays were used to assess cytotoxic synergy between flavopiridol and AG490. Flavopiridol was found to (a) disrupt STAT3/DNA interactions (DNA cellulose affinity chromatography and nuclear elution assay), (b) attenuate STAT3-directed transcription (STAT3 luciferase reporter assay), and (c) down-regulate the STAT3 downstream antiapoptotic polypeptide Mcl-1 at the transcriptional level (reverse transcription-PCR and SDS-PAGE/immunoblotting). Furthermore, flavopiridol, but not the microtubule inhibitor paclitaxel, could be combined with the STAT3 pathway inhibitor AG490 to achieve cytotoxic synergy in A549 human non -small cell lung cancer cells. Collectively, these data suggest that flavopiridol can attenuate STAT3-directed transcription in a targeted fashion and may therefore be exploitable clinically in the development of chemotherapy regimens combining flavopiridol and other inhibitors of STAT3 signaling pathways. [Mol Cancer Ther 2006;5(1):138 -48]

Background:We previously reported that chaetocin has potent and selective anti-myeloma activity attributable to reactive oxygen species (ROS) induction imposed by inhibition of the redox enzyme thioredoxin reductase; we now detail its effects in solid tumours.Methods:Cellular assays, transcriptional profiling and the NCI60 screen were used to assess the effects of chaetocin in solid tumour and endothelial cells.Results:NCI-60 screening demonstrated chaetocin to even more potently inhibit proliferation in solid tumour than in haematological cell lines; transcriptional profiling revealed a signature consistent with induction of inflammatory response and cell death pathways. Chaetocin induced ROS, oxidative damage to cellular proteins and apoptosis, with 2–10 n IC50s (24 h exposures) in all tested solid tumour cell lines. The pan-caspase inhibitor zVAD-fmk did not block chaetocin-induced cell death despite inhibiting mitochondrial membrane depolarisation and apoptosis. Further, Molt-4 rho0 cells lacking metabolically functional mitochondria were readily killed by chaetocin; in addition chaetocin-induced cytotoxicity was unaffected by autophagy inhibitors or hypoxia and consequent HIF-1α upregulation. Moreover, chaetocin inhibited SKOV3 ovarian cancer xenografts producing less vascular tumours, and inhibited human umbilical vein endothelial cell proliferation.Conclusion:Chaetocin has intriguing and wide-ranging in vitro and in vivo anticancer effects, and is an attractive candidate for further preclinical and clinical development.