NF-kappa B activation in vivo in both host and tumour cells by the antivascular agent 5,6-dimethylxanthenone-4-acetic acid (DMXAA)

Carboxyxanthones: Bioactive Agents and Molecular Scaffold for Synthesis of Analogues and Derivatives

Xanthones represent a structurally diverse group of compounds with a broad range of biological and pharmacological activities, depending on the nature and position of various substituents in the dibenzo-γ-pyrone scaffold. Among the large number of natural and synthetic xanthone derivatives, carboxyxanthones are very interesting bioactive compounds as well as important chemical substrates for molecular modifications to obtain new derivatives. A remarkable example is 5,6-dimethylxanthone-4-acetic acid (DMXAA), a simple carboxyxanthone derivative, originally developed as an anti-tumor agent and the first of its class to enter phase III clinical trials. From DMXAA new bioactive analogues and derivatives were also described. In this review, a literature survey covering the report on carboxyxanthone derivatives is presented, emphasizing their biological activities as well as their application as suitable building blocks to obtain new bioactive derivatives. The data assembled in this review intends to highlight the therapeutic potential of carboxyxanthone derivatives and guide the design for new bioactive xanthone derivatives.

Identification of human-selective analogues of the vascular-disrupting agent 5,6-dimethylxanthenone-4-acetic acid (DMXAA)

Background:

Species selectivity of DMXAA (5,6-dimethylxanthenone-4-acetic acid, Vadimezan) for murine cells over human cells could explain in part the recent disappointing phase III trials clinical results when preclinical studies were so promising. To identify analogues with greater human clinical potential, we compared the activity of xanthenone-4-acetic acid (XAA) analogues in murine or human cellular models.

Methods:

Analogues with a methyl group systematically substituted at different positions of the XAA backbone were evaluated for cytokine induction in cultured murine or human leukocytes; and for anti-vascular effects on endothelial cells on matrigel. In vivo antitumour activity and cytokine production by stromal or cancer cells was measured in human A375 and HCT116 xenografts.

Results:

Mono-methyl XAA analogues with substitutions at the seventh and eighth positions were the most active in stimulating human leukocytes to produce IL-6 and IL-8; and for inhibition of tube formation by ECV304 human endothelial-like cells, while 5- and 6-substituted analogues were the most active in murine cell systems.

Conclusion:

Xanthenone-4-acetic acid analogues exhibit extreme species selectivity. Analogues that are the most active in human systems are inactive in murine models, highlighting the need for the use of appropriate in vivo animal models in selecting clinical candidates for this class of compounds.

Dissection of stromal and cancer cell-derived signals in melanoma xenografts before and after treatment with DMXAA

Background:

The non-malignant cells of the tumour stroma have a critical role in tumour biology. Studies dissecting the interplay between cancer cells and stromal cells are required to further our understanding of tumour progression and methods of intervention. For proof-of-principle of a multi-modal approach to dissect the differential effects of treatment on cancer cells and stromal cells, we analysed the effects of the stromal-targeting agent 5,6-dimethylxanthenone-4-acetic acid on melanoma xenografts.

Methods:

Flow cytometry and multi-colour immunofluorescence staining was used to analyse leukocyte numbers in xenografts. Murine-specific and human-specific multiplex cytokine panels were used to quantitate cytokines produced by stromal and melanoma cells, respectively. Human and mouse Affymetrix microarrays were used to separately identify melanoma cell-specific and stromal cell-specific gene expression.

Results:

5,6-Dimethylxanthenone-4-acetic acid activated pro-inflammatory signalling pathways and cytokine expression from both stromal and cancer cells, leading to neutrophil accumulation and haemorrhagic necrosis and a delay in tumour re-growth of 26 days in A375 melanoma xenografts.

Conclusion:

5,6-Dimethylxanthenone-4-acetic acid and related analogues may potentially have utility in the treatment of melanoma. The experimental platform used allowed distinction between cancer cells and stromal cells and can be applied to investigate other tumour models and anti-cancer agents.

Labeling of oxidizable proteins with a photoactivatable analog of the antitumor agent DMXAA: evidence for redox signaling in its mode of action

The signaling pathway(s) and molecular target(s) for 5,6-dimethylxanthenone-4-acetic acid (DMXAA), a tumor vascular disrupting agent in late stages of clinical development, are still undefined. As an approach toward identifying potential targets for DMXAA, a tritiated azido-analog of DMXAA was used to probe for cellular binding proteins. More than 20 cytosolic proteins from murine splenocytes, RAW 264.7 cells, and the HECPP immortalized endothelial cells were photoaffinity-labeled. Although no protein domain, fold, or binding site for a specific ligand was found to be shared by all the candidate proteins, essentially all were noted to be oxidizable proteins, implicating a role for redox signaling in the action of DMXAA. Consistent with this hypothesis, DMXAA caused an increase in concentrations of reactive oxygen species (ROS) in RAW264.7 cells during the first 2 hours. This increase in ROS was suppressed in the presence of the antioxidant, N-acetyl-L-cysteine, which also suppressed DMXAA-induced cytokine production in the RAW 264.7 cells with no effects on cell viability. Short interfering RNA (siRNA)-mediated knockdown of one of the photoaffinity-labeled proteins, superoxide dismutase 1, an ROS scavenger, resulted in an increase in tumor necrosis factor-alpha production by RAW 264.7 cells in response to DMXAA compared with negative or positive controls transfected with nontargeting or lamin A/C-targeting siRNA molecules, respectively. The results from these lines of study all suggest that redox signaling plays a central role in cytokine induction by DMXAA.

The potential of DMXAA (ASA404) in combination with docetaxel in advanced prostate cancer

5,6-Dimethylxanthenone-4-acetic acid (DMXAA) is a vascular disrupting agent that has demonstrated efficacy in combination with taxane-based chemotherapy in patients with advanced cancer. Complementary modes of action, a lack of pharmacokinetic interaction and distinct adverse effect profiles provide a strong rationale for combining these anticancer agents. In a Phase II trial in men with hormone refractory prostate cancer, DMXAA (ASA404) in combination with docetaxel achieved a prostate-specific antigen response in more patients than docetaxel therapy alone, and was generally well tolerated. Further clinical evaluation of this combination in this patient population is warranted.

Role of IKK and oscillatory NFkappaB kinetics in MMP-9 gene expression and chemoresistance to 5-fluorouracil in RKO colorectal cancer cells

Nuclear factor kappa B (NFkappaB) is a central participant in the metastasis and chemoresistance of colorectal cancer (CRC). However, it is not fully understood to what extent NFkappaB contributes to induction of the metastasis-associated matrix metalloprotease-9 (MMP-9) gene and sensitivity to the commonly used chemotherapeutic 5-fluorouracil (5-Fu) in CRC. Using the RKO human CRC cell line and two NFkappaB signaling deficient RKO mutants, we investigated NFkappaB's role in the induction of MMP-9 and 5-Fu sensitivity in RKO CRC cells. NFkappaB plays a predominant role in MMP-9 gene induction in RKO cells, as evidenced by the failure of tumor necrosis factor alpha (TNFalpha) to induce MMP-9 in either of the NFkappaB signaling mutants. RKO cells exhibit a robust, oscillatory NFkappaB activity in response to TNFalpha not seen in either of the NFkappaB mutant cell lines, which instead demonstrate diminished, nonoscillatory NFkappaB activation. Analysis of TNFalpha-induced phosphorylation and MMP-9 promoter recruitment of the p65 NFkappaB subunit revealed a significant reduction in p65 phosphorylation as well as reduced and altered recruitment of p65 to the MMP-9 gene promoter in the mutants compared to the parental RKO cell line. 5-Fu only activated NFkappaB in the parental RKO cells through induction of IkappaB-kinase (IKK) activity and increased sensitivity to 5-Fu is observed in both NFkappaB mutant lines. Our results suggest that TNFalpha-dependent induction of MMP-9 gene expression is tightly regulated by oscillatory/cumulative activation of NFkappaB and that 5-Fu stimulates NFkappaB and RKO CRC cell survival through induction of IKK activity.

The vascular disrupting agent, DMXAA, directly activates dendritic cells through a MyD88-independent mechanism and generates antitumor cytotoxic T lymphocytes

5,6-Di-methylxanthenone-4-acetic acid (DMXAA) is a small molecule in the flavanoid class that has antitumor activity. Although classified as a "vascular disrupting agent," we have recently conducted studies showing that DMXAA has remarkable efficacy in a range of tumors, working primarily as an immune modulator that activates tumor-associated macrophages and induces a subsequent CD8(+) T-cell-mediated response. To more completely analyze the effect of DMXAA on CD8(+) T-cell generation, we treated mice bearing tumors derived from EG7 thymoma cells that express the well-characterized chicken ovalbumin neotumor antigen. Treatment with DMXAA led to cytokine release, tumor cell necrosis, and ultimately reduction in tumor size that was lymphocyte dependent. Within 24 h of administration, we observed dendritic cell activation in tumor-draining lymph nodes (TDLN). This was followed by a rapid and marked increase in the number of tetramer-specific CD8(+) T cells in the spleens of treated animals. In contrast, the vascular disrupting agent combretastatin A4-phosphate, which caused a similar amount of immediate tumor necrosis, did not activate dendritic cells, nor induce an effective antitumor response. Using in vitro systems, we made the observation that DMXAA has the ability to directly activate mouse dendritic cells, as measured by increased expression of costimulatory molecules and proinflammatory cytokine release via a pathway that does not require the Toll-like receptor adaptor molecule MyD88. DMXAA thus has the ability to activate tumor-specific CD8(+) T cells through multiple pathways that include induction of tumor cell death, release of stimulatory cytokines, and direct activation of dendritic cells.

The chemotherapeutic agent DMXAA potently and specifically activates the TBK1-IRF-3 signaling axis

Vascular disrupting agents (VDAs) represent a novel approach to the treatment of cancer, resulting in the collapse of tumor vasculature and tumor death. 5,6-dimethylxanthenone-4-acetic acid (DMXAA) is a VDA currently in advanced phase II clinical trials, yet its precise mechanism of action is unknown despite extensive preclinical and clinical investigations. Our data demonstrate that DMXAA is a novel and specific activator of the TANK-binding kinase 1 (TBK1)–interferon (IFN) regulatory factor 3 (IRF-3) signaling pathway. DMXAA treatment of primary mouse macrophages resulted in robust IRF-3 activation and ∼750-fold increase in IFN-β mRNA, and in contrast to the potent Toll-like receptor 4 (TLR4) agonist lipopolysaccharide (LPS), signaling was independent of mitogen-activated protein kinase (MAPK) activation and elicited minimal nuclear factor κB–dependent gene expression. DMXAA-induced signaling was critically dependent on the IRF-3 kinase, TBK1, and IRF-3 but was myeloid differentiation factor 88–, Toll–interleukin 1 receptor domain–containing adaptor inducing IFN-β–, IFN promoter-stimulator 1–, and inhibitor of κB kinase–independent, thus excluding all known TLRs and cytosolic helicase receptors. DMXAA pretreatment of mouse macrophages induced a state of tolerance to LPS and vice versa. In contrast to LPS stimulation, DMXAA-induced IRF-3 dimerization and IFN-β expression were inhibited by salicylic acid. These findings detail a novel pathway for TBK1-mediated IRF-3 activation and provide new insights into the mechanism of this new class of chemotherapeutic drugs.

Monitoring drug-protein interaction

A variety of therapeutic drugs can undergo biotransformation via Phase I and Phase II enzymes to reactive metabolites that have intrinsic chemical reactivity toward proteins and cause potential organ toxicity. A drug-protein adduct is a protein complex that forms when electrophilic drugs or their reactive metabolite(s) covalently bind to a protein molecule. Formation of such drug-protein adducts eliciting cellular damages and immune responses has been a major hypothesis for the mechanism of toxicity caused by numerous drugs. The monitoring of protein-drug adducts is important in the kinetic and mechanistic studies of drug-protein adducts and establishment of dose-toxicity relationships. The determination of drug-protein adducts can also provide supportive evidence for diagnosis of drug-induced diseases associated with protein-drug adduct formation in patients. The plasma is the most commonly used matrix for monitoring drug-protein adducts due to its convenience and safety. Measurement of circulating antibodies against drug-protein adducts may be used as a useful surrogate marker in the monitoring of drug-protein adducts. The determination of plasma protein adducts and/or relevant antibodies following administration of several drugs including acetaminophen, dapsone, diclofenac and halothane has been conducted in clinical settings for characterizing drug toxicity associated with drug-protein adduct formation. The monitoring of drug-protein adducts often involves multi-step laboratory procedure including sample collection and preliminary preparation, separation to isolate or extract the target compound from a mixture, identification and determination. However, the monitoring of drug-protein adducts is often difficult because of short half-lives of the protein adducts, sampling problem and lack of sensitive analytical techniques for the protein adducts. Currently, chromatographic (e.g. high performance liquid chromatography) and immunological methods (e.g. enzyme-linked immunosorbent assay) are two major techniques used to determine protein adducts of drugs in patients. The present review highlights the importance for clinical monitoring of drug-protein adducts, with an emphasis on methodology and with a further discussion of the application of these techniques to individual drugs and their target proteins.

Transport of the investigational anti-cancer drug 5,6-dimethylxanthenone-4-acetic acid and its acyl glucuronide by human intestinal Caco-2 cells

5,6-Dimethylxanthenone-4-acetic acid (DMXAA), a potent cytokine inducer, exhibited marked antitumor activity when given as multiple oral doses in mice. The aim of this study was to examine the transport of DMXAA and its acyl glucuronide (DMXAA-G) using the human Caco-2 cells. DMXAA was minimally metabolized by Caco-2 cells and both DMXAA and DMXAA-G were taken up to a minor extent by the cells. The permeability coefficient (Papp) values of DMXAA over 10-500 microM were 4x10(-5) cm/s to 4.3x10(-5) cm/s for both apical (AP) to basolateral (BL) and BL-AP transport, while the Papp values for the BL to AP flux of DMXAA-G were significantly greater than those for the AP to BL flux, with Rnet values of 4.5-17.6 over 50-200 microM. The BL to AP active efflux of DMXAA-G followed Michaelis-Menten kinetics, with a Km of 83.5+/-5.5 microM, and Vmax of 0.022+/-0.001 nmol/min. The flux of DMXAA-G was energy and Na+-dependent and MK-571 significantly (P<0.05) inhibited its BL to AP flux, with an estimated Ki of 130 microM. These data indicate that the transport of DMXAA across Caco-2 monolayers was through a passive process, whereas the transport of DMXAA-G was mediated by MRP1/2.

Determination of the investigational anti-cancer drug 5,6-dimethylxanthenone-4-acetic acid and its acyl glucuronide in Caco-2 monolayers by liquid chromatography with fluorescence detection: application to transport studies

5,6-Dimethylxanthenone-4-acetic acid (DMXAA) is a potent cytokine inducer, with a bioavailability of >70% in the mouse. The aim of this study was to develop and validate HPLC methods for the determination of DMXAA and DMXAA acyl glucuronide (DMXAA-G) in the human intestinal cell line Caco-2 monolayers. The developed HPLC methods were sensitive and reliable, with acceptable accuracy (85-115% of true values) and precision (intra- and inter-assay CV < 15%). The total running time was within 6.8 min, with acceptable separation of the compounds of interest. The limit of quantitation (LOQ) values for DMXAA and DMXAA-G were 14.2 and 24 ng/ml, respectively. The validated HPLC methods were applied to examine the epithelial transport of DMXAA and DMXAA-G by Caco-2 monolayers. The permeability coefficient (Papp) values (overall mean +/- S.D., n = 3-9) of DMXAA over 10-500 microM were independent of concentration for both apical (AP) to basolateral (BL) (4.0 +/- 0.4 x 10(-5)cm/s) and BL-AP (4.3 +/- 0.5 x 10(-5)cm/s) transport, and of similar magnitude in either direction, with net efflux ratio (Rnet) values of 1-1.3. However, the Papp values for the BL to AP transport of DMXAA-G were significantly greater than those for the AP to BL transport, with Rnet values of 17.6, 6.7 and 4.5 at 50, 100 and 200 microM, respectively. Further studies showed that the transport of DMXAA-G was Na+- and energy-dependent, and inhibited by MK-571 [a multidrug resistance associated protein (MRP) 1/2 inhibitor], but not by verapamil and probenecid. These data indicate that the HPLC methods for the determination of DMXAA and DMXAA-G in the transport buffer were simple and reliable, and the methods have been applied to the transport study of both compounds by Caco-2 monolayers. DMXAA across Caco-2 monolayers was through a passive transcellular process, whereas the transport of DMXAA-G was mediated by MRP1/2.