Miljenko,
I disagree on the side-effects. Xcytrin has an excellent safety profile, particularly when compared with other chemo drugs and radiation itself.
The mechanism of action does seem unclear. The company seems to be saying it produces apoptosis via oxidative stress, and this seems plausible.
Here's their description of the AACR presentations:
Pharmacyclics, Inc. (Nasdaq: PCYC) today announced preclinical findings demonstrating how its lead product Xcytrin® (motexafin gadolinium) Injection may work to enhance the effects of radiation and chemotherapy. The findings were presented here at the 92nd Annual Meeting of the American Association for Cancer Research (AACR).
One study presented by John E. Biaglow, Ph.D., Professor and Director, Oncology Research Division, Department of Radiation Oncology and Biochemistry, University of Pennsylvania, indicates that Xcytrin is a redox mediator that is extremely reactive with intracellular reducing metabolites, such as ascorbate and dihydrolipoate. This reactivity leads to the production of superoxide and hydrogen peroxide in cancer cells. This process, known as futile redox cycling, may be responsible for Xcytrin's radiation enhancing activity. A separate study showed that Xcytrin localizes in cancer cells, and futile redox cycling renders the cells more responsive to radiation and chemotherapy.
A third study presented during the meeting showed that the catalytic oxidation of endogenous reducing metabolites by Xcytrin enhances the activity of the redox-active chemotherapy agents, bleomycin and doxorubicin. Xcytrin increased cytotoxic response to bleomycin in clonogenic assays and other lab experiments, and increased tumor response to both agents in animal models. A fourth study demonstrated that Xcytrin enhanced tumor response in vitro and in vivo, and concluded that the mechanism appears to involve an increase in apoptotic (i.e., cell death) response to radiation.
Here are the abstracts:
From the Proceedings of the AACR, Vol. 42, March 2001.
Copyright © 2001 by the American Association for Cancer Research.
Online Publication Date: February 27, 2001.
>>Redox Properties of Motexafin Gadolinium (Gd-Tex): A Tumor Selective Radiation Enhancer
John E. Biaglow, Richard Miller, Darren Magda, University of Pennsylvania, Philadelphia, PA; Pharmacyclics, Inc., Sunnyvale, CA.
Motexafin gadolinium (Gd-Tex) is a tumor selective agent that has been shown to enhance the efficacy of radiation in animal tumor models and is currently in Phase III clinical development as an adjuvant to radiation therapy. It was the purpose of this study to investigate Gd-Tex effects on cellular redox reactions involving oxygen consumption, bioreduction of disulfides and reaction with dihydrolipoate and ascorbate. These studies are necessary to define the optimal radiosensitization conditions for Gd-Tex. Gd-Tex does not alter oxygen consumption of the A549 human carcinoma cells. However, it stimulates oxygen consumption in cyanide inhibited cells. The latter effect is typical for many radiosensitizing drugs such as misonidazole and tirapazamine. The stimulation of oxygen consumption occurs when Gd-Tex accepts an electron producing an oxygen reactive radical. This results in superoxide radical anion and peroxide formation. We also tested the effect of Gd-Tex on bioreduction of disulfides (Biaglow, et al., Analytical Biochem. 281, 77-86, 2000). Gd-Tex inhibited the reduction of lipoate to dihydrolipoate, a thioredoxin/thioredoxin reductase linked reaction. It had no effect on glutathione-linked hydroxyethyldisulfide reduction. Lipoate, while not reduced, enhanced the uptake of the green colored Gd-Tex into a number of cell lines. At the present time we do not know if the inhibition of lipoate reduction is due to altered enzyme activity or a rapid reaction of dihydrolipoate with Gd-Tex producing oxygen reactive intermediates. Such a reaction would prevent accumulation of reduced lipoate. Gd-Tex reacts chemically with both dihydrolipoate and ascorbate producing oxygen reactive intermediates. In the presence of cells, incubated with Gd-Tex and ascorbate, oxygen is consumed rapidly producing hypoxic conditions and the accumulation of a new intracellular absorbing species (red). It is obvious from the above information that Gd-Tex is extremely reactive with reducing species and its radiosensitizing effects may depend on these reactions both extracellularly and intracellularly.
Motexafin Gadolinium Increases Apoptotic Response to Radiation in Vitro and in Vivo
Kathryn Woodburn, Yu Luo, Fan Qing, David W. Voehringer, Richard A. Miller, Pharmacyclics, Sunnyvale, CA; Stanford Medical School, Stanford, CA.
Introduction: Motexafin gadolinium (Xcytrin™, Gd-Tex) is an expanded porphyrin molecule that localizes in cancer and enhances tumor response to radiation. Gd-Tex is presently being evaluated clinically as a radiation enhancer for patients with brain metastases. Methods: The intracellular localization of Gd-Tex was investigated in apoptosis-sensitive (LYas) and apoptosis-resistant (LYar) murine lymphoma cell lines using fluorescence microscopy. Apoptosis was measured using caspase 3 activation, morphologic criteria and flow cytometry analysis following radiation exposure and compared to controls. In vivo experiments were performed using the murine tumors EMT6 sarcoma, SCCVII squamous cell carcinoma and RIF-1 fibrosarcoma growing intramuscularly. Human HT-29 colon carcinoma was grown intramuscularly in immunodeficient mice. Gd-Tex was administered IV prior to single or multiple fractions of radiation. TUNEL labeling was performed on EMT6 histologic sections. Results: Gd-Tex localized in extranuclear compartments and uptake increased with time. Gd-Tex increased apoptosis following radiation exposure in both the Gd-Tex treated radiosensitive and radioresistant LY cells based on caspase 3 activation, morphologic criteria and flow cytometry. Gd-Tex increased apoptosis in vivo in irradiated EMT6 tumors measured by TUNEL labeling. Gd-Tex enhanced radiation tumor response in both single fraction HT-29 and RIF-1 models and in multifraction studies with RIF-1 and SCCVII. Conclusions: Gd-Tex enhanced tumor response in several animal models. The mechanism appears to involve an increase in apoptotic response to radiation.
Redox Cycling of Motexafin Gadolinium Leads to Radiation Sensitization in Vitro.
Darren Magda, Cheryl Lepp, Nikolay Gerasimchuk, Intae Lee, Richard Miller, Pharmacyclics, Inc., Sunnyvale, CA.
Motexafin gadolinium, (Xcytrin™, Gd-Tex) has been shown to enhance the efficacy of radiation in animal tumor models and is currently in Phase III clinical development as an adjuvant to radiation therapy. Gd-Tex is electron affinic, with a first reduction potential near -50 mV (NHE). In order to understand better the mechanism of its action as a radiation enhancer, the chemical and biochemical properties of this agent were examined in vitro. GdTex was found to catalyze the oxidation of NADPH, ascorbate, and other reducing metabolites, leading to the formation of superoxide anion and hydrogen peroxide. Decreased cell viability correlated strongly with the presence of reducing metabolites, especially ascorbate, in the culture medium. This effect was cell line dependent: For example, E89, a CHO cell variant deficient in pentose phosphate pathway activity, was found to be more sensitive to Gd-Tex than the wildtype cell line K1 under all conditions tested. Incubation of Gd-Tex in the presence of ascorbate also enhanced cell uptake. Treatment of MES-SA (human uterine cancer line) cells with Gd-Tex in conjunction with L-buthionine-[S,R]-sulfoximine (BSO), diamide, or antimycin A resulted in a cooperative inhibition of cell proliferation, as measured by formazan reduction. Incubation of MES-SA cells with BSO (100 mM) and Gd-Tex (50 mM) for 24 h prior to ionizing radiation reduced clonogenic survival (sensitization enhancement ratio at surviving fraction 0.1 = ca. 1.8) relative to treatment with BSO alone. Exposure of B-human lymphatic cells (LYAS) to Gd-Tex (without BSO) enhanced radiation response (SER at SF 0.1 = ca. 1.4); however, no sensitization was observed under these conditions in an apoptosis-resistant variant of this line (LYAR). These findings lead us to suggest that Gd-Tex sensitizes cells to ionizing radiation through a novel mechanism of action, whereby the catalytic oxidation of endogenous reducing metabolites leads to the formation of reactive oxygen species. Radiation enhancement may involve inhibition of DNA repair or downstream processes such as apoptosis.
Enhancement of Doxorubicin and Bleomycin Activity with Motexafin Gadolinium
Kathryn Wynne Woodburn, Darren Magda, Fan Qing, Intae Lee, George Duran, Branimir Sikic, Richard A. Miller, Pharmacyclics, Sunnyvale, CA; Stanford University School of Medicine, Stanford, CA.
Introduction: Motexafin gadolinium (Xcytrin™, Gd-Tex) is an expanded porphyrin that has been reported to enhance tumor response to ionizing radiation and is currently in Phase III clinical development for the treatment of brain metastases. Gd-Tex has been shown by magnetic resonance imaging to accumulate selectively in both experimental and human tumors. In vitro studies have indicated that Gd-Tex catalyzes the production of superoxide anion and hydrogen peroxide in the presence of reducing metabolites such as ascorbate and NADPH. The mechanisms of action of the chemotherapeutic agents doxorubicin (Dox) and bleomycin (Bleo) have also been reported to involve the production of reactive oxygen species. In this study, we investigated the possibility that Gd-Tex would enhance the activity of Dox, Bleo, and the non-redox active agent, 4-hydroxycyclophosphamide (HCP). Methods: The effect of Gd-Tex on cytotoxic activity was initially evaluated in vitro in a human uterine cancer line (MES-SA) using an MTT assay. The efficacy of the Bleo and Gd-Tex combination was examined further in RIF-1 cells using a clonogenic survival assay. The activity of Dox, Bleo, and cyclophosphamide (CP) in vivo was assessed in the murine EMT6 model. Results: Treatment with Gd-Tex enhanced the in vitro activity of Bleo, but had no effect on that of HCP in the MES-SA MTT assay. We were unable to demonstrate enhancement of Dox activity in vitro by Gd-Tex, due to an apparent inhibitory interaction between the drugs. In clonogenic assays, the surviving fraction of RIF-1 cells after 24 h exposure to Gd-Tex (50 uM) and Bleo (220 nM) was decreased by a factor of 1000 compared to Bleo alone. Significant chemosensitization was observed in vivo with Dox and Bleo, but not CP. EMT6 bearing animals administered 5, 20 or 40 umol/kg of Gd-Tex 5 hours following 7.5 mg /kg of Dox showed a significant increase in median time to tumor regrowth compared to animals receiving Dox alone (e.g. 37 days vs. 22 days at 20umol/kg, p<0.01). Gd-Tex, in combination with Bleo, increased the median time to tumor regrowth of EMT6 tumors compared to Bleo alone (20 days vs. 12 days, p=0.01). Conclusions: Gd-Tex increased Bleo activity in vitro. In vivo tumor response to Bleo and Dox was enhanced by Gd-Tex but no enhancement of CP was observed. We propose that catalytic oxidation of endogenous reducing metabolites by Gd-Tex enhances the activity of the redox-active drugs Bleo and Dox. <<
Peter |
