Elucidation of mode of retroviral-inhibitory effects of imexon through use of immune competent and severe combined immune deficiency (SCID) mice

Imexon-Induced Apoptosis in Multiple Myeloma Tumor Cells Is Caspase-8 Dependent

PURPOSE

Imexon is a 2-cyanoaziridine agent that has been shown to inhibit growth of chemotherapy-sensitive myeloma cells through apoptosis with decreased cellular stores of glutathione and increased reactive oxygen species (ROS). We examined the mechanism of imexon cytotoxicity in a diverse panel of dexamethasone and chemotherapy-sensitive and -resistant myeloma cell lines.

EXPERIMENTAL DESIGN

We examined cellular cytotoxicity, apoptosis, and changes in redox state in dexamethasone-sensitive (C2E3), dexamethasone-resistant (1-310 and 1-414), chemotherapy-sensitive (RPMI-8226), and chemotherapy-resistant (DOX-1V and DOX-10V) myeloma cell lines.

RESULTS

We found significant cytotoxicity after 48-h incubation with imexon (80-160 microM) in dexamethasone and chemotherapy-sensitive and -resistant myeloma cell lines in a time- and dose-dependent manner. The mechanism of imexon cytotoxicity in all cell lines was related to induction of apoptosis with the presence of cleaved caspase-3. Moreover, after imexon exposure in C2E3 and 1-414 cell lines, we demonstrated caspase-8-dependent apoptosis. Bcl-2:bax was proapoptotic with imexon in C2E3, whereas bcl-2:bax was independent of steroid resistance, chemotherapy sensitivity, and chemotherapy resistance. Depletion of intracellular glutathione was documented in RPMI-8226 at high imexon concentrations (>or=225 microM) but not in other cell lines. Furthermore, ROS were found in C2E3, RPMI-8226, and 1-310 only at high imexon concentrations, whereas a sensitive marker of oxidative DNA damage, 8-hydroxydeoxyguanosine, was not increased in any cell line.

CONCLUSIONS

Our results demonstrate that imexon has significant broad antimyeloma activity that is mediated through apoptotic mechanisms that is not dependent on production of ROS. Moreover, we have identified a mechanism of cytotoxicity in dexamethasone-sensitive and -resistant myeloma cells induced by imexon that is caspase-8 dependent.

Thermolabile hemoglobin formation by 2-cyanaziridine derivatives

The erythrocyte toxicity of imexone, azimexone, ciamexone, and its derivatives was assessed by means of the thermolabile hemoglobin formation test. After administration of doses up to 500 mg/kg to mice, imexone had no effect, whereas in accordance with previous studies azimexone strongly and ciamexone moderately enhanced thermolabile hemoglobin formation. With regard to the derivatives (metabolites) of ciamexone, the alcohol was as active as ciamexone itself, whereas ciamexone aldehyde gave a weak reaction. Both ciamexone acid and ciamexone-cysteine had no influence. Demethyl ciamexone, however, was more toxic than ciamexone, further indicating that the metabolic oxidation of the methyl group represents a detoxifying step. In contrast to the in vivo results, after incubation with blood samples solely imexone (in the range of mg/ml blood) but none of the other 2-cyanaziridines enhanced thermolabile globin formation. Preincubation of imexone with cysteine inhibited this effect. The results are discussed in view of the hypothesis that binding of the nitrile group of the 2-cyanaziridines to mercapto groups of proteins precedes their biological effects.

Protection against murine cytomegalovirus infection in aged mice and mice with severe combined immunodeficiency disease with the biological response modifiers polyribosinic-polycytidylic acid stabilized with L-lysine and carboxymethylcellulose, maleic anhydride divinyl ether and colony stimulating factor 1

A variety of biological response modifiers (BRMs) have provided antiviral protection to immunocompetent mice, and this prompted us to determine their efficacy against murine cytomegalovirus (MCMV) infection in immunocompromised mice-including the profoundly immunocompromised SCID mice and C57Bl/6 and B6D2F1 aged mice. SCID mice showed a marked decrease (> 20-fold) in resistance to MCMV, while there was a slight decrease (3-fold) in aged mice. In BRM antiviral protection studies, SCID mice were almost completely protected against MCMV infection by the pleiotropic immunomodulators, MVE-2 and pICLC, but much less by the more selective CSF-1. pICLC-induced IFN and NK cell cytotoxicity were maintained in SCID mice, suggesting that pleiotropic immunomodulatory effects may be required for antiviral protection in such a profoundly immunocompromised model. pICLC also effectively protected aged mice against lethal MCMV infection and effectively induced IFN. These results emphasize the potential for BRM treatment in immunocompromised hosts.

Immunomodulator effects on the Friend virus infection in genetically defined mice

The disease induced by the Friend virus complex (FV) in F1 hybrid mice containing the Rfv-3r/s genotype in the presence of H-2a/a was used to evaluate a variety of immunomodulating substances. In these genetically defined mice, the FV disease results in splenomegaly, early production of high titers of cell-associated and plasma virus, high levels of splenic viral RNA, increased hematocrit, and eventual death. As the disease progresses, reduced levels of infectious virus correlate with development of specific antibody; reduction in T cell populations, increase in B cells, and decrease in T-cell function also occur. The following immunomodulators were evaluated, listed in the order of their ability to inhibit the FV disease: imexon > MVE-2 > human recombinant IFN-alpha A/D > AS101 > ampligen > AM-3 = oxamisole > ImuVert > bropirimine. In fact, bropirimine, used with certain treatment regimens, appeared to enhance the FV disease. These data suggest that certain immunomodulators may have potential value in the treatment of HIV disease, but also indicate that caution should be exercised in their clinical use.

Animal models for anti-AIDS therapy

Primate and non-primate species have been used to study the pathobiology of the simian immunodeficiency virus (SIV) and of the human immunodeficiency virus type 1 (HIV-1), respectively, and to develop new therapeutic regimes. Transgenic mice which express either the entire HIV-1 provirus or subgenomic fragments have been used to analyze viral gene products in vivo and may serve as models for the development of agents targeted to select viral functions. Chimeric mice which were created by transplanting human hematolymphoid cells into mice suffering from congenital severe combined immunodeficiency (scid/scid or so called SCID mice), can be infected with HIV-1 and allow one to study the entire HIV-1 replicative cycle. Type C murine leukemia virus models have been used to develop new prophylactic and therapeutic strategies but their use is restricted to the evaluation of select antiviral drug inhibition, targeted to retroviral genes common to both Lentivirinae and Oncovirinae. The role of various animal model systems in the development of anti-HIV-1 and anti-AIDS therapies is summarized.

Development of antiviral treatment strategies in murine models

Murine models with type C murine leukemia viruses have been used to develop major new prophylactic and therapeutic strategies in vaccination, drug therapy of acute virus exposure and chronic viremia, combination therapy, prevention of maternal transmission, and therapy targeted to the central nervous system. Transgenic mice expressing either the whole human immunodeficiency virus type 1 (HIV-1) provirus or subgenomic sequences allow the in vivo analysis of selected HIV-1 functions. The full replicative cycle of HIV-1 can be studied in human/mouse chimerae which were created by transplanting human hematolymphoid cells into SCID mice. The chimeric SCID mouse models have been used successfully to evaluate anti-HIV-1 drugs. The role of the various murine retrovirus systems in the development of anti-HIV-1 and anti-AIDS therapies is summarized.