Cellular Resistance Mechanisms Research Articles

Chemosensitization of Cancer by Nitric Oxide

Recent evidence from experimental and clinical studies links the development of intratumoral hypoxia and oxidative stress with malignant progression. Cellular adaptation induced by these environmental stresses is also associated with the emergence of drug resistant populations. This adaptation is most likely a multifactorial process involving coordination of various stress-induced signaling pathways, including those regulated by hypoxia-inducible factor-1 (HIF-1) and nuclear factor kappaB (NF-kappaB) together with their downstream targets linked to resistance mechanisms. Experimental data suggest that treatment of human cancer cells with nitric oxide (NO) and NO mimetic agents can effectively restore the sensitivity of resistant populations to the cytotoxic effects of chemotherapeutics both in vitro and in vivo. Furthermore, preliminary results from Phase II clinical trials evaluating NO as an adjuvant to chemotherapy are promising. The present review highlights the significance of intratumoral hypoxia and oxidative stress in the emergence of multidrug resistance, and summarizes the latest data demonstrating the chemosensitizing ability of NO. To date, the specific mechanisms through which NO restores sensitivity to anticancer agents are not clearly understood. However, the data suggest that chemosensitization is likely to involve NO-mediated activities associated with both prevention and inhibition of cellular drug resistance mechanisms. Potential mechanisms contributing to the chemosensitizing activity of NO include vascular changes that promote increased blood delivery and tumor oxygenation, antioxidant effects and down-regulation of the glutathione detoxification/redox buffering system, inhibition of key transcription factors such as HIF-1 and NF-kappaB, as well as inhibition of drug efflux transporters and DNA repair enzymes.

Two Distinct Molecular Mechanisms Underlying Cytarabine Resistance in Human Leukemic Cells

To understand the mechanism of cellular resistance to the nucleoside analogue cytarabine (1-beta-D-arabinofuranosylcytosine, AraC), two resistant derivatives of the human leukemic line CCRF-CEM were obtained by stepwise selection in different concentrations of AraC. CEM/4xAraC cells showed low AraC resistance, whereas CEM/20xAraC cells showed high resistance. Both cell lines showed similar patterns of cross-resistance to multiple cytotoxic nucleoside analogues, with the exception that CEM/20xAraC cells remained sensitive to 5-fluorouridine and 2-deoxy-5-fluorouridine. Both cell lines were sensitive to 5-fluorouracil and to a variety of natural product drugs. Although both CEM/4xAraC and CEM/20xAraC cells displayed reduced intracellular accumulation of [(3)H]AraC, only CEM/4xAraC cells showed reduced uptake of [(3)H]uridine, which was used to assess nucleoside transport activities. Genes encoding proteins known to be involved in nucleoside transport, efflux, and metabolism were analyzed for the presence of mutations in the two cell lines. In CEM/4xAraC cells, independent mutations were identified at each allele of human equilibrative nucleoside transporter 1 (hENT1; SLC29A1), one corresponding to a single-nucleotide change in exon 4, the other being a complex intronic mutation disrupting splicing of exon 13. In contrast to CEM/20xAraC cells, CEM/4xAraC cells did not bind the hENT1/SLC29A1 ligand nitrobenzylmercaptopurine ribonucleoside and lacked detectable hENT1/SLC29A1 protein. In CEM/20xAraC cells, independent intronic mutations impairing splicing of exons 2 and 3 were found at each allele of the deoxycytidine kinase gene. These studies point to at least two distinct mechanisms of AraC resistance in leukemic cells.

Microarray Gene Expression Profiling in Meningiomas and Schwannomas

Microarray gene expression profiling is a high throughput system recently used in basic and applied research. It provides a large amount of data -at molecular level- that once acquired, must be functionally integrated in order to find common patterns within a defined group of biological samples. In addition to identification of differentially expressed genes and the establishment of gene regulation patterns, microarrays may also allow us to discover new tumor markers that could have a great impact on the improvement of clinical practice and therapeutics for cancer. The classification method used for cancer is currently based on the morphological characteristics of the biological samples. The information obtained with this method is limited, omitting many important tumor characteristics like the proliferation rate, the capacity of invasion and metastases, as well as the possible development of mechanisms of cellular resistance to treatment. Microarrays can be used in combination with conventional diagnostics as a helpful complement. In this review we focus on how this technology has contributed to our knowledge of the molecular pathogenesis of meningiomas and schwannomas, its potential role as a useful tool for tumor classification and its application in clinical practice.

Cellular Mechanisms of Resistance to Anthracyclines and Taxanes in Cancer: Intrinsic and Acquired

Taxanes and anthracyclines are two of the most potent and broadly effective classes of chemotherapeutic agents. However, resistance to these agents is common and significantly limits their potential. As such, there is a great need to understand the mechanisms underlying de novo and acquired resistance to these agents. Beyond the resistance barrier lies even greater potential to significantly alter the natural course of human cancer. This review discusses what we currently understand about the mechanisms of resistance to taxanes and anthracyclines. Preclinical models suggest a role for ATP-binding cassette transporters, tubulin isoforms, microtubule-associated proteins, tubulin gene mutations, and mitotic checkpoint signaling proteins in resistance to taxanes. Preclinical models also suggest that drug transport proteins, antioxidant defenses, apoptotic signaling, and topoisomerase modulation may mediate anthracycline resistance. Many of these hypotheses remain untested in appropriately designed clinical studies, but limited clinical evidence will be reviewed. Epothilones represent a novel class of non-taxane microtubule stabilizing agents with distinct drug-resistance profiles. Potential mechanisms behind these differences and their potential role in the treatment of both taxane- and anthracycline-refractory patients are discussed.

Functional identification of genes causing estrogen independence of human breast cancer cells

Endocrine treatment of breast cancer is widely applied and effective. However, in advanced disease cases, the tumors will eventually progress into an estrogen-independent and therapy-resistant phenotype. To elucidate the molecular mechanisms underlying this endocrine therapy failure, we applied retroviral insertion mutagenesis to identify the main genes conferring estrogen independence to human breast cancer cells. Estrogen-dependent ZR-75-1 cells were infected with replication-defective retroviruses followed by selection with the anti-estrogen 4-hydroxy-tamoxifen. In the resulting panel of 79 tamoxifen-resistant cell lines, the viral integrations were mapped within the human genome. Genes located in the immediate proximity of the retroviral integration sites were characterized for altered expression and their capacity to confer anti-estrogen resistance when transfected into breast cancer cells. Out of 15 candidate BCAR (breast cancer anti-estrogen resistance) genes, seven (AKT1, AKT2, BCAR1, BCAR3, EGFR, GRB7, and TRERF1/BCAR2) were shown to directly underlie estrogen independence. Our results show that insertion mutagenesis is a powerful tool to identify BCAR loci, which may provide insights into the molecular and cellular mechanisms of breast tumor progression and therapy resistance thereby offering novel targets for the development of tailor-made therapeutical and prevention strategies.

A role for ceramide in driving cancer cell resistance to doxorubicin

Advanced cancers acquire resistance to chemotherapy, and this results in treatment failure. The cellular mechanisms of chemotherapy resistance are not well understood. Here, for the first time, we show that ceramide contributes to cellular resistance to doxorubicin through up-regulating the gene expression of glucosylceramide synthase (GCS). Ceramide, a cellular lipid messenger, modulates doxorubicin-induced cell death. GCS catalyzes ceramide glycosylation, converting ceramide to glucosylceramide; this process hastens ceramide clearance and limits ceramide-induced apoptosis. In the present study, we evaluated the role of the GCS gene in doxorubicin resistance using several paired wild-type and drug-resistant (doxorubicin-selected) cancer cell lines, including breast, ovary, cervical, and colon. GCS was overexpressed in all drug-resistant counterparts, and suppressing GCS overexpression using antisense oligonucleotide restored doxorubicin sensitivity. Characterizing the effect mechanism showed that doxorubicin exposure increased ceramide levels, enhanced GCS expression, and imparted cellular resistance. Exogenous C(6)-ceramide and sphingomyelinase treatments mimicked the influence of doxorubicin on GCS, activating the GCS promoter and up-regulating GCS gene expression. Fumonisin B(1), an inhibitor of ceramide synthesis, significantly suppressed doxorubicin-up-regulated GCS expression. Promoter truncation, point mutation, gel-shift, and protein-DNA ELISA analysis showed that transcription factor Sp1 was essential for ceramide-induced GCS up-regulation. These data indicate that ceramide-governed GCS gene expression drives cellular resistance to doxorubicin.

Role of adipose hypoxia in endocrine alterations: a possible new anti-inflammatory therapeutic target in obesity?

Dr Jianping Ye is a Professor at the Pennington Biomedical Research Center, Louisiana State University (LA, USA). He completed his graduate education in the medical school, Beijing University (China) in 1989 and moved to the USA to undertake postdoctoral research in 1992. He conducted research in the role of inflammation in cancer and was an instructor at the Health Science Center at San Antonio, University of Texas (TX, USA) in 1997, and assistant professor in the West Virginia University (WV, USA) in 1999. In 2001, he set up his lab at the Pennington Biomedical Research Center, Louisiana State University, where obesity research is the main focus. His current work, supported by NIH and American Diabetes Association, predominantly focuses on insulin resistance in obesity. Recent work from his lab reported that adipose hypoxia may contribute to chronic inflammation and insulin resistance in obesity. The study was based on the hypoxia effect on the endocrine function of adipocytes in a preliminary study of large adipocytes in his laboratory in 2004. Here, Nicole Nogoy talks to Jianping Ye on the future and current issues in the cellular and molecular mechanisms of insulin resistance.

Cellular and molecular mechanisms of resistance to oral Candida albicans infections

Oral candidiasis is a significant health problem in terms of both morbidity and economic outlay. Infections are predominantly caused by the commensal C. albicans, and affect immunocompromised individuals, including HIV-positive and AIDS patients, organ transplant recipients and chemotherapy patients. The molecular and cellular immune mechanisms involved in protection from and responses to oral candidiasis are overlapping, but distinct from those associated with other manifestations of the disease, including systemic, vaginal and gastric candidiasis. In oral candidiasis, clinical observations and experimental mouse models suggest a critical role for cell-mediated immunity. In mice, CD4+ T-cells and the p40 subunit of interleukins 12 and 23 are strict prerequisites for resistance; however abrogation of IFN-gamma does not confer susceptibility. Here, we discuss this apparent inconsistency, and review the experimental evidence that clarifies which immune pathways are specifically involved in resistance and responses to candidiasis of the oral cavity. We also highlight deficiencies in the literature, particularly concerning the putative roles of some relatively new elements in immunobiology: interleukin-23, interleukin-17 and T helper (Th)17 cells.

Mechanisms of cellular resistance to imatinib in human chronic myeloid leukemia cells

A major advancement in the treatment of chronic myeloid leukemia (CML) has been the development of imatinib, which has shown striking activity in the chronic phase and the accelerated phase, but less so in the blast phase of the disease. Despite high rates of hematologic and cytogenetic responses to therapy, the emergence of resistance to imatinib has been recognized as a major problem in the treatment of patients with CML.Various cellular mechanisms may be involved in the nature of cellular resistance. Increased amount of target, alteration in structure of target proteins, decreased drug uptake and increased detoxification are well-known mechanisms of resistance. On the other hand, in some cases, even if anticancer drugs reach their sites of action, bypassing drug efflux system of the cells, some cells still may survive via the dysregulation of apoptotic signalling.In this study, mechanisms of resistance to imatinib-induced apoptosis in human Meg-01 CML cells were examined. Continuous exposure of cells to step-wise increasing concentrations of imatinib resulted in the selection of 200- and 1000 nM imatinib-resistant sub-lines referred to as Meg-01/IMA-0,2 and Meg-01/IMA-1, respectively. MTT cell proliferation, cell cycle analyses and trypan blue dye exclusion analyses showed that Meg-01/IMA-1 cells were resistant to imatinib-induced apoptosis as compared to parental sensitive cells. There was an increased expression of BCR/ABL, Bcl-2 and an increase in mitochondrial membrane potential (MMP) detected in resistant cells comparing to parental sensitive cells. There was no mutation detected in imatinib binding site of ABL kinase region.Various diverse mechanisms have been reported for their involvement in the multidrug resistance. In this study, it has been shown that the degree of BCR/ABL expression appears to be directly proportional to the levels of imatinib resistance. In addition, there have been BCR/ABL-independent mechanisms reported for deriving resistance against imatinib. Our results revealed that besides BCR/ABL overexpression, imatinib resistance also depends on the inhibition of apoptosis as a result of up-regulation of anti-apoptotic stimuli and down-regulation of pro-apoptotic stimuli through MMP but does not depend on any mutation on imatinib binding site of ABL kinase.

Up-regulation of MRP4 and down-regulation of influx transporters in human leukemic cells with acquired resistance to 6-mercaptopurine

To investigate the mechanism of cellular resistance to 6-MP, we established a 6-MP resistant cell line (CEM-MP5) by stepwise selection of the human T-lymphoblastic leukemia cell line (CEM). CEM-MP5 cells were about 100-fold resistant to 6-MP compared with parental CEM cells. Western blot analysis demonstrated that multidrug resistant protein 4 (MRP4) was increased in CEM-MP5 cells, whereas the levels of the nucleoside transporters hENT1, hCNT2 and hCNT3 were decreased compared with those of parental CEM cells. Consistent with the operation of an efflux pump, accumulation of [ 14C]6-MP and/or its metabolites was reduced, and ATP-dependent efflux was increased in CEM-MP5 cells. Taken together these results showed that up-regulation of MRP4 and down-regulation of influx transporters played a major role in 6-MP resistance of CEM-MP5 cells.

Taxane Refractory Prostate Cancer

Although docetaxel based therapy has become established as a front line therapy choice based on large, randomized studies, published studies of second line therapy for taxane refractory disease are limited. The literature on the biology of taxane resistance and studies applied to prostate cancer were reviewed using a PubMed(R) search and proceedings from recent symposia. Although taxane resistance invariably emerges in the treatment of prostate cancer, a consensus working definition or classification does not exist. Although there is a body of knowledge on the mechanisms of action of taxanes and resistance pathways, there are few clinical or translational studies in prostate cancer adequately assessing the modulation of these mechanisms. Results of additional clinical trials are needed to define and improve the standard of care in the second line setting for castration resistant prostate cancer after docetaxel failure. The validation of the microtubule as a target in prostate cancer implies that a finer understanding of specific mechanisms of efficacy and resistance may yield novel strategies. Taxane analogues that have greater antitumor activity and/or are less susceptible to drug resistance mechanisms than their prototypes are in development, as are nontaxane microtubule targeting agents and other agents directed against the mitotic spindle. Combinations of such agents may yield added efficacy but potentially added neurotoxicity. In contrast, combinations with drugs that inhibit cellular mechanisms of taxane resistance and vascular endothelial or tumor-stromal prosurvival interactions may have lower neurotoxic profiles. Although alternate classes of cytotoxic agents, eg satraplatin, are being studied, there is a strong imperative for translational studies in this setting.

Trough plasma imatinib concentrations are associated with responses to standard-dose imatinib in chronic myeloid leukemia and could improve its clinical management

7027 Background: Variable responses to imatinib in the treatment of myelogenous leukemia (CML) are incompletely understood. Previous studies focused on cellular mechanisms of resistance to imatinib. Another hypothesis for variable response lies in pharmacokinetic variability that may reduce drug exposure in patients receiving imatinib. Methods: Using high- performance liquid chromatography-tandem mass spectrometry, we assessed trough plasma imatinib concentrations at steady state (Cmin) in more than 150 CML patients. Major molecular response (MMR) was defined as a 3 logarithm reduction of BCR-ABL transcripts, quantified from peripheral blood using Q-RTPCR. In addition, suboptimal response criteria defined by the European Leukemianet were used to increase doses of imatinib. Results: i) we assessed Cmin in 85 CML patients and demonstated a correlation with imatinib doses (median values 813, 1,135, 1,709 ng/ml for respectively 300,400, 600 mg/day). ii) for 68 patients after at least one year's treatment mean Cmin were significantly higher in the group with MMR (34 patients) than in the group without (1,452.1±649.1 ng per milliliter vs. 869.3±427.5 ng per milliliter, P<0.001) whereas there was no difference in the imatinib daily dose. For Cmin and their discrimination potential for MMR, the area under receiver-operating characteristic curve was 0.775, with best sensitivity (76.5 percent) and specificity (70.6 percent) at a plasma threshold of 1,002 ng per milliliter. iii) dose escalation of imatinid, 400mg up to 600mg was proposed in 8 patients in suboptimal response (3 according to cytogenetic criteria and 5 according to molecular criteria). The median Cmin increase from 573 ng/ml [181–1,376] to 1,139 ng/ml [733–2,262] and 5 patients had a cytogenetic (n=3) or a molecular (n=2) response. Conclusions: Monitoring of imatinib plasma levels could be very useful for the management of CML patients, or should at least be checked in the case of treatment failure or suboptimal response or in patients nonadherent with imatinib or with a drug-drug interaction suspicion. No significant financial relationships to disclose.

A new homolog of FocA transporters identified in cadmium-resistant Euglena gracilis

To better understand the cellular mechanism of stress resistance to various pollutants (cadmium, pentachlorophenol), we undertook a survey of the Euglena gracilis transcriptome by mRNA differential display and cDNA cloning. We performed a real-time RT-PCR analysis upon four selected genes. One of them significantly changed its expression level in response to stress treatments: B25 gene was overexpressed in Cd-resistant cells whereas it was down-regulated in PCP-adapted cells. By Race assays we obtained for B25 a 1093 bp cDNA. The deduced protein was identified as a bacterial formate/nitrite transporter (FocA) homolog and the gene was named EgFth. From all the data, we concluded that EgFth overexpression was related to chronic exposure to cadmium.

Expression of major vault protein gene in osteosarcoma patients

Osteosarcoma (OS) is a primary malignant tumor of bone. Despite the successful use of multiple chemotherapeutic agents in the treatment of OS, more than 30% of OS tumors remain resistant to treatment. Elucidation of cellular resistance mechanisms may lead to better treatments for cancer patients. In this study, we used the low-density expression cDNA array, GEArray Q Series Human Cancer Drug Resistance and Metabolism Gene Array to screen genes related to drug resistance in 15 OS tumors. Expression patterns of the MPV gene were validated by real time PCR on 45 OS patient tumor samples and correlated with clinical and pathological data. Major vault protein (MVP) expression was present in 24 (53%) tumor samples and absent in 21 (47%). Samples from surgery showed correlation between the expression of MVP, metastatic disease at diagnosis and event free survival (EFS). The MVP gene expression correlates with metastatic disease at diagnosis after neoadjuvant chemotherapy (p=0.048), and is also associated with worse EFS (p=0.036). These findings suggest that MVP expression is involved in one of the mechanisms of drug resistance in OS and is induced by chemotherapy.

SEC-ICP-MS and ESI-MS as tools to study the interaction between cisplatin and cytosolic biomolecules

Most of cisplatin’s citotoxic properties are due to the interaction of the drug with DNA. However, other biological molecules present in the cell cytosol, such as MT (metallothionein) and GSH (glutathione), are potential targets for cisplatin and have been related to its side-effects or with the cellular resistance mechanisms to the drug. Experiments simulating physiological conditions have been performed to study the specific cisplatin metabolites which interact with GSH and MT and to characterize the different drug–biomolecule adducts over time. A combination of size exclusion chromatography-inductively coupled plasma mass spectrometry (SEC-ICP-MS) and electrospray ionization-mass spectrometry (ESI-MS) techniques has been used to provide sensible multi-elemental detection and structural information of the species of interest. Time dependent transformation of 10 μM cisplatin at neutral pH (7.4) produces different concentrations of the mono-aquo and oligomeric derivatives, as could be confirmed by ESI-MS. No di-aquo derivative was seen to be produced under these conditions at any of the incubation times used. Cisplatin and the oligomeric derivative were incubated with GSH and MT at different drug:biomolecule ratios. Adducts from cisplatin–GSH (1:500) and from cisplatin–MT (1:10) incubations were characterized by SEC-ICP-MS. While both GSH and MT reacted with cisplatin producing different compounds, only GSH reacted with the oligomeric derivative of cisplatin. SEC-ICP-MS experiments showed that, under neutral pH conditions, Cd atoms remained bound to the cisplatin:MT adducts, but Zn atoms were lost. Results were compared with those obtained by in vitro and in vivo experiments with rat kidney, liver and inner ear cytosols.

Impaired Glycogen Synthase Activity and Mitochondrial Dysfunction in Skeletal Muscle: Markers or Mediators of Insulin Resistance in Type 2 Diabetes?

Insulin resistance in skeletal muscle is a major hallmark of type 2 diabetes and an early detectable abnormality in the development of this disease. The cellular mechanisms of insulin resistance include impaired insulin-mediated muscle glycogen synthesis and increased intramyocellular lipid content, whereas impaired insulin activation of muscle glycogen synthase represents a consistent, molecular defect found in both type 2 diabetic and high-risk individuals. Despite several studies of the insulin signaling pathway believed to mediate dephosphorylation and hence activation of glycogen synthase, the molecular mechanisms responsible for this defect remain unknown. Recently, the use of phospho-specific antibodies in human diabetic muscle has revealed hyperphosphorylation of glycogen synthase at sites not regulated by the classical insulin signaling pathway. In addition, novel approaches such as gene expression analysis and proteomics have pointed to abnormalities in mitochondrial oxidative phosphorylation and cellular stress in muscle of type 2 diabetic subjects, and recent work suggests that impaired mitochondrial activity is another early defect in the pathogenesis of type 2 diabetes. This review will discuss the latest advances in the understanding of the molecular mechanisms underlying insulin resistance in human skeletal muscle in type 2 diabetes with focus on possible links between impaired glycogen synthase activity and mitochondrial dysfunction.

Mechanisms of Cellular Resistance to Camptothecins

The camptothecins are among the most promising antitumor agents endowed with a unique mechanism of action, because they act through inhibition of DNA topoisomerase I, an enzyme involved in regulating critical cellular functions including DNA replication, transcription and recombination. On the basis of the pharmacological interest of camptothecins in cancer chemotherapy, medicinal chemistry has played a crucial role in the development of novel analogs, and recently some compounds have emerged as promising agents for clinical evaluation. A major limitation to the clinical efficacy of camptothecin-containing therapies is represented by drug resistance. As with other cytotoxic drugs, clinical resistance to camptothecins may be a multifactorial phenomenon likely involving pharmacological and tumor-related factors. An additional problem in understanding clinically relevant resistance mechanisms is the observation that preclinical cell/tumor models may be not adequately predictive of clinical resistance. Here, we review the mechanisms of cell sensitivity/resistance to camptothecins and current approaches to overcome specific mechanisms, either by chemical modifications or by combination with modulating agents. In particular, the realization that most camptothecins are substrates for ATP binding cassette transporters has stimulated efforts in molecular design of novel non-cross-resistant analogs. Finally, a better understanding of the mechanism of cell response at a cellular level could help in defining new strategies to overcome resistance as well as chemical features required for efficacy.

Trough Plasma Imatinib Concentrations Are Associated with Both Cytogenetic and Molecular Responses to Standard-Dose Imatinib in Chronic Myeloid Leukemia.

BACKGROUNDVariable responses to imatinib, in the treatment of chronic myeloid leukemia (CML), are incompletely understood. Previous studies focused on cellular mechanisms of resistance to imatinib. Another hypothesis for variable response lies in pharmacokinetic variability that may reduce drug exposure in patients receiving imatinib.METHODSUsing high-performance liquid chromatography-tandem mass spectrometry, we assessed trough plasma imatinib concentrations in CML patients who responded or not to standard-dose imatinib, after at least one year's treatment. Complete cytogenetic response (CCR) was defined as 0 percent Philadelphia-chromosome-positive cells in bone marrow. Major molecular response (MMR) was defined as a 3 logarithm reduction of BCR-ABL transcripts, quantified from peripheral blood using real-time quantitative reverse-transcriptase polymerase chain reaction.RESULTSSixty-eight patients were included. Mean trough plasma imatinib concentrations were significantly higher in the group with CCR (56 patients) than in the group without (P=0.02). Mean (±SD) trough plasma imatinib concentrations were significantly higher in the group with MMR (34 patients) than in the group without (1452.1±649.1 ng per milliliter vs. 869.3±427.5 ng per milliliter, P<0.001) whereas there was no difference in the imatinib daily dose. For trough plasma imatinib concentrations and their discrimination potential for MMR, the area under receiver-operating characteristic curve was 0.775, with best sensitivity (76.5 percent) and specificity (70.6 percent) at a plasma threshold of 1002 ng per milliliter.CONCLUSIONSMonitoring of plasma imatinib concentrations may become part of standard management of CML patients, or should at the very least be checked in the case of treatment failure or suboptimal response.

Administered activity and metastatic cure probability during radioimmunotherapy of ovarian cancer in nude mice with 211At-MX35 F(ab′)2

To elucidate the therapeutic efficacy of alpha-radioimmunotherapy of ovarian cancer in mice. This study: (i) estimated the minimum required activity (MRA), giving a reasonable high therapeutic efficacy; and (ii) calculated the specific energy to tumor cell nuclei and the metastatic cure probability (MCP) using various assumptions regarding monoclonal-antibody (mAb) distribution in measured tumors. The study was performed using the alpha-particle emitter Astatine-211 (211At) labeled to the mAb MX35 F(ab')2. Animals were inoculated intraperitoneally with approximately 1 x 10(7) cells of the cell line NIH:OVCAR-3. Four weeks later animals were treated with 25, 50, 100, or 200 kBq 211At-MX35 F(ab')2 (n = 74). Another group of animals was treated with a nonspecific mAb: 100 kBq 211At-Rituximab F(ab')2 (n = 18). Eight weeks after treatment the animals were sacrificed and presence of macro- and microscopic tumors and ascites was determined. An MCP model was developed and compared with the experimentally determined tumor-free fraction (TFF). When treatment was given 4 weeks after cell inoculation, the TFFs were 25%, 22%, 50%, and 61% after treatment with 25, 50, 100, or 200 kBq (211)At-MX35 F(ab')2, respectively, the specific energy to irradiated cell nuclei varying between approximately 2 and approximately 400 Gy. As a significant increase in the therapeutic efficacy was observed between the activity levels of 50 and 100 kBq (TFF increase from 22% to 50%), the conclusion was that the MRA is approximately 100 kBq (211)At-MX35 F(ab')2. MCP was most consistent with the TFF when assuming a diffusion depth of 30 mum of the mAbs in the tumors.

Nematodes as Models for the Study of the Regulation of Activity of Pglycoproteins in Multidrug Resistance (MDR)

Alterations in drug transport are a major cause of chemotherapy failures due to nonspecific resistance mechanisms. These mechanisms involve several ABC transporter proteins among which the P-glycoproteins have been the most extensively studied in vertebrates. Increased Pgp expression and/or activity in organisms leads to resistance to many chemically unrelated compounds and therapeutic agents. Such resistance has been observed in cancer and in other diseases due to infectious and parasitic pathogens. Most of the available information on these transporters has been obtained from genetic analyses, although useful data for understanding the cellular mechanisms of such resistance remains limited. As resistance affects several types of organism comparative information on the function and activity of these cellular pumps in various biological environments would be welcome. As such, nematodes represent a possible model. Recent data have been obtained for efflux function and modulation by different molecules in these invertebrates. The observed data are summarized here and the future of these studies is discussed. Keywords: ABC transporters, P-glycoprotein, multidrug resistance, nematodes, efflux, pathogens, drugs, lipids