Multidrug resistance in leukemia

Novel insights on [1,2]oxazolo[5,4-e]isoindoles on multidrug resistant acute myeloid leukemia cell line

A series of [1,2]oxazolo[5,4‐e]isoindole derivatives was evaluated against HL‐60 cell line and its multidrug resistance (MDR) variant, HL‐60R, resistant to doxorubicin and to other P‐gp substrates by overexpressing the efflux pump. They displayed antiproliferative activities, with IC₅₀ values ranging from 0.02 to 5.5 µM. In particular, the newly synthesized compound 4k produced synergistic effects in terms of cell growth inhibition and cell death induction either in combination with a Vinca alkaloid, Vinblastine, and a Taxane, Paclitaxel in HL‐60R cells. The study of the mechanism of action indicated that all compounds showed antimitotic activity through inhibition of tubulin polymerization. Thus, [1,2]oxazoles could represent a valuable tool to overcome MDR mechanism, confirming the potential use of this class of compounds.

Molecular Action of Polyphenols in Leukaemia and Their Therapeutic Potential

Leukaemia is a malignant disease of the blood. Current treatments for leukaemia are associated with serious side-effects. Plant-derived polyphenols have been identified as potent anti-cancer agents and have been shown to work synergistically with standard chemotherapy agents in leukaemia cell lines. Polyphenols have multiple mechanisms of action and have been reported to decrease cell proliferation, arrest cell cycle and induce apoptosis via the activation of caspase (3, 8 and 9); the loss of mitochondrial membrane potential and the release of cytochrome c. Polyphenols have been shown to suppress activation of transcription factors, including NF-kB and STAT3. Furthermore, polyphenols have pro-oxidant properties, with increasing evidence that polyphenols inhibit the antioxidant activity of glutathione, causing oxidative DNA damage. Polyphenols also induce autophagy-driven cancer cell death and regulate multidrug resistance proteins, and thus may be able to reverse resistance to chemotherapy agents. This review examines the molecular mechanism of action of polyphenols and discusses their potential therapeutic targets. Here, we discuss the pharmacological properties of polyphenols, including their anti-inflammatory, antioxidant, anti-proliferative, and anti-tumour activities, and suggest that polyphenols are potent natural agents that can be useful therapeutically; and discuss why data on bioavailability, toxicity and metabolism are essential to evaluate their clinical use.

Clinical Significance of ABCB1 in Acute Myeloid Leukemia: A Comprehensive Study

ABCB1 is a member of the ATP binding cassette transporter family and high ABCB1 activity is considered as a poor prognostic factor in acute myeloid leukemia (AML) treated with intensive chemotherapy, its direct relation with drug resistance remains unclear. We evaluated ABCB1 activity in relation with clinical parameters and treatment response to standard chemotherapy in 321 patients with de novo AML. We assessed multiple clinical relationships of ABCB1 activity—ex vivo drug resistance, gene expression, and the ABCB1 inhibitor quinine were evaluated. ABCB1 activity was observed in 58% of AML and was linked to low white blood cell count, high expression of CD34, absence of FLT3-ITD, and absence of mutant NPM1. Moreover, ABCB1 activity was associated with worse overall- and event-free survival. However, ABCB1 activity did not directly lead to ex vivo drug resistance to anthracyclines. We found that ABCB1 was highly correlated with gene expressions of BAALC, CD34, CD200, and CD7, indicating that ABCB1 expression maybe a passenger characteristic of high-risk AML. Furthermore, ABCB1 was inversely correlated to HOX cluster genes and CD33. Thus, low ABCB1 AML patients benefited specifically from anti-CD33 treatment by gemtuzumab ozogamicin in addition to standard chemotherapy. We showed prognostic importance of ABCB1 gene expression, protein expression, and activity. Furthermore, ABCB1 was not directly linked to drug resistance, ABCB1 inhibition did not improve outcome of high ABCB1 AML patients and thus high ABCB1 may represent a passenger characteristic of high-risk AML.

Downregulation of RLIP76 is associated with vincristine resistance in human colorectal cancer HCT-8/VCR cells

RLIP76 is an anti-apoptotic transporter, participating in the multi-specific drug transport and resistance. In the absence of chemotherapy drugs, the knockout or inhibition of RLIP76 leads to pronounced tumor regression. RLIP76 transports anthracycline and vinca alkaloid drugs and mediates the resistance to these drugs. However, functions of RLIP76 in drug resistance colorectal cancer remain unclear. HCT-8 and the vincristine (VCR)-resistant colorectal cancer cell line HCT-8/VCR (HCT-8/V) were used in the present study. The effects of RLIP76 knockdown by the lentivirus were examined in cultured cells, including growth, apoptosis, invasion, and signaling pathways by qRT-PCR, western blot analysis and transwell assay. The relative level of RLIP76 in HCT-8 and HCT-8/V was assessed by western blot analysis, finding RLIP76 was overexpressed in HCT-8/V. Then, HCT-8/V cancer cells were transfected with lentivirus encoding RLIP76-specific shRNA (KD) and the control (NC), and no significant difference of RLIP76 level between the NC cells and cells without transfection was found, but the relative mRNA level decreased to 0.277±0.016 and protein level also reduced in KD cells. Cell functions changed after RLIP76 knockdown in HCT-8/V. The IC50 of VCR decreased from 164.4±1.734 to 13.95±2.008 (µg/ml) (p<0.05) in cell culture. The cell number reduced from 329.67±20.23 to 176.33±2.52 (p<0.05) in migration assay and from 294.67±30.07 to 153±22.11 (p<0.05) in invasion assay. Moreover, apoptotic proteins, including cleaved-caspase-8, cleaved-caspase-9, cleaved-Parp and Bax increased. The phosphorylation level of Erk also reduced significantly. The present study showed that RLIP76 is a key effector of cancer cell survival, invasion, and migration and possibly an important target to improve drug resistance and tumor treatment.

The network of P-glycoprotein and microRNAs interactions

Overexpression of P-glycoprotein (P-gp) contributes to the multidrug resistance (MDR) phenotype found in many cancer cells. P-gp has been identified as a promising molecular target, although attempts to find successful therapies to counteract its function as a drug efflux pump have largely failed to date. Apart from its role in drug efflux, P-gp may have other cellular functions such as being involved in apoptosis, and is found in various locations in the cell. Its expression is highly regulated, namely by microRNAs (miRNAs or miRs). In addition, P-gp may regulate the expression of miRs in the cell. Furthermore, both P-gp and miRs may be found in microvesicles or exosomes and may be transported to neighboring, drug-sensitive cells. Here, we review this current issue together with recent evidence of this network of interactions between P-gp and miRs.

Multidrug resistance in relapsed acute myeloid leukemia: evidence of biological heterogeneity

BACKGROUND

Studies of mechanisms mediating resistance to chemotherapy led to the discovery of the multidrug transporter ABCB1 (ATP-binding cassette, subfamily B, member 1), often expressed in leukemic cells of patients with acute myeloid leukemia (AML). Most clinical trials evaluating the strategy of inhibiting efflux-mediated chemotherapeutic resistance have been unsuccessful, clearly indicating the need for a better approach.

METHODS

This study investigated the clinical relevance of 380 genes whose expression has been shown to affect the response to chemotherapy, mostly through in vitro studies, in 11 paired samples obtained at AML diagnosis and at relapse. The expression profiling of these 380 genes was performed using TaqMan-based quantitative reverse-transcription polymerase chain reaction. Patients had a median age of 58 years at diagnosis, a median duration of complete remission of 284.5 days, and a median overall survival of 563 days. Cytogenetic abnormalities were detected at diagnosis in 4 patients, whereas 5 displayed a normal karyotype and 2 were not investigated.

RESULTS

Hierarchical clustering shows that samples taken at diagnosis and relapse clustered in pairs for 6 patients of the 11 studied, suggesting recurrence of the same leukemic blast, whereas for the other 5 patients, the data indicate their relapse blasts arose from different origins. A patient-by-patient analysis of the paired samples led to the striking observation that each had a unique gene signature representing different mechanisms of resistance.

CONCLUSIONS

The data underline the need for personalized molecular analysis to tailor treatment for patients with AML.

GABAA receptor-binding protein promotes sensitivity to apoptosis induced by chemotherapeutic agents

In the present study, the expression of human γ-aminobutyrate type A (GABAA) receptor-binding protein (GABARBP) is downregulated in ovarian cancer cell lines and tissues. We also found that the specific function of GABAPBP was that of a novel pro-apoptotic protein. Both GABARBP and cisplatin suppressed cancer cell proliferation in a concentration-dependent manner. The combined treatment of GABARBP and cisplatin was more effective in inhibiting cell growth, as well as cell migration, than with either drug treatment alone. At the same time, the treatment combination is correlated with the downregulation of cyclin D1 and CDK4, arrested cell cycle progression in the G₀-G₁ phase and enhancing p53 expression, while also reducing Bcl-2 and Bcl-xL expression. The p53 and p21 promoter luciferase activities were induced by GABARBP, whereas there was no effect on the p53-/- and p21-/- system. In addition, p53 activity was validated with UV irradiation and siGABARBP. Taken together, our results indicate that GABARBP can regulate the pro-apoptotic activity of cisplatin via the upregulation of p53 expression.

Drug resistance: still a daunting challenge to the successful treatment of AML

Resistance to chemotherapy remains a challenging issue for patients and their physicians. P-glycoprotein (Pgp, MDR1, ABCB1), as well as a family of structurally and functionally related proteins, are plasma membrane transporters able to efflux a variety of substrates from the cell cytoplasm, including chemotherapeutic agents. The discovery of ABCB1 made available a potential target for pharmacologic down-regulation of efflux-mediated chemotherapy resistance. In patients with acute myeloid leukemia (AML), a neoplasm characterized by proliferation of poorly differentiated myeloid progenitor cells, leukemic cells often express ABCB1 at high levels, which may lead to the development of resistance to chemotherapy. Thus, AML seemed to be a likely cancer for which the addition of drug efflux inhibitors to the chemotherapeutic regimen would improve outcomes in patients. Despite this rational hypothesis, the majority of clinical trials evaluating this strategy have failed to reach a positive endpoint, most recently the Eastern Cooperative Oncology Group E3999 trial. Here we review data suggesting the importance of ABCB1 in AML, address the failure of clinical trials to support a therapeutic strategy aimed at modulating ABCB1-mediated resistance, and consider the type of research that should be conducted in this field going forward.

Analysis of the interaction of induction regimens with p-glycoprotein expression in patients with acute myeloid leukaemia: results from the MRC AML15 trial

Retrospective analyses in non-randomised cohorts suggest that regimens containing fludarabine/Ara C and/or idarubicin/ara C may be more effective than daunorubicin/AraC (DA)-containing regimens in cases of acute myeloid leukaemia (AML) overexpressing p-glycoprotein (Pgp). We prospectively measured Pgp protein and function by flow cytometry in CD45-gated blasts from 434 AML15 trial patients randomised to remission induction therapy with two courses of FLAG-Ida or DA±etoposide (DA/ADE). In all, 34% were positive for Pgp protein and 38% for function. Pgp protein-positive cases had a higher incidence of resistant disease (14% vs 5%), adjusted odds ratio 2.67 (1.14–6.24). There was a trend towards a higher cumulative incidence of relapse at 5 years for Pgp-positive cases (46% vs 55%), adjusted hazard ratio 1.42 (0.98–2.07) (P=0.06). For patients treated with FLAG-Ida, the complete remission (CR) rate was 86% for both Pgp-positive and Pgp-negative patients. In patients treated with DA/ADE, 78% of Pgp-positive and 90% of Pgp-negative cases achieved CR (P=0.06). In analyses of overall survival, there was no interaction between treatment received and Pgp expression. Data for Pgp function followed similar trends. Our data suggest that FLAG-Ida may improve the remission rate for Pgp-positive AML, but the malignant clone is reduced rather than eradicated such that the relapse rate remains high in Pgp-positive patients.

Sequential influences of leukemia-specific and genetic factors on p-glycoprotein expression in blasts from 817 patients entered into the National Cancer Research Network acute myeloid leukemia 14 and 15 trials

PURPOSE

P-glycoprotein (Pgp) is a major prognostic factor for chemotherapy failure in acute myeloid leukemia (AML). This study compared the influence of genetic and leukemia-specific factors on Pgp.

EXPERIMENTAL DESIGN

Eight hundred and seventeen samples were studied prospectively for Pgp protein expression and function and G1199A, G2677T, and C3435T polymorphisms in the encoding gene ABCB1.

RESULTS

Age, low WBC count, high bcl-2, secondary AML and myelodysplastic syndrome, and adverse cytogenetics all correlated strongly with high Pgp (MRK16) protein expression. However, ABCB1 3435TT homozygosity was negatively correlated with Pgp. Pgp protein is only expressed in 41% of samples such that the negative effect of the polymorphism was not seen at baseline Pgp levels but was marked in the upper 41% of samples (MRK16 Deltamean fluorescence intensity of 75th centile sample = 9 units for TT variant samples and 26 units for CC/CT; P = 0.003). However, no association was found between genetic factors and Pgp function using rhodamine 123 accumulation.

CONCLUSIONS

The genetic polymorphism 3435TT (which results in unstable mRNA) has a significant effect on Pgp expression, but this is only seen in approximately 40% of cases in which mRNA and protein are detectable. Moreover, leukemia-specific factors, such as low WBC count and poor risk cytogenetics, have a much greater effect than genetic polymorphisms on Pgp expression in AML blasts.

Total Synthesis of Dendroamide A: Oxazole and Thiazole Construction Using an Oxodiphosphonium Salt

The total synthesis of dendroamide A (1), a multidrug-resistance reversing bistratamide-type peptide-derived macrocycle, has been accomplished in 19% yield. Fmoc-protected amino acids were condensed into appropriately protected dipeptides which were treated with bis(triphenyl)oxodiphosphonium trifluoromethanesulfonate to afford oxazoles and thiazolines (oxidized to thiazoles) with high chemo- and stereoselectivity. The convergent condensation of three heterocyclic amino acids followed by macrocyclization afforded the natural product.

Resistance to spontaneous apoptosis in acute myeloid leukaemia blasts is associated with p-glycoprotein expression and function, but not with the presence of FLT3 internal tandem duplications

The ability of acute myeloid leukaemia (AML) blasts to survive in culture has been associated with poor patient response to chemotherapy. Other biological factors predicting an adverse outcome include p-glycoprotein (pgp) expression, which is associated with a reduced remission rate, and the presence of fms-like tyrosine kinase 3 gene (FLT3) internal tandem duplications (ITDs), predictive of a high rate of leukaemic relapse. Our previous work has indicated a drug efflux-independent role for pgp in apoptosis resistance. We measured spontaneous in vitro apoptosis in 58 primary AML samples to establish its relationship with functional and phenotypic pgp and with FLT3 ITDs. Cells were incubated for 48 h in a suspension culture, and the remaining viable cells were counted by flow cytometry. Median survival was 38% of baseline values. Resistance to spontaneous apoptosis was strongly associated with pgp (MRK-16 antibody) expression (P = 0.001) and with pgp functional activity (P < 0.001). FLT3 ITDs, found in 20 cases, were inversely associated with functional pgp activity: thus, the median pgp modulation ratio was 2.0 in FLT3 wild-type cases and 1.38 in ITD cases (P = 0.018). Also, the presence of FLT3 ITDs was not associated with in vitro apoptosis resistance. In conclusion, we have found that the presence of FLT3 ITDs is not related to AML blast survival in vitro, and is inversely associated with pgp activity, whereas pgp expression and activity are associated with resistance to spontaneous apoptosis. These results may help to explain the differing adverse effects of pgp (on remission induction) and FLT3 ITDs (on relapse) in AML.

Up-regulation of MDR1 function and expression by cisplatin in LLC-PK1 cells

To examine whether cisplatin affects the multidrug transporter MDR1/P-glycoprotein in the kidneys, the effects of cisplatin on cell sensitivity to an anticancer drug, MDR1 function and expression were examined by assessing the growth inhibition by the MDR1 substrate paclitaxel, the uptake and efflux of the MDR1 substrate Rhodamine123 and the level of MDR1 mRNA, respectively. Porcine kidney epithelial LLC-PK1 cells were used, as they have a structure and function similar to those of renal proximal tubular cells and physiologically express low levels of MDR1. The growth inhibitory curve of LLC-PK1 cells by paclitaxel was shifted to a higher concentration range by pretreatment with 1 micro M cisplatin for 48 h. The uptake and efflux of Rhodamine123 were significantly reduced and enhanced, respectively, by pretreatment with 1 micro M cisplatin for 48 h. This enhanced efflux was suppressed by the representative MDR1 substrate/inhibitor ciclosporin. The expression of MDR1 mRNA was increased by the existence of cisplatin for 48 h. These observations taken together suggested that the transient exposure to cisplatin could cause the up-regulation of MDR1 in LLC-PK1 cells.

Cytotoxic effects of 27 anticancer drugs in HeLa and MDR1-overexpressing derivative cell lines

The cytotoxic effects of 27 anticancer drugs including amrubicin, vinorelbine, paclitaxel, docetaxel, gemcitabine, and irinotecan were evaluated in human cervical carcinoma HeLa cells, and drug-resistant HeLa-derived Hvrl-1, HvrlO-6, and Hvr100-6 cells, which were newly established by stepwise exposure to vinblastine. FACS and RT-PCR analysis indicated that MDR1 (P-glycoprotein) was induced without any alterations in expression of its related transporters. Hvrl00-6 cells showed 2- to 200-fold higher resistance to anthracyclines than HeLa cells, and unexpectedly showed slight resistance to idarubicin and amrubicin. The relative resistance to vinca-alkaloids was 300- to 600,000-fold, and HvrlOO-6 cells showed the highest relative resistance to vinorelbine. HvrlOO-6 cells also showed 4000- and 60000-fold resistance to the taxanes paclitaxel and docetaxel, respectively. Hvr100-6 cells were also resistant to 6-mercaptopurine, actinomycin D, etoposide, and mitomycin C, with relative resistance of 8-, 45000-, 12-, and 9-fold, respectively. In contrast, HvrlOO-6 cells showed no or slight resistance to platinum derivatives, pyrimidine analogues, and alkylating agents or to irinotecan and its active form, or tamoxifen. The cytotoxicity of anthracyclines, vinca-alkaloids, taxanes, actinomycin D, and etoposide was extensively reversed by cyclosporin A. Cyclosporin A had no effect on the cytotoxicity of 6-mercaptopurine or mitomycin C, suggesting that resistance to these drugs was not mediated via MDR1. The alterations in cytotoxicity by overexpression of MDR1 and effects of cyclosporin A could be also qualitatively explained by [3H]vinblastine uptake experiments. The 27 anticancer drugs analyzed here could be classified into substrates and nonsubstrates for MDR1. This will be useful for designing effective regimens for chemotherapy.

The search for optimal treatment in relapsed and refractory acute myeloid leukemia

Despite the significant progress in the treatment of AML during the last 5-10 years, 20-40% of patients still do not achieve remission with standard induction therapy. In addition, 50-70% of patients in CR are likely to relapse. A major limitation of successful AML therapy is intrinsic or acquired drug resistance. Several pharmacological inhibitors of mechanisms inducing chemoresistance in leukemic cells have been investigated. New cytotoxic drugs, agents with novel mechanisms of action, and new treatment strategies are currently being investigated. The management of refractory or relapsed AML patients is reviewed in this study.

Mitochondrial membrane sensitivity to depolarization in acute myeloblastic leukemia is associated with spontaneous in vitro apoptosis, wild-type TP53, and vicinal thiol/disulfide status

Nonresponse to remission-induction chemotherapy, which remains a major problem in acute myeloblastic leukemia (AML), has been linked to cellular resistance to apoptosis. Because the apoptosis induced by chemotherapeutic drugs is mediated by loss of mitochondrial transmembrane potential (MTP), it was postulated that sensitivity to mitochondrial membrane depolarization might be heterogeneous in AML. Using the uncoupling agent carbonyl cyanide m-chlorophenylhydrazone (mClCCP), the mitochondrial membrane sensitivity to depolarization (mClCCP concentrations that inhibit 50% of the transmembrane potential [IC(50)]) in AML blasts was measured and demonstrated marked interclonal heterogeneity, with the existence of comparatively sensitive (median mClCCP IC(50), 4 microM) and resistant (median mClCCP IC(50), 10 microM) clones. Furthermore, the mClCCP IC(50) was inversely associated with spontaneous in vitro apoptosis (P =.001). It was high in cases with mutant TP53 and correlated with the total cellular level of the multidrug resistance-associated protein (P =.019) but not of bcl-2, bax, or bcl-x. It was also found that the dithiol oxidant diamide, in contrast to the monovalent thiol oxidant diethyl maleate, increased the sensitivity of mitochondrial membranes to mClCCP. To confirm that TP53 directly affects MTP in leukemic cells and to establish the role of vicinal thiol oxidation in the TP53-dependent pathway, CEM 4G5 leukemia cells with forced, temperature-dependent expression of TP53 were studied. Monobromobimane, which inhibits mitochondrial membrane depolarization by preventing dithiol cross-linking, inhibited depolarization and apoptosis in 4G5 cells. It was concluded that in leukemia, TP53 and vicinal thiol/disulfide status are determinants of mitochondrial membrane sensitivity to depolarization, which is in turn associated with spontaneous apoptosis.

Intracellular P-gp contributes to functional drug efflux and resistance in acute myeloid leukaemia

Drug compartmentalization as well as drug efflux can contribute to drug resistance. We demonstrate the presence of P-gp in intracellular vesicles in certain AML cell lines and show localization of DNR to a similar subcellular compartment(s) that can be altered in the presence of P-gp inhibitors. Analysis of leukaemic cell lines and 50 AML patient samples showed that the level of P-gp mRNA or total P-gp protein correlated better with drug efflux than surface P-gp protein, suggesting that intracellular P-gp may contribute to MDR in AML. Therefore, the level of total P-gp protein or mRNA may be a better indicator of MDR than surface P-gp protein. In addition, we provide evidence for a novel mechanism of drug sequestration in K562 myeloid leukaemic cells.

Pharmacological basis for cladribine resistance in a human acute T lymphoblastic leukaemia cell line selected for resistance to etoposide

Cross-resistance between different classes of anti-neoplastic agents can jeopardize successful combination cancer chemotherapy. In this study, we observed an unexpected cross-resistance between the podophyllotoxine derivative etoposide (VP) and the nucleoside analogue cladribine (CdA) in CCRF-CEM cells developed for resistance to VP. The resistant cells also displayed 14- and twofold resistance to cytarabine (ara-C) and gemcitabine respectively. Closer analysis of these cells showed that they contained lower amounts of topoisomerase (topo) IIalpha (P < 0.001) and beta protein (P < 0.026), formed substantially lower amounts of the topo II-DNA complex, and had a markedly decreased level of Fas (CD95/APO-1)-ligand mRNA expression. Interestingly, Fas expression in the resistant cells did not differ from that in the parental cell line. No differences were observed in the accumulation/efflux of daunorubicin or in the gene expressions of P-glycoprotein, multidrug resistance-associated protein and the lung resistance-related protein. The activity of deoxycytidine kinase (dCK), responsible for activation of CdA and ara-C, was the same for resistant and wild-type cells. However, there was an increase in the activity of the cytosolic 5'-nucleotidases (5'-NT), responsible for deactivation of nucleotides, amounting to 206% (P < 0.001) for the high Km and 134% (P < 0.331) for the low Km 5'-NT in resistant cells. The high Km 5'-NT is probably responsible for the decreased amount of the active metabolite CdA 5'-triphosphate [40% decreased (P < 0.045)], as well as for other purine ribonucleosides and deoxyribonucleosides triphosphates in the resistant cells. In contrast, a significantly higher deoxycytidine triphosphate (dCTP) level (167%, P < 0.001) was observed in the resistant cells. Thus, this study suggests that the major cause of resistance to the nucleoside analogues CdA and ara-C in cells selected for resistance to VP is a result of metabolic alterations producing increased activity of 5'-NT and higher dCTP levels. Furthermore, these results indicate that there is a common factor in the regulation of nucleotide-degrading enzymes and DNA topoisomerases, which may be altered in cross-resistant cells.

Total Synthesis of Dendroamide A, a Novel Cyclic Peptide That Reverses Multiple Drug Resistance

Dendroamide A (1) was isolated from a blue-green alga on the basis of its ability to reverse drug resistance in tumor cells that overexpress either of the transport proteins, P-glycoprotein or MRP1. Because of this activity, methods for the synthesis of analogues of this oxazole- and thiazole-containing cyclic peptide have been developed, and the total synthesis of 1 has been completed. Highlights of the synthetic strategy are as follows: (1) a dicyclohexylcarbodiimide coupling of D-Ala and L-Thr, followed by reaction with Burgess reagent and DBU-assisted oxidation to form D-Ala-oxazole; (2) formation of D-Val-thiazole and D-Ala-thiazole via modified Hantzsch reactions; and (3) use of molecular modeling to select the preferred precursor for the final cyclization of the peptide analogue. Synthetic 1 demonstrated spectral properties identical to those of the natural product and reversed P-glycoprotein-mediated drug resistance more effectively than MRP1-mediated resistance. Certain of the synthetic precursors had biological activity, indicating that cell permeability and peptide cyclization are necessary for optimal activity. Thus, the structure and the biological activities of the natural product are confirmed, and methods for the synthesis of analogues for further structure-activity explorations are defined.

The importance of drug-transporting P-glycoproteins in toxicology

The importance of specific transport in toxicology is becoming increasingly clear and the work on P-glycoprotein has certainly been a major contribution to these growing insights. P-Glycoproteins were discovered by their ability to confer multidrug resistance in mammalian tumour cells. They are localised in the cell membrane where they actively extrude a wide range of compounds including many anti-cancer drugs from the cell. Besides in tumour cells, drug-transporting P-glycoproteins are also expressed in a polarised fashion in normal tissues that perform an excretory or barrier function, such as the liver, kidneys, intestines, brain endothelial cells. Based on this expression profile, it has been proposed that P-glycoproteins are important in protecting the host by reducing exposure to xenobiotics. Further studies with P-glycoprotein knockout mice have clearly established this protective function. In general, the clearance of substrate drugs is lower in knockout mice due to a diminished hepatobiliary excretion, direct intestinal excretion and/or increased enterohepatic cycling. Moreover, their uptake in sanctuary sites, such as the brain or the foetus, was profoundly higher in P-glycoprotein knockout mice, as was the uptake of drugs from the gastro-intestinal tract into the systemic circulation following oral ingestion. These results clearly highlight the impact that transport proteins can play in toxicology.

Retrovirus insertion and transcriptional activation of the multidrug-resistance gene in leukemias treated by a chemotherapeutic agent in vivo

To understand the molecular basis for multidrug-resistant (MDR) cancer cells in vivo, this study analyzed molecular changes of the mdr1a gene region in leukemia cells in mice during continuous treatment with vincristine. An inverse insertion of murine leukemia retrovirus (MuLV) into the 5'-flanking region of the mdr1a gene was found. This insertion was concomitantly accompanied by up-regulation of the mdr1a gene and the loss of chemosensitivity. Deletion of long-terminal repeat (LTR) sequences dramatically decreased the mdr1a promoter-driven reporter activity. The MuLV LTR insertion appears to exert its enhancer activity on mdr1a transcription during the appearance of MDR leukemia cells. Two mechanisms were postulated to explain the mdr1a gene activation by retrovirus insertion during in vivo chemotreatment: de novo insertion of MuLV induced by vincristine treatment and selection of a small fraction of pre-existing cells carrying MuLV insertion during vincristine treatment. No rearranged sequence was detected by polymerase chain reaction in parental cells. This result argued for the first mechanism. The randomly altered distribution of MuLV during repetitive chemotreatment might also be consistent with this hypothesis. On the other hand, the retrovirus insertion was detected at the same site of the mdr1a promoter region in 2 independent experiments, which suggests the second mechanism. It should be noted that in vivo chemotreatment using vincristine could generate the mdr1a-overexpressing cells through retrovirus insertion and the enhancer effect of the LTR.

Dominant effector genetics in mammalian cells

We have expressed libraries of peptides in mammalian cells to select for trans-dominant effects on intracellular signaling systems. As an example-and to reveal pharmacologically relevant points in pathways that lead to Taxol resistance-we selected for peptide motifs that confer resistance to Taxol-induced cell death. Of several peptides selected, one, termed RGP8.5, was linked to upregulation of expression of the gene ABCB1 (also known as MDR1, for multiple drug resistance) in HeLa cells. Our data indicate that trans-dominant effector peptides can point to potential mechanisms by which signaling systems operate. Such tools may be useful in functional genomic analysis of signaling pathways in mammalian disease processes.

Drug resistance factors in acute myeloid leukemia: a comparative analysis

To compare the clinical relevance of drug resistance factors in de novo acute myeloid leukemia (AML), we determined their relationship to both response to induction chemotherapy and survival of the patients in univariate as well as multivariate analyses. The drug resistance factors immunocytochemically studied in 111 patients at the time of diagnosis included the lung resistance protein (LRP), P-glycoprotein (P-gp), multidrug resistance protein (MRP1) and bcl-2. In the univariate analyses, age (P = 0.005), karyotype (P = 0.03), LRP (P = 0.003), P-gp (P = 0.02) and bcl-2 (P = 0.03) predicted for response to induction chemotherapy, whereas MRP1 had no predictive value. Age (P = 0.05), karyotype (P = 0.05) and LRP (P = 0.03) retained their predictive value in the multivariate logistic regression analyses. With regard to overall survival, age (P = 0. 008), karyotype (P = 0.006), LRP (P = 0.001) and P-gp (P = 0.01) were of prognostic value in the univariate Cox regression analyses but only age (P = 0.01), karyotype (P = 0.02) and LRP (P = 0.01) retained their prognostic significance in the multivariate analyses. A risk score based on the number of independent prognostic factors allowed division of patients into four groups with different outcome. In these groups, the complete remission rates were 93%, 75%, 47% and 33%, respectively, and median overall survival was 2.4, 1.2, 0.6 and 0.2 years, respectively. Thus, several drug resistance factors did predict outcome in the univariate analyses but LRP was the only drug resistance factor with independent predictive and prognostic significance. The proposed risk score might be useful for risk-adapted treatment in the future. Leukemia (2000) 14, 68-76.