Lack of P-glycoprotein induction by rifampicin and phenobarbital in human lymphocytes.

The efficacy of drugs acting within lymphocytes depends on their intracellular concentrations, which could be modulated by membrane efflux transporters including P-glycoprotein (P-gp), encoded by the MDR1 gene. In particular, P-gp induction may compromise the efficacy of its substrates. Rifampicin and phenobarbital have been shown to induce P-gp in hepatic and intestinal cells through the activation of the nuclear receptors PXR and CAR. However, controversial data exist in human lymphocytes. We investigated the effect of these drugs on P-gp activity and expression in lymphocytes in vitro and ex vivo. CCRF-CEM cells and peripheral blood mononuclear cells (PBMCs) from healthy volunteers were incubated in the presence of rifampicin, phenobarbital, or without any drug. P-gp activity was measured by flow cytometry using DiOC(6) efflux. MDR1, PXR and CAR mRNA expression were measured by quantitative RT-PCR. Neither P-gp activity nor MDR1 mRNA expression were modified by rifampicin or phenobarbital both in CCRF-CEM cells and PBMCs. Moreover, P-gp protein expression at the membrane was neither detectable nor induced. The very weak PXR and CAR mRNA expression levels in these cells could partly explain these results. Therefore, P-gp induction by rifampicin and phenobarbital may play a negligible role in drug interactions occurring within lymphocytes.

[A negative first-line work-up of Raynaud's phenomenon: And what if it were cancer?]. / Phénomène de Raynaud avec négativité du bilan de première intention : et si c'était un cancer ?

Raynaud's phenomenon is a transient paroxysmal vasomotor phenomenon affecting the extremities including manifestations of ischemia. It is a common phenomenon in the general population. In a routine clinical situation, the first step is to differentiate Raynaud's disease from a secondary Raynaud's phenomenon, the latter requiring complementary investigations. We report here the case of an 80-year-old woman who presented a secondary Raynaud's phenomenon. First-line investigations remained negative. A mammography was performed and revealed breast cancer. Raynaud's phenomenon disappeared after treatment of the breast carcinoma and did not recur during the 2-year follow-up.

High levels of P-glycoprotein activity in human lymphocytes in the first 6 months of life.

P-glycoprotein (P-gp) is an efflux transporter that controls the intracellular concentrations of drugs. Human development may modulate P-gp function. We investigated the effect of age on P-gp activity and MDR1 gene expression in lymphocytes. We also assessed the influence of human immunodeficiency virus (HIV) infection. We used 3,3'-diethyloxacarbocyanin iodide (DiOC(6)) efflux, estimated by flow cytometry, to quantify P-gp activity in 94 children (age range, 0-18 years) and 25 adults. MDR1 gene expression was quantified using reverse transcription-PCR (RT-PCR). In T and natural killer (NK) cell populations, P-gp activity peaked at birth, decreased between the ages of 0 and 6 months, and stabilized between the ages of 6 months and 2 years (P < 10(-6)). These maturation profiles were also strongly correlated (r = 0.67, P < 10(-6)). HIV infection did not affect P-gp activity in the lymphocytes of children. MDR1 gene expression was not influenced by age, nor was it correlated with P-gp activity. The high levels of P-gp activity observed in the lymphocytes of children ~6 months of age may affect the efficacy of intracellular drugs.

Valproic acid inhibits proliferation and induces apoptosis in acute myeloid leukemia cells expressing P-gp and MRP1.

The multidrug resistance (MDR) phenotype, induced by the overexpression of several ABC transporters or by antiapoptotic mechanisms, has been identified as the major cause of drug resistance in the treatment of patients with acute myeloid leukemia (AML). In this study, we have shown that valproic acid (VPA) (a histone deacetylase inhibitor) can inhibit the proliferation of both P-glycoprotein (P-gp)- and MDR-associated protein 1 (MRP1)-positive and -negative cells. VPA also induced apoptosis of P-gp-positive cells. VPA induced apoptosis in K562 cells led to decrease in Flip (FLICE/caspase-8 inhibitory protein) expression with Flip cleavage, which could not be observed in HL60 cells. In HL60/MRP cell line, which proved to be resistant to apoptosis by VPA, we observed an abnormal expression of apoptotic regulatory proteins, overexpression of Bcl-2 and absence of Bax. Also, the Bcl-2 antagonist HA14-1 rapidly restored apoptosis in this cell line. Cotreatment with cytosine arabinoside induced very strong apoptosis in both K562/DOX and HL60/DNR cell lines. VPA also induced apoptosis in AML patient cells expressing P-gp and/or MRP1. Our findings show VPA as an interesting drug that should be tested in clinical trials for overcoming the MDR phenotype in AML patients.

JC-1: a very sensitive fluorescent probe to test Pgp activity in adult acute myeloid leukemia.

One of the best-characterized resistance mechanisms in acute myeloid leukemia (AML) is the drug extrusion mediated by P-glycoprotein (Pgp). Recently the results of workshops organized by several groups concluded that accurate measurement of low activity of Pgp is a difficult goal in clinical samples. Therefore, highly sensitive and specific assays were developed to assess the functionality of Pgp using JC-1, a fluorescent molecule with the different emission wavelength (green and red fluorescence) according to its concentration in 129 AML samples. It was shown that JC-1 (green and red bands) may define 3 groups of patients: resistant (R) (29% of patients), intermediate (I) (36%), and sensitive (S) (35%). In contrast, rhodamine 123 assay detected only the R group defined by JC-1. Nevertheless, the I group has an intermediate expression of Pgp (0.39, 0.29, and 0.19 for the R, I, and S groups, respectively, P =.002), an intermediate biologic profile (percentage of CD34, 95%, 67%, and 44%, respectively, P <.0001; in vitro resistance to daunorubicin, 94 microM, 20 microM, and 12 microM, respectively, P =. 02), and an intermediate prognosis (achievement of complete remission, 55%, 65%, and 87%, P =.006; 3-year disease-free survival, 11%, 16%, and 36%, respectively, P =.005; and 3-year overall survival, 0%, 20%, and 51%, respectively, P <.0001). Therefore, JC-1 appeared to be a more convenient and simple way to detect a functional Pgp in clinical AML samples than rhodamine 123. (Blood. 2001;97:502-508)

The immunophenotype of 177 adults with acute myeloid leukemia: proposal of a prognostic score.

In acute myeloid leukemia (AML) patients, a variety of clinical and biologic parameters, including phenotype, have been examined for potential value in predicting treatment response and survival. The European Group for the Immunological Classification of Leukaemias (EGIL) has proposed that AML be defined immunologically by the expression of 2 or more of the following myeloid markers: myeloperoxidase, CD13, CD33, CDw65, and CD117. With regard to this classification, the prognostic significance of 21 antigens taken separately and with immunophenotypic subgroups were evaluated and compared with other clinical and biological variables in 177 adult AML patients. None of the antigens tested were associated with treatment outcome. In contrast, patients with blasts disclosing a full expression of panmyeloid phenotype (defined by the expression of all 5 myeloid markers) had a higher complete remission rate (P <. 0001) and differed significantly in disease-free survival (P =.02) and overall survival (P =.008) than patients whose cells expressed fewer than 5 of these markers. In multivariate analysis, only age, panmyeloid phenotype, performance status, and permeability glycoprotein activity influence treatment outcome. Cytogenetics was significant in univariate analysis but not in multivariate analysis, most likely because of the redundancy with panmyeloid phenotype and a higher sensitivity of immunophenotyping. Patients whose cells exhibit the panmyeloid phenotype appear to define a relatively homogeneous biological subset of AML. The 4 independent prognostic factors were used to create a prognostic score, defined by the number of factors present. This score permitted a stratification of patients with AML, thereby allowing for the consideration of innovative therapies to improve outcome in the poorer outcome groups.

Mantle cell lymphoma, in leukaemic phase with prominent splenomegaly. A report of eight cases with similar clinical presentation and aggressive outcome.

Mantle cell lymphoma (MCL) is a well-defined peripheral B-cell lymphoma usually diagnosed upon peripheral lymph node biopsy. We report eight cases of peripheral B-cell leukaemia that demonstrate presumptive evidence of mantle cell characteristics. The patients had a median age of 68.5 years, and five were male. All presented with an enlarged spleen without any peripheral lymphadenopathies, and they were leukaemic at presentation (median lymphocytosis, 38x10(9)/l). Morphological diagnosis of MCL was very difficult in five cases but easier in three because we were able to analyse either pre- or post-mortem lymph nodes and spleen. The immunophenotype of blood lymphocytosis using flow cytometry, the presence of a t(11;14)(q13;q32) and a cyclin D1 expression by leukaemic cells all fit with the diagnosis of MCL. All patients progressed and died with a median overall survival of 8 months. Multifocal areas of transformation in blastoid or large cell variants were observed in the three autopsied patients. In summary, one should consider the diagnosis of MCL at presentation in leukaemic phase even in the absence of peripheral adenopathies.

Both Pgp and MRP1 activities using calcein-AM contribute to drug resistance in AML.

Thirteen cell lines with different levels of Pgp and MRP1 expression were used to assess the ability of calcein-AM uptake and calcein efflux to measure Pgp and MRP1 functions, respectively. There was a good correlation between MRP1 expression and the modulatory effect of probenecid (a specific modulator of MRP1) on the calcein efflux (r = 0.91, p = 0.0003) and between Pgp expression and the modulatory effect of CsA on calcein-AM uptake (r = 0.96, p < 0.0001). On light of the high correlations for both proteins, we tested calcein-AM uptake and efflux in fresh myeloid leukemic cells. In 53 AML patients, there was also a good correlation between MRP1 expression (measured by RT/PCR and by MRPm6 expression by flow cytometry) and the modulatory effect of probenecid on the calcein fluorescence (r = 0.92, p < 0.0001) and between Pgp expression as measured by UIC2 antibody binding on flow cytometry and the modulatory effect of CsA on calcein-AM uptake (r = 0.83, p < 0.0001). Pgp activity was higher in CD34+ leukemia than in CD34- leukemia (2.26 +/- 1.50 vs 1.46 +/- 1.21 respectively, p = 0.003) and MRP1 activity was higher in CD34- leukemia than in CD34+ leukemia (1.77 +/- 0.40 vs 1.4 +/- 0.29 respectively, p = 0.004). Pgp expression and activity (p = 0.004 and p = 0.01, respectively), MRP1 activity (p = 0.03) but not MRP1 expression were prognostic factors for achievement of CR. The effect of probenecid and CsA together were higher than the effect of either probenecid or CsA alone on calcein-AM uptake. These results suggest that functional testing (with calcein-AM +/- modulators) for the presence of both MRP1 and Pgp activities is of prognostic value and that MRP1 contributes to drug resistance in AML.

Cyclin D1 overexpression allows identification of an aggressive subset of leukemic lymphoproliferative disorder.

The conjunction of clinical features, cell morphology and immunological characteristics allows an accurate diagnosis in most cases of B cell chronic lymphoproliferative disorders (CLD). However, the diagnosis remains uncertain in a small percentage of cases, often referred as to unclassified B cell proliferation or atypical chronic lymphocytic leukemia (CLL). We have studied retrospectively the 192 cases of leukemic CLD seen in our institution over a 3-year period, for which both clinical and routine biological data at presentation were available. Forty cases (20%) did not fit into any of the well-identified categories according to the FAB criteria and remained unclassified. We assessed cyclin D1 expression in all of these cases and found that 10 of them expressed a high level of cyclin D1 protein. We compared the characteristics of these 10 cases with those of the 30 cyclin D1 negative CLD. Despite non-distinctive cytological and phenotypic features, the 10 cyclin D1 positive patients exhibited a strikingly uniform clinical presentation with elevated leukocytosis, massive spleen enlargement and no superficial lymphadenopathy. Their outcome was very poor with a median survival of 10 months, contrasting with the prolonged survival of the cyclin D1 negative patients. The cytological features of tumor cells from these 10 patients with cyclin D1 positive unclassified leukemic CLD were similar to those of the circulating lymphoid cells from 15 patients with histologically proven mantle cell lymphoma (MCL) and primary or secondary blood involvement. Therefore, cyclin D1 expression allowed identification among the unclassified CLD, a subset of aggressive disorders which represent a leukemic counterpart of MCL (mantle cell leukemia). We suggest that determination of cyclin D1 expression by any technique available should be systematically included when investigating atypical CLL.

Proapoptotic effects of antiestrogens, progestins and androgen in breast cancer cells.

The promoting action of E2 in breast cancer cells has been, until now, mainly linked to its action on prolifieration. Because of the importance of an increase in apoptosis in breast cancer prevention, we have studied the possible effects of various antiestrogens, progestins and an androgen on its occurrence in three hormone-dependent breast cancer cell lines. The antiestrogens were, a triphenylethylene derivative, 4 hydroxytamoxifen (4OHTAM) and two steroidal antiestrogens, IC1182780 and RU58668. The progestins were Org2058, a pregnane derivative, tibolone (OrgOD14), a normethyltestosterone derivative and OrgOM38 (the delta4 isomer of OrgOD14) and the androgen dihydrotestosterone (DHT). Apoptosis was studied in MCF-7, ZR75-1 and T47-D cells using morphological approaches and flow cytometry. The antiestrogens, the progestins and DHT were proapoptotic but to different potencies according to the cell line studied. Indeed, the 'pure' steroidal antiestrogens were more efficient than 4OHTam in increasing apoptosis. We have also studied the level of expression of some of the proteins involved in the regulation of apoptosis. Bcl-2 and bcxL, two antiapoptotic members of the bcl-2 family proteins, were inhibited by the progestins and the antiestrogens. In contrast, the proapoptotic proteins, bax and bak seemed to be constitutively expressed. Thus, since the ratio of proapoptotic and antiapoptotic proteins determines apoptosis or cell survival, the hormone effects are operating by modulating the antiapoptotic regulators of the balance. These data demonstrate that antiestrogens, progestins, and androgens can promote apoptosis in breast cancer cells, an effect which could be of importance in the therapeutic prevention of breast cancer.

Pentasomy 13q in a case of acute myelogenous leukemia (Mo).

A case of acute myelogenous leukemia Mo FAB subtype with a pentasomy 13q (associated with a trisomy 19 in a subclone) in the initial bone marrow metaphase cells is reported. The pentasomy 13q is the result of the presence of double isochromosome 13q and one normal chromosome 13. In our case, this abnormality had a poor prognosis.

Pgp and MRP activities using calcein-AM are prognostic factors in adult acute myeloid leukemia patients.

Thirteen cell lines with different levels of Pgp and MRP expression were used to assess the ability of calcein acetoxymethyl ester (calcein-AM) uptake and calcein efflux to measure Pgp and MRP functions, respectively. There was a good correlation between MRP expression and the modulatory effect of probenecid (a specific modulator of MRP) on the calcein efflux (r = .91, P = .0003) and between Pgp expression and the modulatory effect of CsA on calcein-AM uptake (r = .96, P < .0001). In light of the high correlations for both proteins, we tested calcein-AM uptake and efflux in fresh myeloid leukemic cells. In 53 acute myeloid leukemia (AML) patients, there was also a good correlation between MRP expression (measured by reverse transcription-polymerase chain reaction and by MRPm6 expression by flow cytometry) and the modulatory effect of probenecid on the calcein fluorescence (r = .92, P < .0001) and between Pgp expression as measured by UIC2 antibody binding on flow cytometry and the modulatory effect of cyclosporin A on calcein-AM uptake (r = .83, P < .0001). Pgp activity was higher in CD34+ leukemia than in CD34- leukemia (2.26 +/- 1.50 v 1.46 +/- 1.21, respectively; P = .003), and MRP activity was higher in CD34- leukemia than in CD34+ leukemia (1.77 +/- 0.40 v 1.4 +/- 0. 29, respectively; P = .004). Pgp expression and activity (P = .004 and P = .01, respectively) and MRP activity (P = .03) but not MRP expression were prognostic factors for achievement of complete remission. These results suggest that functional testing (with calcein-AM +/- modulators) for the presence of both MRP and Pgp activities is of prognostic value and that MRP contributes to drug resistance in AML.

Differential requirement for p56lck in fetal and adult thymopoiesis.

The protein tyrosine kinase p56lck is critical for the generation of mature thymocytes in adult mice. However its requirement during the maturation of thymocytes from the fetal to the adult stage has not been clearly defined. We analyzed prenatal and postnatal thymocyte maturation in mice deficient for p56lck (lck[-/-]). Before birth, lck appears to play a crucial role in the expansion and proliferation of CD4+CD8+ double positive thymocytes, whereas proliferation and absolute numbers of CD4-CD8- double negative thymocyte precursors remained within the normal range until the end of the second week postnatal. Three weeks after birth, the total numbers of double negative and immature single positive thymocytes underwent a dramatic reduction that correlated with a decrease in the double positive population. This ontogenic defect was associated with a significant decrease in the proliferation rates of thymocyte precursors. Our data suggest that signaling via p56lck kinase is differentially required within a given phenotypically defined thymocyte subpopulation, depending on its stage of thymocyte maturation.

Adult biphenotypic acute leukaemia: an entity with poor prognosis which is related to unfavourable cytogenetics and P-glycoprotein over-expression.

Biphenotypic acute leukaemia (BAL) patients represented 8% of the 287 de novo consecutive adult acute leukaemias (23 BAL, 230 acute myeloid leukaemia (AML) and 34 acute lymphoblastic leukaemia (ALL)) referred to our department during the last 4-year period. Of these 23 BAL patients, 14 patients showed myeloid morphology and nine cases lymphoid morphology according to FAB criteria. There were no differences between lymphoid and myeloid BAL according to clinical and biological presentation and treatment outcome. We confirm the poor prognosis of BAL when compared to AML or ALL seen during the same period of time, in terms of complete remission (47%, 62% and 82% respectively, BAL v AML, NS and BAL v ALL, P = 0.006) and 4-year overall survival (8.1%, 25.8% and 23.8% respectively, BAL v AML, P = 0.05 and BAL v ALL, P = 0.003). Comparing adult BAL patients with AML patients, we found an increase in poor prognostic factors: CD34+ phenotype (82% v 60% respectively, P = 0.03), unfavourable karyotype (60% v 20%, P < 0.0001) and Pgp over-expression by RT-PCR (0.705 v 0.107, P < 0.0001) and flow cytometry (0.824 v 0.391, P = 0.0001). MRP and LRP were not found to be poor prognostic factors. Comparing BAL patients with ALL patients, we found also an increase in poor prognostic factors: age (51 v 39, P = 0.003) and CD34+ phenotype (82% v 50%, P = 0.02). We conclude that BAL patients need a more aggressive treatment procedure, including high-dose AraC or the use of Pgp modulators for first-line therapy.

Measuring multidrug resistance expression in human malignancies: elaboration of consensus recommendations.

Before the prognostic significance of P-glycoprotein (P-gp) expression can be properly evaluated in prospective clinical trials of P-gp modulators, standard techniques for the measurement of P-gp must be widely accepted. Several multicenter trials have demonstrated large discrepancies in the observed levels of P-gp expression in the same clinical samples evaluated at different centers. The greatest discrepancies occurred with samples that expressed low levels of P-gp. Although standardized procedures have dramatically increased the interlaboratory reproducibility of flow cytometry and polymerase chain reaction assays, data from immunocytochemistry remain difficult to interpret. Consensus recommendations are presented for improving data reproducibility. These recommendations emphasize the importance of using calibrated batches of antibodies and two different antibodies for immunocytochemistry, the need for an internal standard for reverse transcriptase-polymerase chain reaction (RT-PCR) assays, the need for the presentation of data as a continuous variable, and the need for setting standard parameters for flow cytometry. It is also extremely important for the success of clinical trials that multiple techniques be employed to insure accurate measurement of P-gp expression.

Multicentric evaluation of the MDR phenotype in leukemia. French Network of the Drug Resistance Intergroup, and Drug Resistance Network of Assistance Publique-Hôpitaux de Paris.

The wide discrepancies in the frequency of 'positive' samples for multidrug resistance (MDR) phenotype within the same type of tumor observed in the literature justified the need for the definition of consensus recommendations. To define standard techniques of MDR phenotype measurement, we ran a large multicentric evaluation of the different methods available. Thirty-six French centers participated in the study, and 742 samples of 2-10 x 10(6) viable cells were sent by overnight express mail between December 1993 and February 1996. The same batches of MRK16, 4E3 and UIC2 were used. Nineteen samples of leukemia (12 AML, 1 ALL, 6 lymphoproliferative syndromes) and six leukemic cell lines with different levels of MDR expression were tested. Five meetings reached agreement concerning the guidelines for each technique, except immunocytochemistry. The 19 fresh samples were tested by each center using one to four techniques among cytofluorometry, immunocytochemistry, functional tests and RT-PCR. Five samples were diagnosed as 'negative' according to local criteria, with few discordant results (0 to 16% of 'positive' results). For all the 14 remaining samples, large discrepancies were observed from center to center, and from one technique to another. No correlations could be found between techniques. Flow cytometric analysis of cells already exposed to MRK16 or control IgG2A, fixed in paraformaldehyde and sent to centers did not reduce the discrepancies between centers in two of the four samples with moderate expression, emphasizing the role of histogram interpretation. The use of alternative monoclonal antibodies (4E3 and UIC2) did not reduce the discrepancies observed. In a second step, the K562 parental cell line, a low resistant subline (K562/HHT100, x7 resistance index to DNR) and a high resistant subline (K562/HHT300, x125 resistance index to DNR) were sent blindly three times, with an increasing level of recommendations for flow cytometry. Dramatic improvements were observed in cytometric results when the result was expressed as the ratio of arithmetic mean of fluorescence of antibody (10 microg of MRK16)/arithmetic mean of fluorescence of control (10 microg IgG2A): the proportion of expected results increased from 61 to 100% for K562, and from 37 to 85% for K562/HHT100. For uptake and drug efflux measurements, the use of 1 h uptake of 0.1 microM of rhodamine, followed by 1 h efflux +/-10 microM of verapamil, permitted an increased reproducibility of the technique from 71 to 100% for K562 and K562/HHT100. Whatever the technique used, concordant results were obtained for K562/HHT300. The immunocytochemistry, using several antibodies (MRK16, JSB1 and C219) gave many non-interpretable results (44%), due to a frequent high background and discordant results between antibodies in the same centers, and discordant conclusions between centers. The group does not recommend this technique for circulating tumoral cells.

Functional assay of multidrug resistant cells using JC-1, a carbocyanine fluorescent probe.

Multidrug resistance (MDR) is characterized by a decrease in the efficiency of chemotherapeutic agents correlated with the expression and activity of a membrane protein: the permeability-glycoprotein (Pgp 170). Clinically, detection of MDR can be performed by functional tests based on the accumulation of fluorescent compounds such as rhodamine 123. With the aim of improving the sensitivity of such analysis, we have evaluated JC-1, a fluorescent lipophilic carbocyanine dye. Above a critical concentration, JC-1 aggregates in a 'liquid crystal' form. Aggregates display a specific red emission band centered at 597 nm whereas the monomers display a green emission band centered at 540 nm. JC-1 was avidly accumulated in sensitive K562 cells where it displayed both a green cytoplasmic and red mitochondrial fluorescence. In contrast, JC-1 was poorly accumulated in resistant K562 cells, which displayed only a slight green fluorescence. The level of JC-1 accumulation was correlated with the level of Pgp expression detected by MRK16 and UIC2 antibodies on a set of K562 subclones with increasing resistance levels. The specific fluorescence properties of JC-1 allow accurate discrimination between low-level resistant cells and sensitive cells. Chemosensitizers such as verapamil, cyclosporine A or S9788 restored JC-1 accumulation in resistant cells. The fluorescence properties of JC-1 could therefore be used for monitoring the effects of reversing agents.

Prognostic factors in elderly acute lymphoblastic leukaemia.

A retrospective study was performed on 46 unselected acute lymphoblastic leukaemia (ALL) elderly patients aged 60 years or more. Only 50% of these patients were included in the EORTC cooperative clinical trials, thus confirming the important selection bias in most of the published series on elderly ALL patients. 43% of the elderly patients achieved a complete remission (CR). The median survival was 10 months and the 5-year overall survival was only 7.6 +/- 4%. In multivariate analysis, W.H.O. performance status and peripheral blast counts at day 7 were found to significantly influence achievement of CR and survival. In patients with W.H.O. performance status > or = 2, 35% died during induction treatment versus 4% in patients with W.H.O. performance status < 2. Patients > 70 years old showed a marked drop of the CR rate (27%) compared to those aged 60-69 (67%), and a very high death rate during the induction period (38% versus 4%). This suggests that ALL protocol treatments should be proposed until 70 years in patients with good-performance status, whereas less intensive treatment should be offered to elderly patients with performance status > or = 2 and/or age > or = 70. Peripheral blast counts at day 7 may help to adjust the treatment during induction phase.

Multidrug resistance gene expression in acute myeloid leukemia: major prognosis significance for in vivo drug resistance to induction treatment.

The clinical significance of the multidrug resistance (MDR 1) gene phenotype was investigated in newly diagnosed AML and was compared with other clinical and biological prognostic factors in patients who received at least one course of induction therapy with intercalating agents and conventional doses of Ara-C. MDR 1 gene was overexpressed in 40% of the 110 cases of AML at presentation, MRP in 15% of the 48 patients tested for both markers. Both gene expressions were closely linked (p = 0.008). Except for a lower frequency in the "good risk" cytogenetic group, MDR 1 overexpression was not associated with other prognostic factors. In univariate analysis, MDR 1 overexpression, age over 50 years, and cytogenetic were associated with a higher rate of resistance to induction treatment. The overall survival was shorter in the case of intermediate or poor cytogenetics, high leukocytosis, MDR 1 overexpression, age over 50 years, secondary AML, and poor cytologic differentiation. Using multivariate analysis on 64 patients receiving intensive treatment, MDR 1 overexpression was the first significant prognostic factor for resistance to the first course of induction treatment. Cytogenetic analysis maintained its prognostic value only in MDR 1-negative patients. These data underline the value of MDR 1 gene expression as a powerful prognostic factor in AML for response to the first induction treatment and overall survival, sustaining the use of MDR 1 modulators for first-line therapy in this disease.