Multidrug resistance proteins MRP3, MRP1, and MRP2 in lung cancer: correlation of protein levels with drug response and messenger RNA levels.

Previously (L. C. Young et al., Clin. Cancer Res., 5: 673-680, 1999), we found, in a panel of 23 lung cancer cell lines that had not been selected for in vitro drug resistance, that the mRNA levels of MRP3 and MRP1, two members of the ATP-binding cassette superfamily of transport proteins, correlated with resistance to doxorubicin, vincristine, VP-16, and cis-diamminedicholoroplatinum(II). To extend these studies, we measured multidrug resistance protein (MRP)1, MRP2, and MRP3 protein levels in a panel of 30 lung cancer cell lines that included the original 23 cell lines as well as an additional 7 unselected lung cancer cell lines. In the case of MRP3, a polyclonal antibody was developed that was found to be a sensitive reagent for the detection of MRP3 by Western blot analysis. We found good agreement in the original 23 cell lines between the cognate mRNA and protein levels for MRP1, MRP2, and, especially, MRP3 (r, 0.852), supporting the use of semiquantitative PCR to predict MRP1, MRP2, and MRP3 protein levels in patient samples. There were also strong correlations between the mRNA and protein levels of MRP3 and MRP1, which suggested that these genes might be expressed in a coordinate manner. MRP3, MRP1, and MRP2 protein levels were higher in the non-small cell lung cancer (NSCLC) than in the SCLC cell lines and, in addition, MRP3 and MRP2 were detected almost exclusively in the NSCLC cell lines. Finally, we found that both MRP3 and MRP1, but not MRP2, protein levels correlated with decreased sensitivity of these lung cancer cell lines to doxorubicin, VCR, VP-16, and cis-diamminedicholoroplatinum(II). These findings are consistent with our hypothesis that both MRP3 and MRP1 are components of the multifactorial multidrug resistance phenotype of lung cancer and that MRP3 contributes to the intrinsic resistance of NSCLC cells.

Enhanced anti-cancer activities of some derivatives of titanocene dichloride.

The compounds (eta5-C5H5)2TiCl2 (I), currently undergoing phase II trials, (eta5-C5H5)(eta5-C5H4CO2Me)TiCl2 (II) and C5H4CO2Me)2TiCl2 (III) are assessed for their efficacies against a small lung cancer cell line. It is found that the introduction of the electron withdrawing carbomethoxy group into the cyclopentadienyl rings increases the effectiveness of this class of drugs, such that III compares favorably with the well known cisplatin.

Simultaneous quantitation of topoisomerase II alpha and beta isoform mRNAs in lung tumor cells and normal and malignant lung tissue.

Certain drugs used in the treatment of lung cancer and other human malignancies are cytotoxic because of their ability to interact with the two isoforms of topoisomerase II (topo II), topo IIalpha and topo IIbeta. As part of an effort to evaluate the contribution of topo II alterations to drug sensitivity and resistance in lung cancer, we have developed a semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) assay to measure levels of topo II alpha and beta mRNAs simultaneously using a single pair of primers with sequences common to both isoforms. The PCR products derived from the topo II alpha and beta mRNAs are both 446 bp but have different electrophoretic mobilities in a nondenaturing polyacrylamide gel, allowing sensitive, rapid quantitation when the products are radiolabeled with [35S]-dATP. Using this RT-PCR method, poly(A+) RNA from 13 non-small cell lung cancer (NSCLC) cell lines was analyzed. The results obtained indicated that the cell lines express a wide range of topo II alpha mRNA levels (12-fold) and topo IIbeta mRNA levels (5.5-fold). Tumor and normal lung tissues from 25 patients with NSCLC were also examined. In the tumor samples, the levels of the topo II alpha and beta mRNAs were similar. However, mean topo IIalpha mRNA levels in the tumors were approximately 7-fold higher than those of the paired normal lung tissues. In contrast, topo IIbeta mRNA levels were similar in both tumor and normal lung. Topo II alpha and beta mRNA levels were both significantly lower in the squamous cell tumors than in the adenocarcinoma samples. Topo IIbeta mRNA levels in the squamous cell tumors were also significantly lower than those in paired normal lung tissue. The RT-PCR method described is reliable and convenient, and for the first time, makes the rapid simultaneous direct comparison of topo IIalpha and topo IIbeta mRNA levels feasible in large numbers of clinical samples.

A truncated cytoplasmic topoisomerase IIalpha in a drug-resistant lung cancer cell line is encoded by a TOP2A allele with a partial deletion of exon 34.

To study the problem of acquired resistance to widely used anti-cancer drugs that target the 170 kDa topoisomerase IIalpha (topo IIalpha), a drug-resistant human small-cell lung cancer cell line, H209/VP, was selected in VP-16. H209/VP cells express reduced levels of the 170 kDa topo IIalpha that is localized normally in the nucleus and also express lower levels of a 160 kDa topo IIalpha-related protein that is located predominantly in the cytoplasm. Band depletion immunoblotting experiments suggest that the H209/VP nuclear 170 kDa topo IIalpha is able to form ternary complexes with DNA and VP-16 in intact cells, but the ability of the cytoplasmic 160 kDa protein to do so is greatly diminished. Sequence analysis of the 3; end of the H209/VP mutant topo IIalpha mRNA and the TOP2A gene indicates that the mRNA is missing 200 nt that corresponds to exon 34 because the partial loss of the minimal 3; splice-acceptor sequence at the beginning of exon 34 results in splicing of exon 33 to exon 35. The protein predicted to be encoded by this mutant mRNA does not contain the COOH-terminal 109 amino acids of the wild-type enzyme that we have demonstrated contain a strongly functional nuclear localization signal sequence. Consequently, our data explain both the size and the cytoplasmic localization of the H209/VP mutant topo IIalpha. The mutant TOP2A allele in H209/VP cells differs from those in previously characterized cell lines with cytoplasmic topo IIalpha and extends the number of types of resistance-associated deletions in this region to 4. These findings indicate that this region of the TOP2A gene may be a hot spot for mutations.

Gap junctional communication in cultured human lung carcinoma cells.

Animal tumor models have demonstrated a close correlation between gap junctional, intercellular communication (GJIC) and tumor metastasis. To examine GJIC levels in human lung carcinoma cells, a novel technique was developed: cells were grown on a glass slide, half of which was coated with electrically conductive, optically transparent, indium-tin oxide. An electric pulse which opens transient pores on the plasma membrane was applied in the presence of the fluorescent dye, Lucifer yellow, causing the dye's penetration into the cells growing on the conductive part of the slide. The migration of the dye through gap junctions to the non-electroporated cells growing on the non-conductive area was then observed microscopically under fluorescence illumination. The results show that this is a rapid, precise and highly reproducible assay for GJIC assessment in lines established from lung carcinomas or freshly explanted lung tumor cells. Out of 17 established lines only two had extensive junctional communication, while out of 16 fresh tumor specimens none displayed GJIC. On the other hand, fibroblasts isolated from the same tumors had extensive junctional permeability. The examination of GJIC in a large number of samples could establish a correlation between GJIC and metastasis which might have prognostic value.

Expression of multidrug resistance protein-related genes in lung cancer: correlation with drug response.

Recently, cDNAs have been identified that encode four human proteins (MRP2-5) with structural similarity to the multidrug resistance protein (MRP). Preliminary studies have shown that levels of mRNAs encoding MRP2, MRP3, and MRP5, are increased in some drug-selected cell lines, but the correlation of MRP2-5 mRNA levels with drug resistance has not been examined. Using a collection of small cell lung cancer (SCLC) and non-SCLC patient samples and unselected cell lines established from patients at various stages of treatment, we examined the expression of MRP2, MRP3, MRP4, and MRP5, as well as MDR1 and MRP, by PCR. The levels of individual mRNAs were correlated with the sensitivity of these cell lines to doxorubicin (DOX), vincristine, VP-16, and cis-diamminedichloroplatinum(II), as determined by a modified MTT assay. Using both SCLC and non-SCLC cell lines, we confirmed the previously observed correlation of MRP mRNA levels with resistance to DOX (B. G. Campling et al., Clin. Cancer Res., 3:115-122, 1997) and found a strong correlation of MRP3 mRNA levels with resistance of the cell lines to DOX. In addition, the mRNA levels of both MRP and MRP3 correlated with resistance of the cell lines to vincristine, VP-16, and cis-diamminedichloroplatinum(II). These findings are consistent with the suggestion that MRP3, like MRP, may contribute to the drug resistance phenotype of lung cancer cells.

Coexpression of transcripts encoding EPHB receptor protein tyrosine kinases and their ephrin-B ligands in human small cell lung carcinoma.

The EPH family is the largest subfamily of receptor protein tyrosine kinases, consisting of the EPHA and EPHB subgroups. Ephrin-B1, ephrin-B2, and ephrin-B3 are ligands of the EPHB subgroup and are encoded by the EFNB1, EFNB2, and EFNB3 genes, respectively. We have shown previously that EPHB2 transcripts are expressed in six small cell lung carcinoma (SCLC) cell lines. In this study, we examined the expression of EPHB1, EPHB2, EPHB3, EPHB4, and EPHB6 in 4 SCLC tumor specimens and 14 cell lines including 3 cell lines derived from these tumor specimens. To investigate whether potential autocrine loops of EPHB receptors and ephrin-B ligands exist in SCLC, the expression of EFNB1, EFNB2, and EFNB3 was also examined. Our data show that transcripts encoding multiple members of the EPHB subgroup and the ephrin-B subgroup are coexpressed in SCLC cell lines and tumors. These results suggest that the EPHB subgroup receptor kinases may modulate the biological behavior of SCLC through autocrine and/or juxtacrine activation by ephrin-B ligands that are expressed in the same or neighboring cells.

Immunohistochemical detection of multidrug resistance protein in human lung cancer and normal lung.

Monoclonal antibody QCRL-1 is highly specific for a defined linear epitope in a relatively poorly conserved region of the human multidrug resistance protein (MRP). We have used QCRL-1 to examine MRP expression in archival and fresh snap-frozen samples of untreated small cell (SC) and non-small cell (NSC) lung cancers (LCs), as well as normal lung. We found that the majority (87%) of all histological subtypes of NSCLC had detectable levels of MRP in most of the tumor mass. In a substantial proportion of adenocarcinomas (55%) and squamous cell carcinomas (28%), immunoreactivity approached that obtained with the highly multidrug resistant cell line H69AR from which the MRP was originally cloned. Both the level and frequency of MRP expression in untreated SCLC was significantly lower than in NSCLC. The MRP was detectable in only 56% of SCLC tumors and, in most cases, was expressed in small focal clusters of cells. Immunofluorescence studies of tumor tissue and normal lung confirmed the plasma membrane location of the MRP. However, in normal bronchial epithelium and seromucous glands, unlike in tumor cells, the MRP was detected only on basolateral membranes. In addition, strong MRP immunoreactivity was detected in reactive type II pneumocytes present in hyperplastic alveoli, but not in normal type I and type II pneumocytes. No potentially confounding correlation independent of its possible role in drug resistance was observed between MRP expression in untreated NSCLC and any clinicopathological parameter examined, including overall survival.

Investigation of the genes for RET and its ligand complex, GDNF/GFR alpha-I, in small cell lung carcinoma.

RET is a receptor tyrosine kinase expressed in neuroendocrine cells and in tumors of these cell types. RET activation may be mediated by a ligand complex comprising glial cell line-derived neurotrophic factor (GDNF) and GDNF family receptor alpha-1 (GFR alpha-1). Activating RET mutations are found in the inherited cancer syndrome multiple endocrine neoplasia type 2 and in a subset of the related sporadic tumors, medullary thyroid carcinoma and pheochromocytoma, both being derived from neuroendocrine tissues. In one small study, mutations were identified in another tumor with neuroendocrine features, small cell lung carcinoma (SCLC). To determine whether RET mutations contribute to the pathogenesis of SCLC, we examined a panel of 54 SCLC cell lines. No mutations were identified in RET exons 10, 11, and 13-16, regions previously implicated in SCLC or other neuroendocrine tumors. We further examined the expression pattern of RET and the genes encoding the components of its ligand complex GDNF and GFR alpha-1, in 21 SCLC lines by using RT-PCR. Although we found no consistent pattern of expression for these three genes, RET was expressed in 57% of SCLC lines. Thus, although RET mutations appear unlikely to be an important step in the tumorigenesis of SCLC, the frequent expression of this gene suggests that RET may have a mitogenic role in a subset of SCLC cell lines.

Expression of the MRP and MDR1 multidrug resistance genes in small cell lung cancer.

Acquired multidrug resistance is a major obstacle to a cure for small cell lung cancer (SCLC). Overexpression of the MDR1 gene occurs infrequently in multidrug-resistant SCLC cell lines. The multidrug resistance protein (MRP) can confer multidrug resistance, but its role in clinically acquired drug resistance is unknown. The purpose of this study was to measure expression of MRP and MDR1 mRNA in cell lines and clinical samples from SCLC patients and to correlate the results with drug sensitivity profiles. Twenty-three SCLC cell lines and 10 tumor samples from SCLC patients were examined. Samples expressing MRP and MDR1 were identified by reverse transcription-PCR, and levels of MRP mRNA in the cell lines were measured by quantitative reverse transcription-PCR. One of 23 cell lines (4%) expressed MDR1 mRNA, whereas MRP expression was detected in 19 of 23 cell lines (83%). There was a significant correlation between doxorubicin resistance and MRP expression levels (r = 0.422; P = 0.045). Of the 10 clinical samples, 3 expressed only MRP, 2 expressed only MDR1, and 4 expressed both drug resistance genes. In summary, MRP is frequently expressed in clinical samples and cell lines from SCLC patients, and the levels correlate with doxorubicin resistance in unselected SCLC cell lines. Expression of MDR1 can be detected in clinical samples of SCLC but is rarely found in cell lines from drug-resistant patients. These multidrug resistance proteins may contribute to the multifactorial problem of clinically acquired drug resistance in SCLC.

Mechanisms of resistance of human small cell lung cancer lines selected in VP-16 and cisplatin.

BACKGROUND: The combination of VP-16 and cisplatin is one of the most active regimens available for the treatment of small cell lung cancer (SCLC), however, most tumors eventually become resistant to these drugs. METHODS: To investigate the problem of resistance to VP-16 and cisplatin in patients with SCLC, we established two resistant sublines from the drug sensitive human SCLC line, NCI-H209, by in vitro selection in VP-16 and cisplatin. RESULTS: The VP-16-selected cell line, H209/VP, was more than 100-fold resistant to VP-16, and displayed cross-resistance to VM-26 and other topoisomerase II interactive drugs, but not to vinca alkaloids. There was no difference in accumulation of VP-16 in H209/VP compared with its parent cell line. The level of topoisomerase II-alpha was reduced to 8% of that in the parent cell line, and there was an altered form of this enzyme with a molecular weight of 160 kilodaltons (kDa), in addition to the normal 170 kDa protein. The cisplatin-selected cell line, H209/CP, was 11.5-fold resistant to cisplatin, with only a low level of cross-resistance to other platinum compounds including carboplatin, tetraplatin, iproplatin, and lobaplatin. This line was highly cross-resistant to vinca alkaloids, but not to anthracyclines or epipodophyllotoxins. The H209/CP cell line was not resistant to cadium chloride, suggesting that alterations in metallothionein are unlikely to be a cause of resistance. Although glutathione (GSH) levels were increased nearly 2-fold in H209/CP, there was no difference in levels of the GSH-related enzymes glutathione-S-transferase, glutathione peroxidase, and glutathione reductase, compared with the parent line. The H209/CP line had a 1.4-fold elevation of topoisomerase II-alpha. The accumulation of cisplatin was reduced in this cell line, and there were fewer DNA-interstrand cross links formed in the presence of cisplatin in H209/CP, compared with the parent line. Neither H209/VP nor H209/CP expressed MDR1, the gene for P-glycoprotein. The MRP gene was expressed at a slightly higher level in the H209/VP cell line, but there was no significant increase in expression of this gene in the H209/CP cell line. CONCLUSIONS: The resistance of the H209/VP cell line is associated with an alteration of topoisomerase II-alpha, whereas the resistance in the H209/CP line is associated with reduced drug accumulation.

Flow cytometric technique for quantitating cytotoxic response to photodynamic therapy.

A simple flow cytometric technique for rapid measurement of multilog cytotoxic responses to photosensitization of cellular systems is described. This technique is particularly useful for cell lines with a low colony-forming efficiency, for which a nonclonogenic assay is required. The assay separates cell-sized objects from cellular debris by gating on forward scatter versus side scatter, identifies viable cells by positive calcein AM and negative ethidium homodimer-1 staining and measures cell concentration relative to an internal standard of polystyrene beads. Large numbers of cells can be analyzed rapidly. Two patient-derived small cell lung cancer cell lines, NCI-H209 and SV-E, were used to test the technique. Photordiation survival curves of the response of these cell lines to 5-aminolevulinic acid-induced protoprophyrin IX photosensitization correlated with the extent of photosensitizer accumulation. There was good agreement between the results obtained using the tritiated thymidine incorporation assay and the flow cytometric cytotoxicity assay. The technique can be used to measure cytotoxic responses to photosensitization of cell lines regardless of their plating efficiencies.

ANP secretion from small cell lung cancer cell lines: a potential model of ANP release.

Although atrial distension is widely accepted as the primary stimulus for atrial natriuretic peptide (ANP) release, a number of agonists are also known to induce its secretion. The mechanisms underlying these processes are not well understood. Studies of this nature are hampered by the inherent difficulty in culturing homogeneous populations of cardiac myocytes in sufficient quantities to perform molecular investigations. For this reason, we have examined the possibility of using other cell types as a model of ANP release. It has been reported that a number of tumor samples from small cell lung cancer (SCLC) patients express the ANP gene. Characterization of a large number of cell lines derived from SCLC tumor samples indicated that two of these cell lines, OS-A and SHP-77, secrete ANP at rates of approximately 10(-20) g.cell-1.min-1. This is a sufficient quantity to facilitate secretion studies using a perifusion system. We have demonstrated that ANP is released through regulated secretory pathways, as the Ca2+ ionophore A-23187, arginine vasopressin (AVP), and the sodium ionophore, monensin, were capable of modifying secretion rates. High-pressure liquid chromatography (HPLC) analysis indicated that the primary secretory product is ANP-(99-126), the circulating form of this hormone. Intracellularly, both ANP-(99-126) and ANP-(1-126) were present, suggesting the synthesis and appropriate cleavage of pro-ANP-(1-126). Because both of these cell lines have doubling times in the range of 3-5 days, they could serve as a rapidly proliferating and easily maintainable supply of homogeneous tissue for release studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Secretion of atrial natriuretic peptide and vasopressin by small cell lung cancer.

BACKGROUND: Hyponatremia in patients with small cell lung cancer (SCLC) is a common clinical problem usually attributed to tumor secretion of arginine vasopressin (AVP). It recently was shown that some SCLC cell lines produce atrial natriuretic peptide (ANP). The purpose of this investigation was to determine the frequency and clinical consequences of secretion of ANP by SCLC and the relative contribution of ANP and AVP to the hyponatremia associated with this disease. METHODS: Levels of ANP and AVP were measured in 23 SCLC cell lines and 23 other human tumor cell lines. Also, ANP and AVP levels were determined in plasma samples from 69 patients with active small cell carcinomas. RESULTS: Of the 23 SCLC lines, 16 (70%) had elevated ANP levels. Only two (8.7%) had elevated AVP levels, and these two also had elevated ANP levels. One of the ANP-producing cell lines was derived from a hyponatremic patient with no other apparent explanation for a low sodium level. However, the four cell lines with the highest levels of ANP were derived from patients who were not hyponatremic. Two other human tumor lines also produced ANP. Of the 69 patients with SCLC, 21 (30.4%) had elevated ANP levels, whereas 4 (6%) had elevated AVP levels. Fifteen of these patients were hyponatremic during their clinical course (21.7%). Of the eight patients who were hyponatremic when samples were collected, two had elevated ANP levels, and only one had elevated AVP levels. Six patients (8.7%) had symptoms of postural hypotension, possibly attributable in some cases of tumor secretion of ANP. CONCLUSIONS: The majority of SCLC lines produce ANP, and a minority produce AVP. Secretion of ANP may result in hyponatremia and/or postural hypotension. However, secretion of either or both of these peptides does not account for all cases of hyponatremia in patients with SCLC and does not necessarily cause clinical manifestations.

Do glutathione and related enzymes play a role in drug resistance in small cell lung cancer cell lines?

Small cell lung cancer (SCLC) is treated primarily with combination chemotherapy. Despite high initial response rates, most patients eventually die with drug resistant disease. In some tumours, resistance to multiple chemotherapeutic agents is attributed to overexpression of P-glycoprotein (P-gp). However, this does not appear to be a frequent occurrence in drug resistant SCLC. Increased levels of glutathione (GSH) and related enzymes may play a role in resistance to alkylating agents as well as natural product drugs. We measured levels of GSH, glutathione S-transferase (GST), glutathione reductase (GSH Red), glutathione peroxidase (GSH Px), and gamma-glutamyl transpeptidase (gamma-GT) in a panel of 20 SCLC cell lines. Most of these lines were established from patients treated at this centre. Each cell line had a characteristic and reproducible profile of GSH and related enzyme levels. Immunoblot analysis indicated that the predominant GST in the cell lines was the anionic pi isoenzyme. The relative sensitivity of each of these cell lines to 16 different chemotherapeutic agents was measured using a modified MTT assay. Spearman rank correlation analysis was used to determine the relationships between the relative chemosensitivity of these cell lines and the levels of GSH and related enzymes. The number of positive correlations was no greater than expected by chance alone. Furthermore, there was no correlation with the treatment history of the patients from whom the cell lines were derived. These data suggest that alterations in glutathione metabolism do not play a major role in resistance to chemotherapeutic agents in these human SCLC cell lines.

Establishment and characterization of a panel of human lung cancer cell lines.

The establishment and characterization of 11 human lung cancer cell lines are described in this article. Nine of these cell lines were established over a 5-year period, from 1983 to 1988, from patients treated at the Kingston Regional Cancer Centre. These include eight definite or probable small cell lung cancer (SCLC) lines and one adenocarcinoma line. In addition, two other SCLC cell lines were characterized. All of the lines have been in continuous culture for more than 2 years. The clinical histories of the patients from whom the cell lines were derived are outlined here. Several features of the cell lines are presented, including the following: (1) a comparison of the histologic features of the cell lines with the original biopsy specimens; (2) the expression of various markers, including cytokeratin, carcinoembryonic antigen, calcitonin, and neuron-specific enolase; (3) activities of the enzymes l-dopa decarboxylase and the brain isoenzyme of creatine kinase; (4) growth characteristics; (5) cloning efficiency in soft agar; (6) tumorigenicity in nude mice; and (7) cytogenetic studies. These cell lines, obtained directly from patients with a spectrum of drug-sensitive and drug-resistant tumors, will be valuable in vitro models of sensitivity and resistance to chemotherapy in lung cancer.

Chemosensitivity testing of small cell lung cancer using the MTT assay.

A simple colorimetric test, the MTT assay, has been adapted for chemosensitivity testing of human small cell lung cancer cell lines, and fresh tumour samples. Optimal conditions for clinical chemosensitivity testing were determined using established SCLC lines. Nineteen different chemotherapeutic agents were tested, and sixteen of them were found to be cytotoxic in this assay system. The drug sensitivity of a panel of 16 SCLC cell lines was measured and compared. There was very little intraexperiment variation, but the interexperiment variation was significant. Cell lines which were derived from patients who had not received chemotherapy at the time the cell line was established were more sensitive (to all but one of the drugs) than lines derived from treated patients, and the differences were statistically significant for two of the drugs. One cell line, NCI-H209, which was derived from an untreated patient, stood out as being the most sensitive or among the most sensitive to all of the drugs tested. Another cell line, H69AR, which is a multidrug resistant subline of the cell line NCI-H69, was the most resistant to many of the natural product drugs tested. Multiple drug chemosensitivity testing was performed on eight fresh tumour samples from SCLC patients (five pleural effusions, one lymph node, and two primary tumours). It was possible to perform chemosensitivity testing on all of the clinical samples in which sufficient tumour cells were available. The drug sensitivity of the clinical samples was, in most cases, within the same range as for the cell lines. Since this assay is very rapid and simple to perform, it may have practical applications in clinical drug sensitivity testing of human tumours.

Use of the MTT assay for rapid determination of chemosensitivity of human leukemic blast cells.

A microcytotoxicity assay employing a tetrazolium salt has been adapted for testing the response of human leukemic blast cells to a variety of chemotherapeutic agents. After exposure to various concentrations of drugs, the viability of fresh leukemic blast cells was measured using a tetrazolium salt, MTT, which is converted to blue formazan crystals by living cells. The amount of formazan produced was quantitated using a microtitre plate spectrophotometer. In the present study, optimal conditions for chemosensitivity testing of human leukemia samples were determined, and the relative chemosensitivity of five patient samples was tested.