Flow cytometry-based approach to ABCG2 function suggests that the transporter differentially handles the influx and efflux of drugs

siRNAs targeting multidrug transporter genes sensitise breast tumour to doxorubicin in a syngeneic mouse model

Chemotherapy, the commonly favoured approach to treat cancer is frequently associated with treatment failure and recurrence of disease as a result of development of multidrug resistance (MDR) with concomitant over-expression of drug efflux proteins on cancer cells. One of the most widely used drugs, doxorubicin (Dox) is a substrate of three different ATP-binding cassette (ABC) transporters, namely, ABCB1, ABCG2 and ABCC1, predominantly contributing to MDR phenotype in cancer. To silence these transporter-coding genes and thus enhance the therapeutic efficacy of Dox, pH-sensitive carbonate apatite (CA) nanoparticles (NPs) were employed as a carrier system to co-deliver siRNAs against these genes and Dox in breast cancer cells and in a syngeneic breast cancer mouse model. siRNAs and Dox were complexed with NPs by incubation at 37 °C and used to treat cancer cell lines to check cell viability and caspase-mediated signal. 4T1 cells-induced breast cancer mouse model was used for treatment with the complex to confirm their action in tumour regression. Smaller (∼200 nm) and less polydisperse NPs that were taken up more effectively by tumour tissue could enhance Dox chemosensitivity, significantly reducing the tumour size in a very low dose of Dox (0.34 mg/kg), in contrast to the limited effect observed in breast cancer cell lines. The study thus proposes that simultaneous delivery of siRNAs against transporter genes and Dox with the help of CA NPs could be a potential therapeutic intervention in effectively treating MDR breast cancer.

Quantitative evaluation of ABC transporter-mediated drug resistance based on the determination of the anticancer activity of camptothecin against breast cancer stem cells using TIRF

Total internal reflection fluorescence microscopy (TIRF) and Qdot probe based analytical method for the simultaneous evaluation of the cytotoxic ability of camptothecin and the drug resistance profile upon the inhibition of drug efflux pumps in breast cancer stem cells.

Novel flow cytometric analysis of the blood-brain barrier

The blood-brain barrier (BBB) is primarily comprised of brain microvascular endothelial cells (BMVEC) and astrocytes and serves as a physical and chemical barrier that separates the periphery from the brain. We describe a flow cytometric method using our in vitro model of the human BBB to characterize BMVEC surface junctional proteins critical for maintenance of barrier function, cell viability, and leukocyte adhesion. For this methodology, BMVEC are cocultured with astrocytes in a transwell tissue culture insert to establish the barrier, after which time the BBB are treated with specific agents, and the BMVEC collected for flow cytometric analyses. We use a standard and optimized method to recover the BMVEC from the coculture model that maintains junctional protein expression and cell viability. A novel leukocyte adhesion assay enables a quantitative analysis of peripheral blood mononuclear cell (PBMC) interactions with the BMVEC and can be used to assess the adhesion of many cell types to the BBB. Furthermore, this method enables the concomitant analysis of a large number of adhesion molecules and tight junction proteins on both the BMVEC and adherent PBMC under homeostatic and pathologic conditions. Flow cytometry is an extremely powerful tool, and this technique can also be applied to assess variables not performed in this study, including cell cycle progression, and calcium flux.

Assessment of drug transporter function using fluorescent cell imaging

ATP-binding cassette (ABC) proteins, including the breast cancer resistance protein (BCRP) and multidrug resistance proteins (MDRs), actively transport structurally diverse chemicals from a number of tissues and are being increasingly cited as mediators of clinically relevant drug-drug interactions. The potential outcomes of concomitantly administering two drugs that interact at the same transporter include altered disposition and toxicity and/or efficacy of one or both of the drugs. Research demonstrating the role of transporters in clinical pharmacokinetics has shed light on the need for in vitro screening methods that detect drug-transporter interactions during preclinical development. This unit describes cell-based procedures for detecting functional inhibitors of BCRP and MDR1 by measuring fluorescent substrate accumulation in suspended cells using an automated cell counter, which offers convenience, sensitivity, and speed in measuring intracellular fluorescence and identifying new inhibitors. An alternative method is provided for making similar measurements using a spectrophotometer with fluorescence detection capabilities.

Flow cytometry in cancer stem cell analysis and separation

In recent years, a special type of cancer cell--the cancer stem cell (CSC)--has been identified and characterized for different tumors. CSCs may be responsible for the recurrence of a tumor following a primarily successful therapy and are thought to bear a high metastatic potential. For the development of efficient treatment strategies, the establishment of reliable methods for the identification and effective isolation of CSCs is imperative. Similar to their stem cell counterparts in bone marrow or small intestine, different cluster of differentiation surface antigens have been characterized, thus enabling researchers to identify them within the tumor bulk and to determine their degree of differentiation. In addition, functional properties characteristic of stem cells can be measured. Side population analysis is based on the stem cell-specific activity of certain ATP-binding cassette transporter proteins, which are able to transport fluorescent dyes out of the cells. Furthermore, the stem cell-specific presence of aldehyde dehydrogenase isoform 1 can be used for CSC labeling. However, the flow cytometric analysis of these CSC functional features requires specific technical adjustments. This review focuses on the principles and strategies of the flow cytometric analysis of CSCs and provides an overview of current protocols as well as technical requirements and pitfalls. A special focus is set on side population analysis and analysis of ALDH activity. Flow cytometry-based sorting principles and future flow cytometric applications for CSC analysis are also discussed.

Breast cancer resistance protein BCRP/ABCG2 regulatory microRNAs (hsa-miR-328, -519c and -520h) and their differential expression in stem-like ABCG2+ cancer cells

Recent studies have shown that a number of microRNAs (miRNA or miR) may regulate human breast cancer resistance protein (BCRP/ABCG2), an important efflux transporter responsible for cellular drug disposition, whereas their effects on ABCG2 protein expression are not compared. In this study, we first identified a new proximal miRNA response element (MRE) for hsa-miR-519c within ABCG2 3'-untranslated region (3'UTR) through computational analyses. This miR-519c MRE site was confirmed using dual luciferase reporter assay and site-directed mutagenesis. Immunoblot analyses indicated that ABCG2 protein expression was significantly down-regulated in MCF-7/MX100 cells after transfection with hsa-miR-328- or -519c expression plasmids, and was markedly up-regulated in MCF-7 cells after transfection with miR-328 or -519c antagomir. However, ABCG2 protein expression was unchanged in MCF-7/MX100 cells after transfection with hsa-miR-520h expression plasmids, which was associated with undetectable miR-520h expression. Furthermore, ABCG2 mRNA degradation was accelerated dramatically in cells transfected with miR-519c expression plasmid, suggesting the involvement of mRNA degradation mechanism. Intervention of miR-328 or -519c signaling led to significant change in intracellular mitoxantrone accumulation, as determined by flow cytometry analyses. In addition, we separated RB143 human retinoblastoma cells into stem-like (ABCG2+) and non-stem-like (ABCG2-) populations through immunomagnetic selection, and found that miR-328, -519c and -520h levels were 9-, 15- and 3-fold lower in the ABCG2+ cells, respectively. Our data suggest that miR-519c and -328 have greater impact on ABCG2 expression than miR-520h in MCF-7 human breast cancer cells, and the presence of proximal miR-519c MRE explains the action of miR-519c on shortened ABCG2 3'UTR.

Developing multidrug-resistant cells and exploring correlation between BCRP/ABCG2 over-expression and DNA methyltransferase

Expression of breast cancer resistance protein/ATP-binding cassette sub-family G member 2 (BCRP/ABCG2) is the major cause of chemotherapy failure. It is important to establish and characterize the multidrug resistance cells and to investigate the mechanism of multidrug resistance. Multidrug-resistant cells expressing BCRP/ABCG2 based on human breast cancer MCF-7/wt cells were developed by gradually increasing application of low concentration of mitoxantrone. Real-time quantitative PCR, western blot, and immunofluorescence assay were employed to analyze BCRP mRNA and protein expression. Drug accumulation in the cells was measured by flow cytometry and DNA methyltransferases were analyzed by western blot. The results indicated that the inhibitory ratio of cell proliferative growth exhibited an exponential relation with the concentration of mitoxantrone. The IC₅₀ of MCF-7/wt cells to mitoxantrone was found to be 0.42 μM. 3-(4,5-Dimethylthlthiazol-2-YI)-2,5-Diphenyltetrazolium Bromide assay indicated that the mitoxantrone-resistant cells at different stages exhibited cross-resistance to adriamycin and taxol. BCRP/ABCG2 mRNA and protein levels in the mitoxantrone-resistant cells at different stages increased with increasing concentration of mitoxantrone. Intracellular accumulation of mitoxantrone in the cells decreased with the increase of the BCRP/ABCG2 expression levels. DNA methyltransferase 1 (DNMT1) and DNA methyltransferase 3a (DNMT3a) expressions in the cells at different stages decreased slightly, whereas DNA methyltransferase 3b (DNMT3b) expression decreases significantly. BCRP/ABCG2 overexpression and its drug-efflux function in the drug-resistant cells are the main factors to produce multidrug resistance. Our results suggest that multidrug resistance is related to overexpression of BCRP/ABCG2 and the decrease of DNA methyltransferases, especially DNMT3b.

Lovastatin induces apoptosis of ovarian cancer cells and synergizes with doxorubicin: potential therapeutic relevance

Background

Ovarian carcinoma is a rarely curable disease, for which new treatment options are required. As agents that block HMG-CoA reductase and the mevalonate pathway, the statin family of drugs are used in the treatment of hypercholesterolemia and have been shown to trigger apoptosis in a tumor-specific manner. Recent clinical trials show that the addition of statins to traditional chemotherapeutic strategies can increase efficacy of targeting statin-sensitive tumors. Our goal was to assess statin-induced apoptosis of ovarian cancer cells, either alone or in combination with chemotherapeutics, and then determine these mechanisms of action.

Methods

The effect of lovastatin on ovarian cancer cell lines was evaluated alone and in combination with cisplatin and doxorubicin using several assays (MTT, TUNEL, fixed PI, PARP cleavage) and synergy determined by evaluating the combination index. The mechanisms of action were evaluated using functional, molecular, and pharmacologic approaches.

Results

We demonstrate that lovastatin induces apoptosis of ovarian cancer cells in a p53-independent manner and synergizes with doxorubicin, a chemotherapeutic agent used to treat recurrent cases of ovarian cancer. Lovastatin drives ovarian tumor cell death by two mechanisms: first, by blocking HMG-CoA reductase activity, and second, by sensitizing multi-drug resistant cells to doxorubicin by a novel mevalonate-independent mechanism. This inhibition of drug transport, likely through inhibition of P-glycoprotein, potentiates both DNA damage and tumor cell apoptosis.

Conclusions

The results of this research provide pre-clinical data to warrant further evaluation of statins as potential anti-cancer agents to treat ovarian carcinoma. Many statins are inexpensive, off-patent generic drugs that are immediately available for use as anti-cancer agents. We provide evidence that lovastatin triggers apoptosis of ovarian cancer cells as a single agent by a mevalonate-dependent mechanism. Moreover, we also show lovastatin synergizes with doxorubicin, an agent administered for recurrent disease. This synergy occurs by a novel mevalonate-independent mechanism that antagonizes drug resistance, likely by inhibiting P-glycoprotein. These data raise important issues that may impact how statins can best be included in chemotherapy regimens.

Pim-1 expression and monoclonal antibody targeting in human leukemia cell lines

OBJECTIVE

Based on our previous findings that Pim-1 was expressed on the cell surface and could be targeted with a highly specific anti-Pim-1 monoclonal antibody (P9), this study aims to evaluate the possibility that Pim-1 could be targeted for the treatment of human leukemia.

MATERIALS AND METHODS

Pim-1 expression was investigated in a series of human leukemia cell lines with immunohistochemistry and flow cytometry. The inhibitory effect of P9 on cell proliferation was evaluated with (3)H-thymidine incorporation assay. Cell apoptosis was assayed with Annexin-V/propidium iodide dual staining. The in vivo effect of P9 was evaluated with xenograft tumor models in severe combined immunodeficient mice.

RESULTS

Pim-1 expression varied depending on the cell lines and correlated with the inhibitory effects mediated by P9. An association between Pim-1 expression and drug resistance was observed. Although the drug-resistant CEM/A7R cells were highly resistant to cytotoxic P-glycoprotein substrates, their growth was inhibited by P9 as demonstrated by in vitro proliferation assay and in vivo inhibition of xenograft tumors. P9 had little effect on P-glycoprotein expression and intracellular Rhodamine 123 accumulation, but it inhibited the phosphorylation of Bad and induced apoptosis.

CONCLUSIONS

Pim-1 is variably expressed in leukemia cell lines and associated with drug resistance. Targeting Pim-1 with monoclonal antibody could be explored for the treatment of leukemia and may represent a novel strategy to overcome drug resistance.

Identification of a pancreatic stellate cell population with properties of progenitor cells: new role for stellate cells in the pancreas

Numerous studies conducted in a diversity of adult tissues have shown that certain stem cells are characterized by the expression of a protein known as the ABCG2 transporter (where ABC is ATP- binding cassette). In the adult pancreas, although various multipotent progenitors have been proposed, the ABCG2 marker has only been detected in the so-called 'side population' (a primitive haematopoietic cell population with a multipotential capacity). In the present study we sought to identify new ABCG2+ pancreatic cell populations and to explore whether they exhibit the properties of progenitor cells. We isolated and expanded mitoxantrone-resistant cells from pancreata of lactating rats by drug selection. These cells were characterized and maintained in different stages of differentiation using several media 'cocktails' plus Matrigel (BD Biosciences). Differentiation was assessed by RT-PCR (reverse transcription-PCR), immunocytochemistry, electron microscopy and ELISA. The expanded cell population demonstrated a phenotype of PaSCs (pancreatic stellate cells). Spontaneous cell clusters occurred during cell expansion and they showed weak expression of the transcription factor Pdx1 (pancreatic and duodenal homeobox 1). Moreover, the presence of inductive factors in the Matrigel plus exendin-4 led to an increase in Pdx1 and endocrine genes, such as insulin, islet amyloid polypeptide, glucagon, the glucose transporter GLUT2, chromogranin A and the convertases PC1/3 and PC2 were also detected. Immunocytochemical analysis showed co-localization of insulin and C-peptide, whereas ultrastructural studies revealed the presence of granules. Insulin secretion from cell clusters was detected in the cell culture medium. We identified a population of PaSCs that express the ABCG2+ transporter and have the capacity to transdifferentiate into insulin-producing cells. Although the potential therapeutic application remains to be tested, PaSCs could represent a future option for insulin replacement in diabetes research.

Use of DNA-specific anthraquinone dyes to directly reveal cytoplasmic and nuclear boundaries in live and fixed cells

Image-based, high-content screening assays demand solutions for image segmentation and cellular compartment encoding to track critical events--for example those reported by GFP fusions within mitosis, signalling pathways and protein translocations. To meet this need, a series of nuclear/cytoplasmic discriminating probes have been developed: DRAQ5 and CyTRAK Orange. These are spectrally compatible with GFP reporters offering new solutions in imaging and cytometry. At their most fundamental they provide a convenient fluorescent emission signature which is spectrally separated from the commonly used reporter proteins (e.g. eGFP, YFP, mRFP) and fluorescent tags such as Alexafluor 488, fluorescein and Cy2. Additionally, they do not excite in the UV and thus avoid the complications of compound UV-autofluorescence in drug discovery whilst limiting the impact of background sample autofluorescence. They provide a convenient means of stoichiometrically labelling cell nuclei in live cells without the aid of DMSO and can equally be used for fixed cells. Further developments have permitted the simultaneous and differential labelling of both nuclear and cytoplasmic compartments in live and fixed cells to clearly render the precise location of cell boundaries which may be beneficial for quantitative expression measurements, cell-cell interactions and most recently compound in vitro toxicology testing.

Antipsychotic drugs inhibit the function of breast cancer resistance protein

The ABCG2 transporter breast cancer resistance protein (BCRP) has been identified in several physiological sites. It has been suggested to play an important role in disposition of many drugs and environmental toxins. We investigated the effects of several antipsychotic drugs, including risperidone, 9-hydroxy-risperidone (paliperidone), olanzapine, quetiapine, clozapine, haloperidol and chlorpromazine, and a positive control inhibitor Ko143 on functions of BCRP in MCF7 and BCRP over-expressing MCF7/MX100 cell lines using a BCRP prototypical substrate mitoxantrone. Our findings indicated that the tested antipsychotics rank order of potency of inhibition of BCRP according to concentrations required to reach 50% of maximum inhibition (IC(50)) was as follows: Ko143 (0.07 microM) > risperidone (38.1 microM) > clozapine (42.0 microM) > paliperidone (51 microM) > chlorpromazine (52.2 microM) > quetiapine (66.1 microM) > olanzapine = haloperidol (>100.0 microM). We further tested the effects of various concentrations of risperidone on the BCRP-mediated transport of oestrone-3-sulfate in a colon carcinoma cell line, Caco-2, a widely used model to study drug absorption. Our findings show that risperidone at concentrations ranging from 1 to 100 microM significantly inhibited intracellular accumulation of oestrone-3-sulfate in Caco-2 cell monolayers. The present results suggest that a potential source of pharmacokinetic interactions exists between BCRP substrates and several antipsychotics.

Lovastatin protects human endothelial cells from the genotoxic and cytotoxic effects of the anticancer drugs doxorubicin and etoposide

BACKGROUND AND PURPOSE

3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors (statins) are frequently used lipid-lowering drugs. Moreover, they exert pleiotropic effects on cellular stress responses and death. Here, we analysed whether lovastatin affects the sensitivity of primary human endothelial cells (HUVEC) to the anticancer drug doxorubicin.

EXPERIMENTAL APPROACH

We investigated whether pretreatment of HUVEC with low dose of lovastatin influences the cellular sensitivity to doxorubicin. To this end, cell viability, proliferation and apoptosis as well as DNA damage-triggered stress response were analysed.

KEY RESULTS

Lovastatin reduced the cytotoxic potency of doxorubicin in HUVEC. Lovastatin attenuated the doxorubicin-induced increase in p53 as well as activation of checkpoint kinase (Chk-1) and stress-activated protein kinase/c-Jun-N-terminal kinase (SAPK/JNK). Acquired doxorubicin resistance was independent of alterations in doxorubicin efflux and cell cycle progression. Also, doxorubicin-triggered production of reactive oxygen species (ROS) and formation of oxidative DNA lesions remained unaffected by lovastatin. However, lovastatin impaired DNA strand break formation induced by doxorubicin. Notably, lovastatin also conferred cross-resistance to the cytotoxic and genotoxic effects of etoposide, indicating that lovastatin shields topoisomerase II against poisons.

CONCLUSIONS AND IMPLICATIONS

Based on these data, we suggest that lovastatin-mediated resistance to topoisomerase II inhibitors is due to a reduction in DNA damage and, hence, it attenuates stress responses leading to cell death that are triggered by DNA damage. Therefore, lovastatin might be useful clinically for alleviating side-effects of anticancer therapies that include topoisomerase II inhibitors.

Differential effects of the immunosuppressive agents cyclosporin A, tacrolimus and sirolimus on drug transport by multidrug resistance proteins

PURPOSE

We sought to determine the effects of the immunosuppressants, cyclosporin A (CsA), tacrolimus and sirolimus, on drug transport by the ATP-binding cassette proteins, P-glycoprotein (Pgp; ABCB1), multidrug resistance protein-1 (MRP-1; ABCC1) and breast cancer resistance protein (BCRP; ABCG2), and the major vault protein lung resistance protein (LRP).

METHODS

Cellular content of mitoxantrone, a Pgp, MRP-1 and BCRP substrate, was measured by flow cytometry in cells overexpressing these proteins following incubation with and without CsA, tacrolimus or sirolimus. Interaction of BCRP with these compounds was studied by photolabeling and ATPase assays. Nuclear-cytoplasmic distribution of doxorubicin was studied by confocal microscopy in cells overexpressing LRP.

RESULTS

CsA increased cellular drug uptake in cells overexpressing Pgp, MRP-1 or BCRP and nuclear drug uptake in cells overexpressing LRP at the clinically achievable concentration of 2.5 microM. Tacrolimus enhanced cellular drug uptake at 1 microM, but not at 0.08 microM, its clinically achievable concentration, and did not enhance nuclear drug uptake. Sirolimus enhanced cellular drug uptake in cells overexpressing Pgp, MRP-1 and BCRP with optimal effects at 2.5 microM, but was effective at its clinically achievable concentration of 0.25 microM if cells were pre-incubated for at least 30 min before drug exposure, and also enhanced nuclear drug uptake at 0.25 microM. BCRP modulation by all three immunosuppressive agents was associated with competitive binding to the drug transport sites.

CONCLUSIONS

CsA, tacrolimus and sirolimus modulate drug transport by Pgp, MRP-1 and BCRP and CsA and sirolimus modulate drug transport by LRP at concentrations that differ from immunosuppressive concentrations and maximum tolerated concentrations.

A high-throughput cell-based assay for inhibitors of ABCG2 activity

ABCG2 is a member of the adenosine triphosphate (ATP)-binding cassette family of multidrug transporters associated with resistance of tumor cells to many cytotoxic agents. Evaluation of modulators of ABCG2 activity has relied on methods such as drug sensitization, biochemical characterization, and transport studies. To search for novel inhibitors of ABCG2, a fluorescent cell-based assay was developed for application in high-throughput screening. Accumulation of pheophorbide a (PhA), an ABCG2-specific substrate, forms the basis for the assay in NCI-H460/MX20 cells overexpressing wild-type ABCG2. Treatment of these cells with 10 microM fumitremorgin C (FTC), a specific ABCG2 inhibitor, increased cell accumulation of PhA to 5.6 times control (Z' 0.5). Validation included confirmation with known ABCG2 inhibitors: FTC, novobiocin, tariquidar, and quercetin. Verapamil, reported to inhibit P-glycoprotein but not ABCG2, had insignificant activity. Screening of a library of 3523 natural products identified 11 compounds with high activity (> or = 50% of FTC, confirmed by reassay), including 3 flavonoids, members of a family of compounds that include ABCG2 inhibitors. One of the inhibitors detected, eupatin, was moderately potent (IC50 of 2.2 microM) and, like FTC, restored sensitivity of resistant cells to mitoxantrone. Application of this assay to other libraries of synthetic compounds and natural products is expected to identify novel inhibitors of ABCG2 activity.

Cyclosporin A, tacrolimus and sirolimus are potent inhibitors of the human breast cancer resistance protein (ABCG2) and reverse resistance to mitoxantrone and topotecan

PURPOSE

Several studies have demonstrated significant interactions between immunosuppressants (e.g., cyclosporin A) and chemotherapeutic drugs that are BCRP substrates (e.g., irinotecan), resulting in increased bioavailability and reduced clearance of these agents. One possible mechanism underlying this observation is that the immunosuppressants modulate the pharmacokinetics of these drugs by inhibiting BCRP. Therefore, the aim of this study was to determine whether the immunosuppressants cyclosporin A, tacrolimus and sirolimus are inhibitors and/or substrates of BCRP.

METHODS

First, the effect of the immunosuppressants on BCRP efflux activity in BCRP-expressing HEK cells was measured by flow cytometry.

RESULTS

Cyclosporin A, tacrolimus and sirolimus significantly inhibited BCRP-mediated efflux of pheophorbide A, mitoxantrone and BODIPY-prazosin. The EC(50) values of cyclosporin A, tacrolimus and sirolimus for inhibition of BCRP-mediated pheophorbide A efflux were 4.3 +/- 1.9 microM, 3.6 +/- 1.8 microM and 1.9 +/- 0.4 microM, respectively. Cyclosporin A, tacrolimus and sirolimus also effectively reversed resistance of HEK cells to topotecan and mitoxantrone conferred by BCRP. When direct efflux of cyclosporin A, tacrolimus and sirolimus was measured, these compounds were found not to be transported by BCRP. Consistent with this finding, BCRP did not confer resistance to the immunosuppressants in HEK cells.

CONCLUSION

These results indicate that cyclosporin A, tacrolimus and sirolimus are effective inhibitors but not substrates of BCRP. These findings could explain the altered pharmacokinetics of BCRP substrate drugs when co-administered with the immunosuppressants and suggest that pharmacokinetic modulation by the immunosuppressants may improve the therapeutic outcome of these drugs.

Discrimination of live and early apoptotic mononuclear cells by the fluorescent SYTO 16 vital dye

Accurate detection of apoptotic cells is important for the determination of cell viability. The aim of this study was to compare the sensitivity of the cell permeant SYTO 16 fluorescent dye for detecting early apoptotic mononuclear cells (MNCs) in normal donor blood with other apoptosis assays [i.e. Annexin-V, light scatter/7-amino-actinomycin-D (7-AAD) and chloromethyl-X-rosamine (CMXRos)] and to identify critical parameters for optimal SYTO 16 staining. Apoptosis was induced in normal human leukocytes from adult peripheral blood or cord blood, or the Jurkat T-lymphocytic cell line and assessed by fluorescence microscopy and flow cytometry. Dual labelling showed that SYTO 16 detected more apoptotic MNCs compared to Annexin-V. SYTO 16 staining intensity was consistent with the light scatter profiles expected of live, apoptotic and necrotic MNCs and was more objective than light scatter/7-AAD. CMXRos staining required considerable care and may not be a robust marker of apoptotic primary MNCs. For SYTO 16 flow cytometric analysis, the optimal conditions for staining 1x10(6) leukocytes were 4 nM SYTO 16 in the presence of 30 muM verapamil for 25-45 min at 37 degrees C in media containing calcium/magnesium supplemented with protein. A P-glycoprotein inhibitor, such as verapamil, and calcium/magnesium are essential for optimal loading of SYTO 16 into live MNCs and discrimination of apoptotic MNCs in normal blood samples. SYTO 16 is a sensitive, simple, inexpensive 'live cell' method for the discrimination of live, apoptotic and necrotic normal blood MNCs and is more sensitive for detecting apoptosis in these cells than Annexin-V or light scatter/7-AAD.