Cyclooxygenase (COX) inhibitors induce apoptosis in non-small cell lung cancer through cyclooxygenase independent pathways

Cyclooxygenase (COX) inhibitors are chemopreventive in many tumours but the role of COX inhibition in their effects is contentious. Here we determined if COX inhibitors influenced apoptosis in two non-small cell lung cancer cells one which over expresses COX-2 (MOR-P) and one which expresses neither isoform (H-460). NS398, a selective COX inhibitor, and indomethacin, a non-selective COX inhibitor, were cytotoxic in both cell lines, independently of their COX-2 expression. Furthermore, the cytotoxic concentrations were far greater than the concentrations required to inhibit COX. As indomethacin was more effective we used it in mechanistic studies. Indomethacin induced apoptotic cell death assessed as cytochrome c and apoptotic inducing factor (AIF) release, caspase activation, PARP, lamin B and gelsolin cleavage, chromatin condensation and nuclear fragmentation. The pan-caspase inhibitor, z-VAD, attenuated cell death, and blocked caspase activation, PARP cleavage and nuclear fragmentation without preventing cytochrome c release, suggesting that cytochrome c release is upstream of caspase activation. These observations suggest that COX inhibitors induce apoptosis in non-small lung cancer cells through cytochrome c and AIF release, and subsequent caspase activation, independently of COX-2 expression and prostaglandin production.     

Caspase I-related protease inhibition retards the execution of okadaic acid- and camptothecin-induced apoptosis and PAI-2 cleavage, but not commitment to cell death in HL-60 cells

We have previously reported that the putative cytoprotective protease inhibitor, plasminogen activator inhibitor type 2 (PAI-2), is specifically cleaved during okadaic acid-induced apoptosis in a myeloid leukaemic cell line (Br J Cancer (1994) 70: 834-840). HL-60 cells exposed to okadaic acid and camptothecin underwent morphological and biochemical changes typical of apoptosis, including internucleosomal DNA fragmentation and PAI-2 cleavage. Significant endogenous PAI-2 cleavage was observed 9 h after exposure to okadaic acid; thus correlating with other signs of macromolecular degradation, like internucleosomal DNA fragmentation. In camptothecin-treated cells, PAI-2 cleavage was an early event, detectable after 2 h of treatment, and preceding internucleosomal DNA fragmentation. The caspase I selective protease inhibitor, YVAD-cmk, inhibited internucleosomal DNA fragmentation and PAI-2 cleavage of okadaic acid and camptothecin-induced apoptotic cells. YVAD-cmk rather sensitively and non-toxically inhibited camptothecin-induced morphology, but not okadaic acid-induced morphology. In in vitro experiments recombinant PAI-2 was not found to be a substrate for caspase I. The results suggest that caspase I selective protease inhibition could antagonize parameters coupled to the execution phase of okadaic acid- and camptothecin-induced apoptosis, but not the commitment to cell death.     

Early RB94-produced cytotoxicity in cancer cells is independent of caspase activation or 50 kb DNA fragmentation

RB94, which lacks the N-terminal 112 amino-acid residues of the full-length retinoblastoma protein (RB110) is a more potent inhibitor of cancer cell growth than RB110, being cytotoxic to all cancer cell lines studied, independent of their genetic abnormalities. Although we initially thought RB94-induced cell death was caspase-dependent, such caspase activation now appears to be a late event. Cells that remained attached 48 h after transduction with Ad-RB94 showed, among other changes, nuclear enlargement, peripheral nuclear chromatin condensation and often micronucleation. In addition, the cells were TdT-mediated dUTP nick end labeling (TUNEL) positive but showed no cleavage of caspase 3 or 9. Only after the cells detached was cleavage of both caspase 3 and 9 observed. These TUNEL-positive cells showed neither cytochrome c mitochondrial translocation usually found in typical apoptotic cells nor DNA laddering indicative of oligonucleosomal DNA fragmentation. In addition, although 50 kb DNA fragmentation was produced in these TUNEL-positive cells, which was dependent on apoptosis-inducing factor (AIF), inhibiting this fragmentation by siAIF did not inhibit TUNEL formation or cytotoxicity. As RB94 will soon be used for gene therapy further understanding the molecular basis of these early changes in killing cancer cells is one of our particularly important present goals.     

Caspases as key executors of methyl selenium-induced apoptosis (anoikis) of DU-145 prostate cancer cells

Apoptosis induction may be a mechanism mediating the anticancer activity of selenium. Our earlier work indicated that distinct cell death pathways are likely involved in apoptosis induced by the CH3SeH and the hydrogen selenide pools of selenium metabolites. To explore the role of caspases in cancer cell apoptosis induced by selenium, we examined the involvement of these molecules in the death of the DU-145 human prostate carcinoma cells induced by methylseleninic acid (MSeA), a novel penultimate precursor of the putative critical anticancer metabolite CH3SeH. Sodium selenite, a representative of the genotoxic selenium pool, was used as a reference for comparison. The results show that MSeA-induced apoptosis was accompanied by the activation of multiple caspases (caspase-3, -7, -8, and -9), mitochondrial release of cytochrome c (CC), poly(ADP-ribose) polymerase (PARP) cleavage, and DNA fragmentation. In contrast, selenite-induced apoptotic DNA fragmentation was observed in the absence of these changes, but was associated with the phosphorylation of c-Jun-NH2-terminal kinase 1/2 and p38 mitogen-activated protein kinase/stress-activated protein kinase 2. A general caspase inhibitor, benzyloxycarbonyl-Val-Ala-Asp-(OMe) fluoromethyl ketone, blocked MSeA-induced cleavage of procaspases and PARP, CC release, and DNA nucleosomal fragmentation, but did not prevent cell detachment. Furthermore, PARP cleavage and caspase activation were confined exclusively to detached cells, indicating that MSeA induction of cell detachment was a prerequisite for caspase activation and apoptosis execution. This process therefore resembled "anoikis," a special mode of apoptosis induction in which adherent cells lose contact with the extracellular matrix. Additional experiments with irreversible caspase inhibitors show that MSeA-induced anoikis involved caspase-3- and -7-mediated PARP cleavage that was initiated by caspase-8 and probably amplified through CC-caspase-9 activation and a feedback activation loop from caspase-3. Taken together, the data support a methyl selenium-specific induction of DU-145 cell apoptosis that involves cell detachment as a prerequisite (anoikis) and is executed principally through caspase-8 activation and its cross-talk with multiple caspases.     

Induction of medulloblastoma cell apoptosis by sulforaphane, a dietary anticarcinogen from Brassica vegetables

There is increasing evidence that a variety of natural substances derived from the diet may act as potent chemopreventive agents. In this work, we show that DAOY cells, a widely used model of metastatic medulloblastoma (MBL), are highly sensitive to sulforaphane, a naturally occurring isothiocyanate from Brassica vegetables. Sulforaphane induced DAOY cell death by apoptosis, as determined by DNA fragmentation and chromatin condensation. DAOY apoptosis correlates with the induction of caspase-3 and -9 activities, resulting in the cleavage of PARP and vimentin. Both the cytotoxic effect and apoptotic characteristics induced by sulforaphane were reversed by zVAD-fmk, a broad spectrum caspase inhibitor, demonstrating the important role of caspases in its cytotoxic effect. These results identify sulforaphane as a novel inducer of MBL cell apoptosis, supporting the potential clinical usefulness of diet-derived substances as chemopreventive agents.     

Apoptotic activity of novel bile acid derivatives in human leukemic T cells through the activation of caspases

The therapeutic efficacies of bile acids, such as ursodeoxycholic acid (UDCA) and chenodeoxycholic acid (CDCA), have been widely demonstrated in various liver diseases, suggesting that they might protect hepatocytes against common mechanisms of liver damage. Although they have been shown to prevent apoptotic cell death in certain cell lines, we have previously reported that a novel derivative (HS-1030) of UDCA significantly inhibited cell growth and induced apoptosis in cancer cells. To develop more effective agents, we synthesized several derivatives, named HS-1183, HS-1199 and HS-1200, based on the structure of UDCA and CDCA, and investigated them for anti-proliferative activity in Jurkat cells, a human leukemic T cell line. Whereas UDCA and CDCA had no significant effects on the growth of Jurkat cells in the concentration range tested, both HS-1199 and HS-1200 completely inhibited the cell proliferation, and HS-1183 showed only a weak inhibitory activity. Furthermore, chromatin condensation, DNA ladder formation and proteolytic cleavage of poly(ADP-ribose) polymerase (PARP) were observed after treatment of novel bile acids, indicating the occurrence of apoptotic cell death, which was associated with down-regulation of caspase-3 and -8. The apoptotic manifestations such as PARP cleavage and DNA fragmentation were abolished in the presence of the tripeptide caspase inhibitor zVAD-fmk or the specific caspase-3 inhibitor DEVD-fmk. Our data thus demonstrate that novel bile acid derivatives-induced apoptosis of leukemic T cells is dependent on caspase activation.     

Cleavage of Bcl-2 is an early event in chemotherapy-induced apoptosis of human myeloid leukemia cells.

The proto-oncogene product Bcl-2 protects a wide variety of cell types from apoptosis via a hitherto unknown mechanism. Bcl-2 has been shown to function upstream of the death proteases (caspases) in some, but not all, occurrences of apoptotic cell death. Using the myeloid leukemic cell line P39 we report the chemotherapy-induced caspase-dependent cleavage of endogenous Bcl-2. Etoposide treatment of these cells triggered a time-dependent activation of type II and type III caspases and cleavage of Bcl-2 yielding a 23 kDa cleavage fragment. The emergence of this cleavage product was blocked by the general caspase inhibitor zVAD-fmk, as well as the type III caspase inhibitor IETD-fmk and the caspase-9-selective inhibitor LEHD-fmk, while the type II caspase inhibitor DEVD-fmk proved considerably less efficient. Bcl-2 cleavage preceded cleavage of the known caspase-3 substrate, poly(ADP-ribose) polymerase (PARP), as well as that of the caspase-6 substrate, lamin B, indicating that Bcl-2 cleavage is a relatively early event in the apoptosis cascade in this experimental model. While evidence for cleavage of Bcl-2 in several subcellular compartments of etoposide-treated cells was obtained, this cleavage was detected predominantly in the mitochondrial fraction, thus providing further support for the central role of mitochondria in apoptosis. Caspase-mediated cleavage following etoposide treatment of these myeloid leukemic cells may represent a means for the attenuation of Bcl-2 function upon apoptosis induction.     

Lysosomal proteases as potential targets for the induction of apoptotic cell death in human neuroblastomas

Neuroblastoma is the most common type of cancer in infants. In children this tumor is particularly aggressive; despite various new therapeutic approaches, it is associated with poor prognosis. Given the importance of endosomal-lysosomal proteolysis in cellular metabolism, we hypothesized that inhibition of lysosomal protease would impact negatively on neuroblastoma cell survival. Treatment with E-64 or CA074Me (2 specific inhibitors of cathepsin B) or with pepstatin A (a specific inhibitor of cathepsin D) was cytotoxic for 2 neuroblastoma cell lines having different degrees of malignancy. Cell death was associated with condensation and fragmentation of chromatin and externalization of plasma membrane phosphatidylserine, 2 hallmarks of apoptosis. Concomitant inhibition of the caspase cascade protected neuroblastoma cells from cathepsin inhibitor-induced cytotoxicity. These data indicate that prolonged inhibition of the lysosomal proteolytic pathway is incompatible with cell survival, leading to apoptosis of neuroblastoma cells, and that the cathepsin-mediated and caspase-mediated proteolytic systems are connected and cooperate in the regulation of such an event. Since modern antitumor chemotherapy is aimed at restoring the normal rate of apoptosis in neoplastic tissues, the demonstration that endosomal-lysosomal cathepsins are involved in this process may constitute a basis for novel strategies that include cathepsin inhibitors in the therapeutic regimen.     

Reconstitution of caspase 3 sensitizes MCF-7 breast cancer cells to doxorubicin- and etoposide-induced apoptosis

MCF-7, a breast cancer-derived cell line, is deficient of caspase 3 and relatively insensitive to many chemotherapeutic agents. To study the association of caspase 3 deficiency and chemotherapeutic resistance, we reconstituted caspase 3 in MCF-7 cells and characterized their apoptotic response to doxorubicin and etoposide. Western blots demonstrated that caspase 3 was constitutively expressed in the reconstituted MCF-7 cells. Both morphological observation and survival assays showed that caspase 3 reconstitution significantly sensitized MCF-7 cells to both drugs. Remarkably increased activation of caspases 3, 6, and 7, cleavage of cellular death substrates, and DNA fragmentation were detected in the reconstituted MCF-7 cells after drug treatment. Together, these data demonstrated a specific role for caspase 3 in chemotherapy-induced apoptosis and in activation of caspases 6 and 7. Our results also suggest that caspase 3 deficiency may contribute to chemotherapeutic resistance in breast cancer.     

Aspirin induces cell death and caspase-dependent phosphatidylserine externalization in HT-29 human colon adenocarcinoma cells.

The induction of cell death by aspirin was analysed in HT-29 colon carcinoma cells. Aspirin induced two hallmarks of apoptosis: nuclear chromatin condensation and increase in phosphatidylserine externalization. However, aspirin did not induce either oligonucleosomal fragmentation of DNA, decrease in DNA content or nuclear fragmentation. The effect of aspirin on Annexin V binding was inhibited by the caspase inhibitor Z-VAD.fmk, indicating the involvement of caspases in the apoptotic action of aspirin. However, aspirin did not induce proteolysis of PARP, suggesting that aspirin does not increase nuclear caspase 3-like activity in HT-29 cells. This finding may be related with the 'atypical' features of aspirin-induced apoptosis in HT-29 cells.     

The bisphosphonate zoledronic acid impairs Ras membrane [correction of impairs membrane] localisation and induces cytochrome c release in breast cancer cells

Bisphosphonates are well established in the management of cancer-induced bone disease. Recent studies have indicated that these compounds have direct inhibitory effects on cultured human breast cancer cells. Nitrogen-containing bisphosphonates including zoledronic acid have been shown to induce apoptosis associated with PARP cleavage and DNA fragmentation. The aim of this study was to identify the signalling pathways involved. Forced expression of the anti-apoptotic protein bcl-2 attenuated bisphosphonate-induced loss of cell viability and induction of DNA fragmentation in MDA-MB-231 cells. Zoledronic acid-mediated apoptosis was associated with a time and dose-related release of mitochondrial cytochrome c into the cytosol in two cell lines. Rescue of cells by preincubation with a caspase-3 selective inhibitor and demonstration of pro-caspase-3 cleavage products by immunoblotting suggests that at least one of the caspases activated in response to zoledronic acid treatment is caspase-3. In both MDA-MB-231 and MCF-7 breast cancer cells, zoledronic acid impaired membrane localisation of Ras indicating reduced prenylation of this protein. These observations demonstrate that zoledronic acid-mediated apoptosis is associated with cytochrome c release and consequent caspase activation. This process may be initiated by inhibition of the enzymes in the mevalonate pathway leading to impaired prenylation of key intracellular proteins including Ras.     

Evaluation of 2-deoxy-D-glucose as a chemotherapeutic agent: mechanism of cell death

Nutrient deprivation has been shown to cause cancer cell death. To exploit nutrient deprivation as anti-cancer therapy, we investigated the effects of the anti-metabolite 2-deoxy-D-glucose on breast cancer cells in vitro. This compound has been shown to inhibit glucose metabolism. Treatment of human breast cancer cell lines with 2-deoxy-D-glucose results in cessation of cell growth in a dose dependent manner. Cell viability as measured by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide conversion assay and clonogenic survival are decreased with 2-deoxy-D-glucose treatment indicating that 2-deoxy-D-glucose causes breast cancer cell death. The cell death induced by 2-deoxy-D-glucose was found to be due to apoptosis as demonstrated by induction of caspase 3 activity and cleavage of poly (ADP-ribose) polymerase. Breast cancer cells treated with 2-deoxy-D-glucose express higher levels of Glut1 transporter protein as measured by Western blot analysis and have increased glucose uptake compared to non-treated breast cancer cells. From these results we conclude that 2-deoxy-D-glucose treatment causes death in human breast cancer cell lines by the activation of the apoptotic pathway. Our data suggest that breast cancer cells treated with 2-deoxy-D-glucose accelerate their own demise by initially expressing high levels of glucose transporter protein, which allows increased uptake of 2-deoxy-D-glucose, and subsequent induction of cell death. These data support the targeting of glucose metabolism as a site for chemotherapeutic intervention by agents such as 2-deoxy-D-glucose.     

Phytosphingosine induces apoptotic cell death via caspase 8 activation and Bax translocation in human cancer cells.

PURPOSE: Sphingolipid metabolites, such as sphingosine and ceramide, are highly bioactive compounds and are involved in diverse cell processes, including cell-cell interaction, cell proliferation, differentiation, and apoptosis. However, the physiological roles of phytosphingosine are poorly understood. In this study, we report that phytosphingosine can potently induce apoptotic cell death in human cancer cells via caspase activation and caspase-independent cytochrome c release. EXPERIMENTAL DESIGN: Phytosphingosine-induced apoptosis was determined by Hoechst 33258 staining, flow cytometric analysis, and DNA fragmentation assay. Involvement of caspases was determined by immunoblot analysis and cell death detection assays after treatment with synthetic inhibitor z-Val-Ala-Asp-fluoromethyl ketone, z-DEVD-fmk, or z-IETD-fmk. Death receptor (DR) dependency was analyzed by examining expression of DRs (Fas, DR4, DR5, TNFR1, and R2), and interaction of Fas-associated death domain and caspase 8. Involvement of the mitochondria pathway was examined by monitoring of the mitochondria membrane potential, cytochrome c release, and Bax translocation. RESULTS: Phytosphingosine-treated cells displayed several features of apoptosis, including increase of sub-G(1) population, DNA fragmentation, and poly(ADP-ribose) polymerase cleavage. We observed that phytosphingosine cause activation of caspase 8 in a DR-independent fashion. Phytosphingosine also induced activation of caspase 9 and 3, loss of mitochondrial membrane potential, and the cytochrome c release from mitochondria. However, we failed to detect Bid cleavage. Moreover, caspase 8 inhibitor z-IETD-fmk did not affect phytosphingosine-induced cytochrome c release and caspase 9 activation, suggesting that phytosphingosine-induced cytochrome c release is caused by caspase 8-independent manner. Phytosphingosine induced mitochondrial translocation of Bax from the cytosol without changes in the protein levels of Bcl-2, Bcl-xL, and Bax. In addition, Bcl-2/Bax interaction was diminished after addition of phytosphingosine. CONCLUSION: These findings indicate that phytosphingosine induces apoptotic cell death in human cancer cells by direct activation of caspase 8, and by mitochondrial translocation of Bax and subsequent release of cytochrome c into cytoplasm, providing a potential mechanism for the anticancer activity of phytosphingosine.     

An atypical caspase-independent death pathway for an immunogenic cancer cell line

REGb cell line, a highly immunogenic tumor cell variant isolated from a rat colon cancer, yields regressive tumors when injected into syngeneic hosts. We previously demonstrated that REGb tumor immunogenicity was related to the capacity of releasing dead cells in vivo. Also, in vitro, REGb cell monolayers release dead cells, especially when cultured in serum-free medium. In the current study, we show that the release of dead cells results from an atypical death process associating features of necrosis and apoptosis. In spite of features considered as hallmarks of caspase-dependent apoptosis, including chromatin fragmentation and DNA oligonucleosomal cleavage, caspases are not activated and caspase inhibitors are ineffective to prevent REGb cell death. In contrast with a number of other types of cell death, the spontaneous death of REGb cells in culture depends on de novo protein synthesis as this death is blocked by low doses of the mRNA translation inhibitor cycloheximide. This unusual mode of cell death that associates necrotic and apoptotic features could provide optimal conditions for triggering a specific immune response.     

Polyporenic acid C induces caspase-8-mediated apoptosis in human lung cancer A549 cells

Lung cancer continues to be the leading cause of cancer-related mortality worldwide. This warrants the search for new and effective agents against lung cancer. We and others have recently shown that lanostane-type triterpenoids isolated from the fungal species Poria cocos (P. cocos) can inhibit cancer growth. However, the mechanisms responsible for the anticancer effects of these triterpenoids remain unclear. In this study, we investigated the effect of polyporenic acid C (PPAC), a lanostane-type triterpenoid from P. cocos, on the growth of A549 nonsmall cell lung cancer cells (NSCLC). The results demonstrate that PPAC significantly reduced cell proliferation via induction of apoptosis as evidenced by sub-G1 analysis, annexin V-FITC staining, and increase in cleavage of procaspase-8, -3, and poly(ADP-ribose)-polymerase (PARP). However, unlike our previously reported lanostane-type triterpenoid, pachymic acid, treatment of cells with PPAC was not accompanied by disruption of mitochondrial membrane potential and increase in cleavage of procaspase-9. Further, PPC-induced apoptosis was inhibited by caspase-8 and pan caspase inhibitors but not by a caspase-9 inhibitor. Taken together, the results suggest that PPAC induces apoptosis through the death receptor-mediated apoptotic pathway where the activation of caspase-8 leads to the direct cleavage of execution caspases without the involvement of the mitochondria. Furthermore, suppressed PI3-kinase/Akt signal pathway and enhanced p53 activation after PPAC treatment suggests this to be an additional mechanism for apoptosis induction. Together, these results encourage further studies of PPAC as a promising candidate for lung cancer therapy.     

Anti-proliferative effect of clitocine from the mushroom Leucopaxillus giganteus on human cervical cancer HeLa cells by inducing apoptosis

Clitocine, a natural biologically active substance isolated from the mushroom Leucopaxillus giganteus, possesses several bioactivities including antitumor. Here, for the first time, we studied the molecular mechanism of clitocine-induced apoptosis in human cervical cancer cells (HeLa). Clitocine-induced cell death was characterized with the changes in cell morphology, DNA fragmentation, activation of caspase-3, -8, and -9 (like) activities, poly(ADP-ribose) polymerase (PARP) cleavage, release of cytochrome c (cyt c) into cytosol, and increase of Bax:Bcl-2 ratio. These results indicated that the induction of apoptosis by clitocine involved the multiple pathway including death receptor and mitochondrial pathways, and strongly suggested that the mitochondrial pathways were mediated by down-regulation of Bcl-2 and up-regulation of Bax, release of cytochrome c and subsequent activation of caspase-3 followed by down stream events leading to apoptotic mode of cell death.     

Ciprofloxacin-induced G2 arrest and apoptosis in TK6 lymphoblastoid cells is not dependent on DNA double-strand break formation

Drugs developed for the treatment of conditions other than neoplasia can also show promise as potential antitumor agents. The fluoroquinolone antibiotic ciprofloxacin (CPFX) is known to modulate cycle cell progression and apoptosis in cancer cells, and is thought to induce DNA double-strand breaks (DSBs) via topoisomerase II (topo II) inhibition and stabilized cleavage complex (SCC) formation. DSBs trigger Ser-139 phosphorylation of histone H2AX (gammaH2AX) by PI-3-like kinases including ATM; gammaH2AX can serve as a marker of DNA damage when measured in situ using immunocytochemistry and flow cytometry. The aim of the present study was to investigate the relationship between CPFX-mediated DNA damage and induction of apoptosis in human lymphoblastoid cells and phytohaemagglutinin (PHA)-stimulated lymphocytes (Lymphs). Treatment of TK6 cells (wild-type p53) with 100 microg/ml CPFX for 2-10 h produced no increase in gammaH2AX; to the contrary, its level in S phase cells was reduced at 10 h compared to controls. Nevertheless, stabilization of topo IIalpha, ATM Ser-1981 phosphorylation and G(2) arrest was observed in TK6 cells exposed to CPFX for > or = 4 h. However, following 24 h treatment, gammaH2AX was dramatically increased in a sub-population of cells indicating the onset of apoptosis (confirmed by presence of activated caspase 3). CPFX had a similar lack of effect on induction of gammaH2AX at early time points in WTK1 and NH32 cells (devoid of functional p53) and proliferating Lymphs, however, induction of apoptosis was less pronounced than in TK6 cells. Formation of SCC and activation of ATM (but lack of gammaH2AX induction) indicates topo II-mediated chromatin or DNA changes in the absence of DSBs; ATM activation apparently triggers the G(2)M checkpoint leading to G(2) arrest. The subsequent induction of apoptosis appears to be facilitated by functional p53. CPFX may therefore have a potential use as a chemotherapeutic agent in the treatment of lymphoblast-derived cancer.     

Treatment with inhibitors of caspases, that are substrates of drug transporters, selectively permits chemotherapy-induced apoptosis in multidrug-resistant cells but protects normal cells.

Many chemotherapeutic agents induce apoptosis in tumor cells, but killing of normal cells remains a major obstacle. Development of multidrug resistance further limits chemotherapy in cancer. Here, I show that multidrug resistance can be exploited for selective killing of multidrug-resistant cells by a combination of an apoptosis-inducing agent that is not a substrate of either Pgp or MRP (e.g. flavopiridol) with a caspase inhibitor that is a substrate (e.g. Z-DEVD-fmk). In normal cells, treatment with caspase inhibitors prevented PARP cleavage, nuclear fragmentation, and cell death caused by flavopiridol or epothilone B. In contrast, Pgp- and MRP-expressing cells were not rescued by caspase inhibitors. Furthermore, reversal of drug resistance renders Pgp cells sensitive to caspase inhibitors abolishing therapeutic advantage. Thus, caspase inhibitors, that are inactive in multidrug-resistant cells, protect normal but not multidrug-resistant cells against chemotherapy, permitting selective eradication of multidrug-resistant cells. Clinical application of this approach may diminish the toxic side-effects of chemotherapy in patients with multidrug-resistant tumors.