Selective anti-leukaemic activity of low-dose histone deacetylase inhibitor ITF2357 on AML1/ETO-positive cells

We analysed the in vitro effects of a new hydroxamate derivative, ITF2357, on AML cells. ITF2357 potently induced histone acetylation. ITF2357 0.1 microM blocked proliferation and induced apoptosis in AML1/ETO-positive Kasumi-1 cells, while AML1/ETO-negative HL60, THP1 and NB4 cell lines were sensitive only to 1 microM ITF2357. Apoptosis was induced by 0.1 microM ITF2357 in AML1/ETO-positive primary blasts and U937-A/E cells induced to express AML1/ETO, but not in U937-A/E cells non-expressing AML1/ETO. In Kasumi-1 cells 0.1 microM ITF2357 induced AML1/ETO degradation through a caspase-dependent mechanism. ITF2357 0.1 microM also determined DNMT1 efflux from, and p300 influx to, the nucleus. Moreover, 0.1 microM ITF2357 determined local H4 acetylation and release of DNMT1, HDAC1 and AML1/ETO, paralleled by recruitment of p300 to the IL-3 gene promoter. ITF2357 treatment, however, did not induce re-expression of IL-3 gene. Accordingly, the methylation level of IL-3 promoter, as well as of several other genes, was unmodified. In conclusion, ITF2357 emerged as an anti-leukaemic agent very potent on AML cells, and on AML1/ETO-positive cells in particular. More relevantly, clearly emerged from our results that ITF2357 could be an ideal agent to treat AML subtypes presenting AML1/ETO fusion protein which determine HDAC involvement in leukaemogenesis.     

Histone deacetylase inhibitor NVP-LAQ824 has significant activity against myeloid leukemia cells in vitro and in vivo.

NVP-LAQ824 is a novel potent hydroxamic acid-derived histone deacetylase inhibitor that induces apoptosis in nanomolar concentrations in myeloid leukemia cell lines and patient samples. Here we show the activity of NVP-LAQ824 in acute myeloid leukemia cells and BCR/ABL-expressing cells of mouse and human origin, both sensitive and resistant to imatinib mesylate (Gleevec, STI-571). Whereas imatinib inhibited overall cellular tyrosine phosphorylation in Ba/F3.p210 cells, NVP-LAQ824 did not inhibit tyrosine phosphorylation, and did not affect BCR/ABL or ABL protein expression. Neither compound was able to inhibit cellular tyrosine phosphorylation in the imatinib-resistant Ba/F3.p210-T315I cell line. These data taken together suggest that BCR/ABL kinase activity is not a direct target of NVP-LAQ824. Synergy between NVP-LAQ824 and imatinib was demonstrated against BCR/ABL-expressing K562 myeloid leukemia cell lines. In addition, we show that NVP-LAQ824 was well tolerated in vivo in a pre-clinical murine leukemia model, with antileukemia activity resulting in significant prolongation of the survival of mice when treated with NVP-LAQ824 compared to control mice. Taken together, these findings provide the framework for NVP-LAQ824 as a novel therapeutic in myeloid malignancies.     

Troglitazone inhibits histone deacetylase activity in breast cancer cells

We previously demonstrated that the PPARγ agonist Troglitazone (TRG), a potent antiproliferative agent, in combination with the anthracycline antibiotic Doxorubicin (DOX), is an effective killer of multiple drug resistant (MDR) human cancer cells. Cell killing was accompanied by increased global histone H3 acetylation. Presently, we investigated the epigenetic and cell killing effects of TRG in estrogen receptor (ER) positive MCF7 breast cancer cells. MCF7 cells were treated with the Thiazolidinediones (TZDs) TRG and Ciglitazone (CIG), the non-TZD PPARγ agonist 15PGJ2, and the histone deacetylase inhibitors (HDACi’s) Trichostatin A (TSA), sodium butyrate and PXD101. Using MTT cell viability assays, Western analyzes and mass spectrometry, we showed a dose-dependent increase in cell killing in TRG and HDACi treated cells, that was associated with increased H3 lysine 9 (H3K9) and H3K23 acetylation, H2AX and H3S10 phosphorylation, and H3K79 mono- and di-methylation. These effects were mediated through an ER independent pathway. Using HDAC activity assays, TRG inhibited HDAC activity in cells and in cell lysates, similar to that observed with TSA. Furthermore, TRG and TSA induced a slower migrating HDAC1 species that was refractory to HDAC2 associations. Lastly, TRG and the HDACi’s decreased total and phosphorylated AKT levels. These findings suggest that TRG’s mode of killing may involve downregulation of PI3K signaling through HDAC inhibition, leading to increased global histone post-translational modifications.     

Antitumor activity of histone deacetylase inhibitor trichostatin A in osteosarcoma cells

Background: Histone deacetylase (HDAC) inhibitors have been reported to induce cell growth arrest,apoptosis and differentiation of tumor cells. The present study aimed to examine the effects of trichostatin A(TSA), one such inhibitor, on the cell cycle, apoptosis and invasiveness of osteosarcoma cells. Methods: MG-63 cells were treated with TSA at various concentrations. Then, cell growth and apoptosis were determinedby 3-(4, 5-dimethyl-2-thiazolyl)-2H-tetrazolium bromide (MTT) and TUNEL assays, respectively; cell cyclingwas assessed by flow cytometry; invasion assays were performed with the transwell Boyden Chamber system.Results: MTT assays revealed that TSA significantly inhibited the growth of MG-63 cells in a concentrationand time dependent manner. TSA treated cells demonstrated morphological changes indicative of apoptosisand TUNEL assays revealed increased apoptosis of MG-63 cells after TSA treatment. Flow cytometry showedthat TSA arrested the cell cycle in G1/G2 phase and annexin V positive apoptotic cells increased markedly. Inaddition, the invasiveness of MG-63 cells was inhibited by TSA in a concentration dependent manner. Conclusion:Our findings demonstrate that TSA inhibits the proliferation, induces apoptosis and inhibits invasiveness ofosteosarcoma cells in vitro. HDAC inhibitors may thus have promise to become new therapeutic agents againstosteosarcoma.     

Inhibition of hepatocellular carcinomas in vitro and hepatic metastases in vivo in mice by the histone deacetylase inhibitor HA-But.

PURPOSE: The purpose is to evaluate the CD44-mediated cellular targeting of HA-But, a hyaluronic acid esterified with butyric acid (But) residues, to hepatocellular carcinoma cell lines in vitro and to hepatic tumor metastases in vivo. EXPERIMENTAL DESIGN: In vitro, the CD44-dependent cytotoxicity in two human hepatocellular carcinoma cell lines (HepB3 and HepG2) with high and low CD44 expression was investigated; in vivo, the effect on liver metastases originating from intrasplenic implants of Lewis lung carcinoma (LL3) or B16-F10 melanoma in mice was compared with the pharmacokinetics of organ and tissue distribution using different routes of administration. RESULTS: HepB3 and HepG2 cell lines showed different expression of CD44 (78 and 18%, respectively), which resulted in a CD44-dependent HA-But inhibitory effect as demonstrated also by the uptake analysis performed using radiolabeled HA-But ((99m)Tc-HA-But). Pharmacokinetic studies showed different rates of (99m)Tc-HA-But distribution according to the route of administration (i.v., i.p., or s.c.): very fast (a few minutes) after i.v. treatment, with substantial accumulation in the liver and spleen; relatively slow after i.p. or s.c. treatment, with marked persistence of the drug at the site of injection. The effect of s.c. and i.p. treatment with HA-But on liver metastases originating from intrasplenic implants of LL3 carcinoma or B16-F10 melanoma (both CD44-positive: 68 and 87%, respectively), resulted in 87 and 100% metastases-free animals, respectively (regardless of the route of administration), and a significant prolongation of the life expectancy compared with control groups. CONCLUSIONS: HA-But tends to concentrate in the liver and spleen and appears to be a promising new drug for the treatment of intrahepatic tumor lesions.     

The histone deacetylase inhibitor vorinostat induces calreticulin exposure in childhood brain tumour cells in vitro

Purpose

It has recently been recognised that anticancer chemotherapy can elicit an immunogenic form of apoptosis characterised by the exposure of calreticulin (CRT) on the surface of dying tumour cells, entailing an immune response that contributes to the therapeutic outcome. CRT exposure has been found to be induced by anthracyclins and oxaliplatin, but not by other proapoptotic antineoplastic agents including etoposide, camptothecin and cisplatin. In this study, we examined the histone deacetylase inhibitor vorinostat for its capability to stimulate CRT exposure in tumour cells.

Methods

Childhood tumour cells, i.e. the brain tumour cell lines PFSK and DAOY and the Ewing’s sarcoma cell line CADO-ES-1, were treated with vorinostat, and CRT exposure was determined by flow cytometric analysis of CRT immunofluorescence. Combination effects of vorinostat/TRAIL and vorinostat/bortezomib were also assessed.

Results

Vorinostat treatment induced CRT exposure in PFSK and DAOY cells, but not in caspase-8-deficient CADO-ES-1 cells. CRT exposure could be prevented by the pan-caspase inhibitor z-VAD-fmk and by brefeldin A, an inhibitor of Golgi-mediated transport.

Conclusion

Vorinostat has the capacity to elicit CRT exposure, suggesting its usefulness as immunogenic antitumour agent.     

Significant growth suppression of synovial sarcomas by the histone deacetylase inhibitor FK228 in vitro and in vivo

About 97% of synovial sarcomas harbor the SYT-SSX fusion gene by chromosomal translocation. We found that the histone deacetylase (HDAC) inhibitor FK228 significantly suppressed the growth of synovial sarcoma cells as compared with that of osteosarcoma. The 50% growth inhibition IC50 value we obtained for FK228 was 0.02-0.2 nM, and it indicates that its suppression effect on synovial sarcoma cells is the highest of any of the HDAC inhibitors yet reported. It was not likely that the growth suppression of FK228 depends on the doubling time of these cells. Introduction of SYT-SSX cDNA into HEK293 cells enhanced the sensitivity of the cells for FK228. Immunostaining of the FK228-treated cells using an anti-acetyl-histone H3 antibody showed that FK228 inhibits deacetylation of histone. In a mice assay, the growth of synovial sarcoma cells was markedly inhibited by FK228 treatment, and the invasion of tumors into surrounding tissues was suppressed. These results suggest that FK228 may be useful in developing therapeutic strategies to treat synovial sarcoma.     

DNA甲基化转移酶抑制剂和组蛋白脱乙酰化酶抑制剂的体外抗肿瘤作用

目的:通过在体外分别对DNA甲基化转移酶(DNA methyltransferase,DNMT)抑制剂5-氮杂胞苷(5-azacytidine,5-aza-C)和组蛋白脱乙酰化(histone seacetylase,HDAC)抑制剂苯丁酸钠(sodium phenylbutyrate,SPB)单独及联合用药于人大肠癌细胞LoVo和人肝癌细胞Hep3B的抗肿瘤作用的观察,探讨DNMT抑制剂和HDAC抑制剂联合用药的可能性.方法:使用MTT法测定5-aza-C 和SPB单独用药或联合用药的抗癌活性,用抑制浓度的分数之和(a sum of fractional inhibitory concentration ,SFIC)值及等效剂量分析方法(Isobologram)评价5-aza-C 和SPB联合用药的作用性质.结果:5-aza-C和SPB联用时,LoVo细胞及Hep3B细胞得到的SFIC值均小于1.0,等效剂量分析图的形状呈现凹型,都表明了5-aza-C和SPB联用时两者之间具有明显的增效作用.结论:DNMT抑制剂5aza-C和HDAC抑制剂SPB联合应用于抗肿瘤临床是很有前景的.     

DNA甲基化转移酶抑制剂和组蛋白脱乙酰化酶抑制剂的体外抗肿瘤作用

目的：通过在体外分别对DNA甲基化转移酶（DNAmethyhransferase，DNMT）抑制剂5-氮杂胞苷（5-azacytidine，5-aza-C）和组蛋白脱乙酰化酶（histoneseacetylase，HDAC）抑制剂苯丁酸钠（sodium phenylbutyrate，SPB）单独及联合用药于人大肠癌细胞LoVo和人肝癌细胞Hep3B的抗肿瘤作用的观察，探讨DNMT抑制剂和HDAC抑制剂联合用药的可能性。方法：使用MTT法测定5-aza—C和SPB单独用药或联合用药的抗癌活性，用抑制浓度的分数之和（asum of fractional inhibitory concentration，SFIC）值及等效剂量分析方法（Isobologram）评价5-aza—C和SPB联合用药的作用性质。结果：5-aza—C和SPB联用时，LoVo细胞及Hep3B细胞得到的SFIC值均小于1．0，等效剂量分析图的形状呈现凹型，都表明了5-aza—C和SPB联用时两者之间具有明显的增效作用。结论：DNMT抑制剂5aza—C和HDAC抑制剂SPB联合应用于抗肿瘤临床是很有前景的。     

BRD4 inhibitor and histone deacetylase inhibitor synergistically inhibit the proliferation of gallbladder cancer in vitro and in vivo

Gallbladder cancer (GBC) is the most common malignancy of the bile duct and has a high mortality rate. Here, we demonstrated that BRD4 inhibitor JQ1 and histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA) synergistically inhibited the GBC cells in vitro and in vivo. Our results showed that cotreatment with JQ1 and SAHA significantly inhibited proliferation, cell viability and metastasis, and induced apoptosis and G2/M arrest in GBC cells, with only minor effects in benign cells. In vivo, tumor volumes and weights of GBC xenograft models were significantly decreased after treatment with JQ1 or SAHA; meanwhile, the cotreatment showed the strongest effect. Further study indicated that the above anticancer effects was associated with the downregulation of BRD4 and suppression of PI3K/AKT and MAPK/ERK pathways. These findings highlight JQ1 and SAHA as potential therapeutic agents and their combination as a promising therapeutic strategy for GBC.     

Cytotoxic activity of the histone deacetylase inhibitor panobinostat (LBH589) in anaplastic thyroid cancer in vitro and in vivo

Anaplastic thyroid carcinoma (ATC) has a rapidly fatal clinical course, being resistant to multimodal treatments. Microtubules, α/β tubulin heterodimers, are crucial in cell signaling, division and mitosis and are among the most successful targets for anticancer therapy. Panobinostat (LBH589) is a potent deacetylase inhibitor acting both on histones and nonhistonic proteins, including α-tubulin. In vitro LBH589, evaluated in three ATC cell lines (BHT-101, CAL-62 and 8305C), resulted in impairment of cell viability, inhibition of colony formation, cell cycle arrest and apoptosis induction. Mechanistically, we showed that LBH589 not only affected the expression of p21 and cyclin D1, but markedly determined microtubule stabilization as evidenced by tubulin acetylation and increased tubulin polymerization. In a SCID xenograft model implanted with CAL-62 cells, the cytotoxic properties of LBH589 were confirmed. The drug at the dose of 20 mg/kg significantly impaired tumor growth (final tumor volume 2.5-fold smaller than in untreated animals); at this dose, no relevant side effects were observed. In tumors of treated animals, a significant reduction of Ki67, which was negatively correlated with tubulin acetylation, was observed. Moreover, acetyl-tubulin levels negatively correlated with tumor volume at sacrifice, reinforcing the opinion that tubulin acetylation has a role in the inhibition of tumor growth. In conclusion, LBH589, acting on both histones and nonhistonic proteins in anaplastic thyroid cancer, appears to be a promising therapeutic agent for the treatment of this kind of cancer which is known not to respond to conventional therapy.     

Trichostatin A is a histone deacetylase inhibitor with potent antitumor activity against breast cancer in vivo.

PURPOSE: Trichostatin A (TSA), an antifungal antibiotic with cytostatic and differentiating properties in mammalian cell culture, is a potent and specific inhibitor of histone deacetylase (HDAC) activity. The purpose of this study was to evaluate the antiproliferative and HDAC inhibitory activity of TSA in vitro in human breast cancer cell lines and to assess its antitumor efficacy and toxicity in vivo in a carcinogen-induced rat mammary cancer model. EXPERIMENTAL DESIGN AND RESULTS: TSA inhibited proliferation of eight breast carcinoma cell lines with mean +/- SD IC(50) of 124.4 +/- 120.4 nM (range, 26.4-308.1 nM). HDAC inhibitory activity of TSA was similar in all cell lines with mean +/- SD IC(50) of 2.4 +/- 0.5 nM (range, 1.5-2.9 nM), and TSA treatment resulted in pronounced histone H4 hyperacetylation. In randomized controlled efficacy studies using the N-methyl-N-nitrosourea carcinogen-induced rat mammary carcinoma model, TSA had pronounced antitumor activity in vivo when administered to 16 animals at a dose of 500 microg/kg by s.c. injection daily for 4 weeks compared with 14 control animals. Furthermore, TSA did not cause any measurable toxicity in doses of up to 5 mg/kg by s.c. injection. Forty-one tumors from 26 animals were examined by histology. Six tumors from 3 rats treated with TSA and 14 tumors from 9 control animals were adenocarcinomas. In contrast, 19 tumors from 12 TSA-treated rats had a benign phenotype, either fibroadenoma or tubular adenoma, suggesting that the antitumor activity of TSA may be attributable to induction of differentiation. Two control rats each had tumors with benign histology. CONCLUSIONS: The present studies confirm the potent dose-dependent antitumor activity of TSA against breast cancer in vitro and in vivo, strongly supporting HDAC as a molecular target for anticancer therapy in breast cancer.     

The histone deacetylase inhibitor PXD101 increases the efficacy of irinotecan in in vitro and in vivo colon cancer models

Purpose

Histone deacetylase inhibitors (HDACIs), such as PXD101 and suberoylanilide hydroxamic acid, inhibit proliferation and stimulate apoptosis of tumor cells. The enhanced effectiveness of chemotherapy or radiotherapy when combined with HDACIs has been observed in several cancers. In this study, we investigated the antitumor effect of PXD101 combined with irinotecan in colon cancer.

Methods

HCT116 and HT29 colon cancer cells for cell viability assay were treated with PXD101 and/or SN-38, the active form of irinotecan. Antitumor effects of HCT116 and HT29 xenografts treated with these combinations were evaluated. [¹⁸F]FLT-PET was used to detect early responses to PXD101 and irinotecan in colon cancer.

Results

PXD101 and SN38 possessed dose-dependent antiproliferative activity against HCT116 and HT29 cells and exerted a synergistic effect when used in combination. In xenografted mice, PXD101 in combination with irinotecan dramatically inhibited tumor growth without causing additive toxicity. Apoptotic effects on xenograft tumors were greater with combined treatment than with irinotecan alone. [¹⁸F]FLT-PET imaging revealed a 64% decrease in [¹⁸F]FLT uptake in tumors of HCT116 xenograft-bearing mice treated with a combination of PXD101 and irinotecan, indicating a decrease in thymidine kinase 1 (TK1) activity. These results were supported by Western blot analyses showing a decrease in tumor thymidine kinase 1 protein levels, suggesting that [¹⁸F]FLT-PET can be used to non-invasively detect early responses to these agents.

Conclusions

These data show that PXD101 increases the cytotoxic activity of irinotecan in in vitro and in vivo colon cancer models and suggest these agent combinations should be explored in the treatment of colon cancer.     

Suberoylanilide hydroxamic acid, an inhibitor of histone deacetylase, suppresses the growth of prostate cancer cells in vitro and in vivo

Suberoylanilide hydroxamic acid (SAHA) is the prototype of a family of hybrid polar compounds that induce growth arrest in transformed cells and show promise for the treatment of cancer. SAHA induces differentiation and/or apoptosis in certain transformed cells in culture and is a potent inhibitor of histone deacetylases. In this study, we examined the effects of SAHA on the growth of human prostate cancer cells in culture and on the growth of the CWR22 human prostate xenograft in nude mice. SAHA suppressed the growth of the LNCaP, PC-3, and TSU-Pr1 cell lines at micromolar concentrations (2.5-7.5 microM). SAHA induced dose-dependent cell death in the LNCaP cells. In mice with transplanted CWR222 human prostate tumors, SAHA (25, 50, and 100 mg/kg/day) caused significant suppression of tumor growth compared with mice receiving vehicle alone; treatment with 50 mg/kg/day resulted in a 97% reduction in the mean final tumor volume compared with controls. At this dose, there was no detectable toxicity as evaluated by weight gain and necropsy examination. Increased accumulation of acetylated core histones was detected in the CWR22 tumors within 6 h of SAHA administration. SAHA induced prostate-specific antigen mRNA expression in CWR22 prostate cancer cells, resulting in higher levels of serum prostate-specific antigen than predicted from tumor volume alone. The results suggest that hydroxamic acid-based hybrid polar compounds inhibit prostate cancer cell growth and may be useful, relatively nontoxic agents for the treatment of prostate carcinoma.     

组蛋白去乙酰化酶抑制剂对骨肉瘤MNNG/HOS细胞凋亡的影响

目的:探讨组蛋白去乙酰化酶抑制剂(histone deacetylase inhibitor,HDACI)曲古抑霉素A(trichostatin A,TSA)对体外培养的骨肉瘤MNNG/HOS细胞凋亡的影响.方法:用50、100、200、300和500 nmol/L TSA分别作用体外培养的骨肉瘤MNNG/HOS细胞12、24和48 h,在透射电子显微镜下观察细胞形态学变化,应用MTT、FCM和TUNEL法检测TSA对骨肉瘤细胞增殖、细胞周期和细胞凋亡的影响.结果:TSA具有抑制骨肉瘤MNNG/HOS细胞增殖的作用,并且随着TSA浓度的增加和作用时间的延长而增强(P＜0.01).FCM检测结果显示,与对照组比较,300 nmol/LTSA作用组G1期细胞减少,G2/M期细胞增加,细胞凋亡率增加,差异有统计学意义(P＜0.05).TUNEL法检测结果显示,细胞凋亡指数随着TSA浓度的增加和作用时间的延长而上升,与对照组比较差异有统计学意义(P＜0.05).透射电子显微镜下观察发现,TSA作用后,凋亡的骨肉瘤细胞细胞质浓缩,并可见凋亡小体.结论:TSA可抑制体外培养的骨肉瘤MNNG/HOS细胞增殖并诱导其凋亡,可能与G2/M期细胞周期阻滞有关.HDACI可能成为一种新的骨肉瘤治疗策略及抗肿瘤药物.     

Sulforaphane inhibits histone deacetylase activity in BPH-1, LnCaP and PC-3 prostate epithelial cells

Sulforaphane (SFN), an isothiocyanate first isolated from broccoli, exhibits chemopreventive properties in prostate cancer cells through mechanisms that are poorly understood. We recently reported on a novel mechanism of chemoprotection by SFN in human colon cancer cells, namely the inhibition of histone deacetylase (HDAC). Here, we show that addition of 15 microM SFN also inhibited HDAC activity by 40, 30 and 40% in BPH-1, LnCaP and PC-3 prostate epithelial cells, respectively. The inhibition of HDAC was accompanied by a 50-100% increase in acetylated histones in all three prostate cell lines, and in BPH-1 cells treated with SFN there was enhanced interaction of acetylated histone H4 with the promoter region of the P21 gene and the bax gene. A corresponding 1.5- to 2-fold increase was seen for p21Cip1/Waf1 and Bax protein expression, consistent with previous studies using HDAC inhibitors, such as trichostatin A. The downstream events included cell cycle arrest and activation of apoptosis, as evidenced by changes in cell cycle kinetics and induction of multi-caspase activity. These findings provide new insight into the mechanisms of SFN action in benign prostate hyperplasia, androgen-dependent prostate cancer and androgen-independent prostate cancer cells, and they suggest a novel approach to chemoprotection and chemotherapy of prostate cancer through the inhibition of HDAC.     

The histone deacetylase inhibitor FK228 preferentially enhances adenovirus transgene expression in malignant cells.

PURPOSE: Efficient adenovirus infection requires coxsackie-adenovirus receptor (CAR) and alpha(v) integrin. Whereas many malignant cells express these proteins poorly, normal tissues, especially liver, express high levels and are susceptible to adenovirus infection. Our previous studies showed that treatment of cancer cell lines with low concentrations of the histone deacetylase inhibitor FK228 (FR901228, depsipeptide), a drug in Phase II clinical trials, before infection was associated with an increase in adenovirus transgene expression. The purpose of these studies was to analyze the effects of FK228 on cultured normal human cells before initiating animal studies. EXPERIMENTAL DESIGN: Cancer and normal cells from the corresponding tissue were treated with FK228 and analyzed for the proteins needed for infection and the infection efficiency. RESULTS: Treatment of cancer cell lines with 1 ng/ml FK228 increased CAR RNA, alpha(v) integrin RNA, and histone H3 acetylation levels, and was associated with a 4-10-fold increase in the number of infected cells expressing the transgene. Similar treatment of normal human mammary epithelial cells, renal proximal tubule epithelial cells, and hepatocytes had little effect. The insensitivity of cultured normal cells may be explained, in part, by expression of the drug efflux pump P-glycoprotein, because addition of the P-glycoprotein inhibitor XR9576 (tariquidar) with FK228 resulted in increased histone acetylation and CAR expression. CONCLUSION: These studies suggest that low concentrations of FK228 preferentially increase the efficiency of adenoviral transgene expression in cancer cells compared with cultured normal cells from the corresponding tissue and may increase the efficiency of adenovirus therapies in vivo.