Modulation of multidrug resistance gene (mdr-1) with antisense oligodeoxynucleotides

Use of ribozymes and antisense oligodeoxynucleotides to investigate mechanisms of drug resistance

Chemotherapy can cure a number of human cancers but resistance (either intrinsic or acquired) remains a significant problem in many patients and in many types of solid tumour. Combination chemotherapy (using drugs with different cellular targets/mechanisms) was introduced in order to kill cells which had developed resistance to a specific drug, and to allow delivery of a greater total dose of anti-cancer chemicals by combining drugs with different side-effects (Pratt et al., 1994). Nearly all anti-cancer drugs kill tumour cells by activating an endogenous bio-chemical pathway for cell suicide, known as programmed cell death or apoptosis.

Reversing multidrug resistance in breast cancer cells by silencing ABC transporter genes with nanoparticle-facilitated delivery of target siRNAs

Background

Multidrug resistance, a major impediment to successful cancer chemotherapy, is the result of overexpression of ATP-binding cassette (ABC) transporters extruding internalized drugs. Silencing of ABC transporter gene expression with small interfering RNA (siRNA) could be an attractive approach to overcome multidrug resistance of cancer, although delivery of siRNA remains a major hurdle to fully exploit the potential of siRNA-based therapeutics. Recently, we have developed pH-sensitive carbonate apatite nanoparticles to efficiently carry and transport siRNA across the cell membrane, enabling knockdown of the cyclin B1 gene and consequential induction of apoptosis in synergy with anti-cancer drugs.

Methods and results

We report that carbonate apatite-mediated delivery of the siRNAs targeting ABCG2 and ABCB1 gene transcripts in human breast cancer cells which constitutively express both of the transporter genes dose-dependently enhanced chemosensitivity to doxorubicin, paclitaxel and cisplatin, the traditionally used chemotherapeutic agents. Moreover, codelivery of two specific siRNAs targeting ABCB1 and ABCG2 transcripts resulted in a more robust increase of chemosensitivity in the cancer cells, indicating the reversal of ABC transporter-mediated multidrug resistance.

Conclusion

The delivery concept of multiple siRNAs against ABC transporter genes is highly promising for preclinical and clinical investigation in reversing the multidrug resistance phenotype of breast cancer.

ABCG2 is related with the grade of glioma and resistance to mitoxantone, a chemotherapeutic drug for glioma

OBJECTIVE

The aim of this study is to explore if ABCG2 is related to the grade of glioma and resistance to chemotherapeutic drug for glioma.

METHODS

The ABCG2 expression and distribution among glioma tissues of different grades and other samples were examined using tissue microarray technique. The enhancement of sensitivity of CD133+ glioma stem cells to chemotherapeutic agent, mitoxantone through addition of ABCG2 competitive inhibitor nicardipine was testified by MTT assay and FACS analysis.

RESULTS

The positive immunostaining of ABCG2 was observed in less than 10% of low-grade gliomas (3/31 in grade I + II) and in more than 40% of high-grade gliomas (16/37 in grade III + IV), which was statistically different (chi (2) = 10.710, P = 0.0011). In samples consisting of glioma stem cells (CD133+), the positive-straining rate was 100% (4/4), while in CD133- fraction, no positive staining was observed. A simultaneous treatment of CD133+ tumor cells with concentration-dependent mitoxantone (10(-5)-1 microM) and 2.5/5.0 microM nicardipine resulted in synergistic cytotoxicity. The apoptotic rate of CD133+ cells treated with mitoxantone plus nicardipine was significantly higher than that treated with mitoxantone alone (58.54 +/- 7.06% vs. 30.7 +/- 3.79%, P < 0.01).

CONCLUSIONS

Our results showed that ABCG2 is also expressed in glioma stem cells and the expression level of ABCG2 is positively associated with the increasing pathological grade of glioma (poor cell differentiation). ABCG2 plays a key role in glioma cells resistance to mitoxantone, chemotherapeutic drug for glioma. Thus, inhibition of ABCG2 protein activity by nicardipine in glioma can sensitize it to mitoxantone, which may lead to better treatment strategies for cancers.

Targeting MDR1 gene: synthesis and cellular study of modified daunomycin-triplex-forming oligonucleotide conjugates able to inhibit gene expression in resistant cell lines

Reversal of the multidrug-resistant (MDR) phenotype is very important for chemotherapy success. In fact, the expression of the MDR1 gene-encoded P-glycoprotein (P-gp) actively expels antitumor agents such as daunomycin (DNM) out of the cells, resulting in drug resistance. We show that upon conjugation to triplex-forming oligonucleotides, it is possible to address DNM in resistant cells (MCF7-R and NIH-MDR-G185). The oligonucleotide moiety of the conjugate changes the cellular penetration properties of the antitumor agent that is no more the target of P-gp in resistant cells. We observe an accumulation of conjugated DNM in cells up to 72 h. For more efficient delivery in the cells' nuclei, transfectant agents must be used. In addition, the conjugate recognizes a sequence located in exon 3 of MDR1, and it inhibits its gene expression as measured both by Western blot and by reverse transcription-polymerase chain reaction.

Reversal of MDR1 gene-dependent multidrug resistance using low concentration of endonuclease-prepared small interference RNA

Multidrug resistance following initial chemotherapy is commonly associated with MDR1 gene encoding for P-glycoprotein (P-gp). RNA interference of MDR1 gene expression was used as a strategy to reverse MDR1-mediated multidrug resistance phenotypes. Here we report that endonuclease-prepared small interfering RNA (esiRNA) at concentrations as low as 10 ng/ml (about 0.7 nM) can decrease MDR1 expression and increase chemosensitivity in the Adriamycin-induced resistant MCF-7/R cells. When MCF-7/R cells were transiently transfected with esiRNA of MDR1 (esiMDR1), the MDR1 mRNA was reduced by about 50%, drug accumulation increased by about 30%, and the IC50 for daunorubicin was reduced from 4.5 to 1.2 microM. These results provide evidence that esiRNA of MDR1 could be an alternative to P-gp inhibitors with the advantage of avoiding non-specific suppression with a lower effective dosage than using a single siRNA duplex, offering a potential therapeutic application of siRNA.

Modulation of MDR1 gene expression in multidrug resistant MCF7 cells by low concentrations of small interfering RNAs

MDR1 overexpression is one form of the multidrug resistance (MDR) phenotype, which can be acquired by patients initially responsive to chemotherapy. Because of the high toxicity of the inhibitors of P-glycoprotein (P-gp), the protein encoded by MDR1, attention has been focused on selective modulation of the MDR1 gene. Small interfering RNAs (siRNAs) were shown to be powerful tools for such a purpose, even when used at low concentrations (< or =20 nM) in order to avoid sequence nonspecific effects. Two siRNAs used at 20 nM were shown to lead to efficient down-regulation of MDR1 at the protein level (only ca. 20% total P-gp expression remaining) in the doxorubicin selected MCF7-R human cell line. Cell surface expression of P-gp was inhibited, leading to reversal of the drug efflux phenotype (about 40% reversal with the most efficient siRNA) and enhancement of chemosensitivity (about 35%). At the mRNA level, the down-regulation of MDR1 obtained with the most efficient siRNA increased from about 50% (5 nM siRNA) to 60% (10 or 20 nM). The advantage of using a combination of siRNAs instead of a single one has been suggested.

Reversal of different drug-resistant phenotypes by an autocatalytic multitarget multiribozyme directed against the transcripts of the ABC transporters MDR1/P-gp, MRP2, and BCRP

A "multitarget multiribozyme" (MTMR) was constructed. It consists of three trans-acting hammerhead ribozymes directed against the transcripts of the ABC transporters MDR1/P-gp, BCRP, and MRP2; three cis-acting MDR1/P-gp-specific ribozymes; and three MDR1/P-gp-homologous spacer sequences. The trans-acting hammerhead ribozymes are liberated from the MTMR through autocatalytic self-cleavage by the cis-acting ribozymes. The MTMR was characterized with regard to its kinetic parameters. Comparison of the MTMR-specific kinetic values with those of the corresponding monoribozymes demonstrated that MTMR fragments could cleave their specific substrates without loss of efficiency. The MTMR was applied to three cell models, each overexpressing another ABC transporter, i.e., the gastric carcinoma cell line EPG85-257RDB expresses MDR1/P-gp, the cell variant EPG85-257RNOV synthesizes BCRP, and the ovarian carcinoma line A2780RCIS produces MRP2. In all cellular systems, the MTMR could specifically decrease the expression of the respective ABC transporter at the mRNA level (97% decrease in the MDR1/P-gp mRNA, 80% decrease in the BCRP mRNA, 96% decrease in the MRP2 mRNA) and the protein level. Resistance against the selection drug was reversed completely (100% in EPG85-257RDB) or by 94 (EPG85-257RNOV) or 63% (A2780RCIS). Thus, the MTMR technology provides a novel tool for gene therapeutic applications to reverse different ABC-transporter-dependent drug-resistant phenotypes.

In vivo RNA interference-mediated ablation of MDR1 P-glycoprotein

Multidrug resistance (MDR) remains a major obstacle to successful chemotherapeutic treatment of cancer and can be caused by overexpression of P-glycoprotein, the MDR1 gene product. To further validate a knockdown approach for circumventing MDR, we developed a P-glycoprotein inhibition strategy using short hairpin RNA interference (shRNAi) and now show efficacy and target specificity in vivo. Two of eight tested shRNAi constructs targeted against human MDR1 mRNA inhibited expression of P-glycoprotein by >90%, whereas control shRNAi had no effect. Ablation of P-glycoprotein in cells stably transduced with retroviral-mediated shRNAi was documented by Western blot and functionally confirmed by increased sensitivity of MDR1-transfected cells toward the cytotoxic drugs vincristine, paclitaxel, and doxorubicin as well as by transport of (99m)Tc-Sestamibi. shRNAi-mediated down-regulation of P-glycoprotein transport activity both in cultured cells and in tumor implants in living animals could be followed by direct noninvasive bioluminescence imaging using the Renilla luciferase fluorophore, coelenterazine, a known P-glycoprotein transport substrate. Furthermore, after somatic gene transfer by hydrodynamic infusion of a MDR1-Firefly luciferase (MDR1-FLuc) fusion construct into mouse liver, the effect of shRNAi delivered in vivo on P-glycoprotein-FLuc protein levels was documented with bioluminescence imaging using d-luciferin. ShRNAi against MDR1 reduced bioluminescence output of the P-glycoprotein-FLuc reporter 4-fold in vivo compared with mice treated with control or scrambled shRNAi. Targeted down-regulation of a somatically transferred P-glycoprotein-eGFP fusion reporter also was observed using fluorescence microscopy. Our results show that shRNAi effectively inhibited MDR1 expression and function in cultured cells, tumor implants and mammalian liver, documenting the feasibility of a knockdown approach to reversing MDR in vivo.

Stable suppression of MDR-1 gene using siRNA expression vector to reverse drug resistance in a human uterine sarcoma cell line

OBJECTIVE

Chemotherapy is highly effective in treating a number of gynecologic malignancies; however, its effectiveness diminishes with repeated exposures due to the emergence of multi-drug resistance (MDR). The aim of this study was to establish a permanent MDR gene knockdown model via infection with the siRNA-hairpin expression vector. The impact of transfecting the RNAi upon MDR-1 mRNA and P-glycoprotein expression as well as resultant chemotherapy resistance was assessed.

METHODS

Multi-drug resistant cell line MES-SA/DX5 was transfected with the siRNA-hairpin expression vector (pSMDR-HYG) designed to target MDR-1 mRNA. A negative control was established utilizing a vector lacking the anti-sense component (pSCON-HYG). The LD(50) of doxorubicin for the stable transfectants was determined utilizing a cytotoxic MTT assay. The mRNA expression of MDR-1 gene among those cell lines was evaluated by semi-quantitative RT-PCR. The product of P-glycoprotein (P-gp) was examined by Western blotting hybridization and immunostaining.

RESULTS

Two stable transfected cell lines: MES-SA/DX5-M (with pSMDR-HYG) and MES-SA/DX5-C (with pSCON-HYG) were established. The cell line MES-SA/DX5-M was nearly 7 times more sensitive to doxorubicin than MES-SA/DX5-C and its parent cell line MES-SA/DX5 (P < 0.01). The mRNA expression of the MDR-1 gene in MES-SA/DX5-M was also statistically significantly lower than in the other 2 cell lines (P < 0.01) as assessed by semi-quantitative RT-PCR. A barely detectable signal for P-gp (170 kDa) was observed in MES-SA/DX5-M. The vast majority of MES-SA/DX5-M cells were immunohistochemically negative for P-gp.

CONCLUSIONS

Stable, sequence-specific MDR-1 gene silencing can be demonstrated by inducing the endogenous expression of hairpin siRNA. Hairpin-siRNA-based MDR-1 gene silencing correlated with decreased levels of MDR-1 mRNA and P-gp, thereby restoring permanent native chemosensitivity. This methodologic strategy may have significant clinical impact in reversing chemo-resistance, especially the multi-drug-resistant phenotype, in the treatment of gynecologic malignancies.

Stable and complete overcoming of MDR1/P-glycoprotein-mediated multidrug resistance in human gastric carcinoma cells by RNA interference

Multidrug resistance (MDR) is the major cause of failure of effective chemotherapeutic treatment of disseminated neoplasms. The "classical" MDR phenotype of human malignancies is mediated by drug extrusion by the adenosine triphosphate binding cassette (ABC)-transporter P-glycoprotein (MDR1/P-gp). For stable reversal of "classical" MDR by RNA interference (RNAi) technology, an H1-RNA gene promoter-driven expression vector encoding anti-MDR1/P-gp short hairpin RNA (shRNA) molecules was constructed. By introduction of anti-MDR1/P-gp shRNA expression vectors into the extremely high drug-resistant human gastric carcinoma cell line EPG85-257RDB, the MDR phenotype was completely reversed. The reversal of MDR was accompanied by a complete suppression of MDR1/P-gp expression on mRNA and protein level, and by a considerable increased intracellular anthracyline accumulation in the anti-MDR1/P-gp shRNA-treated cells. The data indicate that stable shRNA-mediated RNAi can be tremendously effective in reversing MDR1/P-gp-mediated MDR and is therefore a promising strategy for overcoming MDR by gene therapeutic applications.

The Reduction of P-Glycoprotein Expression by Small Interfering RNAs Is Improved in Exponentially Growing Cells

Small interfering RNAs (siRNAs) are powerful tools in specifically silencing gene expression. Nevertheless, their efficiency can be limited when targeting proteins with an unusually long half-life, such as P-glycoprotein (P-gp), which is involved in the multidrug resistance phenomenon. P-gp is characterized by a long half-life, which may vary depending on the cell line and, for some of them, on serum deprivation or high cell density. In the present paper, involvement of an exponential cell growth phase in the improvement of siRNA efficiency has been suggested. The doxorubicin-selected human line MCF7-R was shown to be a more adapted model than NIH-MDR-G185 cells stably transfected with human mdr1. Nonspecific effects occurring at moderate (100 nM) siRNA concentration have been shown. Two efficient siRNAs led to a very satisfactory P-gp extinction (only 20% P-gp expression remaining) with siRNA concentration as low as 20 nM.

Strategies for reversing drug resistance

Drug resistance, intrinsic or acquired, is a problem for all chemotherapeutic agents. In this review, we examine numerous strategies that have been tested or proposed to reverse drug resistance. Included among these strategies are approaches targeting the apoptosis pathway. Although the process of apoptosis is complex, it provides several potential sites for therapeutic intervention. A variety of targets and approaches are being pursued, including the suppression of proteins inhibiting apoptosis using antisense oligonucleotides (ASOs), and small molecules targeted at proteins that modulate apoptosis. An alternate strategy is based on numerous studies that have documented methylation of critical regions in the genome in human cancers. Consequently, efforts have been directed at re-expressing genes, including genes that affect drug sensitivity, using 5-azacytidine and 2'-deoxy-5-azacytidine (DAC, decitabine) as demethylating agents. While this strategy may be effective as a single modality, success will most likely be achieved if it is used to modulate gene expression in combination with other modalities such as chemotherapy. At a more basic level, attempts have been made to modulate glutathione (GSH) levels. Owing to its reactivity and high intracellular concentrations, GSH has been implicated in resistance to several chemotherapeutic agents. Several approaches designed to deplete intracellular GSH levels have been pursued including the use of buthionine-(S,R)-sulfoxime (BSO), a potent and specific inhibitor of gamma-glutamyl cysteine synthetase (gamma-GCS), the rate-limiting step in the synthesis of GSH, a hammerhead ribozyme against gamma-GCS mRNA to downregulate specifically its levels and targeting cJun expression to reduce GSH levels. Alternate strategies have targeted p53. The frequent occurrence of p53 mutations in human cancer has led to the development of numerous approaches to restore wild-type (wt) p53. The goals of these interventions are to either revert the malignant phenotype or enhance drug sensitivity. The approach most extensively investigated has utilized one of several viral vectors. An alternate approach, the use of small molecules to restore wt function to mutant p53, remains an option. Finally, the conceptually simplest mechanism of resistance is one that reduces intracellular drug accumulation. Such reduction can be effected by a variety of drug efflux pumps, of which the most widely studied is P-glycoprotein (Pgp). The first strategy utilized to inhibit Pgp function relied on the identification of non-chemotherapeutic agents as competitors. Other approaches have included the use of hammerhead ribozymes against the MDR-1 gene and MDR-1-targeted ASOs. Although modulation of drug resistance has not yet been proven to be an effective clinical tool, we have learned an enormous amount about drug resistance. Should we succeed, these pioneering basic and clinical studies will have paved the road for future developments.

Hydrolytically activated etoposide prodrugs inhibit MDR-1 function and eradicate established MDR-1 multidrug-resistant T-cell leukemia

Effective therapy of high-risk leukemia with established cytotoxic drugs may be limited by poor antitumor efficacy, systemic toxicity, and the induction of drug resistance. Here, we provide the first evidence that hydrolytically activated prodrugs may overcome these problems. For this purpose, VP16 was functionally blocked by hydrolytically cleavable carbonate linkers with unique characteristics to generate 2 novel prodrugs of VP16. First, we established a more than 3-log higher efficacy of the 2 prodrugs compared with VP16 on a panel of naturally drug-resistant tumor cell lines. Second, the prodrugs did overcome VP16-induced multidrug resistance-1 gene (MDR-1)-mediated multidrug resistance in vitro in a newly established VP16-resistant T-cell leukemia cell line MOVP-3 by functionally blocking MDR-1-mediated efflux. Third, in vivo studies showed a maximum tolerated dose of ProVP16-II (> 45mg/kg), which was at least 3-fold higher than that of VP16 (15 mg/kg). Finally, tests of ProVP16-II in a multidrug-resistant xenograft model of T-cell leukemia expressing MDR-1 indicated that only the mice treated with this prodrug revealed a complete and long-lasting regression of established, drug-resistant leukemia. In summary, the hydrolytically activated etoposide prodrugs proved effective against multidrug-resistant T-cell leukemia in vitro and in vivo and provide proof of concept for a highly promising new strategy for the treatment of MDR-1 drug-resistant malignancies.

Modulation of the classical multidrug resistance (MDR) phenotype by RNA interference (RNAi)

For reversal of MDR1 gene-dependent multidrug resistance (MDR), two small interfering RNA (siRNA) constructs were designed to inhibit MDR1 expression by RNA interference. SiRNA duplexes were used to treat human pancreatic carcinoma (EPP85-181RDB) and gastric carcinoma (EPG85-257RDB) cells. In both cellular systems, siRNAs could specifically inhibit MDR1 expression up to 91% at the mRNA and protein levels. Resistance against daunorubicin was decreased to 89% (EPP85-181RDB) or 58% (EPG85-257RDB). The data indicate that this approach may be applicable to cancer patients as a specific means to reverse tumors with a P-glycoprotein-dependent MDR phenotype back to a drug-sensitive one.

Minimally modified phosphodiester antisense oligodeoxyribonucleotide directed against the multidrug resistance gene mdr1

In the perspective of reversing multidrug resistance through antisense strategy while avoiding non-antisense effects of all-phosphorothioate oligonucleotides which non-specifically bind to proteins, a minimally modified antisense phosphodiester oligodeoxyribonucleotide has been designed against mdr1, one of the multidrug resistance genes. Its stability in lysates prepared from NIH/3T3 cells transfected with the human mdr1 gene has already been demonstrated. Confocal microspectrofluorometry using a fluorescence resonance energy transfer technique allowed its stability inside living cells to be proven. Its internalization into the cells was achieved with different delivery agents (addition of a cholesteryl group, Superfect or an amphotericin B cationic derivative) and has been followed by fluorescence imaging. For each of the delivery systems, Western blotting allowed its antisense efficiency to be compared to that of an all-phosphorothioate antisense oligonucleotide. No antisense efficiency was demonstrated for the minimally modified ODN when internalized with Superfect. In both other cases, the best extinction of the P-glycoprotein expression has always been achieved with the all-phosphorothioate antisense. While the difference was significant in the case the amphotericin B derivative was used as delivery agent (20% remaining protein expression with the all-phosphorothioate vs. 40% with the minimally modified antisense), it was negligible for the cholesterol conjugates (2% vs. 6%). It is of great interest to prove that an almost all-phosphodiester oligonucleotide can be an efficient antisense against an overexpressed gene. The reduction of non-antisense effects as non-specific binding to proteins are of importance in the case relatively high ODN concentrations are used, which can prove to be necessary in the case of overexpressed genes.

Reversal of drug resistance of hepatocellular carcinoma cells by adenoviral delivery of anti-MDR1 ribozymes

Human cancers, including hepatocellular carcinoma (HCC), are characterized by a high degree of drug resistance. The multidrug resistance (MDR) transporters MDR1-P-glycoprotein and MRP2 (multidrug-associated protein 2) are expressed in almost 50% of human cancers, including HCCs. In this study, we analyzed the effect of anti-MDR1 ribozymes, especially AFP promoter-driven anti-MDR1 ribozymes, to specifically chemosensitize HCC cells. Epirubicin-selected HB8065/R cells were used as MDR1-P-glycoprotein-overexpressing cells. Adenoviral vectors were constructed to allow an efficient gene transfer of anti-MDR1 ribozyme constructs. AFP promoter-driven anti-MDR1 ribozymes reduced the IC(50) 30-fold for epirubicin in HCC cells, whereas human colorectal cancer cells were unaffected. Target sequences were either the translational start site or codon 196 of the human MDR1 gene. Adenoviral delivery of CMV promoter-driven anti-MDR1 ribozymes resulted in a reduced IC(50) for epirubicin and doxorubicin (60- and 20-fold, respectively). They completely restored chemosensitivity in stably transfected anti-MDR1 ribozyme-expressing HCC cells as well as in HCC cells transduced with adenoviruses expressing wild-type anti-MDR1 ribozymes. Adenoviral delivery of ribozymes was so efficient that chemosensitization of HCC cells could be demonstrated in cell cultures without further selection of transduced cells for single anti-MDR1 ribozyme-expressing HCC cell clones. Northern blots showed a decreased MDR1 mRNA expression, and fluorescence-activated cell sorting (FACS) analysis revealed a significantly reduced expression of MDR1-P-glycoprotein on the cell surface of HB8065/R cells after transduction with the anti-MDR1 ribozymes. In conclusion, our data demonstrate that adenoviral delivery of ribozymes can chemosensitize HCC cells and that chemosensitization can be specifically achieved by ribozymes driven by an AFP promoter directed against human MDR1.

Downregulation of PGY1/MDR1 mRNA level in human KB cells by antisense oligonucleotide conjugates. RNA accessibility in vitro and intracellular antisense activity

Inhibition of PGY1/MDR1 (multidrug resistance gene 1) mRNA expression in multidrug resistant KB-8-5 cells by 5'-bis-pyrenyl-3'-aminohexyl oligodeoxyribonucleotide conjugates targeted to four sites of this mRNA has been investigated. Three of the tested oligonucleotide conjugates specifically inhibited the expression of PGY1/MDR1 mRNA as monitored by the RT-PCR assay. The oligonucleotide conjugate targeted to the region (+178; +194) of the PGY1/MDR1 mRNA decreased level of this mRNA to 10% compared to the control. Nuclease-resistant analogs of oligonucleotide, complementary to this MDR1 mRNA region therefore, might be considered as a prototype compounds for development of gene-targeted therapeutic agents for overcoming the MDR phenotype caused by the overexpression of the PGY1/MDR1 gene.

Best minimally modified antisense oligonucleotides according to cell nuclease activity

Minimally modified oligonucleotides belong to the second-generation antisense class. They are phosphodiester oligonucleotides with a minimum of phosphorothioate linkages in order to be protected against serum and cellular exonucleases and endonucleases. They activate RNase H, have weak interactions with proteins, and have thus a better antisense efficiency. Two of them have been designed from an all-phosphorothioate antisense oligonucleotide directed against mdrl-expressing cells. They are protected against serum and cellular enzymatic degradation by the self-forming hairpin d(GCGAAGC) at their 3'-end and by judiciously located phosphorothioate residues, depending on the cellular composition in exonucleases or endonucleases. Besides their already demonstrated ability to cleave pyrimidine sites, endonucleases show some specificity for CpG sites. Their activity is hindered if specific sites are involved in secondary structure as hairpin.

Antisense therapy in cancer

This review discusses laboratory and clinical studies of antisense oligodeoxynucleotides as potential treatments for haematological malignancies and solid tumours. Mechanisms of action, pharmacokinetics, toxicities and potential clinical applications of these agents are described.

Secondary structure of the 5'-region of PGY1/MDR1 mRNA

In order to identify the optimal target sites for antisense oligonucleotides in the human multiple drug resistance mRNA, the secondary structure of the 5'-terminal part of this mRNA (nucleotides 1-678) was investigated. By using results of probing with ribonucleases T1, ONE and V1 and results of computer simulations, a model of the 5'-region of the PGY1/MDR1 mRNA was built. The molecule is formed by three major domains comprising several hairpins separated by single-stranded fragments. The predicted single-stranded regions of the PGY1/MDR1 mRNA efficiently bind complementary oligonucleotides.

Clinical evaluation of biologically targeted drugs: obstacles and opportunities

Recent insights into the molecular mechanisms of cancer have indicated that a variety of fundamental cellular processes are dysregulated in malignant cells. These processes include cell cycle control, signal transduction pathways, apoptosis, telomere stability, angiogenesis, and interactions with the extracellular matrix. Remarkable advances in molecular genetics, enzymology, and medicinal chemistry have permitted the design of compounds that modulate some of these processes with specificity that was unimaginable a decade ago. As these novel, biologically targeted compounds enter the clinic, they will require a strategy for clinical evaluation and development different from that used commonly for cytotoxic antineoplastic agents. This review examines the development of cancer drugs directed against angiogenesis, metastasis, signal transduction, telomerase, and molecular message (antisense), outlines strategies for the clinical testing of agents directed at these processes, and contrasts these efforts with traditional approaches to cancer drug testing.