Pharmacokinetics of an antiangiogenic ribozyme (ANGIOZYME) in the mouse

Nucleotides Entrapped in Liposome Nanovesicles as Tools for Therapeutic and Diagnostic Use in Biomedical Applications

The use of nucleotides for biomedical applications is an old desire in the scientific community. As we will present here, there are references published over the past 40 years with this intended use. The main problem is that, as unstable molecules, nucleotides require some additional protection to extend their shelf life in the biological environment. Among the different nucleotide carriers, the nano-sized liposomes proved to be an effective strategic tool to overcome all these drawbacks related to the nucleotide high instability. Moreover, due to their low immunogenicity and easy preparation, the liposomes were selected as the main strategy for delivery of the mRNA developed for COVID-19 immunization. For sure this is the most important and relevant example of nucleotide application for human biomedical conditions. In addition, the use of mRNA vaccines for COVID-19 has increased interest in the application of this type of technology to other health conditions. For this review article, we will present some of these examples, especially focused on the use of liposomes to protect and deliver nucleotides for cancer therapy, immunostimulatory activities, enzymatic diagnostic applications, some examples for veterinarian use, and the treatment of neglected tropical disease.

Toxicological and pharmacokinetic properties of chemically modified siRNAs targeting p53 RNA following intravenous administration

We report the toxicological and pharmacokinetic properties of the synthetic, small interfering RNA I5NP following intravenous administration in rodents and nonhuman primates. I5NP is designed to act via the RNA interference (RNAi) pathway to temporarily inhibit expression of the pro-apoptotic protein p53 and is being developed to protect cells from acute ischemia/reperfusion injuries such as acute kidney injury that can occur during major cardiac surgery and delayed graft function that can occur following renal transplantation. Following intravenous administration, I5NP was very rapidly cleared from plasma was distributed predominantly to the kidney, with very low levels in liver and other tissues. Doses of 800 mg/kg I5NP in rodents, and 1,000 mg/kg I5NP in nonhuman primates, were required to elicit adverse effects, which in the monkey were isolated to direct effects on the blood that included a sub-clinical activation of complement and slightly increased clotting times. In the rat, no additional adverse effects were observed with a rat analogue of I5NP, indicating that the effects likely represent class effects of synthetic RNA duplexes rather than toxicity related to the intended pharmacologic activity of I5NP. Taken together, these data support clinical testing of intravenous administration of I5NP for the preservation of renal function following acute ischemia/reperfusion injury.

VEGF receptor inhibition slows the progression of polycystic kidney disease

Although the receptors for vascular endothelial growth factor (VEGF) exert their effects on vasculogenesis and angiogenesis through receptors located on endothelial cells, recent studies have shown that these receptors are also present on renal tubular epithelial cells. We investigated the role of VEGF on increased tubule cell proliferation in the Han:SPRD heterozygous (Cy/+) rat model of polycystic kidney disease. The levels of VEGF in the kidneys and the serum, and the expression of the two receptors on tubules were increased in Cy/+ rats. These rats were given ribozymes that specifically inhibited VEGFR1 and VEGFR2 mRNA expression. Tubule cell proliferation within the cysts was significantly decreased in the ribozyme-treated animals leading to decreased cystogenesis, blunted renal enlargement, and prevented the loss of renal function. Our studies show that inhibition of VEGF function may be an important therapeutic option to delay the progression of polycystic kidney disease.

Anti-angiogenic drugs: from bench to clinical trials

Angiogenesis, the generation of new capillaries through a process of pre-existing microvessel sprouting, is under stringent control and normally occurs only during embryonic and post-embryonic development, reproductive cycle, and wound repair. However, in many pathological conditions (solid tumor progression, metastasis, diabetic retinopathy, hemangioma, arthritis, psoriasis and atherosclerosis among others), the disease appears to be associated with persistent upregulated angiogenesis. The development of specific anti-angiogenic agents arises as an attractive therapeutic approach for the treatment of cancer and other angiogenesis-dependent diseases. The formation of new blood vessels is a complex multi-step process. Endothelial cells resting in the parent vessels are activated by an angiogenic signal and stimulated to synthesize and release degradative enzymes allowing endothelial cells to migrate, proliferate and finally differentiate to give rise to capillary tubules. Any of these steps may be a potential target for pharmacological intervention. In spite of the disappointing results obtained initially in clinical trials with anti-angiogenic drugs, recent reports with positive results in phases II and III trials encourage expectations in their therapeutic potential. This review discusses the current approaches for the discovery of new compounds that inhibit angiogenesis, with emphasis on the clinical developmental status of anti-angiogenic drugs.

Hammerhead ribozymes with cleavage site specificity for NUH and NCH display significant anti-hepatitis C viral effect in vitro and in recombinant HepG2 and CCL13 cells

BACKGROUND/AIMS

Four different ribozymes (Rz) targeting the hepatitis C virus (HCV) 5'-non-coding region (NCR) at nucleotide (nt) positions GUA 165 (Rz1), GUC 270 (Rz2), GUA 330 (Rz3) and GCA 348 (Rz1293) were compared for in vitro cleavage using a 455 nt HCV RNA substrate. The GUA 330 (Rz3) and GCA 348 (Rz1293) ribozymes, both targeting the HCV loop IV region, were found to be the most efficient, and were further analyzed in an in vitro translation system.

METHODS

For this purpose RNA transcribed from a construct encoding a HCV-5'-NCR-luciferase fusion protein was used. Cleavage-inactive (Rz1426), mismatch (Rz1293m) or unrelated ribozymes (Rz1437) were synthesized as controls for Rz-1293. HCV specificity was analysed by competition experiments using sense and mismatch oligodeoxynucleotides HCVrzCI and HCVrzMM, respectively.

RESULTS

A chemically modified nuclease-resistant variant of the GCA 348 cleaving ribozyme was selected for cell culture experiments using recombinant HepG2 or CCL13 cell lines stably transfected with a HCV-5'-NCR-luciferase target construct.

CONCLUSIONS

This ribozyme (Rz1293) showed an inhibitory activity of translation of more than 70% thus verifying that the GCA 348 cleavage site in the HCV loop IV is an accessible target site in vivo and may be suitable for the development of novel optimized hammerhead structures.

A phase I clinical trial of a ribozyme-based angiogenesis inhibitor targeting vascular endothelial growth factor receptor-1 for patients with refractory solid tumors

PURPOSE

This study intended to determine the maximum tolerated dose, safety, pharmacokinetic variables, clinical response, and pharmacodynamic markers of daily s.c. administration of Angiozyme.

PATIENTS AND METHODS

Patients with refractory solid tumors were enrolled in a dose escalation and expanded cohort design. Dose escalation involved cohorts of patients at doses of 10, 30, 100, or 300 mg/m(2)/d for 29 days. A second component enrolled 15 additional patients at a daily dose of 100 mg/m(2). Patients were eligible to continue on therapy until disease progression.

RESULTS

Thirty-one patients were enrolled and 28 were evaluable (range, 29-505 days; median, 89.5 days). A maximum tolerated dose was not defined by toxicity but rather by the maximal deliverable dose of 300 mg/m(2)/d. Grade 1 to 2 injection site reactions were the most common toxicities. One patient in the 300 mg/m(2) group experienced a reversible grade 3 injection site reaction. Angiozyme showed dose-dependent plasma concentrations with good bioavailability. Surrogate markers showed Angiozyme localization in tumor biopsies and a significant increase in serum von Willebrand factor antigen, a marker for endothelial cell dysfunction. Although Angiozyme-reactive antibody production was noted for some patients, no antibody-related adverse events were noted. Seven of 28 (25%) evaluable patients had stable disease for >or =6 months, with the longest treatment duration of > or =16 months. Two patients (nasopharyngeal carcinoma and melanoma) showed minor responses.

CONCLUSION

Angiozyme was well tolerated with satisfactory pharmacokinetic variables for daily s.c. dosing. Results have provided the basis for subsequent clinical trials of this first-of-class biologically targeted therapeutic.

Non-viral delivery of ribozymes for cancer gene therapy

Ribozymes are RNA molecules with the capacity to effect sequence-specific cleavage of other transcripts. Since their initial discovery, there has been considerable interest in the development of ribozymes and other RNA therapeutics for gene therapy, particularly in the realm of cancer. However, as with other gene therapy applications, the delivery of ribozyme-based therapeutics to the target tissues of interest has represented a significant obstacle to the maturation of this technology to the clinical arena. This review will discuss the progress made so far in the use of non-viral methods for the systemic delivery of ribozymes for cancer gene therapy.

Therapeutic gene silencing in the nervous system

Progress in the understanding of RNA biology has brought into focus the prospect of using RNA-based therapeutics as a novel approach to treat human disease. In particular, following the discovery of the RNA interference (RNAi) pathway, the emergence of technology based on small interfering RNA (siRNA) now offers a powerful and highly specific tool for therapeutic gene silencing. Many neurological diseases, including neurodegenerative disorders, tumours and retinal disease are likely candidates to benefit from such advances. The challenges ahead will be to identify appropriate disease gene targets and, crucially, to understand the biological parameters that determine safe, precise and effective delivery and function of RNA-based therapeutic molecules within the unique environment of the nervous system.

Synthesis of New 3‘-, 5-, and N4-Modified 2‘-O-Methylcytidine Libraries on Solid Support

A versatile solid phase combinatorial approach was developed and utilized for the rapid synthesis of new 2'-O-methylcytidine nucleoside libraries 1-7 containing 672 compounds with 3'-deoxy-3'-C-methyl, 3'-deoxy-3'-C-hydroxymethyl, and 5-alkyl/alkynyl modifications. The modified uridine scaffolds 8-10, 23-25, and 31 were loaded onto the 4-methoxytrityl chloride (MMT-Cl) polystyrene resin through the hydroxyl groups at the 5'-position as well as on the substituents at the 3'- and 5-positions. The scaffolds loaded on the resin were orthogonally protected by MMT group on the resin itself and TBDMS or acetyl protecting groups. The 4-position of the uridine derivatives was activated by 2,4,6-triisopropyl benzene sulfonyl chloride for further derivatization. The resins 14-16, 28-30, and 32 loaded with the corresponding activated scaffolds were reacted with the selected and validated amino building blocks in the 96 well format on the semiautomated synthesizer. The high-quality 2'-O-methylcytidine libraries 1-7 were thus generated and characterized by liquid chromatography-mass spectrometry (LC-MS) analysis with 63-99% successful rates.

Therapies directed at vascular endothelial growth factor

The inhibition of angiogenesis through vascular endothelial growth factor (VEGF) receptor targeting is a strategy that is relatively tumour selective. The high selectivity achieved with neutralising antibodies, soluble receptors and ribozymes reduces the risk of adverse reactions not related to VEGF inhibition itself. Small-molecule, orally-active protein kinase inhibitors provide an attractive alternative for chronic therapy, although specifically targeting a small subset of protein kinases from the approximately 550 expressed in mammalian cells is a challenge. Current efforts have resulted in promising clinical data for several synthetic VEGF receptor kinase inhibitors, of which PTK787/ZK222584 and ZD6474 are proceeding into large size clinical trials. It seems likely that blockers of the VEGF signalling pathway will be unsuitable for monotherapy, and that their role will be as an adjunct to additional antiangiogenic agents together with directly-acting antitumour agents or radiation therapy. Caution is needed with combinations of anti-VEGF therapies and cytotoxic agents, as coadministration of cytotoxic agents with either the kinase inhibitor SU5416 or the VEGF antibody avastin appears to be associated with bleeding and thrombotic adverse events.

Hammerhead ribozymes as therapeutic agents for bladder cancer

Hammerhead ribozymes have been investigated extensively as therapeutic agents against cancer. Aberrant or overexpression of genes related to tumorigenicity or cancer growth might be the appropriate targets for ribozyme strategies. Ribozyme-mediated gene therapy should be applied to those diseases that have no successful conventional therapy such as advanced or treatment-resistant bladder cancer. Many genetic alterations have been identified in bladder cancer related to both tumorigenesis and disease progression. Mutated H-ras, fos, and erb-B2 genes have been chosen as targets for ribozymes in previous studies, and antitumor efficacy has been demonstrated by reversion of the malignant phenotypes and by inhibition of tumor growth both in vitro and in vivo. The efficiency of various delivery systems has also been evaluated. An overview of ribozyme strategies, especially for therapeutic applications against bladder cancer, is described here.

The encapsulation of ribozymes in biodegradable polymeric matrices

Ribozymes are catalytic RNA that bind and cleave specific regions of target RNA. Therefore, protein synthesis by the target RNA may be specifically inhibited by ribozymes. However, ribozymes are rapidly cleared from plasma so effective treatment of proliferative diseases may rely on the repeated administration of these agents to maintain therapeutic ribozyme concentrations. Therefore, the objective of this study was to encapsulate ribozymes in injectable polymeric paste and microsphere formulations to allow for the controlled release of these agents over extended periods of time. Ribozymes were effectively encapsulated in poly(L-lactic acid) (PLLA) and poly(lactic-co-glycolic) (PLGA) microspheres in various size ranges using a modified water-in-oil-in-water emulsion system and in poly(epsilon-caprolactone) (PCL) pastes by physical blending. These formulations released non-degraded ribozymes, in vitro, in a controlled manner. PLLA microspheres released the ribozymes rapidly whereas PLGA released drugs more slowly. The release rate of ribozymes from PCL pastes could be effectively controlled by altering the loading concentration of ribozymes in the paste. These polymeric injectable formulations of ribozymes may allow for the extended treatment of localized disease sites, such as cancer and arthritis, without the need for repeated dosing.

The development of a monoclonal antibody specific for a 2(')-C-allyl modification of uridine, and its use in the localization of ribozymes in vivo

Ribozymes are catalytically active RNA molecules that cleave other RNA molecules in a sequence-specific fashion, with significant turnover. The successful design and synthesis of ribozymes with modifications to increase their stability in biological fluids, while maintaining catalytic activity, has been instrumental in moving this technology from the laboratory into clinical trials. With the entry of ribozymes into the clinical setting, the need has arisen for reagents and/or assays to detect these drugs in tissues. We have developed a monoclonal antibody to the 2(')-deoxy-2(')-C-allyl uridine modification present in our synthetic hammerhead ribozymes. The monoclonal antibody, termed CA1USR, is a murine IgG1(k), whose epitope appears to involve both the 2(')-C-allyl modification, and the uridine base. Use of CA1USR for immunohistochemical detection of ribozymes in the tissues of mice which were administered two structurally different ribozymes has demonstrated its utility as a reagent for in vivo localization of ribozymes containing the 2(')-C-allyl uridine modification.

Efficiency of antisense oligonucleotide drug discovery

The costs for discovering and developing new drugs continue to escalate, with current estimates that the average cost is more than $800 million for each new drug brought to the market. Pharmaceutical companies are under enormous pressure to increase their efficiency for bringing new drugs to the market by third-party payers, shareholders, and their patients, and at the same time regulators are placing increased demands on the industry. To be successful in the future, pharmaceutical companies must change how they discover and develop new drugs. So far, new technologies have done little to increase overall efficiency of the industry and have added additional costs. Platform technologies such as monoclonal antibodies and antisense oligonucleotides have the potential of reducing costs for discovery of new drugs, in that many of the steps required for traditional small molecules can be skipped or streamlined. Additionally the success of identifying a drug candidate is much higher with platform technologies compared to small molecule drugs. This review will highlight some of the efficiencies of antisense oligonucleotide drug discovery compared to traditional drugs and will point out some of the current limitations of the technology.

Hammerhead ribozymes for target validation

Expensive failures in the pharmaceutical industry might be avoided by target validation at an early stage. Often, the full consequences of inhibiting a chosen drug target do not emerge until late in the development process. One option is to use hammerhead ribozymes as highly specific ribonucleases targeted exclusively at the mRNA encoding the target protein. The first part of this review is concerned with the mechanism and design of hammerhead ribozymes. This includes the chemistry of their action, specificity of cleavage and ability to discriminate between different mRNAs and selection of suitable cleavage sites. In considering their use for target validation, hammerhead ribozymes are divided into two categories. Endogenous ribozymes are transcribed inside the cell where they act whilst exogenous are introduced into the cell from outside. Exogenous ribozymes are synthesised chemically and must be protected against cellular nucleases. Information is provided on transfection methods and vectors that have been used with endogenous ribozymes as well as synthesis and chemical modification of exogenous ribozymes. Of proteins inhibited in cells or whole organisms, those in animal experiments are emphasised. Comparisons are made with other approaches, especially the use of antisense oligonucleotides or RNA.

Ribozyme gene therapy: applications for molecular medicine

RNA enzymes--ribozymes--are being developed as treatments for a variety of diseases ranging from inborn metabolic disorders to viral infections and acquired diseases such as cancer. Ribozymes can be used both to downregulate and to repair pathogenic genes. In some instances, short-term exogenous delivery of stabilized RNA is desirable, but many treatments will require viral-mediated delivery to provide long-term expression of the therapeutic catalyst. Current gene therapy applications employ variations on naturally occurring ribozymes, but in vitro selection has provided new RNA and DNA catalysts, and research on trans-splicing and RNase P has suggested ways to harness the endogenous ribozymes of the cell for therapeutic purposes.

Angiozyme: a novel angiogenesis inhibitor

Several inhibitors of angiogenesis are being developed for the treatment of cancer. One dominant strategy involves disruption of the vascular endothelial growth factor (VEGF) pathway by inhibition of the receptors for VEGF. Inhibition of the VEGF receptor activity can be accomplished using catalytic RNA molecules known as ribozymes, which downregulate VEGF receptor function by specifically cleaving the mRNAs for the primary VEGF receptors, Flt-1 and KDR. Significant inhibition of angiogenesis using ribozymes against both receptors has been demonstrated. In animal tumor models, antitumor effects are most pronounced with the anti-Flt-1 ribozyme known as Angiozyme (Ribozyme Pharmaceuticals, Boulder, CO). Extensive preclinical studies have demonstrated no significant toxicities. Clinical trials of Angiozyme are currently in progress for patients with advanced malignancy. Preliminary results demonstrate Angiozyme to be well tolerated, without significant side effects. Several phase II trials are underway for patients with advanced malignancy to test therapeutic efficacy.

General aspects of anti-angiogenesis and cancer therapy

Angiogenesis is the outgrowth of new vessels from pre-existing ones. Tumour growth and metastasis is dependent on angiogenesis and many stimulatory and inhibitory factors have been described which play an active role in this process. Inhibition of tumour neovasculature may be one strategy to inhibit tumour growth. Naturally occurring inhibitors of angiogenesis have been discovered and synthetic agents have been designed. Many of these inhibitors are currently being evaluated in clinical trials for the treatment of cancer. This review discusses the mechanism of action of these anti-angiogenics as well as a description of the clinical trials in which they are being evaluated.

Pharmacokinetics and tissue distribution of a ribozyme directed against hepatitis C virus RNA following subcutaneous or intravenous administration in mice

A nuclease resistant ribozyme targeting the 5' untranslated region (5' UTR) of hepatitis C virus (HCV) at site 195 has been identified. To investigate the therapeutic utility of this ribozyme, we evaluated the pharmacokinetics and tissue distribution with two labeled forms of this ribozyme. [(32)P]-labeled ribozyme was administered as a single subcutaneous (SC) or intravenous (IV) bolus at a dose of 10 mg/kg or 30 mg/kg in C57Bl/6 mice. Regardless of route of administration, peak liver concentrations achieved were greater than the concentration necessary to inhibit HCV-IRES-luciferase expression in cell culture. The ribozyme was well absorbed after SC administration (89%) and had an elimination half-life of 23 minutes. To show intracellular localization of the ribozyme in target tissue, a tetramethyl rhodamine (TMR)-labeled ribozyme was administered as a single SC or IV bolus at a dose of 30 mg/kg in C57Bl/6 mice. Mice treated SC or IV with TMR-labeled ribozyme had positive fluorescence in the liver from 15 minutes to 48 hours after dosing. Definite positive fluorescence was still present at 72 hours in the mice dosed via the IV route. At early time points (15 and 30 minutes postinjection), nuclear and possibly cytoplasmic fluorescence was present in the hepatocytes, and sinusoidal fluorescence was intense. At the later time points, fluorescence became more punctate. Abundant staining was often present in Kupffer cells. This study confirms the retention of ribozyme in liver cells and supports the potential of an anti-HCV ribozyme as a therapeutic agent for treatment of chronic hepatitis C.

Quantitative determination of a chemically modified hammerhead ribozyme in blood plasma using 96-well solid-phase extraction coupled with high-performance liquid chromatography or capillary gel electrophoresis

Versatile bioanalytical assays to detect chemically stabilized hammerhead ribozyme and putative ribozyme metabolites from plasma are described. The extraction protocols presented are based on serial solid-phase extractions performed on a 96-well plate format and are compatible with either IEX-HPLC or CGE back-end analysis. A validation of both assays confirmed that both the HPLC and the CGE methods possess the required linearity, accuracy, and precision to accurately measure concentrations of hammerhead ribozyme extracted from plasma. These methods should be of general use to detect and quantitate ribozymes from other biological fluids such as serum and urine.

Acute toxicology and pharmacokinetic assessment of a ribozyme (ANGIOZYME) targeting vascular endothelial growth factor receptor mRNA in the cynomolgus monkey

The potential acute toxicity of a ribozyme (ANGIOZYME) targeting the flt-1 vascular endothelial growth factor (VEGF) receptor mRNA was evaluated in cynomolgus monkeys following i.v. infusion or s.c. injection. ANGIOZYME was administered as a 4-hour i.v. infusion at doses of 10, 30, or 100 mg/kg or a s.c. bolus at 100 mg/kg. End points included blood pressure, electrocardiogram (ECG), clinical chemistry, hematology, complement factors, coagulation parameters, and ribozyme plasma concentrations. ANGIOZYME was well tolerated, with no drug-associated morbidity or mortality. There was no clear evidence of ANGIOZYME-related adverse effects in this study. Slight increases in spleen weight and lymphoid hyperplasia were observed in several animals. However, these changes were not dose dependent. Steady-state concentrations of ANGIOZYME were achieved during the 4-hour infusion of 10, 30, or 100 mg/kg. Dose-dependent elimination of ANGIOZYME was observed, with faster clearance at the two highest doses. ANGIOZYME was slowly absorbed after s.c. administration, resulting in steady-state concentrations for the 9-hour sampling period. Monkeys in this toxicology study received significant plasma ANGIOZYME exposure by both the s.c. and i.v. routes.

Therapeutic potential of ribozymes in haematological disorders

Ribozymes are RNA molecules that possess the ability to cleave and thus destroy other RNA molecules. As a result of this ability, they are ideal specific agents to use against the messenger RNAs of important genes found to be linked with disease (of cellular and viral origin). This review will briefly describe the different types of ribozyme and the potential they have as therapeutic compounds against viruses, oncogenes and drug resistance in haematological settings. The latest news from the various Phase I and II ribozyme clinical trials is discussed, as is the potential for the ribozymes' future as therapeutic agents.

In vitro selection of a novel nuclease-resistant RNA phosphodiesterase

BACKGROUND

Ribonucleotide-based enzymes (ribozymes) that cleave pathological RNAs are being developed as therapeutic agents. Chemical modification of the hammerhead ribozyme has produced nuclease-resistant catalysts that cleave targeted mRNAs in cell culture and exhibit antitumor activity in animals. Unfortunately, stabilizing modifications usually reduce the catalytic rate in vitro. An alternative to rationally designed chemical modifications of existing ribozymes is to identify novel motifs through in vitro selection of nuclease-stable sequence space. This approach is desirable because the catalysts can be optimized to function under simulated physiological conditions.

RESULTS

Utilizing in vitro selection, we have identified a nuclease-stable phosphodiesterase that demonstrated optimal activity at simulated physiological conditions. The initial library of 10(14) unique molecules contained 40 randomized nucleotides with all pyrimidines in a nuclease-stabilized 2'-deoxy-2'-amino format. The selection required trans-cleaving activity and base-pairing specificity towards a resin-bound RNA substrate. Initial selective pressure was permissive, with a 30 min reaction time and 25 mM Mg(2+). Stringency of selection pressure was gradually increased until final conditions of 1 mM Mg(2+) and less than 1 min reaction times were achieved. The resulting 61-mer catalyst required the 2'-amino substitutions at selected pyrimidine positions and was stable in human serum (half-life of 16 h).

CONCLUSIONS

We demonstrated that it is possible to identify completely novel, nuclease-resistant ribozymes capable of trans-cleaving target RNAs at physiologically relevant Mg(2+) concentrations. The new ribozyme motif has minimal substrate requirements, allowing for a wide range of potential RNA targets.

Inhibition of hepatitis C virus (HCV)-RNA-dependent translation and replication of a chimeric HCV poliovirus using synthetic stabilized ribozymes

Ribozymes are catalytic RNA molecules that can be designed to cleave specific RNA sequences. To investigate the potential use of synthetic stabilized ribozymes for the treatment of chronic hepatitis C virus (HCV) infection, we designed and synthesized hammerhead ribozymes targeting 15 conserved sites in the 5' untranslated region (UTR) of HCV RNA. This region forms an internal ribosome entry site that allows for efficient translation of the HCV polyprotein. The 15 synthetic ribozymes contained modified nucleotides and linkages that stabilize the molecules against nuclease degradation. All 15 ribozymes were tested for their ability to reduce expression in an HCV 5' UTR/luciferase reporter system and for their ability to inhibit replication of an HCV-poliovirus (HCV-PV) chimera. Treatment with several ribozymes resulted in significant down-regulation of HCV 5' UTR/luciferase reporter expression (range 40% to 80% inhibition, P <.05). Moreover, several ribozymes showed significant inhibition (>90%, P <.001) of chimeric HCV-PV replication. We further show that the inhibitory activity of ribozymes targeting site 195 of HCV RNA exhibits a sequence-specific dose response, requires an active catalytic ribozyme core, and is dependent on the presence of the HCV 5' UTR. Treatment with synthetic stabilized anti-HCV ribozymes has the potential to aid patients who are infected with HCV by reducing the viral burden through specific targeting and cleavage of the viral genome.