Antisense oligonucleotides as therapeutics for hyperlipidaemias

Mechanisms of Antisense Transcription Initiation with Implications in Gene Expression, Genomic Integrity and Disease Pathogenesis

Non-coding antisense transcripts arise from the strand opposite the sense strand. Over 70% of the human genome generates non-coding antisense transcripts while less than 2% of the genome codes for proteins. Antisense transcripts and/or the act of antisense transcription regulate gene expression and genome integrity by interfering with sense transcription and modulating histone modifications or DNA methylation. Hence, they have significant pathological and physiological relevance. Indeed, antisense transcripts were found to be associated with various diseases including cancer, diabetes, cardiac and neurodegenerative disorders, and, thus, have promising potentials for prognostic and diagnostic markers and therapeutic development. However, it is not clearly understood how antisense transcription is initiated and epigenetically regulated. Such knowledge would provide new insights into the regulation of antisense transcription, and hence disease pathogenesis with therapeutic development. The recent studies on antisense transcription initiation and its epigenetic regulation, which are limited, are discussed here. Furthermore, we concisely describe how antisense transcription/transcripts regulate gene expression and genome integrity with implications in disease pathogenesis and therapeutic development.

Mechanisms of antisense transcription initiation from the 3' end of the GAL10 coding sequence in vivo

In spite of the important regulatory functions of antisense transcripts in gene expression, it remains unknown how antisense transcription is initiated. Recent studies implicated RNA polymerase II in initiation of antisense transcription. However, how RNA polymerase II is targeted to initiate antisense transcription has not been elucidated. Here, we have analyzed the association of RNA polymerase II with the antisense initiation site at the 3' end of the GAL10 coding sequence in dextrose-containing growth medium that induces antisense transcription. We find that RNA polymerase II is targeted to the antisense initiation site at GAL10 by Reb1p activator as well as general transcription factors (e.g., TFIID, TFIIB, and Mediator) for antisense transcription initiation. Intriguingly, while GAL10 antisense transcription is dependent on TFIID, its sense transcription does not require TFIID. Further, the Gal4p activator that promotes GAL10 sense transcription is dispensable for antisense transcription. Moreover, the proteasome that facilitates GAL10 sense transcription does not control its antisense transcription. Taken together, our results reveal that GAL10 sense and antisense transcriptions are regulated differently and shed much light on the mechanisms of antisense transcription initiation.

Modulation of lipoprotein metabolism by antisense technology: preclinical drug discovery methodology

Antisense oligonucleotides (ASOs) are a new class of specific therapeutic agents that alter the intermediary metabolism of mRNA, resulting in the suppression of disease-associated gene products. ASOs exert their pharmacological effects after hybridizing, via Watson-Crick base pairing, to a specific target RNA. If appropriately designed, this event results in the recruitment of RNase H, the degradation of targeted mRNA or pre-mRNA, and subsequent inhibition of the synthesis of a specific protein. A key advantage of the technology is the ability to selectively inhibit targets that cannot be modulated by traditional therapeutics such as structural proteins, transcription factors, and, of topical interest, lipoproteins. In this chapter, we will first provide an overview of antisense technology, then more specifically describe the status of lipoprotein-related genes that have been studied using the antisense platform, and finally, outline the general methodology required to design and evaluate the in vitro and in vivo efficacy of those drugs.

Therapeutic potential of mipomersen in the management of familial hypercholesterolaemia

High levels of low-density lipoprotein cholesterol (LDL-C) and lipoprotein(a) [Lp(a)] are associated with early morbidity and mortality caused by cardiovascular disease (CVD). There are hints that a reduction of LDL-C levels beyond currently advocated targets, and the use of drugs that also have Lp(a)-lowering potential, could provide further clinical benefit. Today, LDL apheresis is the only available treatment option to achieve further lowering of apolipoprotein-B (apo-B)-containing lipoproteins, especially Lp(a). Mipomersen is currently being studied in patients with mild to severe hypercholesterolaemia as add-on therapy to other lipid-lowering therapy, as monotherapy in patients who are intolerant of HMG-CoA reductase inhibitors (statins) and who are at high risk for CVD. Patients affected by homozygous or heterozygous familial hypercholesterolaemia (FH), which are inherited autosomal co-dominant disorders characterized by a marked elevation of serum LDL-C concentration, remain a clinical challenge, especially when their CVD risk is aggravated by additionally elevated Lp(a) levels. Mipomersen is a 20-mer oligonucleotide [2'-O-(2-methoxy) ethyl-modified oligonucleotide], a second-generation antisense oligonucleotide (AOS), complementary to the coding region for human-specific apo-B-100 messenger RNA (mRNA). Mipomersen inhibits apo-B-100 synthesis and is consequently a new treatment strategy to lower apo-B-containing lipoproteins like LDL-C and Lp(a) in patients at high risk for CVD not on target or intolerant to statins. This article focuses on mipomersen and gives an overview of the current status of mipomersen as a promising treatment option. Recent studies have shown a decrease in LDL-C levels of 22-42.2% and in Lp(a) of 19.6-31.1% from baseline, depending on study design. Dose-dependent reductions of very low-density lipoprotein cholesterol (VLDL-C) and triglyceride levels have also been observed. Although the short-term efficacy and safety of mipomersen have been proven, side effects like injection-site reactions (up to 90-100%), increased liver enzymes, cephalgias, nasopharyngitis, myalgia, nausea and fatigue must be mentioned and critically discussed. Furthermore, we need more data on the long-term side effects, especially regarding the long-term potential for hepatic steatosis. Data on cardiovascular outcomes with mipomersen are also not yet available.

A review of the role of apolipoprotein C-II in lipoprotein metabolism and cardiovascular disease

The focus of this review is on the role of apolipoprotein C-II (apoC-II) in lipoprotein metabolism and the potential effects on the risk of cardiovascular disease (CVD). We searched PubMed/Scopus for articles regarding apoC-II and its role in lipoprotein metabolism and the risk of CVD. Apolipoprotein C-II is a constituent of chylomicrons, very low-density lipoprotein, low-density lipoprotein, and high-density lipoprotein (HDL). Apolipoprotein C-II contains 3 amphipathic α-helices. The lipid-binding domain of apoC-II is located in the N-terminal, whereas the C-terminal helix of apoC-II is responsible for the interaction with lipoprotein lipase (LPL). At intermediate concentrations (approximately 4 mg/dL) and in normolipidemic subjects, apoC-II activates LPL. In contrast, both an excess and a deficiency of apoC-II are associated with reduced LPL activity and hypertriglyceridemia. Furthermore, excess apoC-II has been associated with increased triglyceride-rich particles and alterations in HDL particle distribution, factors that may increase the risk of CVD. However, there is not enough current evidence to clarify whether increased apoC-II causes hypertriglyceridemia or is an epiphenomenon reflecting hypertriglyceridemia. A number of pharmaceutical interventions, including statins, fibrates, ezetimibe, nicotinic acid, and orlistat, have been shown to reduce the increased apoC-II concentrations. An excess of apoC-II is associated with increased triglyceride-rich particles and alterations in HDL particle distribution. However, prospective trials are needed to assess if apoC-II is a CVD marker or a risk factor in high-risk patients.

Antisense therapy and emerging applications for the management of dyslipidemia

BACKGROUND

Because a significant percentage of patients who require high-dose statin therapy for dyslipidemia experience treatment-related muscle symptoms and an inconsistent clinical response, alternative or adjunctive approaches to the management of dyslipidemia are needed. One alternative approach, antisense therapy, may offer an effective and well-tolerated option for patients not satisfactorily responsive to or intolerant to standard pharmacologic dyslipidemia therapies.

OBJECTIVE

This review provides an overview of antisense technology and its potential role in the management of dyslipidemia.

METHODS

Source material was obtained primarily from the published literature identified through a search of the PubMed database.

RESULTS

Antisense technology is an evolving approach to therapy that has gone through a series of refinements to enhance molecular stability, potency, and tolerability. Mipomersen is an antisense molecule capable of producing clinically meaningful reductions in low-density lipoprotein cholesterol in patients with severe familial hypercholesterolemia. Further long-term clinical studies are required to more clearly quantify its impact on risk for cardiovascular events and establish whether it increases risk for hepatosteatosis.

CONCLUSION

Antisense therapy represents a potentially effective and well-tolerated emerging treatment modality for numerous diseases. In the treatment of hypercholesterolemia, the antisense therapy mipomersen may provide a possible treatment option for patients with treatment-resistant dyslipidemia.

Mipomersen, an antisense apolipoprotein B synthesis inhibitor

INTRODUCTION

mipomersen is a second-generation antisense oligonucleotide (ASO) targeted to human apolipoprotein (apo) B-100, a large protein synthesized by the liver that plays a fundamental role in human lipoprotein metabolism. Mipomersen predominantly distributes to the liver and decreases the production of apoB-100, the primary structural protein of the atherogenic lipoproteins including low density lipoprotein (LDL), thereby reducing plasma LDL-cholesterol and apoB-100 concentrations.

AREAS COVERED

the mode of action, preclinical development and clinical trials of mipomersen, an antisense apoB synthesis inhibitor. The paper provides an understanding of the pharmacokinetic and pharmacodynamic characteristics of mipomersen and insight into its clinical efficacy and safety. In clinical trials, mipomersen produced dose-dependent and prolonged reductions in LDL-cholesterol and other apoB-containing lipoproteins, including lipoprotein (a) [Lp(a)] in healthy volunteers and in patients with mild to moderate hypercholesterolemia. Mipomersen has been shown to decrease apoB, LDL-cholesterol and Lp(a) in patients with heterozygous and homozygous familial hypercholesterolemia on maximally tolerated lipid-lowering therapy.

EXPERT OPINION

mipomersen shows promise as an adjunctive agent by reducing apoB-containing lipoproteins in patients at high risk of atherosclerotic cardiovascular disease who are not at target or are intolerant of statins. Although the short-term efficacy and safety of mipomersen has been established, concern exists regarding the long-term potential for hepatic steatosis with this ASO.

Development of apolipoprotein B antisense molecules as a therapy for hyperlipidemia

As new studies demonstrate that lower levels of low-density lipoprotein cholesterol (LDL-C) reduce cardiovascular disease, and as goals for LDL-C in high-risk individuals are reduced further and further, reaching those goals becomes more difficult for a significant percentage of the population. New therapeutic approaches to lower LDL-C would, therefore, be advantageous, particularly in those who are most likely to suffer cardiovascular disease-associated morbidity and mortality. Mouse and human genetic models suggest that decreasing hepatic apolipoprotein B (apoB) production may be a therapeutic approach for the treatment of dyslipidemia. Because antisense oligonucleotides naturally distribute to the liver and can specifically inhibit synthesis of proteins from their messenger RNAs, antisense oligonucleotides represent a potential approach for decreasing the biosynthesis of apoB, and thereby, the production of both very low density lipoprotein (VLDL) and LDL. Newly developed apoB antisense approaches have produced results in animal models and humans, providing proof of concept regarding reductions in LDL-C concentrations. Surprisingly, despite prior experience with inhibitors of microsomal triglyceride transfer protein, which also inhibits the secretion of VLDL, apoB antisense-mediated reduction in VLDL secretion does not appear to cause marked steatosis. The mechanisms whereby two different approaches for inhibiting apoB and triglyceride secretion have different effects on hepatic triglycerides are currently being examined.

Apolipoprotein B synthesis inhibition: results from clinical trials

PURPOSE OF REVIEW

Mipomersen is a second-generation antisense oligonucleotide developed to inhibit the synthesis of apolipoprotein B-100 in the liver. In this review we will summarize the results of recent preclinical and clinical studies addressing safety and low-density lipoprotein-cholesterol (LDL-c) lowering efficacy of this new compound.

RECENT FINDINGS

In phase 3 clinical trials, mipomersen has been shown to significantly reduce LDL-c in patients with homozygous and heterozygous familial hypercholesterolemia on maximally tolerated lipid-lowering therapy. Injection site reactions, flu-like symptoms and increases in liver transaminases were the main adverse events. A recent safety study, designed to investigate the effects of mipomersen on intrahepatic triglyceride content, failed to show evidence of clinically relevant hepatic steatosis after 13 weeks of treatment.

SUMMARY

Mipomersen is a new agent to lower LDL-c in patients at increased risk of cardiovascular disease and/or intolerant to statins. Whereas safety concerns have focused on hepatic fat accumulation, to date no evidence of clinically relevant increases of intrahepatic triglyceride content are reported. Ongoing and future studies are eagerly awaited to assess the impact of mipomersen on hepatic triglyceride content after prolonged exposure.

Gene therapy for dyslipidemia: a review of gene replacement and gene inhibition strategies

Despite numerous technological and pharmacological advances and more detailed knowledge of molecular etiologies, cardiovascular diseases remain the leading cause of morbidity and mortality worldwide claiming over 17 million lives a year. Abnormalities in the synthesis, processing and catabolism of lipoprotein particles can result in severe hypercholesterolemia, hypertriglyceridemia or low HDL-C. Although a plethora of antidyslipidemic pharmacological agents are available, these drugs are relatively ineffective in many patients with Mendelian lipid disorders, indicating the need for new and more effective interventions. In vivo somatic gene therapy is one such intervention. This article summarizes current strategies being pursued for the development of clinical gene therapy for dyslipidemias that cannot effectively be treated with existing drugs.

Antisense oligonucleotide directed to human apolipoprotein B-100 reduces lipoprotein(a) levels and oxidized phospholipids on human apolipoprotein B-100 particles in lipoprotein(a) transgenic mice

BACKGROUND

Lipoprotein (a) [Lp(a)] is a genetic cardiovascular risk factor that preferentially binds oxidized phospholipids (OxPL) in plasma. There is a lack of therapeutic agents that reduce plasma Lp(a) levels.

METHODS AND RESULTS

Transgenic mice overexpressing human apolipoprotein B-100 (h-apoB-100 [h-apoB mice]) or h-apoB-100 plus human apo(a) to generate genuine Lp(a) particles [Lp(a) mice] were treated with the antisense oligonucleotide mipomersen directed to h-apoB-100 mRNA or control antisense oligonucleotide for 11 weeks by intraperitoneal injection. Mice were then followed up for an additional 10 weeks off therapy. Lp(a) levels [apo(a) bound to apoB-100] and apo(a) levels ["free" apo(a) plus apo(a) bound to apoB-100] were measured by chemiluminescent enzyme-linked immunoassay and commercial assays, respectively. The content of OxPL on h-apoB-100 particles (OxPL/h-apoB) was measured by capturing h-apoB-100 in microtiter wells and detecting OxPL by antibody E06. As expected, mipomersen significantly reduced plasma h-apoB-100 levels in both groups of mice. In Lp(a) mice, mipomersen significantly reduced Lp(a) levels by approximately 75% compared with baseline (P<0.0001) but had no effect on apo(a) levels or hepatic apo(a) mRNA expression. OxPL/h-apoB levels were much higher at baseline in Lp(a) mice compared with h-ApoB mice (P<0.0001) but decreased in a time-dependent fashion with mipomersen. There was no effect of the control antisense oligonucleotide on lipoprotein levels or oxidative parameters.

CONCLUSIONS

Mipomersen significantly reduced Lp(a) and OxPL/apoB levels in Lp(a) mice. The present study demonstrates that h-apoB-100 is a limiting factor in Lp(a) particle synthesis in this Lp(a) transgenic model. If applicable to humans, mipomersen may represent a novel therapeutic approach to reducing Lp(a) levels and their associated OxPL.

ISIS 301012 gene therapy for hypercholesterolemia: sense, antisense, or nonsense?

OBJECTIVE

To present an overview of antisense technology and to review and assess available literature on the chemistry, pharmacology, pharmacokinetics, drug interactions, preclinical and clinical studies, dosing, and adverse events of ISIS 301012 in the treatment of hyperlipidemia.

DATA SOURCES

PubMed database searches were conducted from 1966 to May 2007 using the search terms ISIS 301012, antisense, oligonucleotide, hypercholesterolemia, hyperlipidemia, and apolipoprotein B. Bibliographies of relevant review articles and information from the manufacturer were reviewed for additional references.

STUDY SELECTION AND DATA EXTRACTION

Available English-language literature, including abstracts, preclinical, and clinical trials, review articles, and scientific presentations were examined.

DATA SYNTHESIS

Apolipoprotein B is an important structural protein on the surface of atherogenic lipoproteins such as remnant very-low-density lipoprotein and low-density lipoprotein and facilitates the clearance of these particles from the circulation by binding to the low-density lipoprotein receptor. Overproduction of apolipoprotein B or reduced receptor-mediated clearance of lipoproteins leads to elevated serum cholesterol levels and premature atherosclerosis. ISIS 301012 is an antisense oligonucleotide that inhibits apolipoprotein B production by binding directly to and reducing the expression of apolipoprotein B messenger RNA. In a clinical trial, ISIS 301012 50-400 mg administered weekly via subcutaneous injection for 4 weeks reduced apolipoprotein B by 14.3-47.4% and low-density lipoprotein cholesterol by 5.9-40% at 55 days. The most frequent adverse event was injection-site erythema that resolved spontaneously. Studies are ongoing to further define the safety, efficacy, and pharmacokinetics of ISIS 301012 as add-on therapy in patients with heterozygous and homozygous familial hypercholesterolemia. No pharmacokinetic interactions have been demonstrated with ezetimibe and simvastatin.

CONCLUSIONS

ISIS 301012 is the first agent to enter clinical trials utilizing an antisense mechanism for reducing the production of apolipoprotein B. Further studies are needed to verify its safety, efficacy, and position of therapy in the dyslipidemic patient.

Antisense apolipoprotein B therapy: where do we stand?

PURPOSE OF REVIEW

Antisense oligonucleotides are novel therapeutic agents that reduce the number of specific mRNAs available for translation of the encoded protein. ISIS 301012 is an antisense oligonucleotide developed to reduce the hepatic synthesis of apolipoprotein B-100. Apolipoprotein B-100 is made in the liver, and antisense oligonucleotides preferentially distribute to that organ, so antisense apolipoprotein B-100 may have potential as an efficacious lipid-lowering agent.

RECENT FINDINGS

Recently, in healthy volunteers and in mild dyslipidaemic patients, this strategy as monotherapy or in conjunction with statins has shown unparalleled efficacy in reducing apolipoprotein B-100 and LDL-cholesterol. Tolerance for this novel therapy is encouraging and safety concerns currently only relate to mild injection-site reactions and rare liver-function test abnormalities. It should be noted, however, that these safety results were obtained in relatively few individuals.

SUMMARY

ISIS 301012 has initially shown promising results in experimental animal models, and in clinical trials in humans. Besides the effect of reducing apolipoprotein B-100 and LDL-cholesterol, this compound also significantly lowers plasma triglycerides. Safety concerns related to the drug include increased liver-function tests. To date no evidence of hepatic steatosis has been reported. Nonetheless, clinical trials of longer duration are required to demonstrate further safety.

MTP inhibition as a treatment for dyslipidaemias: time to deliver or empty promises?

The development of cholesterol-lowering drugs, including a statins, bile acid sequestrants and cholesterol absorption inhibitors has expanded the options for cardiovascular prevention. Recent treatment guidelines emphasise that individuals at substantial risk for atherosclerotic coronary heart disease should meet defined lipid targets. Combination therapy with drugs that have different and complementary mechanisms of action is often needed to achieve these goals. Existing approaches to the treatment of hypercholesterolaemia are still ineffective in halting the progression of coronary artery disease in some patients despite combination therapies. Other patients are resistant to, or intolerant of, conventional pharmacotherapy and remain at high-risk of atherosclerotic cardiovascular disease, so that alternative approaches are needed. New agents, including inhibitors of microsomal triglyceride transfer protein (MTP), may play a future role, either alone or in combination, in the treatment of hyperlipidaemias. This review focuses on novel approaches to treat dyslipidaemias via the inhibition of MTP. Patients most suitable for use of MTP inhibitors include those with hepatic hypersecretion of apoB, including the metabolic syndrome, Type 2 diabetes mellitus and familial combined hyperlipidaemia, as well as homozygous and heterozygous familial hypercholesterolaemia. However, certain safety issues with these agents need resolving, particularly fatty liver disease.

Liver-specific inhibition of acyl-coenzyme a:cholesterol acyltransferase 2 with antisense oligonucleotides limits atherosclerosis development in apolipoprotein B100-only low-density lipoprotein receptor-/- mice

OBJECTIVE

The purpose of this study was to determine the effects of liver-specific inhibition of acyl-coenzyme A:cholesterol acyltransferase 2 (ACAT2) on the development of hypercholesterolemia and atherosclerosis in mice.

METHODS AND RESULTS

Apolipoprotein B100-only low-density lipoprotein (LDL) receptor-/- mice were given saline, a nontargeting control antisense oligonucleotide (ASO), or ASOs targeting ACAT2 biweekly for a period spanning 16 weeks. Mice treated with ACAT2 targeting ASOs had liver-specific reduction in ACAT2 mRNA, yet intestinal ACAT2 and cholesterol absorption was left undisturbed. ASO-mediated knockdown of ACAT2 resulted in reduction of total plasma cholesterol, increased levels of plasma triglyceride, and a shift in LDL cholesteryl ester (CE) fatty acid composition from mainly saturated and monounsaturated to polyunsaturated fatty acid enrichment. Furthermore, the liver-specific depletion of ACAT2 resulted in protection against diet-induced hypercholesterolemia and aortic CE deposition. This is the first demonstration that specific pharmacological inhibition of ACAT2, without affecting ACAT1, is atheroprotective.

CONCLUSIONS

Hepatic ACAT2 plays a critical role in driving the production of atherogenic lipoproteins, and therapeutic interventions, such as the ACAT2-specific ASOs used here, which reduce acyltransferase 2 (ACAT2) function in the liver without affecting ACAT1, may provide clinical benefit for cardiovascular disease prevention.