Critical issues in the antisense inhibition of brain gene expression in vivo: experiences targetting the 5-HT1A receptor

Intracerebral and intrathecal infusion of the TGF-beta 2-specific antisense phosphorothioate oligonucleotide AP 12009 in rabbits and primates: toxicology and safety

Here, we provide first evidence that long-term continuous infusion of highly purified antisense phosphorothioate oligodeoxynucleotides (S-ODN) into brain parenchyma is well tolerated and thus highly suitable for in vivo application. AP 12009 is an S-ODN for the therapy of malignant glioma. It is directed against human transforming growth factor-beta (TGF-beta2) mRNA. In the clinical setting, AP 12009 is administered intratumorally by continuous infusion directly into the brain tumor. In view of this clinical application, the focus of our data is on local toxicology studies in rabbits and monkeys to evaluate the safety of AP 12009. AP 12009 was administered either by intrathecal bolus injection into the subarachnoidal space of the lumbar region of both cynomolgus monkeys and rabbits or by continuous intraparenchymatous infusion directly into the brain tissue of rabbits. Intrathecal bolus administration of 0.1 ml of 500 microM AP 12009 showed neither clinical signs of toxicity nor macroscopically visible or histomorphologic changes. After a 7-day intraparenchymatous continuous infusion of 500 microM AP 12009 at 1 microl/h in rabbits, there was no evidence of toxicity except for local mild to moderate lymphocytic leptomeningoencephalitis. Additionally, AP 12009 showed good tolerability in safety pharmacology as well as in acute toxicity studies and 4-week subchronic toxicity studies in mice, rats, and monkeys. This favorable safety profile proves the suitability of AP 12009 for local administration in brain tumor patients from the point of view of toxicology.

5-hydroxytryptamine1A receptor is involved in the bee venom induced inflammatory pain

Injection of bee venom into one hindpaw of rat can elicit acute inflammation together with spontaneous pain, heat hyperalgesia and mechanical hyperalgesia/allodynia in the injected paw. 5-hydroxytryptamine (5-HT)1A receptor is the predominant receptor subtype in the spinal dorsal horn mediating the function of 5-HT in nociception. The goal of the present study is to assess the role of 5-HT1A receptor in the pain associated with the bee venom induced inflammation. Here we showed that 1 or 4 h after a subcutaneous bee venom challenge, expression of 5-HT1A receptor mRNA in the ipsilateral lumbar spinal cord increased significantly by 80.94 or 37.86%, respectively. Antisense oligodeoxynucleotide knockdown of spinal 5-HT1A receptor attenuated spontaneous pain and reversed heat hyperalgesia in rats injected with bee venom. Thus, the present data suggest a facilitating role for 5-HT1A receptor in bee venom induced inflammatory pain.

GeneBlocs are powerful tools to study and delineate signal transduction processes that regulate cell growth and transformation

The study of signal transduction processes using antisense oligonucleotides is often complicated by low intracellular stability of the antisense reagents or by nonspecific effects that cause toxicity. Here, we introduce a new class of antisense molecules, so-called GeneBlocs, which are characterized by improved stability, high target RNA specificity, and low toxicity. GeneBlocs allow for efficient downregulation of mRNA expression at nanomolar concentrations, and they do not interfere with cell proliferation. We demonstrate these beneficial properties using a positive readout system. GeneBloc-mediated inhibition of tumor suppressor PTEN (phosphatase and tension homologue detected on chromosome 10) expression leads to hyperactivation of the phosphatidylinositol (PI) 3-kinase pathway, thereby mimicking the loss of PTEN function and its early consequences observed in mammalian cancer cells. Specifically, cells treated with PTEN GeneBlocs show functional activation of Akt, a downstream effector of PI 3-kinase signaling, and exhibit enhanced proliferation when seeded on a basement membrane matrix. In addition, GeneBlocs targeting the catalytic subunit of PI 3-kinase, p110, specifically inhibit signal transduction of endogenous or recombinant PI 3-kinase. This demonstrates that GeneBlocs are powerful tools to analyze and to modulate signal transduction processes and, therefore, represent alternative reagents for the validation of gene function.

Evidence for sequence-dependent and reversible nonspecific effects of PS-capped antisense treatment after intracerebral administration

Phosphorothioate (PS)-capped phosphodiester (PE) oligodeoxynucleotides (ODNs) were used to determine whether the dopamine-dependent locomotor-stimulant effect of nicotine is mediated via a4 subunit-containing nicotinic receptors. To this end, rats received direct intraventral tegmental area infusion of a4 antisense via osmotic minipump, and their locomotor response to nicotine (0.2 mg/kg, s.c.) was tested. Eight antisense ODNs were screened, but only one inhibited nicotine-induced locomotion. This inhibition was reversible and selective, insofar as basal (saline) activity was unaffected, and a mismatch ODN was without effect. However, antisense treatment also caused sequence-dependent toxic effects, including neuronal degeneration in the ventral tegmental area, dopaminergic denervation, and weight loss. We conclude that despite previous reports, PS-capped PE-ODNs can cause severe neurotoxicity on chronic infusion into brain tissue. Moreover, sequence dependence and temporal reversibility, two generally accepted criteria of antisense action, may sometimes reflect the occurrence of toxic effects and resultant functional compensation.

The cellular delivery of antisense oligonucleotides and ribozymes

The design and development of antisense oligonucleotides and ribozymes for the treatment of diseases arising from genetic abnormalities has become a real possibility over the past few years. Improvements in oligonucleotide chemistry have led to the synthesis of nucleic acids that are relatively stable in the biological milieu. However, advances in cellular targeting and intracellular delivery will probably lead to more widespread clinical applications. This review looks at recent advances in the in vitro and in vivo delivery of antisense oligodeoxynucleotides and ribozymes.

GABAA receptor antisense epilepsy: histological changes following infusion of antisense oligodeoxynucleotide to GABAA receptor gamma 2 subunit into rat hippocampus

A deficiency of neuronal inhibition mediated by gamma-aminobutyric acid (GABA) via the GABAA receptor complex has been hypothesised to be a central factor in epileptogenesis. Intrahippocampal infusion of antisense oligodeoxynucleotide (ODN) to the GABAA receptor gamma 2 subunit in rats leads to electrographic limbic status epilepticus. In this model, epileptic phenomena are accompanied by loss of hippocampal neurones. The purpose of the present study was to investigate the time-course of morphological changes following hippocampal antisense 'knockdown' of the GABAA receptor gamma 2 subunit. gamma 2 subunit antisense ODN was infused continuously into the right hippocampus for periods between 1 and 5 days. After about 4 days of infusion, pronounced neurodegenerative changes were consistently observed within the ipsilateral hippocampus. In general, marked loss of CA3 pyramidal cells was found. The notion that the histological changes induced by the antisense ODN were specific to the applied ODN sequence was supported by the finding that a mismatch control ODN did not induce neurodegenerative changes, except for a small lesion in the immediate vicinity of the infusion site. Extensive ipsilateral hippocampal infiltration with monocytes and macrophages was a feature of antisense ODN infusion, but was considerably less pronounced after the infusion of control ODN. Immunocytochemistry using an antibody labeling glial fibrillary acidic protein (GFAP), revealed marked astroglial hypertrophy/proliferation after 4 days of antisense treatment, i.e., coincident with the development of neurodegeneration, in the ipsilateral hippocampus. At this time GFAP-immunoreactivity was also evident in the contralateral hippocampus, indicating contralateral spread of seizure activity.

The delivery of antisense therapeutics

Antisense oligonucleotides, ribozymes and DNAzymes have emerged as novel, highly selective inhibitors or modulators of gene expression. Indeed, their use in the treatment of diseases arising from genetic abnormalities has become a real possibility over the past few years. The first antisense drug molecule is now available for clinical use in Europe and USA. However, their successful application in the clinic will require improvements in cellular targeting and intracellular delivery. This review aims to look at recent advances in the in vitro and in vivo delivery of antisense oligodeoxynucleotides and ribozymes.

Synthetic oligonucleotides as therapeutics: the coming of age

Synthetic oligonucleotides (ODNs) are short nucleic acid chains that can act in a sequence specific manner to control gene expression. Significant progress has been made in the development of synthetic ODN therapeutics since the first demonstration of gene inhibition by antisense ODNs in a cell culture system two decades ago. This new class of therapeutic agents can potentially target any abnormally expressed genes in a broad range of diseases from viral infections to psychoneurological disorders. A number of "first" generation synthetic ODNs have entered into human clinical trials in the last few years. The eminent approval of the first ODN for the treatment of cytomaglovirus retinitis by the FDA in USA will provide much excitement that this new class of compounds holds great promise as a therapeutic "magic bullet". However, many obstacles still exist in the development of this technology. In this review, the current status of synthetic ODN chemistry, drug delivery methods, mechanisms of ODN action, potential clinical applications and its limitations in a wide range of human disorders will be described.

Behavioural and physiological effects induced by an infusion of antisense to alpha(2D)-adrenoceptors in the rat

1. The aim of this study was to investigate the behavioural and physiological effects of an i.c.v. infusion of antisense oligonucleotide to the alpha(2D)-adrenoceptor subtype. Behavioural and physiological parameters were monitored for 2 days before the infusion, throughout the 3-day infusion period and for 3 days following the end of the infusion. 2. The antisense infusion resulted in a significant increase in behavioural activity characterized by increased locomotion and grooming scores. Behavioural activity scores of rats treated with antisense to alpha(2D)-adrenoceptors were significantly higher than those of rats treated with vehicle (H(2)O) or the mismatch toxicity control on day 4 and day 5 and, significantly higher than vehicle controls on day 6. 3. Body weight gain was significantly reduced in the antisense-treated rats at the end of the study compared to the vehicle (34%) and mismatch groups (30%), although daily food and water intakes were not significantly different at any time point. 4. Pupil diameters of rats infused with antisense to alpha(2D)-adrenoceptors were significantly greater than those of animals treated either with vehicle or mismatch oligonucleotide on day 5 of the study. On day 6, the pupil diameters of these animals were still significantly greater than the mismatch group. 5. In conclusion, an i.c.v. infusion of antisense to the alpha(2D)-adrenoceptor induced behavioural activation in rats, increased pupil diameter and reduced total weight gain. These effects were specific to the antisense-treated group and were fully reversed post-infusion.

Brain as a unique antisense environment

During the last few years, antisense oligodeoxyribonucleotides (asODN) have become a commonly used tool for blocking of gene expression in the mammalian central nervous system. Successful gene inhibition has been reported for such diverse targets as those encoding neurotransmitter receptors, neuropeptides, trophic factors, transcription factors, cytokines, transporters, ion channels, and others. This review presents a discussion of recent studies on ODN in the brain, with a focus on specific approaches taken by the researchers in this field and especially on peculiar features of this organ as a milieu for asODN action. It is concluded that from the presented literature survey no coherent view on how to rationally design ODN for brain studies has emerged.

Limited inhibition of 5-HT1A receptor expression in the rat brain by antisense RNA and oligodesoxynucleotides

The efficiency of antisense approaches to produce a selective regional inhibition of the expression of brain 5-HT1A receptors was tested in the rat. In vivo ICV injections of modified antisense oligodesoxynucleotides yielded at most an 18% specific decrease in 5-HT1A receptor expression in the hippocampus only, as measured by [3H]8-OH-DPAT autoradiographic labeling. In vitro, when 5-HT1A receptors were transiently expressed in LLC-PK1 cells, co-transfection with antisense RNA encoding plasmids resulted in a marked reduction (50-70%) in the density of 5-HT1A binding sites. In vivo stereotaxic injections of the same constructs into the hippocampus, but not in the raphe, which contains 5-HT1A autoreceptors, were shown to produce a approximately 20% reduction in local 5-HT1A receptor density. These data show that antisense strategies could be used to inhibit 5-HT1A receptors expression in the rat hippocampus, but with a limited efficacy.

Drug delivery of antisense molecules to the brain for treatment of Alzheimer's disease and cerebral AIDS

Antisense oligonucleotides (ODNs) and peptide nucleic acids (PNAs) are potential therapeutics for eradication of malignancies, viral infections, and other pathologies. However, ODNs and PNAs in general are unable to cross cellular membranes and blood-tissue barriers, such as the blood-brain barrier (BBB), which is only permeable to lipophilic molecules of molecular weight <600 Da. Cellular delivery systems based on conjugates of streptavidin (SA) and the OX26 monoclonal antibody directed to the transferrin receptor may be employed as a universal carrier for the transport of mono-biotinylated peptides, ODNs, or PNAs. 3'-Biotinylation of phosphodiester (PO)-ODN produces complete protection of ODN against serum and cellular 3'-exonucleases, facilitating the conjugation to avidin-based delivery systems and maintaining the activation of RNase H. These delivery systems markedly increased the cellular uptake and antisense efficacy of 3'-biotinylated ODNs in models of Alzheimer's disease and HIV-AIDS. In vivo brain delivery studies demonstrated that 3'-protected PO-ODNs and PO-phosphorothioate(PS)-ODN hybrids containing a single PO linkage are subjected to endonuclease degradation in vivo. On the contrary PS-ODNs, which were also protected at 3'-terminus by biotinylation, are metabolically stable in vivo and resistant to exo/endonuclease degradation. However, because of the strong binding of these oligomers to plasma protein, PS-ODNs are poorly transported into the brain through the BBB by the OX26-SA delivery vector following intravenous administration. PNAs are also resistant to exo/endonuclease and protease degradation, and these molecules biotinylated at the amino terminal group were transported into the brain by the OX26-SA delivery system with brain uptake levels comparable to that of morphine. Using the rev gene of HIV as a model target, RNase protection assays and cell-free translation arrest showed that the PNA-OX26-SA conjugate maintained active recognition and inactivation of target mRNA, respectively. The overall experimental evidence suggests that PNA-OX26-SA conjugates represent optimal antisense molecules for drug delivery to the brain.