Therapeutic Approaches to Prion Diseases.

Therapy of prion diseases represents an extremely challenging effort for scientists working in the field. These challenges are epitomized by 20 years of failures in clinical trials and preclinical investigations. However, the discovery that misfolded proteins involved in other neurodegenerative diseases show a prion-like mechanism of spreading, is positively impacting the prion drug discovery field. Herein, we describe those efforts that have contributed to strengthen the drug discovery process in prion diseases. Accordingly, we analyze the historical course of clinical trials that have assessed the efficacy of several chemically unrelated repositioned drugs. Unfortunately, none of them resulted successful. Thus, alternative approaches aiming at identifying innovative drugs with a completely new mechanism of action, have been recently pursued. In this respect, the multifactorial nature of prion diseases provides a strong foundation to the development of small molecules directed to two or multiple pathological targets, critically involved in the intricate disease pathogenesis (i.e., multitarget compounds). Second, the fact that misfolded proteins can be considered not only as therapeutic target, but also as neuropathological hallmark, lends support to the development of theranostics, i.e., single molecules with concomitant therapeutic and diagnostic properties. Although nobody knows whether these innovative tools will be brought to clinical trials, and the process is certainly time-consuming and demanding, the rewards are well worth the effort.

Convection-enhanced delivery of AAV2-PrPshRNA in prion-infected mice.

Prion disease is caused by a single pathogenic protein (PrPSc), an abnormal conformer of the normal cellular prion protein PrPC. Depletion of PrPC in prion knockout mice makes them resistant to prion disease. Thus, gene silencing of the Prnp gene is a promising effective therapeutic approach. Here, we examined adeno-associated virus vector type 2 encoding a short hairpin RNA targeting Prnp mRNA (AAV2-PrP-shRNA) to suppress PrPC expression both in vitro and in vivo. AAV2-PrP-shRNA treatment suppressed PrP levels and prevented dendritic degeneration in RML-infected brain aggregate cultures. Infusion of AAV2-PrP-shRNA-eGFP into the thalamus of CD-1 mice showed that eGFP was transported to the cerebral cortex via anterograde transport and the overall PrPC levels were reduced by ∼ 70% within 4 weeks. For therapeutic purposes, we treated RML-infected CD-1 mice with AAV2-PrP-shRNA beginning at 50 days post inoculation. Although AAV2-PrP-shRNA focally suppressed PrPSc formation in the thalamic infusion site by ∼ 75%, it did not suppress PrPSc formation efficiently in other regions of the brain. Survival of mice was not extended compared to the untreated controls. Global suppression of PrPC in the brain is required for successful therapy of prion diseases.

[Innovation of therapeutics and prophylaxis for prion diseases].

There is no established treatment for prion diseases; however, recently several drug candidates, including pentosan polysulfate and doxycycline, have been clinically used on a trial basis to prevent accumulation of abnormal prion protein in the brain. So far the outcome of the trials is still very far from the goal where a complete cure of the diseases is expected. In order to bridge the gap between the reality and the ideal, the followings are suggested. First, combination therapy needs to be developed against multi-targets: inhibition of prion replication; degradation and scavengery of prion; inhibition of prion-related neurodegeneration. Secondly, preclinical diagnostic means, by which healthy prion-carriers can be revealed before the onset of the diseases, should be explored for earlier therapeutic interventions. The last is to disclose intrinsic disease susceptibility factors and environmental factors, both of which could solely or jointly facilitate in suppressing prion replication and disease progress. Exploitation of these items should be tough but will be deserved for overcoming the fatal diseases.

[Systematic review of the therapeutics for prion diseases].

Prion diseases are fatal infectious neurodegenerative disorders; examples include the Creutzfeldt-Jakob disease affecting humans and bovine spongiform encephalopathy in cattle. The causative agents of these diseases--the prions--are thought to consist of the pathogenic isoform of the prion protein PrP(Sc), which is produced by the conformational conversion of the normal isoform PrP(c). Many lines of evidence indicate that the constitutive conversion of PrP(c) to PrP(Sc), resulting in a marked accumulation of PrP(Sc) in the brain, is a central event in the pathogenesis of prion diseases. A large number of compounds have been identified as anti-prion agents and capable of reducing the PrP(Sc) levels in infected cells. Some of these compounds have been found to be partially effective in infected animals, thus resulting in the prolongation of the incubation or survival times and a few of these compounds were or are under clinical trials. However, none of these compounds have proven to be therapeutically effective against this group of diseases. This is probably because (1) these compounds fail to cross the blood-brain barrier and (2) their effectiveness is reduced because they are administered only to patients with clinically advanced disease owing to a lack of diagnostic indicators for presymptomatic individuals. In this communication, we systematically list these anti-prion compounds and summarize their effectiveness and possible mechanisms of action.

Therapeutic interventions ameliorating prion disease.

Of the many unresolved issues in relation to prion diseases, effective treatments remain an elusive exigency, although some progress has been made. This review describes disease-ameliorating therapeutic strategies reported to date in animal models of prion disease, as well as providing a brief overview of selected completed human treatment trials. Included in vivo studies have been broadly dichotomized according to the time of introduction of the treatment in relation to animal inoculation and also according to their possible principal mechanism of action, although the latter is not always entirely clear, and often there is likely to be more than one mechanism. Consequent to the pathogenic primacy of cellular prion protein (PrP(c))-to-scrapie PrP(c) (PrP(sc)) conversion, most reported treatments appear to directly target this replication process, although various other strategies, such as depletion of reaction substrates and abrogation of downstream effector pathways, have been utilized. Many factors, including experimental design, militate against reliable extrapolation of study results to the routine clinical setting or limit easy translational application to human disease. Notably problematic are approaches wherein benefit has been shown but the treatment was initiated before, at or soon after inoculation of experimental animals.

LRP/LR as an alternative promising target in therapy of prion diseases, Alzheimer's disease and cancer.

The 37 kDa/67 kDa laminin receptor (LRP/LR) represents a key player for cell adhesion, is associated with the metastatic potential of solid tumors and is required for maintenance of cell viability by preventing apoptosis. LRP/LR acts as a receptor for viruses such as Sindbis virus, Venezuelean Equine Encephalitis (VEE) virus, Adeno-associated-viruses (AAV) and Dengue Virus, the latter causing 50 to 100 million infections in humans per year. LRP/LR acts further as a receptor for prions and represents a multifunctional protein subcellularly located to the nucleus, the cytoplasm and the cell surface. The receptor represents an alternative target for therapy of viral infections, cancer and prion disorders and might play additional roles in further neurodegenerative diseases such as Alzheimer's disease. The species barrier in prion disorders might be at least in part determined by the presence of LRP/LR in enterocytes of the intestinal epithelium. Anti-LRP/LR antibodies, siRNAs directed against LRP mRNA, polysulfated glycanes such as pentosan polysulfate and heparan mimetics and LRP decoy mutants are promising tools for blocking or downregulating the receptor and may represent alternative therapeutics for the treatment of prion disorders, Alzheimer's Disease and metastatic cancer.

Targeting prion proteins in neurodegenerative disease.

BACKGROUND: Spongiform neurodegeneration is the pathological hallmark of individuals suffering from prion disease. These disorders, whose manifestation is sporadic, familial or acquired by infection, are caused by accumulation of the aberrantly folded isoform of the cellular prion protein (PrP(c)), termed PrP(Sc). Although usually rare, prion disorders are inevitably fatal and transferrable by infection. OBJECTIVE: Pathology is restricted to the central nervous system and premortem diagnosis is usually not possible. Yet, promising approaches towards developing therapeutic regimens have been made recently. METHODS: The biology of prion proteins and current models of neurotoxicity are discussed and prophylactic and therapeutic concepts are introduced. RESULTS/CONCLUSIONS: Although various promising drug candidates with antiprion activity have been identified, this proof-of-concept cannot be transferred into translational medicine yet.

[Recent advances in the diagnosis and therapeutics for human prion diseases].

Prion diseases, or transmissible spongiform encephalopathies, are fatal, neurodegenerative disorders associated with the accumulation of a misfolded infectious prion protein which is made by a posttranslational conformational change of the host-encoded cellular prion protein. A large number of studies to reveal the pathogenesis of prion diseases have been done using such experimental models as animals, cell cultures and cell-free systems over the past 30 years. The prion pathogenesis is still enigmatic, but current explosion of the knowledge about prion biology has led to the discovery of either more reliable diagnostic measurements or more beneficial therapeutic candidates. Here, the recent advances are reviewed in the diagnostics and the therapeutics for prion diseases.

Therapeutic approaches for prion and Alzheimer's diseases.

Alzheimer's and prion diseases belong to a category of conformational neurodegenerative disorders [Prusiner SB (2001) N Eng J Med344, 1516-1526; Sadowski M & Wisniewski T (2007) Curr Pharm Des 13, 1943-1954; Beekes M (2007) FEBS J 274, 575]. Treatments capable of arresting or at least effectively modifying the course of disease do not yet exist for either one of these diseases. Alzheimer's disease is the major cause of dementia in the elderly and has become an ever greater problem with the aging of Western societies. Unlike Alzheimer's disease, prion diseases are relatively rare. Each year only approximately 300 people in the USA and approximately 100 people in the UK succumb to various forms of prion diseases [Beekes M (2007) FEBS J 274, 575; Sigurdsson EM & Wisniewski T (2005) Exp Rev Vaccines 4, 607-610]. Nevertheless, these disorders have received great scientific and public interest due to the fact that they can be transmissible among humans and in certain conditions from animals to humans. The emergence of variant Creutzfeld-Jakob disease demonstrated the transmissibility of the bovine spongiform encephalopathy to humans [Beekes M (2007) FEBS J 274, 575]. Therefore, the spread of bovine spongiform encephalopathy across Europe and the recently identified cases in North America have put a large human population at risk of prion infection. It is estimated that at least several thousand Britons are asymptomatic carriers of prion infections and may develop variant Creutzfeld-Jakob disease in the future [Hilton DA (2006) J Pathol 208, 134-141]. This delayed emergence of human cases following the near elimination of bovine spongiform encephalopathy in the UK may occur because prion disease have a very prolonged incubation period, ranging from months to decades, which depends on the amount of inoculum, the route of infection and the genetic predisposition of the infected subject [Hilton DA (2006) J Pathol 208, 134-141]. Therefore, there is a great need for effective therapies for both Alzheimer's disease and prion diseases.

Anti-PrP antibodies block PrPSc replication in prion-infected cell cultures by accelerating PrPC degradation.

The use of anti-PrP antibodies represents one of the most promising strategies for the treatment of prion diseases. In the present study, we screened various anti-PrP antibodies with the aim of identifying those that would block PrP(Sc) replication in prion-infected cell culture. Two antibodies, SAF34 recognizing the flexible octarepeats region on HuPrP protein, and SAF61 directed against PrP amino acid residues (144-152), not only inhibited PrP(Sc) formation in prion-infected neuroblastoma cells but also decreased the PrP(C) levels in non-infected N2a cells. In addition, treatment with both SAF34 and SAF61 antibodies decreased PrP(C) and PrP(Sc) levels in the cells synergistically. In the presence of both antibodies, our results showed that the mode of action which leads to the disappearance of PrP(Sc) in cells is directly coupled to PrP(C) degradation by reducing the half-life of the PrP(C) protein.

Approaches to prophylaxis and therapy.

Despite important progress in experimental treatment of neurodegenerative diseases, no therapeutic strategy has today proven its capability to cure or even to stabilise human TSEs. Pathogenesis experiments performed in rodent TSE models have shown that central nervous system damages are detectable long before the appearance of the clinical symptoms. At the time of disease onset, PrP(Sc) accumulation has almost reached its highest level, and the neuropathological lesions (spongiosis, gliosis) are as intense as they are at the time of death. Therefore, the neurodegeneration that is present at the onset of the disease is beyond therapy, and, in theory, only a preclinical diagnosis of TSEs would permit the prevention (or delay) of neurodegeneration. Unfortunately, there are no diagnostic tests that can be used to show TSE agent infection during the preclinical phase of the disease. Nevertheless, since the appearance of variant Creutzfeldt-Jakob disease (vCJD), those in the scientific community working on experimental therapy have increased their efforts. Tens of drugs have been tested in several experimental models, and there are some high-output screening platforms being used in Europe and in the US. Any rational therapeutic strategy needs to be based on pathogenesis data and/or knowledge on the nature of the causative agent. Therefore, progress in therapy is tightly linked to a better understanding of the basic science of TSE.