Current Topics in Medicinal Chemistry - Volume 13, Issue 19, 2013

Prion diseases are a group of fatal neurodegenerative diseases caused by the misfolding of cellular prion protein (PrPC) into pathogenic conformers (PrPSc). Although no effective therapies for prion diseases are currently available, a number of small molecule inhibitors have been identified that are capable of reducing or eliminating PrPSc in prion infected cells. However, recent experiments have shown that upon sustained treatment, prions have the capacity to evolve into drug resistant conformations. These studies suggest that the mechanism of prion strain adaptation involves rare conformational conversions followed by competitive selection among the heterogeneous pool of PrPSc conformers. The plasticity of prion conformers makes PrPSc a particularly challenging drug target and suggests that combination drug therapies or targeting of PrPC may be required for effective therapy. In this review, we highlight recent literature that demonstrate the phenomenon of prion drug resistance and strain specificity, and discuss potential ramifications for therapeutic efforts against prion diseases.

Prion diseases or transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders associated with the conformational conversion of the cellular prion protein, PrPC, into a pathological form known as prion or PrPSc. They can be classified into sporadic, inherited and infectious forms. Spontaneous generation of PrPSc in inherited forms of prion diseases is caused by mutations in the human prion protein gene (PRNP). A major goal in prion biology is unraveling the molecular mechanism by which PrPC misfolds and leads to development of diseases. Structural characterization of various human PrP (HuPrP) variants may be helpful for better understanding of the earliest stages of the conformational changes leading to spontaneous generation of prions. Here, we review the results of the recent high-resolution nuclear magnetic resonance (NMR) structural studies on HuPrPs with pathological Q212P and V210I mutations linked with Gerstmann-Straussler-Scheinker (GSS) syndrome and familial Creutzfeldt-Jakob disease (fCJD), respectively, and HuPrP carrying naturally occurring E219K polymorphism considered to protect against sporadic CJD (sCJD). We describe subtle local differences between the three-dimensional (3D) structures of HuPrP mutants and the wild-type (WT) protein, providing new insights into the possible key structural determinants underlying conversion of PrPC into PrPSc. Also highlighted are the most recent findings from NMR studies about the effect of pH on the structural features of HuPrP with V210I mutation.

Prion diseases are rare neurodegenerative diseases characterized by the conversion of the prion protein from its native state (PrPC) towards the so-called 'scrapie form', rich in β-strands. Computational approaches, here briefly reviewed, are instrumental to understand the intrinsic instability of PrPC fold and how the latter is affected by mutations, binding of metals as well as by different environmental conditions, such as pH and temperature. These studies also provide a structural basis for the binding of anti-prion compounds, which may block the conversion to the scrapie form and, consequently, may inhibit fibril formation.

A strategy of logical drug design (LDD) and its application to prion diseases are reviewed. LDD is primarily based on the localizability of a hot spot which initiates structural instability in the target protein. It is also based on the regulability of the hot spot by small compounds, their designabilty by a computer, their organic synthesizability and the specificity of their functions once administered to the biological organisms. Unification of localizability, regulability, producibility and specificity is the central theme of LDD. Theoretical foundation of LDD based on quantum theories is initially outlined. The localizability using nuclear magnetic resonance (NMR), the regulability by a medical chaperone, the synthesizability, and the functional specificity accomplished thus far, are then described.

Prion diseases, also known as transmissible spongiform encephalopathies (TSEs) are infectious and fatal neurodegenerative diseases. So far, there is no therapy available with clinical efficacy. A detailed survey on the discovery of major classes of small molecule antiprion compounds is documented in this review in the hope that it may shine some light on the future direction of drug discovery against prion and other neurodegenerative diseases.

In the last two decades, knowledge of the neurobiology of prion diseases or transmissible spongiform encephalopathies (TSE) has significantly advanced, but a successful therapy to stop or delay the progression of these disorders remains one of the most challenging goals of biomedical research. Several obstacles to this achievement are in common with other neurodegenerative disorders: difficulties to move from experimental level to clinical stage; appropriate timing of intervention; correct set up of clinical trial. Also in terms of molecular bases of disease, TSE and the other neurodegenerative disorders associated with protein misfolding such as Alzheimer, Parkinson and Huntington diseases, share a central pathogenic role of soluble small aggregates, named oligomers, considered the culprit of neuronal dysfunction: accordingly, these disorders could by termed oligomeropathies. However, the rapid progression of TSE, together with their clinical and molecular heterogeneity, make the therapeutic approach particularly problematic. The main target of the antiprion strategy has been the pathological form of the cellular prion protein (PrPC) termed PrPSc, invariably associated with the diseases. Several compounds have been found to affect PrPSc formation or enhance its clearance in in vitro models, and prolong survival in experimental animals. However, few of them such as quinacrine and pentosan polysulfate have reached the clinical evaluation; more recently, we have conducted a clinical trial with doxycycline in patients with Creutzfeldt-Jakob disease without satisfactory results. In experimental conditions, active and passive immunization with antibodies against PrP and mucosal vaccination have shown to protect from peripheral infection. Other studies have proposed new potentially effective molecules targeting PrP oligomers. Furthermore, the possibility to interfere with PrPC to PrPSc conversion by an active control of PrPC is another interesting approach emerging from experimental studies. However, in common with the other oligomeropathies, early diagnosis allowing to treat at risk population in a preclinical stage represent the more realistic perspective for efficient TSE therapy.

Transmissible spongiform encephalopathies (TSEs), also called prion diseases, are fatal, infectious, genetic or sporadic neurodegenerative disorders of humans and animals. In humans, TSEs are represented by Creutzfeldt-Jakob disease (CJD), Gerstmann-Straussler-Scheinker syndrome, Fatal Familial Insomnia and Kuru. In animals, the most prominent prion diseases are scrapie of sheep and goats, bovine spongiform encephalopathy (BSE) of cattle and chronic wasting disease (CWD) of deer and elk. A critical event in prion diseases is the accumulation in the central nervous system (CNS) of the abnormally folded PrPSc protein that is the protease-resistant isoform of a normal cellular protein encoded by the host and called PrPC. PrPSc (also known as rPrPSc or PrP27-30) represents the main marker of prion diseases and is routinely used in the reference method for the diagnosis of prion diseases. Most of the therapeutic strategies developed so far aimed at identifying compounds that diminish the levels of PrPSc, with variable success when tested in vivo. In this review, we present an alternative approach in which small molecules that induce PrPSc oligomers are identified. By using virtual and cellular screenings, we found several thienyl pyrimidine compounds that trigger PrPSc oligomerization and trap prion infectivity.

An emerging common feature of the age-associated neurodegenerative disorders like Alzheimer's disease (AD) and Creutzfeldt-Jakob disease (CJD) is the ability of many disease-associated protein aggregates to induce conversion of a normal counterpart conformer leading to an acceleration of disease progression. Curative pharmacotherapy has not been achieved so far despite successes in elucidating pathomechanisms. Here, we review the pharmaceutical strategy of generating hybrid compounds, i.e. compounds consisting of several independently acting moieties with synergistic effects, on key molecular players in AD and CJD. For prion diseases, we review hybrid compounds consisting of two different heterocyclic compounds, their synergistic effects on prion replication in a cell culture model and their ability to prolong survival of experimentally prion-infected mice in vivo. While a combination therapy of several antiprion compounds including quinacrine, clomipramine, simvastatin and tocopherol prolonged survival time to 10-25%, administration of hybrid compound quinpramine alone, a chimera of acridine and iminodibenzyl scaffolds, led to 10% survival time extension. For AD, we review a hybrid compound consisting of an Aβ recognizing D-peptide fused to a small molecule β-sheet breaker, an aminopyrazole. This molecule was able to diminish Aβ oligomers in cell culture and significantly decrease synaptotoxicity as measured by miniature excitatory postsynaptic responses in vitro. Hybrid compounds can dramatically increase potency of their single moieties and lead to novel functions when they act in a simultaneous or sequential manner thereby revealing synergistic properties. Their systematic generation combining different classes of compounds from peptides to small molecules has the potential to significantly accelerate drug discovery.

Prion diseases are fatal neurodegenerative disorders that affect humans and animals and for which no pharmacological treatment is available. Compounds consisting of two identical moieties joined via an appropriate spacer (i.e. bivalent compounds) have turned out to be effective tools to prevent prion fibril formation and exhibit an improved biological profile with regard to the corresponding monovalent derivatives. In this review we discuss the importance of the bivalent strategy as a viable approach to design new chemical entities to combat prion diseases.

Prion diseases belong to a group of fatal infectious diseases with no effective therapies available. Throughout the last 35 years, less than 50 different drugs have been tested in different experimental animal models without hopeful results. An important limitation when searching for new drugs is the existence of appropriate models of the disease. The three different possible origins of prion diseases require the existence of different animal models for testing anti-prion compounds. Wild type, over-expressing transgenic mice and other more sophisticated animal models have been used to evaluate a diversity of compounds which some of them were previously tested in different in vitro experimental models. The complexity of prion diseases will require more pre-screening studies, reliable sporadic (or spontaneous) animal models and accurate chemical modifications of the selected compounds before having an effective therapy against human prion diseases. This review is intended to put on display the more relevant animal models that have been used in the search of new antiprion therapies and describe some possible procedures when handling chemical compounds presumed to have anti-prion activity prior to testing them in animal models.

Prion diseases, or transmissible spongiform encephalopathies, are characterized by abnormal prion protein accumulation in the brain. Abnormal prion proteins, having properties of amyloids when extracted from the brain, are observed as amyloid plaque deposits in the brain in some prion diseases such as variant Creutzfeldt–Jakob disease and Gerstmann–Straussler–Scheinker syndrome. This article reviews amyloid-binding compounds from the perspective of their usefulness for diagnosis and therapy of prion diseases. Styrylbenzoazole derivatives and phenylhydrazine derivatives are recently developed amyloid binding compounds that present benefits for prion-disease-related medicinal applications. For instance, styrylbenzoazole derivative BF-227, currently used as an amyloid imaging probe of positron emission tomography in Alzheimer disease, is useful also for the diagnosis of Gerstmann–Straussler–Scheinker syndrome. A phenylhydrazine derivative, compB, has remarkable prophylactic effects on intracerebrally infected animals with certain prion strains, even when administered orally. These amyloid-binding compounds, however, are not applicable to prion strains or prion diseases of all types. For example, amyloid-binding compounds are ineffective for inhibiting prion strains such as 263K. They are not feasible for detecting abnormal prion protein accumulation in the brain for prion diseases having no amyloid plaques. To elucidate the limitations of amyloid-binding compounds, further investigation is necessary to clarify the binding mode of the compounds to abnormal prion protein structures at an atomic level.