Enhanced antiscrapie effect using combination drug treatment

Combination of Styrylbenzoazole Compound and Hydroxypropyl Methylcellulose Enhances Therapeutic Effect in Prion-Infected Mice

Prion diseases are fatal transmissible neurodegenerative disorders. Tremendous efforts have been made for prion diseases; however, no effective treatment is available. Several anti-prion compounds have a preference for which prion strains or prion-infected animal models to target. Styrylbenzoazole compound called cpd-B is effective in RML prion-infected mice but less so in 263K prion-infected mice, whereas hydroxypropyl methylcellulose is effective in 263K prion-infected mice but less so in RML prion-infected mice. In the present study, we developed a combination therapy of cpd-B and hydroxypropyl methylcellulose expecting synergistic effects in both RML prion-infected mice and 263K prion-infected mice. A single subcutaneous administration of this combination had substantially a synergistic effect in RML prion-infected mice but had no additive effect in 263K prion-infected mice. These results showed that the effect of cpd-B was enhanced by hydroxypropyl methylcellulose. The complementary nature of the two compounds in efficacy against prion strains, chemical properties, pharmacokinetics, and physical properties appears to have contributed to the effective combination therapy. Our results pave the way for the strategy of new anti-prion agents.

Human cerebral organoids as a therapeutic drug screening model for Creutzfeldt-Jakob disease

Creutzfeldt-Jakob Disease (CJD) is a fatal, currently incurable, neurodegenerative disease. The search for candidate treatments would be greatly facilitated by the availability of human cell-based models of prion disease. Recently, an induced pluripotent stem cell derived human cerebral organoid model was shown to take up and propagate human CJD prions. This model offers new opportunities to screen drug candidates for the treatment of human prion diseases in an entirely human genetic background. Here we provide the first evidence that human cerebral organoids can be a viable model for CJD drug screening by using an established anti-prion compound, pentosan polysulfate (PPS). PPS delayed prion propagation in a prophylactic-like treatment paradigm and also alleviated propagation when applied following establishment of infection in a therapeutic-like treatment paradigm. This study demonstrates the utility of cerebral organoids as the first human 3D cell culture system for screening therapeutic drug candidates for human prion diseases.

Strain variation in treatment and prevention of human prion diseases

Transmissible spongiform encephalopathies or prion diseases describe a number of different human disorders that differ in their clinical phenotypes, which are nonetheless united by their transmissible nature and common pathology. Clinical variation in the absence of a conventional infectious agent is believed to be encoded by different conformations of the misfolded prion protein. This misfolded protein is the target of methods designed to prevent disease transmission in a surgical setting and reduction of the misfolded seed or preventing its continued propagation have been the focus of therapeutic strategies. It is therefore possible that strain variation may influence the efficacy of prevention and treatment approaches. Historically, an understanding of prion disease transmission and pathogenesis has been focused on research tools developed using agriculturally relevant strains of prion disease. However, an increased understanding of the molecular biology of human prion disorders has highlighted differences not only between different forms of the disease affecting humans and animals but also within diseases such as Creutzfeldt-Jakob Disease (CJD), which is represented by several sporadic CJD specific conformations and an additional conformation associated with variant CJD. In this chapter we will discuss whether prion strain variation can affect the efficacy of methods used to decontaminate prions and whether strain variation in pre-clinical models of prion disease can be used to identify therapeutic strategies that have the best possible chance of success in the clinic.

Cell-free prion protein conversion assays in screening for anti-prion drug candidates

The search for medications to treat prion diseases has lasted more than 30 years but no clinically validated treatments for prion diseases of humans or livestock have been realized. A primary strategy has been to identify molecules that can inhibit the formation of pathological forms of prion protein, for example, protease-resistant forms called PrPres. Such inhibitors can prolong the lives of experimental animals inoculated peripherally with prions, but the practical therapeutic efficacy of known inhibitors against ongoing brain infections has so far been limited by toxicity, insufficient bioavailability to the CNS, and/or strain specificities. Thus, the search continues for clinically applicable inhibitors of PrPres accumulation. Here we highlight key cell-free assays that are useful for the initial screening and mechanistic characterization of such compounds and are relatively high throughput, rapid, and cost-effective. These include cell-free conversions, protein misfolding cyclic amplification (PMCA), real time quaking-induced conversion (RT-QuIC), and fluorescence correlation-based competitive binding assays.

Clinical trials

Arguably the most important goal of prion research is the discovery of a safe and effective treatment for the human diseases. The final stages of the pathway to develop a treatment require clinical trials. Choices about how a trial is designed and conducted have a large impact on the chances of success. The gold-standard large randomized double-blind placebo-controlled study, which minimizes sources of bias and has been incredibly successful in other diseases, has been hard to achieve in Creutzfeldt-Jakob disease principally because of the rarity and rapidity of the clinical syndrome. To date, clinical trials have been restricted to repurposed compounds, doxycycline, quinacrine, pentosan polysulfate (PPS), and flupertine. In most cases, these trials have used survival as an endpoint, which, whilst clearcut, has limitations. Biomarkers have played a strong role in diagnosis and entry criteria, but only a limited role as secondary outcome measures. Recent developments suggest some possible improvements in trial design by use of new outcome measures that have more favorable properties, and biomarkers of neuronal damage and/or prion seeding activity. Alternative patient populations, including those at risk of genetic forms of prion disease, warrant more consideration. In the future, improved trial designs will be employed to test compounds designed specifically to treat prion diseases.

Insights from Therapeutic Studies for PrP Prion Disease

Although an effective therapy for prion disease has not yet been established, many advances have been made toward understanding its pathogenesis, which has facilitated research into therapeutics for the disease. Several compounds, including flupirtine, quinacrine, pentosan polysulfate, and doxycycline, have recently been used on a trial basis for patients with prion disease. Concomitantly, several lead antiprion compounds, including compound B (compB), IND series, and anle138b, have been discovered. However, clinical trials are still far from yielding significantly beneficial results, and the findings of lead compound studies in animals have highlighted new challenges. These efforts have highlighted areas that need improvement or further exploration to achieve more effective therapies. In this work, we review recent advances in prion-related therapeutic research and discuss basic scientific issues to be resolved for meaningful medical intervention of prion disease.

Antiprion compounds that reduce PrP(Sc) levels in dividing and stationary-phase cells

During prion diseases, a normally benign, host protein, denoted PrP(C), undergoes alternative folding into the aberrant isoform, PrP(Sc). We used ELISA to identify and confirm hits in order to develop leads that reduce PrP(Sc) in prion-infected dividing and stationary-phase mouse neuroblastoma (ScN2a-cl3) cells. We tested 52,830 diverse small molecules in dividing cells and 49,430 in stationary-phase cells. This led to 3100 HTS and 970 single point confirmed (SPC) hits in dividing cells, 331 HTS and 55 confirmed SPC hits in stationary-phase cells as well as 36 confirmed SPC hits active in both. Fourteen chemical leads were identified from confirmed SPC hits in dividing cells and three in stationary-phase cells. From more than 682 compounds tested in concentration-effect relationships in dividing cells to determine potency (EC50), 102 had EC50 values between 1 and 10 μM and 50 had EC50 values of <1 μM; none affected cell viability. We observed an excellent correlation between EC50 values determined by ELISA and Western immunoblotting for 28 representative compounds in dividing cells (R(2)=0.75; p <0.0001). Of the 55 confirmed SPC hits in stationary-phase cells, 23 were piperazine, indole, or urea leads. The EC50 values of one indole in stationary-phase and dividing ScN2a-cl3 cells were 7.5 and 1.6 μM, respectively. Unexpectedly, the number of hits in stationary-phase cells was ~10% of that in dividing cells. The explanation for this difference remains to be determined.

Pharmacokinetics and metabolism of 2-aminothiazoles with antiprion activity in mice

PURPOSE

To discover drugs lowering PrP(Sc) in prion-infected cultured neuronal cells that achieve high concentrations in brain to test in mouse models of prion disease and then treat people with these fatal diseases.

METHODS

We tested 2-AMT analogs for EC50 and PK after a 40 mg/kg single dose and 40-210 mg/kg/day doses for 3 days. We calculated plasma and brain AUC, ratio of AUC/EC50 after dosing. We reasoned that compounds with high AUC/EC50 ratios should be good candidates going forward.

RESULTS

We evaluated 27 2-AMTs in single-dose and 10 in 3-day PK studies, of which IND24 and IND81 were selected for testing in mouse models of prion disease. They had high concentrations in brain after oral dosing. Absolute bioavailability ranged from 27-40%. AUC/EC50 ratios after 3 days were >100 (total) and 48-113 (unbound). Stability in liver microsomes ranged from 30->60 min. Ring hydroxylated metabolites were observed in microsomes. Neither was a substrate for the MDR1 transporter.

CONCLUSIONS

IND24 and IND81 are active in vitro and show high AUC/EC50 ratios (total and unbound) in plasma and brain. These will be evaluated in mouse models of prion disease.

Emergence and natural selection of drug-resistant prions

Drug resistance is a refractory barrier in the battle against many fatal diseases caused by rapidly evolving agents, including HIV, apicomplexans and specific cancers. Emerging evidence suggests that drug resistance might extend to lethal prion disorders and related neurodegenerative amyloidoses. Prions are self-replicating protein conformers, usually 'cross-beta' amyloid polymers, which are naturally transmitted between individuals and promote phenotypic change. Prion conformers are catalytic templates that specifically convert other copies of the same protein to the prion form. Once in motion, this chain reaction of conformational replication can deplete all non-prion copies of a protein. Typically, prions exist as ensembles of multiple structurally distinct, self-replicating forms or 'strains'. Each strain confers a distinct phenotype and replicates at different rates depending on the environment. As replicators, prions are units of selection. Thus, natural selection inescapably enriches or depletes various prion strains from populations depending on their conformational fitness (ability to self-replicate) in the prevailing environment. The most successful prions confer advantages to their host as with numerous yeast prions. Here, I review recent evidence that drug-like small molecules can antagonize some prion strains but simultaneously select for drug-resistant prions composed of mammalian PrP or the yeast prion protein, Sup35. For Sup35, the drug-resistant strain configures original intermolecular amyloid contacts that are not ordinarily detected. Importantly, a synergistic small-molecule cocktail counters prion diversity by eliminating multiple Sup35 prion strains. Collectively, these advances illuminate the plasticity of prionogenesis and suggest that synergistic combinatorial therapies might circumvent this pathological vicissitude.

A gamma-secretase inhibitor and quinacrine reduce prions and prevent dendritic degeneration in murine brains

In prion-infected mice, both the Notch-1 intracellular domain transcription factor (NICD) and the disease-causing prion protein (PrP(Sc)) increase in the brain preceding dendritic atrophy and loss. Because the drug LY411575 inhibits the gamma-secretase-catalyzed cleavage of Notch-1 that produces NICD, we asked whether this gamma-secretase inhibitor (GSI) might prevent dendritic degeneration in mice with scrapie. At 50 d postinoculation with Rocky Mountain Laboratory (RML) prions, mice were given GSI orally for 43-60 d. Because we did not expect GSI to produce a reduction of PrP(Sc) levels in brain, we added quinacrine (Qa) to the treatment regimen. Qa inhibits PrP(Sc) formation in cultured cells. The combination of GSI and Qa reduced PrP(Sc) by approximately 95% in the neocortex and hippocampus but only approximately 50% in the thalamus at the site of prion inoculation. The GSI plus Qa combination prevented dendritic atrophy and loss, but GSI alone did not. Even though GSI reduced NICD levels to a greater extent than GSI plus Qa, it was unable to prevent dendritic degeneration. Whether a balance between NICD and dendrite growth-stimulating factors was achieved with GSI plus Qa but not GSI alone remains to be determined. Although the combination of GSI and Qa diminished PrP(Sc) in the brains of RML-infected mice, GSI toxicity prevented us from being able to assess the effect the GSI plus Qa combination on incubation times. Whether less toxic GSIs can be used in place of LY411575 to prolong survival remains to be determined.

NIAID resources for developing new therapies for severe viral infections

Severe viral infections, including hemorrhagic fever and encephalitis, occur throughout the world, but are most prevalent in developing areas that are most vulnerable to infectious diseases. Some of these can also infect related species as illustrated by the threatened extinction of gorillas by Ebola infection in west and central Africa. There are no safe and effective treatments available for these serious infections. In addition to the logistical difficulties inherent in developing a drug for infections that are sporadic and occur mainly in the third world, there is the overwhelming barrier of no hope for return on investment to encourage the pharmaceutical industry to address these unmet medical needs. Therefore, the National Institute of Allergy and infectious Disease (NIAID) has developed and supported a variety of programs and resources to provide assistance and lower the barrier for those who undertake these difficult challenges. The primary programs relevant to the development of therapies for severe viral infections are described and three case studies illustrate how they have been used. In addition, contact information for accessing these resources is supplied.

Cyclic tetrapyrrole sulfonation, metals, and oligomerization in antiprion activity

Cyclic tetrapyrroles are among the most potent compounds with activity against transmissible spongiform encephalopathies (TSEs; or prion diseases). Here the effects of differential sulfonation and metal binding to cyclic tetrapyrroles were investigated. Their potencies in inhibiting disease-associated protease-resistant prion protein were compared in several types of TSE-infected cell cultures. In addition, prophylactic antiscrapie activities were determined in scrapie-infected mice. The activity of phthalocyanine was relatively insensitive to the number of peripheral sulfonate groups but varied with the type of metal bound at the center of the molecule. The tendency of the various phthalocyanine sulfonates to oligomerize (i.e., stack) correlated with anti-TSE activity. Notably, aluminum(III) phthalocyanine tetrasulfonate was both the poorest anti-TSE compound and the least prone to oligomerization in aqueous media. Similar comparisons of iron- and manganese-bound porphyrin sulfonates confirmed that stacking ability correlates with anti-TSE activity among cyclic tetrapyrroles.

Experimental treatments for human transmissible spongiform encephalopathies: is there a role for pentosan polysulfate?

Human transmissible spongiform encephalopathies (TSEs), also known as prion diseases, are caused by the accumulation of an abnormal isoform of the prion protein in the CNS. Creutzfeldt-Jakob disease in its sporadic form is the most frequent type of human TSE. At present, there is no proven specific or effective treatment available for any form of TSE. Pentosan polysulfate (PPS) has been shown to prolong the incubation period when administered to the cerebral ventricles in a rodent TSE model. Cerebroventricular administration of PPS has been carried out in 26 patients with TSEs and has been shown to be well tolerated in doses < or = 220 microg/kg/day. Proof of efficacy has been difficult because the specific and objective criteria for measurement of response have not been established yet. Preliminary clinical experience confirms extended survival in patients with variant Creutzfeldt-Jakob disease receiving intraventricular PPS; however, it is still not clear if this is due to PPS itself. Further prospective investigations of long-term intraventricular PPS administration are essential for the assessment of its effects.