Polyanions (Table IV), including heteropolyanion 23 (HPA-23), Dextran Sulfate 500 (DS 500), pentosan polysulfate (SP54) and heparin are known to bind the prion protein and/or prevent PrPSc accumulation in animals and cell systems (Brimacombe et al., 1999; Caughey
and Raymond, 1993; Diringer and Ehlers, 1991; Ehlers and Diringer, 1984; Farquhar et al., 1999; Gabizon et al., 1993; Kimberlin and Walker, 1983; Kimberlin and Walker, 1986; Ladogana et al., 1992). The first polyanion denoted as an anti-scrapie drug was HPA-23 (Kimberlin and Walker, 1983; Kimberlin and Walker, 1986). The effect of HPA-23 was tested in several different scrapie strains, such as 139A, ME7, 22A and 263K. HPA-23 was effective in all these strains and prolonged the lifetimes of the animals significantly after scrapie injection. Less effect was observed when scrapie material was injected intraperitoneally or if the drug was given more than 48 hours after scrapie infection. Injection before to infection with scrapie is not effictive, owing to the rapid metabolization or excretion of HPA-23. HPA- 23 is thought to interfere with early replication of PrPSc in the lymphoreticular system,
reducing the efficiency of scrapie infection. These results, together with the brain toxicity of this molecule suggest, that HPA-23 has limited therapeutic value.
Two high-molecular-weight polyanions, carrageenan and DS 500, were shown to be highly efficient in reducing scrapie titers in mice infected with the 139A strain of scrapie (Ehlers and Diringer, 1984; Kimberlin and Walker, 1986). All intravenous or intraperitoneal combinations of injecting DS 500 or scrapie reduced the effective titer about 100- to 200 fold. The effect of DS 500 is long-lasting. Application of DS 500 up to 10 weeks before to infection increases
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the incubation period in mice. However, DS 500 itself is highly toxic and causes up to 50% mortality at a dose of 2 mg per mouse. Like HPA-23, DS 500 is thought to prevent PrPSc
replication in spleen and lymph nodes and its mode of action is likely to be independent of its activity as a B-cell mitogen. The high-molecular-weight and negative charge may represent important factors in the anti-scrapie effect of DS 500. SP54 (Pentosan Polysulfate, Fig. 7A) has an anti-scrapie effect comparable to DS 500, but is less toxic. It has been shown that SP54 significally increases scrapie incubation period in hamsters infected with 263K scrapie strain and in mice infected with the 139A, Me7 and 22A strains of scrapie (Ehlers and Diringer, 1984; Farquhar et al., 1999; Ladogana et al., 1992). SP54 is even effective if only a single low-dose is injected after infection. A single injection of 250 µg of SP54 increased the mean incubation period of the ME7 strain by up to 66% and 1 mg of SP54 protected mice completely from the 22A scrapie strain. SP54 is thought be effective during the very early events of pathogenesis by interfering with the uptake of PrPSc by nerve endings and/or carrier
cells. The low-dose effect and the lower in vivo toxicity compared to other polyanions make SP54 a promising candidate in the field of anti-scrapie polyanions.
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Table IV Antiscrapie drugs likely to interact directly with PrP
Drug Tested scrapie
strain
Successfull y treated animals
Suggested mode of action Comments References
HPA-23 139A, ME7, 22A and 263K
Mouse and hamster
Prevents early agent replication in the LRS, competes with GAG (glycosaminoglycan) binding site
Effective in a lot of scrapie strains, rapid metabolism and excretion, toxic Kimberlin and Walker (1983; Kimberlin and Walker (1986 DS 500 139A Mouse Prevents agent replication in the
LRS due to its high molecular weight and negative charge, competes with GAG
(glycosaminoglycan) binding site
Long-lasting anti- scrapie effect but toxic at therapeutic doses
Ehlers and Diringer (1984); Kimberlin and Walker (1986) Pentosan Polysulfate 139A, ME7, 22A and 263K Mouse and hamster
Interferes with PrPSc uptake from nerve endings, competes with GAG (glycosaminoglycan) binding site
Very promising drug, effective at extreme low dose
Ehlers and Diringer (1984); Farquhar et al. (1999); Ladogana et al. (1992) Amphotericin B C506M3 and 263K Mouse and hamster
Direct prevention of PrP conversion or interference with PrPSc uptake
Acute nephrotoxicity and low solubility, widely used for the treatment of fungals Pocchiari et al. (1987); Xi et al. (1992) MS-8209 C506M3 and 263K Mouse and hamster
Same as for AmB Lower toxicity than AmB
Adjou et al. (1995); Demaimay et al. (1997)
Adjou et al. (1999) Congo Red 263K and
139A
Hamster Binding to PrPc with polyanion-like behavior, or binding to PrPSc (overstabilisation)
Dyes amyloid Caspi et al. (1998); Caughey et al. (1993); Ingrosso et al. (1995) Anthrycycline 263K Hamster Binding to PrPSc, preventing
amyloid deposition
Used for the treatment of malignancies Tagliavini et al. (1997) Porphyrins and Phtalocyans 263K Mouse expressing hamster PrP
Binding to PrPSc Inhibits cell free PrPc/Sc conversion
Caughey et al. (1998); Priola et al. (2000)
Cp-60/Cp-62 ScN2a cells None Mimicking dominant negative inhibition of prion replication
Identified by using a computational database search Perrier et al. (2000) IPrP13 (b-sheet breaker)
139A Mouse Direct change of PrP secondary structure
Synthetic peptide Soto et al. (2000) Clusterin [apolipoprotein J (apo J)] --- None, prevents aggregation of PrP106- 126
Binding to PrPc/Sc Binds to extraneuronal PrPBSE
McHattie and Edington (1999)
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All anti-scrapie polyanions published so far might act by competing directly with the binding of cellular glycoaminoglycans (GAGs) to PrPc (see chapter II. A) and/or PrPSc (Brimacombe
et al., 1999; Caughey et al., 1994). Indeed, GAGs are involved in the metabolism of PrPc (see
chapter II.A) and thus in the biogenesis of PrPSc. It was shown by surface plasmon resonance,
that pentosan polysulfate shows the strongest binding to recombinant PrP followed by heparin and dermatan sulfate. This correlates to the ability of the molecules to delay scrapie disease and reduce PrPSc accumulation in scrapie-infected cell lines (Caughey and Raymond, 1993).
2. Congo Red
Congo red (Fig. 7C, Table IV) is a dye that can be used as a diagnosic stain for amyloids. It is well known that Congo red can inhibit PrPres accumulation in Sc+-MNB cells and PrPSc
replication in 263K and 139H treated hamsters (Caspi et al., 1998; Caughey et al., 1994; Caughey et al., 1993; Ingrosso et al., 1995). The mechanism of the Congo red anti-scrapie effect probably involves direct binding to PrPc, which again is thought to block the binding of
cellular GAGs to PrPc, as described for polyanions (Caughey et al., 1994). The proposed
direct binding of Congo red to PrPSc is thought to stabilize PrP Sc, the abnormal isoform of the
prion protein, and prevents its partial denaturation, which could be necessary for agent replication (Caspi et al., 1998).