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Journal of Virology, January 2006, p. 1044-1046, Vol. 80, No. 2
0022-538X/06/$08.00+0     doi:10.1128/JVI.80.2.1044-1046.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Mefloquine, an Antimalaria Drug with Antiprion Activity In Vitro, Lacks Activity In Vivo

David A. Kocisko1* and Byron Caughey1

National Institute of Allergy & Infectious Diseases, National Institutes of Health, Hamilton, Montana 598401

Received 25 August 2005/ Accepted 27 October 2005


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ABSTRACT
 
In view of the effectiveness of antimalaria drugs inhibiting abnormal protease-resistant prion protein (PrP-res) formation in scrapie agent-infected cells, we tested other antimalarial compounds for similar activity. Mefloquine (MF), a quinoline antimalaria drug, was the most active compound tested against RML and 22L mouse scrapie agent-infected cells, with 50% inhibitory concentrations of ~0.5 and ~1.2 µM, respectively. However, MF administered to mice did not delay the onset of intraperitoneally inoculated scrapie agent, the result previously observed with quinacrine. While most anti-scrapie agent compounds inhibit PrP-res formation in vitro, many PrP-res inhibitors have no activity in vivo. This underscores the importance of testing promising candidates in vivo.


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TEXT
 
The transmissible spongiform encephalopathies (TSEs) or prion diseases show a common and unique posttranslational conversion of normal, host-encoded, protease-sensitive prion protein (PrP-sen or PrPC) to an abnormal disease-associated isoform (PrP-res or PrPSc). The latter is an aggregation-prone and detergent-insoluble polymer resistant to proteolysis (5). Human TSEs include Gerstmann-Straussler-Scheinker disease, fatal familial insomnia, Creutzfeldt-Jakob disease (CJD), and kuru. The epidemic nature of prion diseases in domestic and wild animals could constitute serious health problems. Scrapie is a TSE of sheep which has been experimentally adapted to rodents, and bovine spongiform encephalopathy (BSE) is prominent in Europe and has also occurred in other continents, including North America. The appearance of a new form of CJD, presumably due to consumption of BSE-contaminated beef, created a troubling new scenario in the transmission of fatal prion diseases. As there is no deployable therapeutic TSE intervention immediately available, it is important to continue to pursue TSE drug development (reviewed in references 4, 12, and 17).

Compounds including polyene antibiotics, such as amphotericin B (18, 23); cyclic tetrapyrroles, such as porphyrins (7, 24); and polyanions, such as pentosan polysulfate (6, 9), inhibit PrP-res formation in infected cells and have also demonstrated antiscrapie activity in vivo. Many antimalarial compounds and related acridine and quinoline analogs have been shown to be effective inhibitors of PrP-res formation in infected mouse neuroblastoma (N2a) cells (11, 16, 19, 20). Thus, we were particularly interested in testing other antimalarial compounds, as many are FDA-approved drugs and some also cross the blood-brain barrier (BBB). Here we demonstrate mefloquine (MF) as an effective inhibitor of PrP-res in N2a cells infected with RML and 22L mouse strains of scrapie agent. We also tested MF, the most potent inhibitor found, against intraperitoneal (i.p.) scapie infection in mice as a further evaluation of its potential as an anti-TSE drug.

Antimalarial compounds were tested for the ability to inhibit PrP-res formation in infected cells as described previously (14). MF was supplied by Roche, and other compounds tested were included in the Spectrum Collection from Microsource Discovery (Groton, CT). As shown in Table 1 with new and published data, many antimalarial molecules can inhibit RML PrP-res accumulation in N2a cells. The ability is especially pronounced for quinoline, 4-aminoquinoline, 8-aminoquinoline, and acridine analogs. Many more quinoline and acridine compounds have been reported as inhibitors than are listed here (16, 19, 20). MF was the most effective new inhibitor, so it was also tested against 22L-infected N2a cells. MF also inhibited 22L PrP-res, with a 50% inhibitory concentration (IC50) of 1.2 µM. Interestingly, antimalarial compounds not of the above-mentioned classes demonstrated no activity at concentrations lower than those toxic to the cells. Doxycycline, which has been reported to render preexisting PrP-res sensitive to proteolysis at concentrations approaching 1 mM (13), had no PrP-res inhibitory activity at concentrations lower than that toxic to cells. These results emphasize that not all antimalarial compounds inhibit PrP-res accumulation and suggest additionally that the presence of a quinoline or acridine ring system is advantageous.


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  		TABLE 1. Inhibition of PrP-res in infected cells by various antimalarial compounds

Because MF is an FDA-approved antimalaria drug that potently inhibits PrP-res formation in cells and crosses the BBB, it was an excellent TSE therapeutic candidate. MF was tested for scrapie prophylaxis in transgenic mice (Tg7) (25) that are very susceptible to hamster 263K scrapie agent. Mice were first given a loading dose of MF consisting of three daily i.p. injections of 5 mg of MF per kg of body weight. Immediately after the third MF dose, the mice were inoculated i.p. with 50 µl of 1% 263K-infected brain homogenate (~1,000 50% infective doses). Based on pharmacokinetic studies of MF in mice (1), blood and brain levels should exceed 22L- or RML-PrP-res IC50 values. Inoculation was on a Friday, and 5-mg/kg i.p. MF dosing continued on Mondays, Wednesdays, and Fridays for the next 4 weeks. As shown in Table 2, MF was not able to delay the onset of scrapie in mice. A similar prophylaxis test with different cyclic tetrapyrroles has shown a significant delay in scrapie onset (24), but amodiaquine in this type of test was also ineffective (15). It remains possible that prophylactic effects of MF or amodiaquine could be seen in different in vivo models having greater lymphoreticular involvement than 263K scrapie agent; however, effects on established central nervous system infections will be required to treat most CJD patients. Since treating such advanced TSE disease is likely to be even more challenging than prophylaxis, MF and amodiaquine were not considered further as potential therapeutic agents.


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  		TABLE 2. Test of MF for scrapie prophylaxis in Tg7 mice inoculated on day 0 with 50 µl of 1% 263K brain homogenatea

Quinacrine, another FDA-approved antimalaria drug that inhibits mouse PrP-res formation in cells about as potently as MF (11) and crosses the BBB, also was an excellent TSE therapeutic candidate (16). However, no antiscrapie activity has been observed in mice tested for prophylaxis by quinacrine oral gavage (8) and i.p. injections (2) and no therapeutic effects have been observed against existing mouse brain infections by infusion pumping of quinacrine into the brain (10). Additionally, quinacrine has been dosed experimentally to a limited number of human TSE patients, with no benefit to some and limited transient benefit to others (3, 21, 22). Liver dysfunction was also a common side effect of the quinacrine treatment. Surprisingly, it is now being considered for expanded clinical trials in the United Kingdom and United States.

Screening compounds for PrP-res inhibitory activity in infected cell cultures has successfully found classes of compounds with in vivo antiscrapie activity, such as the cyclic tetrapyrroles and sulfonated dyes. Antimalarials have been tested as TSE therapeutic candidates because of such screening. Most compounds with in vivo antiscrapie activity also inhibit PrP-res formation in cells, regardless of how they were initially discovered. For instance, pentosan polysulfate demonstrated antiscrapie activity before it was found to inhibit PrP-res formation in cell culture (6, 9). Although in vitro tests are useful as initial compound screens, they cannot substitute for in vivo tests against actual TSE disease. Also, specific in vitro assays cannot be expected to test for all possible therapeutic mechanisms or provide information on optimum dosages for in vivo use. A compound that does not inhibit PrP-res in cells might have activity in vivo through a mechanism that does not involve the inhibition of PrP-res accumulation. In light of the fact that much is still unknown concerning the mechanisms of infection and disease processes of the TSEs, it would be prudent to demonstrate anti-TSE activity in vivo before a therapeutic candidate is advanced to clinical use.


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ACKNOWLEDGMENTS
 
This work was funded in part by the Intramural Research Program of the NIH, NIAID, and by U.S. DOD prion interagency transfer.


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FOOTNOTES
 
* Corresponding author. Mailing address: Rocky Mountain Laboratories, 903 S. 4th Street, Hamilton, MT 59840. Phone: (406) 375-9692. Fax: (406) 363-9286. E-mail: dkocisko{at}niaid.nih.gov. Back


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Journal of Virology, January 2006, p. 1044-1046, Vol. 80, No. 2
0022-538X/06/$08.00+0     doi:10.1128/JVI.80.2.1044-1046.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

  • Trevitt, C. R, Collinge, J. (2006). A systematic review of prion therapeutics in experimental models. Brain 129: 2241-2265 [Abstract] [Full Text]  

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