Continuous Quinacrine Treatment Results in the Formation of Drug-Resistant Prions

Abstract

Quinacrine is a potent antiprion compound in cell culture models of prion disease but has failed to show efficacy in animal bioassays and human clinical trials. Previous studies demonstrated that quinacrine inefficiently penetrates the blood-brain barrier (BBB), which could contribute to its lack of efficacy in vivo. As quinacrine is known to be a substrate for P-glycoprotein multi-drug resistance (MDR) transporters, we circumvented its poor BBB permeability by utilizing MDR0/0 mice that are deficient in mdr1a and mdr1b genes. Mice treated with 40 mg/kg/day of quinacrine accumulated up to 100 µM of quinacrine in their brains without acute toxicity. PrPSc levels in the brains of prion-inoculated MDR0/0 mice diminished upon the initiation of quinacrine treatment. However, this reduction was transient and PrPSc levels recovered despite the continuous administration of quinacrine. Treatment with quinacrine did not prolong the survival times of prion-inoculated, wild-type or MDR0/0 mice compared to untreated mice. A similar phenomenon was observed in cultured differentiated prion-infected neuroblastoma cells: PrPSc levels initially decreased after quinacrine treatment then rapidly recovered after 3 d of continuous treatment. Biochemical characterization of PrPSc that persisted in the brains of quinacrine-treated mice had a lower conformational stability and different immunoaffinities compared to that found in the brains of untreated controls. These physical properties were not maintained upon passage in MDR0/0 mice. From these data, we propose that quinacrine eliminates a specific subset of PrPSc conformers, resulting in the survival of drug-resistant prion conformations. Transient accumulation of this drug-resistant prion population provides a possible explanation for the lack of in vivo efficacy of quinacrine and other antiprion drugs.

Quinacrine promotes replication and conformational mutation of chronic wasting disease prions

“…The contradictory effects of quinacrine on mouse and CWD prions seem unrelated to effects specific to RK13 cells because we show that quinacrine reduces mouse PrP Sc in RK13 cells expressing mouse PrP C infected with RML prions, while improving CWD replication in RK13 cells expressing either elk or deer PrP C . The failure of long-term quinacrine treatment to affect disease outcome in CWD-infected Tg(ElkPrP)5037 +/− mice is consistent with previous results showing a similar lack of effect of quinacrine on mouse prions in vivo (17,27,28).…”

“…Tragically, in no case did treatment provide discernible benefits (10)(11)(12)(13)(14)(15). Possibly related to this failure, studies in mice lacking the multiple drug resistant gene inoculated with RML prions and orally dosed with quinacrine resulted in selection and propagation of quinacrineresistant prions (28). Similarly, IND24, although effective at prolonging the lifespan of RML-infected mice, also produced drug-resistant prions but had no effect in CJD-infected Tg mice (37).…”

Drug resistance confounding prion therapeutics

“…However, when mice were inoculated with Rocky Mountain Laboratory (RML) prions, a mouse-adapted scrapie strain, and treated with quinacrine, there was no extension in survival and drug-resistant prions emerged (12,13). When Weissmann and co-workers added swainsonine (swa) to RML-infected cultured cells, they were able to isolate swa-resistant and swa-dependent RML strains (14).…”

Insights from Therapeutic Studies for PrP Prion Disease

“…also showed the ineffectiveness of mefloquine, an antimalarial drug approved by the U.S. Food and Drug Administration that crosses the blood-brain barrier . It is interesting to note that quinacrine eliminates and/or modifies a specific subset of PrP Sc conformers, resulting in the survival of drug-resistant PrP Sc conformers (Ghaemmaghami et al 2009) or selective amplification of drug-modified PrP Sc conformers (Bian et al 2014).…”

“…Another issue is the strain dependency of antiprion compounds, which causes the emergence of drug-resistant prion conformers in prion-infected cells or animals (Kawasaki et al 2007;Ghaemmaghami et al 2009;Berry et al 2013;Miller-Vedam and Ghaemmaghami 2013). This issue is predicted based on similar drug-resistant phenomena in chemotherapy with antibiotic, antiviral, or anticancer drugs.…”