Quinacrine failure to treat prion diseases : a possible pharmacological explanation

Abstract

Given the epidemiological uncertainty of the prion diseases, there is an urgent need to discover and to develop antiprion therapeutics in humans. The efficacy of candidate molecules is predominantly tested in vitro in neuroblastoma cells. For several molecules, including quinacrine, a discrepancy has been observed between a proven antiprion action in vitro and its lack of clinical efficacy in the Creutzfeldt-Jakob disease. To further investigate the possible pharmacokinetic origin of the lack of clinical efficacy of quinacrine (and then predictable), we studied the quinacrine disposition both in vivo and in vitro. Ultimately, our experiment was aimed at determining the dosage regimen, which should be administered in vivo to obtain efficacious concentrations in the biophase. We used a model of naturally scrapie-affected ewes. First, we performed a standard clinical trial in scrapie-affected ewes and, we confirmed the absence of therapeutic benefit of quinacrine, as previously shown in humans. In in vitro experiments reproducing the princeps culture conditions in which 50% of antiprion action has been observed, namely a nominal quinacrine concentration of 300 nM, we re-evaluated the EC₅₀ for the potential biophases of quinacrine effect. Quinacrine was assayed by HPLC. We showed that the actual extracellular and intracellular quinacrine neuroblastoma concentrations were 120 nM and 3700 nM, respectively. Quinacrine concentrations in cerebrospinal fluid concentrations and brain tissue, corresponding to the extracellular and intracellular biophases respectively, were measured in healthy ewes after quinacrine administration. The cerebrospinal fluid quinacrine concentrations (<2.1 nM and 55 nM after therapeutic and toxic quinacrine exposure, respectively) were lower than the actual quinacrine extracellular neuroblastoma concentrations corresponding to the measured EC₅₀ (120 nM). Tissue brain quinacrine concentration (1040 nM) after a therapeutic dose of quinacrine was lower than the actual active quinacrine intracellular neuroblastoma concentration (6700 nM) and, only a toxic quinacrine dose allowed to obtain efficacious quinacrine tissular concentrations (53800 nM). Finally, whatever the actual quinacrine biophase, intra- versus extracellular, efficacious quinacrine concentrations cannot be achieved in vivo with a safe dosage regimen. In the future, in order to void in vivo studies for which failure can be predicted, in particular in humans, it is recommended to measure the actual anti-prion EC₅₀ in the biophase in vitro, in order to determine if the in vitro anti-prion action is achievable in vivo.