Emergence of prions selectively resistant to combination drug therapy.

Author summary Prions are unusual infectious agents that lack DNA or RNA blueprints and cause fatal brain diseases that are currently incurable. Combining two or more drugs has proven to be an effective strategy for treating other infectious agents and cancer cells, and here we evaluated the effectiveness of treating experimental prion disease with a combination of two oral drugs previously shown to prolong the lives of prion-infected mice. Our results show that the combination regimen was no more effective than either of the individual drugs. Combination therapy induced the formation of a new prion strain that is specifically resistant to the combination regimen (i.e. resistant to the combination but not to the individual drug give alone). To our knowledge, this is the first report of an infectious agent with selective resistance to combination therapy in the absence of resistance to an individual component, and suggests that combination therapy may not be a good strategy for combating prions because of their unique malleability. Prions are unorthodox infectious agents that replicate by templating misfolded conformations of a host-encoded glycoprotein, collectively termed PrPSc. Prion diseases are invariably fatal and currently incurable, but oral drugs that can prolong incubation times in prion-infected mice have been developed. Here, we tested the efficacy of combination therapy with two such drugs, IND24 and Anle138b, in scrapie-infected mice. The results indicate that combination therapy was no more effective than either IND24 or Anle138b monotherapy in prolonging scrapie incubation times. Moreover, combination therapy induced the formation of a new prion strain that is specifically resistant to the combination regimen but susceptible to Anle138b. To our knowledge, this is the first report of a pathogen with specific resistance to combination therapy despite being susceptible to monotherapy. Our findings also suggest that combination therapy may be a less effective strategy for treating prions than conventional pathogens.