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drug resistance

Combination antiviral therapy for hepatitis C

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Ledipasvir and SofosbuvirThe Food and Drug Administration has approved the use of a single pill containing two different antiviral drugs for the treatment for hepatitis C. It is the first combination pill approved for the disease, and also the first treatment that does not contain interferon or ribavirin.

The new hepatitis C drug, called Harvoni, is a mixture of the antiviral drugs ledipasvir and sofosbuvir. Ledipasvir (pictured) is an inhibitor of the hepatitis C virus protein NS5A, which has multiple roles in the viral replication cycle that include RNA synthesis and virus particle assembly. The mechanism of NS5A inhibition by ledipasvir is not known. Sofosbuvir is a previously licensed inhibitor that targets the viral RNA-dependent RNA polymerase. It is an analog of the nucleoside uridine, one of the four building blocks of RNA. Sofosbuvir is utilized by the viral RNA polymerase, leading to inhibition of viral RNA synthesis.

The use of single antiviral drugs (monotherapy) to treat RNA virus infections is always problematic because resistance usually arises rapidly. Dual-therapy pills like Harvoni are better, but the best are triple-therapy pills. Triple therapy formulations such as Atripla have been used successfully to treat infections with HIV-1, and presumably there will be mixtures of three antiviral drugs for treating hepatitis C.

Let’s use HIV-1 to illustrate the value of treating infections with multiple antiviral drugs. The HIV-1 viral genome, like that of HCV, is slightly less than 10,000 bases long. Assume that one mutation in the viral genome is needed for drug resistance. If the RNA polymerase mutation rate is 1 out of every 10,000 bases synthesized, then each base in the viral genome is substituted in a collection of 10,000 viruses. An HIV-1 infected person can make as many as 10,000,000,000 virus particles each day, so 1010/104 = one million viruses will be produced each day with resistance to one drug.

If we use two antiviral drugs, developing resistance to both occurs in every 104 x 104 = 108 viruses. In this case 1010/108 = 100 viruses will be produced each day with resistance to two drugs.

If we use three antiviral drugs, developing resistance occurs in every 104 x 104 x 104= 1012 viruses, which is more than what is produced each day.

This is why triple antiviral therapy has been so successful for the treatment of AIDS.

And yes, I’m sure someone has tested Sofosbuvir for inhibition of Ebola virus replication.

Futures in Biotech 56: RNA viruses and more

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I joined Marc Pelletier on futures-in-biotechepisode 56 of Futures in Biotech for a conversation with Stanford University School of Medicine Professor Karla Kirkegaard.  We talked about RNA viruses – where they came from, where they are going, and Dr. Kirkegaard’s unique approach to discovering antiviral drugs. Don’t miss this episode: Dr. Kirkegaard is a brilliant and eloquent virologist who makes complicated science easy to understand.

Video courtesy of Team ODTV

 

Download video (114 MB .mp4)

Tamiflu-resistant pandemic influenza H1N1 virus selected by prophylaxis

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viral_loadsThe emergence of oseltamivir (Tamiflu)-resistant 2009 H1N1 influenza virus in a Canadian family illustrates the basic concept that viral loads depend on the dose of antiviral drug.

Neuraminidase inhibitors like Tamiflu and Relenza are used to treat severe illness caused by the 2009 pandemic influenza A (H1N1) virus. The antiviral drugs may also be used to prevent infection in high-risk persons, a use called postexposure prophylaxis. For Tamiflu, that means taking 75 mg a day, compared with the same dose twice a day for treating a confirmed infection. Unfortunately, using sub-optimal levels of an antiviral drug is a recipe for disaster.

In this case, a boy with asthma developed confirmed H1N1 influenza and was given Tamiflu twice a day. Tamiflu was also prescribed once a day for all members of his household; this was presumably done to protect the boy’s father who has chronic obstructive pulmonary disease. Eight days after initiation of prophylaxis the father developed laboratory confirmed influenza. Sequence analysis revealed that the NA gene of the virus isolated from the boy and his father differed by just one amino acid change, H275Y, known to confer resistance to Tamiflu.

This experience illustrates the important fact that antiviral therapy has the potential to promote or prevent the emergence of drug resistant viruses. When viral replication is blocked, no drug resistant mutants can emerge. If an antiviral drug is given after the viral population has expanded, or if the amount of drug is not sufficient to block viral replication, genomes that contain mutations will replicate. If the number of viral genomes is small, the infection may be cleared by the immune response.

This concept is illustrated in the figure, in which median viral load is graphed versus time. Low doses of antiviral drug are not sufficient to block viral replication; there may be a transient drop in viral load but then replication resumes. At an intermediate dose of drug, viral load is initially reduced, but since replication is not completely blocked, resistant viruses emerge. At an optimal dose of drug all viral replication is blocked, viral loads drop dramatically, and no drug resistant mutants emerge.

In the Canadian family it’s likely that the sub-inhibitory (prophylactic) amount of Tamiflu taken by the father allowed some viral replication and hence the emergence of resistant mutants. The authors of the study correctly conclude:

These observations support the need for limiting the indications for postexposure prophylaxis. It also seems reasonable to rapidly convert prophylactic (once daily) regimens to therapeutic (twice daily) regimens as soon as influenza-like symptoms develop in a patient receiving prophylactic treatment. Monitoring for the H275Y mutation during outbreaks of 2009 H1N1 virus is important in order to rapidly identify transmission events that could lead to large-scale dissemination of an oseltamivir-resistant 2009 H1N1 virus, similar to what occurred with recent H1N1 virus seasonal strains.

Baz M, Abed Y, Papenburg J, Bouhy X, Hamelin ME, & Boivin G (2009). Emergence of Oseltamivir-Resistant Pandemic H1N1 Virus during Prophylaxis. The New England journal of medicine PMID: 19907034