L31V) in the replicon had less than a 100 fold increase in resistance to the ledipasvir in Harvoni, while a two amino acid substitution had over a 1000 fold increase in resistance. Genotype 1a had a similar but more substantial increase in resistance with each respective increase in amino acid substitution with resistance associated substitutions at K24R, M28T/V, Q30R/H/K/L, L31M, and or Y93H/N.NS5A polymorphisms also have an effect on viral resistance with the most common resistance-associated amino acid substitutions detected at Q30R, Y93H or N, and L31M in patients with a rapid virological response (RVR). The specific baseline NS5A resistance-associated polymorphisms observed in clinical trials were M28T/V, Q30H, Q30R, L31M, H58P, Y93H, and Y93N in genotype 1a and L28M, A92T, and Y93H in genotype 1b. Patients with multiple baseline NS5A polymorphisms tend to have higher relapse rates when using ledipasvir/sofosbuvir. The difference in relapse rates between treatment naive and treatment experience groups with baseline NS5A polymorphisms ranges from 1% after a 12-week regimen and 0% after a 24-week regimen respectively. NS5B mutations
A single amino acid substitution S282T contributes to viral resistance and decreases the activity of sofosbuvir in ledipasvir/sofosbuvir by approximately 2 to 18 fold. Cross resistance
No cross resistance was seen in the combination of ledipasvir and sofosbuvir in ION phase 1,2 and, 3 clinical trials as ledipasvir was fully active against sofosbuvir-resistance-associated substitutions and vice versa.
Mechanisms of action
The most commonly associated mechanism associated with ledipasvir/sofosbuvir is the hyperphosphorylation of NS5A, a viral polymerase important in proper viral assembly and interferes with proper liver metabolism. Ledipasvir/sofosbuvir inhibits the proper viral assembly by re-positioning NS5As sub-cellular localization.NS5B, a viral polymerase that can initiate RNA synthesis de novo, is also allosterically inhibited by ledipasvir/sofosbuvir.NS5A and NS5B inhibitors in combination have a synergistic effect. Pharmacokinetics
Sofosbuvir is absorbed fast in the plasma with a peak concentration (Cmax) at 0.8 to 1 hour after the administered dosage and undergoes extra hepatic metabolism with 61 to 65% bound to human plasma proteins. It is then predominantly converted to the inactive phosphate free circulating metabolite GS-331007 (eliminated 76% through renal passive filtration) which has a median peak plasma concentration at 3.5 to 4 hours after the medication is ingested. Sofosbuvir does not appear to be affected by different levels of macronutrients when compared with fasting states.Ledipasvir has a maximum concentration at 4 to 4.5 hours after ingestion and is not affected by macronutrients. It is more than 98% protein bound and is predominantly eliminated fecally, with minimal metabolism in the liver. Elimination
The median terminal half life after a dosage of ledipasvir/sofosbuvir for 90 mg of [14C]-Ledipasvir is 47 hours; for 400 mg of [14C]-Sofosbuvir it is 0.5 hours (after the initial distribution of medication in body tissue) and 27 hours (the eventual excretion of the medication).