Coadministration of elvitegravir/cobicistat/emtricitabine/tenofovir (150/150/200/300 mg once daily) and ledipasvir/sofosbuvir (90/400 mg once daily) was studied. Elvitegravir Cmax decreased by 12% but AUC and Cmin increased by 2% and 36%; cobicistat Cmax, AUC and Cmin increased by 25%, 59% and 325%. Ledipasvir Cmax, AUC and Cmin increased by 63%, 78% and 91%; sofosbuvir Cmax and AUC increased by 33% and 36%; Cmax, AUC and Cmin of GS-331007 increased by 33%, 44% and 53%. When given with elvitegravir/ cobicistat/ emtricitabine/ tenofovir disoproxil fumarate, ledipasvir/sofosbuvir is expected to increase the concentration of tenofovir but have no effect on emtricitabine exposure. The safety of tenofovir disoproxil fumarate in the setting of ledipasvir/sofosbuvir and a pharmacokinetic enhancer (e.g. ritonavir or cobicistat) has not been established. The combination should be used with caution with frequent renal monitoring, if other alternatives are not available.
Harvoni Summary of Product Characteristics, Gilead Sciences Ltd, November 2014.

Coadministration of elvitegravir/cobicistat (150/150 mg once daily) and ledipasvir/sofosbuvir (90/400 mg once daily) was studied in 29 subjects. Ledipasvir Cmax, AUC and Cmin increased by 63%, 78% and 91%, respectively. Sofosbuvir Cmax and AUC increased by 33% and 36%; GS-331007 Cmax, AUC and Cmin increased by 33%, 44% and 53%, respectively. Elvitegravir Cmax decreased by 12%, but AUC and Cmin increased 2% and 36%; cobicistat Cmax, AUC and Cmin increased by 25%, 59% and 325%, respectively. Coadministration may increase tenofovir concentrations. The safety of increased tenofovir concentrations in the setting of ledipasvir/sofosbuvir and the combination of elvitegravir, cobicistat, emtricitabine and tenofovir has not been established. Coadministration is not recommended.
Harvoni US Prescribing Information, Gilead Sciences, October 2014.