And is there some specific reason as to why ENTA-based regimen works in this specific population where SOF does not?
I don't think it is Enanta per se.
I think it is about where drugs are metabolized; Sovaldi places more burden on the kidneys. I think/thought Harvoni was still effective, it was just too rough on the kidneys for some patients.
"Severe Renal Impairment and End Stage Renal Disease
No dosage recommendation can be given for patients with severe renal impairment (estimated Glomerular Filtration Rate [eGFR] less than 30 mL/min/1.73m2) or with end stage renal disease (ESRD) due to higher exposures (up to 20-fold) of the predominant sofosbuvir metabolite."