He said that before VRUS thought it would not partner 938 before 7977 but basically the 938 program moved much faster than they expected and you had data which showed you could get a 2 log drop in 1 day with 938 and a better resistance profile compared to a 2 log drop in 3 days with 7977. So, basically, the 938 data made 7977 look a little less attractive between the two for partnering
Just a thought on comparing the values of PSI-7977 and PSI-938. The purine PSI-938 may have shown greater efficacy than the pyrimidine PSI-7977 but the safety of the pyrimidine nuke class (as seen in RG7128) is the envy of the industry IMO. The pyrimidine class is also underrepresented and lacks competition from other developers which in turn makes PSI-7977 the more valuable asset.