I am sure it's been noted before, but the CSO will pay a substantial income tax on the spread. Assuming the total spread on the $2.8M purchase is $2.750M, his combined federal and state tax, at a 45% overall rate, will be about $1,237,500. He has presumably decided that it's worth paying this tax now, and having future appreciation above $49/share taxed at lower capital gains rates.
IMO, this shows his confidence in the future prospects for ENTA's pipeline, which seem uncertain to me as an outside investor for years beyond 2017.