RMB, the entire purpose of a share buyback is to increase the stock price, through reducing the float. Sarissa likely wants to time any re-purchase to correspond with either with other major news, or when they are actively seeking to sell the company. Since neither is imminent, beginning a share buyback now would be poor timing.
I don't believe they are trying to conserve cash for operational reasons. If anything, they eventually want to use that cash the way they are using cash at their largest holding (INVA).