Activist investor Carl Icahn is dropping his campaign to have Apple buy back $50 billion of its stock from shareholders. His forfeit comes after the prominent proxy firm Institutional Shareholder Services (ISS) — an influential adviser to investors — recommended that shareholders vote against his proposal. In an open letter to Apples investors, Icahn cites both ISS opposition and some of ISS arguments against his plan as reasons for ending his aggressive buyback campaign. Apple was also opposed to the proposal.