Activist investor Bill Ackman said Tuesday his Pershing Square Holdings Ltd. hedge fund would take a "much more proactive role" in protecting its losing investment in Valeant Pharmaceuticals International Inc., after a grim outlook and fears of a possible default sent that stock sharply lower. closed down 51% at $33.51, marking their worst-ever one-day performance, after the company slashed earnings and sales guidance and said it may breach loan covenants if it fails to produce an already delayed annual report by April.