NEW YORK -- Billionaire investor Carl Icahn has raised his stake in Nuance Communications ( NUAN) over the past month to almost 17%, nearing a threshold that would trigger defensive mechanisms to thwart a hostile takeover. A regulatory filing shows that Icahn held about 52.4 million shares, or 16.9 percent of Nuance's outstanding shares, as of Thursday. The disclosure tallied share purchases over several weeks, beginning Aug. 7. Earlier this month, the voice recognition software maker adopted a plan to shield itself against a hostile takeover in the event a person or group acquired 20% or more of the company's stock without board approval. When such "poison pills" kick in, they enable existing stockholders to acquire additional shares at lower prices. Shares of Nuance Communications Inc. are down 14% this year.