MannKind Corp.'s post-split stock had a difficult initial day of trading on the market Friday, closing down nearly 13 percent. The stock opened Friday at $2.38 but closed at $2.12. It had closed at 49 cents the previous day before the reverse stock split went into effect. Late last month, shareholders voted to implement the 1-for-5 reverse split to prevent the struggling company from being delisted on the Nasdaq Stock Market. Reverse splits are typically held to increase a stock price above the Nasdaq's listing threshold of $1 a share. This is accomplished by reducing the number of shares but increasing the value of each share proportionately. Prior to Friday, MannKind's stock hadn't traded above $1 since August. The Valencia biotech has had trouble getting its only commercial product, inhalable insulin Afrezza, off the ground, negatively affecting stock price.