Shares of MannKind Corp. fell nearly 25 percent Friday, after the company announced a $28 million stock offering. The Westlake Village biotech on Thursday said it planned a direct offering of 14 million shares at a purchase price of $2 each, also with warrants to purchase 14 million additional shares at a price of $2.38. The company planned to market the offering to select health care institutional investors. MannKind estimated that after deducting the placement agent’s fees and estimated offering expenses, the net proceeds from the offering would be $26.3 million. The money would go towards the marketing of Afrezza, the inhalable insulin that is the company’s only commercial product, as well as clinical trials and research and development. In a post on the Motley Fool investment website, George Budwell said that the main reason for MannKind’s stock price slump was that the offering was priced at a substantial discount to the trading price, indicating that institutional investors needed some enticement to buy the shares. “The brutal truth is that MannKind’s inhaled insulin endeavor has failed, leaving the company no available options but to continue diluting shareholders on a regular basis just to keep its doors open,” Budwell wrote. Shares of MannKind (MNKD) closed Friday down 59 cents, or 24.8 percent, to $1.79 on the Nasdaq.