MannKind Corporation (NASDAQ:MNKD) shares soared 9.60% on Monday to $10.96. The biopharmaceutical company announced Friday that the USFDA has finally approved its inhaled insulin drug Afrezza. RBC Capital Markets said that the drug has immense revenue potential, and its sales could eventually reach $5-$7 billion. However, James Brumley of InvestorPlace says that we might have already seen the best of MannKind stock. Buying the stock at this point carries more risk than rewards.

MannKind Afrezza

Not too many buyers left to bid MannKind up

It’s not about MannKind’s long-term growth or Afrezza’s market potential. It’s the sheer optimism around MannKind and Afrezza that makes the stock vulnerable to a pullback. Ever since the FDA advisory committee voted unanimously in favor of the drug in April, investors were widely expecting the final approval of Afrezza for diabetes treatment. That’s the reason the stock has rallied about 172% since the beginning of April 2014.

Many believe that investors would wait until the final approval before pulling the trigger. But the reality is that investors who were interested in post-approval MannKind also willing to bet on the stock before approval. That means there may not be too many buyers left at this time. Brumley believes that there are a large number of would-be sellers willing to exit the stock at a moment’s notice.

MannKind’s drug acts faster than Exubera

The diabetes drug has a huge market potential. According to the Center for Disease Control and Prevention, about 29.1 million Americans are suffering from diabetes mellitus. Despite approving Afrezza, the FDA has asked MannKind to put a warning on the box for people with known lung diseases. Notably, many experts believe that there could be little demand for the inhaled insulin product Afrezza.

Pfizer Inc. (NYSE:PFE) had once launched its own inhaled insulin product Exubera. But the pharma giant pulled the drug out of the market due to poor sales. But MannKind founder Alfred Mann says Afrezza is going to succeed as it acts faster than Exubera.

MannKind shares were up 0.18% to $10.98 in pre-market trading Tuesday.