Source: MannKind. 

What: Shares of biopharmaceutical company MannKind (NASDAQ:MNKD), which focuses on the development of inhaled therapies to treat diabetes, sank 18% in April, according to data from S&P Capital IQ following a disappointing sales update on Afrezza.

So what: Approved in June 2014, Afrezza is an inhalable, fast-acting powder for type 1 and 2 diabetics that was clinically shown to leave the body in a quicker timeframe, thus reducing instances where the pendulum is swung too far and excess blood glucose is removed, rendering the patient hypoglycemic. All told, the prospect of no needles and fast-acting formulation demonstrated plenty of promise. However, thus far sales of Afrezza have largely flopped.

According to the first-quarter earnings results from Sanofi (NASDAQ:SNY), MannKind's licensing partner for Afrezza, the inhaled diabetes drug brought in just $1 million when Wall Street analysts had been expecting something closer to $3 million to $4 million. Understandably, there's some sampling going on aimed at promoting long-term sales of the drug, but this was still weak by practically all standards. With MannKind lined up for just 31.5% of this revenue, don't expect much in the way of top-line product sales for MannKind.

Source: Afrezza Users via Facebook.

Now what: If Afrezza's weak sales data wasn't already a primary concern, MannKind's short interest continues to rise, implying that investors are turning more bearish on the company. The latest short interest update shows that 95.7 million shares are currently being held short by investors, up by more than 4.8 million shares two weeks prior.

On one hand, MannKind and Sanofi certainly have a drug that could revolutionize diabetes treatment. However, I've been concerned that insurance coverage, pricing, and consumer interest could be an issue. While one quarter doesn't make Afrezza a failure, I've witnessed enough launch failures in recent years that there's certainly cause for concern. As such, I would continue to suggest investors keep their distance from MannKind stock and wait for stronger sales data from Afrezza before taking a position.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.