Take Long View

What: After disclosing in an SEC filing that Chief Operating Officer Caeser Belbel sold 153,333 shares, Ziopharm Oncology (NASDAQ:ZIOP) stock slumped by 15% earlier today.

So what: Ziopharm Oncology is a clinical-stage biotech company developing next-generation therapies designed to supercharge a cancer patient's immune system.

Those therapies include chimeric antigen receptor T-cell (CAR-T) medicines that re-engineer the body's T-cells to allow them to bind to and destroy cancer cells. Ziopharm is using switching technology developed by Intrexon (NYSE:XON) to enhance the efficacy and safety of its CAR-T approach. Earlier this year Intrexon and Ziopharm expanded their collaboration to include Merck Serono, a German drugmaker, to further bolster their product development.

Because there's a considerable amount of industry enthusiasm surrounding CAR-T research and Ziopharm's programs are in the early stages of human clinical testing, the disclosure that one of its senior C-suite leaders unloaded shares has surprised investors.

Specifically, Belbel exercised options on 153,333 shares at $2.30 per share and sold them for $12.84 per share on Nov. 19. The sale generated total proceeds of about $1.97 million and the options had an exercise expiration date of June 2023.

Now what:  It's not uncommon for insiders to want to reap some of the benefit of their hard work and sell shares, and while this isn't the first time that Belbel has sold some of his stock, it is his biggest sale so far and significantly reduces his financial exposure to the company.

Having said that, Belbel still owns 59,201 shares in Ziopharm Oncology directly and he still has options to buy an additional 76,667 shares, too. Because he didn't exercise all of his options, or sell the 59,201 stock that he already owned, investors may want to take this sale with a grain of salt.

What may be more concerning to Ziopharm Oncology investors, however, is that Belbel's sale continues a trend of insider selling at the company that includes an option exercise and sale by Chief Medical Officer Francois Lebel in June. Lebel exercised options on 50,000 shares at $4.34 on June 9 and then sold those shares at $10.12. Lebel still has options on another 100,000 shares, but he doesn't own any shares in the company directly. Collaboration partner Intrexon also completed the distribution of its $172 million equity ownership stake in Ziopharm Oncology to its investors in June.

An argument can certainly be made that if Ziopharm's technology is a game-changer, insiders ought to be content to sit on their options for years to come. But since we don't know the precise reason why these insiders sold shares (and there can be plenty of personal reasons for them to do so), the sales do create some uncertainty that increases pressure for Ziopharm Oncology to deliver solid trial results, particularly for its lead drug, Ad-RTS-hIL-12.

Todd Campbell has no position in any stocks mentioned. Todd owns E.B. Capital Markets, LLC. E.B. Capital's clients may have positions in the companies mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.