What happened

Atara Biotherapeutics (NASDAQ:ATRA), a clinical-stage cell therapy company, has lost an eye-popping 42% of its value over the first six months of 2019, according to data from S&P Global Market Intelligence. What's been weighing on the biotech's shares this year?

Atara's shares have slumped in response to the news that its experimental, off-the-shelf cell therapy tab-cel -- indicated for patients with Epstein-Barr virus-associated post-transplant lymphoproliferative disease -- is experiencing slower-than-expected enrollment in its late-stage program. Compounding matters, the biotech's founder and first CEO, Isaac Ciechanover, announced his retirement earlier this year, which is a rather unsettling development ahead of a key data readout.

Magnified round cell-like balls.

Image Source: Getty Images.

So what

Atara, though, appears to have landed firmly on its feet with the hire of Novartis executive Dr. Pascal Touchon as its new president, CEO, and member of the board of directors. Touchon is clearly a first-round draft pick, given his leadership role in overseeing the launch of Novartis' adoptive cell therapy Kymriah. In short, this is a stellar addition to Atara's brain trust from an experience standpoint.

Having said that, the biotech also lost its global head of research and development, Dr. Dietmar Berger, at the end of May. That's a lot of managerial turnover for a small-cap biotech barreling toward its first major top-line data release.

Now what

Should investors take advantage of this hefty drop in Atara's share price? While there's a lot to be said about the first company that can successfully bring an off-the-shelf T cell therapy to market, Atara's ongoing management churn and clinical trial enrollment issues are worrisome. These issues may not be linked at the end of the day, but two top executives leaving at this critical juncture is an odd occurrence, to put it mildly. Investors, in kind, might want to stick to the safety of the sidelines while this story plays out.