Here's the cancer data that led AbbVie to make the second-largest acquisition in the history of venture capital

Until now, the second-largest acquisition in the history of venture capital was largely based on clinical trial data that hadn't been released.

In April, Stemcentrx, a startup with backing from Silicon Valley investors including Artis Ventures and Founders Fund, was acquired by AbbVie for $5.8 billion in cash and stock in a deal that in total could be worth up to $10.2 billion. The deal officially closed June 1.

In data released Sunday at the American Society of Clinical Oncology conference, Stemcentrx showed that its drug could extend the lives of certain patients with small cell lung cancer. All of those patients had higher amounts of a specific protein.

Stemcentrx is researching drugs that are designed to treat cancer - specifically solid tumors - by targeting cancer stem cells. The intent is that by attacking those cells, it can stop the cancer from spreading. Unlike general adult stem cells, which are found all over the body as unspecified cells that can regenerate, cancer stem cells are a specific type of cancer cell that some research suggests could play a key role in defeating the disease.

The Stemcentrx drug that's farthest along thus far is called rovalpitzumab tesirine, or Rova-T. It's currently in trials to treat small-cell lung cancer, a type of lung cancer that makes up about 10% to 15% of all cases. The targeted drug goes after a special protein that may be linked to cancer stem cells. The protein, called DLL3, is highly expressed (meaning a genetic variation is telling the body to make a lot of it) in about 80% of tumors, Stemcentrx chief scientific officer Scott Dylla said on Friday at AbbVie's research and development day based on data that he's seen. Rova-T works by latching onto the protein, delivering the deadly toxin to the cell, and cutting off the tumor growth at its source.

The Phase 1 study released Sunday, which had the goal of showing that Rova-T is effective and safe to use, looked at 74 people with small cell lung cancer whose disease had gotten worse even with at least one other treatment option. Of the 60 who were able to be evaluated by the end of the trial, 11 saw tumor shrinkage. Those results started to look better when you look at the smaller group of 26 people with high levels of DLL3. Then, about 10 of those 26 responded to the treatment, and about 1/3 of those 26 people were alive after a year.

"Although these results are preliminary, rovalpituzumab tesirine seems to be the first targeted therapy to show efficacy in small cell lung cancer, and we may have identified DLL3 as the first predictive biomarker in this disease," Dr. Charles Rudin, an oncologist at Memorial Sloan Kettering who led the study, said in a release. Rudin said later in a press briefing that the results should be taken with caution as it was still just a small group and the first study done in humans.

What's next

San Francisco-based Stemcentrx, which has been been working on its drugs since 2008 but only launched publicly in September 2015. Since the acquisition by AbbVie, CEO Brian Slingerland told Business Insider, not much has changed in the day-to-day operations.

Stemcentrx is currently enrolling patients in a phase 2 trial for Rova-T as a third line treatment (meaning patients have already tried other already-approved drugs) in small cell lung cancer, and AbbVie expects the drug to make it to market by 2018.

Beyond Rova-T, Stemcentrx has four other drugs in clinical development and is also collecting a library of patient-derived xenografts (PDX for short), which is a way to evaluate what other highly expressed genes beyond DLL3 might be worth by looking at human tumors in mice models.

Gary Gordon, AbbVie's vice president of oncology told Business Insider that the addition of Stemcentrx is more exciting based on that library. "For us thinking long term, it's being able to find those tumor-initiating cells and then say what other types of targets exist and how else can we target them."