A narrowing of its collaboration with biotech Goliath Celgene Corp (NASDAQ:CELG) led shares of bluebird bio (NASDAQ:BLUE) to tumble 13.3% in June (via data from Capital IQ).

Casting aside CAR-T
Bluebird bio and Celgene had previously been collaborating on the development of immunotherapies that re-engineer a patient's T-cells to better identify and kill cancer cells.

Specifically, under a deal cut between the two companies in 2013, the two were partnering on chimeric antigen receptor, or CAR-T, cell approaches in which T-cells are removed from a patient, altered to recognize a key protein expressed on the surface of cancer cells, and then reintroduced to the patient.

CAR-T therapies are among the most promising advances in cancer treatment; however, Celgene restructured its deal with bluebird bio in June, effectively ending its collaboration on them.

Going forward, Celgene and bluebird bio will focus instead on the development of B-cell maturation antigen, or BCMA, medicine targeting the BCMA protein selectively expressed by B-cell cancer cells, such as in multiple myeloma -- the indication for which Celgene already generates the majority of its annual revenue.

Looking ahead
After discontinuing its CAR-T research with bluebird bio, Celgene then went on to ink a massive $1 billion, 10-year CAR-T collaboration with Juno Therapeutics that includes having Celgene take a nearly 10% equity stake in the company.

That Celgene discontinued researching CAR-T with bluebird bio only to team up with Juno Therapeutics suggests that, in Celgene's view, bluebird bio's CAR-T program wasn't as good as Juno Therapeutics'.

Although that's a disappointing revelation for those hoping that bluebird bio's relationship with Celgene would lead to a blockbuster CAR-T therapy, bluebird bio still has some fascinating research under way that could reward investors.

For example, bluebird bio is in late-stage studies for Lentiglobin BB305, a treatment for beta-thalessemia, a rare blood disorder. In May, bluebird bio outlined a regulatory strategy for BB305 including plans to apply for conditional approval in Europe.

Overall, bluebird bio remains an intriguing company, but a $5.8 billion market cap and uncertainty as to when it could eventually turn profitable suggests investors should approach it with a hefty dose of caution.