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Pipeline vs. pipeline.

That's the match-up investors must consider when deciding which clinical-stage biotech stock to buy. Two such biotech stocks attracting plenty of investor attention are Celldex Therapeutics (NASDAQ:CLDX) and bluebird bio (NASDAQ:BLUE). Which stock is the better buy? It comes down to the pipelines. 

Why buy Celldex?

Unfortunately for Celldex, the company can no longer tout Rintega as a promising pipeline candidate. In March, Celldex discontinued its phase 3 study of the glioblastoma drug after Rintega failed to improve overall survival rates in patients. Celldex's stock cratered on the news and is now down close to 80% year to date.

There are still several reasons for investors to consider Celldex, though. Probably the best thing the biotech has going for it currently is cancer drug glembatumumab vedotin, or glemba for short. Five clinical studies are in progress for glemba, four of them in phase 2 and another in phase 1.

The most important of these studies right now is evaluating glemba in treating triple-negative breast cancer. Celldex initiated the study in December 2013 on an accelerated approval path. Enrollment for patients with triple-negative breast cancer that overexpress gpNMB continues. The biotech expects the study to wrap up in 2018.

Celldex recently reported positive results from another study of glemba in treating metastatic melanoma. 52% of the 62 patients in the study experienced tumor shrinkage. The disease also didn't progress for at least three months in 52% of patients. Most importantly, the primary endpoint of the study (at least six objective responses in the first 52 patients) was exceeded.

Another promising pipeline candidate is varlilumab. The drug is in a phase 2 study in combination with Opdivo in treating multiple solid tumors plus three other early stage studies. One of those phase 1 studies combines varlilumab with another Celldex drug, CDX-1401. Like glemba, CDX-1401 is also in a phase 2 study targeting treatment of metastatic melanoma.

Celldex reported cash, cash equivalents, and marketable securities of $220.1 million at the end of June. That amount, combined with another potential stock offering under its $60 million agreement with Cantor Fitzgerald, should be enough to fund operations through 2018. 

Why buy Bluebird?

Bluebird's pipeline includes two candidates that should particularly appeal to investors. Lenti-D is in a phase 2/3 study for treatment of cerebral adrenoleukodystrophy (CALD), an often fatal, rare genetic disease. LentiGlobin is in a phase 2/3 clinical study for treating rare genetic blood disorder transfusion-dependent beta thalassemia and a phase 1/2 study for treating severe sickle cell disease. 

Interim data from the phase 2/3 Lenti-D study were encouraging. All 17 patients in the study were free of major functional disabilities. Sixteen patients had stabilization of their neurological function scores, and 14 patients had stable a Loes score (a measurement of demyelination and atrophy in patients with CALD).

Bluebird recently announced changes that should help advance LentiGlobin. The company is implementing a new manufacturing process to increase the percentage of cells successfully transduced. It is also changing the protocol for the phase 1/2 study of the drug in treating sickle cell disease, a move that could lead to better outcomes in the trial. Perhaps the most important update, though, is that Bluebird is working out the details with U.S. and European regulators to advance LentiGlobin into late-stage clinical studies in treating beta thalassemia.

The biotech's only other clinical program is for experimental cancer drug bb2121. The potential for the chimeric antigen receptor T cell (CAR T) therapy is significant enough that it attracted the interest of Celgene. Earlier this year, Celgene exercised its option to exclusively license bb2121. 

How does Bluebird's cash position look? Pretty good. The company reported cash, cash equivalents, and marketable securities of $779 million as of June 30. Bluebird expects this should be enough to carry it through 2018.

Better buy

Had Rintega not flopped, I would have picked Celldex in a heartbeat. Without Rintega, though, the choice is more difficult. All things considered, I think Bluebird gets the nod as the better buy.

Significant risks remain for both stocks. Celldex has already experienced the agony of a late-stage clinical failure. Glemba, varlilumab, and CDX-1401 still have a long way to go. Of course, Bluebird's Lenti-D and LentiGlobin have plenty of hurdles to jump as well.

However, Bluebird seems to be making the right moves to increase its likelihood of success with LentiGlobin. If the drug ultimately wins approval for both beta thalassemia and sickle cell disease, Bluebird will have a major blockbuster on its hands. That's still a big "if" for now, but I like Bluebird's chances.

Keith Speights owns shares of Celgene, and The Motley Fool owns shares of and recommends the stock. The Motley Fool has the following options: short October 2016 $95 puts on Celgene. The Motley Fool recommends Bluebird Bio and Celldex Therapeutics. 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.