With the Nasdaq Biotechnology Index up 65% since the start of the year, it's clear that the biotech sector performed extraordinarily well in 2013 - but which stocks were the biggest winners? Several small-cap and mid-cap biotech companies posted returns of 200% or more through the middle of December this year, and, in this series, I review the 15 biggest movers of 2013. Let's continue this review with number 12 on the list, Celldex Therapeutics, Inc. (Nasdaq: CLDX).

Targeting specific genetic markers is a common theme among top performing biotechs this year. Its probably not surprising that investors rallied Celldex Therapeutics soared 236% through mid-December on hopes its gene-specific immunotherapy drugs can thwart tough-to-treat cancers.

Striving to cure rare cancer
Celldex's most advanced drug is rindopepimut, which is in Phase 3 trials for glioblastoma, a rare brain cancer. The drug targets a specific mutation of the EGFR protein that occurs in about 30% of glioblastoma patients.

Celldex had previously partnered with Pfizer (PFE -0.12%) on rindopepimut's development. However, the tough-to-treat disease became less of a focus for Pfizer as it shifted attention toward easier indications in hopes of more quickly blunting lost sales tied to high-profile patent expirations. As a result, Pfizer returned the drug to Celldex in 2010.

Celldex continued rindopepimut's development and reported Phase 2 data in the fall of 2012 which showed that patients treated with rindopepimut saw median survival of 24.6 months, compared to 15.2 months on the current standard of care. Celldex updated that data in December, showing 51% of those treated with the drug survived two years, far above the historical 6% rate associated with the disease.

Celldex also has an ongoing Phase 2 trial studying its CDX-011 for breast cancer. That drug uses technology from Seattle Genetics to target glycoprotein NMB, a protein associated with cancer cells ability to metastasize.  

In Phase 2, Celldex found the drug was most effective in treating triple-negative breast cancer, which represents roughly 15%-25% of all breast cancer cases.  That prompted the company to initiate a Phase 3 trial in that patient population, in which CDX-011 takes on Roche's (RHHBY -0.10%) Xeloda -- a drug that goes off-patent this year and generated $182 million in third quarter sales   in the United States. The FDA approved the first generic version, made by Teva Pharmaceuticals, in September.

Fool-worthy final thoughts
Glioblastoma is exceptionally tough to treat, suggesting a significant unmet need and a greater risk final data won't pan out as Celldex hopes. Since spending on brain cancer is expected to grow from $4.4 billion in 2010 to as much as $8 billion in 2020, success in phase 3 could prove significant for Celldex. If not, investors will need to pin faith on CDX-011, which means keeping a close eye on that drug's clinical trials too.

Top Stock of 2014