Kite Pharma's (KITE) CAR-T therapy demonstrated a 39% response rate, including a 33% complete response rate, in patients with an aggressive form of lymphoma, the company said Tuesday.

The interim results from the phase II clinical trial are strong enough for Kite to seek marketing approval for the therapy, known as KTE-C19, with the U.S. Food and Drug Administration, the company said.

If approved, Kite will be the first drug company to secure approval of a CAR-T therapy, ahead of rivals Novartis (NVS - Get Report) and Juno Therapeutics (JUNO) .

Kite shares closed Monday at $54.97 ahead of the data announcement. The stock was halted in after-hours trading. 

CAR-T, or chimeric antigen receptor T-cells, is a new form of cancer immunotherapy in which a patient's own T cells are removed and then engineered to identify and kill malignant blood cancer cells.

In the study, 51 patients with advanced diffuse large B-cell lymphoma (DLBCL) were treated with Kite's KTE-C19. Initially, 76% of the patients responded to treatment, including 47% complete remissions.

After three months of follow-up, some patients relapsed, reducing the overall response rate to 39% and the complete remission rate to 33%.

The sliding response rates will raise questions about the durability of Kite's CAR-T therapy. The company contends that KTE-C19 deserves to be approved because the patients treated in the study have aggressive lymphoma which no longer responds to any other treatments. As a comparison, similar DLBCL patients treated again with chemotherapy in previous studies only achieve an 8% complete response rate, Kite said.

Kite continues to follow the patients in the study and will report response rates after six and nine months of follow up. These data will also be submitted to the FDA and could impact the approval decision. A recent research note published by Jefferies, citing an FDA reviewer, said duration of response to CAR-T therapy of six months or more would be an important factor in the agency's approval decision.

Two lymphoma patients in the study died due to treatment with KTE-C19, the company said. Serious neurological toxicity was reported in 34% of patients.

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