LONDON (Reuters) - French cell therapy specialist Cellectis, which is developing a gene-modified cancer treatment similar to Novartis’s recently approved Kymriah, has been forced to suspend testing following a patient death.

Cellectis said on Tuesday it was working closely with the U.S. Food and Drug Administration in order to resume trials with a lower dose of the medicine UCART123.

Shares in the company fell 26 percent in morning trade following the U.S. regulator’s decision to place a so-called clinical hold on two early-stage trials of the medicine in blood cancers.

Novartis made history last week when it won approval for its $475,000 drug Kymriah, the first in a new class of treatments called CAR-T immunotherapies that use modified disease-fighting T cells to attack cancer.

While Novartis and rivals such as Juno and Kite use cells from the patient’s own body, Cellectis’s gene edited cell therapy product offers an “off-the-shelf”, or allogeneic, option by deriving cells from healthy donors.

It is designed to help patients with acute myeloid leukemia (AML) and blastic plasmacytoid dendritic cell neoplasm (BPDCN).

However, the first patient treated in the BPDCN study, a 78-year-old man, died after experiencing cytokine release syndrome (CRS), a dangerous release of cell-signaling proteins.

The first patient treated in the AML trial, a 58-year-old woman, also experienced CRS and other symptoms but recovered.

Jefferies analysts said there was a chance that the adverse CRS events could be mitigated by lowering the dose and treating symptoms more aggressively, but more information was needed to assess prospects for UCART123.

The side effects could be caused in part by the fact that UCART123 cells come from a healthy donor, rather than the patient’s own body, they added.

Cellectis, which was founded in 1999, is also working on another off-the-shelf cell therapy called UCART19, which is being developed with Servier and Pfizer. That product is now being tested in trials for acute lymphoblastic leukemia.

UCART19 has already rescued two babies treated at London’s Great Ormond Street Hospital from previously incurable cancer.