BioWorld International Correspondent

PARIS - A clinical trial testing a gene therapy for severe combined immune deficiency linked to the X chromosome, SCID-X1, a rare genetic disease, has been halted indefinitely after a third subject suffered a serious complication.

The Phase I/II trial was being conducted at the Necker children's hospital in Paris. It began in March 1999 and already was suspended in 2002 after two infants developed an anomalous proliferation of T lymphocytes.

SCID-X1 is characterized by an inability to generate T lymphocytes, making those afflicted susceptible to infection. It is caused by the faulty expression or function of a protein called gamma-c, which acts as a receptor of signals needed for the differentiation of cells that are precursors to lymphocytes.

The gene therapy entailed the ex vivo transfer of a normal copy of the gamma-c gene into medullar cells taken from the patient and reintroduced using a viral vector. It was developed by researchers at the French National Institute of Health and Medical Research with the support of the French Muscular Dystrophy Association. The trial initially yielded positive results, since most of the 10 babies treated in the first two years developed an effective immune system after acquiring the ability to generate T lymphocytes, enabling them to lead a normal life.

But two and a half years after receiving treatment, two of the subjects had a disturbingly high level of abnormal, monoclonal T lymphocytes in the blood. The trial was suspended and the protocol modified to exclude babies less than 6 months old and to modify the dosages.

The French Health Safety Agency authorized the resumption of the trial in May, but another patient, who was 9 months old at the time he was treated, developed the same complication in January. The trial was immediately suspended again. The latest setback could put a definitive end to the Necker hospital trial and prompted some members of the gene therapy fraternity in France to voice fears that it could be the nail in the coffin - at least for now.

In the case of SCID-X1, the only treatment available is a transplant of bone marrow from a family member who ideally shares the same tissular antigens of HLA histocompatibility, but that is possible in only 20 percent of cases. Without treatment, the disease results in death within a year of birth.