By David N. Leff

The nine boys and girls, ages 4 to 16, and three young adults in their 20s, had one thing in common: All 12 of them faced a life expectancy of six weeks at most. They were all in the terminal stages of leukemias or lymphomas when they reached the clinical service of pediatric oncologist Eva Guinan at the Dana-Farber Cancer Institute in Boston.

Dana-Farber oncologist/immunologist John Gribben explained what had brought them to this point of death.

"These were young people," he told BioWorld Today, "who were invariably going to die of their disease. They did not have a brother or sister whose immune system cells was a match for theirs, to make a bone-marrow transplant practical. And they did not have a match on the National Marrow Donor Registry, a database of 4 million potential donors."

Gribben made a related point: "It turns out if you're a Caucasian in this society you have a fairly good chance of finding somebody in the registry who's a match The more you go into ethnic minority groups, the harder and harder it becomes to find matching donors. The 12 patients included a good number of Hispanic and African-American origin. Those were the ones we anticipated would have the most difficulty finding matches from the registry.

"To have any chance of survival," he went on, "these children needed a bone-marrow transplant (BMT). There was no conventional BM donor. Therefore, we were trying to see if we could do a transplant from partially matched family donors, but do it in a way that made the toxicity of the procedure acceptable.

"That is, the patient shares half of the immune-system antigens with one relative, but the other half is uniformly different. A BMT from a parent to a child under such circumstances is almost invariably associated with death from graft-vs.-host disease (GVHD)."

GVHD is the recipient body's lethal backlash against what it perceives as enemy antigens from a mismatched donor. Its severity - depending on how imperfect the match - varies from a mild skin rash to gut and liver disorders to rapid death.

"Even in a fully HLA-matched sibling donor," Gribben related, "without doing something drastic trying to prevent GVHD, that would invariably be fatal, because what you are doing in the end is attempting to switch off this GVHD response. So you have to put patients on such severe immunosuppression that they risk dying of infection. But if you don't switch off the process itself, the cause of death will be that these cells are going to attack the host, who is going to be eaten from the inside out, literally."

Study Enrolls Its In Extremis Cohort

Between March 1996 and October 1998, the 12 patients underwent the last-resort, half-mismatched, experimental bone-marrow transplantation. It differed from conventional BMT by adding to the donor marrow a proprietary, recombinant fusion protein, CTLA4-Ig, donated to the Dana-Farber group by Repligen Corp., of Needham, Mass.

CTL stands for "cytotoxic T lymphocytes," the killer T cells that wreak GVHD's havoc. Repligen's protein is designed to tame those ravaging T cells, and educate them to attack only their legitimate targets - the leukemia or lymphoma antigens of the patients' cancers. This selective immune aggression is known as anergy.

It relies on two basic facts of T lymphocyte life: First, that a cytotoxic T cell turns its attention to a target antigen only when it is brought to the killer's notice by so-called antigen-presenting cells (APC). And second, it takes two separate stimulatory signals beamed at the T cell receptor complex to activate that aggressor.

"It turns out," Gribben explained, "that the most important costimulatory signals are delivered by two molecules, B7-1 and B7-2, on the APC, to the CD28 that's on the T cell surface. Signal 1: The T cell receptor sees the antigen being presented by the major histocompatibility complex molecule. Signal 2: the interaction of B7-1 and B7-2 with CD 28.

"To have a functional immune response," he pointed out, "you have to have both those signals. Preclinical data told us that if you delivered Signal 1 in the absence of Signal 2, you did not get an immune response, but instead the cells became anergic or tolerant. So we used the CTLA4-Ig to bind to B7-1 and B7-2 on antigen-presenting cells."

Pulling Killer T Cells' Fangs

"So now," Gribben continued, "the T cell sees, in this particular case, the alloantigen of the recipient child against its donor relative. But it doesn't see the B7-1 or B7-2. So we paralyzed those cells, making them unable to react against that antigen in the future."

Gribben is senior author, and Eva Guinan first author, of a paper in today's New England Journal of Medicine, dated June 3, 1999. Its title is, "Ex vivo blockade of B7 mediated costimulation greatly diminishes in vivo alloreactivity of a histocompatible stem cell allograft." Their report recounts the treatment and outcomes of those 12 young patients in the Phase I pilot trial:

¿ Only three patients had any acute GVHD.

¿ None died of it.

¿ Five of 12 patients are alive and in remission.

Bigger Market Looms: Solid Organ Grafts

Now Repligen is planning a larger Phase II BMT study of its CTLA4-Ig agent, to start before the end of 1999. "We will probably enroll between 35 and 50 patients," the company's president and CEO, Walter Herlihy, told BioWorld Today. "It will certainly be a multicenter trial," he added, "at well-recognized U.S. centers. It will be using - as did the Dana-Farber study - mismatched donor-recipient pairs. We may broaden the focus a little bit, in terms of mismatch."

On that theme of broadening CTLA4-Ig's clinical application, Herlihy observed: "As for solid-organ transplantation, there's a wealth of information from animal models for solid organs, as well as insulin-producing pancreatic islet cells."

Anent which, Gribben observed, "I do BMTs for a living, but I'm very well aware that heart, liver and kidney transplants are where the real market is. And finding a way to exploit this type of methodology into the solid organ transplant, I'm sure, from a company perspective, that market is where Repligen would be wanting to go in the long term."