By David N. Leff

A woman in her mid-forties is alive today, two years after completing a landmark trial of in vivo gene transfer combined with ex vivo immunotherapy to treat end-stage malignant melanoma.

She was one of 10 patients who volunteered to test gene therapy as a last resort for the untreatable, lethal skin cancer.

Molecular geneticist Gary Nabel, at the University of Michigan, Ann Arbor, assembled this cohort in 1992. Results of his Phase I/II clinical study appear in the current Proceedings of the National Academy of Sciences (PNAS), dated Dec. 24. Its title: "Immune response in human melanoma after transfer of an allogeneic class I histocompatibility complex gene with DNA-liposome complexes."

"This particular patient," Nabel told BioWorld Today, "has actually been in complete remission for more than two years. It's a bright spot, but we shouldn't read too much into it at this point. What we've done, basically, is use the genetic approach to create a more effective immunotherapy. Her case is a lead that will give us an important area to investigate further."

What the Michigan gene therapists did, in a sense, was convince that woman's immune system to regard her solid tumor as if it were a transplanted donor organ, ripe for graft rejection.

"What we're really trying to do," Nabel explained, "is to harness the ability of the body to respond to transplants, and then direct that immune response against the tumor."

When a foreign donor heart or kidney, for example, gets stitched into a recipient's body, a complex of genes on the short arm of human chromosome 6 turns on, and expresses a set of major histocompatibility complex (MHC) Class I transplantation antigens. These identify and take into custody samples of the foreign donor organ's own antigens, and "present" them, with extreme prejudice, to the immune system's stand-by T lymphocyte arsenal.

Those T cells recognize the alien enemy, report to action stations, and expand their population into an army of killers, which deploys to destroy the invaders.

What Nabel and his colleagues have done is inject melanotic tumor nodules with plasmids containing those human transplantation antigen genes, wrapped in liposome microcarriers.

"This is a way of attracting the immune defenses to the tumor," Nabel explained, "and immunologically uncloaking it, by providing a very strong immuno-stimulatory signal in the tumor."

Not Just Any Gene Will Do

That Class I MHC contains an assortment of genetic variants, each fitting one or another individual in the human population. "The point is," Nabel pointed out, "if there's a mismatch between the MHC molecules in the donor and the host, that's what gives you rejection."

One of those variant molecules is known as HLA-B7 (human leukocyte antigen, the human form of MHC).

Because 85 percent of all Americans lack that particular B-7 variant of HLA, their immune systems are primed to attack this antigen when they see it, courtesy of the MHC uncloaking mechanism. All 10 of the Michigan melanoma patients were B-7 negative, which set up their tumors for destruction by the T killer cells.

In two of their 10 patients * six women and four men, aged 29 to 72 * they saw either stabilization or regression of local disease. And nine of the 10 patients expressed recombinant B-7 protein in their tumors.

"I think the news is somewhat encouraging here," Nabel observed, "and what's a little bit different from past experiments, is the fact that we were able to combine * in that one woman patient * gene transfer with immunotherapy."

After several courses of plasmid injection to tumors on her thigh, buttock and shoulder, Nabel recalled, "We looked at her cytolytic [killer] T lymphocytes, and found she had killer cells that were able to kill her tumor."

So, with no other treatment available to her, they removed T lymphocytes from her lesions, expanded those cells in tissue culture to much larger populations, then returned these tumor-infiltrating lymphocytes to the patient, plus a helping of interleukin-2 to enhance their activity.

She thereupon achieved complete remission, Nabel said, "and that experience has now prompted us to go on and develop a protocol in which we're looking at this approach in larger numbers of patients."

The Bystander Effect

Curiously, this patient eliminated disease from one distant tumor site that was not treated. "It supports the concept, Nabel observed, "that lymphocytes are able to travel to other sites, and affect the disease elsewhere."

That bystander effect, he added, "is the whole goal of this approach. We don't need to get every last tumor cell transduced. We need only do a small percentage, and in doing that educate the lymphocytes to recognize the tumor antigen."

Plasmid DNA and liposomes for the Michigan melanoma program comes from Vical Inc., of San Diego, which has licensed Nabel's patents from the university. (See BioWorld Today, May 24, 1995, p. 3.)

Under the trade name Allovectin-7, Vical is sponsoring Phase II clinical trials of Nabel's approach in larger melanoma cohorts at several cancer centers * most recently at the University of Colorado, in Boulder * as well as trying the plasmid to treat other malignancies.

"So far," the company's vice president of business development, Robert Zaugg, told BioWorld Today, "melanoma appears to be the one tumor type where we get the best response." He cited experimental oncologist Evan Hersh, at the University of Arizona in Tucson, who is treating some 30 patients with Allovectin-7.

"Evan has a number of partial remissions," Zaugg said, "among patients who should have expired long ago, after failing other therapies."

He estimates that there are a quarter of a million melanoma cases in the U.S. today, with their number increased by some 32,000 annually in recent years. That incidence itself is going up by five percent a year, Zaugg added. Other figures cite the number of deaths per annum at 6,600. *