By Debbie Strickland

Special To BioWorld Today

WASHINGTON - Researchers at the City of Hope National Medical Center's Beckman Research Institute reported early success in a proof-of-concept trial of an anti-HIV strategy that inserts a ribozyme gene against the virus into pluripotent stem cells - the parent cells to the immune cells' HIV targets.

John Rossi, director of the department of molecular biology at Beckman, presented the data at the American Society of Gene Therapy's second annual meeting.

The gene contains two ribozymes, which act as "molecular scissors," cleaving two critical HIV genes and rendering the virus ineffective. The process starts with peripheral stem cell harvest, in which patients are treated with granulocyte macrophage colony-stimulating factor to knock stem cells out of the bone marrow and into the circulation, from which they can be withdrawn.

A retrovirus vector is used to insert the ribozyme gene into the cells ex vivo, and then the modified cells are reinfused.

A Phase I trial launched in April 1997 confirmed that the concept was safe, but it failed to achieve the goal of engraftment of the cells into the bone marrow to produce HIV-resistant daughter cells.

"The bone marrow of these HIV-positive patients was intact," Rossi said. "There was no space for the reinfused immune cells containing the therapeutic gene to find a home. This realization prompted us to modify our strategy to prepare bone marrow to receive the reinfused cells."

Intensive chemotherapy is a well-known way to destroy the existing marrow, but such a drastic treatment would be indicated only for cancer patients. So the researchers have turned their attention to AIDS-related lymphoma - such patients already receive high-dose chemotherapy - and launched a new trial in November.

"What we really want to prove in this trial is that mobilized peripheral stem cells can be transduced with a gene and engraft," Rossi said. "If we were to see engraftment of these long-term progenitors ... this could be a lifetime treatment for [HIV] disease."

The data are dramatic in the two patients enrolled thus far. The number of cells carrying the ribozyme, as measured by blood samples, expanded by three orders of magnitude in one patient and by two orders of magnitude in the other. The trial will add another three patients as it moves forward.

"We're seeing long-term persistence of reinfused cells and evidence suggesting they are differentiating into multi-cell lineages, including T cells and monocytes," Rossi said.

Of course, he said, "the goal is to be able to do a stem-cell therapy in non-lymphoma patients."

Partial ablation offers a possible strategy for non-cancer AIDS patients, he suggested, noting that a Phase II trial to explore that technique is on the drawing board, even as the researchers continue to tweak the vector.

Approach May Conquer HIV Resistance

Despite the success of highly active antiretroviral therapy (HAART) in treating HIV infection, "thousands of patients will fail drug therapy," Rossi said. Moreover, "if there's a way to have a one-time treatment, that's a better prospect for patient welfare."

The ribozyme approach also offers a way of attacking multiple HIV sites, which might offer a means to combat the virus's notorious ability to develop resistance, he said.

Chiron Corp., of Emeryville, Calif., manufactured the vectors used in the initial studies, and Ribozyme Pharmaceuticals Inc., of Boulder, Colo., provided help with garnering the necessary FDA approvals for testing. The program, however, does not have a commercial development partner. Rossi holds the patent on the ribozymes.

While this still early-stage program has potential for treating HIV/AIDS with ribozymes, the basic concept holds much broader promise, Rossi noted.

"This is an important proof of concept, that genetically modified stem cells can engraft and produce long-term expansion." The result has implications for other diseases that potentially could be treated by modified stem cells, from leukemia to multiple sclerosis.