February 1, 2007 (Vol. 27, No. 3)
Aldehyde Dehydrogenase Marks the Cells that Respond to Damage Signals
Stem cell therapy promises to provide replacement cells to rebuild damaged spinal cords and hearts and solutions to other serious disorders. Biotechnology companies are searching for ways to optimize adult stem cells obtained from human bone marrow or blood as a source of therapeutic cells. The approach at Aldagen (www.aldagen.com) starts with identifying adult stem cells that express high levels of the enzyme aldehyde dehydrogenase (ALDH), which is highly expressed in blood and other stem cells and serves as a marker for progenitor cells.
“No one knows exactly why, but the enzyme is a marker for active stem cells that respond to damage signals and repair tissues,” says Andrew Balber, Ph.D., vp of preclinical development at Aldagen. The company reports that there is evidence to suggest that ALDH may be involved in the production of retinoids that regulate gene expression.
Aldagen’s technology, licensed from Duke University, relies on a substrate that detects cells expressing high levels of ALDH by generating an intense green fluorescence. These so-called ALDH bright cells are isolated and sorted using three different products based on the same technology. The company’s Aldesort product, sold to laboratories that process stem cells for clinical applications, includes all the reagents to carry out the reaction in human bone marrow, peripheral blood, or cord blood. The green fluorescence is detected using a cell sorter that separates the ALDH bright cells from other cells.
Another product, Aldecount, is FDA-approved for in vitro diagnostics to identify and enumerate ALDH bright cells by flow cytometry. The product can be used with fresh or frozen human peripheral blood, leukapheresis, umbilical cord blood, and bone marrow.
Obtaining a Spectrum of Cell Types
Many stem cell studies rely on unfractionated stem cells, which are obtained from bone marrow, then centrifuged before injection. Alternatively, researchers focus on a particular lineage of stem cells that must be cultured for four to eight weeks to obtain large amounts for infusion. In contrast, the population of cells isolated with Aldesort contains a mixture of different cells, rather than a homogenous cell type. “We’re more versatile and capture more of the important players,” says Dr. Balber.
The spectrum of cell types captured by Aldesort depends on the starting tissue, such as cord blood, peripheral blood, or bone marrow. ALDH bright cells contain different progenitors that give rise to a variety of important cell lines, including neurological and endothelial cells. The progenitor cells also communicate with each other to produce a variety of cytokines that mobilize endogenous stem cells and control the activity of other cells in the population. “In one pot we have all the possible players that can regenerate tissues,” Dr. Balber adds.
The first clinical application of Aldesort is under way at Duke University Medical School. Joanne Kurtzberg, M.D., director of the Duke Pediatric Blood and Marrow Transplant Program, isolates ALDH bright cells with Aldesort from cord blood to treat children who have leukemia or who were born with genetic diseases. Dr. Kurtzberg has treated 14 patients in this study, and “the results are looking very encouraging,” says Thomas Amick, CEO of Aldagen.
At the Texas Heart Institute in Houston, Emerson Perin, M.D., and James Willerson, M.D., are injecting unfractionated bone marrow cells into patients’ hearts to repair damage from heart failure. In September 2006, they injected the first heart disease patient with ALDH bright cells to see if they more efficiently restore blood flow to damaged tissue. The pilot study, the first of its type in the U.S., according to the company, involves 10 patients.
Aldagen also has an approved IND to evaluate ALDH bright cells as a treatment for critical limb ischemia. Ten patients will receive injections of ALDH bright cells directly into muscle in the ischemic leg. Another 10 patients will receive bone marrow injections of unfractionated cells that are not processed with Aldesort. Patients will be monitored for safety and the ability of ALDH bright cells to reduce pain and increase exercise capacity.
Preclinical animal studies at Aldagen demonstrate that ALDH bright cells perform better than unfractionated cells. In animal models for angiogenesis, head-to-head comparisons of ALDH bright cells with unfractionated bone marrow cells show that Aldesort promotes more efficient healing. Although ALDH bright cells account for only about 1% of stem cells, Aldesort “pulls out concentrated cells that do the work, and they may get rid of other cells that get in the way of healing,” says Balber.
Cardiologists at Duke University Medical Center measure endothelial progenitor cells (EPC) with Aldecount. EPC are produced in bone marrow and travel to sites of arterial damage. Patients with lower levels of circulating EPC are less likely to repair cardiac damage, raising the risk for cardiac events. As few as one endothelial progenitor cell can occur in 10,000 blood cells, and the current methods for detection are subjective and time consuming. Aldecount offers a better method for identifying and measuring levels of EPC, according to Duke cardiologist Thomas Povsic, M.D. “EPC may be used to assess a patient’s degree of coronary artery disease or risk for suffering a heart attack.”
“We feel that we are heading in the right direction,” says Amick, “and the clinical work will back this up.” By focusing on ALDH bright cells, “which do everything, we ask them to do,” Amick adds that other applications for Aldagen’s technology will open up.