Philadelphia, PA - A blood test that measures the activity of genes related to allograft rejection could potentially help heart-transplant patients avoid some endomyocardial biopsies and improve their clinicians' ability to make management decisions, suggested several analyses presented here recently at the International Society for Heart and Lung Transplantation (ISHLT) 2005 annual meeting.

Earlier research had shown a high correlation between results of the AlloMap™ "molecular-expression" blood test (XDx, South San Francisco, CA) and histologic rejection based on endomyocardial biopsy, an invasive procedure with an infection risk that poses a threat to immune-suppressed organ recipients.

I think this will pan out as a huge advance. We may be able to decrease the number of biopsies dramatically.

The new studies fine-tune the recently introduced assay's potential and show how it might augment or improve on biopsy in the context of contemporary heart-transplant programs, where predicting and managing organ rejection can be imprecise and highly variable. The AlloMap test assesses the expression of 20 genes, about half of which are directly involved in rejection while the remainder provide other information needed for risk assessment. It yields a score that reflects the degree to which the body has mobilized the cellular and biochemical agents that cause allograft rejection, potentially before the process can be detected histologically.

Data from each of the analyses derive from the Cardiac Allograft Rejection Gene Expression Observational Study (CARGO) database, encompassing more than 600 transplant recipients at eight US centers, parts of which were presented at last year's ISHLT meeting and reported by heartwire.

Dr Howard J Eisen

"I think this will pan out as a huge advance. We may be able to decrease the number of biopsies dramatically," Dr Howard J Eisen (Drexel University, Philadelphia, PA) told heartwire. The AlloMap test could allow "more accurate and frequent assessment of immune activation" and perhaps forecast episodes of rejection and guide antirejection drug therapy, he said.

He pointed to a study at the recent ISHLT meeting that illustrated the imprecise nature of conventional histologic testing for rejection and suggested that pairing it with the AlloMap test could potentially sharpen the evaluation, especially in more equivocal cases.

That investigation, from Dr Daniel Bernstein (Stanford University, CA) and colleagues, focused on the AlloMap test's ability to differentiate mild rejection, for which histologic findings may be the least accurate. Most of the time, their data suggest, pathologists will be unable to agree on whether a specimen indicates mild or more advanced rejection, while AlloMap testing may be more precise.

For each of the >5000 biopsy specimens obtained from 650 adults and 105 children, the assessment of the local pathologist at each of eight CARGO transplant centers was compared with the consensus of a centralized three-member panel of pathologists who were blinded to all clinical data. The two diagnostic judgments disagreed on the presence of mild rejection in 77% of cases, virtually all because the centralized pathologists reclassified the initial decision of mild rejection as more advanced.

The implication is that biopsy diagnosis is very subjective and may be inaccurate, and we may be overtreating people.

On the other hand, AlloMap testing identified a subgroup of biopsies judged histologically as showing mild rejection that had a gene-expression profile nearly identical to those the pathologists considered quiescent. The other histologically mild cases showed a significantly greater AlloMap score, indicating increased rejection, which was similar to scores for histologically advanced rejection. The findings were similar for adults and children.

"The implication is that biopsy diagnosis is very subjective and may be inaccurate, and we may be overtreating people," said Eisen, whose own research team helped to define how the AlloMap test might be used to improve rejection assessments and help guide management.

They followed the progress of 50 CARGO patients using the AlloMap test plus biopsy and the range of standard hemodynamic, laboratory, and clinical evaluations and compared their observations of six patients who did and six who did not experience significant rejection episodes. Over a mean 16-month follow-up, the group found that:

• Baseline AlloMap scores for stable heart recipients can vary widely across a group but remain consistent within individuals.
• Significant rejection episodes and often clinical events are both preceded and followed by changes in the AlloMap test score.
• AlloMap scores reflecting increased rejection-related gene expression usually precede rejection episodes.

The last observation suggests that the assay can warn of an impending episode of rejection that warrants steroid therapy before it can be seen histologically, according to the group. In addition, Eisen said when interviewed, the test might show how patients respond to steroid therapy and potentially—by titrating the drugs according to serial assay results—guide dose reductions to the lowest effective level or until they can be withdrawn completely.

Dr Randall C Starling (Source: Cleveland Clinic)

That strategy's underpinnings were explored elsewhere at the ISHLT sessions by Dr Randall C Starling (Cleveland Clinic, OH) and colleagues, who confirmed that AlloMap indeed has the potential for monitoring physiologic responses to steroid weaning. Among 124 allograft recipients, serial test scores for the 62 patients who developed rejection according to standard criteria were, not surprisingly, consistently and significantly higher (p=0.009) than those for the 122 who were histologically quiescent. But the investigators observed that the expression of only five genes was significantly correlated with histologic responses to steroid therapy. The functions those genes controlled included T-cell regulation and migration, hematopoiesis, and inhibition of cytokine signaling. Changes in steroid dose were also linearly and inversely correlated with AlloMap scores: every 10-mg increase corresponded to a 2.5-point score reduction.

Traditionally, steroid immunosuppression has been "one size fits all" unless frequent biopsies are obtained to help tailor therapy, an uncommon and risky strategy, Starling told heartwire, saying patients therefore tend to be "overimmunosuppressed."

"There has been no way to push the envelope and reduce the immunosuppression to the lowest possible dose," according to Starling, who said a tool that can guide steroid doses to the lowest effective level would theoretically reduce the risk of infection and other long-term complications. That's one of the "holy grails" of transplant medicine, he said, and a role that the AlloMap test could potentially fill.

Starling noted the assay does have limitations. For example, it's not effective at monitoring rejection within the first six months of transplantation, and "we don't really understand yet what a high AlloMap score might mean in the setting of no histologic rejection," he said. "We're in the early exploratory phases with this test."