MI blazes critical path to speedy assessments

Molecular imaging will figure prominently in collaborations among public and private interests to modernize the drug development process and make new therapies available more quickly and at lower cost.

In March, the Critical Path Initiative, an FDA program created to refine the development process for medical products, released its initial priority list of projects. The list includes 76 projects. Nine of these involve use of modern imaging techniques to advance translational research. One of the nine imaging-related research priorities focuses on cancer applications. One of the first investigations within that priority will examine the ability of FDG-PET imaging to predict response to therapy in lymphoma patients. Persistent FDG uptake might identify patients with unresponsive disease early in their treatment and enable a switch to other more effective therapies.

Speaking at the Academy of Molecular Imaging meeting in March, Dr. Sandra Horning, president of the American College of Clinical Oncology, made note of the predictive value of PET scanning performed in the middle of treatment for non-Hodgkin's lymphoma (NHL). Cumulative data from multiple sources suggest PET imaging has prognostic value after as few as one or two cycles of therapy, reflected in a positive predictive value of 100% and a negative predictive value of 84%, she said.

While encouraging, the data offer no proof that switching from one therapy to another on the basis of PET imaging will improve patient outcomes.

"We need a randomized study in which PET-positive patients are assigned to continue with conventional therapy or cross over to a different treatment," she said.

The Eastern Cooperative Oncology Group based in Boston and Philadelphia has just begun such a trial. Patients will have PET scans after three cycles of rituximab-CHOP chemoimmunotherapy. Those with negative scans will continue treatment, while patients with positive scans will be switched to another therapy.

Several recent studies show that FDG-PET, whether performed at the end of therapy or in midcourse, can identify lymphoma patients whose disease is not responding to treatment. Belgian investigators examined the ability of end-of-treatment FDG-PET to predict relapse (J Clin Oncol 2001; 19:414-419). Of 67 patients who had normal PET scans after completion of initial chemotherapy, 56 remained in complete remission during median follow-up of 653 days. By contrast, 26 patients had persistent abnormal FDG uptake, and all relapsed. In 14 of the 26 patients, only PET imaging predicted the presence of persistent disease.

A study from the University of Iowa examined the impact of adding FDG-PET results to the International Workshop Criteria (IWC) for classification of treatment response (J Clin Oncol 2005;23:4652-4661). The study involved 54 patients with aggressive NHL treated with four to eight cycles of chemotherapy. FDG-PET imaging was performed one to 16 weeks after completion of therapy. Comparison of IWC alone and IWC with FDG-PET showed that the addition of PET led to a more accurate response classification.

More recently, clinical investigators have explored the ability of FDG-PET imaging to predict response to therapy before the end of the treatment. A group at Guy's and St. Thomas's Hospital in London performed FDG-PET studies after two or three cycles of chemotherapy in 121 patients with high-grade NHL (Ann Oncol 2005;16:1514-1523). Fifty patients had negative scans, 19 had minimal residual uptake (MRU) of FDG, and 52 had positive scans. Estimates of five-year survival correlated with PET results: 88.8% for the PET-negative patients, 59.3% for patients with MRU, and 16.2% for PET-positive patients.

Finally, a group at Memorial Sloan-Kettering Cancer Center in New York City evaluated the intensity of FDG uptake as a means of distinguishing between indolent and aggressive NHL (J Clin Oncol 2005;23:4643-4651). The results of FDG-PET studies in 97 patients showed that a standardized uptake value greater than 10 was associated with a high likelihood of aggressive disease.

In an accompanying editorial, Dr. Malik Juweid, a radiologist at the University of Iowa, and Dr. Bruce Cheson, an oncologist at Georgetown University, said more research is needed to determine the prognostic value of FDG-PET imaging in NHL. Juweid and Cheson noted, however, that the role of PET for monitoring tumor response is likely to increase once clinical trials demonstrate that information provided by PET imaging affects patient management and outcomes.