Building on the Promise of Radioimmunotherapy
Building on the Promise of Radioimmunotherapy
Radioimmunotherapy (RIT) is a method of targeted therapy that utilizes a monoclonal antibody as a radionuclide carrier to deliver high-energy radiation over a short path length to the tumor cell while sparing the normal tissues. From a patient's perspective, RIT is an ideal therapy. The treatment is delivered on an outpatient basis with few immediate side effects beyond the length of the infusion itself. The entire RIT treatment program is complete in 1 week, allowing a rapid return to work and daily activities. The response rate to treatment is high and can be durable. Because RIT does not produce alopecia, there is no outward sign that the patient has recently been in treatment. The only side effect is delayed myelosuppression, which is usually manageable, again through outpatient treatment.
Dr. Schaefer-Cutillo et al have provided an excellent and comprehensive review of the clinical development of the first two US Food and Drug Administration (FDA)-approved radioimmunoconjugates for the treatment of CD20+ B-cell non-Hodgkin's lymphoma (NHL). They also provide an overview of where the field of radioimmunotherapy is heading.
The development of radioimmunotherapy in the 1990s somewhat paralleled the development of the unlabeled human chimeric anti-CD20 antibody rituximab (Rituxan). Rituximab was FDA-approved in 1997 and was in wide use by the time the FDA approved ibritumomab tiuxetan (Zevalin) in 2002 and tositumomab/iodine-131 (131I) tositumomab (Bexxar) in 2003. Rituximab and the two RIT agents are clearly the most important new agents for the treatment of NHL and appear to be having an impact on survival.[2,3] The clinical trials that were designed to demonstrate the efficacy and toxicity of yttrium-90 (90Y)-ibritumomab tiuxetan and 131I-tositumomab were performed in a rational and methodical manner in order to maximize safety and to address the relevant questions of that time. These trials for the most part were focused on patients who had failed other therapies (and, in some cases, rituximab).
As outlined in the review by Schaefer-Cutillo et al, in relapsed follicular NHL, the overall response rate to RIT is approximately 80% with 30% complete remissions, and approximately 20% of patients are achieving a very long progression-free survival. It is gratifying to observe some of these patients still free of relapse 10 years out from treatment. However, as might be expected, most patients have required additional therapy and have relapsed, as these trials focused on heavily pretreated relapsed patients.
How can this highly active and well-tolerated therapy be further utilized to enhance antitumor response and prolong survival? When dealing with patients with relapsed indolent NHL, physicians must take many factors into consideration before choosing treatment, such as the patient's age, performance status, and other comorbidities. In general, when patients relapse they should be treated with the next most effective therapy associated with the fewest side effects. RIT should be considered early in the treatment sequence for the relapsed patient when the disease is nonbulky and the marrow reserve is adequate. It is clear that if RIT is reserved until the patient is very heavily pretreated, the marrow may be compromised with involvement of lymphoma or the platelet count may be too low for RIT.
Because of the high response rates in relapsed patients and the fact that RIT works optimally in patients without bulky disease, trials have been designed that use RIT as adjuvant therapy after chemotherapy or chemo-immunotherapy. The key ongoing trials in Europe and the United States randomize patients with either indolent or aggressive NHL to adjuvant RIT after the completion of standard therapy. If these studies demonstrate a prolongation of disease-free or overall survival, then the use of RIT will move from end-stage to upfront, resulting in a new standard of care for patients and more widespread use of RIT.
The study by Kaminski et al in previously untreated patients has provided evidence that some patients who achieve a complete response with RIT alone can have prolonged disease-free survival, whereas patients who only achieve a partial response will relapse rather promptly. RIT as a single-agent therapy is therefore an option for untreated patients with low-bulk, asymptomatic disease who choose not to be observed but also want to avoid chemotherapy.