Optimizing radioimmunotherapy by matching dose distribution with tumor structure using 3D reconstructions of serial images

Research Spotlight - Radioimmunotherapy: optimizing delivery to solid tumors

Radioimmunotherapy (RIT) is a cancer treatment that exploits the specific targeting capability of monoclonal antibodies to deliver cytotoxic radionuclides to antigen-expressing tumor cells or stromal targets. While this has been extremely successful in the treatment of hematologic malignancies, RIT of solid tumors has produced less prolonged effects. In our laboratory, we have developed a bench-to-bedside translational pipeline with the aim of optimizing RIT for solid tumors. We will show how preclinical models of colorectal adenocarcinoma were initially used to study reciprocal interactions between elements of the tumor microenvironment and RIT and to test novel therapeutic strategies. These studies were then used to facilitate the design of novel trials carried out in close collaboration with our clinical colleagues.

Three-dimensional imaging-based radiobiological dosimetry

Targeted radionuclide therapy holds promise as a new treatment for cancer. Advances in imaging are making it possible for researchers to evaluate the spatial distribution of radioactivity in tumors and normal organs over time. Matched anatomical imaging, such as combined single-photon emission computed tomography/computed tomography and positron emission tomography/computed tomography, has also made it possible to obtain tissue density information in conjunction with the radioactivity distribution. Coupled with sophisticated iterative reconstruction algorithms, these advances have made it possible to perform highly patient-specific dosimetry that also incorporates radiobiological modeling. Such sophisticated dosimetry techniques are still in the research investigation phase. Given the attendant logistical and financial costs, a demonstrated improvement in patient care will be a prerequisite for the adoption of such highly-patient specific internal dosimetry methods.

Recombinant antibodies for cancer diagnosis and therapy

Recombinant antibodies currently represent over 30% of biopharmaceuticals in clinical trials, highlighted by the recent Food and Drug Administration (FDA) approvals of Zevalin(TM) (ibritumomab-tiuxetan; IDEC Pharmaceuticals, San Dieago, CA, USA) for cancer radioimmunotherapy and Humira(TM) (adalimumab; Abbott Laboratories, IL, USA) for rheumatoid arthritis. Together, these FDA approvals have excited the biotechnology industry, particularly since sales of recombinant antibodies are increasing rapidly to a predicted US dollar 4 billion per annum worldwide in 2003. To date, 10 engineered therapeutic antibodies have gained FDA approval and many others are in Phase III trials. Many recent FDA-approved antibodies are simple molecular designs that have taken 10 years to be developed into effective therapeutic reagents. Emerging new technologies have created a vast range of recombinant, antibody-based reagents, which specifically target clinical biomarkers of disease. Radiolabelling of antibodies has increased their potential for cancer imaging and targeting. Recombinant antibodies have also been reduced in size and rebuilt into multivalent molecules for higher affinity. In addition, antibodies have been fused with many molecules, including toxins, enzymes, drugs and viruses, for prodrug therapy, cancer treatment and gene delivery. Recombinant antibody technology has enabled clever manipulations in the construction of complex in vitro libraries for the selection of high-affinity reagents against refractory targets. Furthermore, innovative affinity maturation methods have been developed which enable rapid selection of extremely high-affinity reagents. This review focuses on developments in the last 12 months and describes the latest developments in the design, production and clinical use of recombinant antibodies for cancer diagnosis and therapy.

Synergy between vascular targeting agents and antibody-directed therapy

PURPOSE

Tumor heterogeneity necessitates the use of combined therapies. We have shown that combining antibody-directed therapy with antivascular agents converts a subcurative to a curative treatment. The purpose of this study was to investigate, by radioluminographic and microscopic techniques, the regional effects of the two complementary therapies.

METHODS AND MATERIALS

Nude mice bearing colorectal tumors were injected with 125I-labeled anti-carcinoembryonic antigen antibody, and images were obtained for antibody distribution and modeling studies using radioluminography. For therapy studies, the mice were given radioimmunotherapy alone (131I-A5B7 anti-carcinoembryonic antigen antibody), the antivascular agent combretastatin A-4 3-0-phosphate (200 mg/kg), or both. Extra mice were used to study the regional tumor effects of these therapies over time: relevant histochemical procedures were performed on tissue sections to obtain composite digital microscopic images of apoptosis, blood vessels, perfusion, hypoxia, and morphology.

RESULTS

Antibody distribution, modeling, and immunohistochemistry showed how radioimmunotherapy (7.4 MBq/40 microg antibody) effectively treated the outer, well-oxygenated tumor region only. Combretastatin A-4 3-0-phosphate treated the more hypoxic center, and in doing so altered the relationship between tumor parameters.

CONCLUSION

The combined complementary therapies produced cures by destroying tumor regions with different pathophysiologies. Relating these regional therapeutic effects to the relevant tumor parameters microscopically allows optimization of therapy and improved translation to clinical trials.

Spatial accuracy of 3D reconstructed radioluminographs of serial tissue sections and resultant absorbed dose estimates

Many agents using tumour-associated characteristics are deposited heterogeneously within tumour tissue. Consequently, tumour heterogeneity should be addressed when obtaining information on tumour biology or relating absorbed radiation dose to biological effect. We present a technique that enables radioluminographs of serial tumour sections to be reconstructed using automated computerized techniques, resulting in a three-dimensional map of the dose-rate distribution of a radiolabelled antibody. The purpose of this study is to assess the reconstruction accuracy. Furthermore, we estimate the potential error resulting from registration misalignment, for a range of beta-emitting radionuclides. We compare the actual dose-rate distribution with that obtained from the same activity distribution but with manually defined translational and rotational shifts. As expected, the error produced with the short-range 14C is much larger than that for the longer range 90Y; similarly values for the medium range 131I are between the two. Thus, the impact of registration inaccuracies is greater for short-range sources.