A novel nuclear medicine approach is showing great promise for precision treatment of solid tumors in many types of cancer—including lung, breast, pancreas, and ovarian in adults and glioma, neuroblastoma, and sarcoma in children. The research was presented yesterday at the SNMMI 2018 Annual Meeting, June 23-26 in Philadelphia.

In 2017, researchers from Memorial Sloan Kettering Cancer Center developed a novel approach to pretargeted radioimmunotherapy (DOTA-PRIT) that demonstrated pre-clinically, complete responses, including cures, in several solid tumor types using the beta-emitting lutetium-177 (¹⁷⁷Lu)-DOTA-hapten. In the research presented yesterday, the researchers expanded the DOTA-PRIT approach to actinium-225 (²²⁵Ac), an alpha-emitting isotope.

“Targeted alpha radiotherapy has shown considerable promise for patients, especially for those with advanced castration-resistance prostate cancer,” said Steven M. Larson, MD, and Sarah M. Cheal, PhD, of the Program in Molecular Pharmacology and Department of Radiology at Memorial Sloan Kettering Cancer Center in New York. “By combining DOTA-PRIT with ²²⁵Ac-proteus-DOTA hapten, we can potentially target a wide array of cancer types for which we have validated DOTA-PRIT bispecific antibodies (GD2-expressing, HER2-expressing, and GPA33-expressing cancers).”

DOTA PRIT has a major advantage over other forms of radioimmunotherapy because of its very high ability to deliver radiation to tumors, while sparing normal tissues, such as kidney and bone marrow. Larson and Cheal explained, “Creating a targeting alpha radiohapten greatly expands the potential for killing small nests of cells and even single cancer cells, which is likely to be important early in the course of metastatic spread.”

Researchers synthesized proteus-DOTA, radiolabeled it with ²²⁵Ac, and conducted in vitro and in vivo studies of a mouse model with colorectal cancer to determine if pretargeting the tumor with ²²⁵Ac-proteus-DOTA hapten was feasible. A toxicity study was performed in normal tumor-free athymic nude mice with varying dose levels of ²²⁵Ac-proteus-DOTA given as a single IV injection. Mice were monitored daily for 145 days postinjection and weighed up to twice weekly for evidence of treatment-induced toxicity.

Researchers found that their new approach, ²²⁵Ac-proteus-DOTA, mimics the behavior of the previously developed approach, ¹⁷⁷Lu-DOTA-hapten, with high tumor uptake, minimal accumulation in normal tissue, good whole-body clearance, no acute toxicity, and no chronic radiation damage. In addition, the new approach offers greater versatility of treatment for a wide variety of solid tumors and clinical situations.

Larson and Cheal pointed out, “When solid tumors spread beyond surgical control, they are the deadliest tumors for cancer patients. Using the DOTA-PRIT approach, we hope to greatly expand the potential of delivering precision radioimmunotherapy to human solid tumors.”