A new pretargeted radioimmunotherapy (PRIT) technique has been shown to be safe and effective in eradicating tumours from a preclinical colorectal cancer model.

The multi-step theranostic approach delivers alpha-emitting radiation directly to tumours while limiting exposure to healthy tissues.

This research was published in the May issue of The Journal of Nuclear Medicine.

Radiopharmaceutical therapy, especially with molecularly targeted alpha-emitting radionuclides, is proving to be transformative in oncology.

Among the most promising is the matched ²⁰³Pb/²¹²Pb theranostic pair.

The toxic effects of ²¹²Pb on the kidneys, however, remain a concern.

"With the toxicity issues that come along with Pb-based radionuclides, PRIT can be especially helpful in making sure the radiation only goes where it is needed,"  said Sarah M.

Cheal, PhD, assistant professor at Weill Cornell Medicine in New York.

"In this study, my colleagues and I developed a novel PRIT approach for colorectal cancer, assessed its biodistribution, and tested various treatment regimens to see what was most effective."

The PRIT approach targeted GPA33, an antigen that is overexpressed in 95 percent of colorectal tumours.

Researchers first performed serial biodistribution and SPECT/CT imaging studies of GPA33-targeted ²⁰³Pb/²¹²Pb-DOTA-based PRIT to assess feasibility.

Once confirmed, dosimetry was evaluated and various treatment regimens were evaluated in mice bearing human colorectal cancer tumours.

Researchers established that two consecutive doses separated by 48 hours led to prolonged survival--including histologic cures in three out of five mice in the cohort.

Mice in this treatment group exhibited normal bone marrow and overall preserved kidney function.

"This PRIT approach produced a combination of effective tumour targeting with minimal toxicity, demonstrating a highly favourable therapeutic window for curative radioimmunotherapy," said Nai-Kong V.

Cheung, MD, PhD, Enid A. Haupt Chair in Paediatric Oncology at Memorial Sloan Kettering Cancer Centre in New York.

"We know there is a significant unmet need for more effective and low-toxicity therapies for advanced colorectal cancer. This study represents an early but important step towards developing a radiopharmaceutical strategy that can meet that need."

"What's more,"  he said, "because this platform is modular, it can be adapted to many different tumour targets. What is learned for one target could be exploited for others, either for the same tumour, or across a wider range of cancers. The ²⁰³/²¹²Pb theranostic pairing is especially promising, and our findings support its emerging potential in nuclear medicine and precision oncology."

Source: Society of Nuclear Medicine and Molecular Imaging