Combining radionuclide therapy with a PARP inhibitor slows neuroendocrine tumor growth

The Canadian study evaluated lutetium-177 (¹⁷⁷Lu)-octreotate PRRT, which has been found effective for slowing the growth of malignant peptide-receptor positive NETs. The challenge is that damaged cells can naturally repair their DNA, which limits the effectiveness of PRRT. By blocking a protein called poly(ADP-ribose) polymerase, or PARP, scientists can effectively override this process and induce more cell death -- the objective being fewer and smaller tumors and longer lives for neuroendocrine cancer patients.

"The use of PARP inhibitors is a promising approach to enhance targeted radionuclide therapy for neuroendocrine cancer," said Samuel Adant, a member of the research team from the CHU de Québec -- Université Laval Research Center in Quebec City, Quebec, Canada. "Furthermore, PARP inhibitors could enhance many other current and emerging radionuclide therapies."

The researchers used 3D spheroid models (proven superior for replicating human tumor growth), rather than 2D monolayer culture, for two separate human NET cell lines -- one of gastroenteropancreatic origin and one of bronchopulmonary origin. These models were dosed for five days and then monitored for 15 days. Results showed that more NET cancer cells died when PRRT and the PARP inhibitor were used together than when they were used separately. The combined treatment led to significantly inhibited cell proliferation, and it increased programmed cell death.

In the gastroenteropancreatic cell line, untreated control spheres increased in size by 16.7 times their original volume within 15 days. Spheres treated with ¹⁷⁷Lu-octreotate alone grew by more than 5-fold their original volume, and those treated with only the PARP inhibitor grew 11-fold. Those with the combined treatment grew by only 1.9-fold. The bronchopulmonary cell line revealed 11.4-fold growth for untreated controls, 4.6-fold growth for spheres treated with PRRT alone, 3.5-fold growth for those treated with only the PARP inhibitor, and 2.8-fold growth for those treated with both PRRT and the PARP inhibitor.

This method of combining ¹⁷⁷Lu-octreotate and PARP inhibitors could be translated readily for improved management of peptide-receptor positive neuroendocrine cancer, because both drugs are already approved for use in cancer patients.

Link to the abstract: http://jnm.snmjournals.org/content/58/supplement_1/243.abstract?sid=acd3e42e-6163-419c-9c38-c7a09d823b4b