Muscarinic inhibition of salivary glands with glycopyrronium bromide does not reduce the uptake of PSMA-ligands or radioiodine

Rationale Salivary glands are highly perfused and express the prostate-specific membrane antigen (PSMA) receptor as well as the sodium—iodide symporter. As a consequence, treatment with 177 Lu/ 225 Ac-PSMA for prostate cancer or 131 I for thyroid cancer leads to a high radiation dose in the salivary glands, and patients can be confronted with persistent xerostomia and reduced quality of life. Salivation can be inhibited using an antimuscarinic pharmaceutical, such as glycopyrronium bromide (GPB), which may also reduce perfusion. The primary objective of this work was to determine if inhibition with GPB could provide a considerable (> 30%) reduction in the accumulation of administered 123 I or 68 Ga-PSMA-11 in salivary glands. Methods Ten patients who already received a whole-body 68 Ga-PSMA-11 PET/CT scan for (re)staging of prostate cancer underwent a repeat PET/CT scan with tracer administration at 90 min after intravenous injection of 0.2 mg GPB. Four patients in follow-up after thyroid cancer, who had been treated with one round of ablative 131 I therapy with curative intent and had no signs of recurrence, received 123 I planar scintigraphy at 4 h after tracer administration without GPB and a repeated scan at least one week later, with tracer administration at 30 min after intramuscular injection of 0.4 mg GPB. Tracer uptake in the salivary glands was quantified on PET and scintigraphy, respectively, and values with and without GPB were compared. Results No significant difference in PSMA uptake in the salivary glands was seen without or with GPB (Mean SUL mean parotid glands control 5.57, intervention 5.72, p = 0.50. Mean SUL mean submandibular glands control 6.25, intervention 5.89, p = 0.12). Three out of 4 patients showed increased 123 I uptake in the salivary glands after GPB (Mean counts per pixel control 8.60, intervention 11.46). Conclusion Muscarinic inhibition of salivation with GPB did not significantly reduce the uptake of PSMA-ligands or radioiodine in salivary glands, and can be dismissed as a potential strategy to reduce toxicity from radionuclide therapies.