Sn-117m-DTPA, also known as Pentetate stannic Sn-117m, is an emerging radiopharmaceutical being studied for its potential use in treating painful bone metastases, particularly in patients with metastatic castration-resistant prostate cancer (mCRPC). This agent combines diagnostic imaging and targeted radiotherapy, a combined approach referred to as “theranostics.”

Sn-117m-DTPA includes the chelating agent DTPA, which directs the radioactive tin isotope (Sn-117m) to specific areas in the body. It has established applications in diagnostic imaging, including functional studies of organs such as the kidneys. Its gamma photon emission (158.6–159 keV) allows detection with standard gamma cameras, enabling clinicians to monitor its distribution and confirm uptake in bone lesions.

In addition to its imaging function, Sn-117m-DTPA emits low-energy conversion electrons and Auger electrons when localized in bone. These emissions have a short tissue penetration range (0.2–0.3 mm), concentrating radiation in the immediate vicinity of the target area. This delivery mechanism may reduce radiation exposure to surrounding tissues, including the bone marrow, and has been associated with low rates of myelotoxicity in early studies.

Ongoing clinical trials are evaluating its safety and therapeutic potential. A Phase II trial (NCT04616547) is investigating its effectiveness in relieving bone pain in 25 patients with mCRPC. Separately, a Phase I trial (NCT06982222) is assessing safety, dosing, and anti-tumor activity in patients with advanced prostate, breast, or non-small cell lung cancers with bone metastases.

Data from previous Phase I/II trials involving over 120 patients have reported pain relief in a majority of participants, with approximately 30% experiencing complete pain resolution. Pain relief typically occurred within a few days of administration and lasted for over three months in some cases. Adverse events were infrequent, with minimal bone marrow toxicity observed.

In simple terms, research in this case is moving from using a palliative agent to relieve bone metastasis pain to developing one that may actually help fight the metastases themselves.

Clinical trial 1. (terminated due to low drug supply, but the production has been improved now)

Clinical trial 2.