Erbium-169 (169Er)

Properties and Characteristics

Erbium-169 (¹⁶⁹Er) is a radioactive isotope that emits both gamma and beta radiation. It has a half-life of approximately 9.4 days, during which it releases gamma rays with an energy of 8 keV (0.16% emission probability) and beta particles at energies of 351 keV (55% intensity) and 343 keV (45% intensity).

Manufacturing Process

¹⁶⁹Er is synthesized by exposing Erbium-168 (¹⁶⁸Er) to thermal neutrons, resulting in the nuclear reaction ¹⁶⁸Er(n,γ)¹⁶⁹Er. Due to this production method, it is not feasible to obtain the isotope in a carrier-free form.

Source and Availability

The production of ¹⁶⁹Er is geographically constrained, with a single irradiation facility located in Russia. The demand for this radionuclide remains minimal, and competition within this market is virtually nonexistent. However, the production of ¹⁶⁹Er faces significant challenges due to the dwindling availability of the precursor ¹⁶⁸Er. The scarcity of ¹⁶⁸Er has led to a sharp increase in its cost, further complicating the production process.

Applications and Derivatives

¹⁶⁹Er has limited practical applications, with its primary use being in radiosynovectomy—a procedure for treating joint disorders, particularly in small finger joints. This application is prevalent in parts of Europe and certain Asian countries, such as the Philippines, where ¹⁶⁹Er is administered in the form of Erbium citrate. The average dosage used per joint is less than 1 mCi.

Pricing

The bulk cost of ¹⁶⁹Er is currently estimated at approximately EUR 10 per mCi (equivalent to around USD 13 per mCi). However, this price is expected to rise significantly in the coming years due to the limited availability of ¹⁶⁸Er.

Challenges and Considerations

Several issues limit the widespread adoption of ¹⁶⁹Er:

1. Limited Supply: The isotope is only available as a carrier-added bulk product, with no option for a carrier-free form.
2. Application Constraints: Its primary use is restricted to localized injections.
3. Physical Limitations: The combination of a long half-life, low energy emissions, and very low specific activity makes ¹⁶⁹Er unsuitable for therapeutic applications involving labeled molecules. There are no known development programs exploring the use of ¹⁶⁹Er in molecular labeling for therapy.
4. Precursor Scarcity: The dwindling availability of ¹⁶⁸Er, along with its rising cost, poses a severe threat to the future production of ¹⁶⁹Er.

Alternative Solutions

Given the challenges associated with ¹⁶⁹Er, alternative radionuclides are being considered for radiosynovectomy applications. Companies such as Clear Vascular and R-NAV (now known as Serene) have proposed the use of Tin-117m (^117mSn) as a potential substitute for ¹⁶⁹Er, offering a more viable option in light of the current production and economic hurdles.