Comparative Radiolabelling of PSMA I&T and Somatostatin Analogues with ¹⁷⁷Lu and ⁹⁰Y: The Impact of SPE Purification

Technical Summary: Comparative Radiolabelling of PSMA I&T and Somatostatin Analogues with ¹⁷⁷Lu and ⁹⁰Y

Problem Statement
The increasing demand for radiometal-based radiopharmaceuticals for radioligand therapy (RLT), such as [¹⁷⁷Lu]Lu-PSMA I&T, [¹⁷⁷Lu]Lu-DOTA-TATE, and [⁹⁰Y]Y-DOTA-TOC, necessitates frequent, high-volume production. While automated synthesis modules have improved reproducibility and radiation safety compared to manual methods, a divergence exists in production protocols regarding post-synthesis purification. Some commercial systems incorporate a Solid Phase Extraction (SPE) step (typically C-18) to remove unbound radiometals, while others formulate the product directly after radiolabelling. The necessity of this purification step for ensuring product quality, particularly regarding radiochemical purity (RCP), ethanol content, and the formation of radiolytic byproducts, requires systematic evaluation to optimize production workflows without compromising regulatory compliance.

Methodology
The study conducted a comparative evaluation of two fully automated, cassette-based synthesis processes on an Eckert & Ziegler Modular-Lab PharmTracer module. Three radiopharmaceuticals were produced:

1. [¹⁷⁷Lu]Lu-PSMA I&T
2. [¹⁷⁷Lu]Lu-DOTA-TATE
3. [⁹⁰Y]Y-DOTA-TOC

Two production workflows were compared:

• With SPE: Utilizing commercially available cassettes with a C-18 Sep-Pak® Plus Light cartridge to purify the product post-incubation.
• Without SPE: Utilizing custom-made cassettes designed to bypass the SPE step, transferring the crude product directly to the final vial via sterile filtration.

Both methods employed identical radiolabelling conditions (specific temperatures and times for each tracer) and final formulation steps, including the addition of ascorbic acid and DTPA. The study analyzed 80 batches from clinical routine (20–30 batches per radiopharmaceutical), covering low, medium, and high starting activity levels.

Quality control (QC) was performed using:

• RP-HPLC: To determine radiochemical purity (RCP) and identify radiolytic side products.
• iTLC: To quantify unbound radiometal species using sodium citrate and ammonium acetate/methanol mobile phases.
• Gas Chromatography (GC): To measure ethanol concentrations in the final product.

Statistical analysis was conducted using the Mann–Whitney U test, with significance set at p < 0.05.

Key Contributions and Results
The study provided a direct, data-driven comparison of purification versus non-purification workflows for three distinct RLT agents.

• Radiochemical Purity (RCP):

  • [¹⁷⁷Lu]Lu-PSMA I&T: A statistically significant difference was observed; batches produced without SPE exhibited slightly higher RCP than those with SPE. Specifically, the SPE process was associated with a slight increase in a radiolytic side product (de-iodination product) visible at a retention time of ~6.0 minutes, whereas this product was significantly reduced when using the production method without SPE purification. Despite this, all batches met the pharmacopeial limit of 95% RCP.
  • [¹⁷⁷Lu]Lu-DOTA-TATE & [⁹⁰Y]Y-DOTA-TOC: No significant differences in RCP were found between the two methods. All batches met the 95% release specification. A minor, uncharacterized peak (0.5% impurity) was noted in non-SPE [⁹⁰Y]Y-DOTA-TOC batches but did not affect compliance.

• Unbound Radiometal:

  • iTLC analysis confirmed that unbound radiometal levels were negligible (<0.2%) across all batches and methods, regardless of the presence of SPE.

• Ethanol Concentration:

  • A significant difference was found in ethanol content. The SPE method resulted in higher ethanol concentrations (approx. 2.3–2.6%) due to the elution step with 50% ethanol. The non-SPE method resulted in significantly lower ethanol levels (0.14–0.50%).

• Activity Yield:

  • Yields were comparable, with non-SPE methods showing marginally higher mean yields (e.g., 94.6% vs. 92.7% for PSMA I&T), though all batches were within acceptable ranges.

Significance and Claims
The authors conclude that, within their specific setting and for the three radiotracers evaluated, the omission of the SPE purification step did not lead to significant changes in the quality of the final products. All batches produced without SPE met pharmacopeial and internal release specifications.

The paper claims that product purification is not universally required for all automated radiopharmaceutical production procedures involving radioligand therapy. However, the authors maintain a modest and cautious stance, emphasizing that the decision to omit purification must be based on a case-by-case evaluation for each specific synthesis module and radiopharmaceutical. The study suggests that while SPE can be omitted to simplify workflows and reduce ethanol content, the potential for specific side reactions (as seen with PSMA I&T) must be monitored during validation.