Development of alpha particle emitting radionuclide therapy according to GMP guidelines for treatment of mCRPC

Introduction: Patients with metastatic castration resistant prostate cancer (mCRPC) have a poor prognosis. Recently, promising results of antitumor effects were observed in patients with mCRPC treated with Lutetium-177 linked to PSMA. Efficacy of radionuclide therapy might improve with the alpha emitting radionuclide actinium-225 (Ac-225). Ac-225 surfaced as a candidate radionuclide for targeted alpha-particle therapy (TAT) due to the preferable characteristics regarding high LET energies, deposit of the energy at a close range and an appropriate half-life for treatment (T½=9.92 days). Therefore, [²²⁵Ac]Ac-PSMA-I&T emerged as a possible  effective treatment of mCRPC.
Objectives: The aim of this study is to manufacture [²²⁵Ac] Ac-PSMA-I&T according to Good Manufacturing Practice (GMP) guidelines for the translation of [²²⁵Ac]Ac-PSMA-I&T (8-12 MBq) into the clinic, for a phase I dose escalation study.
Methods: Labelling of PSMA-I&T with Ac-225 was optimized to ensure stability and release criteria. Labelling solution before heating consisted of 15 MBq Ac-225 (~25L 0.1M HCl), 180g/300L PSMAI&T in 0.1M TRIS buffer (pH 9) and 180L ultrapure water. After labelling, 500L ascorbate (pH5.8) and 15 L 4mg/mL DTPA were
added. For first labelling, SPE Sep-pak C₁₈ purification was performed to identify and quantify radiolysed PSMA-I&T. Quencher addition (1M ascorbate (pH5.8) 0-100L) during labelling was evaluated. Quality control of [²²⁵Ac]Ac-PSMA-I&T is based on measurement of Fransium-221 (Fr-221, 218KeV), which forms an equilibrium with Ac-225 in ~6 T½ of Fr-221 (4.8 min). Detection methods of Ac-225 (Fr-221) are validated according to EANM guidelines. Radio-(I)TLC methods are utilized for identification of different chemical forms. Separation methods; sodium citrate (0.5M pH5) and acetonitrile/water (50:50v/v) are evaluated. Radio-(I)TLC methods  identified free Ac-225, [²²⁵Ac]Ac-DTPA and [²²⁵Ac]Ac-DTPA (0.5M sodium citrate, pH5). The gamma counter is used for concentration quantification (kBq/L) and HPLC fractions measurement. RCY results are confirmed
by HPGe-detector. RCP is determined by HPLC. [²²⁵Ac]Ac-PSMA-I&T for injection is prepared by addition of an equal ratio quencher as after stabilization, addition of ethanol (5% final concentration) and saline solution, followed by sterile filtration. Final patient dose is confirmed by dose calibrator.
Results: SPE Sep-pak C₁₈ purification confirmed presence of radiolysed [²²⁵Ac]Ac-PSMA-I&T (RCP ~82%). After introduction of quencher (100L 1M ascorbate, pH5.8) during labelling, stability increased (RCP >90%). Quality control radio-(I)TLC (Optimal separation method sodium citrate (0.5M, pH 5)), resulted in a RCY >95%, which is confirmed by HPGe detector. Stability could be maintained with a RCP >90% up to 3 hours after labelling (HPLC). After dilution into saline, stability decreased rapidly. Therefore, dilution of labelling solution for injection was prepared by addition equal ratio quencher as after stabilization. [²²⁵Ac]Ac-PSMA-I&T (8-12 MBq) solution for injection is produced, containing ~100 g PSMA-I&T per dose.
Conclusion: [²²⁵Ac]Ac-PSMA-I&T has been produced according to GMP guidelines, conform the release criteria: RCY > 95% and RCP > 90% up to 3 hours after synthesis. The final patient dose [²²⁵Ac] Ac-PSMA-I&T (8-12 MBq, 5 mL) allows the start of the phase I dose escalation study, which is planned in the near future.