⁶⁸Ga-PSMA and ¹⁷⁷Lu-PSMA of high specific activity for targeted diagnosis and therapy of prostate cancer in patients

Background: Prostate-specific membrane antigen (PSMA) is a membrane enzyme expressed in nearly all prostate cancers with increased expression in poorly differentiated, metastatic and hormone-refractory carcinomas. The urea-based inhibitors of PSMA exhibiting Glu-NH-CO-NH-Lys are excellent pharmacophores to bind this enzyme. Its conjugation to HBED-CC or DOTA and its labelling with ⁶⁸Ga or ¹⁷⁷Lu allower to obtain radiopharmaceuticals for PET/CT diagnosis or β^- emitter therapy.

Objectives: Development of a pair of radiopharmaceuticals with high specific activity (SA) and radiochemical purity (RP), preserving high recognition by its target (PSMA) suitable for diagnotherapy. This includes: radiolabelling optimization of Glu-NH-CO-NH-Lys(Ahx)-HBED-CC with ⁶⁸Ga (⁶⁸Ga-PSMA), and PSMA-617 (2-[3-(1-Carboxy-5-{3-naphthalen-2-yl-2-[(4-{[2-(4,7,10-tris-carboxymethyl-1,4,7,10-tetraaza-cyclododec-1-yl)-acetylamino]-methyl}-cyclohexanecarbonyl)-amino] -propionylamino}-pentyl)-ureido]-pentanedioic acid)-DOTA with ¹⁷⁷Lu (¹⁷⁷Lu-PSMA), stability determination, evaluation of the RP identifying the main potential impurities, and binding studies with cells expressing PSMA. To finally develop the production under GMP, with maximum specific activity (SA) for use in patients.

Methods: Labelling of Glu-NH-CO-NH-Lys(Ahx)-HBED-CC with ⁶⁸GaCl₃ (360MBq in 1000µL HCl 0.05M) was optimized for 0.04 to 1.25µg (0.04-1.32nmol) of precursor. The pH was adjusted to 4.0 with 250µL NaOAc 0.25M, the mixture was incubated at 100ºC for 5 min and purified by Sep-Pak-C18-light. Alternatively, generator eluate was purified and concentrated by anionic method (PS-HCO3), pH was adjusted to 4.0 with NaOAc 2.5M and precursor was added and incubated for 5 min at 100ºC. RP was controlled by RP-HPLC and ITLC-SG. The scaling up production of ⁶⁸Ga-PSMA for patients was done with a microfluidics system (ITG) inside a shielded laminar flow hood Class A. Labelling of DOTA-PSMA-617 with ¹⁷⁷LuCl₃ nca (345MBq) was optimised using 0.56-4.46nmol at pH:5.0 (NaOAc and gentisic acid), incubated at 90ºC or 100ºC for 5-15 minutes. RP and stability was evaluated by RP-HPLC and ITLC-SG up to 8 days. ¹⁷⁷LuCl₃ with lower radionuclide SA was also evaluated. Binding to LnCAP cells was done, and maximum binding and internalisation was determined.

Results: ⁶⁸Ga-PSMA maximum SA was 1171MBq/nmol. For patients production 1131±29MBq of ⁶⁸Ga and 2,64nmol of precursor were used. A global yield of 90±1% (ndc), a RP of 97±1% and a SA of 418±15MBq/nmol were obtained. ¹⁷⁷Lu-PSMA maximum SA was 292MBq/nmol. For patients production 372±18MBq of ¹⁷⁷Lu and 2,9nmol of precursor were used. A global yield of 88±3% (ndc), a RP>99.9% and a SA of 128±6 MBq/nmol were obtained. Maximum binding to LNCaP cells was higher than 48% while internalization was higher than 52%.

Conclusion: The high specific activities required for the clinical application of these radiopharmaceuticals for prostate cancer diagnosis and therapy require careful control of the ratio activity/precursor as well as incubation temperature and time. ⁶⁸Ga-PSMA of high RP and SA is being used in patients at our Centre with excellent results. ¹⁷⁷Lu-PSMA is ready to start a pilot study with patients.