Targeted Radionuclide Therapy in Metastatic Prostate Cancer Using a New PSMA Ligand Radiolabelled With Terbium-161 (161Tb-SibuDAB) - Dose Identification/Escalation Phase Ia/b Study

Inclusion Criteria: * Consent form signed * Male patients with age \> 18 years * Clinical indication for RLT with 177Lu-PSMA-I\&T (progressive PSMA-positive mCRPC patients after androgen receptor signalling pathway inhibitor and taxan-based chemotherapy or patient unfit for chemotherapy) * Patients will be included in Phase Ia while being under active therapy with 177Lu-PSMA-I\&T (SoC) and after they have completed the first two cycles of 177Lu-PSMA-I\&T RLT * At least 3 measurable tumours on PSMA PET/CT (\>1.5 cm) with sufficiently intense PSMA uptake (SUVmax\>20) * ECOG Performance status: 0-1 * Blood parameters: a) Leucocytes ≥ 3 G/L; b) Haemoglobin ≥ 100 g/L; c) Thrombocytes ≥ 100 G/L * Estimated glomerular filtration rate (eGFR) \> 45 ml/min * Albumin \> 25 g/L * ALT, AST, AP: ≤ 5 times upper standard value * Bilirubin ≤ 2 times upper standard value * For male patients who are not surgically sterilized (orchiectomy or vasectomy), appropriate contraceptive measures must be taken during RLT and until 4 months after completion of RLT. As acceptable contraceptive count sexual abstinence or double contraceptive methods: hormonal contraceptive (oral, transdermal, implants or injections) in combination with barrier methods (spiral, condom, diaphragm) Exclusion Criteria: * Prior PSMA-targeted RLT (except for the 2 first cycles in Part Ia) * PSMA-negative (or PSMA-negative / FDG-positive) disease * Known intolerance against DOTA, DOTAGA, urea-based analogues or against any of the components/formulation of 177Lu-PSMA-I\&T or 161Tb-SibuDAB solutions. * Ongoing infection at the screening visit or a serious infection in the past 4 weeks * Administration of another investigational product in the last 60 days before Visit 1 Day 1 * Prior or planed administration of a therapeutic radiopharmaceutical during 8 half-lives of the used radio-pharmaceutical's radionuclide, also during the ongoing study * Any extensive radiotherapy involving bone marrow over the last 3 months before inclusion to the study * Chemotherapy in the last 4 weeks before inclusion * Any uncontrolled significant medical, psychiatric or surgical condition (active infection, unstable angina pectoris, cardiac arrhythmia, poorly controlled hypertension, poorly controlled diabetes mellitus \[HbA1c ≥ 9%\], uncontrolled congestive heart disease, etc.) or laboratory findings that might jeopardize the patient's safety or that would limit compliance with the objectives and assessments of the study. Any mental conditions which prevent the patient from understanding the type, extent and possible consequences of the study and/or an uncooperative attitude from the patient. * Current history of any malignancy other than prostate cancer within 5 years of enrolment except for fully resected non-melanoma skin cancer