177Lu-DOTA-HYNIC-Lys(Nal)-Urea-Glu: Biokinetics, Dosimetry, and Evaluation in Patients with Advanced Prostate Cancer.

SPECT/CT images in patients have demonstrated the ability of [^99mTc]Tc-EDDA/HYNIC-Lys(Nal)-Urea-Glu ([^99mTc]Tc-iPSMA) to detect tumors and metastases of prostate cancer. Considering that theranostics combines the potential of therapeutic and diagnostic radionuclides in the same molecular probe, the aim of this research was to estimate the biokinetics and dosimetry of ¹⁷⁷Lu-DOTA-HYNIC-Lys(Nal)-Urea-Glu (¹⁷⁷Lu-iPSMA) in healthy subjects and analyze the response in patients receiving ¹⁷⁷Lu-iPSMA therapeutic doses. ¹⁷⁷Lu-iPSMA was obtained from lyophilized formulations with radiochemical purities >98%. Whole-body images from five healthy subjects were acquired at 20 min, 6, 24, 48, and 120 h after ¹⁷⁷Lu-iPSMA administration (185 MBq). The image sequence was used to extrapolate the ¹⁷⁷Lu-iPSMA time-activity curves of each organ to adjust the biokinetic model and calculate the total number of disintegrations (N) that occurred in the source regions. N data were the input for the OLINDA/EXM code to calculate internal radiation doses. Ten patients (median age: 68 y; range 58-86 y) received from 1 to 4 cycles of ¹⁷⁷Lu-iPSMA (3.7 or 7.4 GBq) every 8-10 weeks. Response was evaluated using the ⁶⁸Ga-PSMA-ligand-PET/CT or ^99mTc-iPSMA-SPECT/CT diagnostic images and serum PSA levels before and after ¹⁷⁷Lu-iPSMA treatment. The blood activity showed a half-life value of 1.1 h for the fast component (T_1/2α = ln2/0.614), 9.2 h for the first slow component (T_1/2β = ln2/0.075), and 79.6 h for the second slow component (T_1/2γ = ln2/0.008). The average absorbed doses were 0.23, 0.28, 0.88, and 1.17 Gy/GBq for the spleen, liver, kidney, and salivary glands. A total of 18 cycles were performed in 10 patients. A PSA decrease and some reduction of the radiotracer uptake (SUV) in tumor lesions occurred in 60% and 70% of the patients, respectively. ¹⁷⁷Lu-iPSMA obtained from kit formulations showed high tumor uptake with good response rates in patients. The results obtained in this study warrant further clinical studies to establish the optimal number of treatment cycles and for evaluating the effect of this therapeutic agent on survival of patients.