Post-therapeutic dosimetry of 177Lu-DKFZ-PSMA-617 in the treatment of patients with metastatic castration-resistant prostate cancer

Repeated 177Lu-Labeled PSMA-617 Radioligand Therapy Using Treatment Activities of Up to 9.3 GBq

Current treatment protocols for ¹⁷⁷Lu-labeled PSMA-617 therapies were cautiously derived from dosimetry data, but their practical appropriateness has not yet been proven clinically. We retrospectively report our clinical observations using 4 different treatment activities. Methods: Forty patients with advanced prostate cancer and positive uptake in prostate-specific membrane antigen (PSMA) imaging were treated with 4 GBq of ¹⁷⁷Lu activity/80 nmol of precursor, 6 GBq of ¹⁷⁷Lu activity/120 nmol of precursor, 7.4 GBq of ¹⁷⁷Lu activity/150 nmol of precursor, or 9.3 GBq of ¹⁷⁷Lu activity/150 nmol of precursor (10 patients per group) every 2 mo. Safety was checked every 2 wk by laboratory tests, the prostate-specific antigen response was checked every 4 wk, and other effects were assessed by anamnesis. Results: The initial prostate-specific antigen response showed no correlation with treatment activity. However, 2 of 10, 4 of 10, 4 of 10, and 7 of 10 patients receiving doses of 4, 6, 7.4, and 9.3 GBq, respectively, were in partial remission 8 wk after completing all 3 cycles. This finding would be in line with but-because of low patient numbers-would not prove a positive dose-response relationship. Acute hematologic toxicity was also not correlated with treatment activity, and no more than 1 patient per group had grade 3/4 toxicity. Nevertheless, in contrast to the findings for the other groups, the mean platelet count in the 9.3-GBq group decreased chronically over time. Conclusion: If patients with diffuse red marrow infiltration and extensive chemotherapeutic pretreatments are excluded, then treatment activities of up to 3 injections of 9.3 GBq of ¹⁷⁷Lu-PSMA-617 every 2 mo are tolerated well. Further dose escalation should be conducted with care, as the highest dose seems to be close to the maximum tolerable dose.

Targeted α-Therapy of Metastatic Castration-Resistant Prostate Cancer with 225Ac-PSMA-617: Dosimetry Estimate and Empiric Dose Finding

The aim of this study was to develop a treatment protocol for ²²⁵Ac-PSMA-617 α-radiation therapy in advanced-stage, metastatic castration-resistant prostate cancer patients with prostate-specific membrane antigen (PSMA)-positive tumor phenotype. Methods: A dosimetry estimate was calculated on the basis of time-activity curves derived from serially obtained ¹⁷⁷Lu-PSMA-617 scans extrapolated to the physical half-life of ²²⁵Ac, assuming instant decay of unstable daughter nuclides. Salvage therapies empirically conducted with 50 (n = 4), 100 (n = 4), 150 (n = 2), and 200 kBq/kg (n = 4) of ²²⁵Ac-PSMA-617 were evaluated retrospectively regarding toxicity and treatment response. Eight of 14 patients received further cycles in either 2- or 4-mo intervals with identical or deescalated activities. Results: Dosimetry estimates for 1 MBq of ²²⁵Ac-PSMA-617 assuming a relative biologic effectiveness of 5 revealed mean doses of 2.3 Sv for salivary glands, 0.7 Sv for kidneys, and 0.05 Sv for red marrow that are composed of 99.4% α, 0.5% β, and 0.1% photon radiation, respectively. In clinical application, severe xerostomia became the dose-limiting toxicity if treatment activity exceeded 100 kBq/kg per cycle. At 100 kBq/kg, the duration of prostate-specific antigen decline was less than 4 mo, but if therapy was repeated every 2 mo patients experienced additive antitumor effects. Treatment activities of 50 kBq/kg were without toxicity but induced insufficient antitumor response in these high-tumor-burden patients. Remarkable antitumor activity by means of objective radiologic response or tumor marker decline was observed in 9 of 11 evaluable patients. Conclusion: For advanced-stage patients, a treatment activity of 100 kBq/kg of ²²⁵Ac-PSMA-617 per cycle repeated every 8 wk presents a reasonable trade-off between toxicity and biochemical response.