Salivary glands as the primary dose-limiting organ: an integrated dosimetry, efficacy, and toxicity study of Ac-PSMA-617 in prostate cancer xenografts.

[BACKGROUND] Targeted alpha therapy with Ac-PSMA-617 is a promising strategy for metastatic prostate cancer. However, physiological uptake in dose-limiting organs, particularly the salivary glands and kidneys, necessitates precise dosimetry to define the therapeutic window. Therefore, the objective of this study was to perform integrated dosimetry, quantify the therapeutic index (TI), and establish dose-response relationships for the tumor control and organ toxicity of Ac-PSMA-617 in a PSMA-positive xenograft model.

[METHODS] In male nude mice bearing C4-2 xenografts, a dosimetry group (n = 32) received 5 kBq Ac-PSMA-617 for multi-time-point biodistribution. Time-activity curves were fitted with bi-exponential models, and absorbed doses were calculated using the MIRD formalism. A therapy group (n = 12/arm) received a single 40 kBq dose of the therapy or vehicle. Efficacy was assessed by tumor growth inhibition (TGI) and time-to-progression (TTP) of 500 mm. Toxicity was evaluated via renal function, histopathology (kidney: Day 56; salivary glands: Day 28), and pilocarpine-stimulated salivary secretion. The therapeutic index (TI_min) and Spearman correlations between absorbed dose and biological endpoints were calculated.

[RESULTS] The radiopharmaceutical demonstrated high purity and stability. Dosimetry revealed favorable pharmacokinetics, with the highest absorbed dose coefficient for the tumor (588 Gy/GBq). The mean absorbed dose to the parotid gland (0.56 Gy at 5 kBq) exceeded that to the kidneys (0.29 Gy). The TI_min was >1 for all animals. The 40 kBq dose induced significant tumor growth inhibition (TGI: 75.2%, P < 0.001) and delayed progression (P < 0.001). While renal function remained normal, histology revealed mild tubular injury. Salivary gland function, however, showed a profound and sustained decrease (>60%, P < 0.001). Strong correlations were found between tumor dose and TGI (ρ = 0.720), kidney dose and histology score (ρ = 0.643), and salivary gland dose and functional impairment (ρ = 0.776) (all P < 0.05).

[CONCLUSION] Ac-PSMA-617 provides a significant therapeutic window (TI > 1) with potent efficacy. The salivary glands, not the kidneys, are identified as the primary dose-limiting organ based on severe functional toxicity strongly correlated with absorbed dose. These quantitative dose-response correlations establish a foundation for clinical dose optimization.