What a difference a methylene makes: replacing Glu with Asp or Aad in the Lys-urea-Glu pharmacophore of PSMA-targeting radioligands to reduce kidney and salivary gland uptake

The aim of this study was to investigate the effect of replacing Glu in the Lys-urea-Glu PSMA-targeting pharmacophore of [ 68 Ga]Ga-HTK03041 with a close analog on the uptake of kidneys, salivary glands and PSMA-expressing tumor xenografts.Methods: HTK03161, HTK03149 and HTK03189A/B were obtained by replacing Glu in HTK03041 with Asp, Aad (L-2-aminoadipic acid) and Api (2-aminopimelic acid), respectively.PSMA binding affinities were measured by competition binding assays.PET imaging and biodistribution studies of 68 Ga-labeled ligands were performed in LNCaP tumor-bearing mice.The best candidate HTK03149 was selected and radiolabeled with 177 Lu, and SPECT imaging and biodistribution studies were performed in LNCaP tumor-bearing mice.Radiation dosimetry calculation was conducted using the OLINDA software.Radioligand therapy study was performed in LNCaP tumor-bearing mice treated with [ 177 Lu]Lu-HTK03149 (9.3-148 MBq), [ 177 Lu]Lu-PSMA-617 (37 MBq) or nat Lu-HTK03149 (500 pmol).Results: PSMA binding affinities (Ki) of Ga-HTK03161, Ga-HTK03149, Ga-HTK03189A and Lu-HTK03149 were 3.88±0.66,6.99±0.80,550±35.7 and 1.57±0.42nM, respectively.PET imaging showed that all 68 Ga-labeled HTK03161, HTK03149 and HTK03189A/B were excreted mainly via the renal pathway and had minimal uptake in all organs/tissues including kidneys and salivary glands.Tumor xenografts were clearly visualized in PET images of [ 68 Ga]Ga-HTK03161 and [ 68 Ga]Ga-HTK03149 but were barely visualized using [ 68 Ga]Ga-HTK03189A/B.Tumor uptake values for [ 68 Ga]Ga-HTK03161, [ 68 Ga]Ga-HTK03149, [ 68 Ga]Ga-HTK0189A and [ 68 Ga]Ga-HTK03189B were 12.7±1.91,19.1±6.37,2.10±0.28 and 0.67±0.15%IA/g, respectively at 1h post-injection, and their average kidney and salivary gland uptake values were 2.13-4.41 and 0.20-0.23 %IA/g, respectively.Longitudinal SPECT imaging studies showed that [ 177 Lu]Lu-HTK03149 was excreted mainly through the renal pathway with high uptake in LNCaP tumors and minimal uptake in all normal organs/tissues.The tumor uptake of [ 177 Lu]Lu-HTK03149 peaked at 4h post-injection (20.9±2.99 %IA/g) and the uptake was sustained over time.Compared to [ 177 Lu]Lu-PSMA-617, [ 177 Lu]Lu-HTK03149 had 145% increase in tumor absorbed dose but 70% less in kidney absorbed dose, leading to an 7.1-fold increase in tumor-to-kidney absorbed dose ratio.Radioligand therapy studies showed that only half of the [ 177 Lu]Lu-PSMA-617 injected dosage was needed for [ 177 Lu]Lu-HTK03149 to achieve the same median survival. Conclusion:Replacing Glu in the PSMA-targeting Lys-urea-Glu pharmacophore of [ 68 Ga]Ga-HTK03041 with Asp and Aad generates [ 68 Ga]Ga-HTK03161 and [ 68 Ga]Ga-HTK03149, respectively, and the new derivatives retain high uptake in LNCaP tumors and have minimal uptake in normal organs/tissues including kidneys and salivary glands.[ 177 Lu]Lu-HTK03149 also retain high uptake in LNCaP tumors and has minimal uptake in normal organs/tissues, and is, therefore, promising for clinical translation to treat prostate cancer. Ivyspring