GCP III is not the "off-target" for urea-based PSMA ligands

Searching for Protein Off-Targets of Prostate-Specific Membrane Antigen-Targeting Radioligands in the Salivary Glands

Background: Prostate specific membrane antigen (PSMA)-targeted radioligand therapies represent a highly effective treatment for metastatic prostate cancer. However, high and sustain uptake of PSMA-ligands in the salivary glands led to dose limiting dry mouth (xerostomia), especially with α-emitters. The expression of PSMA and histologic analysis couldn't directly explain the toxicity, suggesting a potential off-target mediator for uptake. In this study, we set out to search for possible off-target non-PSMA protein(s) in the salivary glands. Methods: A machine-learning based quantitative structure activity relationship (QSAR) model was built for seeking the possible off-target(s). The resulting target candidates from the model prediction were subjected to further analysis for salivary protein expression and structural homology at key regions required for PSMA-ligand binding. Furthermore, cellular binding assays were performed utilizing multiple cell lines with high expression of the candidate proteins and low expression of PSMA. Finally, PSMA knockout (PSMA-/-) mice were scanned by small animal PET/MR using [68Ga]Ga-PSMA-11 for in-vivo validation. Results: The screening of the trained QSAR model did not yield a solid off-target protein, which was corroborated in part by cellular binding assays. Imaging using PSMA-/- mice further demonstrated markedly reduced PSMA-radioligand uptake in the salivary glands. Conclusion: Uptake of the PSMA-targeted radioligands in the salivary glands remains primarily PSMA-mediated. Further investigations are needed to illustrate a seemingly different process of uptake and retention in the salivary glands than that in prostate cancer.

Biological determinants of PSMA expression, regulation and heterogeneity in prostate cancer

Prostate-specific membrane antigen (PSMA) is an important cell-surface imaging biomarker and therapeutic target in prostate cancer. The PSMA-targeted theranostic 177Lu-PSMA-617 was approved in 2022 for men with PSMA-PET-positive metastatic castration-resistant prostate cancer. However, not all patients respond to PSMA-radioligand therapy, in part owing to the heterogeneity of PSMA expression in the tumour. The PSMA regulatory network is composed of a PSMA transcription complex, an upstream enhancer that loops to the FOLH1 (PSMA) gene promoter, intergenic enhancers and differentially methylated regions. Our understanding of the PSMA regulatory network and the mechanisms underlying PSMA suppression is evolving. Clinically, molecular imaging provides a unique window into PSMA dynamics that occur on therapy and with disease progression, although challenges arise owing to the limited resolution of PET. PSMA regulation and heterogeneity - including intertumoural and inter-patient heterogeneity, temporal changes, lineage dynamics and the tumour microenvironment - affect PSMA theranostics. PSMA response and resistance to radioligand therapy are mediated by a number of potential mechanisms, and complementary biomarkers beyond PSMA are under development. Understanding the biological determinants of cell surface target regulation and heterogeneity can inform precision medicine approaches to PSMA theranostics as well as other emerging therapies.

Discovery of Glutamate Carboxypeptidase III Ligands to Compete the Uptake of [177Lu]Lu-PSMA-617 in Healthy Organs

Prostate-specific membrane antigen (PSMA)-targeted radio ligand therapeutics (RLTs), such as [177Lu]Lu-PSMA-617 (Pluvicto), have been shown to accumulate in salivary glands and kidneys, potentially leading to undesired side effects. As unwanted accumulation in normal organs may derive from the cross-reactivity of PSMA ligands to glutamate carboxypeptidase III (GCPIII), it may be convenient to block this interaction with GCPIII-selective ligands. Parallel screening of a DNA-encoded chemical library (DEL) against GCPIII and PSMA allowed the identification of GCPIII binders. Structure-activity relationship (SAR) studies resulted in the identification of nanomolar GCPIII ligands with up to 1000-fold selectivity over PSMA. We studied the ability of GCPIII ligands to counteract the binding of [177Lu]Lu-PSMA-617 to human salivary glands by autoradiography and could demonstrate a partial radioprotection.

Hematological and renal toxicity in mice after three cycles of high activity [177Lu]Lu-PSMA-617 with or without human α1-microglobulin

Radioligand therapy with [177Lu]Lu-PSMA-617 can be used to prolong life and reduce tumor burden in terminally ill castration resistant prostate cancer patients. Still, accumulation in healthy tissue limits the activity that can be administered. Therefore, fractionated therapy is used to lower toxicity. However, there might be a need to reduce toxicity even further with e.g. radioprotectors. The aim of this study was to (i). establish a preclinical mouse model with fractionated high activity therapy of three consecutive doses of 200 MBq [177Lu]Lu-PSMA-617 in which we aimed to (ii). achieve measurable hematotoxicity and nephrotoxicity and to (iii). analyze the potential protective effect of co-injecting recombinant α1-microglobulin (rA1M), a human antioxidant previously shown to have radioprotective effects. In both groups, three cycles resulted in increased albuminuria for each cycle, with large individual variation. Another marker of kidney injury, serum blood urea nitrogen (BUN), was only significantly increased compared to control animals after the third cycle. The number of white and red blood cells decreased significantly and did not reach the levels of control animals during the experiment. rA1M did reduce absorbed dose to kidney but did not show significant protection here, but future studies are warranted due to the recent clinical studies showing a significant renoprotective effect in patients.

Human ABC and SLC Transporters: The Culprit Responsible for Unspecific PSMA-617 Uptake?

[177Lu]Lu-PSMA-617 has recently been successfully approved by the FDA, the MHRA, Health Canada and the EMA as Pluvicto®. However, salivary gland (SG) and kidney toxicities account for its main dose-limiting side-effects, while its corresponding uptake and retention mechanisms still remain elusive. Recently, the presence of different ATP-binding cassette (ABC) transporters, such as human breast cancer resistance proteins (BCRP), multidrug resistance proteins (MDR1), multidrug-resistance-related proteins (MRP1, MRP4) and solute cassette (SLC) transporters, such as multidrug and toxin extrusion proteins (MATE1, MATE2-K), organic anion transporters (OAT1, OAT2v1, OAT3, OAT4) and peptide transporters (PEPT2), has been verified at different abundances in human SGs and kidneys. Therefore, our aim was to assess whether [177Lu]Lu-PSMA-617 and [225Ac]Ac-PSMA-617 are substrates of these ABC and SLC transporters. For in vitro studies, the novel isotopologue ([α,β-3H]Nal)Lu-PSMA-617 was used in cell lines or vesicles expressing the aforementioned human ABC and SLC transporters for inhibition and uptake studies, respectively. The corresponding probe substrates and reference inhibitors were used as controls. Our results indicate that [177Lu]Lu-PSMA-617 and [225Ac]Ac-PSMA-617 are neither inhibitors nor substrates of the examined transporters. Therefore, our results show that human ABC and SLC transporters play no central role in the uptake and retention of [177Lu]Lu-PSMA-617 and [225Ac]Ac-PSMA-617 in the SGs and kidneys nor in the observed toxicities.

Palladium-Mediated S-Arylation of Cysteine Residues with 4-[18F]Fluoroiodobenzene ([18F]FIB)

Transition-metal-mediated bioconjugation chemistry has been used extensively to design and synthesize molecular probes to visualize, characterize, and quantify biological processes within intact living organisms at the cellular and subcellular levels. We demonstrate the development and validation of chemoselective [18F]fluoro-arylation chemistry of cysteine residues using Pd-mediated S-arylation chemistry with 4-[18F]fluoroiodobenzene ([18F]FIB) as an aryl electrophile. The novel bioconjugation technique proceeded in excellent radiochemical yields of 73-96% within 15 min under ambient and aqueous reaction mixture conditions, representing a versatile novel tool for decorating peptides and peptidomimetics with short-lived positron emitter 18F. The chemoselective S-arylation of several peptides and peptidomimetics containing multiple reactive functional groups confirmed the versatility and functional group compatibility. The synthesis and radiolabeling of a novel prostate-specific membrane antigen (PSMA) binding radioligand [18F]6 was accomplished using the novel labeling protocol. The validation of radioligand [18F]6 in a preclinical prostate cancer model with PET resulted in favorable accumulation and retention in PSMA-expressing LNCaP tumors. At the same time, a significantly lower salivary gland uptake was observed compared to clinical PSMA radioligand [18F]PSMA-1007. This finding coincides with ongoing discussions about the molecular basis of the off-target accumulation of PSMA radioligands currently used for clinical imaging and therapy of prostate cancer.

A novel PSMA-targeting tracer with highly negatively charged linker demonstrates decreased salivary gland uptake in mice compared to [68Ga]Ga-PSMA-11

BACKGROUND

The current generation of radiolabeled PSMA-targeting therapeutic agents is limited by prominent salivary gland binding, which results in dose-limiting xerostomia from radiation exposure. JB-1498 is a urea-based small molecule with a highly negatively charged linker targeting prostate specific membrane antigen (PSMA). Prior work on a similar tracer with the same negatively charged linker demonstrated low normal organ/soft tissue background uptake compared to [68Ga]Ga-PSMA-11. The purpose of this study was to investigate if [68Ga]Ga-JB-1498 had reduced salivary gland uptake in mice compared to [68Ga]Ga-PSMA-11.

RESULTS

JB-1498 demonstrated high affinity for PSMA binding and tumor uptake in a murine tumor model. In an initial biodistribution study with low molar activity, [68Ga]Ga-JB-1498 demonstrated salivary gland uptake of 0.13 ± 0.01%ID/g. In a second biodistribution study in non-tumor-bearing mice with high molar activity, [68Ga]Ga-JB1498 demonstrated salivary gland uptake of 0.39 ± 0.24% ID/g and kidney activity of 10.12 ± 1.73% ID/g at one hour post IV injection. This salivary gland uptake is significantly less than the published uptake of [68Ga]Ga-PSMA-11. Micro-PET visually confirmed the findings of the biodistribution studies. Dynamic micro-PET imaging demonstrated gradually decreasing [68Ga]Ga-JB1498 activity in salivary glands and kidneys, compared to gradually increasing [68Ga]Ga-PSMA-11 activity in these two organs during the first hour.

CONCLUSION

Biodistribution and micro-PET imaging of [68Ga]Ga-JB-1498 demonstrate significantly decreased salivary gland uptake and different pharmacokinetic behavior in kidneys and salivary glands in mice compared to [68Ga]Ga-PSMA-11. Our findings suggest that constructing a PSMA-targeting molecule with a highly negatively charged linker is a promising strategy to reduce salivary gland uptake of GCP-II/PSMA ligands in theranostic applications.

Lys-urea-Aad, Lys-urea-Cmc and Lys-urea-Cms as potential pharmacophores for the design of PSMA-targeted radioligands to reduce off-target uptake in kidneys and salivary glands

High kidney and salivary gland uptake is a common feature of prostate-specific membrane antigen (PSMA)-targeted radioligands derived from the lysine-urea-glutamic acid (Lys-urea-Glu) pharmacophore. In this study we investigated if radioligands derived from lysine-urea-2-aminoadipic acid (Lys-urea-Aad), lysine-urea-S-carboxylmethylcysteine (Lys-urea-Cmc) and lysine-urea-O-carboxylmethylserine (Lys-urea-Cms) pharmacophores with/without an albumin binder could retain good PSMA-targeting capability but with minimized kidney and salivary gland uptake. Methods: HTK03177 and HTK03187 were obtained by replacing Aad in the previously reported Lys-urea-Aad-derived HTK03149 with Cmc and Cms, respectively. HTK03170, HTK04048 and HTK04028 were derived from HTK03149, HTK03177 and HTK03187, respectively, with the conjugation of an albumin-binding moiety, 4-(p-methoxyphenyl)butyric acid. In vitro competition binding assays were conducted using PSMA-expressing LNCaP prostate cancer cells and [18F]DCFPyL as the radioligand. Imaging and biodistribution studies of 68Ga-labeled HTK03177 and HTK03187, and 177Lu-labeled HTK03170, HTK04048 and HTK04028 were performed in LNCaP tumor-bearing mice. Radioligand therapy study of [177Lu]Lu-HTK03170 was carried out in LNCaP tumor-bearing mice and [177Lu]Lu-PSMA-617 was used for comparison. Results: The calculated Ki(PSMA) values of Ga-HTK03177, Ga-HTK03187, Lu-HTK03170, Lu-HTK04048 and Lu-HTK04028 were 5.0±2.4, 10.6±2.0, 1.6±0.4, 1.4±1.0 and 13.9±3.2 nM, respectively. PET Imaging and biodistribution studies at 1 h post-injection showed that both [68Ga]Ga-HTK03177 and [68Ga]Ga-HTK03187 had high uptake in LNCaP tumor xenografts (24.7±6.85 and 21.1±3.62 %ID/g, respectively) but minimal uptake in normal organs/tissues including kidneys (7.76±1.00 and 2.83±0.45 %ID/g, respectively) and salivary glands (0.22±0.02 and 0.16±0.02 %ID/g, respectively). SPECT imaging and biodistribution studies showed that the LNCaP tumor uptake of 177Lu-labeled HTK03170, HTK04048 and HTK04028 peaked at 4-24 h post-injection at ~43-65 %ID/g and was relatively sustained over time. Their peaked average uptake in kidneys (≤ 17.4 %ID/g) and salivary glands (≤ 2.92 %ID/g) was lower and continuously reduced over time. Radioligand therapy study showed that compared with [177Lu]Lu-PSMA-617 (37 MBq), a quarter dose of [177Lu]Lu-HTK03170 (9.3 MBq) led to a better median survival (63 vs 90 days). Conclusions: Our data demonstrate that that Lys-urea-Aad, Lys-urea-Cmc and Lys-urea-Cms are promising pharmacophores for the design of PSMA-targeted radioligands especially for radiotherapeutic applications to minimize toxicity to kidneys and salivary glands.

Synthesis and Evaluation of 99mTc-Labeled PSMA-Targeted Tracers Based on the Lys-Urea-Aad Pharmacophore for Detecting Prostate Cancer with Single Photon Emission Computed Tomography

Prostate-specific membrane antigen (PSMA) is a well-validated prostate cancer marker but reported PSMA-targeted tracers derived from the Lys-urea-Glu pharmacophore including the clinically validated [^99mTc]Tc-EDDA/HYNIC-iPSMA have high off-target uptake in kidneys, spleen, and salivary glands. In this study, we synthesized and evaluated three novel ^99mTc-labeled PSMA-targeted tracers and investigated if the tracers derived from the Lys-urea-Aad pharmacophore could have minimized uptake in off-target organs/tissues. In vitro competition binding assays showed that compared with HYNIC-iPSMA, the three novel ligands had slightly weaker PSMA binding affinity (average K_i = 3.11 vs. 8.96-11.6 nM). Imaging and ex vivo biodistribution studies in LNCaP tumor-bearing mice showed that [^99mTc]Tc-EDDA/HYNIC-iPSMA and the three novel tracers successfully visualized LNCaP tumor xenografts in SPECT images and were excreted mainly via the renal pathway. The average tumor uptake at 1 h post-injection varied from 5.40 to 18.8%ID/g, and the tracers derived from the Lys-urea-Aad pharmacophore had much lower uptake in the spleen and salivary glands. Compared with the clinical tracer [^99mTc]Tc-EDDA/HYNIC-iPSMA, the Lys-urea-Aad-derived [^99mTc]Tc-EDDA-KL01127 had lower background uptake and superior tumor-to-background contrast ratios and is therefore promising for clinical translation to detect prostate cancer lesions with SPECT.