Stanford Advisors

Current Research and Scholarly Interests

Current research projects include the development of:
1) Gastrin-releasing peptide receptor (GRPR) targeted radiotheranostics (Cu-64, Ga-68, Tb-161, Lu-177, amongst others)
2) Radiohybrid-based cholecystokinin-2 receptor (CCK-2R) targeted radiotheranostics (F-18, Lu-177)
3) Radiotherapeutics for targeted alpha-particle therapy
4) Radiotheranostics for novel targets
All projects have a strong focus on clinical translation

All Publications

  • Preclinical Evaluation of Minigastrin Analogs and Proof-of-Concept [68Ga]Ga-DOTA-CCK-66 PET/CT in 2 Patients with Medullary Thyroid Cancer. Journal of nuclear medicine : official publication, Society of Nuclear Medicine Günther, T., Holzleitner, N., Viering, O., Beck, R., Wienand, G., Dierks, A., Pfob, C. H., Bundschuh, R. A., Kircher, M., Lapa, C., Wester, H. J. 2023


    Because of the need for radiolabeled theranostics for the detection and treatment of medullary thyroid cancer (MTC), and the yet unresolved stability issues of minigastrin analogs targeting the cholecystokinin-2 receptor (CCK-2R), our aim was to address in vivo stability, our motivation being to develop and evaluate DOTA-CCK-66 (DOTA-γ-glu-PEG3-Trp-(N-Me)Nle-Asp-1-Nal-NH2, PEG: polyethylene glycol) and DOTA-CCK-66.2 (DOTA-glu-PEG3-Trp-(N-Me)Nle-Asp-1-Nal-NH2), both derived from DOTA-MGS5 (DOTA-glu-Ala-Tyr-Gly-Trp-(N-Me)Nle-Asp-1-Nal-NH2), and clinically translate [68Ga]Ga-DOTA-CCK-66. Methods: 64Cu and 67Ga labeling of DOTA-CCK-66, DOTA-CCK-66.2, and DOTA-MGS5 was performed at 90°C within 15 min (1.0 M NaOAc buffer, pH 5.5, and 2.5 M 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid buffer, respectively). 177Lu labeling of these 3 compounds was performed at 90°C within 15 min (1.0 M NaOAc buffer, pH 5.5, 0.1 M sodium ascorbate). CCK-2R affinity of natGa/natCu/natLu-labeled DOTA-CCK-66, DOTA-CCK-66.2, and DOTA-MGS5 was examined on AR42J cells. The in vivo stability of 177Lu-labeled DOTA-CCK-66 and DOTA-MGS5 was examined at 30 min after injection in CB17-SCID mice. Biodistribution studies at 1 h ([67Ga]Ga-DOTA-CCK-66) and 24 h ([177Lu]Lu-DOTA-CCK-66/DOTA-MGS5) after injection were performed on AR42J tumor-bearing CB17-SCID mice. In a translation to the human setting, [68Ga]Ga-DOTA-CCK-66 was administered and whole-body PET/CT was acquired at 120 min after injection in 2 MTC patients. Results: Irrespective of the metal or radiometal used (copper, gallium, lutetium), high CCK-2R affinity (half-maximal inhibitory concentration, 3.6-6.0 nM) and favorable lipophilicity were determined. In vivo, increased numbers of intact peptide were found for [177Lu]Lu-DOTA-CCK-66 compared with [177Lu]Lu-DOTA-MGS5 in murine urine (23.7% ± 9.2% vs. 77.8% ± 2.3%). Overall tumor-to-background ratios were similar for both 177Lu-labeled analogs. [67Ga]Ga-DOTA-CCK-66 exhibited accumulation (percentage injected dose per gram) that was high in tumor (19.4 ± 3.5) and low in off-target areas (blood, 0.61 ± 0.07; liver, 0.31 ± 0.02; pancreas, 0.23 ± 0.07; stomach, 1.81 ± 0.19; kidney, 2.51 ± 0.49) at 1 h after injection. PET/CT examination in 2 MTC patients applying [68Ga]Ga-DOTA-CCK-66 confirmed multiple metastases. Conclusion: Because of the high in vivo stability and favorable overall preclinical performance of [nat/67Ga]Ga-/[nat/177Lu]Lu-DOTA-CCK-66, a proof-of-concept clinical investigation of [68Ga]Ga-DOTA-CCK-66 was completed. As several lesions could be identified and excellent biodistribution patterns were observed, further patient studies applying [68Ga]Ga- and [177Lu]Lu-DOTA-CCK-66 are warranted.

    View details for DOI 10.2967/jnumed.123.266537

    View details for PubMedID 37945383

  • CCK2 Receptor-Targeted PET/CT in Medullary Thyroid Cancer Using [68Ga]Ga-DOTA-CCK-66. Journal of nuclear medicine : official publication, Society of Nuclear Medicine Viering, O., Günther, T., Holzleitner, N., Dierks, A., Wienand, G., Pfob, C. H., Bundschuh, R. A., Wester, H. J., Enke, J. S., Kircher, M., Lapa, C. 2023

    View details for DOI 10.2967/jnumed.123.266380

    View details for PubMedID 37918864

  • Synthesis of 177Lu-Labeled, Somatostatin-2 Receptor-Targeted Metalla-Assemblies: Challenges in the Design of Supramolecular Radiotherapeutics. Inorganic chemistry Deiser, S., Drexler, M., Moreno-Alcántar, G., Irl, M., Schmidt, C., Günther, T., Casini, A. 2023


    Self-assembled supramolecular coordination complexes (SCCs) hold promise for biomedical applications in cancer therapy, although their potential in the field of nuclear medicine is still substantially unexplored. Therefore, in this study an exo-functionalized cationic [Pd2L2]4+ metallacycle (L = 3,5-bis(3-ethynylpyridine)phenyl), targeted to the somatostatin-2 receptor (sst2R) and featuring the DOTA chelator (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) in order to bind the β-- and γ-emitter lutetium-177, was synthesized by self-assembly following ligand synthesis via standard solid-phase peptide synthesis (SPPS). This metallacycle was then characterized by reverse-phase high-performance liquid chromatography (RP-HPLC), electrospray ionization mass spectrometry (ESI-MS), and 1H and 1H-DOSY NMR (DOSY = diffusion-ordered spectroscopy). A procedure for the radiolabeling of the metallacycle with 177Lu was also optimized. The resulting [nat/177Lu]Lu-DOTA-metallacycle, termed [nat/177Lu]Lu-Cy, was evaluated concerning its stability and in vitro properties. The compound was more lipophilic compared to the reference [177Lu]Lu-DOTA-TATE (logPOct/H2O = -0.85 ± 0.10 versus -3.67 ± 0.04, respectively). While [natLu]Lu-Cy revealed low stability in a DMEM/F12 GlutaMax medium, it demonstrated good stability in other aqueous media as well as in DMSO. A high sst2R binding affinity (expressed as IC50) was determined in CHOsst2 cells (Chinese hamster ovary cells that were stably transfected with human sst2R). Moreover, the metallacycle exhibited high human serum albumin binding, as assessed by high-performance affinity chromatography (HPAC), and moderate stability in human serum compared to [177Lu]Lu-DOTA-TATE (TATE = (Tyr3)-octreotate). In order to improve stability, a heteroleptic approach was used to develop a less sterically hindered cage-like SCC that is potentially endowed with host-guest chemistry capability, which has been preliminarily characterized by RP-HPLC and ESI-MS. Overall, our initial results encourage future studies on sst2R-directed SCCs and have led to new insights into the chemistry of ss2R-directed SCCs for radiopharmaceutical applications.

    View details for DOI 10.1021/acs.inorgchem.3c02090

    View details for PubMedID 37556427

  • Preclinical Comparison of the 64Cu- and 68Ga-Labeled GRPR-Targeted Compounds RM2 and AMTG, as Well as First-in-Humans [68Ga]Ga-AMTG PET/CT. Journal of nuclear medicine : official publication, Society of Nuclear Medicine Koller, L., Joksch, M., Schwarzenböck, S., Kurth, J., Heuschkel, M., Holzleitner, N., Beck, R., von Amsberg, G., Wester, H. J., Krause, B. J., Günther, T. 2023


    Despite the recent success of prostate-specific membrane antigen (PSMA)-targeted compounds for theranostic use in prostate cancer (PCa), alternative options for the detection and treatment of PSMA-negative lesions are needed. We have recently developed a novel gastrin-releasing peptide receptor (GRPR) ligand with improved metabolic stability, which might improve diagnostic and therapeutic efficacy and could be valuable for PSMA-negative PCa patients. Our aim was to examine its suitability for theranostic use. We performed a comparative preclinical study on [64Cu]Cu-/[68Ga]Ga-AMTG ([64Cu]Cu-/[68Ga]Ga-α-Me-l-Trp8-RM2) using [64Cu]Cu-/[68Ga]Ga-RM2 ([64Cu]Cu-/[68Ga]Ga-DOTA-Pip5-Phe6-Gln7-Trp8-Ala9-Val10-Gly11-His12-Sta13-Leu14-NH2) as a reference compound and investigated [68Ga]Ga-AMTG in a proof-of-concept study in a PCa patient. Methods: Peptides were labeled with 64Cu (80 °C, 1.0 M NaOAc, pH 5.50) and 68Ga (90 °C, 0.25 M NaOAc, pH 4.50). GRPR affinity (half-maximal inhibitory concentration, room temperature, 2 h) and GRPR-mediated internalization (37 °C, 60 min) were examined on PC-3 cells. Biodistribution studies were performed at 1 h after injection in PC-3 tumor-bearing mice. For a first-in-humans application, 173 MBq of [68Ga]Ga-AMTG were administered intravenously and whole-body PET/CT scans were acquired at 75 min after injection. Results: 64Cu- and 68Ga-labeling proceeded almost quantitatively (>98%). All compounds revealed similarly high GRPR affinity (half-maximal inhibitory concentration, 1.5-4.0 nM) and high receptor-bound fractions (79%-84% of cell-associated activity). In vivo, high activity levels (percentage injected dose per gram) were found in the PC-3 tumor (14.1-15.1 %ID/g) and the pancreas (12.6-30.7 %ID/g), whereas further off-target accumulation was low at 1 h after injection, except for elevated liver uptake observed for both 64Cu-labeled compounds. Overall biodistribution profiles and tumor-to-background ratios were comparable but slightly enhanced for the 68Ga-labeled analogs in most organs. [68Ga]Ga-AMTG confirmed the favorable pharmacokinetics-as evident from preclinical studies-in a patient with metastasized castration-resistant PCa showing intense uptake in several lesions. Conclusion: AMTG is eligible for theranostic use, as labeling with 64Cu and 68Ga, as well as 177Lu (known from previous study), does not have a negative influence on its favorable biodistribution pattern. For this reason, further clinical evaluation is warranted.

    View details for DOI 10.2967/jnumed.123.265771

    View details for PubMedID 37474269

  • Investigation of the structure-activity relationship at the N-terminal part of minigastrin analogs. EJNMMI research Holzleitner, N., Günther, T., Daoud-Gadieh, A., Lapa, C., Wester, H. J. 2023; 13 (1): 65


    Over the last years, several strategies have been reported to improve the metabolic stability of minigastrin analogs. However, currently applied compounds still reveal limited in vitro and in vivo stability. We thus performed a glycine scan at the N-terminus of DOTA-MGS5 (DOTA-D-Glu-Ala-Tyr-Gly-Trp-(N-Me)Nle-Asp-1-Nal) to systematically analyze the peptide structure. We substituted N-terminal amino acids by simple PEG spacers and investigated in vitro stability in human serum. Furthermore, we evaluated different modifications on its tetrapeptide binding sequence (H-Trp-(N-Me)Nle-Asp-1-Nal-NH2).Affinity data of all glycine scan peptides were found to be in a low nanomolar range (4.2-8.5 nM). However, a truncated compound lacking the D-γ-Glu-Ala-Tyr sequence revealed a significant loss in CCK-2R affinity. Substitution of the D-γ-Glu-Ala-Tyr-Gly sequence of DOTA-γ-MGS5 (DOTA- D-γ-Glu-Ala-Tyr-Gly-Trp-(N-Me)Nle-Asp-1-Nal-NH2) by polyethylene glycol (PEG) spacers of different length exhibited only a minor influence on CCK-2R affinity and lipophilicity. However, in vitro stability of the PEG-containing compounds was significantly decreased. In addition, we confirmed that the tetrapeptide sequence H-Trp-Asp-(N-Me)Nle-1-Nal-NH2 is indeed sufficient for high CCK-2R affinity.We could demonstrate that a substitution of D-γ-Glu-Ala-Tyr-Gly by PEG spacers simplified the peptide structure of DOTA-MGS5 while high CCK-2R affinity and favorable lipophilicity were maintained. Nevertheless, further optimization with regard to metabolic stability must be carried out for these minigastrin analogs.

    View details for DOI 10.1186/s13550-023-01016-y

    View details for PubMedID 37421545

    View details for PubMedCentralID PMC10329608

  • Development of the First 18F-Labeled Radiohybrid-Based Minigastrin Derivative with High Target Affinity and Tumor Accumulation by Substitution of the Chelating Moiety. Pharmaceutics Günther, T., Holzleitner, N., Di Carlo, D., Urtz-Urban, N., Lapa, C., Wester, H. J. 2023; 15 (3)


    In order to optimize elevated kidney retention of previously reported minigastrin derivatives, we substituted (R)-DOTAGA by DOTA in (R)-DOTAGA-rhCCK-16/-18. CCK-2R-mediated internalization and affinity of the new compounds were determined using AR42J cells. Biodistribution and µSPECT/CT imaging studies at 1 and 24 h p.i. were carried out in AR42J tumor-bearing CB17-SCID mice. Both DOTA-containing minigastrin analogs exhibited 3- to 5-fold better IC50 values than their (R)-DOTAGA-counterparts. natLu-labeled peptides revealed higher CCK-2R affinity than their natGa-labeled analogs. In vivo, tumor uptake at 24 h p.i. of the most affine compound, [19F]F-[177Lu]Lu-DOTA-rhCCK-18, was 1.5- and 13-fold higher compared to its (R)-DOTAGA derivative and the reference compound, [177Lu]Lu-DOTA-PP-F11N, respectively. However, activity levels in the kidneys were elevated as well. At 1 h p.i., tumor and kidney accumulation of [19F]F-[177Lu]Lu-DOTA-rhCCK-18 and [18F]F-[natLu]Lu-DOTA-rhCCK-18 was high. We could demonstrate that the choice of chelators and radiometals has a significant impact on CCK-2R affinity and thus tumor uptake of minigastrin analogs. While elevated kidney retention of [19F]F-[177Lu]Lu-DOTA-rhCCK-18 has to be further addressed with regard to radioligand therapy, its radiohybrid analog, [18F]F-[natLu]Lu-DOTA-rhCCK-18, might be ideal for positron emission tomography (PET) imaging due to its high tumor accumulation at 1 h p.i. and the attractive physical properties of fluorine-18.

    View details for DOI 10.3390/pharmaceutics15030826

    View details for PubMedID 36986687

    View details for PubMedCentralID PMC10054553

  • Synthesis and preclinical evaluation of novel Tc-99m-labeled PSMA ligands for radioguided surgery of prostate cancer EJNMMI RESEARCH Kunert, J., Mueller, M., Guenther, T., Stopper, L., Urtz-Urban, N., Beck, R., Wester, H. 2023; 13 (1): 2


    Radioguided surgery (RGS) has recently emerged as a valuable new tool in the management of recurrent prostate cancer (PCa). After preoperative injection of a 99mTc-labeled prostate-specific membrane antigen (PSMA) inhibitor, radioguided intraoperative identification and resection of lesions is facilitated by means of suitable γ-probes. First clinical experiences show the feasibility of RGS and suggest superiority over conventional lymph node dissection in recurrent PCa. However, commonly used [99mTc]Tc-PSMA-I&S exhibits slow whole-body clearance, thus hampering optimal tumor-to-background ratios (TBR) during surgery. We therefore aimed to develop novel 99mTc-labeled, PSMA-targeted radioligands with optimized pharmacokinetic profile to increase TBR at the time of surgery.Three 99mTc-labeled N4-PSMA ligands were preclinically evaluated and compared to [99mTc]Tc-PSMA-I&S. PSMA affinity (IC50) and internalization were determined on LNCaP cells. Lipophilicity was assessed by means of the distribution coefficient logD7.4 and an ultrafiltration method was used to determine binding to human plasma proteins. Biodistribution studies and static µSPECT/CT-imaging were performed at 6 h p.i. on LNCaP tumor-bearing CB17-SCID mice.The novel N4-PSMA tracers were readily labeled with [99mTc]TcO4- with RCP > 95%. Comparable and high PSMA affinity was observed for all [99mTc]Tc-N4-PSMA-ligands. The ligands showed variable binding to human plasma and medium to low lipophilicity (logD7.4 - 2.6 to - 3.4), both consistently decreased compared to [99mTc]Tc-PSMA-I&S. Biodistribution studies revealed comparable tumor uptake among all [99mTc]Tc-N4-PSMA-ligands and [99mTc]Tc-PSMA-I&S, while clearance from most organs was superior for the novel tracers. Accordingly, increased TBR were achieved. [99mTc]Tc-N4-PSMA-12 showed higher TBR than [99mTc]Tc-PSMA-I&S for blood and all evaluated tissue. In addition, a procedure suitable for routine clinical production of [99mTc]Tc-N4-PSMA-12 was established. Labeling with 553 ± 187 MBq was achieved with RCP of 98.5 ± 0.6% (n = 10).High tumor accumulation and favorable clearance from blood and non-target tissue make [99mTc]Tc-N4-PSMA-12 an attractive tracer for RGS, possibly superior to currently established [99mTc]Tc-PSMA-I&S. Its GMP-production according to a method presented here and first clinical investigations with this novel radioligand is highly recommended.

    View details for DOI 10.1186/s13550-022-00942-7

    View details for Web of Science ID 000913195400001

    View details for PubMedID 36645586

    View details for PubMedCentralID PMC9842843

  • Introduction of a SiFA Moiety into the D-Glutamate Chain of DOTA-PP-F11N Results in Radiohybrid-Based CCK-2R-Targeted Compounds with Improved Pharmacokinetics In Vivo PHARMACEUTICALS Holzleitner, N., Guenther, T., Beck, R., Lapa, C., Wester, H. 2022; 15 (12)


    In order to enable 18F- and 177Lu-labelling within the same molecule, we introduced a silicon-based fluoride acceptor (SiFA) into the hexa-D-glutamate chain of DOTA-PP-F11N. In addition, minigastrin analogues with a prolonged as well as γ-linked D-glutamate chain were synthesised and evaluated. CCK-2R affinity (IC50, AR42J cells) and lipophilicity (logD7.4) were determined. Biodistribution studies at 24 h post-injection (p.i.) and µSPECT/CT imaging at 1, 4 and 24 h p.i. were carried out in AR42J tumour-bearing CB17-SCID mice. CCK-2R affinity of (R)-DOTAGA-rhCCK-1 to 18 was enhanced with increasing distance between the SiFA building block and the binding motif. Lipophilicity of [177Lu]Lu-(R)-DOTAGA-rhCCK-1 to 18 was higher compared to that of [177Lu]Lu-DOTA-PP-F11N and [177Lu]Lu-CP04. The respective α- and γ-linked rhCCK derivatives revealing the highest CCK-2R affinity were further evaluated in vivo. In comparison with [177Lu]Lu-DOTA-PP-F11N, [177Lu-]Lu-(R)-DOTAGA-rhCCK-9 and -16 exhibited three- to eight-fold increased activity levels in the tumour at 24 h p.i. However, activity levels in the kidneys were elevated as well. We could show that the introduction of a lipophilic SiFA moiety into the hydrophilic backbone of [177Lu]Lu-DOTA-PP-F11N led to a decelerated blood clearance and thus improved tumour retention. However, elevated kidney retention has to be addressed in future studies.

    View details for DOI 10.3390/ph15121467

    View details for Web of Science ID 000902941000001

    View details for PubMedID 36558917

    View details for PubMedCentralID PMC9783573

  • Substitution of L-Tryptophan by a-Methyl-L-Tryptophan in 177Lu-RM2 Results in 177Lu-AMTG, a High-Affinity Gastrin-Releasing Peptide Receptor Ligand with Improved In Vivo Stability JOURNAL OF NUCLEAR MEDICINE Guenther, T., Deiser, S., Felber, V., Beck, R., Wester, H. 2022; 63 (9): 1364-1370


    Theranostic applications targeting the gastrin-releasing peptide receptor (GRPR) have shown promising results. When compared with other peptide ligands for radioligand therapy, the most often used GRPR ligand, DOTA-Pip5-d-Phe6-Gln7-Trp8-Ala9-Val10-Gly11-His12-Sta13-Leu14-NH2 (RM2), may be clinically impacted by limited metabolic stability. With the aim of improving the metabolic stability of RM2, we investigated whether the metabolically unstable Gln7-Trp8 bond within the pharmacophore of RM2 can be stabilized via substitution of l-Trp8 by α-methyl-l-tryptophan (α-Me-l-Trp) and whether the corresponding DOTAGA analog might also be advantageous. A comparative preclinical evaluation of 177Lu-α-Me-l-Trp8-RM2 (177Lu-AMTG) and its DOTAGA counterpart (177Lu-AMTG2) was performed using 177Lu-RM2 and 177Lu-NeoBOMB1 as reference compounds. Methods: Peptides were synthesized by solid-phase peptide synthesis and labeled with 177Lu. Lipophilicity was determined at pH 7.4 (logD 7.4). Receptor-mediated internalization was investigated on PC-3 cells (37°C, 60 min), whereas GRPR affinity (half-maximal inhibitory concentration) was determined on both PC-3 and T-47D cells. Stability toward peptidases was examined in vitro (human plasma, 37°C, 72 ± 2 h) and in vivo (murine plasma, 30 min after injection). Biodistribution studies were performed at 24 h after injection, and small-animal SPECT/CT was performed on PC-3 tumor-bearing mice at 1, 4, 8, 24, and 28 h after injection. Results: Solid-phase peptide synthesis yielded 9%-15% purified labeling precursors. 177Lu labeling proceeded quantitatively. Compared with 177Lu-RM2, 177Lu-AMTG showed slightly improved GRPR affinity, a similar low internalization rate, slightly increased lipophilicity, and considerably improved stability in vitro and in vivo. In vivo, 177Lu-AMTG exhibited the highest tumor retention (11.45 ± 0.43 percentage injected dose/g) and tumor-to-blood ratio (2,702 ± 321) at 24 h after injection, as well as a favorable biodistribution profile. As demonstrated by small-animal SPECT/CT imaging, 177Lu-AMTG also revealed a less rapid clearance from tumor tissue. Compared with 177Lu-AMTG, 177Lu-AMTG2 did not show any further benefits. Conclusion: The results of this study, particularly the superior metabolic stability of 177Lu-AMTG, strongly recommend a clinical evaluation of this novel GRPR-targeted ligand to investigate its potential for radioligand therapy of GRPR-expressing malignancies.

    View details for DOI 10.2967/jnumed.121.263323

    View details for Web of Science ID 000859967100012

    View details for PubMedID 35027371

    View details for PubMedCentralID PMC9454457

  • Optimization of the Pharmacokinetic Profile of [(99)mTc]Tc-N-4-Bombesin Derivatives by Modification of the Pharmacophoric Gln-Trp Sequence PHARMACEUTICALS Guenther, T., Konrad, M., Stopper, L., Kunert, J., Fischer, S., Beck, R., Casini, A., Wester, H. 2022; 15 (9)


    Current radiolabeled gastrin-releasing peptide receptor (GRPR) ligands usually suffer from high accumulation in GRPR-positive organs (pancreas, stomach), limiting tumor-to-background contrast in the abdomen. In novel N4-bombesin derivatives this was addressed by substitutions at the Gln7-Trp8 site within the MJ9 peptide (H-Pip5-phe6-Gln7-Trp8-Ala9-Val10-Gly11-His12-Sta13-Leu14-NH2) either by homoserine (Hse7), β-(3-benzothienyl) alanine (Bta8) or α-methyl tryptophan (α-Me-Trp8), with the aim of optimizing pharmacokinetics. We prepared and characterized the peptide conjugates 6-carboxy-1,4,8,11-tetraazaundecane (N4)-asp-MJ9, N4-asp-[Bta8]MJ9, N4-[Hse7]MJ9 and N4-[α-Me-Trp8]MJ9, and evaluated these compounds in vitro (GRPR affinity via IC50,inverse; internalization; lipophilicity via logD7.4) and in vivo (biodistribution and μSPECT/CT studies at 1 h post injection (p.i.) in PC-3 tumor-bearing CB17-SCID mice). 99mTc-labeling resulted in radiochemical yields (RCYs) > 95%. All 99mTc-labeled MJ9 analogues showed comparable or higher GRPR affinity than the external reference [99mTc]Tc-Demobesin 4. Receptor-bound fractions were noticeably higher than that of the reference. Despite a slightly enhanced lipophilicity, all novel MJ9 derivatives revealed improved in vivo pharmacokinetics compared to the reference. The Bta8-modified ligand revealed the most favorable tumor-to-abdomen contrast at 1 h p.i. Substitutions at the Gln7-Trp8 site within GRPR ligands hold great potential to modify pharmacokinetics for improved imaging.

    View details for DOI 10.3390/ph15091133

    View details for Web of Science ID 000856781600001

    View details for PubMedID 36145354

    View details for PubMedCentralID PMC9500665

  • Preclinical comparison of four [F-18, Ga-nat]rhPSMA-7 isomers: influence of the stereoconfiguration on pharmacokinetics EJNMMI RESEARCH Wurzer, A., Parzinger, M., Konrad, M., Beck, R., Guenther, T., Felber, V., Faerber, S., Di Carlo, D., Wester, H. 2020; 10 (1): 149


    Radiohybrid (rh) ligands, a novel class of prostate-specific membrane antigen (PSMA)-targeted radiopharmaceuticals, can be labeled either with [18F]fluorine via isotopic exchange or with radiometals (such as [68Ga]Gallium, [177Lu]Lutetium, [225Ac]Actinium). Among these, [18F, natGa]rhPSMA-7 has recently entered clinical assessment.Since [18F, natGa]rhPSMA-7 is composed of four stereoisomers ([18F, natGa]rhPSMA-7.1, -7.2, -7.3 and -7.4), we initiated a preclinical selection process to identify the isomer with the most favorable pharmacokinetics for further clinical investigation.A synthetic protocol for enantiopure [19F, natGa]rhPSMA-7 isomers has been developed. The comparative evaluation of the four isomers comprised human serum albumin binding, lipophilicity, IC50, internalization and classical biodistribution studies and competition experiments in LNCaP tumor-bearing CB-17 SCID mice. In addition, a radio high-performance liquid chromatography-based method was developed allowing quantitative, intraindividual comparison of [18F, natGa]rhPSMA-7.1 to -7.4 in LNCaP tumor-bearing mice.Cell studies revealed high PSMA affinity and internalization for [18/19F, natGa]rhPSMA-7.2, -7.3 and -7.4, whereas [18/19F, natGa]rhPSMA-7.1 showed approximately twofold lower values. Although the biodistribution profile obtained was typical of PSMA inhibitors, it did not allow for selection of a lead candidate for clinical studies. Thus, an intraindividual comparison of all four isomers in LNCaP tumor-bearing mice was carried out by injection of a diastereomeric mixture, followed by analysis of the differential uptake and excretion pattern of each isomer. Based on its high tumor accumulation and low uptake in blood, liver and kidneys, [18F, natGa]rhPSMA-7.3 was identified as the preferred isomer and transferred into clinical studies.[18F, natGa]rhPSMA-7.3 has been selected as a lead compound for clinical development of a [18F]rhPSMA-based candidate. The intraindividual differential uptake and excretion analysis in vivo allowed for an accurate comparison and assessment of radiopharmaceuticals.

    View details for DOI 10.1186/s13550-020-00740-z

    View details for Web of Science ID 000598723200003

    View details for PubMedID 33284394

    View details for PubMedCentralID PMC7721954

  • Synthesis and in vitro and in vivo evaluation of urea-based PSMA inhibitors with increased lipophilicity EJNMMI RESEARCH Wirtz, M., Schmidt, A., Schottelius, M., Robu, S., Guenther, T., Schwaiger, M., Wester, H. 2018; 8: 84


    Several radiolabeled prostate-specific membrane antigen (PSMA) inhibitors based on the lysine-urea-glutamate (KuE) motif as the pharmacophore proved to be suitable tools for PET/SPECT imaging of the PSMA expression in prostate cancer patients. PSMA I&T, a theranostic tracer developed in our group, was optimized through alteration of the peptidic structure in order to increase the affinity to PSMA and internalization in PSMA-expressing tumor cells. However, further structural modifications held promise to improve the pharmacokinetic profile.Among the investigated compounds 1-9, the PSMA inhibitors 5 and 6 showed the highest PSMA affinity (lowest IC50 values) after the introduction of a naphthylalanine modification. The affinity was up to three times higher compared to the reference PSMA I&T. Extended aromatic systems such as the biphenylalanine residue in 4 impaired the interaction with the lipophilic binding pocket of PSMA, resulting in a tenfold lower affinity. The IC50 of DOTAGA-conjugated 10 was slightly increased compared to the acetylated analog; however, efficient PSMA-mediated internalization and 80% plasma protein binding of 68Ga-10 resulted in effective tumor targeting and low uptake in non-target tissues of LNCaP tumor-bearing CD-1 nu/nu mice at 1 h p.i., as determined by small-animal PET imaging and biodistribution studies. For prolonged tumor retention, the plasma protein binding was increased by insertion of 4-iodo-D-phenylalanine resulting in 97% plasma protein binding and 16.1 ± 2.5% ID/g tumor uptake of 177Lu-11 at 24 h p.i.Higher lipophilicity of the novel PSMA ligands 10 and 11 proved to be beneficial in terms of affinity and internalization and resulted in higher tumor uptake compared to the parent compound. Additional combination with para-iodo-phenylalanine in the spacer of ligand 11 elevated the plasma protein binding and enabled sustained tumor accumulation over 24 h, increasing the tumor uptake almost fourfold compared to 177Lu-PSMA I&T. However, high renal uptake remains a drawback and further studies are necessary to elucidate the responsible mechanism behind it.

    View details for DOI 10.1186/s13550-018-0440-2

    View details for Web of Science ID 000442817300001

    View details for PubMedID 30136051

    View details for PubMedCentralID PMC6104465

  • Synthesis and preclinical evaluation of novel F-18-labeled Glu-urea-Glu-based PSMA inhibitors for prostate cancer imaging: a comparison with F-18-DCFPyl and F-18-PSMA1007 EJNMMI RESEARCH Robu, S., Schmidt, A., Eiber, M., Schottelius, M., Guenther, T., Yousefi, B., Schwaiger, M., Wester, H. 2018; 8: 30


    Due to its high and consistent expression in prostate cancer (PCa), the prostate-specific membrane antigen (PSMA) represents an ideal target for molecular imaging and targeted therapy using highly specific radiolabeled PSMA ligands. To address the continuously growing clinical demand for 18F-labeled PSMA-probes, we developed two novel Glu-urea-Glu-(EuE)-based inhibitors, EuE-k-18F-FBOA (1) and EuE-k-β-a-18F-FPyl (2), both with optimized linker structure and different 18F-labeled aromatic moieties. The inhibitors were evaluated in a comparative preclinical study with 18F-DCFPyl and 18F-PSMA-1007.Radiolabeling procedures allowed preparation of (1) and (2) with high radiochemical yields (67 ± 7 and 53 ± 7%, d.c.) and purity (> 98%). When compared with 18F-DCFPyl (IC50 = 12.3 ± 1.2 nM) and 18F-PSMA-1007 (IC50 = 4.2 ± 0.5 nM), both metabolically stable EuE-based ligands showed commensurable or higher PSMA affinity (IC50 = 4.2 ± 0.4 nM (1), IC50 = 1.1 ± 0.2 nM (2)). Moreover, 1.4- and 2.7-fold higher internalization rates were observed for (1) and (2), respectively, resulting in markedly enhanced tumor accumulation in LNCaP-tumor-bearing mice ((1) 12.7 ± 2.0% IA/g, (2) 13.0° ± 1.0% IA/g vs. 7.3 ± 1.0% IA/g (18F-DCFPyl), 7.1 ± 1.5% IA/g (18F-PSMA-1007), 1 h p.i.). In contrast to (1), (2) showed higher kidney accumulation and delayed clearance kinetics. Due to the high hydrophilicity of both compounds, almost no unspecific uptake in non-target tissue was observed. In contrast, due to the less hydrophilic character (logP = - 1.6) and high plasma protein binding (98%), 18F-PSMA-1007 showed uptake in non-target tissue and predominantly hepatobiliary excretion, whereas, 18F-DCFPyl exhibited pharmacokinetics quite similar to those obtained with (1) and (2).Both 18F-labeled EuE-based PSMA ligands showed excellent in vitro and in vivo PSMA-targeting characteristics. The substantially higher tumor accumulation in mice compared to recently introduced 18F-PSMA-1007 and 18F-DCFPyl suggests their high value for preclinical studies investigating the effects on PSMA-expression. In contrast to (2), (1) seems to be more promising for further investigation, due to the more reliable 18F-labeling procedure, the faster clearance kinetics with comparable high tumor uptake, resulting therefore in better high-contrast microPET imaging as early as 1 h p.i.

    View details for DOI 10.1186/s13550-018-0382-8

    View details for Web of Science ID 000431695500001

    View details for PubMedID 29651565

    View details for PubMedCentralID PMC5897267