Preclinical Evaluation of Gastrin-Releasing Peptide Receptor Antagonists Labeled with 161Tb and 177Lu: A Comparative Study

To elucidate potential benefits of the Auger-electron-emitting radionuclide 161Tb, we compared the preclinical performance of the gastrin-releasing peptide receptor antagonists RM2 (DOTA-Pip5-d-Phe6-Gln7-Trp8-Ala9-Val10-Gly11-His12-Sta13-Leu14-NH2) and AMTG (α-Me-Trp8-RM2), each labeled with both 177Lu and 161Tb. Methods: 161Tb/177Lu labeling (90°C, 5 min) and cell-based experiments (PC-3 cells) were performed. In vivo stability (30 min after injection) and biodistribution studies (1-72 h after injection) were performed on PC-3 tumor-bearing CB17-SCID mice. Results: Gastrin-releasing peptide receptor affinity was high for all compounds (half-maximal inhibitory concentration [nM]: [161Tb]Tb-RM2, 2.46 ± 0.16; [161Tb]Tb-AMTG, 2.16 ± 0.09; [177Lu]Lu-RM2, 3.45 ± 0.18; [177Lu]Lu-AMTG, 3.04 ± 0.08), and 75%-84% of cell-associated activity was receptor-bound. In vivo, both AMTG analogs displayed distinctly higher stability (30 min after injection) and noticeably higher tumor retention than their RM2 counterparts. Conclusion: On the basis of preclinical results, [161Tb]Tb-/[177Lu]Lu-AMTG might reveal a higher therapeutic efficacy than [161Tb]Tb-/[177Lu]Lu-RM2, particularly [161Tb]Tb-AMTG because of additional Auger-electron emissions at the cell membrane level.

Diagnostic accuracy of fully hybrid [68Ga]Ga-PSMA-11 PET/MRI and [68Ga]Ga-RM2 PET/MRI in patients with biochemically recurrent prostate cancer: a prospective single-center phase II clinical trial

PURPOSE

To compare the diagnostic accuracy and detection rates of PET/MRI with [68Ga]Ga-PSMA-11 and [68Ga]Ga-M2 in patients with biochemical recurrence of prostate cancer (PCa).

METHODS

Sixty patients were enrolled in this prospective single-center phase II clinical trial from June 2020 to October 2022. Forty-four/60 completed all study examinations and were available at follow-up (median: 22.8 months, range: 6-31.5 months). Two nuclear medicine physicians analyzed PET images and two radiologists interpreted MRI; images were then re-examined to produce an integrated PET/MRI report for both [68Ga]Ga-PSMA-11 and [68Ga]Ga-RM2 examinations. A composite reference standard including histological specimens, response to treatment, and conventional imaging gathered during follow-up was used to validate imaging findings. Detection rates, accuracy, sensitivity, specificity, positive, and negative predictive value were assessed. McNemar's test was used to compare sensitivity and specificity on a per-patient base and detection rate on a per-region base. Prostate bed, locoregional lymph nodes, non-skeletal distant metastases, and bone metastases were considered. p-value significance was defined below the 0.05 level after correction for multiple testing.

RESULTS

Patients' median age was 69.8 years (interquartile range (IQR): 61.8-75.1) and median PSA level at time of imaging was 0.53 ng/mL (IQR: 0.33-2.04). During follow-up, evidence of recurrence was observed in 31/44 patients. Combining MRI with [68Ga]Ga-PSMA-11 PET and [68Ga]Ga-RM2 PET resulted in sensitivity = 100% and 93.5% and specificity of 69.2% and 69.2%, respectively. When considering the individual imaging modalities, [68Ga]Ga-RM2 PET showed lower sensitivity compared to [68Ga]Ga-PSMA-11 PET and MRI (61.3% vs 83.9% and 87.1%, p = 0.046 and 0.043, respectively), while specificity was comparable among the imaging modalities (100% vs 84.6% and 69.2%, p = 0.479 and 0.134, respectively).

CONCLUSION

This study brings further evidence on the utility of fully hybrid PET/MRI for disease characterization in patients with biochemically recurrent PCa. Imaging with [68Ga]Ga-PSMA-11 PET showed high sensitivity, while the utility of [68Ga]Ga-RM2 PET in absence of a simultaneous whole-body/multiparametric MRI remains to be determined.

Side-by-side comparison of the two widely studied GRPR radiotracers, radiolabeled NeoB and RM2, in a preclinical setting

INTRODUCTION

NeoB and RM2 are the most investigated gastrin-releasing peptide receptor (GRPR)-targeting radiotracers in preclinical and clinical studies. Therefore, an extensive side-by-side comparison of the two radiotracers is valuable to demonstrate whether one has advantages over the other. Accordingly, this study aims to compare the in vitro and in vivo characteristics of radiolabeled NeoB and RM2 to guide future clinical studies.

METHOD

The stability of the radiolabeled GRPR analogs was determined in phosphate buffered saline (PBS), and commercially available mouse and human serum. Target affinity was determined by incubating human prostate cancer PC-3 cells with [177Lu]Lu-NeoB or [177Lu]Lu-RM2, + / - increasing concentrations of unlabeled NeoB, RM2, or Tyr4-bombesin (BBN). To determine uptake and specificity cells were incubated with [177Lu]Lu-NeoB or [177Lu]Lu-RM2 + / - Tyr4-BBN. Moreover, in vivo studies were performed to determine biodistribution and pharmacokinetics. Finally, radiotracer binding to various GRPR-expressing human cancer tissues was investigated.

RESULTS

Both radiotracers demonstrated high stability in PBS and human serum, but stability in mouse serum decreased substantially over time. Moreover, both radiotracers demonstrated high GRPR affinity and specificity, but a higher uptake of [177Lu]Lu-NeoB was observed in in vitro studies. In vivo, no difference in tumor uptake was seen. The most prominent difference in uptake in physiological organs was observed in the GRPR-expressing pancreas; [177Lu]Lu-RM2 had less pancreatic uptake and a shorter pancreatic half-life than [177Lu]Lu-NeoB. Furthermore, [177Lu]Lu-RM2 presented with a lower tumor-to-kidney ratio, while the tumor-to-blood ratio was lower for [177Lu]Lu-NeoB. The autoradiography studies revealed higher binding of radiolabeled NeoB to all human tumor tissues.

CONCLUSION

Based on these findings, we conclude that the in vivo tumor-targeting capability of radiolabeled NeoB and RM2 is similar. Additional studies are needed to determine whether the differences observed in physiological organ uptakes, i.e., the pancreas, kidneys, and blood, result in relevant differences in organ absorbed doses when the radiotracers are applied for therapeutic purposes.

The effects of novel macrocyclic chelates on the targeting properties of the 68Ga-labeled Gastrin releasing peptide receptor antagonist RM2

BACKGROUND

The gastrin-releasing peptide receptor (GRPr) is a molecular target for the visualization of prostate cancer. Bombesin (BN) analogs are short peptides with a high affinity for GRPr. RM2 is a bombesin-based antagonist. It has been demonstrated that RM2 have superior in vivo biodistribution and targeting properties than high-affinity receptor agonists. This study developed new RM2-like antagonists by introducing the novel bifunctional chelators AAZTA⁵ and DATA^5m to RM2.

RESULTS

The effects of different macrocyclic chelating groups on drug targeting properties and the possibility of preparing ⁶⁸Ga-radiopharmaceuticals in a kit-based protocol were investigated using ⁶⁸Ga-labeled entities. Both new RM2 variants were labelled with ⁶⁸Ga³⁺ resulting in high yields, stability, and low molarity of the ligand. DATA^5m-RM2 and AAZTA⁵-RM2 incorporated ⁶⁸Ga³⁺ nearly quantitatively at room temperature within 3-5 min, and the labelling yield for ⁶⁸Ga-DOTA-RM2 was approximately 10% under the same conditions. ⁶⁸Ga-AAZTA⁵-RM2 showed stronger hydrophilicity according to partition coefficient. Although the maximal cellular uptake values of the three compounds were similar, ⁶⁸Ga-AAZTA⁵-RM2 and ⁶⁸Ga-DATA^5m-RM2 peaked more rapidly. Biodistribution studies showed high and specific tumor uptake, with a maximum of 9.12 ± 0.81 percentage injected activity per gram of tissue (%ID/g) for ⁶⁸Ga-DATA^5m-RM2 and 7.82 ± 0.61%ID/g for ⁶⁸Ga-AAZTA⁵-RM2 at 30 min after injection.

CONCLUSIONS

The conditions for complexation of DATA^5m-RM2 and AAZTA⁵-RM2 with gallium-68 are milder, faster and require less amount of precursors than DOTA-RM2. Chelators had an evident influence on the pharmacokinetics and targeting properties of ⁶⁸Ga-X-RM2 derivatives. Positively charged ⁶⁸Ga-DATA^5m-RM2 provided a high tumor uptake, high image contrast and good capability of targeting GRPr.

Substitution of l-Tryptophan by -Methyl-l-Tryptophan in 177Lu-RM2 Results in 177Lu-AMTG, a High-Affinity Gastrin-Releasing Peptide Receptor Ligand with Improved In Vivo Stability

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.

68Ga-PSMA and 68Ga-DOTA-RM2 PET/MRI in Recurrent Prostate Cancer: Diagnostic Performance and Association with Clinical and Histopathological Data

Simple Summary

Prostate cancer (PCa) relapse occurs in up to 50% of patients after radical treatment. Once PCa recurrence is detected, a precise identification of the number and sites of recurrence is necessary to tailor the treatment on the patient’s needs. Positron emission tomography (PET) plays a pivotal role in this clinical setting and new radiotracers have been developed to improve its performance. While ⁶⁸Ga-PSMA is a well-established radiotracer for PCa recurrence detection, ⁶⁸Ga-DOTA-RM2 is a recently proposed tracer that targets the gastrin-releasing peptide receptors that are overexpressed in prostate cancer. In this work, the performance of ⁶⁸Ga-PSMA and ⁶⁸Ga-DOTA-RM2 PET/MRI in identifying recurrent disease were compared on the same cohort, using the same study protocol, as this is the only way to assess whether one outperforms the other and therefore should be preferred in clinical practice. Furthermore, the association between PET findings and clinical and histopathological characteristics was investigated to find potential biomarkers.

Abstract

The aim of the present study is to investigate and compare the performances of ⁶⁸Ga-PSMA and ⁶⁸Ga-DOTA-RM2 PET/MRI in identifying recurrent prostate cancer (PCa) after primary treatment and to explore the association of dual-tracer PET findings with clinical and histopathological characteristics. Thirty-five patients with biochemical relapse (BCR) of PCa underwent ⁶⁸Ga PSMA PET/MRI for restaging purpose, with 31/35 also undergoing ⁶⁸Ga-DOTA-RM2 PET/MRI scan within 16 days (mean: 3 days, range: 2–16 days). Qualitative and quantitative image analysis has been performed by comparing ⁶⁸Ga-PSMA and ⁶⁸Ga-DOTA-RM2 PET/MRI findings both on a patient and lesion basis. Clinical and instrumental follow-up was used to validate PET findings. Fisher’s exact test and Mann-Whitney U test were used to investigate the association between dual-tracer PET findings, clinical and histopathological data. p-value significance was defined below the 0.05 level. Patients’ mean age was 70 years (range: 49–84) and mean PSA at time of PET/MR scans was 1.88 ng/mL (range: 0.21–14.4). A higher detection rate was observed for ⁶⁸Ga-PSMA PET/MRI, with more lesions being detected compared to ⁶⁸Ga-DOTA-RM2 PET/MRI (26/35 patients, 95 lesions vs. 15/31 patients, 41 lesions; p = 0.016 and 0.002). ⁶⁸Ga-PSMA and ⁶⁸Ga-DOTA-RM2 PET/MRI findings were discordant in 11/31 patients; among these, 10 were ⁶⁸Ga-PSMA positive (9/10 confirmed as true positive and 1/10 as false positive by follow-up examination). Patients with higher levels of PSA and shorter PSA doubling time (DT) presented more lesions on ⁶⁸Ga-PSMA PET/MRI (p = 0.006 and 0.044), while no association was found between PET findings and Gleason score. ⁶⁸Ga-PSMA has a higher detection rate than ⁶⁸Ga-DOTA-RM2 in detecting PCa recurrence. The number of ⁶⁸Ga-PSMA PET positive lesions is associated with higher levels of PSA and shorter PSA DT, thus representing potential prognostic factors.

Preliminary Results of an Ongoing Prospective Clinical Trial on the Use of 68Ga-PSMA and 68Ga-DOTA-RM2 PET/MRI in Staging of High-Risk Prostate Cancer Patients

The aim of the present study is to investigate the synergic role of ⁶⁸Ga-PSMA PET/MRI and ⁶⁸Ga-DOTA-RM2 PET/MRI in prostate cancer (PCa) staging. We present pilot data on twenty-two patients with biopsy-proven PCa that underwent ⁶⁸Ga-PSMA PET/MRI for staging purposes, with 19/22 also undergoing ⁶⁸Gaa-DOTA-RM2 PET/MRI. TNM classification based on image findings was performed and quantitative imaging parameters were collected for each scan. Furthermore, twelve patients underwent radical prostatectomy with the availability of histological data that were used as the gold standard to validate intraprostatic findings. A DICE score between regions of interest manually segmented on the primary tumour on ⁶⁸Ga-PSMA PET, ⁶⁸Ga-DOTA-RM2 PET and on T2 MRI was computed. All imaging modalities detected the primary PCa in 18/19 patients, with ⁶⁸Ga-DOTA-RM2 PET not detecting any lesion in 1/19 patients. In the remaining patients, ⁶⁸Ga-PSMA and MRI were concordant. Seven patients presented seminal vesicles involvement on MRI, with two of these being also detected by ⁶⁸Ga-PSMA, and ⁶⁸Ga-DOTA-RM2 PET being negative. Regarding extraprostatic disease, ⁶⁸Ga-PSMA PET, ⁶⁸Ga-DOTA-RM2 PET and MRI resulted positive in seven, four and five patients at lymph-nodal level, respectively, and at a bone level in three, zero and one patients, respectively. These preliminary results suggest the potential complementary role of ⁶⁸Ga-PSMA PET, ⁶⁸Ga-DOTA-RM2 PET and MRI in PCa characterization during the staging phase.

Voxel-based comparison of [68Ga]Ga-RM2-PET/CT and [68Ga]Ga-PSMA-11-PET/CT with histopathology for diagnosis of primary prostate cancer

BACKGROUND

Focal therapies or focally escalated therapies of primary prostate cancer are becoming more and more important. This increases the need to identify the exact extension of the intraprostatic tumor and possible dominant intraprostatic lesions by imaging techniques. While the prostate-specific membrane antigen (PSMA) is already a well-established target for imaging of prostate cancer cells, the gastrin-releasing peptide receptor (GRPR) seems to provide interesting additional information. Histopathology was used to examine the extent to which the single and combined image information of PET scans targeting GRPR and PSMA might lead to better tumor delineation.

METHODS

Eight patients with histologically proven primary prostate cancer underwent two positron emission tomography with computer tomography scans, [68Ga]Ga-RM2-PET/CT (RM2-PET) and [68Ga]Ga-PSMA-11-PET/CT (PSMA-PET), prior to radical prostatectomy. RM2-PET data were correlated voxel-wise to a voxel-based model of the histopathologic tumor volume information. The results were compared to, correlated to, and combined with the correlation of PSMA-PET data analyzed analogously.

RESULTS

In 4/8 patients, RM2-PET showed a higher signal in histologically proven tumor regions compared to PSMA. There were also tumor regions where PSMA-PET showed a higher signal than GRPR in 4/8 patients. A voxel-wise correlation of RM2-PET against histopathology yielded similar results compared to the correlation of PSMA-PET against histopathology, while PSMA-PET is the slightly better performing imaging technique. The combined information of both tracers yielded the best overall result, although this effect was not statistically significant compared to RM2-PET alone.

CONCLUSIONS

Qualitative and quantitative findings in this preliminary study with 8 patients indicate that RM2-PET and PSMA-PET partially show not only the same, but also distinct regions of prostate cancer. Patients with pPCa might profit from information given by tracers targeting GRPR and PSMA simultaneously, in terms of a better delineation of the gross tumor volume.

Matched-pair, 86Y/90Y-labeled, bivalent RGD/bombesin antagonist, [RGD-Glu-[DO3A]-6-Ahx-RM2], as a potential theranostic agent for prostate cancer

INTRODUCTION

In this study, we describe development of a true matched-pair theranostic agent that is able to target the αVβ3 integrin and the gastrin releasing peptide receptor (GRPR). We herein describe methods to metallate and characterize the new conjugate and to validate its biological efficacy by in vitro and in vivo methods.

METHODS

We have previously described the development of [RGD-Glu-6Ahx-RM2] (where RGD: Arg-Gly-Asp; Glu: glutamic acid; 6-Ahx: 6-amino hexanoic acid; RM2: (D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2)) that has been conjugated to a DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) bifunctional chelating agent (BFCA) to afford [RGD-Glu-[DO3A]-6-Ahx-RM2] peptide. In this study, we have radiolabeled [RGD-Glu-[DO3A]-6-Ahx-RM2] peptide with 86Y or 90Y. Natural-metallated (natY) conjugates were assessed for binding affinity for the αVβ3 integrin or GRPR in human glioblastoma U87-MG and prostate PC-3 cell lines, respectively. The effective stability of the new tracers was also evaluated prior to in vivo evaluation in normal CF-1 mice and SCID mice bearing xenografted tumors.

RESULTS

Competitive displacement binding assays in PC-3 cells showed high binding affinity for the GRPR (IC50, 5.65 ± 0.00 nM). On the other hand, competitive displacement binding assays in U87-MG cells revealed only moderate binding to the αVβ3 integrin (IC50, 346 ± 5.30 nM). Biodistribution studies in PC-3 tumor-bearing mice [RGD-Glu-[[90Y]Y-DO3A]-6-Ahx-RM2] showed high tumor uptake (8.70 ± 0.35%ID/g at 1 h post-intravenous injection) and retention of tracer (5.28 ± 0.12%ID/g) at 24 h post-intravenous injection. Micro-positron emission tomography (microPET) in PC-3 tumor-bearing mice using [RGD-Glu-[[86Y]Y-DO3A]-6-Ahx-RM2] correlated well with biodistribution investigations over the various time points that were studied.

CONCLUSIONS

The [RGD-Glu-[[86Y]Y-DO3A]-6-Ahx-RM2] and [RGD-Glu-[[90Y]Y-DO3A]-6-Ahx-RM2] matched-pair conjugates described herein exhibit favorable microPET and pharmacokinetic profiles and merit further investigations for molecular imaging and/or therapeutic evaluation in larger animal models and potentially humans.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

The theranostic, heterobivalent, agents described herein perform comparably with other mono- and multivalent conjugates we have reported and offer the potential of improved sensitivity for detecting prostate cancer cells that might exhibit differing profiles of receptor expression on tumor cells in human patients.

Prospective Evaluation of 68Ga-RM2 PET/MRI in Patients with Biochemical Recurrence of Prostate Cancer and Negative Findings on Conventional Imaging

⁶⁸Ga-labeled DOTA-4-amino-1-carboxymethyl-piperidine-d-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH₂ (⁶⁸Ga-RM2) is a synthetic bombesin receptor antagonist that targets gastrin-releasing peptide receptor (GRPr). GRPr proteins are highly overexpressed in several human tumors, including prostate cancer (PCa). We present data from the use of ⁶⁸Ga-RM2 in patients with biochemical recurrence (BCR) of PCa and negative findings on conventional imaging. Methods: We enrolled 32 men with BCR of PCa, who were 59-83 y old (mean ± SD, 68.7 ± 6.4 y). Imaging started at 40-69 min (mean, 50.5 ± 6.8 min) after injection of 133.2-151.7 MBq (mean, 140.6 ± 7.4 MBq) of ⁶⁸Ga-RM2 using a time-of-flight-enabled simultaneous PET/MRI scanner. T1-weighted, T2-weighted, and diffusion-weighted images were acquired. Results: All patients had a rising level of prostate-specific antigen (PSA) (range, 0.3-119.0 ng/mL; mean, 10.1 ± 21.3 ng/mL) and negative findings on conventional imaging (CT or MRI, and a ^99mTc-methylene diphosphonate bone scan) before enrollment. The observed ⁶⁸Ga-RM2 PET detection rate was 71.8%. ⁶⁸Ga-RM2 PET identified recurrent PCa in 23 of the 32 participants, whereas the simultaneous MRI scan identified findings compatible with recurrent PCa in 11 of the 32 patients. PSA velocity was 0.32 ± 0.59 ng/mL/y (range, 0.04-1.9 ng/mL/y) in patients with negative PET findings and 2.51 ± 2.16 ng/mL/y (range, 0.13-8.68 ng/mL/y) in patients with positive PET findings (P = 0.006). Conclusion:⁶⁸Ga-RM2 PET can be used for assessment of GRPr expression in patients with BCR of PCa. High uptake in multiple areas compatible with cancer lesions suggests that ⁶⁸Ga-RM2 is a promising PET radiopharmaceutical for localization of disease in patients with BCR of PCa and negative findings on conventional imaging.

Characterization and evaluation of DOTA-conjugated Bombesin/RGD-antagonists for prostate cancer tumor imaging and therapy

INTRODUCTION

Here we present the metallation, characterization, in vivo and in vitro evaluations of dual-targeting, peptide-based radiopharmaceuticals with utility for imaging and potentially treating prostate tumors by virtue of their ability to target the αVβ3 integrin or the gastrin releasing peptide receptor (GRPr).

METHODS

[RGD-Glu-6Ahx-RM2] (RGD: Arg-Gly-Asp; Glu: glutamic acid; 6-Ahx: 6-amino hexanoic acid; RM2: (D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2)) was conjugated to a DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) bifunctional chelator (BFCA) purified via reversed-phase high-performance liquid chromatography (RP-HPLC), characterized by electrospray ionization-mass spectrometry (ESI-MS), and radiolabeled with (111)In or (177)Lu. Natural-metallated compounds were assessed for binding affinity for the αVβ3 integrin or GRPr in human glioblastoma U87-MG and prostate PC-3 cell lines and stability prior to in vivo evaluation in normal CF-1 mice and SCID mice xenografted with PC-3 cells.

RESULTS

Competitive displacement binding assays with PC-3 and U87-MG cells revealed high to moderate binding affinity for the GRPr or the αVβ3 integrin (IC50 range of 5.39±1.37 nM to 9.26±0.00 nM in PC-3 cells, and a range of 255±47 nM to 321±85 nM in U87-MG cells). Biodistribution studies indicated high tumor uptake in PC-3 tumor-bearing mice (average of 7.40±0.53% ID/g at 1h post-intravenous injection) and prolonged retention of tracer (mean of 4.41±0.91% ID/g at 24h post-intravenous injection). Blocking assays corroborated the specificity of radioconjugates for each target. Micro-single photon emission computed tomography (microSPECT) confirmed favorable radiouptake profiles in xenografted mice at 20h post-injection.

CONCLUSIONS

[RGD-Glu-[(111)In-DO3A]-6-Ahx-RM2] and [RGD-Glu-[(177)Lu- DO3A]-6-Ahx-RM2] show favorable pharmacokinetic and radiouptake profiles, meriting continued evaluation for molecular imaging in murine U87-MG/PC-3 xenograft models and radiotherapy studies with (177)Lu and (90)Y conjugates.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

These heterovalent, peptide-targeting ligands perform comparably with many mono- and multivalent conjugates with the potential benefit of increased sensitivity for detecting cancer cells exhibiting differential expression of target receptors.