Approaches to improve metabolic stability of a statine-based GRP receptor antagonist

Optimization of the Pharmacokinetic Profile of [99mTc]Tc-N4-Bombesin Derivatives by Modification of the Pharmacophoric Gln-Trp Sequence

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.

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.

Radiolabeled Bombesin Analogs

Simple Summary

Recent medical advancements have strived for a personalized medicine approach to patients, aimed at optimizing therapy outcomes with minimum toxicity. In this respect, nuclear medicine methodologies have been playing increasingly important roles. For example, the overexpression of peptide receptors, such as the gastrin-releasing peptide receptor (GRPR), on tumor cells as opposed to their lack of expression in healthy surrounding tissues can be elegantly exploited with the aid of “smart” peptide carriers, such as the analogs of the amphibian 14-peptide bombesin (BBN). These molecules can bring clinically attractive radionuclides to malignant lesions in prostate, breast, and other human cancers, sparing healthy tissues. Depending upon the radionuclide in question, diagnostic imaging with single-photon emission computed tomography (SPECT) or positron emission tomography (PET) has been pursued, identifying patients who are eligible for peptide radionuclide receptor therapy (PRRT) in an integrated “theranostic” approach. In the present review, we (i) discuss the major steps taken in the development of anti-GRPR theranostic radioligands, with a focus on those selected for clinical testing; (ii) comment on the present status in this field of research; and (iii) reflect on the current limitations as well as on new opportunities for their broader and more successful clinical applications.

Abstract

The gastrin-releasing peptide receptor (GRPR) is expressed in high numbers in a variety of human tumors, including the frequently occurring prostate and breast cancers, and therefore provides the rationale for directing diagnostic or therapeutic radionuclides on cancer lesions after administration of anti-GRPR peptide analogs. This concept has been initially explored with analogs of the frog 14-peptide bombesin, suitably modified at the N-terminus with a number of radiometal chelates. Radiotracers that were selected for clinical testing revealed inherent problems associated with these GRPR agonists, related to low metabolic stability, unfavorable abdominal accumulation, and adverse effects. A shift toward GRPR antagonists soon followed, with safer analogs becoming available, whereby, metabolic stability and background clearance issues were gradually improved. Clinical testing of three main major antagonist types led to promising outcomes, but at the same time brought to light several limitations of this concept, partly related to the variation of GRPR expression levels across cancer types, stages, previous treatments, and other factors. Currently, these parameters are being rigorously addressed by cell biologists, chemists, nuclear medicine physicians, and other discipline practitioners in a common effort to make available more effective and safe state-of-the-art molecular tools to combat GRPR-positive tumors. In the present review, we present the background, current status, and future perspectives of this endeavor.

68Ga-Radiolabeling and Pharmacological Characterization of a Kit-Based Formulation of the Gastrin-Releasing Peptide Receptor (GRP-R) Antagonist RM2 for Convenient Preparation of [68Ga]Ga-RM2

Background: [⁶⁸Ga]Ga-RM2 is a potent Gastrin-Releasing Peptide-receptor (GRP-R) antagonist for imaging prostate cancer and breast cancer, currently under clinical evaluation in several specialized centers around the world. Targeted radionuclide therapy of GRP-R-expressing tumors is also being investigated. We here report the characteristics of a kit-based formulation of RM2 that should ease the development of GRP-R imaging and make it available to more institutions and patients. Methods: Stability of the investigated kits over one year was determined using LC/MS/MS and UV-HPLC. Direct ⁶⁸Ga-radiolabeling was optimized with respect to buffer (pH), temperature, reaction time and shaking time. Conventionally prepared [⁶⁸Ga]Ga-RM2 using an automated synthesizer was used as a comparator. Finally, the [⁶⁸Ga]Ga-RM2 product was assessed with regards to hydrophilicity, affinity, internalization, membrane bound fraction, calcium mobilization assay and efflux, which is a valuable addition to the in vivo literature. Results: The kit-based formulation, kept between 2 °C and 8 °C, was stable for over one year. Using acetate buffer pH 3.0 in 2.5–5.1 mL total volume, heating at 100 °C during 10 min and cooling down for 5 min, the [⁶⁸Ga]Ga-RM2 produced by kit complies with the requirements of the European Pharmacopoeia. Compared with the module production route, the [⁶⁸Ga]Ga-RM2 produced by kit was faster, displayed higher yields, higher volumetric activity and was devoid of ethanol. In in vitro evaluations, the [⁶⁸Ga]Ga-RM2 displayed sub-nanomolar affinity (K_d = 0.25 ± 0.19 nM), receptor specific and time dependent membrane-bound fraction of 42.0 ± 5.1% at 60 min and GRP-R mediated internalization of 24.4 ± 4.3% at 30 min. The [^natGa]Ga-RM2 was ineffective in stimulating intracellular calcium mobilization. Finally, the efflux of the internalized activity was 64.3 ± 6.5% at 5 min. Conclusion: The kit-based formulation of RM2 is suitable to disseminate GRP-R imaging and therapy to distant hospitals without complex radiochemistry equipment.

Internal Radiation Therapy

Targeted therapies are applied to increase the efficiency of antitumor treatment by simultaneously decreasing side effects. This can be achieved using carrier molecules which specifically bind to target structures or areas with remodeling activity. These carrier molecules may be coupled to chemotherapeutic drugs or to radioactive isotopes. In most cases, these carrier molecules are antibodies against tumor antigens, peptides, or small molecules which are binders for overexpressed receptors on tumor cells. The paradigm of endoradiotherapy is exemplified by the peptidic tracer DOTATOC which binds to somatostatin receptors and recently also small molecule inhibitors with high affinity for the prostate-specific membrane antigen.

Methods to Enhance the Metabolic Stability of Peptide-Based PET Radiopharmaceuticals

The high affinity and specificity of peptides towards biological targets, in addition to their favorable pharmacological properties, has encouraged the development of many peptide-based pharmaceuticals, including peptide-based positron emission tomography (PET) radiopharmaceuticals. However, the poor in vivo stability of unmodified peptides against proteolysis is a major challenge that must be overcome, as it can result in an impractically short in vivo biological half-life and a subsequently poor bioavailability when used in imaging and therapeutic applications. Consequently, many biologically and pharmacologically interesting peptide-based drugs may never see application. A potential way to overcome this is using peptide analogues designed to mimic the pharmacophore of a native peptide while also containing unnatural modifications that act to maintain or improve the pharmacological properties. This review explores strategies that have been developed to increase the metabolic stability of peptide-based pharmaceuticals. It includes modifications of the C- and/or N-termini, introduction of d- or other unnatural amino acids, backbone modification, PEGylation and alkyl chain incorporation, cyclization and peptide bond substitution, and where those strategies have been, or could be, applied to PET peptide-based radiopharmaceuticals.

Development and Characterization of a Novel, High-Affinity, Specific, Radiolabeled Ligand for BRS-3 Receptors

Bombesin (Bn) receptor subtype 3(BRS-3) is an orphan G-protein-coupled receptor of the Bn family, which does not bind any natural Bn peptide with high affinity. Receptor knockout studies show that the animals develop diabetes, obesity, altered temperature control, and other central nervous system (CNS)/endocrine/gastrointestinal changes. It is present in CNS, peripheral tissues, and tumors; however, its role in normal physiology/pathophysiology, as well as its receptor localization/pharmacology is largely unknown, in part due to the lack of a convenient, specific, direct radiolabeled ligand. This study was designed to address this problem and to develop and characterize a specific radiolabeled ligand for BRS-3. The peptide antagonist Bantag-1 had >10,000-fold selectivity for human BRS-3 (hBRS-3) over other mammalian Bn receptors (BnRs) [i.e., gastrin-releasing peptide receptor (GRPR) and neuromedin B receptor (NMBR)]. Using iodogen and basic conditions, it was radiolabeled to high specific activity (2200 Ci/mmol) and found to bind with high affinity/specificity to hBRS-3. Binding was saturable, rapid, and reversible. The ligand only interacted with known BRS-3 ligands, and not with other specific GRPR/NMBR ligands or ligands for unrelated receptors. The magnitude of ¹²⁵I-Bantag-1 binding correlated with BRS-3 mRNA expression and the magnitude of activation of phospholipase C in lung cancer cells, as well as readily identifying BRS-3 in lung cancer cells and normal tissues, allowing the direct assessment of BRS-3 receptor pharmacology/numbers on cells containing BRS-3 with other BnRs, which is usually the case. This circumvents the need for subtraction assays, which are now frequently used to assess BRS-3 indirectly using radiolabeled pan-ligands, which interact with all BnRs.

The pharmacological properties of 3-arm or 4-arm DOTA constructs for conjugation to α-melanocyte-stimulating hormone analogues for melanoma imaging

Background

Although a 3-arm DOTA construct, which has three carboxylic acids, h has been applied for conjugation to many peptides, we investigated if a 4-arm DOTA construct conjugated to peptides improves chemical properties for melanoma imaging of the melanocortin 1 receptor compared to 3-arm DOTA-conjugated peptides.

Methods

Specific activities, radiolabeling efficiencies, and partition coefficients were evaluated using ¹¹¹In-labeled 3-arm and 4-arm DOTA-α-melanocyte-stimulating hormone (MSH). For assessment of MC1-R affinity and accumulation in tumor cells in vitro, B16-F1 melanoma and/or 4T1 breast cancer cells were incubated with ¹¹¹In-labeled 3-arm and 4-arm DOTA-α-MSH with and without α-MSH as a substrate. The stability was evaluated using mouse liver homogenates and plasma. Biological distribution and whole-body single photon emission computed tomography imaging of ¹¹¹In-labeled 3-arm and 4-arm DOTA-α-MSH were obtained using B16-F1 melanoma-bearing mice.

Results

Specific activities and radiolabeling efficiencies of both radiotracers were about 1.2 MBq/nM and 90–95%, respectively. The partition coefficients were −0.28 ± 0.03 for ¹¹¹In-labeled 3-arm DOTA-α-MSH and −0.13 ± 0.04 for ¹¹¹In-labeled 4-arm DOTA-α-MSH. Although accumulation was significantly inhibited by α-MSH in B16-F1 cells, the inhibition rate of ¹¹¹In-labeled 4-arm DOTA-α-MSH was lower than that of ¹¹¹In-labeled 3-arm DOTA-α-MSH. ¹¹¹In-labeled 4-arm DOTA-α-MSH was taken up early into B16-F1 cells and showed higher accumulation than ¹¹¹In-labeled 3-arm DOTA-α-MSH after 10 min of incubation. Although these stabilities were relatively high, the stability of ¹¹¹In-labeled 4-arm DOTA-α-MSH was higher than that of ¹¹¹In-labeled 3-arm DOTA-α-MSH. Regarding biological distribution, ¹¹¹In-labeled 4-arm DOTA-α-MSH showed significantly lower average renal accumulation (1.38-fold) and significantly higher average melanoma accumulation (1.32-fold) than ¹¹¹In-labeled 3-arm DOTA-α-MSH at all acquisition times. ¹¹¹In-labeled 4-arm DOTA-α-MSH showed significantly higher melanoma-to-kidney, melanoma-to-blood, and melanoma-to-muscle ratios than ¹¹¹In-labeled 3-arm DOTA-α-MSH.

Conclusions

The 4-arm DOTA construct has better chemical properties for peptide radiotracers than the 3-arm DOTA construct.

Exploiting cancer's phenotypic guise against itself: targeting ectopically expressed peptide G-protein coupled receptors for lung cancer therapy

Lung cancer, claiming millions of lives annually, has the highest mortality rate worldwide. This advocates the development of novel cancer therapies that are highly toxic for cancer cells but negligibly toxic for healthy cells. One of the effective treatments is targeting overexpressed surface receptors of cancer cells with receptor-specific drugs. The receptors-in-focus in the current review are the G-protein coupled receptors (GPCRs), which are often overexpressed in various types of tumors. The peptide subfamily of GPCRs is the pivot of the current article owing to the high affinity and specificity to and of their cognate peptide ligands, and the proven efficacy of peptide-based therapeutics. The article summarizes various ectopically expressed peptide GPCRs in lung cancer, namely, Cholecystokinin-B/Gastrin receptor, the Bombesin receptor family, Bradykinin B1 and B2 receptors, Arginine vasopressin receptors 1a, 1b and 2, and the Somatostatin receptor type 2. The autocrine growth and pro-proliferative pathways they mediate, and the distinct tumor-inhibitory effects of somatostatin receptors are then discussed. The next section covers how these pathways may be influenced or 'corrected' through therapeutics (involving agonists and antagonists) targeting the overexpressed peptide GPCRs. The review proceeds on to Nano-scaled delivery platforms, which enclose chemotherapeutic agents and are decorated with peptide ligands on their external surface, as an effective means of targeting cancer cells. We conclude that targeting these overexpressed peptide GPCRs is potentially evolving as a highly promising form of lung cancer therapy.

Microgels of silylated HPMC as a multimodal system for drug co-encapsulation

Combined therapy is a global strategy developed to prevent drug resistance in cancer and infectious diseases. In this field, there is a need of multifunctional drug delivery systems able to co-encapsulate small drug molecules, peptides, proteins, associated to targeting functions, nanoparticles. Silylated hydrogels are alkoxysilane hybrid polymers that can be engaged in a sol-gel process, providing chemical cross linking in physiological conditions, and functionalized biocompatible hybrid materials. In the present work, microgels were prepared with silylated (hydroxypropyl)methyl cellulose (Si-HPMC) that was chemically cross linked in soft conditions of pH and temperature. They were prepared by an emulsion templating process, water in oil (W/O), as microreactors where the condensation reaction took place. The ability to functionalize the microgels, so-called FMGs, in a one-pot process, was evaluated by grafting a silylated hydrophilic model drug, fluorescein (Si-Fluor), using the same reaction of condensation. Biphasic microgels (BPMGs) were prepared to evaluate their potential to encapsulate lipophilic model drug (Nile red). They were composed of two separate compartments, one oily phase (sesame oil) trapped in the cross linked Si-HPMC hydrophilic phase. The FMGs and BPMGs were characterized by different microscopic techniques (optic, epi-fluorescence, Confocal Laser Scanning Microscopy and scanning electronic microscopy), the mechanical properties were monitored using nano indentation by Atomic Force Microscopy (AFM), and different preliminary tests were performed to evaluate their chemical and physical stability. Finally, it was demonstrated that it is possible to co-encapsulate both hydrophilic and hydrophobic drugs, in silylated microgels, that were physically and chemically stable. They were obtained by chemical cross linking in soft conditions, and without surfactant addition during the emulsification process. The amount of drug loaded was in favor of further biological activity. Mechanical stimulations should be necessary to trigger drug release.

Amide-to-triazole switch vs. in vivo NEP-inhibition approaches to promote radiopeptide targeting of GRPR-positive tumors

INTRODUCTION

Radiolabeled bombesin (BBN)-analogs have been proposed for diagnosis and therapy of gastrin-releasing peptide receptor (GRPR)-expressing tumors, such as prostate, breast and lung cancer. Metabolic stability represents a crucial factor for the success of this approach by ensuring sufficient delivery of circulating radioligand to tumor sites. The amide-to-triazole switch on the backbone of DOTA-PEG₄-[Nle¹⁴]BBN(7-14) (1) was reported to improve the in vitro stability of resulting ¹⁷⁷Lu-radioligands. On the other hand, in-situ inhibition of neutral endopeptidase (NEP) by coinjection of phosphoramidon (PA) was shown to significantly improve the in vivo stability and tumor uptake of biodegradable radiopeptides. We herein compare the impact of the two methods on the bioavailability and localization of ¹⁷⁷Lu-DOTA-PEG₄-[Nle¹⁴]BBN(7-14) analogs in GRPR-positive tumors in mice.

METHODS

The 1,4-disubstituted [1-3]-triazole was used to replace one (2: Gly¹¹-His¹²; 3: Ala⁹-Val¹⁰) or two (4: Ala⁹-Val¹⁰ and Gly¹¹-His¹²) peptide bonds in 1 (reference) and all compounds were labeled with ¹⁷⁷Lu. Each of [¹⁷⁷Lu]1-[¹⁷⁷Lu]4 was injected without (control) or with PA in healthy mice. Blood samples collected 5min post-injection (pi) were analyzed by HPLC. Biodistribution of [¹⁷⁷Lu]1-[¹⁷⁷Lu]4 was conducted in SCID mice bearing human prostate adenocarcinoma PC-3 xenografts at 4h pi. Groups of 4 animals were injected with radioligand, alone (controls), or with coinjection of PA, or of a mixture of PA and excess and [Tyr⁴]BBN to determine GRPR-specificity of uptake (Block).

RESULTS

The in vivo stability of the radioligands was: [¹⁷⁷Lu]1 (25% intact), [¹⁷⁷Lu]2 (45% intact), [¹⁷⁷Lu]3 (30% intact) and [¹⁷⁷Lu]4 (40% intact). By PA-coinjection these values notably increased to 90%-93%. Moreover, treatment with PA induced an impressive and GRPR-specific uptake of all radioligands in the PC-3 xenografts at 4h pi: [¹⁷⁷Lu]1: 4.7±0.4 to 24.8±4.9%ID/g; [¹⁷⁷Lu]2: 8.3±1.2 to 26.0±1.1%ID/g; [¹⁷⁷Lu]3: 6.6±0.4 to 21.3±4.4%ID/g; and [¹⁷⁷Lu]4: 4.8±1.6 to 13.7±3.8%ID/g.

CONCLUSIONS

This study has shown that amide-to-triazole substitutions in ¹⁷⁷Lu-DOTA-PEG₄-[Nle¹⁴]BBN(7-14) induced minor effects on bioavailability and tumor uptake in mice models, whereas in-situ NEP-inhibition(s) by PA impressively improved in vivo profiles.