Synthesis of a ⁶⁸Ga-Labeled Peptoid−Peptide Hybrid for Imaging of Neurotensin Receptor Expression in Vivo

Abstract

The neurotensin receptor subtype 1 (NTS1) represents an attractive molecular target for imaging various tumors. Positron emission tomography (PET) gained widespread importance due to its sensitivity. We combined the design of a metabolically stable neurotensin analogue with a ⁶⁸Ga-radiolabeling approach. The ⁶⁸Ga-labeled peptoid−peptide hybrid [⁶⁸Ga]3 revealed high stability, specific tumor uptake (0.7%ID/g, 65 min p.i.), and advantageous biokinetics in vivo using HT29 tumor-bearing nude mice. Because of the ability to internalize into NTS1-expressing tumor cells, [⁶⁸Ga]3 proved to be highly suitable for a reliable and practical visualization of NTS1-expressing tumors in vivo by small animal PET.

Positron emission tomography (PET) has emerged as an imaging modality with superior sensitivity. It has been used to study the expression and activity of a variety of molecular targets such as receptors in vivo.¹ Bioactive peptides that specifically recognize their corresponding receptors in vivo represent a class of PET tracers with growing importance for imaging therapy response in the clinical surrounding.² To date, the majority of peptide-based radiopharmaceuticals are labeled with single photon emitters, such as technetium-99 m, indium-111, or iodine-123. Prominent examples are a ^99mTc-labeled bombesin derivative ([^99mTc]Leu13-BN), [¹¹¹In]octreotide (OctreoScan), or ¹²³I-labeled vasoactive intestinal peptide ([¹²³I]VIP).² Recent progress has also been achieved with regard to labeling peptides with the positron emitter ¹⁸F (t_1/2 = 110 min).^2,3 These approaches include rapid, chemoselective ligation methods with ¹⁸F-labeled prosthetic groups.⁴ One major obstacle to developing and using PET tracers of peptide receptors is the limited availability of the radionuclides suitable for PET imaging. One solution to this problem is offered by the ⁶⁸Ge/⁶⁸Ga generator,^5,6 providing frequent and easy access to the short-lived ⁶⁸Ga isotope (t_1/2 = 68 min) multiple times a day within the generator's lifetime of about 1 year. Therefore, the ⁶⁸Ge/⁶⁸Ga generator system provides increased availability of the short-lived ⁶⁸Ga and significant cost reduction in comparison with cyclotron-produced positron emitters, such as ¹⁸F. The chemistry of ⁶⁸Ga relies on the use of chelators that provide advantages, such as the use of very low amounts of chelator-linked peptides in aqueous buffer solution for quantitative radiolabeling reactions. However, very few radiolabeled peptides suitable for PET imaging studies have been developed so far.² Thus, the majority of peptide receptors that are potential molecular targets for imaging of, for example, tumor response or tumor progression, have not yet been addressed by adequate peptide-based PET imaging probes.

The pivotal role of neurotensin (NT) and its receptors (NTS1, NTS2, and NTS3) in oncogenesis relies on autocrine, paracrine, and endocrine mechanisms. The crucial importance of the NTS1 expression has been demonstrated for various types of human tumors, such as breast cancer, prostate cancer, and colorectal tumors.⁷ The NT carboxy terminus NT(8−13) represents the truncated binding sequence of the natural agonist NT; however, the versatility of NTS1 imaging agents derived from NT(8−13) is frequently hampered by the low resistance of NT(8−13) toward degradation by endogenous peptidases. A variety of studies addressed this issue by modifying the amino acid sequence of NT(8−13).⁷ Among these, radiolabeled peptide candidates for therapy and in vivo imaging applications by single photon emitter tomography (SPECT) have been developed.⁸⁻¹⁵ However, PET offers significant advantages over SPECT, such as improved spatial resolution and sensitivity combined with the option to quantify tracer concentration in absolute units in vivo.¹⁶

On the basis of our studies on the influence of peptide backbone modifications and ligand conformation on affinity changes for a series of NT(8−13) analogues,^17,18 we recently discovered a metabolically stabilized amino acid sequence by alteration of three amino acids in the sequence of NT(8−13), most importantly including the replacement of the N-terminal Arg-Arg by the peptoid-like residue N-(4-aminobutyl)Gly-Lys (NLys-Lys).^17,19 These studies led to the characterization of the first metabolically stable ¹⁸F-labeled NTS1 imaging probe for PET ([¹⁸F]FGlc-NT4; see Figure 1) when we applied the strategy of an efficient click chemistry-based ¹⁸F-labeling and glycosylation method²⁰ to a NT(8−13) related amino acid sequence. The ¹⁸F-glycopeptide mimetic [¹⁸F]FGlc-NT4 displayed a K_i value of 16 nM for hNTS1 and was successfully applied to small-animal PET (μPET) studies for imaging hNTS1 expression in vivo.¹⁹ However, [¹⁸F]FGlc-NT4 showed high kidney uptake in vivo and suffered from insufficient clearance properties. Because elongation of the N-terminal ending of NT(8−13) is generally well-tolerated with respect to NTS recognition, we aimed at the synthesis of an N-terminal chelator-linked peptoid−peptide hybrid, which could provide both the opportunity for simple ⁶⁸Ga-radiolabeling by the formation of stable complexes with ⁶⁸Ga and improved biokinetic properties of the resulting ⁶⁸Ga-labeled peptide mimetic [⁶⁸Ga]3 (Figure 1).

Figure 1.

Chemical structure of the glycopeptoid [¹⁸F]FGlc-NT4¹⁹ and the DOTA-conjugated derivative [⁶⁸Ga]3.

We here report the first ⁶⁸Ga-labeled NT surrogate suitable for imaging of NT receptor expression in vivo by PET. After demonstration of binding affinity to hNTS1, radiolabeling with ⁶⁸Ga, and investigation of metabolic stability in vitro, [⁶⁸Ga]3 was successfully applied to biodistribution and μPET studies using a HT29 xenograft nude mice model.

Our plan of synthesis was based on our previous publications on metabolically stabilized NT(8−13) derivatives (Scheme 1).^17,19 In detail, we applied solid-phase methods with repetitive cycles of Fmoc deprotection with piperidine and acylation with the Fmoc-protected amino acids in the presence of PyBOP/HOBt, thus allowing us to incorporate t-leucine, proline, lysine, and N-(4-aminobutyl)-glycine. The peptide coupling of Fmoc-Tyr(tBu)-OH to the sterically demanding t-leucyl residue was achieved by activation with HATU. The attachment of the chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) was performed employing the commercially available tri-t-butyl ester derivative. Microwave acceleration proved to be advantageous for both Fmoc deprotection of the resin and acylation to afford 1 (Scheme 1). The direct cleavage of the peptide from the resin with TFA resulted in the formation of hardly separable t-butylated byproducts. This obstacle was overcome by following a previously reported protocol using 10% sulfuric acid in dioxan at 8 °C, which allowed us to safely remove the t-butyl-based protective groups while preventing liberation of the peptide from the solid support.²¹ Thereafter, TFA-mediated cleavage furnished the DOTA derivative 2 sufficiently pure to allow successful high-performance liquid chromatography (HPLC) purification.

Scheme 1.

The synthesis of the Ga complex 3 was accomplished by stirring 2 with an excess of Ga(NO₃)₃ (1.4 equiv) in acetate buffer (pH 4) at 90 °C, followed by RP-HPLC purification (Scheme 1). Electrospray ionization−mass spectrometry revealed the [M]⁺ peak of 3, confirming previous studies on DOTA-conjugated peptides with one free deprotonated carboxylate group at physiological pH that does not participate with the pseudo-octahedral geometry of the Ga−DOTA complex.

After confirmation of the high purity of 3 (>99%) by liquid chromatography−mass spectrometry (LC-MS), we proceeded with NT receptor binding studies using a Chinese hamster ovary (CHO) cell line, stably expressing the human NTS1 as described previously.¹⁹ The Ga−DOTA conjugate 3 turned out to be significantly less potent in comparison with NT(8−13) and the previously described fluoroglycosyl-conjugated peptide mimetic FGlc-NT4 (Table 1). Notably, the DOTA-linked precursor 2 displayed a 13-fold lower NTS1 affinity when compared with the Ga−DOTA complex 3. Thus, small amounts of 2 within the radiolabeled product solution of [⁶⁸Ga]3 could be tolerated without significantly suppressing the specific binding of [⁶⁸Ga]3 in vivo. The K_i values of 3 and FGlc-NT4 differed by a factor of 10. This effect could be due to the sterically demanding Ga−DOTA moiety directly linked to the pharmacophoric residue 8 of the parent peptide.¹⁴ On the other hand, the ability of agonist ligands to internalize into tumor cells via NTS1 could provide sufficient tumor labeling in vivo, even for ligands with lower affinities. Thus, we continued our efforts on the development of the ⁶⁸Ga-labeled analogue.

Table 1. Receptor Binding Data of NT Analogues in Comparison with the Reference NT(8-13) as Determined by Displacement of [³H]NT Using hNTS1-Expressing CHO Cells^a.

^aK_i values are based on the means of 2−7 individual experiments, each done in triplicate.

^bK_i ± SEM (nM).

^cValues are from ref (19).

^dK_i ± SD (nM).

The radiosynthesis of [⁶⁸Ga]3 was performed by the addition of an aliquot of the [⁶⁸Ga]GaCl₃ eluate, freshly obtained from the ⁶⁸Ge/⁶⁸Ga generator, to an aqueous-buffered solution of 2 (20 nmol, acetate buffer, pH 4.5) at 95 °C (Scheme 1). The ⁶⁸Ga-labeled peptide complex [⁶⁸Ga]3 was obtained in a radiochemical yield (RCY) and purity of >98% (radio-HPLC) within a total synthesis time of only 16 min. Because of the low amount of the peptide precursor (2, 20 nmol) that was needed for the labeling reaction and the low NTS1 affinity of 2, we could omit a time-consuming subsequent HPLC separation of 2 and [⁶⁸Ga]3. The use of aqueous buffer allowed an easy formulation of [⁶⁸Ga]3 by pH adjustment of the final product solution and a sufficient radioactivity concentration for further experimental use. The specific activity of [⁶⁸Ga]3 was about 10 GBq/μmol at the end of radiosynthesis, rendering [⁶⁸Ga]3 suitable for PET imaging of apparently high NT receptor densities on tumors grown as subcutaneous xenografts.

After the successful radiosynthesis of [⁶⁸Ga]3, we turned our attention to the stability of the ⁶⁸Ga-labeled NT analogue toward degradation by peptidases present in serum. The stability of [⁶⁸Ga]3 was investigated in human serum at 37 °C. The HPLC data indicated high metabolic stability of 96% after an incubation time of 60 min (see the Supporting Information). Moreover, the lipophilicity (logD_7.4) of [⁶⁸Ga]3 was determined to be −4.35 ± 0.26 (n = 6), and internalization studies with [⁶⁸Ga]3 were performed in vitro using HT29 cells. The HT29 cell line is a human colon cancer cell line that expresses the hNTS1 similarly to cells from normal intestinal mucosa.²² The internalization rate of the [⁶⁸Ga]3/NTS1 complex into HT29 cells was approximately 70% as determined by the acid wash procedure^19,23 after incubation with the tracer for 30 min at room temperature (Figure 2). The ability of [⁶⁸Ga]3 to internalize in vitro suggested enhanced retention of this tracer in NTS1-expressing tumors in vivo, thus prompting us to perform biodistribution studies.

Figure 2.

Time dependency of internalization of [⁶⁸Ga]3 in HT29 cells. Data are expressed as mean values ± SDs from two independent experiments, each performed in quadruplicate.

We investigated the biodistribution of [⁶⁸Ga]3 in HT29 xenografted nude mice at 10, 30, and 65 min postinjection (p.i.; Figure 3). [⁶⁸Ga]3 demonstrated metabolic stability in vivo (90% intact tracer in blood, 10 min p.i.; see the Supporting Information), fast blood clearance, high uptake in the kidneys, and low uptake in the liver, suggesting that hepatobiliary clearance of [⁶⁸Ga]3 did not play a role. The markedly dominating excretion kinetics of [⁶⁸Ga]3 occurred via fast renal clearance (Figure 3a). This notion is consistent with various other studies on DOTA-conjugated peptides,^23,24 reflecting the effect of the free carboxylate group of the DOTA chelator that is unprotonated under physiological conditions. Most importantly, [⁶⁸Ga]3 showed significantly lower kidney uptake and an improved kidney clearance [14.1% ID/g (10 min p.i.); 8.1% ID/g (65 min p.i.); Figure 3a] when compared to the previously described [¹⁸F]FGlc-NT4 [data from ref (19): 57.1% ID/g (10 min p.i.); 50.0% ID/g (65 min p.i.)]. The uptake of [⁶⁸Ga]3 in HT29 tumors was similar (1.2% ID/g at 30 min p.i.; 0.7% ID/g at 65 min p.i.) to our ¹⁸F-labeled glycopeptide mimetic,¹⁹ with tumor/blood ratios rapidly increasing from 1.1 to 5.4 (10−65 min p.i.; Figure 3b). This indicates a suitable signal/noise ratio for in vivo imaging applications at early time points after injection. The in vivo specificity of NTS1-mediated uptake of [⁶⁸Ga]3 was proven by the analysis of tumor uptake in animals that were coinjected with [⁶⁸Ga]3 and the high affinity NTS1 ligand NT4¹⁹ (Table 1; 100 μg/animal). The coinjected animals revealed a significantly reduced tumor uptake in the biodistribution studies at 65 min p.i. (P < 0.05; Figure 3a).

Figure 3.

Biodistribution of [⁶⁸Ga]3 in nude mice bearing HT29 xenografts (a) and tumor/blood ratio (b). Data are expressed as mean values ± standard deviations (SDs) from 3 to 4 animals [*P < 0.05, 65 min p.i.: 0.72 ± 0.18% ID/g (n = 3) vs 0.34 ± 0.17% ID/g (n = 4)].

Using a μPET scanner and HT29 tumor-bearing nude mice, we also demonstrated the specific visualization of NTS1 expression in vivo by [⁶⁸Ga]3 (Figure 4). One day after administration of [⁶⁸Ga]3 in a first PET scan, the experiment was repeated with the same animal coinjected with [⁶⁸Ga]3 and an excess of NT4. A representative set of PET images including transaxial and coronal image projections is shown in Figure 4a, clearly indicating specific binding of [⁶⁸Ga]3 in HT29 tumors grown as subcutaneous xenografts, when 20 min static PET images were acquired at 45 min after injection. For each PET scan, regions of interest were drawn around the tumor and analyzed on decay-corrected whole-body coronal images. The statistical analysis of the animal group injected with the tracer alone (n = 3) and the coinjected animals (n = 4) confirmed the results obtained by the dissection analysis and revealed a significantly reduced standard uptake value (SUV) for the tumor region in coinjected animals (Figure 4b). These results provide evidence for the specificity of [⁶⁸Ga]3 for imaging NTS1 expression in vivo.

Figure 4.

Static μPET images (transaxial and coronal projections) of HT29 tumor-bearing nude mice at 45−65 min postinjection of [⁶⁸Ga]3 with (right) and without (left) coinjection of NT4 (100 μg/mouse) (a) (white arrows indicate the tumor). Statistical analysis of the animal group injected with the tracer alone ([⁶⁸Ga]3, n = 3) and the coinjected animals ([⁶⁸Ga]3 + NT4, n = 4) (b). Maximum standard uptake values (SUV) were determined by μPET data analysis of the tumor region in each animal using the software ASI Pro. Data are expressed as mean values ± SDs from all animals in each group.

In conclusion, we successfully developed [⁶⁸Ga]3 as the first ⁶⁸Ga-labeled tracer for imaging NTS1 expression in vivo by PET. Our data confirmed that the in vitro binding affinity of a radiotracer candidate is not the only parameter that determines its suitability for in vivo imaging applications. Other factors such as the ability to internalize into cells or its lipophilicity also contribute to the in vivo binding potential of a suitable peptide radiopharmaceutical. Using [⁶⁸Ga]3 as a lead compound, work is in progress toward the development of analogues with high NTS1 binding affinity to facilitate PET imaging of low receptor densities in vivo. We anticipate that [⁶⁸Ga]3 and derivatives thereof have the potential to become valuable imaging agents for the exploration of NTS1 expression in various tumor types by experimental small animal PET and potentially in human PET imaging studies.

Supporting Information Available

Complete experimental procedures and characterization data for compounds 1−3 and [⁶⁸Ga]3. This material is available free of charge via the Internet at http://pubs.acs.org.