Rapid and high-yield solution-phase synthesis of DOTA-Tyr3-octreotide and DOTA-Tyr3-octreotate using unprotected DOTA

Effect and mechanism of actionin vitroof cyclodextrin derivative nanoparticles loaded with tyroserleutide on hepatoma

In this study, a cyclodextrin derivative (R6RGD-CMβCD) nanoparticle with tumor targeting and cell penetration ability was successfully synthesized and loaded with tyroserleutide (YSL) to obtain YSL-loaded nanoparticles (YSL/R6RGD-CMβCD NPs). The characterization of these NPs revealed a smooth surfaces and an average diameter of approximately 170 nm. YSL/R6RGD-CMβCD NPs increased the NP uptake in Caco-2 cells. As regard the mechanism of action, the cell uptake was related to endocytosis mediated by reticulin and megacytosis. In addition, YSL/R6RGD-CMβCD NPs induced significantly higher cytotoxicity on tumor cells and better tumor targeting compared with the effect of CMβCD NPs. Most importantly, the good anti-cancer effect of YSL/R6RGD-CMβCD NPs might be due to the interference with the function of mitochondria. On the other hand, YSL/R6RGD-CMβCD NPs were not toxic for normal cells. Taken together, our results indicated that R6RGD-CMβCD could be considered as a nanopharmaceutical material with good tumor targeting abilities, and their combination with YSL could represent an effective anti-cancer system.

A New Turn in Peptide-Based Imaging Agents: Foldamers Afford Improved Theranostics Targeting Cholecystokinin-2 Receptor-Positive Cancer

Proteins adopt unique folded secondary and tertiary structures that are responsible for their remarkable biological properties. This structural complexity is key in designing efficacious peptides that can mimic the three-dimensional structure needed for biological function. In this study, we employ different chemical strategies to induce and stabilize a β-hairpin fold of peptides targeting cholecystokinin-2 receptors for theranostic application (combination of a targeted therapeutic and a diagnostic companion). The newly developed peptides exhibited enhanced folding capacity as demonstrated by circular dichroism (CD) spectroscopy, ion-mobility spectrometry-mass spectrometry, and two-dimensional (2D) NMR experiments. Enhanced folding characteristics of the peptides led to increased biological potency, affording four optimal Ga-68 labeled radiotracers ([⁶⁸Ga]Ga-4b, [⁶⁸Ga]Ga-11b-13b) targeting CCK-2R. In particular, [⁶⁸Ga]Ga-12b and [⁶⁸Ga]Ga-13b presented improved metabolic stability, enhanced cell internalization, and up to 6 fold increase in tumor uptake. These peptides hold great promise as next-generation theranostic radiopharmaceuticals.

Ultrasonic-Assisted Solid-Phase Peptide Synthesis of DOTA-TATE and DOTA-linker-TATE Derivatives as a Simple and Low-Cost Method for the Facile Synthesis of Chelator-Peptide Conjugates

Peptides have been widely adopted as biological targeting vectors for applications in molecular imaging and peptide-receptor radionuclide therapy (PRRT). Somatostatin (SST) analogues such as octreotate (TATE) are exogenous ligands for somatostatin receptors (SSTRs), which are highly expressed on neuroendocrine tumors (NETs). Recently, both [⁶⁸Ga]Ga-DOTA-TATE (NETSPOT) and [¹⁷⁷Lu]Lu-DOTA-TATE (LUTATHERA) received U.S. Food and Drug Administration approval for positron emission tomography (PET) imaging and PRRT of NETs, respectively. However, to the best of our knowledge a well-described synthesis of DOTA-TATE has not been reported in the literature. Herein, we report a fully reoptimized DOTA-TATE synthesis, including the application of a simple ultrasonic bath to greatly improve yields, reduce coupling times, and decrease the amount of reagents required for each coupling step by a half. The most prevalently used cyclizing agents such as iodine, thallium(III) trifluoroacetate, hydrogen peroxide, and dimethyl sulfoxide were compared. On-resin cyclizations using mechanical agitation showed higher yields (23% and 25% using I₂ and Tl(III), respectively) than off-resin (1.3% and 11% using DMSO and H₂O₂, respectively), and the total synthesis time of DOTA-TATE was ∼540 min excluding the cyclization step, with a total synthesis yield of ∼23%. The same manual SPPS methods/reagents were reoptimized with ultrasonic (US) agitation, resulting in an immense reduction in the total synthesis time by ∼8-fold to ∼70 min for DOTA-TATE with a higher yield (∼29% yield), and ∼13-fold to 105 min for DOTA-PEG₄-TATE (∼29% yield). Also, the use of US agitation reduces the need for excess molar equivalents of the reagents to a half, which is particularly important when coupling expensive or custom-synthesized groups such as bifunctional chelators and linkers. Finally, the synthesized DOTA-TATE was successfully radiolabeled with [⁶⁸Ga]Ga³⁺ (t_1/2 = 68 min) with high radiochemical yields (30 min, 95 °C). We believe this work opens the door to the facile and low-cost synthesis of many new chelator-linker-peptide conjugates that were previously cumbersome or cost-prohibitive to produce with manual SPPS.

Functionally Versatile and Highly Stable Chelator for 111In and 177Lu: Proof-of-Principle Prostate-Specific Membrane Antigen Targeting

Here, we present the synthesis and characterization of a new potentially nonadentate chelator H₄pypa and its bifunctional analogue ^tBu₄pypa-C7-NHS conjugated to prostate-specific membrane antigen (PSMA)-targeting peptidomimetic (Glu-urea-Lys). H₄pypa is very functionally versatile and biologically stable. Compared to the conventional chelators (e.g., DOTA, DTPA), H₄pypa has outstanding affinities for both ¹¹¹In (EC, t_1/2 ≈ 2.8 days) and ¹⁷⁷Lu (β^-,γ, t_1/2 ≈ 6.64 days). Its radiolabeled complexes were achieved at >98% radiochemical yield, RT within 10 min, at a ligand concentration as low as 10^-6 M, with excellent stability in human serum over at least 5-7 days (<1% transchelation). The thermodynamic stabilities of the [M(pypa)]^- complexes (M³⁺ = In³⁺, Lu³⁺, La³⁺) were dependent on the ionic radii, where the smaller In³⁺ has the highest pM value (30.5), followed by Lu³⁺ (22.6) and La³⁺ (19.9). All pM values are remarkably higher than those with DOTA, DTPA, H₄octapa, H₄octox, and H₄neunpa. Moreover, the facile and versatile bifunctionalization enabled by the p-OH group in the central pyridyl bridge of the pypa scaffold (compound 14) allows incorporation of a variety of linkers for bioconjugation through easy nucleophilic substitution. In this work, an alkyl linker was selected to couple H₄pypa to a PSMA-targeting pharmacophore, proving that the bioconjugation sacrifices neither the tumor-targeting nor the chelation properties. The biodistribution profiles of ¹¹¹In- and ¹⁷⁷Lu-labeled tracers are different, but promising, with the ¹⁷⁷Lu analogue particularly outstanding.

The influence of different metal-chelate conjugates of pentixafor on the CXCR4 affinity

Background

The overexpression of the chemokine receptor 4 (CXCR4) in different epithelial, mesenchymal, and hematopoietic cancers makes CXCR4 an attractive diagnostic and therapeutic target. However, targeting the CXCR4 receptor with small cyclic pentapeptide-based radiopharmaceuticals remains challenging because minor structural modifications within the ligand-linker-chelate structure often significantly affect the receptor affinity. Based on the excellent in vivo properties of CXCR4-directed pentapeptide [⁶⁸Ga]pentixafor (cyclo(-d-Tyr-N-Me-d-Orn(AMB-DOTA)-l-Arg-l-2-Nal-Gly-)), this study aims to broaden the spectrum of applicable (radio)metal-labeled pentixafor analogs.

Methods

Cyclic pentapeptides, based on the pentixafor scaffold, were synthesized by a combined solid- and solution-phase peptide synthesis. The CXCR4 receptor affinities of the cold reference compounds were determined in competitive binding assays using CXCR4-expressing Jurkat T - cell leukemia cells and [¹²⁵I]FC131 as the radioligand.

Results

Metalated pentixafor derivatives with cyclic and acyclic chelators were synthesized by solid-phase peptide synthesis and evaluated in vitro. The resulting CXCR4 affinities (IC₅₀) were highly dependent on the chelator and metal used. Two pentapeptides, Ga-NOTA and Bi-DOTA conjugates, offer an improved affinity compared to [⁶⁸Ga]pentixafor.

Conclusions

Based on the pentapeptide [⁶⁸Ga]pentixafor, a broad range of metal-labeled analogs were investigated. The affinities of the new compounds were found to be strongly dependent on both the chelator and the metal used. Bi-labeled pentixafor showed high receptor affinity and seems to be a promising ligand for further preclinical evaluation and future α-emitter-based endoradiotherapy.

Twins in spirit - episode I: comparative preclinical evaluation of [(68)Ga]DOTATATE and [(68)Ga]HA-DOTATATE

Background

Recently, an intra-patient comparison demonstrated that the somatostatin (sst) ligand [⁶⁸Ga]HA-DOTATATE ([⁶⁸Ga]DOTA-3-iodo-Tyr³-octreotate) provides PET images comparable to or superior to those obtained with [⁶⁸Ga]DOTATATE. To provide a comprehensive basis for nevertheless observed slight differences in tracer biodistribution and dosimetry, the characteristics of [⁶⁸Ga]HA-DOTATATE were investigated in a detailed preclinical study.

Methods

Affinities of ^natGa-HA-DOTATATE and ^natGa-DOTATATE to sst_1–5 were determined using membrane preparations and [¹²⁵I]SST-28 as radioligand. Internalization into AR42J cells was studied in dual-tracer studies with [¹²⁵I]TOC as internal reference. Biodistribution was investigated using AR42J tumor-bearing CD1 mice, and specificity of tracer uptake was confirmed in competition studies by coinjection of 0.8 mg TOC/kg.

Results

Sst₂ affinities (IC₅₀) of [^natGa]HA-DOTATATE (1.4 ± 0.8 nM, logP: −3.16) and [^natGa]DOTATATE (1.2 ± 0.6 nM, logP: −3.69) were nearly identical. Both compounds displayed IC₅₀ > 1 μM for sst_1,3,4, while sst₅ affinity was markedly increased for ^natGa-HA-DOTATATE (102 ± 65 nM vs >1 μM for ^natGa-DOTATATE). [^natLu]HA-DOTATATE and [^natLu]DOTATATE showed slightly lower, identical sst₂ affinities (2.0 ± 1.6 and 2.0 ± 0.8 nM, respectively) and sst₃ affinities of 93 ± 1 and 162 ± 16 nM. Internalization of [⁶⁸Ga]HA-DOTATATE was tenfold higher than that of [¹²⁵I]TOC but only sixfold higher for [⁶⁸Ga]DOTATATE and [¹⁷⁷Lu]HA-DOTATATE. While [⁶⁸Ga]HA-DOTATATE and [⁶⁸Ga]DOTATATE had shown similar target- and non-target uptake in patients, biodistribution studies in mice at 1 h post injection (n = 5) revealed slightly increased non-specific uptake of [⁶⁸Ga]HA-DOTATATE in the blood, liver, and intestines (0.7 ± 0.3, 1.0 ± 0.2, and 4.0 ± 0.7 %iD/g vs 0.3 ± 0.1, 0.5 ± 0.1, and 2.7 ± 0.8 %iD/g for [⁶⁸Ga]DOTATATE). However, sst-mediated accumulation of [⁶⁸Ga]HA-DOTATATE in the pancreas, adrenals, and tumor was significantly enhanced (36.6 ± 4.3, 10.8 ± 3.2, and 33.6 ± 10.9 %iD/g vs 26.1 ± 5.0, 5.1 ± 1.4, and 24.1 ± 4.9 %iD/g, respectively). Consequently, tumor/background ratios for [⁶⁸Ga]HA-DOTATATE in the AR42J model are comparable or slightly increased compared to [⁶⁸Ga]DOTATATE.

Conclusions

The present preclinical data fully confirm the general biodistribution pattern and excellent in vivo sst-targeting characteristics previously observed for [⁶⁸Ga]HA-DOTATATE in patients. The effect of slightly enhanced lipophilicity on background accumulation and normal organ dose is compensated by the high uptake of [⁶⁸Ga]HA-DOTATATE in tumor. Thus, [⁶⁸Ga]HA-DOTATATE represents a fully adequate, freely available substitute for [⁶⁸Ga]DOTATATE and, given the superb sst-targeting characteristics of [¹⁷⁷Lu]HA-DOTATATE in vitro, potential applicability for sst-targeted PRRT.

Synthesis of peptide radiopharmaceuticals for the therapy and diagnosis of tumor diseases

Despite the advances in molecular biology and biochemistry, the prognosis of patients suffering from tumor diseases remains poor. The limited therapeutic success can be explained by the insufficient performance of the common chemotherapeutic drugs that lack the ability to specifically target tumor tissues. Recently peptide radiopharmaceuticals have been developed that enable the concurrent imaging and therapy of tumors expressing a specific target. Here, with a special emphasis on the synthesis of the building blocks required for the complexation of metallic radioisotopes, the requirements to the design and synthesis of radiolabeled peptides for clinical applications are described.

SIB-DOTA: a trifunctional prosthetic group potentially amenable for multi-modal labeling that enhances tumor uptake of internalizing monoclonal antibodies

A major drawback of internalizing monoclonal antibodies (mAbs) radioiodinated with direct electrophilic approaches is that tumor retention of radioactivity is compromised by the rapid washout of iodo-tyrosine, the primary labeled catabolite for mAbs labeled via this strategy. In our continuing efforts to develop more versatile residualizing labels that could overcome this problem, we have designed SIB-DOTA, a prosthetic labeling template that combines the features of the prototypical, dehalogenation-resistant N-succinimidyl 3-iodobenzoate (SIB) with DOTA, a useful macrocyclic chelator for labeling with radiometals. Herein we describe the synthesis of the unlabeled standard of this prosthetic moiety, its protected tin precursor, and radioiodinated SIB-DOTA. An anti-EGFRvIII-reactive mAb, L8A4 was radiolabeled with [(131)I]SIB-DOTA in 27.1±6.2% (n=2) conjugation yields and its targeting properties to the same mAb labeled with [(125)I]SGMIB both in vitro and in vivo using U87MG·ΔEGFR cells and xenografts were compared. In vitro paired-label internalization assays showed that the intracellular radioactivity from [(131)I]SIB-DOTA-L8A4 was 21.4±0.5% and 26.2±1.1% of initially bound radioactivity at 16 and 24h, respectively. In comparison, these values for [(125)I]SGMIB-L8A4 were 16.7±0.5% and 14.9±1.1%. Similarly, the SIB-DOTA prosthetic group provided better tumor targeting in vivo than SGMIB over 8 d period. These results suggest that SIB-DOTA warrants further evaluation as a residualizing agent for labeling internalizing mAbs including those targeted to EGFRvIII.

Design, synthesis and in vitro characterization of Glucagon-Like Peptide-1 derivatives for pancreatic beta cell imaging by SPECT

Novel Glucagon-Like Peptide-1 (GLP-1) derivatives containing the metal chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and naturally occurring Indium ((113/115)In) were prepared using solid-phase Fmoc methods. All synthesized peptides contained d-Ala-8, a modification known to improve resistance towards degradation by dipeptidyl peptidase-IV. The effect of increased distance between DOTA and the peptide chain was investigated using an (aminoethyl) ethoxy acetyl linker, in order to reduce steric effects imposed by DOTA. Placement of linker and DOTA moieties were also varied within the GLP-1 sequence to test for optimal metal-complex location. The binding affinity of the peptide derivatives was determined in vitro with Chinese hamster ovary cells stably transfected with a human GLP-1 receptor (CHO/GLP-1R) cell line and was shown to be in the nM range. Gamma camera imaging of an insulinoma cell line was carried out using (111)In-labeled peptides. Our results suggest that the prepared GLP-1 derivatives are suitable imaging probes for studying pancreatic islet function in vivo.

New enzyme-activated solubility-switchable contrast agent for magnetic resonance imaging: from synthesis to in vivo imaging

We designed and synthesized a novel contrast agent (CA) to image the activity of matrix metalloproteinase-2 (MMP-2) in a tumor, noninvasively using magnetic resonance imaging (MRI). We exploited the concept of solubility-switchable CAs in the design of a protease-modulated CA (PCA), referred to as PCA2-switch. This PCA contains a paramagnetic gadolinium chelate (Gd-DOTA), which was attached to the N-terminus of a MMP-2 cleavable peptide sequence via a hydrophobic chain. The aqueous solubility of the CA depends on the presence of a polyethylene glycol chain (PEG) on the C-terminus of the peptide. Upon proteolytic cleavage of the peptide by MMP-2, the PEG chain is detached from the CA, which becomes less water soluble. This compound and control compounds were successfully tested in an animal model bearing two tumors with different levels of MMP-2 activity.

Synthesis of novel 1,4,7,10-tetraazacyclodecane-1,4,7,10-tetraacetic acid (DOTA) derivatives for chemoselective attachment to unprotected polyfunctionalized compounds

A convenient synthesis of novel bifunctional poly(amino carboxylate) chelating agents allowing chemoselective attachment to highly functionalized biomolecules is described. Based on the well known chelator 1,4,7,10-tetraazacyclodecane-1,4,7,10-tetraacetic acid (DOTA), we synthesized novel bifunctional chelating agents bearing additional functional groups by alkylating 1,4,7,10-tetraazacyclododecane (cyclen) with one equivalent of para-functionalized alkyl 2-bromophenyl-acetate and three equivalents of tert-butyl 2-bromoacetate. The resulting compounds, which contain an additional carbonyl or alkyne functionality, allow site specific labeling of appropriately functionalized unprotected biomolecules in a rapid manner via click reactions. This was demonstrated by the attachment of our new DOTA derivatives to the somatostatin analogue Tyr3-octreotate by chemoselective oxime ligation and CuI-catalyzed azide-alkyne cycloaddition. Initial biodistribution studies in mice with the radiometalated compound demonstrated the applicability of the described DOTA conjugation.

Renal accumulation of [111In]DOTATOC in rats: influence of inhibitors of the organic ion transport and diuretics

AIM

Radiation exposure to the kidney limits therapy with radiometal labelled DOTATOC. This study evaluates the organic anion and cation transport (inhibitors: probenecid and cimetidine/dexamethasone) as well as diuresis (furosemide and mannitol) regarding renal uptake of [(111)In]DOTATOC.

METHODS

One hundred eight male Fisher rats were injected with [(111)In]DOTATOC via the tail vein. Prior to activity injection a total of 84 rats underwent injection with probenecid vs. sodium chloride 0.9% (48 rats), cimetidine vs. dexamethasone vs. sodium chloride 0.9% (18 rats), and furosemide vs. mannitol vs. sodium chloride 0.9% (18 rats). Rats were sacrificed at predetermined time points up to 48 h after activity injection. Kidneys, adrenal glands, pancreas, spleen, blood, liver, and muscle were harvested and injected activity per gram tissue was determined. Autoradiographic images of the kidneys were acquired in a total of 24 rats.

RESULTS

Probenecid led to a reduction in renal uptake by up to 30% while not significantly changing the activity accumulation in the other organs investigated. This reduction was attributable to the renal cortex (ratio cortex/medulla 1.72 vs. 1.99; p = 0.006). Cimetidine and dexamethasone had no effect in any of the organs. Furosemide led to a 44% increase in renal activity accumulation attributable to enhanced renal medullary uptake (ratio cortex/medulla 1.44 versus 1.69; p = 0.006). Mannitol had no effect on renal activity uptake.

CONCLUSION

Inhibition of the organic anion transport by probenecid may help reduce renal uptake regarding therapy with radiometal labelled DOTATOC. The enhancing effect of furosemide may be unfavourable for therapy. The results must be confirmed by human studies.

Peptidyl molecular imaging contrast agents using a new solid-phase peptide synthesis approach

A versatile method is disclosed for solid-phase peptide synthesis (SPPS) of molecular imaging contrast agents. A DO3A moiety was derivatized to introduce a CBZ-protected amino group and then coupled to a polymeric support. CBZ cleavage with Et2AlCl/thioanisole was optimized for SPPS. Amino acids were then coupled to the aminoDOTA-loaded resin using conventional stepwise Fmoc SPPS to create a product with DOTA coupled to the C-terminus of the peptide. In a second study, the DO3A moiety was coupled to a glycine-loaded polymeric support, and amino acids were then coupled to the amino-DOTA-peptide-loaded resin using SPPS to incorporate DOTA within the peptide sequence. The peptide-(Tm3+-DOTA) amide showed a paramagnetic chemical exchange saturation transfer (PARACEST) effect, which demonstrated the utility of this contrast agent for molecular imaging. These results demonstrate the advantages of exploiting SPPS methodologies through development of unique DOTA derivatives to create peptide-based molecular imaging contrast agents.

[(111)In]DOTATOC as a dosimetric substitute for kidney dosimetry during [(90)Y]DOTATOC therapy: results and evaluation of a combined gamma camera/probe approach

PURPOSE

During [(90)Y]DOTATOC therapy, for determination of kidney doses a conventional approach using co-injected [(111)In]DOTATOC was evaluated for validity, reliability and reproducibility as well as for the influence of methodological variations and bremsstrahlung. Biologically effective doses were estimated by calculating the relative effectiveness (RE) of kidney doses.

METHODS

Fractionated [(90)Y]DOTATOC therapy (n=20 patients, 3.1+/-0.7 GBq/therapy cycle, 45 therapy cycles) included co-injection of 157+/-37 MBq [(111)In]DOTATOC and a nephroprotective infusion regimen. From serial gamma camera/probe measurements, individual region of interest (ROI) sets were established and kidney doses were determined according to MIRDOSE3 (corrected for individual kidney mass) by use of three methodological variants: (1) correction for interfering organs (liver/spleen) and background activity, (2) correction for interfering organs alone and (3) no corrections. A phantom study was performed with (111) In alone and with (111)In +(90)Y to estimate the influence of (90)Y bremsstrahlung.

RESULTS

Mean kidney dose (method 1, n=20 patients, 20 therapy cycles) was 1.51+/-0.60 Gy/GBq [(90)Y]DOTATOC (1.41+/-0.48 Gy/GBq for n=20 patients, 45 therapy cycles). With partial correction (method 2) or no correction (method 3) for interfering activity, kidney doses increased significantly, to 2.71+/-1.26 Gy/GBq and 3.15+/-1.22 Gy/GBq, respectively. The span of REs ranged from 1.02 to 1.24. Inter-observer variability was as high as +/-32% (+/-2SD). (90)Y bremsstrahlung accounted for a 4-11% underestimation of obtained target activity.

CONCLUSION

The obtained kidney doses are highly influenced by methodological variations. Full correction for interfering activity clearly underestimates kidney doses. By comparison, (90)Y bremsstrahlung and variability in the relative effectiveness of kidney doses cause minor bias. Inter-observer variability must be considered when interpreting kidney doses.