Total Solid-Phase Synthesis of NOTA-Functionalized Peptides for PET Imaging

Development and characterization of two novel 68 Ga-labelled neuropeptide Y short analogues with potential application in breast cancer imaging

In vivo receptor targeting with radiolabelled peptide-based probes is an attractive approach for the development of novel radiotracers for molecular imaging. This work presents the development and characterization of two novel neuropeptide Y analogues labelled with a positron emitter 68 Ga, for potential use in breast cancer imaging. Both analogues share the same amino acid sequence and were derivatized with NOTA through either a lysine linker (L1) or an acetylated lysine (L2). In both cases, a single product with radiochemical purity higher than 95% was obtained. The two complexes were hydrophilic, showed remarkable in vitro stability, good cellular uptake, binding affinity in the nanomolar range and high cellular internalization rate. Biodistribution studies revealed low blood uptake and elimination through the urinary tract. The addition of an acetyl group in the spacer increased the lipophilicity of C2 and modified the reactivity of the ε-amino group of the lysine which resulted in lower protein binding and lower percentage of injected dose in bladder and urine. The tumour versus muscle ratio was (3.8 ± 0.4) for 68 Ga-L1 and (4.7 ± 0.4) for 68 Ga-L2. These results encourage performing further studies in order to complete the evaluation of both tracers as potential radiopharmaceutical for breast cancer imaging.

Synthesis and Characterization of Radiogallium-Labeled Cationic Amphiphilic Peptides as Tumor Imaging Agents

SVS-1 is a cationic amphiphilic peptide (CAP) that exhibits a preferential cytotoxicity towards cancer cells over normal cells. In this study, we developed radiogallium-labeled SVS-1 (⁶⁷Ga-NOTA-KV6), as well as two SVS-1 derivatives, with the repeating KV residues replaced by RV or HV (⁶⁷Ga-NOTA-RV6 and ⁶⁷Ga-NOTA-HV6). All three peptides showed high accumulation in epidermoid carcinoma KB cells (53-143% uptake/mg protein). Though ⁶⁷Ga-NOTA-RV6 showed the highest uptake among the three CAPs, its uptake in 3T3-L1 fibroblasts was just as high, indicating a low selectivity. In contrast, the uptake of ⁶⁷Ga-NOTA-KV6 and ⁶⁷Ga-NOTA-HV6 into 3T3-L1 cells was significantly lower than that in KB cells. An endocytosis inhibition study suggested that the three ⁶⁷Ga-NOTA-CAPs follow distinct pathways for internalization. In the biodistribution study, the tumor uptakes were found to be 4.46%, 4.76%, and 3.18% injected dose/g of tissue (% ID/g) for ⁶⁷Ga-NOTA-KV6, ⁶⁷Ga-NOTA-RV6, and ⁶⁷Ga-NOTA-HV6, respectively, 30 min after administration. Though the radioactivity of these peptides in tumor tissue decreased gradually, ⁶⁷Ga-NOTA-KV6, ⁶⁷Ga-NOTA-RV6, and ⁶⁷Ga-NOTA-HV6 reached high tumor/blood ratios (7.7, 8.0, and 3.8, respectively) and tumor/muscle ratios (5.0, 3.3, and 4.0, respectively) 120 min after administration. ⁶⁷Ga-NOTA-HV6 showed a lower tumor uptake than the two other tracers, but it exhibited very low levels of uptake into peripheral organs. Overall, the replacement of lysine in SVS-1 with other basic amino acids significantly influenced its binding and internalization into cancer cells, as well as its in vivo pharmacokinetic profile. The high accessibility of these peptides to tumors and their ability to target the surface membranes of cancer cells make radiolabeled CAPs excellent candidates for use in tumor theranostics.

A comparison of labelling characteristics of manual and automated synthesis methods for gallium-68 labelled ubiquicidin

Gallium-68 labelled 1,4,7-triazacyclononane-1,4,7-triacetic acid ubiquicidin (NOTA-UBI) is currently investigated as a PET radiopharmaceutical for the imaging of infections. The aim of this study was to compare the labelling characteristics of an optimized manual radiosynthesis method with those of optimized automated synthesis methods. Data from this study suggest that automated radiosynthesis of [⁶⁸Ga]Ga-NOTA-UBI provides a higher degree of robustness and repeatability than the manual method. Our results also suggest that for our full-scale automated synthesis, radical scavengers should be considered to reduce radiolysis. Automated synthesis methods have the advantage of markedly reducing radiation exposure to operators. Standardised automation also makes the synthesis more reliably compliant with Good Manufacturing Practice guidelines.

Heterodimeric Analogues of the Potent Y1R Antagonist 1229U91, Lacking One of the Pharmacophoric C-Terminal Structures, Retain Potent Y1R Affinity and Show Improved Selectivity over Y4R

The cyclic dimeric peptide 1229U91 (GR231118) has an unusual structure and displays potent, insurmountable antagonism of the Y₁ receptor. To probe the structural basis for this activity, we have prepared ring size variants and heterodimeric compounds, identifying the specific residues underpinning the mechanism of 1229U91 binding. The homodimeric structure was shown to be dispensible, with analogues lacking key pharmacophoric residues in one dimer arm retaining high antagonist affinity. Compounds 11d-h also showed enhanced Y1R selectivity over Y4R compared to 1229U91.

Solid phase synthesis in the development of magnetic resonance imaging probes

We review the use of the solid phase synthesis methodology for the preparation of diverse and potent MRI probes.

A convenient and efficient total solid-phase synthesis of DOTA-functionalized tumor-targeting peptides for PET imaging of cancer

Introduction

An efficient and cost-effective synthesis of the metal chelating agents that couple to tumor-targeting peptides is required to enhance the process of preclinical research toward the clinical translation of molecular imaging agents. DOTA is one of the most widely used macrocyclic ligands for the development of new metal-based imaging and therapeutic agents owing to its ability to form stable and inert complexes under physiological conditions. Although solid-phase synthesis compatible DOTA-tris-(t-Bu ester) is a commercial product, it is expensive and contain chemical impurities. There is a need to explore new and cost-effective methods for the preparation of metal chelating agents, i.e., DOTA, directly on solid support to facilitate rapid, cost-effective, and high purity preparation of DOTA-linked peptides for imaging and therapy. In the present study, we describe a facile synthetic strategy of DOTA preparation and its linkage to peptides directly on solid-phase support.

Methods

Bombesin (BN) peptides were functionalized with DOTA chelator prepared from cyclen precursor on solid-phase and from commercial DOTA-tris and radiolabeled with ⁶⁸Ga. In vitro BN/GRP receptor binding affinities of the corresponding radiolabeled peptides were determined by saturation binding assays on human breast MDA-MB-231, MCF7, T47D, and PC3 prostate cancer cells. Pharmacokinetics were studied in Balb/c mice and in vivo tumor targeting in MDA-MB-231 tumor-bearing nude mice.

Results

DOTA was prepared successfully from cyclen on solid-phase support, linked specifically to BN peptides and resultant DOTA-coupled peptides were radiolabeled efficiently with ⁶⁸Ga. The binding affinities of all the six BN peptides were comparable and in the low nanomolar range. All ⁶⁸Ga-labeled peptides showed high metabolic stability in plasma. These radiopeptides exhibited rapid pharmacokinetics in Balb/c mice with excretion mainly through the urinary system. In nude mice, MDA-MB-231 tumor uptake profiles were slightly different; the BN peptide with Ahx spacer and linked to DOTA through cyclen exhibited higher tumor uptake (2.32% ID/g at 1 h post-injection) than other radiolabeled BN peptides investigated in this study. The same leading BN peptide also displayed favorable pharmacokinetic profile in Balb/c mice. The PET images clearly visualized the MDA-MB-231 tumor.

Conclusions

DOTA prepared from cyclen on solid-phase support showed comparable potency and efficiency to DOTA-tris in both in vitro and in vivo evaluation. The synthetic methodology described here allows versatile, site-specific introduction of DOTA into peptides to facilitate the development of DOTA-linked molecular imaging and therapy agents for clinical translation.

Targeting IL-5Rα with antibody-conjugates reveals a strategy for imaging and therapy for invasive bladder cancer

Despite the high interest and concern due to an increasing incidence and death rate, patients who develop muscle invasive bladder cancer (MIBC) have few options available. However, the past decade has produced many candidate bladder tumor-specific markers but further development of these markers is still needed for creating effective targeted medications to solve this urgent need. Interleukin-5 receptor α-subunit (IL-5Rα) has recently been reported to be involved in MIBC progression. Thus, we aimed to validate IL-5Rα as a target for antibody-conjugates to better manage patients with MIBC. Patients were recruited and their tumors were processed for IL-5Rα immunohistochemical analysis. NOD/SCID mice were also heterotopically implanted with the human MIBC HT-1376 and HT-B9 cell lines and established xenografts immunohistochemically evaluated for IL-5Rα and compared against patient tumors. Using the mAb A14, an antibody-drug conjugate (ADC) and a radiolabeled immunoconjugate (RIC) were developed by conjugating to vinblastine and to the positron emitter copper-64 (⁶⁴Cu), respectively. As a proof-of-concept for ADC and RIC efficacy, in vitro cytotoxicity and in vivo positron emission tomography (PET) imaging in tumor-bearing mice were performed, respectively. In addition, as rapid internalization and accumulation are important components for effective antibody-conjugates, we evaluated these aspects in response to IL-5 and ⁶⁴Cu-A14 treatments. Our findings suggest that although IL-5Rα protein expression is preferentially increased in MIBC, it is rapid IL-5Rα-mediated internalization allowing vinblastine-A14 to have cytotoxic activity and ⁶⁴Cu-A14 to detect MIBC tumors in vivo. This is the first report to elucidate the potential of IL-5Rα as an attractive MIBC target for antibody-conjugate applications.

Peptide-based PET imaging of the tumor restricted IL13RA2 biomarker

Peptides that target cancer cell surface receptors are promising platforms to deliver diagnostic and therapeutic payloads specifically to cancer but not normal tissue. IL13RA2 is a tumor-restricted receptor found to be present in several aggressive malignancies, including in the vast majority of high-grade gliomas and malignant melanoma. This receptor has been successfully targeted for diagnostic and therapeutic purposes using modified IL-13 ligand and more recently using a specific peptide, Pep-1L. In the current work, we establish the in vitro and in vivo tumor binding properties of radiolabeled Pep-1L, designed for tumor imaging. We radiolabeled Pep-1L with Copper-64 and demonstrated specific cell uptake in the IL13RA2-over expressing G48 glioblastoma cell line having abundant IL13RA2 expression. [⁶⁴Cu]Pep-1L binding was blocked by unlabeled ligand, demonstrating specificity. To demonstrate in vivo tumor uptake, we intravenously injected into tumor-bearing mice and demonstrated that [⁶⁴Cu]Pep-1L specifically bound tumors at 24 hours, which was significantly blocked (3-fold) by pre-injecting unlabeled peptide. To further demonstrate specificity of Pep-1L towards IL13RA2 in vivo, we exploited an IL13RA2-inducible melanoma tumor model that does not express receptor at baseline but expresses abundant receptor after treatment with doxycycline. We injected [⁶⁴Cu]Pep-1L into mice bearing IL13RA2-inducible melanoma tumors and performed in vivo PET/CT and post-necropsy biodistribution studies and found that tumors that were induced to express IL13RA2 receptor by doxycycline pretreatment bound radiolabeled Pep-1L 3-4 fold greater than uninduced tumors, demonstrating receptor specificity. This work demonstrates that [⁶⁴Cu]Pep-1L selectively binds hIL13RA2-expressing tumors and validates Pep-1L as an effective platform to deliver diagnostics and therapeutics to IL13RA2-expressing cancers.

PACE4 inhibitors and their peptidomimetic analogs block prostate cancer tumor progression through quiescence induction, increased apoptosis and impaired neovascularisation

Prostate cancer is the leading cancer in North American men. Current pharmacological treatments are limited to anti-androgen strategies and the development of new therapeutic approaches remains a challenge. As a fundamentally new approach, we propose the inhibition of PACE4, a member of the proprotein convertases family of enzymes, as a therapeutic target in prostate cancer. We developed an inhibitor named the Multi-Leu peptide, with potent in vitro anti-proliferative effects. However, the Multi-Leu peptide has not been tested under in vivo conditions and its potency under such conditions is most likely limited, due to the labile characteristics of peptides in general. Using a peptidomimetic approach, we modified the initial scaffold, generating the analog Ac-[DLeu]LLLRVK-Amba, which demonstrates increased inhibitory potency and stability. The systemic administration of this peptidomimetic significantly inhibits tumor progression in the LNCaP xenograft model of prostate cancer by inducing tumor cell quiescence, increased apoptosis and neovascularization impairment. Pharmacokinetic and biodistribution profiles of this inhibitor confirm adequate tumor delivery properties of the compound. We conclude that PACE4 peptidomimetic inhibitors could result in stable and potent drugs for a novel therapeutic strategy for prostate cancer.

PACE4-based molecular targeting of prostate cancer using an engineered ⁶⁴Cu-radiolabeled peptide inhibitor

The potential of PACE4 as a pharmacological target in prostate cancer has been demonstrated as this proprotein convertase is strongly overexpressed in human prostate cancer tissues and its inhibition, using molecular or pharmacological approaches, results in reduced cell proliferation and tumor progression in mouse tumor xenograft models. We developed a PACE4 high-affinity peptide inhibitor, namely, the multi-leucine (ML), and sought to determine whether this peptide could be exploited for the targeting of prostate cancer for diagnostic or molecular imaging purposes. We conjugated a bifunctional chelator 1,4,7-triazacyclononane-1,4,7- triacetic acid (NOTA) to the ML peptide for copper-64 ((64)Cu) labeling and positron emission tomography (PET)- based prostate cancer detection. Enzyme kinetic assays against recombinant PACE4 showed that the NOTA-modified ML peptide displays identical inhibitory properties compared to the unmodified peptide. In vivo biodistribution of the (64)Cu/NOTA-ML peptide evaluated in athymic nude mice bearing xenografts of two human prostate carcinoma cell lines showed a rapid and high uptake in PACE4-expressing LNCaP tumor at an early time point and in PACE4-rich organs. Co-injection of unlabeled peptide confirmed that tumor uptake was target-specific. PACE4-negative tumors displayed no tracer uptake 15 minutes after injection, while the kidneys, demonstrated high uptake due to rapid renal clearance of the peptide. The present study supports the feasibility of using a (64)Cu/NOTA-ML peptide for PACE4-targeted prostate cancer detection and PACE4 status determination by PET imaging but also provides evidence that ML inhibitor-based drugs would readily reach tumor sites under in vivo conditions for pharmacological intervention or targeted radiation therapy.

Radiometals: towards a new success story in nuclear imaging?

The use of radiometal isotopes in positron emission tomography: a new success story in nuclear imaging?

Synthetic routes to the Neuropeptide Y Y1 receptor antagonist 1229U91 and related analogues for SAR studies and cell-based imaging

The potent Y1 receptor antagonist, 1229U91 has an unusual cyclic dimer structure that makes syntheses of analogue series quite challenging. We have examined three new routes to the synthesis of such peptides that has given access to novel structural variants including heterodimeric compounds, ring size variants and labelled conjugates. These compounds, including a fluorescently labelled analogue VIII show potent antagonism that can be utilised in studying Y1 receptor pharmacology.

Preclinical evaluation of 68Ga-labeled 1,4,7-triazacyclononane-1,4,7-triacetic acid-ubiquicidin as a radioligand for PET infection imaging

Antimicrobial peptides such as ubiquicidin (UBI) are believed to differentiate between mammalian and bacterial or fungal cells. (99m)Tc-UBI29-41 was previously tested for detecting infection in humans using SPECT. For the present study, the UBI fragment UBI29-41 (TGRAKRRMQYNRR) was conjugated to 1,4,7-triazacyclononane-triacetic acid (NOTA), radiolabeled with (68)Ga, and investigated in a rabbit infection model.

METHODS

(68)Ga was obtained from a 1.85-GBq (68)Ge/(68)Ga generator. New Zealand White rabbits were anesthetized with ketamine/medetomidine before tracer administration and placed in a clinical PET/CT scanner. (68)Ga-1,4,7-triazacyclononane-1,4,7-triacetic-acid-ubiquicidin29-41 ((68)Ga-NOTA-UBI29-41) was formulated in saline solution, and 101 ± 41 MBq were administered intravenously. The tracer distribution was studied by PET/CT imaging in animals (a) that were healthy, (b) bearing muscular Staphylococcus aureus infections and turpentine oil-induced muscular inflammations, and (c) bearing ovalbumin-induced lung inflammations. Static PET/CT imaging was performed at different time intervals up to 120 min after injection. For calculation of target-to-nontarget ratios, standardized uptake values were normalized against healthy thigh muscle, representing nontargeted tissue.

RESULTS

PET/CT images of healthy animals showed predominant distribution in the kidneys, liver, and bladder; heart and spleen showed moderate, declining uptake, only. The biologic half-life in blood was 29 min. Urinary accumulation of (68)Ga-NOTA-UBI29-41 peaked at 3.8 ± 0.91 percentage injected dose per gram (%ID) at 120 min, and 88 ± 5.2 %ID was recovered in total urine. (68)Ga-NOTA-UBI29-41 imaging in (b) selectively visualized the muscular infection site and was differentiated from sterile inflammatory processes. Standardized uptake value ratios for muscles (infected/inflamed) were 2.9 ± 0.93, 2.9 ± 0.50, 3.5 ± 0.86, and 3.8 ± 0.90 at 5, 30, 60, and 90 min after injection, respectively. Rabbit lungs with asthma showed insignificant uptake.

CONCLUSION

(68)Ga-NOTA-UBI29-41 was strongly localized in bacteria-infected areas and minimally detected in a sterile inflammation area in rabbit muscles. The findings propose this compound to be an excellent first-line PET/CT tracer to allow the distinguishing of infection from inflammation.

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.

Comparative study of 64Cu/NOTA-[D-Tyr6,βAla11,Thi13,Nle14]BBN(6-14) monomer and dimers for prostate cancer PET imaging

Background

Gastrin-releasing peptide receptors [GRPR] are highly over-expressed in multiple cancers and have been studied as a diagnostic target. Multimeric gastrin-releasing peptides are expected to have enhanced tumor uptake and affinity for GRPR. In this study, a ⁶⁴Cu-labeled 1,4,7-triazacyclononane-1,4,7-triacetic acid [NOTA]-monomer and two NOTA-dimers of [D-Tyr⁶,βAla¹¹, Thi¹³, Nle¹⁴]bombesin(6-14) ] [BBN(6-14)] were compared.

Methods

Monomeric and dimeric peptides were synthesized on solid phase support and radiolabeled with ⁶⁴Cu. NOTA-dimer 1 consists of asymmetrically linked BBN(6-14), while NOTA-dimer 2 has similar spacer between the two BBN(6-14) ligands and the chelator. In vitro GRPR-binding affinities were determined with competitive binding assays on PC3 human prostate cancer cells. In vivo stability and biodistribution of radiolabeled compounds were assessed in Balb/c mice. Cellular uptake and efflux were measured with radiolabeled NOTA-monomer and NOTA-dimer 2 on PC3 cells for up to 4 h. In vivo biodistribution kinetics were measured in PC3 tumor-bearing Balb/c nude mice by μ-positron emission tomography [μPET] imaging and confirmed by dissection and counting.

Results

NOTA-monomer, NOTA-dimers 1 and 2 were prepared with purity of 99%. The inhibition constants of the three BBN peptides were comparable and in the low nanomolar range. All ⁶⁴Cu-labeled peptides were stable up to 24 h in mouse plasma and 1 h in vivo. ⁶⁴Cu/NOTA-dimer 2 featuring a longer spacer between the two BBN(6-14) ligands is a more potent GRPR-targeting probe than ⁶⁴Cu/NOTA-dimer 1. PC3 tumor uptake profiles are slightly different for ⁶⁴Cu/NOTA-monomer and ⁶⁴Cu/NOTA-dimer 2; the monomeric BBN-peptide tracer exhibited higher tumor uptake during the first 0.5 h and a fast renal clearance resulting in higher tumor-to-muscle ratio when compared to ⁶⁴Cu/NOTA-dimer 2. The latter exhibited higher tumor-to-blood ratio and was retained longer at the tumor site when compared to ⁶⁴Cu/NOTA-monomer. Lower ratios of tumor-to-blood and tumor-to-muscle in blocking experiments showed GRPR-dependant tumor uptake for both tracers.

Conclusion

Both ⁶⁴Cu/NOTA-monomer and ⁶⁴Cu/NOTA-dimer 2 are suitable for detecting GRPR-positive prostate cancer in vivo by PET. Tumor retention was improved in vivo with ⁶⁴Cu/NOTA-dimer 2 by applying polyvalency effect and/or statistical rebinding.

Evaluation of 64Cu-labeled bifunctional chelate-bombesin conjugates

Several bifunctional chelates (BFCs) were investigated as carriers of (64)Cu for PET imaging. The most widely used chelator for (64)Cu labeling of BFCs is DOTA (1,4,7,10-tetraazacyclododecane-N,N',N″,N'''-tretraacetic acid), even though this complex exhibits only moderate in vivo stability. In this study, we prepared a series of alternative chelator-peptide conjugates labeled with (64)Cu, measured in vitro receptor binding affinities in human breast cancer T47D cells expressing the gastrin-releasing peptide receptor (GRPR) and compared their in vivo stability in mice. DOTA-, NOTA-(1,4,7-triazacyclononane-1,4,7-triacetic acid), PCTA-(3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-3,6,9-triacetic acid), and Oxo-DO3A-(1-oxa-4,7,10-triazacyclododecane-4,7,10-triacetic acid) peptide conjugates were prepared using H(2)N-Aoc-[d-Tyr(6),βAla(11),Thi(13),Nle(14)]bombesin(6-14) (BBN) as a peptide template. The BBN moiety was selected since it binds with high affinity to the GRPR, which is overexpressed on human breast cancer cells. A convenient synthetic approach for the attachment of aniline-BFC to peptides on solid support is also presented. To facilitate the attachment of the aniline-PCTA and aniline-Oxo-DO3A to the peptide via an amide bond, a succinyl spacer was introduced at the N-terminus of BBN. The partially protected aniline-BFC (p-H(2)N-Bn-PCTA(Ot-Bu)(3) or p-H(2)N-Bn-DO3A(Ot-Bu)(3)) was then coupled to the resulting N-terminal carboxylic acid preactivated with DEPBT/ClHOBt on resin. After cleavage and purification, the peptide-conjugates were labeled with (64)Cu using [(64)Cu]Cu(OAc)(2) in 0.1 M ammonium acetate buffer at 100 °C for 15 min. Labeling efficacy was >90% for all peptides; Oxo-DO3A-BBN was incubated an additional 150 min at 100 °C to achieve this high yield. Specific activities varied from 76 to 101 TBq/mmol. Competition assays on T47D cells showed that all BFC-BBN complexes retained high affinity for the GRPR. All BFC-BBN (64)Cu-conjugates were stable for over 20 h when incubated at 37 °C in mouse plasma samples. However, in vivo, only 37% of the (64)Cu/Oxo-DO3A complex remained intact after 20 h while the (64)Cu/DOTA-BBN complex was completely demetalated. In contrast, both (64)Cu/NOTA- and (64)Cu/PCTA-BBN conjugates remained stable during the 20 h time period. Our results indicate that it is possible to successfully conjugate aniline-BFC with peptide on solid support. Our data also show that (64)Cu-labeled NOTA- and PCTA-BBN peptide conjugates are promising radiotracers for PET imaging of many human cancers overexpressing the GRP receptor.

Characterization of metal-labelled peptides by matrix-assisted laser desorption/ionization mass spectrometry and tandem mass spectrometry

Metal labelling of peptides and proteins using high-affinity metal-chelating compounds has found widespread applications in the medical and bioanalytical fields. In the present study we investigated the analysis of peptides derivatized either with cysteine- or amino group-directed metal-bound DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) chelators in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). The metal complexes of DOTA were shown to be stable under MALDI-MS conditions. The introduction of the metal label led in a number of cases to significantly increased signal-to-noise (S/N) values and thus improved sensitivity of the labelled peptides compared to their unlabelled counterparts, especially for multiply labelled peptides. The presence of the labels did alter the tandem mass spectrometric (MS/MS) behaviour, namely the formation of sequence specific a-, b- and y-ion series, in dependence of the position of the label within the peptide sequence. For cysteine-derivatized peptides several label-specific reporter ions and characteristic immonium ions could be identified. Amino-directed labelling led only to the formation of characteristic immonium ions in ε-amino groups of lysine, whereas N-terminal labelling in some cases led to the formation of a(1)- and b(1)-ions. The results clearly show that MALDI-MS is suitable for the analysis of metal-labelled peptides, which was also confirmed in liquid chromatography (LC)/MALDI-based identification of proteins in a model protein mixture labelled with Cys-reactive DOTA. Here, in comparison to a run with alkylated cysteines, more than 50% more cysteine-containing peptides were identified.