The discovery and evaluation of [18F]BMS-986229, a novel macrocyclic peptide PET radioligand for the measurement of PD-L1 expression and in-vivo PD-L1 target engagement.

PURPOSE: A same-day PET imaging agent capable of measuring PD-L1 status in tumors is an important tool for optimizing PD-1 and PD-L1 treatments. Herein we describe the discovery and evaluation of a novel, fluorine-18 labeled macrocyclic peptide-based PET ligand for imaging PD-L1. METHODS: [18F]BMS-986229 was synthesized via copper mediated click-chemistry to yield a PD-L1 PET ligand with picomolar affinity and was tested as an in-vivo tool for assessing PD-L1 expression. RESULTS: Autoradiography showed an 8:1 binding ratio in L2987 (PD-L1 (+)) vs. HT-29 (PD-L1 (-)) tumor tissues, with >90% specific binding. Specific radioligand binding (>90%) was observed in human non-small-cell lung cancer (NSCLC) and cynomolgus monkey spleen tissues. Images of PD-L1 (+) tissues in primates were characterized by high signal-to-noise, with low background signal in non-expressing tissues. PET imaging enabled clear visualization of PD-L1 expression in a murine model in vivo, with 5-fold higher uptake in L2987 (PD-L1 (+)) than in control HT-29 (PD-L1 (-)) tumors. Moreover, this imaging agent was used to measure target engagement of PD-L1 inhibitors (peptide or mAb), in PD-L1 (+) tumors as high as 97%. CONCLUSION: A novel 18F-labeled macrocyclic peptide radioligand was developed for PET imaging of PD-L1 expressing tissues that demonstrated several advantages within a nonhuman primate model when compared directly to adnectin- or mAb-based ligands. Clinical studies are currently evaluating [18F]BMS-986229 to measure PD-L1 expression in tumors.

Development, Characterization, and Radiation Dosimetry Studies of 18F-BMS-986229, a 18F-Labeled PD-L1 Macrocyclic Peptide PET Tracer.

PURPOSE: In cancer immunotherapy, the blockade of the interaction between programmed death-1 and its ligand (PD-1:PD-L1) has proven to be one of the most promising strategies. However, as mechanisms of resistance to PD-1/PD-L1 inhibition include variability in tumor cell PD-L1 expression in addition to standard tumor biopsy PD-L1 immunohistochemistry (IHC), a comprehensive and quantitative approach for measuring PD-L1 expression is required. Herein, we report the development and characterization of an 18F-PD-L1-binding macrocyclic peptide as a PET tracer for the comprehensive evaluation of tumor PD-L1 expression in cancer patients. PROCEDURES: 18F-BMS-986229 was characterized for PD-L1 expression assessment by autoradiography or PET imaging. 18F-BMS-986229 was utilized to evaluate tumor PD-L1 target engagement in competition with a macrocyclic peptide inhibitor of PD-L1 (BMS-986189) over a range of doses using PET imaging. A whole-body radiation dosimetry study of 18F-BMS-986229 in healthy non-human primates (NHPs) was performed. RESULTS: In vitro autoradiography showed an 8:1 binding ratio in L2987(PD-L1 +) vs. HT-29 (PD-L1-) tumors, more than 90% of which could be blocked with 1 nM of BMS-986189. Ex vivo autoradiography showed that 18F-BMS-986229 detection was penetrant over a series of sections spanning the entire L2987 tumor. In vivo PET imaging in mice demonstrated a 5:1 tracer uptake ratio (at 90-100 min after tracer administration) in L2987 vs. HT-29 tumors and demonstrated 83%-93% specific binding of BMS-986189 within those dose ranges. In a healthy NHP dosimetry study, the resultant whole-body effective dose was 0.025 mSv/MBq. CONCLUSION: 18F-BMS-986229 has been preclinically characterized and exhibits high target specificity, low background uptake, and a short blood half-life supportive of same day imaging in the clinic. As the PET tracer, 18F-BMS-986229 shows promise in the quantification of PD-L1 expression, and its use in monitoring longitudinal changes in patients may provide insights into PD-1:PD-L1 immuno-therapy treatment outcomes.

Synthesis of a dual carbon-14-labeled calcitonin gene-related peptide receptor antagonist for use in a human absorption-distribution-metabolism-elimination study.

Oral calcitonin gene-related peptide (CGRP) receptor antagonists have been shown to be effective in the acute and preventive treatment of migraine. CGRP receptor antagonists offer safety advantages over triptans because they are not active vasoconstrictors, which reduces cardiovascular risks. Bristol Myers Squibb discovered a high affinity CGRP receptor antagonist BMS-927711 for the treatment of migraine now FDA approved as Nurtec® ODT (rimegepant). Dual-labeled [14 C]-BMS-927711 was prepared and used in a human absorption-distribution-metabolism-elimination (ADME) study. A dual-labeled analog of BMS-927711 was required to fully track the compound's metabolic transformation. The carbon-14-labeled synthesis of both right side and left side portions of [14 C]-BMS-927711 is described.

Synthesis of carbon-14 and stable isotope labeled Censavudine.

Censavudine is a nucleoside reverse transcriptase inhibitor (NRTI) explored clinically by Bristol Myers Squibb for the treatment of human immunodeficiency virus-1 (HIV-1). As part of the development process, a carbon-14 labeled analog was synthesized for use in a human absorption, distribution, metabolism, and excretion (ADME) study. A stable isotope labeled analog was also synthesized for use as a mass spectrum internal standard in bioanalytical assays to accurately quantify the concentration of the drug in biological samples. Carbon-14 labeled Censavudine was synthesized in 10 steps in a 9% overall yield from carbon-14 labeled trimethylsilylacetylene. A total of 4.44 mCi of material was prepared with a specific activity of 0.25 µCi/mg. The radiochemical and UV purities were 99% and it met all of the specifications for use in a human clinical study. Deuterium labeled Censavudine was synthesized in two steps in a 68% overall yield from [D4 ]-thymine. A total of 237 mg were prepared with a UV purity of 99%.

Synthesis of tritium labeled 30-KDa PEG and conjugation to form [3 H]Pegbelfermin.

Non-alcoholic steatohepatitis (NASH) is the most chronic liver condition in the western population and is fueled by the obesity and type 2 diabetes epidemic. Pegbelfermin (1), a PEGylated human fibroblast growth factor 21 (FGF21) analogue, has previously been shown to improve markers of metabolism and liver fibrosis in obese patients with type 2 diabetes. Radiolabeled Pegbelfermin was needed to access the accumulation of intact drug and metabolized PEG. In an effort to accomplish both goals with one labeled synthesis, the isotopic label was positioned in the PEG. A total of 21 mCi of tritium labeled Pegbelfermin was synthesized having a specific activity of 21.6 Ci/mmol for use in animal studies.

Synthesis and Preclinical Evaluation of a 68Ga-Labeled Adnectin, 68Ga-BMS-986192, as a PET Agent for Imaging PD-L1 Expression.

Blocking the interaction of the immune checkpoint molecule programmed cell death protein-1 and its ligand, PD-L1, using specific antibodies has been a major breakthrough for immune oncology. Whole-body PD-L1 expression PET imaging may potentially allow for a better prediction of response to programmed cell death protein-1-targeted therapies. Imaging of PD-L1 expression is feasible by PET with the adnectin protein 18F-BMS-986192. However, radiofluorination of proteins such as BMS-986192 remains complex and labeling yields are low. The goal of this study was therefore the development and preclinical evaluation of a 68Ga-labeled adnectin protein (68Ga-BMS-986192) to facilitate clinical trials. Methods:68Ga labeling of DOTA-conjugated adnectin (BXA-206362) was performed in NaOAc-buffer at pH 5.5 (50°C, 15 min). In vitro stability in human serum at 37°C was analyzed using radio-thin layer chromatography and radio-high-performance liquid chromatography. PD-L1 binding assays were performed using the transduced PD-L1-expressing lymphoma cell line U-698-M and wild-type U-698-M cells as a negative control. Immunohistochemical staining studies, biodistribution studies, and small-animal PET studies of 68Ga-BMS-986192 were performed using PD-L1-positive and PD-L1-negative U-698-M-bearing NSG mice. Results:68Ga-BMS-986192 was obtained with quantitative radiochemical yields of more than 97% and with high radiochemical purity. In vitro stability in human serum was at least 95% after 4 h of incubation. High and specific binding of 68Ga-BMS-986192 to human PD-L1-expressing cancer cells was confirmed, which closely correlates with the respective PD-L1 expression level determined by flow cytometry and immunohistochemistry staining. In vivo, 68Ga-BMS-986192 uptake was high at 1 h after injection in PD-L1-positive tumors (9.0 ± 2.1 percentage injected dose [%ID]/g) and kidneys (56.9 ± 9.2 %ID/g), with negligible uptake in other tissues. PD-L1-negative tumors demonstrated only background uptake of radioactivity (0.6 ± 0.1 %ID/g). Coinjection of an excess of unlabeled adnectin reduced tumor uptake of PD-L1 by more than 80%. Conclusion:68Ga-BMS-986192 enables easy radiosynthesis and shows excellent in vitro and in vivo PD-L1-targeting characteristics. The high tumor uptake combined with low background accumulation at early imaging time points demonstrates the feasibility of 68Ga-BMS-986192 for imaging of PD-L1 expression in tumors and is encouraging for further clinical applications of PD-L1 ligands.

Synthesis of stable-isotope-labeled N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide and N-(3-dimethylaminopropyl)-N'-ethylurea.

N-(3-Dimethylaminopropyl)-N'-ethylcarbodiimide (EDC) is a carbodiimide coupling reagent commonly used for the preparation of amides from carboxylic acids and amines. Because of initial concerns regarding the genotoxicity of EDC and its use in GMP syntheses at Bristol Myers Squibb, the quantitation of residual EDC and its by-product N-(3-dimethylaminopropyl)-N'-ethylurea (EDU) by liquid chromatography-mass spectrometry (LCMS) impurity analysis was required. These analyses required the use of stable-isotope-labeled EDC and EDU to serve as internal standards. To meet this need, stable-isotope-labeled EDC 9 and EDU 10 were prepared from [1,2-13 C2 ] ethylene glycol and [13 C,15 N] potassium cyanide in overall yields of 6% and 8%, respectively.

Nucleophilic (Radio)Fluorination of Redox-Active Esters via Radical-Polar Crossover Enabled by Photoredox Catalysis.

We report a redox-neutral method for nucleophilic fluorination of N-hydroxyphthalimide esters using an Ir photocatalyst under visible light irradiation. The method provides access to a broad range of aliphatic fluorides, including primary, secondary, and tertiary benzylic fluorides as well as unactivated tertiary fluorides, that are typically inaccessible by nucleophilic fluorination due to competing elimination. In addition, we show that the decarboxylative fluorination conditions are readily adapted to radiofluorination with [18F]KF. We propose that the reactions proceed by two electron transfers between the Ir catalyst and redox-active ester substrate to afford a carbocation intermediate that undergoes subsequent trapping by fluoride. Examples of trapping with O- and C-centered nucleophiles and deoxyfluorination via N-hydroxyphthalimidoyl oxalates are also presented, suggesting that this approach may offer a general blueprint for affecting redox-neutral SN1 substitutions under mild conditions.

LC-MS/MS bioanalysis of plasma 1, 14-tetradecanedioic acid and 1, 16-hexadecanedioic acid as candidate biomarkers for organic anion-transporting polypeptide mediated drug-drug interactions.

AIM: A robust LC-MS/MS assay was developed to quantify endogenous 1, 14-tetradecanedioic acid (TDA) and 1, 16-hexadecanedioic acid (HDA) in human plasma as potential biomarkers for evaluating drug-drug interactions mediated by the hepatic drug transporters, organic anion-transporting polypeptides. RESULTS: This assay was validated using fit-for-purpose approach over standard curve range of 2.5-1000 nM for TDA and HDA using analyte-free charcoal-stripped human plasma as the surrogate matrix. Chromatographic separation condition was successfully optimized to separate TDA from an interference peak while maintaining both analytes in neutral forms to minimize carryover issue. CONCLUSION: The described assay is currently applied to a clinical study for evaluating TDA/HDA as potential substitute biomarkers for drug-drug interaction studies.

Synthesis and Biologic Evaluation of a Novel 18F-Labeled Adnectin as a PET Radioligand for Imaging PD-L1 Expression.

The programmed death protein (PD-1) and its ligand (PD-L1) play critical roles in a checkpoint pathway cancer cells exploit to evade the immune system. A same-day PET imaging agent for measuring PD-L1 status in primary and metastatic lesions could be important for optimizing drug therapy. Herein, we have evaluated the tumor targeting of an anti-PD-L1 adnectin after 18F-fluorine labeling. Methods: An anti-PD-L1 adnectin was labeled with 18F in 2 steps. This synthesis featured fluorination of a novel prosthetic group, followed by a copper-free click conjugation to a modified adnectin to generate 18F-BMS-986192. 18F-BMS-986192 was evaluated in tumors using in vitro autoradiography and PET with mice bearing bilateral PD-L1-negative (PD-L1(-)) and PD-L1-positive (PD-L1(+)) subcutaneous tumors. 18F-BMS-986192 was evaluated for distribution, binding, and radiation dosimetry in a healthy cynomolgus monkey. Results:18F-BMS-986192 bound to human and cynomolgus PD-L1 with a dissociation constant of less than 35 pM, as measured by surface plasmon resonance. This adnectin was labeled with 18F to yield a PET radioligand for assessing PD-L1 expression in vivo. 18F-BMS-986192 bound to tumor tissues as a function of PD-L1 expression determined by immunohistochemistry. Radioligand binding was blocked in a dose-dependent manner. In vivo PET imaging clearly visualized PD-L1 expression in mice implanted with PD-L1(+), L2987 xenograft tumors. Two hours after dosing, a 3.5-fold-higher uptake (2.41 ± 0.29 vs. 0.82 ± 0.11 percentage injected dose per gram, P < 0.0001) was observed in L2987 than in control HT-29 (PD-L1(-)) tumors. Coadministration of 3 mg/kg ADX_5322_A02 anti-PD-L1 adnectin reduced tumor uptake at 2 h after injection by approximately 70%, whereas HT-29 uptake remained unchanged, demonstrating PD-L1-specific binding. Biodistribution in a nonhuman primate showed binding in the PD-L1-rich spleen, with rapid blood clearance through the kidneys and bladder. Binding in the PD-L1(+) spleen was reduced by coadministration of BMS-986192. Dosimetry estimates indicate that the kidney is the dose-limiting organ, with an estimated human absorbed dose of 2.20E-01 mSv/MBq. Conclusion:18F-BMS-986192 demonstrated the feasibility of noninvasively imaging the PD-L1 status of tumors by small-animal PET studies. Clinical studies with 18F-BMS-986192 are under way to measure PD-L1 expression in human tumors.

Synthesis of unlabelled and stable-isotope-labelled glucuronide metabolites of dapagliflozin and synthesis of stable-isotope-labelled dapagliflozin.

Two regioisomeric glucuronide metabolites of dapagliflozin (BMS-512148) were synthesized and used to elucidate the structures of dapagliflozin metabolites observed in human urine samples. The structures of the synthetic metabolites were assigned by heteronuclear multiple-bond correlation, ROESY, and total correlation spectroscopy experiments. Analogues of these metabolites containing carbon-13 as a stable label were also prepared for use as internal standards for the analysis of urine samples obtained from patients participating in clinical studies.

Tissue distribution and tumor uptake of folate receptor-targeted epothilone folate conjugate, BMS-753493, in CD2F1 mice after systemic administration.

To assess targeting of an epothilone folate conjugate (BMS-753493) to the folate receptor (FR)-overexpressed tumor in mice bearing both FR+ and FR- tumors, a series of experiments were conducted by quantitative whole-body autoradiography (QWBA) and LC-MS/MS following i.v. administration of BMS-753493 or its active moiety, BMS-748285 in mice bearing FR+ (98M109) and FR- (M109) tumors. QWBA showed [3H]BMS-753493-derived radioactivity was extensively distributed to various tissues. The FR over-expressing 98M109 tumors showed consistently higher level of radioactivity than FR-negative tumors (i.e., M109 tumors) up to 48 h post dose of [3H]BMS-753493, despite the magnitude of difference between the tumors is relatively small (generally 3~5-fold). The radioactivity level in 98M109 tumors was 2~12-fold of normal tissues except intestine/content at 48 h post dose. No selective radioactivity uptake into 98M109 tumors over M109 or normal tissues was observed after i.v. administration of the active epothilone, [3H]BMS-748285. LC-MS/MS measurements demonstrated that the concentrations of BMS-748285, presumably from hydrolysis of the folate conjugate, in 98M109 tumors were greater than those in M109 tumors after i.v. administration of BMS-753493 (2-3-fold) whereas no differential uptake in the tumors following BMS-748285 administration. Those data were consistent with radioactivity determinations. Those results demonstrated that the folate conjugation in BMS-753493 enabled moderately preferential distribution of the active epothilone to FR over-expressing 98M109 tumors, thereby supporting targeted delivery of cytotoxics through the folate receptor.

The syntheses of isotopically labelled CB-1 antagonists for the treatment of obesity.

BMS-725519, BMS-811064, and BMS-812204 are potent and selective central cannabinoid receptor antagonists that have been investigated for the treatment of human obesity. To further understand their biotransformation profiles, radiolabelled and stable-labelled products were required. This paper describes the utility of [14 C]1,1-carbonyldiimidazole as a radiolabelling reagent for the syntheses of carbonyl-labelled [14 C]BMS-725519, [14 C]BMS-811064, and [14 C]BMS-812204. The syntheses of stable-labelled [13 C6 ]BMS-725519 and [13 CD313 CD2 ]BMS-812204 synthesized from of [13 C6 ]4-chloroacetophenone and [13 CD313 CD2 ]iodoethane, respectively, are also described.

The synthesis and analysis of [phenyl-14 C(U)]BMS-770767 and [13 C6 ]BMS-770767 for use in discovery biotransformation, human ADME and bioanalytical studies.

Type 2 diabetes is a significant worldwide health problem. To support the development of BMS-770767 as an inhibitor of 11ß-hydroxysteroid dehydrogenase type 1 (11ß-HSD1) for type 2 diabetes was required the synthesis of carbon-14-labelled material for use in metabolic profiling and for the human adsorption, distribution, metabolism and excretion (ADME) study. Initially, [phenyl-14 C(U)]BMS-770767 was synthesized in two steps from a late-stage intermediate and [14 C(U)]2-chlorophenol to give the desired final product in 18% yield. Later, the synthesis was completed for the human ADME clinical study using a combination of the discovery and process chemistry routes under cGMP to prepare [phenyl-14 C(U)]BMS-770767. The radiochemical purity of the synthesized [phenyl-14 C(U)]BMS-770767 after dilution with unlabelled clinical grade BMS-770767 was 99.1% having a specific activity of 1.61 µCi/mg. In addition, to support the quantification of BMS-770767 in LC/MS analyses, [13 C6 ]BMT-770767 was prepared in two steps from a late-stage intermediate and [13 C6 ]2-chlorophenol.

The syntheses of [(14) C]BMS-823778 for use in a human ADME clinical study and of [(13) CD3 (13) CD2 ]BMT-094817, a stable-isotope labeled standard of a newly detected human metabolite.

Type 2 diabetes is a significant worldwide health problem. To support the development of BMS-823778 as an inhibitor of 11ß-hydroxysteroid dehydrogenase type 1 for type 2 diabetes, the synthesis of carbon-14-labeled material was required for use in a human adsorption, distribution, metabolism, and excretion (ADME) study. The HCl salt form of [(14) C]BMS-823778 was synthesized in two steps from commercially available [2-(14) C]acetone. The radiochemical purity of the synthesized [(14) C]BMS-823778 after dilution with unlabeled clinical-grade BMS-823778 was 99.5% having a specific activity of 7.379 µCi/mg. One result of the human ADME study was the detection of a new human metabolite, BMT-094817. To support the quantification of BMT-094817 in clinical samples, it was necessary to synthesize [(13) CD3 (13) CD2 ]BMT-094817 for use as a liquid chromatography/mass spectrometry standard. [(13) CD3 (13) CD2 ]BMT-094817 was prepared in five labeled steps from [(13) CD3 ]iodomethane.

Synthesis of isotopically labeled daclatasvir for use in human clinical studies.

Daclatasvir is a novel hepatitis C virus NS5A inhibitor developed by Bristol-Myers Squibb and marketed as Daklinza®. The need to support the development of daclatasvir required the synthesis of carbon-14 labeled material for use in human absorption, distribution, metabolism, and excretion studies. A total of 7.53 mCi of [(14) C]-daclatasvir was synthesized in eight steps from commercially available [(14) C]-copper cyanide. The radiochemical purity was 99.6%, and specific activity was 3.86 µCi/mg. To support a human absolute bioavailability study, 5.56 g of [(13) C2 , (15) N4 ]-daclatasvir was synthesized in four steps.

Synthesis of stable isotope-labeled epothilone D using a degradation-reconstruction approach.

The stabilization of microtubules using epothilones represents a novel mechanism of action to treat Alzheimer's disease. Epothilone D is one such microtubule-stabilizing drug that has been investigated by Bristol-Myers Squibb. An important step in the development process was the synthesis of a stable isotope-labeled analog for use in bioanalytical assays to accurately quantify the concentration of the drug in biological samples. A novel synthetic route to stable isotope-labeled epothilone D is described. The synthetic route was based on a strategy to degrade epothilone B and then use that key intermediate to reconstruct stable isotope-labeled epothilone D. Epothilone B was treated with potassium osmate and sodium periodate. The thiazole moiety in epothilone B was efficiently cleaved to give (1S,3S,7S,10R,11S,12S,16R)-3-acetyl-7,11-dihydroxy-8,8,10,12,16-pentamethyl-4,17-dioxabicyclo[14.1.0]heptadecane-5,9-dione. The epoxide in the macrocyclic ring of that intermediate was cleanly removed by treatment with tungsten hexachloride and n-butyllithium to give the corresponding olefin (4S,7R,8S,9S,16S,Z)-16-acetyl-4,8-dihydroxy-5,5,7,9,13-pentamethyloxacyclohexadec-13-ene-2,6-dione. Bis(triethylsilyl) protection produced (4S,7R,8S,9S,16S,Z)-16-acetyl-5,5,7,9,13-pentamethyl-4,8-bis(triethylsilyloxy)-oxacyclohexadec-13-ene-2,6-dione. This intermediate was coupled to a stable isotope-labeled thiazole using a Wittig reaction as the key step to provide (13)C5, (15)N-labeled epothilone D. In summary, the synthesis was completed in nine total steps, only six of which involved isotopically labeled reagents. A total of 168 mg of (13)C5, (15)N-labeled epothilone D was prepared in an 8% overall yield from (13)C2, (15)N-labeled thioacetamide and (13)C3-labeled ethyl bromopyruvate.

The synthesis of 14C-labeled N-succinimidyl-3-maleimidopropionate, a linker molecule for PEGylated biologics.

Carbon-14 labeled linker molecule, N-succinimidyl-3-maleimidopropionate was prepared for disposition studies of PEGylated biologics. Our new route started with 100 mCi of carbon-14 labeled Potassium cyanide (KCN) to prepare 55 mCi of [1-(14)C]N-succinimidyl-3-maleimidopropionate, 6 in five steps. This represents a multiple of 5.5× improvement in the yield of the desired labeled product compared with our previous synthesis.

Synthesis of carbon-14 and stable isotope labeled Avagacestat: a novel gamma secretase inhibitor for the treatment of Alzheimer's disease.

Bristol-Myers Squibb and others are developing drugs that target novel mechanisms to combat Alzheimer's disease. γ-Secretase inhibitors are one class of potential therapies that have received considerable attention. (R)-2-(4-Chloro-N-(2-fluoro-4-(1,2,4-oxadiazol-3-yl)benzyl)phenylsulfonamido)-5,5,5-trifluoropentanamide (Avagacestat) is a γ-secretase-inhibiting drug that has been investigated by Bristol-Myers Squibb in preclinical and clinical studies. An important step in the development process was the synthesis of a carbon-14-labeled analog for use in a human absorption, distribution, metabolism, and excretion study and a stable isotope labeled analog for use as a standard in bioanalytical assays to accurately quantify the concentration of the drug in biological samples. Carbon-14 labeled Avagacestat was synthesized in seven steps in a 33% overall yield from carbon-14 labeled potassium cyanide. A total of 5.95 mCi was prepared with a specific activity of 0.81 µCi/mg and a radiochemical purity of 99.9%. (13) C6 -Labeled Avagacestat was synthesized in three steps in a 15% overall yield from 4-chloro[(13) C6 ]aniline. A total of 585 mg was prepared with a ultraviolet purity of 99.9%.

Synthesis of a stable isotopically labeled universal surrogate peptide for use as an internal standard in LC-MS/MS bioanalysis of human IgG and Fc-fusion protein drug candidates.

The synthesis of a 16-residue, stable isotopically labeled peptide is described for use as a LC-MS/MS (Liquid chromatography-mass spectrometry/mass spectrometry) internal standard in bioanalytical studies. This peptide serves as a single universal surrogate peptide capable of quantifying a wide variety of immunoglobulin G and Fc-fusion protein drug candidates in animal species used in pre-clinical drug development studies. An efficient synthesis approach for this peptide was developed using microwave-assisted solid phase peptide synthesis (SPPS) techniques, which included the use of a pseudoproline dipeptide derivative. The corresponding conventional room temperature SPPS was unsuccessful and gave only mixtures of truncated products. Stable-labeled leucine was incorporated as a single residue via manual coupling of commercially available Fmoc-[(13) C6 , (15) N]-l-leucine onto an 11-unit segment followed by automated microwave-assisted elaboration of the final four residues. Using this approach, the desired labeled peptide was prepared in high purity and in sufficient quantities for long-term supplies as a bioanalytical internal standard. The results strongly demonstrate the importance of utilizing both microwave-assisted peptide synthesis and pseudoproline dipeptide techniques to allow the preparation of labeled peptides with highly lipophilic and sterically hindered side-chains.