Deuterium MRI of serine metabolism in mouse models of glioblastoma.

PURPOSE: Serine is a major source of one-carbon units needed for the synthesis of nucleotides and the production of intramitochondrial nicotinamide adenine dinucleotide phosphate (NADPH), and it plays an important role in cancer cell proliferation. The aim of this study was to develop a deuterium (2H) MRS imaging method for imaging tumor serine metabolism. METHODS: Sequential (2H) spectra and spectroscopic images were used to monitor the metabolism of [2,3,3-2H3]serine in patient-derived glioblastoma cells in vitro and in tumors obtained by their orthotopic implantation in mouse brain. RESULTS: [14,14-2H2] 5,10-methylene-tetrahydrofolate, [2H]glycine, [2H]formate, and labeled water were detected in cell suspensions and water labeling in spectroscopic images of tumors. Studies in cells and tumors with variable mitochondrial content and inhibitor studies in cells demonstrated that most of the labeled serine was metabolized in the mitochondria. Water labeling in the cell suspensions was correlated with formate labeling; therefore, water labeling observed in tumors could be used to provide a surrogate measure of flux in the pathway of one-carbon metabolism in vivo. CONCLUSION: The method has the potential to be used clinically to select patients for treatment with inhibitors of one-carbon metabolism and subsequently to detect their early responses to such treatment.

Deuterium Metabolic Imaging Differentiates Glioblastoma Metabolic Subtypes and Detects Early Response to Chemoradiotherapy.

Metabolic subtypes of glioblastoma (GBM) have different prognoses and responses to treatment. Deuterium metabolic imaging with 2H-labeled substrates is a potential approach to stratify patients into metabolic subtypes for targeted treatment. In this study, we used 2H magnetic resonance spectroscopy and magnetic resonance spectroscopic imaging (MRSI) measurements of [6,6'-2H2]glucose metabolism to identify metabolic subtypes and their responses to chemoradiotherapy in patient-derived GBM xenografts in vivo. The metabolism of patient-derived cells was first characterized in vitro by measuring the oxygen consumption rate, a marker of mitochondrial tricarboxylic acid cycle activity, as well as the extracellular acidification rate and 2H-labeled lactate production from [6,6'-2H2]glucose, which are markers of glycolytic activity. Two cell lines representative of a glycolytic subtype and two representative of a mitochondrial subtype were identified. 2H magnetic resonance spectroscopy and MRSI measurements showed similar concentrations of 2H-labeled glucose from [6,6'-2H2]glucose in all four tumor models when implanted orthotopically in mice. The glycolytic subtypes showed higher concentrations of 2H-labeled lactate than the mitochondrial subtypes and normal-appearing brain tissue, whereas the mitochondrial subtypes showed more glutamate/glutamine labeling, a surrogate for tricarboxylic acid cycle activity, than the glycolytic subtypes and normal-appearing brain tissue. The response of the tumors to chemoradiation could be detected within 24 hours of treatment completion, with the mitochondrial subtypes showing a decrease in both 2H-labeled glutamate/glutamine and lactate concentrations and glycolytic tumors showing a decrease in 2H-labeled lactate concentration. This technique has the potential to be used clinically for treatment selection and early detection of treatment response. SIGNIFICANCE: Deuterium magnetic resonance spectroscopic imaging of glucose metabolism has the potential to differentiate between glycolytic and mitochondrial metabolic subtypes in glioblastoma and to evaluate early treatment responses, which could guide patient treatment.

Metabolic imaging with deuterium labeled substrates.

Deuterium metabolic imaging (DMI) is an emerging clinically-applicable technique for the non-invasive investigation of tissue metabolism. The generally short T1 values of 2H-labeled metabolites in vivo can compensate for the relatively low sensitivity of detection by allowing rapid signal acquisition in the absence of significant signal saturation. Studies with deuterated substrates, including [6,6'-2H2]glucose, [2H3]acetate, [2H9]choline and [2,3-2H2]fumarate have demonstrated the considerable potential of DMI for imaging tissue metabolism and cell death in vivo. The technique is evaluated here in comparison with established metabolic imaging techniques, including PET measurements of 2-deoxy-2-[18F]fluoro-d-glucose (FDG) uptake and 13C MR imaging of the metabolism of hyperpolarized 13C-labeled substrates.

Assessment of the sensitivity of 2 H MR spectroscopy measurements of [2,3-2 H2 ]fumarate metabolism for detecting tumor cell death.

Imaging the metabolism of [2,3-2 H2 ]fumarate to produce malate can be used to detect tumor cell death post-treatment. Here, we assess the sensitivity of the technique for detecting cell death by lowering the concentration of injected [2,3-2 H2 ]fumarate and by varying the extent of tumor cell death through changes in drug concentration. Mice were implanted subcutaneously with human triple negative breast cancer cells (MDA-MB-231) and injected with 0.1, 0.3, and 0.5 g/kg [2,3-2 H2 ]fumarate before and after treatment with a multivalent TRAlL-R2 agonist (MEDI3039) at 0.1, 0.4, and 0.8 mg/kg. Tumor conversion of [2,3-2 H2 ]fumarate to [2,3-2 H2 ]malate was assessed from a series of 13 spatially localized 2 H MR spectra acquired over 65 min using a pulse-acquire sequence with a 2-ms BIR4 adiabatic excitation pulse. Tumors were then excised and stained for histopathological markers of cell death: cleaved caspase 3 (CC3) and DNA damage (terminal deoxynucleotidyl transferase dUTP nick end labeling [TUNEL]). The rate of malate production and the malate/fumarate ratio plateaued at tumor fumarate concentrations of 2 mM, which were obtained with injected [2,3-2 H2 ]fumarate concentrations of 0.3 g/kg and above. Tumor malate concentration and the malate/fumarate ratio increased linearly with the extent of cell death determined histologically. At an injected [2,3-2 H2 ]fumarate concentration of 0.3 g/kg, 20% CC3 staining corresponded to a malate concentration of 0.62 mM and a malate/fumarate ratio of 0.21. Extrapolation indicated that there would be no detectable malate at 0% CC3 staining. The use of low and nontoxic fumarate concentrations and the production of [2,3-2 H2 ]malate at concentrations that are within the range that can be detected clinically suggest this technique could translate to the clinic.

Preclinical PET Imaging of Tumor Cell Death following Therapy Using Gallium-68-Labeled C2Am.

There is an unmet clinical need for imaging agents capable of detecting early evidence of tumor cell death, since the timing, extent, and distribution of cell death in tumors following treatment can give an indication of treatment outcome. We describe here 68Ga-labeled C2Am, which is a phosphatidylserine-binding protein, for imaging tumor cell death in vivo using positron emission tomography (PET). A one-pot synthesis of 68Ga-C2Am (20 min, 25 °C, >95% radiochemical purity) has been developed, using a NODAGA-maleimide chelator. The binding of 68Ga-C2Am to apoptotic and necrotic tumor cells was assessed in vitro using human breast and colorectal cancer cell lines, and in vivo, using dynamic PET measurements in mice implanted subcutaneously with the colorectal tumor cells and treated with a TRAIL-R2 agonist. 68Ga-C2Am showed predominantly renal clearance and low retention in the liver, spleen, small intestine, and bone and generated a tumor-to-muscle (T/m) ratio of 2.3 ± 0.4, at 2 h post probe administration and at 24 h following treatment. 68Ga-C2Am has the potential to be used in the clinic as a PET tracer for assessing early treatment response in tumors.

Imaging Glioblastoma Response to Radiotherapy Using 2H Magnetic Resonance Spectroscopy Measurements of Fumarate Metabolism.

Early detection of tumor cell death in glioblastoma following treatment with chemoradiation has the potential to distinguish between true disease progression and pseudoprogression. Tumor cell death can be detected noninvasively in vivo by imaging the production of [2,3-2H2]malate from [2,3-2H2]fumarate using 2H magnetic resonance (MR) spectroscopic imaging. We show here that 2H MR spectroscopy and spectroscopic imaging measurements of [2,3-2H2]fumarate metabolism can detect tumor cell death in orthotopically implanted glioblastoma models within 48 hours following the completion of chemoradiation. Following the injection of [2,3-2H2]fumarate into tumor-bearing mice, production of [2,3-2H2]malate was measured in a human cell line-derived model and in radiosensitive and radioresistant patient-derived models of glioblastoma that were treated with temozolomide followed by targeted fractionated irradiation. The increase in the [2,3-2H2]malate/[2,3-2H2]fumarate signal ratio posttreatment, which correlated with histologic assessment of cell death, was a more sensitive indicator of treatment response than diffusion-weighted and contrast agent-enhanced 1H MRI measurements, which have been used clinically to detect responses of glioblastoma to chemoradiation. Overall, early detection of glioblastoma cell death using 2H MRI of malate production from fumarate could help improve the clinical evaluation of response to chemoradiation. SIGNIFICANCE: 2H magnetic resonance imaging of labeled fumarate metabolism can detect early evidence of tumor cell death following chemoradiation, meeting a clinical need to reliably detect treatment response in glioblastoma.

Deuterium MRSI of tumor cell death in vivo following oral delivery of 2 H-labeled fumarate.

PURPOSE: There is an unmet clinical need for direct and sensitive methods to detect cell death in vivo, especially with regard to monitoring tumor treatment response. We have shown previously that tumor cell death can be detected in vivo from 2 H MRS and MRSI measurements of increased [2,3-2 H2 ]malate production following intravenous injection of [2,3-2 H2 ]fumarate. We show here that cell death can be detected with similar sensitivity following oral administration of the 2 H-labeled fumarate. METHODS: Mice with subcutaneously implanted EL4 tumors were fasted for 1 h before administration (200 µl) of [2,3-2 H2 ]fumarate (2 g/kg bodyweight) via oral gavage without anesthesia. The animals were then anesthetized, and after 30 min, tumor conversion of [2,3-2 H2 ]fumarate to [2,3-2 H2 ]malate was assessed from a series of 13 2 H spectra acquired over a period of 65 min. The 2 H spectra and 2 H spectroscopic images were acquired using a surface coil before and at 48 h after treatment with a chemotherapeutic drug (etoposide, 67 mg/kg). RESULTS: The malate/fumarate signal ratio increased from 0.022 ± 0.03 before drug treatment to 0.12 ± 0.04 following treatment (p = 0.023, n = 4). Labeled malate was undetectable in spectroscopic images acquired before treatment and increased in the tumor area following treatment. The increase in the malate/fumarate signal ratio was similar to that observed previously following intravenous administration of labeled fumarate. CONCLUSION: Orally administered [2,3-2 H2 ]fumarate can be used to detect tumor cell death noninvasively following treatment with a sensitivity that is similar to that obtained with intravenous administration.

Genetic algorithm-based optimization of pulse sequences.

PURPOSE: The performance of pulse sequences in vivo can be limited by fast relaxation rates, magnetic field inhomogeneity, and nonuniform spin excitation. We describe here a method for pulse sequence optimization that uses a stochastic numerical solver that in principle is capable of finding a global optimum. The method provides a simple framework for incorporating any constraint and implementing arbitrarily complex cost functions. Efficient methods for simulating spin dynamics and incorporating frequency selectivity are also described. METHODS: Optimized pulse sequences for polarization transfer between protons and X-nuclei and excitation pulses that eliminate J-coupling modulation were evaluated experimentally using a surface coil on phantoms, and also the detection of hyperpolarized [2-13 C]lactate in vivo in the case of J-coupling modulation-free excitation. RESULTS: The optimized polarization transfer pulses improved the SNR by ~50% with a more than twofold reduction in the B1 field, and J-coupling modulation-free excitation was achieved with a more than threefold reduction in pulse length. CONCLUSION: This process could be used to optimize any pulse when there is a need to improve the uniformity and frequency selectivity of excitation as well as to design new pulses to steer the spin system to any desired achievable state.

Labile Photo-Induced Free Radical in α-Ketoglutaric Acid: a Universal Endogenous Polarizing Agent for In†Vivo Hyperpolarized 13 C Magnetic Resonance.

Hyperpolarized (HP) 13 C magnetic resonance enables non-invasive probing of metabolism in vivo. To date, only 13 C-molecules hyperpolarized with persistent trityl radicals have been injected in humans. We show here that the free radical photo-induced in alpha-ketoglutaric acid (α-KG) can be used to hyperpolarize photo-inactive 13 C-molecules such as [1-13 C]lactate. α-KG is an endogenous molecule with an exceptionally high radical yield under photo-irradiation, up to 50 %, and its breakdown product, succinic acid, is also endogenous. This radical precursor therefore exhibits an excellent safety profile for translation to human studies. The labile nature of the radical means that no filtration is required prior to injection while also offering the opportunity to extend the 13 C relaxation time in frozen HP 13 C-molecules for storage and transport. The potential for in vivo metabolic studies is demonstrated in the rat liver following the injection of a physiological dose of HP [1-13 C]lactate.

Labile Photo-Induced Free Radical in α-Ketoglutaric Acid: a Universal Endogenous Polarizing Agent for In†Vivo Hyperpolarized 13C Magnetic Resonance.

Hyperpolarized (HP) 13C magnetic resonance enables non-invasive probing of metabolism in vivo. To date, only 13C-molecules hyperpolarized with persistent trityl radicals have been injected in humans. We show here that the free radical photo-induced in alpha-ketoglutaric acid (α-KG) can be used to hyperpolarize photo-inactive 13C-molecules such as [1-13C]lactate. α-KG is an endogenous molecule with an exceptionally high radical yield under photo-irradiation, up to 50 %, and its breakdown product, succinic acid, is also endogenous. This radical precursor therefore exhibits an excellent safety profile for translation to human studies. The labile nature of the radical means that no filtration is required prior to injection while also offering the opportunity to extend the 13C relaxation time in frozen HP 13C-molecules for storage and transport. The potential for in vivo metabolic studies is demonstrated in the rat liver following the injection of a physiological dose of HP [1-13C]lactate.

Monitoring tumor cell death in murine tumor models using deuterium magnetic resonance spectroscopy and spectroscopic imaging.

2H magnetic resonance spectroscopic imaging has been shown recently to be a viable technique for metabolic imaging in the clinic. We show here that 2H MR spectroscopy and spectroscopic imaging measurements of [2,3-2H2]malate production from [2,3-2H2]fumarate can be used to detect tumor cell death in vivo via the production of labeled malate. Production of [2,3-2H2]malate, following injection of [2,3-2H2]fumarate (1 g/kg) into tumor-bearing mice, was measured in a murine lymphoma (EL4) treated with etoposide, and in human breast (MDA-MB-231) and colorectal (Colo205) xenografts treated with a TRAILR2 agonist, using surface-coil localized 2H MR spectroscopy at 7 T. Malate production was also imaged in EL4 tumors using a fast 2H chemical shift imaging sequence. The malate/fumarate ratio increased from 0.016 ± 0.02 to 0.16 ± 0.14 in EL4 tumors 48 h after drug treatment (P = 0.0024, n = 3), and from 0.019 ± 0.03 to 0.25 ± 0.23 in MDA-MB-231 tumors (P = 0.0001, n = 5) and from 0.016 ± 0.04 to 0.28 ± 0.26 in Colo205 tumors (P = 0.0002, n = 5) 24 h after drug treatment. These increases were correlated with increased levels of cell death measured in excised tumor sections obtained immediately after imaging. 2H MR measurements of [2,3-2H2]malate production from [2,3-2H2]fumarate provide a potentially less expensive and more sensitive method for detecting cell death in vivo than 13C MR measurements of hyperpolarized [1,4-13C2]fumarate metabolism, which have been used previously for this purpose.

18F-C2Am: a targeted imaging agent for detecting tumor cell death in vivo using positron emission tomography.

INTRODUCTION: Trialing novel cancer therapies in the clinic would benefit from imaging agents that can detect early evidence of treatment response. The timing, extent and distribution of cell death in tumors following treatment can give an indication of outcome. We describe here an 18F-labeled derivative of a phosphatidylserine-binding protein, the C2A domain of Synaptotagmin-I (C2Am), for imaging tumor cell death in vivo using PET. METHODS: A one-pot, two-step automated synthesis of N-(5-[18F]fluoropentyl)maleimide (60 min synthesis time, > 98% radiochemical purity) has been developed, which was used to label the single cysteine residue in C2Am within 30 min at room temperature. Binding of 18F-C2Am to apoptotic and necrotic tumor cells was assessed in vitro, and also in vivo, by dynamic PET and biodistribution measurements in mice bearing human tumor xenografts treated with a TRAILR2 agonist or with conventional chemotherapy. C2Am detection of tumor cell death was validated by correlation of probe binding with histological markers of cell death in tumor sections obtained immediately after imaging. RESULTS: 18F-C2Am showed a favorable biodistribution profile, with predominantly renal clearance and minimal retention in spleen, liver, small intestine, bone and kidney, at 2 h following probe administration. 18F-C2Am generated tumor-to-muscle (T/m) ratios of 6.1 ± 2.1 and 10.7 ± 2.4 within 2 h of probe administration in colorectal and breast tumor models, respectively, following treatment with the TRAILR2 agonist. The levels of cell death (CC3 positivity) following treatment were 12.9-58.8% and 11.3-79.7% in the breast and colorectal xenografts, respectively. Overall, a 20% increase in CC3 positivity generated a one unit increase in the post/pre-treatment tumor contrast. Significant correlations were found between tracer uptake post-treatment, at 2 h post-probe administration, and histological markers of cell death (CC3: Pearson R = 0.733, P = 0.0005; TUNEL: Pearson R = 0.532, P = 0.023). CONCLUSION: The rapid clearance of 18F-C2Am from the blood pool and low kidney retention allowed the spatial distribution of cell death in a tumor to be imaged during the course of therapy, providing a rapid assessment of tumor treatment response. 18F-C2Am has the potential to be used in the clinic to assess early treatment response in tumors.

The Contorsbody, an antibody format for agonism: Design, structure, and function.

The careful design of the antibody architecture is becoming more and more important, especially when the purpose is agonism. We present the design of a novel antibody format that is able to promote receptor dimerization and induce signal transduction resulting in cell proliferation. Mono-specific bivalent Y-shape IgGs made of two light chains and two heavy chains are engineered into single chain dimers of two modified heavy chains, resulting in the fixation of the two Fab fragments along the Fc dimerizing moiety. By this, an antagonist of the Her-receptor family, Trastuzumab, is re-formatted into an agonist by simply incorporating the original binding motif into a different geometrically and sterically constrained conformation. This novel format, named Contorsbody, retains antigen binding properties of the parental IgGs and can be produced by standard technologies established for recombinant IgGs. Structural analyses using molecular dynamics and electron microscopy are described to guide the initial design and to confirm the Contorsbody as a very compact molecule, respectively. Contorsbodies show increased rigidity compared to IgGs and their Fab moieties are positioned parallel and adjacent to each other. This geometry has an increased potential to trigger cell surface antigen or receptor 'cis'-dimerization without 'trans'-bridging of cells or mere receptor blockade.

A multi spin echo pulse sequence with optimized excitation pulses and a 3D cone readout for hyperpolarized 13 C imaging.

PURPOSE: Imaging tumor metabolism in vivo using hyperpolarized [1-13 C]pyruvate is a promising technique for detecting disease, monitoring disease progression, and assessing treatment response. However, the transient nature of the hyperpolarization and its depletion following excitation limits the available time for imaging. We describe here a single-shot multi spin echo sequence, which improves on previously reported sequences, with a shorter readout time, isotropic point spread function (PSF), and better signal-to-noise ratio. METHODS: The sequence uses numerically optimized spectrally selective excitation pulses set to the resonant frequencies of pyruvate and lactate and a hyperbolic secant adiabatic refocusing pulse, all applied in the absence of slice selection gradients. The excitation pulses were designed to be resistant to the effects of B0 and B1 field inhomogeneity. The gradient readout uses a 3D cone trajectory composed of 13 cones, all fully refocused and distributed among 7 spin echoes. The maximal gradient amplitude and slew rate were set to 4 G/cm and 20 G/cm/ms, respectively, to demonstrate the feasibility of clinical translation. RESULTS: The pulse sequence gave an isotropic PSF of 2.8 mm. The excitation profiles of the optimized pulses closely matched simulations and a 46.10 ± 0.04% gain in image SNR was observed compared to a conventional Shinnar-Le Roux excitation pulse. The sequence was demonstrated with dynamic imaging of hyperpolarized [1-13 C]pyruvate and [1-13 C]lactate in vivo. CONCLUSION: The pulse sequence was capable of dynamic imaging of hyperpolarized 13 C labeled metabolites in vivo with relatively high spatial and temporal resolution and immunity to system imperfections.

Measuring Tumor Glycolytic Flux in Vivo by Using Fast Deuterium MRI.

Background Tumor cells frequently show high rates of aerobic glycolysis, which provides the glycolytic intermediates needed for the increased biosynthetic demands of rapid cell growth and proliferation. Existing clinical methods (fluorodeoxyglucose PET and carbon 13 MRI and spectroscopy) do not allow quantitative images of glycolytic flux. Purpose To evaluate the use of deuterium (hydrogen 2 [2H]) MR spectroscopic imaging for quantitative mapping of tumor glycolytic flux and to assess response to chemotherapy. Materials and Methods A fast three-dimensional 2H MR spectroscopic imaging pulse sequence, with a time resolution of 10 minutes, was used to image glycolytic flux in a murine tumor model after bolus injection of D-[6,6'-2H2]glucose before and 48 hours after treatment with a chemotherapeutic agent. Tumor lactate labeling, expressed as the lactate-to-water and lactate-to-glucose signal ratios, was also assessed in localized 2H MR spectra. Statistical significance was tested with a one-sided paired t test. Results 2H MR spectroscopic imaging showed heterogeneity in glycolytic flux across the tumor and an early decrease in flux following treatment with a chemotherapeutic drug. Spectroscopy measurements on five animals showed a decrease in the lactate-to-water signal ratio, from 0.33 ± 0.10 to 0.089 ± 0.039 (P = .005), and in the lactate-to-glucose ratio, from 0.27 ± 0.12 to 0.12 ± 0.06 (P = .04), following drug treatment. Conclusion Rapidly acquired deuterium (hydrogen 2) MR spectroscopic images can provide quantitative and spatially resolved measurements of glycolytic flux in tumors that can be used to assess treatment response. Published under a CC BY 4.0 license. Online supplemental material is available for this article. See also the editorial by Ouwerkerk in this issue.

Characterization of a re-engineered, mesothelin-targeted Pseudomonas exotoxin fusion protein for lung cancer therapy.

Mesothelin overexpression in lung adenocarcinomas correlates with the presence of activating KRAS mutations and poor prognosis. Hence SS1P, a mesothelin-targeted immunotoxin, could offer valuable treatment options for these patients, but its use in solid tumor therapy is hampered by high immunogenicity and non-specific toxicity. To overcome both obstacles we developed RG7787, a de-immunized cytotoxic fusion protein comprising a humanized SS1 Fab fragment and a truncated, B-cell epitope silenced, 24 kD fragment of Pseudomonas exotoxin A (PE24). Reactivity of RG7787 with sera from immunotoxin-treated patients was >1000 fold reduced. In vitro RG7787 inhibited cell viability of lung cancer cell lines with picomolar potency. The pharmacokinetic properties of RG7787 in rodents were comparable to SS1P, yet it was tolerated up to 10 fold better without causing severe vascular leak syndrome or hepatotoxicity. A pharmacokinetic/pharmacodynamic model developed based on NCI-H596 xenograft studies showed that for RG7787 and SS1P, their in vitro and in vivo potencies closely correlate. At optimal doses of 2-3 mg/kg RG7787 is more efficacious than SS1P. Even large, well established tumors (600 mm(3)) underwent remission during three treatment cycles with RG7787. Also in two patient-derived lung cancer xenograft models, Lu7336 and Lu7187, RG7787 showed anti-tumor efficacy. In monotherapy two treatment cycles were moderately efficacious in the Lu7336 model but showed good anti-tumor activity in the KRAS mutant Lu7187 model (26% and 80% tumor growth inhibition, respectively). Combination of RG7787 with standard chemotherapies further enhanced efficacy in both models achieving near complete eradication of Lu7187 tumors.

Aggregation and chemical modification of monoclonal antibodies under upstream processing conditions.

PURPOSE: To investigate antibody stability and formation of modified species under upstream processing conditions. METHODS: The stability of 11 purified monoclonal human IgG1 and IgG4 antibodies, including an IgG1-based bispecific CrossMab, was compared in downscale mixing stress models. One of these molecules was further evaluated in realistic bioreactor stress models and in cell culture fermentations. Analytical techniques include size exclusion chromatography (SEC), turbidity measurements, cation exchange chromatography (cIEX), dynamic light scattering (DLS) and differential scanning calorimetry (DSC). RESULTS: Sensitivity in downscale stress models varies among antibodies and results in formation of high molecular weight (HMW) aggregates. Stability is increased in cell culture medium and in bioreactors. Media components stabilizing the proteins were identified. Extensive chemical modifications were detected both in stress models as well as during production of antibodies in cell culture fermentations. CONCLUSIONS: Protective compounds must be present in chemically defined fermentation media in order to stabilize antibodies against the formation of HMW aggregates. An increase in chemical modifications is detectable in bioreactor stress models and over the course of cell culture fermentations; this increase is dependent on the expression rate, pH, temperature and fermentation time. Consequently, product heterogeneity increases during upstream processing, and this compromises the product quality.

Separation of fast from slow anabolism by site-specific PEGylation of insulin-like growth factor I (IGF-I).

Insulin-like growth factor I (IGF-I) has important anabolic and homeostatic functions in tissues like skeletal muscle, and a decline in circulating levels is linked with catabolic conditions. Whereas IGF-I therapies for musculoskeletal disorders have been postulated, dosing issues and disruptions of the homeostasis have so far precluded clinical application. We have developed a novel IGF-I variant by site-specific addition of polyethylene glycol (PEG) to lysine 68 (PEG-IGF-I). In vitro, this modification decreased the affinity for the IGF-I and insulin receptors, presumably through decreased association rates, and slowed down the association to IGF-I-binding proteins, selectively limiting fast but maintaining sustained anabolic activity. Desirable in vivo effects of PEG-IGF-I included increased half-life and recruitment of IGF-binding proteins, thereby reducing risk of hypoglycemia. PEG-IGF-I was equipotent to IGF-I in ameliorating contraction-induced muscle injury in vivo without affecting muscle metabolism as IGF-I did. The data provide an important step in understanding the differences of IGF-I and insulin receptor contribution to the in vivo activity of IGF-I. In addition, PEG-IGF-I presents an innovative concept for IGF-I therapy in diseases with indicated muscle dysfunction.