Cerebellar Brain Inhibition Is Associated With the Severity of Cervical Dystonia

PURPOSE

Cerebellar connectivity is thought to be abnormal in cervical dystonia (CD) and other dystonia subtypes, based on evidence from imaging studies and animal work. The authors investigated whether transcranial magnetic stimulation-induced cerebellar brain inhibition (CBI), a measure of cerebellar efficiency at inhibiting motor outflow, is abnormal in patients with CD and/or is associated with clinical features of CD. Because of methodological heterogeneity in CBI reporting, the authors deployed additional controls to reduce potential sources of variability in this study.

METHODS

Cerebellar brain inhibition was applied in 20 CD patients and 14 healthy control subjects. Cerebellar brain inhibition consisted of a cerebellar conditioning stimulus delivered at four different interstimulus intervals (ISIs) before a test stimulus delivered to hand muscle representation in the motor cortex. The average ratio of conditioned to unconditioned motor evoked potential was computed for each ISI. Cervical dystonia clinical severity was measured using the Toronto Western Spasmodic Torticollis Rating Scale. Control experiments involved neuronavigated transcranial magnetic stimulation, neck postural control in patients, and careful screening for noncerebellar pathway inhibition via cervicomedullary evoked potentials.

RESULTS

There was no difference between CBI measured in healthy control subjects and CD patients at any of the four ISIs; however, CBI efficiency was significantly correlated with worsening CD clinical severity at the 5 ms ISI.

CONCLUSIONS

Cerebellar brain inhibition is a variable measure in both healthy control subjects and CD patients; much of this variability may be attributed to experimental methodology. Yet, CD severity is significantly associated with reduced CBI at the 5 ms ISI, suggestive of cerebello-thalamo-cortical tract dysfunction in this disorder.

Robotic mapping of motor cortex in children with perinatal stroke and hemiparesis

Brain stimulation combined with intensive therapy may improve hand function in children with perinatal stroke‐induced unilateral cerebral palsy (UCP). However, response to therapy varies and underlying neuroplasticity mechanisms remain unclear. Here, we aimed to characterize robotic motor mapping outcomes in children with UCP. Twenty‐nine children with perinatal stroke and UCP (median age 11 ± 2 years) were compared to 24 typically developing controls (TDC). Robotic, neuronavigated transcranial magnetic stimulation was employed to define bilateral motor maps including area, volume, and peak motor evoked potential (MEP). Map outcomes were compared to the primary clinical outcome of the Jebsen–Taylor Test of Hand Function (JTT). Maps were reliably obtained in the contralesional motor cortex (24/29) but challenging in the lesioned hemisphere (5/29). Within the contralesional M1 of participants with UCP, area and peak MEP amplitude of the unaffected map were larger than the affected map. When comparing bilateral maps within the contralesional M1 in children with UCP to that of TDC, only peak MEP amplitudes were different, being smaller for the affected hand as compared to TDC. We observed correlations between the unaffected map when stimulating the contralesional M1 and function of the unaffected hand. Robotic motor mapping can characterize motor cortex neurophysiology in children with perinatal stroke. Map area and peak MEP amplitude may represent discrete biomarkers of developmental plasticity in the contralesional M1. Correlations between map metrics and hand function suggest clinical relevance and utility in studies of interventional plasticity.

Theta-Burst Stimulation for Cognitive Enhancement in Parkinson's Disease With Mild Cognitive Impairment: A Randomized, Double-Blind, Sham-Controlled Trial

Background: Mild cognitive impairment is a common non-motor symptom of Parkinson's disease (PD-MCI) and has minimal treatment options.

Objective: In this double-blind, randomized, sham-controlled trial, we assessed the effect of repeated sessions of intermittent theta-burst stimulation over the left dorsolateral prefrontal cortex on cognition and brain connectivity in subjects with PD-MCI.

Methods: Forty-one subjects were randomized to receive real (n = 21) or sham stimulation (n = 20). All subjects underwent neuropsychological assessments before, 1 day, and 1 month after stimulation. Subjects also underwent resting-state functional magnetic resonance imaging before and 48 h after stimulation. The primary outcome was the change in the cognitive domain (executive function, attention, memory, language, and visuospatial abilities) z-scores across time.

Results: There was an insignificant effect on cognitive domain z-scores across time when comparing real with sham stimulation and correcting for multiple comparisons across cognitive domains (p > 0.05 Bonferroni correction). However, the real stimulation group demonstrated a trend toward improved executive functioning scores at the 1-month follow-up compared with sham (p < 0.05 uncorrected). After real stimulation, the connectivity of the stimulation site showed decreased connectivity to the left caudate head. There was no change in connectivity within or between the stimulation network (a network of cortical regions connected to the stimulation site) and the striatal network. However, higher baseline connectivity between the stimulation network and the striatal network was associated with improved executive function scores at 1 month.

Conclusions: These results suggest that intermittent theta-burst stimulation over the dorsolateral prefrontal cortex in subjects with PD-MCI has minimal effect on cognition compared with sham, although there were trends toward improved executive function. This intervention may be more effective in subjects with higher baseline connectivity between the stimulation network and the striatal network. This trial supports further investigation focusing on executive function and incorporating connectivity-based targeting.

Clinical Trial Registration:www.ClinicalTrials.gov, identifier NCT03243214.

Preoperative Transcranial Direct Current Stimulation in Glioma Patients: A Proof of Concept Pilot Study

Background: Transcranial direct current stimulation (tDCS) has been used extensively in patient populations to facilitate motor network plasticity. However, it has not been studied in patients with brain tumors. We aimed to determine the feasibility of a preoperative motor training and tDCS intervention in patients with glioma. In an exploratory manner, we assessed changes in motor network connectivity following this intervention and related these changes to predicted electrical field strength from the stimulated motor cortex.

Methods: Patients with left-sided glioma (n=8) were recruited in an open label proof of concept pilot trial and participated in four consecutive days of motor training combined with tDCS. The motor training consisted of a 60-min period where the subject learned to play the piano with their right hand. Concurrently, they received 40 min of 2 mA anodal tDCS of the left motor cortex. Patients underwent task and resting state fMRI before and after this intervention. Changes in both the connectivity of primary motor cortex (M1) and general connectivity across the brain were assessed. Patient specific finite element models were created and the predicted electrical field (EF) resulting from stimulation was computed. The magnitude of the EF was extracted from left M1 and correlated to the observed changes in functional connectivity.

Results: There were no adverse events and all subjects successfully completed the study protocol. Left M1 increased both local and global connectivity. Voxel-wide measures, not constrained by a specific region, revealed increased global connectivity of the frontal pole and decreased global connectivity of the supplementary motor area. The magnitude of EF applied to the left M1 correlated with changes in global connectivity of the right M1.

Conclusion: In this proof of concept pilot study, we demonstrate for the first time that tDCS appears to be feasible in glioma patients. In our exploratory analysis, we show preoperative motor training combined with tDCS may alter sensorimotor network connectivity. Patient specific modeling of EF in the presence of tumor may contribute to understanding the dose-response relationship of this intervention. Overall, this suggests the possibility of modulating neural networks in glioma patients.

Impact of Peritumoral Edema During Tumor Treatment Field Therapy: A Computational Modelling Study

BACKGROUND

Tumor treatment fields (TTFie-lds) are an approved adjuvant therapy for glioblastoma (GBM). The magnitude of applied electrical field has been shown to be related to the anti-tumoral response. However, peritumoral edema may result in shunting of electrical current around the tumor, thereby reducing the intra-tumoral electric field. In this study, we systematically address this issue with computational simulations.

METHODS

Finite element models are created of a human head with varying amounts of peritumoral edema surrounding a virtual tumor. The electric field distribution was simulated using the standard TTFields electrode montage. Electric field magnitude was extracted from the tumor and related to edema thickness. Two patient specific models were created to confirm these results.

RESULTS

The inclusion of peritumoral edema decreased the average magnitude of the electric field within the tumor. In the model considering a frontal tumor and an anterior-posterior electrode configuration, ≥6 mm of peritumoral edema decreased the electric field by 52%. In the patient specific models, peritumoral edema decreased the electric field magnitude within the tumor by an average of 26%. The effect of peritumoral edema on the electric field distribution was spatially heterogenous, being most significant at the tissue interface between edema and tumor.

CONCLUSIONS

The inclusion of peritumoral edema during TTFields modelling may have a dramatic effect on the predicted electric field magnitude within the tumor. Given the importance of electric field magnitude for the anti-tumoral effects of TTFields, the presence of edema should be considered both in future modelling studies and when planning TTField therapy.

Case studies in neuroscience: deep brain stimulation changes upper limb cortical motor maps in dystonia

Deep brain stimulation of the globus pallidus pars interna (GPi-DBS) is an effective treatment for primary dystonia; however, its therapeutic mechanism is poorly understood. Because improvement is gradual, GPi-DBS treatment likely involves short- and long-term mechanisms. Abnormal plasticity resulting in somatotopic reorganization is involved in the development of dystonia and has been proposed as a possible mechanism for this gradual improvement, yet it has not been directly investigated. We hypothesized that GPi-DBS will lead to progressive changes in the cortical representations (motor maps) of upper limb muscles. Neuronavigated robotic transcranial magnetic stimulation was used to map the cortical representation of five upper limb muscles in six healthy controls and a 45-yr-old female cervical dystonia patient before (Pre) and at four time points (Post5 to Post314), 5 to 314 days after GPi-DBS. Motor map area and volume decreased in all muscles following GPi-DBS, while changes in overlap and center of gravity distance between muscles were variable. Despite these motor map changes, only dystonic tremor improved after a year of DBS; neck position worsened slightly. These preliminary findings suggest that GPi-DBS may reduce the cortical representation and excitability of upper limb muscles in dystonia and that these changes can occur without clinical improvement.NEW & NOTEWORTHY Neuronavigated robotic transcranial magnetic stimulation was used to investigate changes in upper limb muscle representation in a cervical dystonia patient before and at four time points up to 314 days after globus pallidus pars interna deep brain stimulation (GPi-DBS). GPi-DBS altered excitability and motor cortical representation of upper limb muscles; however, these changes were not associated with clinical improvement.

Treatment of Persistent Post-Traumatic Headache and Post-Concussion Symptoms Using Repetitive Transcranial Magnetic Stimulation: A Pilot, Double-Blind, Randomized Controlled Trial

Persistent post-traumatic headache (PTH) after mild traumatic brain injury is one of the most prominent and highly reported persistent post-concussion symptoms (PPCS). Non-pharmacological treatments, including non-invasive neurostimulation technologies, have been proposed for use. Our objective was to evaluate headache characteristics at 1 month after repetitive transcranial magnetic stimulation (rTMS) treatment in participants with PTH and PPCS. A double-blind, randomized, sham-controlled, pilot clinical trial was performed on 20 participants (18-65 years) with persistent PTH (International Classification of Headache Disorders, 3rd edition) and PPCS (International Classification of Diseases, Tenth Revision). Ten sessions of rTMS therapy (10 Hz, 600 pulses, 70% resting motor threshold amplitude) were delivered to the left dorsolateral pre-frontal cortex. The primary outcome was a change in headache frequency or severity at 1 month post-rTMS. Two-week-long daily headache diaries and clinical questionnaires assessing function, PPCS, cognition, quality of life, and mood were completed at baseline, post-treatment, and at 1, 3, and 6 months post-rTMS. A two-way (treatment × time) mixed analyisis of variance indicated a significant overall time effect for average headache severity (F_(3,54) = 3.214; p = 0.03) and a reduction in headache frequency at 1 month post-treatment (#/2 weeks, REAL -5.2 [standard deviation {SD} = 5.8]; SHAM, -3.3 [SD = 7.7]). Secondary outcomes revealed an overall time interaction for headache impact, depression, post-concussion symptoms, and quality of life. There was a significant reduction in depression rating in the REAL group between baseline and 1 month post-treatment, with no change in the SHAM group (Personal Health Questionnaire-9; REAL, -4.3 [SD = 3.7[ p = 0.020]; SHAM, -0.7 [SD = 4.7; p = 1.0]; Bonferroni corrected). In the REAL group, 60% returned to work whereas only 10% returned in the SHAM group (p = 0.027). This pilot study demonstrates an overall time effect on headache severity, functional impact, depression, PPCS, and quality of life after rTMS treatment in participants with persistent PTH; however, findings were below clinical significance thresholds. There was a 100% response rate, no dropouts, and minimal adverse effects, warranting a larger phase II study. Clinicaltrials.gov: NCT03691272.

Tool-Tissue Interaction Forces in Brain Arteriovenous Malformation Surgery

OBJECTIVE

Surgical resection of a brain arteriovenous malformation (AVM) poses a technical challenge because of the fragility and number of small feeding and draining vessels around the nidus. Acquiring knowledge of the optimal force applied to such tissue is important in surgical performance and education.

METHODS

A force-sensing bipolar forceps was developed through installation of strain gauge sensors, and force profiles were obtained from 2 AVM surgeries. The force data associated with vessel injury, unsuccessful trial, was compared with that from successful trials. Receiver operating curve analysis was used for determining optimal force threshold and evaluating the discriminative accuracy of measurement.

RESULTS

Force data from 519 trials was collected, of which 16 (3.1%) were unsuccessful. The mean and maximum forces in successful trials were 0.23 ± 0.06 N and 0.35 ± 0.11 N compared with unsuccessful trials of 0.33 ± 0.05 N and 0.53 ± 0.11 N, respectively (P < 0.001). There was a strong association of mean and maximum force peaks with unsuccessful trials as reflected by the area under the curve of 0.91 and 0.87, respectively. Threshold analysis showed that the rate of unsuccessful trials and error forces tended to increase with surgical time.

CONCLUSIONS

Excessive force at the tool tip may result in injury to fragile vessels during AVM surgery. A quantifiable metric through force sensing instruments can detect and predict the occurrence of such injury. Such an instrument may be ideal for resident training and evaluation.

Treatment of Glioma Using neuroArm Surgical System

The use of robotic technology in the surgical treatment of brain tumour promises increased precision and accuracy in the performance of surgery. Robotic manipulators may allow superior access to narrow surgical corridors compared to freehand or conventional neurosurgery. This paper reports values and ranges of tool-tissue interaction forces during the performance of glioma surgery using an MR compatible, image-guided neurosurgical robot called neuroArm. The system, capable of microsurgery and stereotaxy, was used in the surgical resection of glioma in seven cases. neuroArm is equipped with force sensors at the end-effector allowing quantification of tool-tissue interaction forces and transmits force of dissection to the surgeon sited at a remote workstation that includes a haptic interface. Interaction forces between the tool tips and the brain tissue were measured for each procedure, and the peak forces were quantified. Results showed maximum and minimum peak force values of 2.89 N (anaplastic astrocytoma, WHO grade III) and 0.50 N (anaplastic oligodendroglioma, WHO grade III), respectively, with the mean of peak forces varying from case to case, depending on type of the glioma. Mean values of the peak forces varied in range of 1.27 N (anaplastic astrocytoma, WHO grade III) to 1.89 N (glioblastoma with oligodendroglial component, WHO grade IV). In some cases, ANOVA test failed to reject the null hypothesis of equality in means of the peak forces measured. However, we could not find a relationship between forces exerted to the pathological tissue and its size, type, or location.

Robotics in the neurosurgical treatment of glioma

Background:

The treatment of glioma remains a significant challenge with high recurrence rates, morbidity, and mortality. Merging image guided robotic technology with microsurgery adds a new dimension as they relate to surgical ergonomics, patient safety, precision, and accuracy.

Methods:

An image-guided robot, called neuroArm, has been integrated into the neurosurgical operating room, and used to augment the surgical treatment of glioma in 18 patients. A case study illustrates the specialized technical features of a teleoperated robotic system that could well enhance the performance of surgery. Furthermore, unique positional and force information of the bipolar forceps during surgery were recorded and analyzed.

Results:

The workspace of the bipolar forceps in this robot-assisted glioma resection was found to be 25 × 50 × 50 mm. Maximum values of the force components were 1.37, 1.84, and 2.01 N along x, y, and z axes, respectively. The maximum total force was 2.45 N. The results indicate that the majority of the applied forces were less than 0.6 N.

Conclusion:

Robotic surgical systems can potentially increase safety and performance of surgical operation via novel features such as virtual fixtures, augmented force feedback, and haptic high-force warning system. The case study using neuroArm robot to resect a glioma, for the first time, showed the positional information of surgeon's hand movement and tool-tissue interaction forces.

Forces exerted during microneurosurgery: a cadaver study

Background

A prerequisite for the successful design and use of robots in neurosurgery is knowledge of the forces exerted by surgeons during neurosurgical procedures. The aim of the present cadaver study was to measure the surgical instrument forces exerted during microneurosurgery.

Methods

An experimental apparatus was set up consisting of a platform for human cadaver brains, a Leica microscope to provide illumination and magnification, and a Quanser 6 Degrees-Of-Freedom Telepresence System for tissue manipulation and force measurements.

Results

The measured forces varied significantly depending on the region of the brain (P = 0.016) and the maneuver performed (P < 0.0001). Moreover, blunt arachnoid dissection was associated with greater force exertion than sharp dissection (0.22 N vs. 0.03 N; P = 0.001).

Conclusions

The forces necessary to manipulate brain tissue were surprisingly low and varied depending on the anatomical structure being manipulated, and the maneuver performed. Knowledge of such forces could well increase the safety of microsurgery. © 2014 The Authors. The International Journal of Medical Robotics and Computer Assisted Surgery published by John Wiley & Sons, Ltd.

Merging machines with microsurgery: clinical experience with neuroArm

Object

It has been over a decade since the introduction of the da Vinci Surgical System into surgery. Since then, technology has been advancing at an exponential rate, and newer surgical robots are becoming increasingly sophisticated, which could greatly impact the performance of surgery. NeuroArm is one such robotic system.

Methods

Clinical integration of neuroArm, an MR-compatible image-guided robot, into surgical procedure has been developed over a prospective series of 35 cases with varying pathology.

Results

Only 1 adverse event was encountered in the first 35 neuroArm cases, with no patient injury. The adverse event was uncontrolled motion of the left neuroArm manipulator, which was corrected through a rigorous safety review procedure. Surgeons used a graded approach to introducing neuroArm into surgery, with routine dissection of the tumor-brain interface occurring over the last 15 cases. The use of neuroArm for routine dissection shows that robotic technology can be successfully integrated into microsurgery. Karnofsky performance status scores were significantly improved postoperatively and at 12-week follow-up.

Conclusions

Surgical robots have the potential to improve surgical precision and accuracy through motion scaling and tremor filters, although human surgeons currently possess superior speed and dexterity. Additionally, neuroArm's workstation has positive implications for technology management and surgical education. NeuroArm is a step toward a future in which a variety of machines are merged with medicine.

First permanent human implant of the Stimulus Router System, a novel neuroprosthesis: preliminary testing of a polarity reversing stimulation technique

Neuroprostheses (NPs) are electrical stimulators that help to restore sensory or motor functions lost as a result of neural damage. The Stimulus Router System (SRS) is a new type of NP developed in our laboratory. The system uses fully implanted, passive leads to "capture" and "route" some of the current flowing between pairs of surface electrodes to the vicinity of the target nerves, hence eliminating the need for an implanted stimulator. In June 2008, 3 SRS leads were implanted in a tetraplegic man for restoration of grasp and release. To reduce the size of the external wristlet and thereby optimize usability, we recently implemented a polarity reversing stimulation technique that allowed us to eliminate a reference electrode. Selective activation of three target muscles was achieved by switching the polarities of the stimulus current delivered between pairs of surface electrodes located over the pick-up terminals of the implanted leads and reducing the amplitude of the secondary phases of the stimulus pulses.

Nerve lesioning with direct current

Spastic hypertonus (muscle over-activity due to exaggerated stretch reflexes) often develops in people with stroke, cerebral palsy, multiple sclerosis and spinal cord injury. Lesioning of nerves, e.g. with phenol or botulinum toxin is widely performed to reduce spastic hypertonus. We have explored the use of direct electrical current (DC) to lesion peripheral nerves. In a series of animal experiments, DC reduced muscle force by controlled amounts and the reduction could last several months. We conclude that in some cases controlled DC lesioning may provide an effective alternative to the less controllable molecular treatments available today.

Nerve lesioning with direct current

Spastic hypertonus (muscle over-activity due to exaggerated stretch reflexes) often develops in stroke and spinal cord injury (SCI) survivors and individuals who suffer from multiple sclerosis. In previous published experiments we have shown that Direct Current (DC), when used to lesion nerves, can attenuate muscle force in a gradual manner, and this attenuation can last for several months. In this paper we present initial experimental results that profile the current required to cause controlled nerve ablation.

A new means of transcutaneous coupling for neural prostheses

Neural prostheses are electronic stimulators that activate nerves to restore sensory or motor functions. Implanted neural prostheses receive command signals and in some cases energy to recharge their batteries through the skin by telemetry. Here, we describe a new approach that eliminates the implanted stimulator. Stimulus pulse trains are passed between two surface electrodes placed on the skin. An insulated lead with conductive terminals at each end is implanted inside the body. One terminal is located under the cathodal surface electrode and the other is attached to a nerve targeted for stimulation. A fraction (10%-15%) of the current flowing between the surface electrodes is routed through the implanted lead. The nerve is stimulated when the amount of routed current is sufficient. The aims of this study were to establish some basic electrical properties of the system and test long-term stability in chronic implants. Stimulation of the nerve innervating the ankle flexors produced graded force over the full physiological range at amplitudes below threshold for evoking muscle contractions under the surface electrodes. Implants remained stable for over 8 mo. The findings provide the basis for a new family of neural prostheses.

Formation of unusual glutamate conjugates of 1-[3-(aminomethyl)phenyl]-N-[3-fluoro-2'-(methylsulfonyl)-[1,1'-biphenyl]-4-yl]-3-(trifluoromethyl)-1H-pyrazole-5-carboxamide (DPC 423) and its analogs: the role of gamma-glutamyltranspeptidase in the biotransformation of benzylamines

The role of gamma-glutamyltranspeptidase (GGT) in transferring glutamate from endogenous glutathione (GSH) to the benzylamine moiety of a compound, such as 1-[3-(aminomethyl)phenyl]-N-[3-fluoro-2'-(methylsulfonyl)-[1,1'-biphenyl]-4-yl]-3-(trifluoromethyl)-1H-pyrazole-5-carboxamide (DPC 423), is described. Studies were performed with structurally related analogs of DPC 423 to demonstrate that this type of reaction was common to compounds possessing a benzylamine group. Synthesizing appropriate standards and confirming by liquid chromatography (LC)/mass spectroscopy and LC/NMR made unambiguous assignments of the structures of glutamate conjugates of DPC 423. The use of stable isotope-labeled GSH for metabolism studies has not been described before. In the present study, we report the novel use of deuterated GSH in conjunction with mass spectral analysis to demonstrate the glutamate transfer to the benzylamines in the presence of GGT. To further demonstrate that the alpha protons on the benzylamines and glutamate (as part of glutathione) were unaffected during the transpeptidation, these protons were replaced with deuterium. Acivicin (AT-125), a potent and selective inhibitor of GGT, was used to abolish the formation of the glutamate conjugates of DPC 423 in vitro and in vivo. This provided further evidence of the role of GGT in forming the glutamate conjugates of benzylamines. This study demonstrated conclusively that GGT was responsible for mediating the transfer of glutamic acid from GSH to the benzylamine moiety of a series of structurally related compounds.

The species-dependent metabolism of efavirenz produces a nephrotoxic glutathione conjugate in rats

Efavirenz, a potent nonnucleoside reverse transcriptase inhibitor widely prescribed for the treatment of HIV infection, produces renal tubular epithelial cell necrosis in rats but not in cynomolgus monkeys or humans. This species selectivity in nephrotoxicity could result from differences in the production or processing of reactive metabolites, or both. A detailed comparison of the metabolites produced by rats, monkeys, and humans revealed that rats produce a unique glutathione adduct. The mechanism of formation and role of this glutathione adduct in the renal toxicity were investigated using both chemical and biochemical probes. Efavirenz was labeled at the methine position on the cyclopropyl ring with the stable isotope deuterium, effectively reducing the formation of the cyclopropanol metabolite, an obligate precursor to the glutathione adduct. This substitution markedly reduced both the incidence and severity of nephrotoxicity as measured histologically. Further processing of this glutathione adduct was also important in producing the lesion and was demonstrated by inhibiting gamma-glutamyltranspeptidase with acivicin pretreatment (10 mg/kg, IV) prior to dosing with efavirenz. Again, both the incidence and severity of the nephrotoxicity were reduced, such that four of nine rats given acivicin were without detectable lesions. These studies provide compelling evidence that a species-specific formation of glutathione conjugate(s) from efavirenz is involved in producing nephrotoxicity in rats. Mechanisms are proposed for the formation of reactive metabolites that could be responsible for the renal toxicity observed in rats.

Disposition of glutathione conjugates in rats by a novel glutamic acid pathway: characterization of unique peptide conjugates by liquid chromatography/mass spectrometry and liquid chromatography/NMR

With the advent of liquid chromatography/mass spectrometry and liquid chromatography/NMR, it has become easier to characterize metabolites that were once difficult to isolate and identify. These techniques have enabled us to uncover the existence of an alternate pathway for the disposition of glutathione adducts of several structurally diverse compounds. Studies were carried out using acetaminophen as a model compound to investigate the role of the glutamic acid pathway in disposition of the glutathione adducts. Although the mercapturic acid pathway was the major route of degradation of the glutathione adducts, it was found that the conjugation of the glutathione, cysteinylglycine, and cysteine adducts of acetaminophen with the gamma-carboxylic acid of the glutamic acid was both interesting and novel. The coupling of the glutathione adduct and the products from the mercapturic acid pathway with the glutamic acid led to unusual peptide conjugates. The natures of these adducts were confirmed unequivocally by comparisons with synthetic standards. This pathway (addition of glutamic acids) led to larger peptides, in contrast to the mercapturic acid pathway, in which the glutathione adducts are broken down to smaller molecules. The enzyme responsible for the addition of glutamic acid to the different elements of the mercapturic acid pathway is currently unknown. It is postulated that the gamma-carboxylic acid is activated (perhaps by ATP) before enzymatic addition to the alpha-amino group of cysteine or glutamate takes place. The discovery of these peptide conjugates of acetaminophen represents a novel disposition of glutathione adducts of compounds. The formation of such conjugates may represent yet another pathway by which drugs could produce covalent binding via their reactive intermediates.

Characterization of novel glutathione adducts of a non-nucleoside reverse transcriptase inhibitor, (S)-6-chloro-4-(cyclopropylethynyl)-4-(trifluoromethyl)-3, 4-dihydro-2(1H)-quinazolinone (DPC 961), in rats. Possible formation of an oxirene metabolic intermediate from a disubstituted alkyne

The postulated formation of oxirene-derived metabolites from rats treated with a disubstituted alkyne, (S)-6-chloro-4-(cyclopropylethynyl)-4-(trifluoromethyl)-3, 4-dihydro-2(1H)-quinazolinone (DPC 961), is described. The reactivity of this postulated oxirene intermediate led to the formation of novel glutathione adducts whose structures were confirmed by LC/MS and by two-dimensional NMR experiments. These metabolites were either excreted in rat bile or degraded to mercapturic acid conjugates and eliminated in urine. To demonstrate the oxidation of the triple bond, an analogue of DPC 961 was synthesized, whereby the two carbons of the alkyne moiety were replaced with (13)C stable isotope labels. Rats were orally administered [(13)C]DPC 961 and glutathione adducts isolated from bile. The presence of an oxygen atom on one of the (13)C labels of the alkyne was demonstrated unequivocally by NMR experiments. Administration of (14)C-labeled DPC 961 showed that biliary elimination was the major route of excretion with the 8-OH glucuronide conjugate (M1) accounting for greater than 90% of the eliminated radioactivity. On the basis of radiochemical profiling, the glutathione-derived metabolites were minor in comparison to the glucuronide conjugate. Studies with cDNA-expressed rat enzymes, polyclonal antibodies, and chemical inhibitors pointed to the involvement of P450 3A1 and P450 1A2 in the formation of the postulated oxirene intermediate. The proposed mechanism shown in Scheme 1 begins with P450-catalyzed formation of an oxirene, rearrangement to a reactive cyclobutenyl ketone, and a 1,4-Michael addition with endogenous glutathione to produce two isomeric adducts, GS-1 and GS-2. The glutathione adducts were subsequently catabolized via the mercapturic acid pathway to cysteinylglycine, cysteine, and N-acetylcysteine adducts. The transient existence of the alpha,beta-unsaturated cyclobutenyl ketone was demonstrated by incubating the glutathione adduct in the presence of N-acetylcysteine and monitoring the formation of N-acetylcysteine adducts by LC/MS. Epimerization of GS-1 to GS-2 was also observed when N-acetylcysteine was omitted from the incubation.

Identification and characterization of efavirenz metabolites by liquid chromatography/mass spectrometry and high field NMR: species differences in the metabolism of efavirenz

Efavirenz (Sustiva, Fig. 1) is a potent and specific inhibitor of HIV-1 reverse transcriptase approved for the treatment of HIV infection. To examine the potential differences in the metabolism among species, liquid chromatography/mass spectrometry profiles of efavirenz metabolites in urine of rats, guinea pigs, hamsters, cynomolgus monkeys, and humans were obtained and compared. The metabolites of efavirenz were isolated, and structures were determined unequivocally by mass spectral and NMR analyses. Efavirenz was metabolized extensively by all the species as evidenced by the excretion of none or trace quantities of parent compound in urine. Significant species differences in the metabolism of efavirenz were observed. The major metabolite excreted in the urine of all species was the O-glucuronide conjugate (M1) of the 8-hydroxylated metabolite. Efavirenz was also metabolized by direct conjugation with glucuronic acid, forming the N-glucuronide (M2) in all five species. The sulfate conjugate of 8-OH efavirenz (M3) was found in the urine of rats and cynomolgus monkeys but not in humans. In addition to the aromatic ring-hydroxylated products, metabolites with a hydroxylated cyclopropane ring (at C14) were also isolated. GSH-related products of efavirenz were identified in rats and guinea pigs. The cysteinylglycine adduct (M10), formed from the GSH adduct (M9), was found in significant quantities in only rat and guinea pig urine and was not detected in other species. In vitro metabolism studies were conducted to show that the GSH adduct was produced from the cyclopropanol intermediate (M11) in the presence of only rat liver and kidney subcellular fractions and was not formed by similar preparations from humans or cynomolgus monkeys. These studies indicated the existence of a specific glutathione-S-transferase in rats capable of metabolizing the cyclopropanol metabolite (M11) to the GSH adduct, M9. The biotransformation pathways of efavirenz in different species were proposed based on some of the in vitro results.

Liquid chromatography/mass spectrometry and high-field nuclear magnetic resonance characterization of novel mixed diconjugates of the non-nucleoside human immunodeficiency virus-1 reverse transcriptase inhibitor, efavirenz

Efavirenz (Sustiva) is a potent and specific inhibitor of the HIV-1 reverse transcriptase and is approved for the treatment of HIV infection. The metabolism of efavirenz in different species has been described previously. Efavirenz is primarily metabolized in rats to the glucuronide conjugate of 8-OH efavirenz. Electrospray ionization-liquid chromatography/mass spectrometry analyses of bile samples from rats dosed with either efavirenz or with 8-OH efavirenz revealed three polar metabolites, M9, M12, and M13, with pseudomolecular ions [M-H](-) at m/z 733, 602, and 749, respectively. The characteristic mass spectral fragmentation patterns obtained for metabolites M9 and M13 suggested that these were glutathione-sulfate diconjugates, and the presence of a glutathione moiety in metabolite M9 was confirmed by liquid chromatograpy/nuclear magnetic resonance (NMR) analysis of bile extracts. Metabolite M12 was characterized by liquid chromatography/mass spectrometry as a glucuronide-sulfate diconjugate. Unambiguous structures of M9, M12, and M13 were obtained from one-dimensional proton and carbon NMR as well as proton-proton (correlated spectroscopy, two-dimensional shift correlation), proton-carbon heteronuclear multiple quantum correlation, and long-range proton-carbon (heteronuclear multiple bond correlation) correlated two-dimensional NMR analyses of metabolites isolated from rat bile. The mass spectral and NMR analyses of M10, which was isolated from rat urine, suggested a cysteinylglycine-sulfate diconjugate. The isolation of these polar metabolites for further characterization by NMR was aided by mass spectral analyses of HPLC fractions and solid phase extraction extracts during the isolation steps. The complete characterization of these novel diconjugates demonstrates that further phase II metabolism of polar conjugates such as sulfates could take place in vivo.

Biliary excretion in primary rat hepatocytes cultured in a collagen-sandwich configuration

The objective of the present investigation was to examine the functional reestablishment of polarity in freshly isolated hepatocytes cultured between 2 layers of gelled collagen (sandwich configuration). Immunoblot analysis demonstrated that the canalicular multispecific organic anion transport protein (multidrug resistance-associated protein, Mrp2) was partially maintained in day 5 hepatocytes cultured in a sandwich configuration. Fluorescein-labeled taurocholate and carboxydichlorofluorescein were excreted into and concentrated in the bile canalicular lumen of day 5 sandwich-cultured hepatocytes, resulting in formation of fluorescent networks in standard buffer (intact bile canaliculi). Confocal microscopy studies demonstrated that 1) carboxydichlorofluorescein that had concentrated in the canalicular lumen was released into the incubation buffer in the presence of Ca(2+)-free buffer (disrupted bile canaliculi), and 2) rhodamine-dextran, an extracellular space marker, was only able to diffuse into the canalicular lumen in the presence of Ca(2+)-free buffer. The cumulative uptake of [(3)H]taurocholate in day 5 sandwich-cultured hepatocytes was significantly higher in standard buffer compared with Ca(2+)-free buffer, due to accumulation of taurocholate in canalicular spaces. When [(3)H]taurocholate was preloaded in the day 5 sandwich-cultured hepatocytes, taurocholate efflux was greater in Ca(2+)-free compared with standard buffer. The biliary excretion index of taurocholate, equivalent to the percentage of retained taurocholate in the canalicular networks, increased from approximately 8% at day 0 to approximately 60% at day 5 in sandwich-cultured hepatocytes. In summary, hepatocytes cultured in a collagen-sandwich configuration for up to 5 days establish intact canalicular networks, maintain Mrp2, reestablish polarized excretion of organic anions and bile acids, and represent a useful in vitro model system to investigate the hepatobiliary disposition of substrates.

Discovery of bioavailable inhibitors of secretory phospholipase A2

Substrate-mimetic inhibitors of sPLA2 with submicromolar in vitro potency were discovered by use of a novel dual substrate screening strategy. In vivo evaluation of selected inhibitors in the rat carrageenan paw edema model of inflammation, however, indicated that in vitro potency was not a good predictor of in vivo activity. Studies of the metabolic stability of early examples of these inhibitors suggested that the metabolic lability of these compounds was a major contributing factor to the observed weak in vivo activity. In an attempt to achieve improved in vivo activity, we prepared and tested compounds designed to overcome the observed metabolic instability. The design of the new compounds involved two types of changes in the inhibitor molecules. First, the C-2 ester moiety was replaced with an amide function so that direct cleavage by stomach acid and blood esterases at this site was minimized. Second, omega-oxidation of the decanamide moiety was eliminated by substitution of hydrogen with fluorine in this position. Compounds containing fluorine in the terminal positions of the alkyl chain retained sPLA2 inhibitory activity and also possessed improved in vitro metabolic stability and pharmacokinetic parameters relative to nonfluorinated inhibitors in this series. As exemplified by GW 4776, improvements in metabolic stability alone, however, were not sufficient to ensure oral activity. Thus, GW 4776 did not show oral activity in the carrageenan edema model and had only modest activity after i.v. dosing in the same model. In fact, the results for GW 9624 and GW 8219 suggested that factors in addition to potency of sPLA2 inhibition and metabolism affect the observed in vivo activity. Despite the fact that these two compounds varied only by a single oxygen-to-sulfur substitution, one was active whereas the other was not. One possible explanation for the observed variability is a compound-dependent difference in the rate of equilibration into tissue. This possibility is relevant as both the carrageenan paw edema model and the phorbol ester edema model involve a localized inflammation. No measurements were made to assess differences in the distribution of the different inhibitors between the blood and the localized site of inflammation. In summary, a series of bioavailable inhibitors of sPLA2 was prepared using an iterative approach that combined medicinal chemistry, in vitro and in vivo evaluation of biological activity, and metabolic and pharmacokinetic studies. Although some compounds in the series showed in vivo activity, the anti-inflammatory effect observed in animal models was modest and a decision was made to abandon sPLA2 as a molecular target for the development of anti-inflammatory agents.

Partial maintenance of taurocholate uptake by adult rat hepatocytes cultured in a collagen sandwich configuration

PURPOSE

This study was designed to characterize taurocholate uptake properties in primary cultures of rat hepatocytes maintained under different matrix conditions.

METHODS

Hepatocytes isolated from male Wistar rats (230-280 g) were cultured on a simple collagen film, on a substratum of gelled collagen or between two layers of gelled collagen (sandwich configuration). Hepatocyte morphology, taurocholate uptake properties, and expression of the sinusoidal transport protein. Na+/taurocholate-cotransporting polypeptide (Ntcp) were examined in these cultures at day 0 and day 5.

RESULTS

By day 5, monolayer integrity had deteriorated in simple collagen cultures. In contrast, cell morphology was preserved in hepatocytes maintained in a sandwich configuration. At day 5, taurocholate accumulation at 5 min in hepatocytes cultured on a simple collagen film, on a substratum of gelled collagen, and in a sandwich configuration was approximately 13%, 20% and 35% of day-0 levels, respectively, and occurred predominately by a Na+-dependent mechanism. The initial taurocholate uptake rate vs. concentration (1-200 microM) profile was best described by a combined Michaelis-Menten and first-order function. In all cases, the estimated apparent Km values were comparable for day-0 and day-5 hepatocytes (3241 microM). In contrast, the Vmax values of hepatocytes cultured on a simple collagen film, on gelled collagen and in a sandwich configuration were approximately 5, 6 and 14% of the values at day 0, respectively; values for the first-order rate constant were 5-, 3- and 2-fold lower, respectively. Immunoblot analysis indicated that at day 5 Ntcp expression in hepatocytes cultured in a sandwich configuration was greater than in hepatocytes cultured on a simple collagen film.

CONCLUSIONS

A collagen sandwich configuration reestablishes normal morphology and partially restores bile acid uptake properties in primary cultures of rat hepatocytes.

Modulation of the tight junctions of the Caco-2 cell monolayers by H2-antagonists

PURPOSE

The tight junctions in the intestinal epithelium represent highly specialized intercellular junctions. Ranitidine, an H2-antagonist, causes a tightening of the tight junctions. Hence, we have investigated the effect of ranitidine and other H2-antagonists on the function of the intestinal tight junctions.

METHODS

Effect of the H2-antagonists on the tight junctions has been investigated using the transepithelial electrical resistance (TEER) and the transport of mannitol across the Caco-2 cell monolayers.

RESULTS

Four different H2-antagonists caused an increase in the TEER across the Caco-2 cell monolayers, accompanied by a decrease in the permeability for mannitol. The effect was concentration-dependent and saturable. Ranitidine and famotidine, caused a decrease in their own transport rate across the Caco-2 cells. Ranitidine competitively inhibited the increase in TEER caused by famotidine, whereas compounds which represent molecular fragments of ranitidine had no effect. The relative potency of the four H2-antagonists in causing an increase in the TEER correlated inversely with the oral bioavailability of these compounds in humans.

CONCLUSIONS

We hypothesize that the H2-antagonists exert their effect on the tight junctions of Caco-2 cells by modulation of interactions among proteins associated with the tight junctional complex.

CYP3A-like cytochrome P450-mediated metabolism and polarized efflux of cyclosporin A in Caco-2 cells

Transport of cyclosporin A (CsA) across Caco-2 cells is modulated by its directional efflux, mediated by a p-glycoprotein-like pump (Augustijns et al., Biochem. Biophys. Res. Comm. 197:360-365, 1994). In addition to this unidirectional flux, oxidative metabolism of CsA by cytochrome P450 is likely to influence the absorption of this cyclic peptide across intestinal mucosa. Thus, metabolism of CsA in the in vitro Caco-2 cell culture system was investigated. Formation of several metabolites was observed during the course of CsA transport across Caco-2 cell monolayers. Results from LC/MS/MS experiments revealed that the major metabolite was 1eta-hydroxy CsA (M-17), one of the three major metabolites produced by CYP3A4 present in both the liver and small intestine in humans. Preincubation of Caco-2 cell monolayers with troleandomycin, a specific inhibitor for the microsomal CYP3A protein, reduced the formation of the metabolite M-17, suggesting that an enzyme that functionally resembles CYP3A is responsible for the formation of this metabolite. However, formation of only the M-17 metabolite suggests that the isozyme present in the Caco-2 cells is distinct from CYP3A4, which also catalyzes the formation of significant quantities of the metabolites 9gamma-hydroxy cyclosporin A (M-1) and 4N-desmethyl cyclosporin A (M-21) from CsA. Interestingly, the amount of M-17 accumulating on the apical (AP) side was much greater than that on the basolateral (BL) side during the AP --> BL transport of CsA across Caco-2 cell monolayers. This is consistent with p-glycoprotein pump-mediated efflux of the metabolite to the apical side. Furthermore, formation of the M-17 metabolite on the AP side of cell monolayers during the AP --> BL transport of CsA was much greater than that during the BL --> AP transport. This result suggests that the p-glycoprotein efflux pump causes an increase in the metabolism of CsA during the course of its AP --> BL transport by effectively slowing down the transport of CsA molecules across Caco-2 cells. Thus, Caco-2 cells serve as an excellent model to dissect the relative roles played by p-glycoprotein-mediated efflux and CYP3A-catalyzed oxidation in modulating the overall absorption of CsA and other such compounds.

High-performance liquid chromatographic purification, optimization of the assay, and properties of ribonucleoside diphosphate reductase from rabbit bone marrow

Partial purification of ribonucleoside diphosphate reductase from rabbit bone marrow was achieved by size exclusion HPLC of the crude homogenate. This step, requiring < 15 min, led to 9- to 13-fold purification of the reductase and removal of 64% of the contaminating kinase/phosphatase activities, which in the crude extract degrade > 95% of substrate CDP when reductase is assayed. A systematic study was conducted to evaluate the influence of contaminating kinase/phosphatase activities on CDP concentration during the reductase-catalyzed reaction with either ATP or its kinase-inhibiting analog, 5'-adenylylimidodiphosphate (AMP-PNP), as the allosteric effector. Our studies demonstrated that in the presence of ATP, CDP levels fell instantly to < 24% but thereafter remained fairly constant due to recycling via CTP. In contrast, in the presence of AMP-PNP, CDP levels decreased continuously. The Km values of the reductase for CDP determined in the presence of ATP were significantly higher than those in the presence of AMP-PNP. Furthermore, we also found that the concentration of the ultimate electron donor dithiothreitol (DTT) required for optimum activity of the reductase varies significantly with the level of purity of the reductase preparation. Interestingly, DTT is an inhibitor of the reductase above the optimum concentration. This purification method and the optimized assay together with the understanding of the fate of CDP in partially purified preparations should find application in studies with reductases from other eukaryotic sources.

Evidence for predominantly paracellular transport of thyrotropin-releasing hormone across CACO-2 cell monolayers

Uptake, transport, and metabolism of tripeptide thyrotropin-releasing hormone were examined in the human intestinal epithelial cell line, Caco-2. A linear relationship between rate and concentration was observed for both the uptake and the transport of thyrotropin-releasing hormone across Caco-2 cell monolayers. Transport of thyrotropin-releasing hormone was not affected by the presence of dipeptide glycylsarcosine, amino acid glycine, tripeptide thyrotropin-releasing hormone free acid as well as active transport inhibitors 2,4-dinitrophenol, sodium azide, ouabain, and amiloride. There was no formation of metabolites during the course of thyrotropin-releasing hormone transport across Caco-2 cells. Incubation of Caco-2 cell homogenate with thyrotropin-releasing hormone, however, showed a time-dependent hydrolysis of thyrotropin-releasing hormone and the formation of thyrotropin-releasing hormone free acid. Increased rate of transport in the presence of EDTA indicates a paracellular passive diffusion as the major route for the transport of TRH. The hydrolytic enzyme present in Caco-2 cells appeared to have little or no access to TRH during the transcellular transport across Caco-2 cell monolayers.

Evidence for a polarized efflux system in CACO-2 cells capable of modulating cyclosporin A transport

The characteristics of cyclosporin A (CsA) transport across Caco-2 monolayers were investigated. CsA (0.25-5.0 microM) was transported in a time and concentration dependent manner. The total amount of apical (AP) to basolateral (BL) transport was non-linearly related to CsA concentration from 0.25 to 1 microM and was linear from about 1 to 5 microM. Average permeability coefficient (Papp) values obtained in the AP to BL direction showed CsA concentration (0.5 and 5.0 microM) dependence, whereas those of the reverse (BL to AP) process did not. Papp values for the AP to BL direction were also markedly lower. When the P-glycoprotein pump inhibitors, chlorpromazine and progesterone, were included in the transport medium we observed a significant increase in CsA (0.5 and 5.0 microM) transport from the AP to BL direction; transport was decreased in the reverse direction. This study suggests that CsA is transported across Caco-2 cells by passive diffusion, but that a polarized efflux system (presumably a P-glycoprotein pump) located at the apical membrane can attenuate the net AP to BL transport.

Mechanism of intestinal absorption of ranitidine and ondansetron: transport across Caco-2 cell monolayers

We have investigated the transport of ranitidine and ondansetron across the Caco-2 cell monolayers. The apparent permeability co-efficients (Papp) were unchanged throughout the concentration range studied, indicating a passive diffusion pathway across intestinal mucosa. No metabolism was observed for ranitidine and ondansetron during the incubation with Caco-2 cell monolayers. Papp values for ranitidine and ondansetron (bioavailability of 50 and approximately 100% in humans, respectively) were 1.03 +/- 0.17 x 10(-7) and 1.83 +/- 0.055 x 10(-5) cm/sec, respectively. The Papp value for ranitidine was increased by 15- to 20-fold in a calcium-free medium or in the transport medium containing EDTA, whereas no significant change occurred with ondansetron, indicating that paracellular passive diffusion is not rate determining for ondansetron. Uptake of ondansetron by Caco-2 cell monolayers was 20- and 5-fold higher than that of ranitidine when the uptake study was carried out under sink conditions and at steady state. These results suggest that ranitidine and ondansetron are transported across Caco-2 cell monolayers predominantly via paracellular and transcellular pathways, respectively.

Gas chromatographic-mass spectrometric analysis of diols and tetrols from reactions of polycyclic aromatic hydrocarbon epoxides with hemoglobin

We have evaluated both electron ionization (EI) and negative-ion chemical ionization (NICI) methods for the analysis of trimethylsilyl derivatives of a series of polycyclic aromatic hydrocarbon (PAH) alcohols including styrene diol, benzo[e]pyrene diol and tetrols, cyclopenta[c,d]pyrene diols, benzo[a]pyrene-4,5-diols, chrysene tetrols, benz[a]anthracene tetrols I and II, and syn- and anti-benzo[a]pyrene tetrols. NICI is the more sensitive method for all compounds except styrene diol. Detection limits are compound-dependent and range from 1 fmol for cyclopenta[c,d]pyrene diol to 1 pmol for benzo[e]pyrene diol. The EI detection limit for styrene diol is 60 fmol. PAH alcohols related to the compounds listed above were observed following hydrolysis of hemoglobin which had been reacted with PAH epoxides in vitro. Benzo[a]pyrene tetrols and a chrysene tetrol were observed following hydrolysis of hemoglobin isolated from human smokers' blood. Hydrolysis of styrene oxide treated hemoglobin in 18O-labeled water revealed at least two mechanisms of ester hydrolysis, including the BAL 1 pathway.

Molecular dosimetry of polycyclic aromatic hydrocarbon epoxides and diol epoxides via hemoglobin adducts

Ten reactive metabolites of five polycyclic aromatic hydrocarbons and styrene were investigated to determine the generality of ester adduct formation with human hemoglobin in the form of RBC and hydrolysis to the corresponding tetrahydrotetrols or dihydrodiols. No exceptions were noted among the compounds tested, which included the anti-diol epoxides of benzo[a]pyrene (BaP), chrysene, and benz[a]anthracene; the syn-diol epoxide of BaP; a mixture of syn- and anti-diol epoxides of benzo[e]pyrene; and epoxides of styrene, benzo[e]pyrene, BaP, and cyclopenta[c,d]pyrene. A test of the propensity of the simplest benzylic epoxide, styrene oxide, to form esters that hydrolyze via a BAL1 mechanism was performed. Hydrolysis of styrene oxide-adducted hemoglobin in H2(18)O at neutral pH yielded 18O incorporation results that suggest this mechanism of hydrolysis is operant to a minor degree in styrene oxide-hemoglobin ester adducts. A method was developed for the isolation and quantification of the polycyclic aromatic alcohols, which consists of enzymatic proteolysis, immunoaffinity chromatography, and gas chromatography-mass spectrometry or fluorimetry. The method allows for routine analysis of hemoglobin from individual samples as small as 1 ml of whole blood. Analysis of blood from different human populations revealed that hemoglobin adducts of the anti-diol epoxide of BaP dominated the spectrum of adducts formed by the selected metabolites.

Benzo[a]pyrene diol epoxide adduct formation in mouse and human hemoglobin: physicochemical basis for dosimetry

The interactions of human hemoglobin (hHb) with the anti-diol epoxide of benzo[a]pyrene (aBaPDE) and with the corresponding tetrols formed by hydrolysis of the epoxide were investigated with the aim of characterizing the covalent adducts formed by reaction of the epoxide with the protein. The major product (80% of the total) was determined to be an ester resulting from oxirane ring opening by one or several (unidentified) carboxylate group(s). Minor products were characterized as adducts formed by reaction with amino or heterocyclic nitrogen by comparison of their UV spectra with those of model compounds. There was no evidence for reaction with cysteine. Formation of ester adducts by aBaPDE with mouse hemoglobin (mHb) following administration of BaP to mice was also investigated. It was found that esters constituted the majority of the adducts formed by aBaPDE and a substantial fraction of the total adducts formed. The esters formed by mHb were significantly less stable than those formed by hHb, both in vivo and in vitro. The instability of mHb ester adducts is believed by the responsible for differences among previous descriptions of the in vivo binding of BaP to mHb.

A rapid method for isolation of human hemoglobin benzo[a]pyrene diol epoxide derived adducts using high performance liquid chromatography

A method is described to isolate rapidly human hemoglobin-benzo[a]pyrene diol epoxide adducts. A combination of 300 A pore size C4 reversed phase HPLC to effect separation of adducted protein from native protein, and mu-bore C18 reversed phase HPLC to isolate and partially characterize proteolytic peptide adducts (by UV), was used.

Quantitation of methylated hemoglobin via hydrolysis of methyl esters to yield methanol

Methyl esters formed by methylation of carboxylic acid groups on hemoglobin can be quantitated via GC/MS negative-ion chemical ionization analysis of the pentafluorobenzoate derivative of the methanol released from the hemoglobin upon base hydrolysis. Although the method requires extensive cleanup of extracting solvents because of the ubiquitous presence of trace levels of methanol, it is possible to quantitate adducted hemoglobin at levels as low as 4 ng per gram of hemoglobin using [13C, 2H3]methyl alcohol as the internal standard. In principle this method can be used in humans as a dosimeter for exposure to environmental methylating or alkylating agents.

Serum albumin adducts in the molecular epidemiology of aflatoxin carcinogenesis: correlation with aflatoxin B1 intake and urinary excretion of aflatoxin M1

Aflatoxin-serum albumin adducts in the blood of 42 residents of Guangxi Province, People's Republic of China, were determined and compared with intake of aflatoxin B1 (AFB1) and excretion of aflatoxin M1 (AFM1) in urine. Blood specimens were obtained during the same period that urine was collected and that diet was sampled. Serum albumin was isolated from blood by affinity chromatography on Reactive Blue 2-Sepharose and subjected to enzymatic proteolysis using Pronase. Immunoreactive products were purified by immunoaffinity chromatography and quantified by competitive radioimmunoassay. A highly significant correlation (r = 0.60, P less than 0.00003) of adduct level with AFM1 excretion was observed. An equally highly significant correlation of adduct level with intake (r = 0.69, P less than 0.000001) was also observed. From the slope of the regression line for adduct level as a function of intake, it was calculated that 1.4-2.3% of ingested AFB1 becomes covalently bound to serum albumin, a value very similar to that observed when rats are administered AFB1.

Effects of acetylenic and olefinic pyrenes upon cytochrome P-450 dependent benzo[a]pyrene hydroxylase activity in liver microsomes

1-Ethynylpyrene, trans-, & cis-1-(2-bromovinyl)pyrene, methyl 1-pyrenyl acetylene, and phenyl 1-pyrenyl acetylene are substrates for cytochrome P-450 dependent monooxygenases and also inhibitors of cytochrome P-450 dependent benzo[a]pyrene hydroxylase activities in liver microsomes from 5,6-benzoflavone or phenobarbital pretreated rats. 1-Ethynylpyrene, trans-1-(2-bromovinyl)pyrene, and methyl 1-pyrenyl acetylene cause a mechanism based inhibition (suicide inhibition) of the benzo[a]pyrene hydroxylase activities in microsomes from 5,6-benzoflavone or phenobarbital pretreated rats, while cis-1-(2-bromovinyl)pyrene only causes suicide inhibition of the hydroxylse activities in the 5,6-benzoflavone induced microsomes and phenyl 1-pyrenyl acetylene does not cause a detectable suicide inhibition of these activities in either type of microsome. Incubation with NADPH and 1-ethynylpyrene, trans-, or cis-1-(2-bromovinyl)pyrene causes a loss of the P-450 content in the microsomes from 5,6-benzoflavone or phenobarbital pretreated rats, but incubations with methyl 1-pyrenyl acetylene or phenyl 1-pyrenyl acetylene did not cause a loss of the P-450 content of either microsomal preparation.

1-Ethynylpyrene, a suicide inhibitor of cytochrome P-450 dependent benzo[a]pyrene hydroxylase activity in liver microsomes

The preparation of 1-ethynylpyrene (EP) is described. Incubation of EP with liver microsomes in the presence of NADPH yields fluorescent products, but the production of these products ceases after 15 min. Addition of fresh microsomes restores the original rate of EP metabolism. The metabolism of EP is initially more rapid in microsomes from 5,6-benzoflavone- (BF) pretreated rats than in microsomes from phenobarbital- (PB) pretreated rats or from untreated, control animals. EP strongly inhibits the hydroxylation of benzo[a]pyrene (BP) by liver microsomes; after 20 min in the presence of EP, BP metabolism nearly ceases. Addition of fresh microsomes restores the original rate of BP hydroxylation. EP more effectively inhibits the oxidation of BP in liver microsomes from rats pretreated with BF than from rats pretreated with PB or from untreated, control animals. The inhibition of BP hydroxylation activity due to EP is dependent upon NADPH and is apparently irreversible. Kinetic analyses demonstrate that the observed inhibition of BP hydroxylation is due to loss of the enzymatic activity by a process that is first order in EP and that reaches a limiting value at infinite EP concentrations. One such first-order process, with a t 1/2 of 3.5 min and a Ks for EP of 40 microM, is observed in microsomes from BF-pretreated rats. Two such first-order processes, one with t 1/2 of 6.9 min and Ks of 46 microM and one with t 1/2 of 12.7 min and Ks of 33 microM, are observed in microsomes from PB-pretreated rats. It is proposed that a self-catalyzed inhibition (suicide inhibition) of the cytochrome P-450 dependent BP hydroxylation occurs in the presence of EP. Incubation with EP under conditions that result in loss of about 90% of the BP hydroxylase activity in microsomes from BF-pretreated rats and about 66% of the activity in microsomes from PB-pretreated rats causes the loss of only 6 and 12% of the cytochrome P-450, respectively. It is concluded that loss of P-450 content is an insensitive measure of the effect of this inhibitor upon this cytochrome P-450 dependent enzyme activity. The selectivity of the loss of P-450 due to the incubation of the different microsomal preparations with EP is also observed to be different than the selectivity for loss of BP hydroxylase activity. It is proposed that the suicide inhibition of cytochrome P-450 dependent enzymes by alkynes need not involve heme alkylation and a resulting loss of P-450 content.(ABSTRACT TRUNCATED AT 400 WORDS)