Alexander Tzabazis

Pharmacokinetics and pharmacodynamics of the new propofol prodrug GPI 15715 in rates

Background and objective: We studied the pharmacokinetics and pharmacodynamics of GPI 15715 (Aquavan® injection), a new water-soluble prodrug metabolized to propofol by hydrolysis. Methods: Nine adult male Sprague-Dawley rats (398 ± 31 g) received a bolus dose of 40 mg GPI 15715. The plasma concentrations of GPI 15715 and propofol were determined from arterial blood samples, and the pharmacokinetics of both compounds were investigated using compartment models whereby the elimination from the central compartment of GPI 15715 was used as drug input for the central compartment of propofol. Pharmacodynamics were assessed using the median frequency of the EEG power spectrum. Results: A maximum propofol concentration of 7.1 ± 1.7 μg mL−1 was reached 3.7 ± 0.2 min after bolus administration. Pharmacokinetics were best described by two-compartment models. GPI 15715 showed a short half-life (2.9 ± 0.2 and 23.9 ± 9.9 min), an elimination rate constant of 0.18 ± 0.01 min−1 and a central volume of distribution of 0.25 ± 0.02 L kg−1. For propofol, the half-life was 1.9 ± 0.1 and 45 ± 7 min, the elimination rate constant was 0.15 ± 0.02 min−1 and the central volume of distribution was 2.3 ± 0.6 L kg−1. The maximum effect on the electroencephalogram (EEG) - EEG suppression for >4 s - occurred 6.5 ± 1.2 min after bolus administration and baseline values of the EEG median frequency were regained 30 min later. The EEG effect could be described by a sigmoid Emax model including an effect compartment (E0 = 16.9 ± 7.9 Hz, EC50 = 2.6 ± 0.8 μg mL−1, ke0 = 0.35 ± 0.04 min−1). Conclusions: Compared with known propofol formulations, propofol from GPI 15715 showed a longer half-life, an increased volume of distribution, a delayed onset, a sustained duration of action and a greater potency with respect to concentration.

Differential activation of trigeminal C or Aδ nociceptors by infrared diode laser in rats: behavioral evidence

Radiant heat is often used for studying thermal nociception, although inherent characteristics such as the broad spectrum of applied wavelengths of typical light sources limit control over and repeatability of stimuli. To overcome these problems, we used a diode infrared laser-based stimulator (wavelength: 980 nm) for selectively stimulating trigeminal Adelta or C thermonociceptors in rats. To provide indirect evidence for nociceptor-selective stimulation, we tested the effects of capsaicin, dimethylsulfoxide (DMSO), and morphine on withdrawal latencies for long pulses with a low current (hypothesized to selectively stimulate C nociceptors) and for threshold currents of short pulses with high current (hypothesized to selectively stimulate Adelta nociceptors) in lightly anesthetized rats. Nonmem analysis was used to perform pharmacodynamic modeling. The measured baseline withdrawal latency for long pulses was 12.5 +/- 0.3 s which was changed significantly to 6.7 +/- 0.4 s after applying topical capsaicin which selectively sensitizes C nociceptors and to 16.5 +/- 1.3 s after 1.0 mg/kg morphine which preferentially attenuates C fiber nociception. Topical DMSO which appears to selectively sensitize Adelta afferents did not significantly alter withdrawal latencies to the long pulses. Fitted threshold currents for short pulses after DMSO were however significantly lower (974 +/- 53 mA vs. 1113 +/- 12 mA for baseline) indicating Adelta sensitization. Capsaicin and morphine did not significantly change threshold currents. Best Nonmem fits for the long pulse were obtained using a model assuming no DMSO effect, but a different inter-individual variability after applying this substance. For the short pulse, a model assuming no capsaicin or morphine effect, but again allowing different inter-individual variabilities after applying these drugs, best described the data. We conclude that different settings of the stimulator used in this study were capable of selectively activating trigeminal Adelta or C thermonociceptors.

The sitting position in neurosurgery: indications, complications and results. a single institution experience of 600 cases

BackgroundThe benefit of the sitting position for surgery of the posterior fossa and cervical spine is still a matter of controversy. In our study we analyzed the outcome after sitting position surgery at our institution. We compared the incidence of venous air embolism (VAE) as recognized with different monitoring techniques and the severity of complications.MethodsWe retrospectively analyzed 600 patients, who underwent surgery for different posterior fossa and cervical spine pathologies, respectively, in the sitting position at our institution from 1995 to 2011. Intraoperative monitoring for VAE included endtidal CO2 level, Doppler ultrasound or intraoperative transesophageal echocardiography (TEE). We defined VAE as a decrease of the endtidal CO2 levels by more than 4xa0mm Hg, a characteristic sound in the thoracic Doppler, or any sign of air in the TEE.ResultsWe found an overall incidence of VAE in 19xa0% of all patients, whereas the rate of severe complications associated with VAE such as a decline of partial oxygen pressure (pO2) or a drop of blood pressure was only 3.3xa0% in all patients. Only three out of 600 operations had to be terminated because of non-controllable VAE (0.5xa0%). There was no mortality resulting from VAE in our series. We also found a difference in the incidence of VAE depending on the monitoring technique. The VAE rate as monitored with TEE was 25.6xa0% whereas the incidence of VAE in patients monitored with Doppler ultrasound was 9.4xa0%. The rate of a significant VAE was comparable in both methods 4.8xa0% vs. 1.2xa0%. All patients were preoperatively screened for persisting foramen ovale (PFO); 24 patients with clinically confirmed PFO were included in this series. There was no case of paradox air embolism.ConclusionsIn our series, VAE was detected in 19xa0% of all patients in the sitting position. However, in only 0.5xa0% of cases a termination of the surgical procedure became necessary. In all other cases, the cause of air embolism could be found and eliminated during surgery. TEE was found to be the monitoring technique with the highest sensitivity. In our opinion, the sitting position is a safe positioning technique if TEE monitoring is used.

Propofol in rats: testing for nonlinear pharmacokinetics and modelling acute tolerance to EEG effects.

BACKGROUND AND OBJECTIVEnPharmacokinetics of propofol in rats have usually been described using linear models. Furthermore, there are only a few investigations for a pharmacodynamic model of the electroencephalographic effects of propofol in rats. We investigated pharmacokinetics and pharmacodynamics of propofol in rats with special regard to linearity in pharmacokinetics and development of tolerance.nnnMETHODSnTwelve adult male Sprague-Dawley rats received propofol in three successive infusion periods of 30 min each with infusion rates of 0.5, 1 and 0.5 mg kg(-1) min(-1). Propofol plasma concentrations were determined from arterial blood samples. Pharmacokinetics were tested for linearity using the ratio of the concentrations at the end of the first and second infusion interval as a model independent criterion. Several linear and nonlinear models were investigated with population pharmacokinetic analysis. Pharmacodynamics were analysed using the median frequency of the electroencephalographic power spectrum as a quantitative measure of the hypnotic effect.nnnRESULTSnPharmacokinetics were found to be nonlinear and were best described by a two-compartment model with Michaelis-Menten elimination (Vm = 2.17 microg mL(-1) min(-1), Km = 2.65 microg mL(-1), k12 = 0.30 min(-1), k21 0.063 min(-1), Vc = 0.13 L). Acute tolerance to the electroencephalographic effect of propofol was observed. The hypnotic effect was best described by a sigmoid Emax model (E0 = 17.8 Hz, Emax = 17.7 Hz, EC50 = 4.1 microg mL(-1), gamma = 2.3, ke0 = 0.36 min(-1)) with competitive antagonism of propofol and a hypothetical drug in an additional tolerance compartment.nnnCONCLUSIONSnFor the applied infusion scheme, propofol pharmacokinetics in rats were nonlinear and a development of tolerance to the electroencephalographic effect of propofol was observed during an infusion time of 90 min.

Shaped magnetic field pulses by multi-coil repetitive transcranial magnetic stimulation (rTMS) differentially modulate anterior cingulate cortex responses and pain in volunteers and fibromyalgia patients

BackgroundRepetitive transcranial magnetic stimulation (rTMS) has shown promise in the alleviation of acute and chronic pain by altering the activity of cortical areas involved in pain sensation. However, current single-coil rTMS technology only allows for effects in surface cortical structures. The ability to affect activity in certain deep brain structures may however, allow for a better efficacy, safety, and tolerability. This study used PET imaging to determine whether a novel multi-coil rTMS would allow for preferential targeting of the dorsal anterior cingulate cortex (dACC), an area always activated with pain, and to provide preliminary evidence as to whether this targeted approach would allow for efficacious, safe, and tolerable analgesia both in a volunteer/acute pain model as well as in fibromyalgia chronic pain patients.MethodsPart 1: Different coil configurations were tested in a placebo-controlled crossover design in volunteers (Nu2009=u200916). Tonic pain was induced using a capsaicin/thermal pain model and functional brain imaging was performed by means of H215O positron emission tomography – computed tomography (PET/CT) scans. Differences in NRS pain ratings between TMS and sham treatment (NRSTMS-NRSplacebo) which were recorded each minute during the 10xa0minute PET scans. Part 2: 16 fibromyalgia patients were subjected to 20 multi-coil rTMS treatments over 4xa0weeks and effects on standard pain scales (Brief Pain Inventory, item 5, i.e. average pain NRS over the last 24xa0hours) were recorded.ResultsA single 30xa0minute session using one of 3 tested rTMS coil configurations operated at 1xa0Hz consistently produced robust reduction (mean 70% on NRS scale) in evoked pain in volunteers. In fibromyalgia patients, the 20 rTMS sessions also produced a significant pain inhibition (43% reduction in NRS pain over last 24xa0hours), but only when operated at 10xa0Hz. This degree of pain control was maintained for at least 4xa0weeks after the final session.ConclusionMulti-coil rTMS may be a safe and effective treatment option for acute as well as for chronic pain, such as that accompanying fibromyalgia. Further studies are necessary to optimize configurations and settings as well as to elucidate the mechanisms that lead to the long-lasting pain control produced by these treatments.

Antihyperalgesic Effect of a Recombinant Herpes Virus Encoding Antisense for Calcitonin Gene–related Peptide

Background:Calcitonin gene–related peptide (CGRP) is contained in and released by small-diameter, nociceptive primary afferent sensory neurons. Upon spinal release, one of the effects of CGRP seems to be to sensitize dorsal horn neurons to subsequent input from nociceptive afferents and, consequently, to induce a behavioral hyperalgesia. Therefore, attenuating evoked release of CGRP from central terminals of nociceptors should have an antihyperalgesic effect. Methods:The authors applied a recombinant herpes vector, encoding an antisense sequence to the whole CGRP gene, to the dorsal surface of the hind paw of mice to knock down expression of the peptide selectively in primary afferents innervating this tissue. Results:Herpes virus–based vector encoding an antisense sequence for the whole CGRP clearly reduced CGRP immunoreactivity in the infected spinal dorsal horn levels as well as in cultured dorsal root ganglia neurons. Selective knockdown of CGRP in primary afferents significantly attenuated the thermal, C-fiber hyperalgesia normally observed after topical application of capsaicin. The effect of viral vector–mediated knockdown of CGRP was comparable to the effect of intrathecal application of the CGRP antagonist CGRP8-37, but lasted for 14 weeks after one single application. Conclusion:Viral vector–mediated knockdown of CGRP in primary afferent neurons provides a promising tool for treatment of chronic pain states as well as for studies investigating the pathophysiology underlying these conditions.

Selective nociceptor activation in volunteers by infrared diode laser

BackgroundTwo main classes of peripheral sensory neurons contribute to thermal pain sensitivity: the unmyelinated C fibers and thinly myelinated Aδ fibers. These two fiber types may differentially underlie different clinical pain states and distinctions in the efficacy of analgesic treatments. Methods of differentially testing C and Aδ thermal pain are widely used in animal experimentation, but these methods are not optimal for human volunteer and patient use. Thus, this project aimed to provide psychophysical and electrophysiological evidence that whether different protocols of infrared diode laser stimulation, which allows for direct activation of nociceptive terminals deep in the skin, could differentially activate Aδ or C fiber thermonociceptors in volunteers.ResultsShort (60 ms), high intensity laser pulses (SP) evoked monomodal pricking pain which was not enhanced by topical capsaicin, whereas longer, lower power pulses (LP) evoked monomodal burning pain which was enhanced by topical capsaicin. SP also produced cortical evoked EEG potentials consistent with Aδ mediation, the amplitude of which was directly correlated with pain intensity but was not affected by topical capsaicin. LP also produced a distinct evoked potential pattern the amplitude of which was also correlated with pain intensity, which was enhanced by topical capsaicin, and the latency of which could be used to estimate the conduction velocity of the mediating nociceptive fibers.ConclusionsPsychophysical and electrophysiological data were consistent with the ability of short high intensity infrared laser pulses to selectively produce Aδ mediated pain and of longer pulses to selectively produce C fiber mediated thermal pain. Thus, the use of these or similar protocols may be useful in developing and testing novel therapeutics based on the differential molecular mechanisms underlying activation of the two fiber types (e.g., TRPV1, TRPV2, etc). In addition, these protocol may be useful in determining the fiber mediation of different clinical pain types which may, in turn be useful in treatment choice.

Oxytocin receptor: Expression in the trigeminal nociceptive system and potential role in the treatment of headache disorders.

Aims Our studies investigated the location of oxytocin receptors in the peripheral trigeminal sensory system and determined their role in trigeminal pain. Methods Oxytocin receptor expression and co-localization with calcitonin gene-related peptide was investigated in rat trigeminal ganglion using immunohistochemistry. Enzyme-linked immunosorbent assay was used to determine the effects of facial electrocutaneous stimulation and adjuvant-induced inflammation of the temporomandibular joint on oxytocin receptor expression in the trigeminal ganglion. Finally, the effects of oxytocin on capsaicin-induced calcitonin gene-related peptide release from dural nociceptors were investigated using isolated rat dura mater. Results Oxytocin receptor immunoreactivity was present in rat trigeminal neurons. The vast majority of oxytocin receptor immunoreactive neurons co-expressed calcitonin gene-related peptide. Both electrocutaneous stimulation and adjuvant-induced inflammation led to a rapid upregulation of oxytocin receptor protein expression in trigeminal ganglion neurons. Oxytocin significantly and dose-dependently decreased capsaicin-induced calcitonin gene-related peptide release from dural nociceptors. Conclusion Oxytocin receptor expression in calcitonin gene-related peptide containing trigeminal ganglion neurons, and the blockade of calcitonin gene-related peptide release from trigeminal dural afferents suggests that activation of these receptors may provide therapeutic benefit in patients with migraine and other primary headache disorders.

Visualizing Dermal Permeation of Sodium Channel Modulators by Mass Spectrometric Imaging

Determining permeability of a given compound through human skin is a principal challenge owing to the highly complex nature of dermal tissue. We describe the application of an ambient mass spectrometry imaging method for visualizing skin penetration of sodium channel modulators, including novel synthetic analogs of natural neurotoxic alkaloids, topically applied ex vivo to human skin. Our simple and label-free approach enables successful mapping of the transverse and lateral diffusion of small molecules having different physicochemical properties without the need for extensive sample preparation.

Trigeminal antihyperalgesic effect of intranasal carbon dioxide.

AIMSnClinical studies demonstrate attenuation of trigeminal-related pain states such as migraine by intranasal CO(2) application. This study investigated the underlying mechanisms of this observation and its potential use to reverse trigeminal pain and hypersensitivity.nnnMAIN METHODSnWe used a behavioral rat model of capsaicin-induced trigeminal thermal hyperalgesia, intranasal CO2 application and several pharmacologic agents such as carbonic anhydrase, acid-sensing ion channels (ASICs), and TRPV1 blocker as well as acidic buffer solutions to investigate and mimic the underlying mechanism.nnnKEY FINDINGSnIntranasal CO(2) application produced a robust dose-dependent antihyperalgesic effect in rats that lasted at least one hour. Blockade of nasal carbonic anhydrase with a dorzolamide solution (Trusopt ophthalmic solution) showed only a non-significant decrease of the antihyperalgesic effect of intranasal CO(2) application. Pharmacologic blockade of ASICs or TRPV(1) receptor significantly attenuated the antihyperalgesic effect of CO(2) application. The effect of intranasal CO(2) application could be mimicked by application of pH 4, but not pH 5, buffer solution to the nasal mucosa. As with CO(2) application, the antihyperalgesic effect of intranasal pH 4 buffer was blocked by nasal application of antagonists to ASICs and TRPV(1) receptors.nnnSIGNIFICANCEnOur results indicate that intranasal CO(2) application results in a subsequent attenuation of trigeminal nociception, mediated by protonic activation of TRPV(1) and ASIC channels. A potential central mechanism for this attenuation is discussed. The antihyperalgesic effects of intranasal CO(2) application might be useful for the treatment of trigeminal pain states.