Application of in vitro data in physiologically-based kinetic models for quantitative in vitro-in vivo extrapolation: A case-study for baclofen.

Physiologically-based kinetic (PBK) models can simulate concentrations of chemicals in tissues over time without animal experiments. Nevertheless, in vivo data are often used to parameterise PBK models. This study aims to illustrate that a combination of kinetic and dynamic readouts from in vitro assays can be used to parameterise PBK models simulating neurologically-active concentrations of xenobiotics. Baclofen, an intrathecally administered drug to treat spasticity, was used as a proof-of-principle xenobiotic. An in vitro blood-brain barrier (BBB) model was used to determine the BBB permeability of baclofen needed to simulate plasma and cerebrospinal concentrations. Simulated baclofen concentrations in individuals and populations of adults and children generally fall within 2-fold of measured clinical study concentrations. Further, in vitro micro-electrode array recordings were used to determine the effect of baclofen on neuronal activity (cell signalling). Using quantitative in vitro-in vivo extrapolations (QIVIVE) corresponding doses of baclofen were estimated. QIVIVE showed that up to 4600 times lower intrathecal doses than oral and intravenous doses induce comparable neurological effects. Most simulated doses were in the range of administered doses. This show that PBK models predict concentrations in the central nervous system for various routes of administration accurately without the need for additional in vivo data.

Cerebrospinal fluid and spinal cord distribution of hyperbaric bupivacaine and baclofen during slow intrathecal infusion in pigs.

BACKGROUND: Despite the widespread use of implanted pumps for continuous intrathecal drug delivery, there have been no studies aimed at defining the effect of baricity and posture on drug distribution in the cerebrospinal fluid and spinal cord during the very slow infusion rates typically used for chronic intrathecal drug administration. METHODS: Intrathecal microdialysis probes were placed at six points along the neuraxis in both the anterior and posterior intrathecal space of anesthetized pigs to permit cerebrospinal fluid sampling. Animals were then positioned either vertically or horizontally (prone), and a hyperbaric solution containing bupivacaine (7.5 mg/ml) and baclofen (2 mg/ml) was infused at 20 microl/h for 6 h, while the cerebrospinal fluid was collected for measurement of drug concentration. At the end of the experiment, the animals were killed, and the spinal cord was removed and divided into 1-cm sections that were further divided into anterior and posterior portions for measurement of drug concentration. RESULTS: Bupivacaine and baclofen distribution was biased caudally in the vertical group and cephalad in the horizontal group. Drug concentration decreased rapidly in the cerebrospinal fluid and spinal cord as a function of distance from the site of administration in both groups, resulting in most drugs being located in very close proximity to the site of infusion. CONCLUSION: Even at very slow infusion rates, drug distribution within the cerebral spinal fluid and spinal cord are affected by baricity/posture. These findings suggest that patient position and solution baricity may be important clinical factors determining the distribution and ultimate efficacy of chronic intrathecal drug infusions.

Cerebrospinal fluid baclofen concentrations in patients undergoing continuous intrathecal baclofen therapy.

The aim of this study was to report concentrations of cerebrospinal fluid (CSF) baclofen in children undergoing chronic intrathecal baclofen (ITB) infusions. CSF baclofen concentrations were analyzed in 53 specimens obtained by intrathecal catheter aspiration from 43 participants (28 males, 15 females; range 3-44y, mean 16y [SD 8y 11mo]), with functioning baclofen pumps and catheters. Daily ITB doses ranged from 70 to 1395 microg per day (mean 607 microg per day [SD 363], median 575). Baclofen concentration was quantified by high-pressure liquid chromatography and confirmed by injection onto a gas chromatograph. CSF baclofen concentrations from children receiving either simple continuous or complex infusions ranged from 0.2 to 20.0 microg/ml (mean 4.64 microg/ml, median 3.3 microg/ml). CSF baclofen concentrations from children receiving simple continuous infusions ranged from 0.5 to 12.9 (mean 4.7 microg/ml, median 3.55 microg/ml). There was no correlation between ITB dosage and CSF baclofen concentration. We conclude that baclofen concentration can be measured to determine if baclofen is present in CSF. However, there appears to be no correlation between the ITB dose infused and the corresponding CSF baclofen level.

Interaction of ventriculoperitoneal shunt and baclofen pump.

OBJECTIVE: The authors present three cases of children with shunted hydrocephalus and intrathecal delivery of baclofen via an implanted pump. Each case illustrates a potential interaction of these devices. MATERIALS AND METHODS: A review of the pertinent charts, operative notes, and discharge summaries was performed, along with examination of the pertinent literature. CONCLUSION: The first case illustrates that a change in the pharmacology or effect of intrathecal baclofen may be caused by a failure of a shunt affecting cerebrospinal fluid clearance. The second case illustrates that patients can have ventricular changes with any procedure that accesses the thecal space, even with a functioning shunt. The third case illustrates that ensuring that a patient's shunt is functional before implantation of a baclofen pump is imperative. As the potential pool of patients with both shunted hydrocephalus and need for intrathecal baclofen grows, clinicians should be aware of potential serious interactions.

Simple and sensitive liquid chromatography-tandem mass spectrometry method for determination of the S(+)- and R(-)-enantiomers of baclofen in human plasma and cerebrospinal fluid.

A simple and sensitive liquid chromatography-tandem mass spectrometry (LC/MS/MS) method to determine the enantiomers of the muscle relaxant baclofen in human plasma and cerebrospinal fluid (CSF) has been developed. A commercially available ultrafiltration membrane is used to prepare the sample. A chiral CROWNPAK CR(+) stationary phase column is then used to perform complete resolution of the S(+)- and R(-)-enantiomers of baclofen. This method was used to analyze human plasma and CSF spiked with baclofen, and the calibration curves for both biologic samples were linear over a concentration range of 0.15-150 ng enantiomer/ml. The lower limit of quantification was 0.15 ng enantiomer/ml in both fluids. Finally, the method was tested with an artificial CSF as an alternative to authentic human CSF. The results showed that no matrix effects and no interfering peaks were observed using this artificial CSF.

Role of thiopentone, nitrous oxide and relaxant anaesthesia in causing the syndrome of post-operative paralysis in man.

Forty-seven patients undergoing elective/emergency surgery were investigated for the recovery pattern by numerically scoring the state of consciousness, skeletomuscular tone, respiration and blood pressure after the neuromuscular transmission at the level of thenar muscles returned to normalcy. Anaesthesia in them consisted of thiopentone induction and passive ventilation with nitrous oxide and oxygen mixtures (4 1/2:2 1/2 1) with consequent changes in PaCO-2 (22.0 to 90 mm Hg) after using 0.43 to 0.68 mg/kg d-tubocurarine or 2.3 to 3.8 mg/kg gallamine. In this series twelve patients were selected at random and biological assay of cerebrospinal fluid in them for curare/gallamine after 15 min anaesthesia and in the recovery phase was carried out on frog rectus muscle. All the patients recovered satisfactorily and did not present clinical signs of depression of central nervous system, even though all of them showed the presence of curare (ranging from 0.05 to 0.33 mug/ml) and gallamine (from 0.1 to 0.75 mug/ml) in the cerebrospinal fluid. This study therefore indicates that thiopentone, nitrous oxide and relaxant type of anaesthesia does not cause clinical syndrome of post-operative paralysis even when mild to moderate degree of hypocapnia is present and even when such a technique of anaesthesia is administered in poor-risk patients with associated changes in acid-base balance, electrolytes etc. Significant quantities of skeleto-muscular relaxant drug (used during the technique) when found in cerebrospinal fluid after the technique of anaesthesia need not induce post-operative paralysis in man.