Optimal drug therapy and therapeutic drug monitoring after spinal cord injury: a population-specific approach

Changes in renal function during acute spinal cord injury: implications for pharmacotherapy

STUDY DESIGN

Laboratory investigation in rats submitted to experimental spinal cord injury (SCI).

OBJECTIVE

To characterize changes in renal function during acute SCI.

METHODS

Sprague Dawley rats were subjected to severe spinal cord contusion at T8 level or to laminectomy as control. Twenty-four hours after spine surgery, clearance assessments of a single dose of iohexol (120 mg kg(-1)) or of p-aminohippuric acid (PAH, 100 mg kg(-1)) were used to evaluate glomerular filtration rate (GFR) and tubular secretion (TS), respectively. Blood sampling was used to determine concentrations of both compounds by high-performance liquid chromatography for pharmacokinetic measurements.

RESULTS

Iohexol clearance decreased significantly after injury, which resulted in increased concentrations and half-life of iohexol in blood; PAH clearance remained unchanged.

CONCLUSION

GFR but not TS is altered during spinal shock. These observations should be of interest to professionals caring for early cord-injured patients, in order to prevent toxicity and therapeutic failure when administering drugs eliminated by the kidney.

Theophylline for bradycardia secondary to cervical spinal cord injury

BACKGROUND

Spinal cord injury (SCI) is a devastating disease process that can occur as a consequence of motor vehicle collisions, falls, or other traumatic injuries. Persistent bradycardia was found to be universally present in all high cervical SCI patients. Limited data exists to suggest the most effective therapy for the bradycardia associated with high cervical SCI. Treatment includes atropine, epinephrine, dopamine, and even implantable cardiac pacemakers, all of which have their risks and side effects. There are no prospective studies to evaluate methylxanthines for the treatment of bradycardia secondary to cervical SCI.

METHODS

We report on four patients in whom Theophylline was successfully used enterally as a second line agent to treat bradycardia secondary to cervical SCI. We also reported on two patients in whom Theophylline was successfully used as a first line agent. Bradycardia resolved in all patients

RESULTS

Theophylline levels were below toxic levels in all of the patients and no side effects from theophylline were observed.

CONCLUSIONS

Theophylline's use via enteral route can successfully and safely treat SCI-related bradycardia, and may help avoid the long term use of inotropic and chronotropic infusions and pacemakers and their associated risks and complications. We strongly recommend further studies to establish the role of this agent as a first line therapy in this specific patient population. Optimal dosing and duration of therapy will also need to be established.

Single-dose gentamicin clearance is a predictor of creatinine clearance in spinal man

To identify a radionuclide-free, SCI (spinal cord injury) population-specific method for estimating renal function utilizing the total body clearance of gentamicin as a measure of creatinine clearance (CL(cr)) and glomerular filtration rate (GFR). To incorporate SCI population-specific patient variables into a predictive model for estimating CLcr and GFR. A retrolective study of gentamicin clearance as a measure of GFR in patients with SCI. Each patient had received a single, intravenous dose of gentamicin. Simultaneously, a 24-hour creatinine clearance (CL(cr-24)) was obtained. Experimentally measured total body clearance of a single intravenous administration of gentamicin (CL(gent-iv)) was estimated from PK analyses of gentamicin disposition in patients with SCI. Comparisons were made between CL(gent-iv) and predictors of GFR derived from a SCI population-specific nomogram (SCI-psn), the method of Cockcroft and Gault (C-G), CL(cr-24), and a multiplicative statistical model. A multiplicative model, calculated gentamicin clearance (CL(gent-calc)), that incorporates SCI population-specific characteristics to predict CL(cr-24) and GFR was derived from a multivariate nonlinear regression analysis, and the strengths of association between CL(gent-iv) and CL(gent-calc), and estimates of GFR derived from the method of C-G, SCI-psn, and CL(cr-24) were tested. The best predictive performance was demonstrated for CL(gent-iv) and the multiplicative model, CL(gent-calc), when tested against CL(cr-24) or the SCI-psn, supporting our premise that CL(gent-iv) can be used to estimate GFR in SCI. CL(gent-iv) and CL(gent-calc) outperformed CL(cr-24), the method of C-G, and the SCI-psn as predictors of GFR. The multiplicative model produced estimates of CL(gent-iv) which were more precise and less biased than CL(cr-24), the standard radiation-free predictor of GFR in patients with SCI, the SCI-psn, and the C-G equation. In this preliminary study, the clearance of gentamicin administered intravenously, CL(gent-iv), correlated highly with simultaneously measured CL(cr-24). CL(gent-iv) and our predictive model, CL(gent-calc), are more precise and less biased predictors of GFR in SCI than CL(cr-24). CL(gent-iv) and CL(gent-calc) seem to be clinically useful alternatives to CL(cr-24), the method of C-G, and the SCI-psn.

Recombinant Human Erythropoietin Protects against Experimental Spinal Cord Trauma Injury by Regulating Expression of the Proteins MKP-1 and p-ERK

The present study explored the tissue-protective effect of erythropoietin in rats after experimental spinal cord injury (SCI) produced by dropping a weight onto surgically exposed spinal cord. Sixty rats were randomized to sham operation (spinal cord exposure; control), SCI plus intraperitoneal saline injection, or SCI plus intraperitoneal erythropoietin injection. Locomotor function was evaluated with Basso, Beattie and Bresnahan scores 1 day (24 h) and 7 days later, and rats were then killed for analysis of lesion site tissue. Compared with saline-treated SCI rats, erythropoietin-treated SCI rats showed significantly less locomotor dysfunction and faster locomotor recovery. Immunohistochemistry showed that erythropoietin-treated SCI rats had a significantly lower phospho-extracellular signal-regulated kinase (p-ERK) protein expression and a significantly higher mitogen-activated protein kinase phosphatase-1 (MKP-1) protein expression than saline-treated SCI rats. Haematoxylin–eosin staining showed progressive disruption of dorsal white matter and neuron loss after SCI; lesions were less severe and there was more neuron regeneration in the erythropoietin group than in the saline group. It is concluded that erythropoietin reduces pathological changes and SCI severity via down-regulation of p-ERK and up-regulation of MKP-1.

Understanding Drug Disposition Alterations Induced by Acute Spinal Cord Injury: Role of Injury Level and Route of Administration for Agents Submitted to Extensive Liver Metabolism

It is known that acute spinal cord injury (SCI) produces hemodynamic alterations, including a reduction in liver blood flow that is more pronounced after high-thoracic than after low-thoracic injury. To determine if these changes have an impact in the pharmacokinetics of high extraction drugs (i.e., those drugs which clearance mainly depends on liver blood flow), we studied the pharmacokinetics of a model compound, phenacetin, and of its main metabolite, acetaminophen, in rats 24 h after a high (T1) or a low (T8) SCI, as well as in sham-lesioned controls. After intravenous administration to animals with SCI, reductions in drug clearance and distribution led to an increase in blood concentrations. These alterations were more pronounced after high than after low SCI, as expected from hemodynamic changes. After oral administration, phenacetin blood levels were similar in sham-lesioned and T1-injured animals, but decreased by injury at T8. This is likely due to a reduction in drug absorption which compensates the changes in distribution and elimination induced by injury at T1, whereas it prevails in T8-lesioned animals. Acetaminophen blood concentrations observed after intravenous or oral phenacetin, or after the oral administration of acetaminophen by itself, were increased or reduced, depending on the overall effect of the alterations on absorption, first pass metabolism, distribution and elimination induced by high and low SCI. Results demonstrate that acute SCI significantly alters the pharmacokinetics of high extraction drugs. The outcome of such alterations depends on the level of SCI and on the route of administration.

Effects of 4-aminopyridine on cardiac repolarization, PR interval, and heart rate in patients with spinal cord injury

STUDY OBJECTIVE

To determine the effects of 4-aminopyridine (4-AP) on heart rate and PR, QT, and QTc intervals in patients with longstanding spinal cord injury (SCI).

DESIGN

Randomized, active-treatment-controlled, dose level-blinded study, with allocation concealed.

SETTING

University-affiliated, tertiary care medical center.

PATIENTS

Sixty otherwise healthy male and female outpatients with traumatic SCI of more than 1 year's duration. Intervention. Oral administration and dose titration to tolerance of an immediate-release formulation of 4-AP.

MEASUREMENTS AND MAIN RESULTS

The PR interval, heart rate, QT interval, and QTc interval obtained from standard 12-lead electrocardiograms (ECGs) at baseline (before administration of 4-AP) and after 1 month of treatment were compared. The QTc intervals were derived by using Bazett's formula (equation) incorporated into standard computerized analyses of 12-lead ECG printouts. The paired t test was performed to test for the significance of differences between means and variances. No statistically significant differences were noted in heart rate or between ECG time intervals measured at baseline and after 1 month of treatment with 4-AP among all patients with SCI or between subgroups stratified by injury level (tetraplegia, paraplegia) or sex.

CONCLUSION

During the 1-month period that 4-AP was administered, the heart rate and PR, QT, and QTc intervals all remained unchanged and stayed well within normal range in comparison to literature-derived control values. 4-Aminopyridine does not appear to influence the length of cardiac time intervals or heart rate and, hence, is unlikely to cause potentially life-threatening ventricular dysrrhythmias when administered long-term and taken orally in dosages of up to 30 mg/day. Specifically, cardiac repolarization (QTc interval) is unaffected in patients with SCI who continuously receive 4-AP for up to 1 month.