Absorption characteristics of sustained-release 4-aminopyridine (fampridine SR) in patients with chronic spinal cord injury

Pharmacology of riluzole in acute spinal cord injury

OBJECT

The aim of this paper was to characterize individual and population pharmacokinetics of enterally administered riluzole in a Phase 1 clinical trial of riluzole as a neuroprotective agent in adults 18-70 years old with acute spinal cord injury (SCI).

METHODS

Thirty-five individuals with acute SCI, American Spinal Injury Association Impairment Scale Grades A-C, neurological levels from C-4 to T-12, who were enrolled in the Phase 1 clinical trial sponsored by the North American Clinical Trials Network for Treatment of Spinal Cord Injury, received 50 mg riluzole twice daily for 28 doses. The first dose was administered at a mean of 8.7 ± 2.2 hours postinjury. Trough plasma samples were collected within 1 hour predose, and peak plasma samples were collected 2 hours postdose on Days 3 and 14 of treatment. Riluzole concentrations were quantified by high-performance liquid chromatography assay. The data were analyzed for individual and population pharmacokinetics using basic structural and covariate models. The pharmacokinetic measures studied were the peak concentration (C(max)), trough concentration (C(min)), systemic exposure (AUC(0-12)), clearance (CL/F), and volume of distribution (V_F) normalized by the bioavailability (F).

RESULTS

The C(max) and AUC(0-12) achieved in SCI patients were lower than those in ALS patients on the same dose basis, due to a higher CL and larger V. The pharmacokinetics of riluzole (C(max), C(min), AUC(0-12), CL, and V) changed during the acute and subacute phases of SCI during the 14 days of therapy. It was consistently observed in patients at all clinical sites that C(max), C(min), and AUC(0-12) (128.9 ng/ml, 45.6 ng/ml, and 982.0 ng × hr/ml, respectively) were significantly higher on Day 3 than on Day 14 (76.5 ng/ml, 19.1 ng/ml, and 521.0 ng × hr/ml, respectively). These changes resulted from lower CL (49.5 vs 106.2 L/hour) and smaller V (557.1 vs 1297.9/L) on Day 3. No fluid imbalance or cytochrome P 1A2 induction due to concomitant medications was identified during the treatment course to account for such increases in V and CL, respectively. Possible mechanisms underlying these changes are discussed.

CONCLUSIONS

This is the first report of clinical pharmacokinetics of riluzole in patients with SCI. The C(max) and AUC(0-12) achieved in SCI patients were lower than those in ALS patients on the same dose basis, due to a higher clearance and larger volume of distribution in SCI patients. The finding in SCI patients of an increase in the clearance and distribution of riluzole between the 3rd and 14th days after SCI, with a lower plasma concentration of riluzole on the 14th day, stresses the importance of monitoring changes in drug metabolism after SCI in interpreting the safety and efficacy of therapeutic drugs that are used in clinical trials in SCI. Clinical trial registration no.: NCT00876889.

Clinical overview of dalfampridine: an agent with a novel mechanism of action to help with gait disturbances

BACKGROUND

Medication used to treat multiple sclerosis (MS) can be categorized as disease-modifying therapies, symptomatic therapies, or treatment of acute exacerbations. Dalfampridine is the first symptomatic therapy approved by the Food and Drug Administration to improve walking in patients with MS.

OBJECTIVE

This article reviews the pharmacology, pharmacodynamic properties, and pharmacokinetic properties of dalfampridine, as well as its clinical efficacy, safety profile, pharmacoeconomic considerations, and place in therapy.

METHODS

Three PubMed searches were conducted for original articles published in English between 1966 and August 2012 with human study participants. Articles concerning the pharmacology, pharmacokinetic properties, pharmacodynamic properties, efficacy, and safety profile of dalfampridine were evaluated.

RESULTS

Dalfampridine theoretically works to improve conduction and enhance walking by inhibiting potassium channels in the axonal membrane and by prolonging action potentials in demyelinated neurons. The efficacy of dalfampridine has been reported in 2 Phase III clinical trials in patients with MS. When comparing dalfampridine 10 mg twice daily with placebo, these studies found a statistically significant improvement in walking (42.9% vs 9.3% and 35% vs 8%; P < 0.001). However, clinical trials and postmarketing surveillance have shown an increased risk of seizures with dalfampridine use that appears to be dose related [corrected].

CONCLUSIONS

Dalfampridine has a unique mechanism of action, leading to its approval as the first symptomatic therapy for MS to improve walking speed. The increased risk of seizures can be a safety concern and will require health care providers to be diligent in monitoring patients and to ensure adequate patient education [corrected]. The addition of dalfampridine as symptomatic therapy for MS may lead to additional novel products in the future.

Steady-state pharmacokinetics and tolerability of orally administered fampridine sustained-release 10-mg tablets in patients with multiple sclerosis: a 2-week, open-label, follow-up study

BACKGROUND

Fampridine sustained release (SR) has generally been well tolerated in clinical studies in patients with multiple sclerosis (MS) at doses < or = 20 mg/d. The pharmacokinetics of single escalating doses of fampridine SR (5, 10, 15, and 20 mg) were evaluated in a companion study.

OBJECTIVES

The primary objective of this study, which followed on from the single-dose pharmacokinetic study, was to assess the steady-state pharmacokinetics of fampridine in patients with MS over 2 weeks of oral administration of open-label fampridine SR 20 mg BID. Tolerability was also evaluated.

METHODS

The dose of fampridine SR was titrated upward to twice-daily administration over days 1 and 2, with 20 mg given in the morning and 10 mg in the evening. From days 3 to 14, patients received fampridine SR 20 mg BID (total daily dose, 40 mg). Fampridine pharmacokinetic parameters were determined on days 1, 8, and 15. The parameters of interest included C(max), T(max), C(min), AUC, and apparent t((1/2)) These parameters were compared with those from the single-dose study. Tolerability was assessed based on adverse events, physical examinations, vital signs, laboratory tests, and ECGs.

RESULTS

Twenty-one white patients who participated in the single-dose study were enrolled in the steady-state study (52.4% female; mean [SD] age, 45.1 [7.4] years; weight range, 54-87 kg). Fampridine pharmacokinetic parameters on day 1 were consistent with those obtained in the single-dose study. The T(max) did not differ significantly between day 1, day 8, day 15, and the single-dose study (range, 3.25-3.78 hours). C(max) values on days 8 and 15 (66.7 and 62.6 ng/mL, respectively) were significantly higher than those on day 1 (48.6 ng/mL) and in the single-dose study (50.5 ng/mL) (all, P < 0.001), reflecting accumulation of fampridine with multiple dosing. Values for C(max)/C(min) did not differ significantly between day 1 and the single-dose study (2.44 and 2.43, respectively) or between days 8 and 15 (2.90 and 2.88, respectively); however, the single-dose and day-1 values differed significantly from the day-8 and day-15 values (P < or = 0.001). There were no significant differences with respect to any other pharmacokinetic parameters. One hundred adverse events were reported by 21 patients. With the exception of 1 case of severe nausea, all adverse events were of mild to moderate severity. Thirty-five events reported by 14 subjects were considered treatment related. Dizziness was the most common treatment-related adverse event, with 11 episodes reported by 8 patients. No clinically significant changes were found in clinical laboratory values, vital signs, or physical examination findings from baseline to the last visit, and there were no clinically significant changes in QTc intervals (Bazett's correction).

CONCLUSIONS

In these patients with MS, the steady-state pharmacokinetic profile of fampridine SR 20 mg BID administered for 2 weeks appeared to support the use of twice-daily dosing in this population. This dosage was generally well tolerated.

Pharmacokinetics and tolerability of single escalating doses of fampridine sustained-release tablets in patients with multiple sclerosis: a Phase I-II, open-label trial

BACKGROUND

Fampridine (4-aminopyridine) is a potassium channel-blocking agent that has been reported to have therapeutic potential for improving walking and mobility in patients with multiple sclerosis (MS). A sustained-release (SR) formulation of fampridine was developed to improve the agent's pharmacokinetic profile by extending its t((1/2)) relative to that of immediate-release fampridine.

OBJECTIVES

The primary study objective was to examine the pharmacokinetics of fampridine SR tablets after single escalating doses in patients with MS. Tolerability was evaluated as a secondary end point.

METHODS

This multicenter, Phase I-II, open-label trial evaluated the dose proportionality and tolerability of 4 single doses of fampridine SR (5, 10, 15, and 20 mg) in patients with MS. There was a 4-day washout between doses. Blood samples were collected immediately before drug administration, hourly for the first 8 hours after administration, and at 10, 12, 14, 18, and 24 hours after administration. The pharmacokinetic parameters evaluated included C(max), T(max), AUC, elimination rate constant, apparent elimination t((1/2)), and apparent CL/F. Twelve-lead ECGs were obtained at baseline (0.5 hour before dosing) and at 1, 4, 12, and 24 hours after drug administration to evaluate potential effects on the QTc interval. All adverse events, abnormal laboratory values, and ECG abnormalities were recorded and evaluated for clinical relevance. Adverse-event data were monitored for 24 hours after the last dose, and patients were instructed to report any adverse events for 14 days after the conclusion of the study.

RESULTS

Twenty-four white patients were enrolled (58% female; mean [SD] age, 45.4 [7.3] years; weight range, 47.8-87.1 kg), and 23 completed the study. Mean plasma concentrations and AUC values were dose proportional. T(max) occurred at 3.36 to 3.92 hours after dosing; the apparent elimination t((1/2)) was 5.47 hours. Both sex and weight affected the pharmacokinetic parameters of fampridine SR. Eleven treament-related adverse events were reported in 10 patients, with dizziness being the most common (7 incidents reported by 6 patients [1 at 10 mg, 3 at 15 mg, and 3 at 20 mg]). Other adverse events included amblyopia, asthenia, headache, and ataxia. All treatment-related adverse events were mild to moderate in severity, with the exception of 1 case of dizziness (20 mg) that was considered severe. No serious adverse events were reported, and no clinically significant changes in corrected QT intervals were observed. No patients with-drew due to treatment-related adverse events.

CONCLUSION

In these patients with MS, fampridine SR (5-20 mg) had a potentially advantageous pharmacokinetic profile relative to that associated with immediate-release fampridine and was generally well tolerated.

Single-Dose pharmacokinetics of sustained-release fampridine (Fampridine-SR) in healthy volunteers and adults with renal impairment

Fampridine-SR is a sustained-release formulation of fampridine (4-aminopyridine), a potassium channel blocker demonstrated to improve walking ability in patients with multiple sclerosis. This study evaluated the pharmacokinetics of fampridine and its metabolites after administration of fampridine-SR 10 mg in healthy volunteers and in subjects with mild, moderate, or severe renal impairment (5 per group). Analysis of variance was used to calculate 90% confidence intervals (CIs) for the ratios (impaired/healthy) of least squares mean in maximum plasma concentration (C(max)) and area under the plasma concentration-time curve (AUC). Clearance was primarily through urinary excretion. In renally impaired subjects, fampridine plasma concentrations were consistently higher than in healthy individuals: ratios for C(max) ranged from 166.5% to 199.9% for mild and severe renal impairment, respectively. AUC(0-infinity) ratios ranged from 175.3% to 398.7%, respectively, for mild and severe renal impairment. Mean terminal disposition half-life was 6.4 hours in healthy individuals, compared with 7.4, 8.1, and 14.3 hours in patients with mild, moderate, and severe renal impairment, respectively. Regression analysis confirmed the significant relationship between creatinine clearance and extent of exposure as quantified by AUC for fampridine and its metabolites, suggesting cautious use in patients with mild renal impairment and avoidance in cases of moderate or severe renal impairment.

Spinal cord transection modifies ileal fluid and electrolyte transport in rats

Spinal cord injury (SCI) is associated with severe autonomic changes, including inhibition of gastrointestinal (GI) motility. GI motility changes are known to affect electrolytes transport and these changes have not been adequately studied after SCI. We studied the ileal permeability to fluid and electrolytes in rats submitted to experimental spinal cord transection (SCT), between T4 and T5, throughout the first week after SCT. SCT increased ileal secretion of Na+ (P<0.05) and decreased the Cl(-) absorption during the first week post SCI (P<0.05). Water transport was also significantly altered, leading to increased water secretion following the Na+ gradient. Ileal secretion of K+ was significantly increased 1 and 7 days after spinal cord injury. To our knowledge, the present findings are the first direct evidence that SCT alters ileal electrolyte transport in rats. Further studies are necessary to evaluate the mechanisms involved in this phenomenon.

Effects of long-term 4-aminopyridine therapy on glucose tolerance and glucokinetics in patients with spinal cord injury

STUDY OBJECTIVE

To assess the effects of the potassium channel blocker, 4-aminopyridine, on glucose tolerance and glucokinetics in patients with spinal cord injury.

DESIGN

Prospective, dose level-blinded study.

SETTING

University-affiliated, tertiary-level care, Veterans Affairs medical center, and a university-affiliated research center.

PATIENTS

Thirty-one patients with spinal cord injury of more than 1 year's duration.

INTERVENTION

In a fasting state, patients ingested 75 g of glucose and completed a 5-hour oral glucose tolerance test before and after 6 months of treatment with an oral, immediate-release formulation of 4-aminopyridine.

MEASUREMENTS AND MAIN RESULTS

The time course of glucose plasma concentrations during the oral glucose tolerance tests was profiled for each patient, and glucokinetic parameters were estimated. Results were compared at baseline and after 6 months of treatment with 4-aminopyridine. Of the 31 patients, 29 (94%) had impaired glucose tolerance at baseline. After 6 months of treatment with 4-aminopyridine, 12 (41%) of the 29 patients had a 2-hour postprandial glucose level that no longer supported a diagnosis of impaired glucose tolerance. No significant changes or clinically important trends were seen in fasting blood glucose concentrations or in other glucokinetic parameters in these patients.

CONCLUSIONS

The long-term administration of an oral, immediate-release formulation of 4-aminopyridine to patients with longstanding spinal cord injury was associated with readily discernible, potentially clinically significant improvements in glucose tolerance. Because impaired glucose tolerance is a common finding in patients with spinal cord injury, more research, including randomized controlled trials with large study populations, is warranted on this potential treatment.

Fampridine-SR for multiple sclerosis and spinal cord injury

Fampridine-SR is a sustained-release tablet form of the K(+) channel-blocking compound 4-aminopyridine that has been shown to restore conduction in focally demyelinated axons, to enhance synaptic transmission in many types of neurons and to potentiate muscle contraction. The present review describes the mechanism of action and chemistry of Fampridine-SR, its pharmacokinetics and safety, and the outcomes of clinical trials of its safety and efficacy for enhancing neuromuscular function in patients with multiple sclerosis or spinal cord injury. Randomized clinical trials completed to date indicate that this form of K(+) channel blockade may be useful for the improvement of walking ability in patients with multiple sclerosis.

Guidelines for the conduct of clinical trials for spinal cord injury as developed by the ICCP panel: clinical trial design

The International Campaign for Cures of Spinal Cord Injury Paralysis established a panel tasked with reviewing the methodology for clinical trials for spinal cord injury (SCI), and making recommendations on the conduct of future trials. This is the fourth of four papers. Here, we examine the phases of a clinical trial program, the elements, types, and protocols for valid clinical trial design. The most rigorous and valid SCI clinical trial would be a prospective double-blind randomized control trial utilizing appropriate placebo control subjects. However, in specific situations, it is recognized that other trial procedures may have to be considered. We review the strengths and limitations of the various types of clinical trials with specific reference to SCI. It is imperative that the design and conduct of SCI clinical trials should meet appropriate standards of scientific inquiry to insure that meaningful conclusions about efficacy and safety can be achieved and that the interests of trial subjects are protected. We propose these clinical trials guidelines for use by the SCI clinical research community.

An Appraisal of Ongoing Experimental Procedures in Human Spinal Cord Injury

Clinicians and scientists in the field of spinal cord injury research and medicine are poised to begin translating promising new experimental findings into treatments for people. Advances in experimental regeneration research have led to several transplantation strategies that promote axonal regrowth and partial functional recovery in animal models of injury. In this review, we summarize current knowledge regarding various invasive experimental treatments that have been or are now being applied clinically. Various questions about the timeliness, safety, and benefits of the procedures are under discussion within the spinal cord injury (SCI) research community. We also describe guidelines for carrying out optimal clinical trials and efforts to establish specific international guidelines to translate preclinical treatment strategies into clinical trials in SCI. The clinical trial process and the role that clinical professionals have in advising individuals regarding participation in experimental procedures also is discussed.

Immunoactivation and Altered Intercellular Communication Mediate the Pathophysiology of Spinal Cord Injury

Evidence and inferences from clinical research, clinical observation, and literature review support an etiologic paradigm for the pathophysiology of spinal cord injury (SCI). According to this paradigm, changes in immunoregulation and in the activation of cytokines or intercellular adhesion molecules (ICAMs) contribute to many of the comorbidities, metabolic changes, and pathophysiologic sequelae observed after traumatic SCI. Cytokines and ICAMs are endogenously secreted molecules that serve as intercellular signals and immunoregulators. They modulate the activity of cells and influence the organization and function of tissues or organs. These intercellular signals are posited as molecular links between the damaged, decentralized nervous system of SCI and the acquired autonomic failure, neuroendocrine-immunoregulatory dysfunction, diminished central nervous system (CNS) regenerative capacity, and broad spectrum of pathology, organ failure, and generalized impairment of homeostasis caused by trauma to the spinal cord. These highly bioactive molecules may also mediate or facilitate the intralesional CNS axonal damage and peripheral neurologic deficits sustained at time of acute CNS injury. Ultimately, it should be possible to develop treatments that will block or modulate the local and systemic expression of cytokine or ICAM bioactivity. Such treatments might aid victims of SCI by diminishing overall morbidity or mortality, helping restore sensorimotor function and homeostasis, and enhancing longevity and quality of life.

Effect of 4-aminopyridine on gait in ambulatory spinal cord injuries: a double-blind, placebo-controlled, crossover trial

Animal and human research have shown that the drug 4-aminopyridine (4-AP) may improve gait in spinal cord lesions by enhancing nerve transmission to affected muscles.

STUDY DESIGN

Prospective, randomized, double-blind, placebo-controlled, crossover trial.

OBJECTIVES

To determine the efficacy of 4-AP in improving lower limb muscle strength and biomechanical gait patterns of chronic spinal cord injuries (SCI).

SETTING

The Rehabilitation Centre (Ottawa, Canada).

METHODS

In all, 15 chronic, ambulatory SCI persons were randomized to an initial 2 weeks of 40 mg/day, oral medication of either placebo or immediate-release, 4-AP and subsequently crossed over to the alternate medication for the following 2 weeks. Evaluations were conducted at baseline (before starting 4-AP or placebo medication), 2 weeks, and 4 weeks. Measures included dynamometer lower limb isometric muscle force and biomechanical gait measures including temporal-spatial parameters, electromyographic activation patterns, joint kinematics and kinetics. Subjective impressions of the drug by the participants were obtained from an exit survey.

RESULTS

Despite some positive comments from subjects, statistical and clinical analyses showed no within-subject differences between placebo and 4-AP measures of lower limb muscle force and objective gait analyses (ANOVA statistic P>0.05).

CONCLUSION

Results demonstrated the importance of placebo-controlled trials and quantitative outcome measures for the evaluation of 4-AP aimed to enhance gait for chronic, ambulatory SCI persons. Energy expenditure measures and mood may relate more to subjective comments and is suggested for future investigations.

Pharmacokinetics and safety of multiple oral doses of sustained-release 4-aminopyridine (fampridine-sr) in subjects with chronic, incomplete spinal cord injury 11A commercial party with a direct financial interest in the results of the research supporting this article has conferred or will confer a financial benefit on the author or 1 or more of the authors

OBJECTIVE

To examine the pharmacokinetics and safety of sustained-release 4-aminopyridine (Fampridine-SR), a potassium channel blocker, in subjects with chronic, incomplete spinal cord injury (SCI).

DESIGN

Open-label.

SETTING

Clinical research unit in Ontario.

PARTICIPANTS

Sixteen neurologically stable subjects with chronic, incomplete SCI (American Spinal Injury Association Impairment Scale grade B, C, or D).

INTERVENTION

Oral administration of Fampridine-SR (25, 30, 35, 40, 50, 60 mg twice daily, each for 1 wk).

MAIN OUTCOME MEASURES

Steady-state pharmacokinetic parameters: maximum observed plasma concentration (Cmax), minimum observed plasma concentration (Cmin), average observed plasma concentration (Cav), area under the plasma concentration-time curve from 0 to 12 hours (AUC(0-12)), time to Cmax (tmax), plasma half-life (t(1/2)), apparent volume of distribution (Vd/F), and apparent total clearance (Cl/F). Safety assessments: physical examinations, vital sign measurements, clinical laboratory tests, electrocardiogram recordings, and adverse events.

RESULTS

Mean steady-state Cmax, Cmin, Cav, and AUC(0-12) increased over the entire Fampridine-SR dosage range and were dosage dependent up to 50 mg twice daily. Fampridine-SR had a mean tmax of 2.2 to 3.0 hours and a mean t(1/2) of 5.7 to 6.9 hours. Mean Vd/F (415.4-528.0 L) and Cl/F (51.4-57.7 L/h) were independent of dosage, as were mean tmax and t(1/2) across dosages. Adverse events were mild or moderate and were not dosage related. During the entire study period (17 wk), dizziness was the most frequently reported adverse event, followed by urinary tract infection, paresthesia, ataxia, and insomnia.

CONCLUSION

In subjects with chronic, incomplete SCI, Fampridine-SR was slowly absorbed and eliminated, which will allow Fampridine-SR to be administered in a convenient twice-daily manner. Fampridine-SR was well tolerated at dosages from 25 to 60 mg twice daily.

Low micromolar concentrations of 4-aminopyridine facilitate fictive locomotion expressed by the rat spinal cord in vitro

Upregulating the operation of spinal locomotor networks is one mechanism to restore, at least partially, lesion-impaired locomotion. We investigated if the K+ channel blocker 4-aminopyridine (4-AP) could facilitate spinal locomotor networks in addition to its well-known effect on motor nerve conduction. Fictive locomotor patterns were recorded from ventral roots (VRs) of the isolated spinal cord of the neonatal rat. 4-AP (0.1-50 microM) produced synchronous VR oscillations which did not develop into fictive locomotion. These oscillations had network origin, required intact glutamatergic transmission and were probably amplified via electrotonic coupling because of their depression by the selective gap junction blocker carbenoxolone. 4-AP (5 microM) slightly increased input resistance of lumbar motoneurons without affecting their action or resting potentials. Dorsal root (DR) evoked synaptic responses were enhanced (217 +/- 65%) by 5 microM 4-AP without changes in axon conduction. 4-AP (5 microM) accelerated fictive locomotion induced by N-methyl-d-aspartate (NMDA) and serotonin (5-HT) without altering cycle amplitude and facilitated the onset of fictive locomotion in the presence of sub-threshold concentrations of NMDA and 5-HT. Furthermore, in the presence of 4-AP, weak DR stimuli, previously insufficient to activate locomotor patterns, generated alternating VR discharges. Thus, although 4-AP per se could not directly activate the locomotor network of the spinal cord, it could strongly facilitate the locomotor program initiated by neurochemicals or electrical stimuli. These data suggest that the reported improvement by 4-AP in locomotor activity of spinal-injury patients may include activation of locomotor networks when low concentrations of this drug are administered in coincidence with appropriate stimuli.

The use of 4-aminopyridine (fampridine) in demyelinating disorders

4-Aminopyridine (4-AP or fampridine) is a potassium channel-blocking agent that has been shown to restore conduction in focally demyelinated axons. A sustained-release matrix tablet form of 4-AP (fampridine-SR) is currently undergoing multicenter clinical trials in patients with multiple sclerosis or chronic spinal cord injury. This review describes the pharmacology and mechanisms of action of 4-AP, its pharmacokinetics in human subjects, and the outcomes of clinical trials employing either immediate-release or sustained-release formulations of the drug. The randomized clinical trials that have been completed to date indicate that K+ channel blockade may prove to be a useful strategy for ameliorating central conduction deficits due to demyelination. Diverse neurological gains have been reported for both motor and sensory domains. At the present time, however, the clinical trials have not provided sufficiently robust or definitive evidence of efficacy to gain regulatory approval for the symptomatic management of patients with either multiple sclerosis or spinal cord injury.

Pharmacokinetic studies of single and multiple oral doses of fampridine-SR (sustained-release 4-aminopyridine) in patients with chronic spinal cord injury

Fampridine (4-aminopyridine) is a potassium channel blocking agent that restores conduction in demyelinated axons and improves neurologic function in patients with chronic spinal cord injury (SCI). Based on the pharmacokinetic profile of orally administered fampridine, multiple daily doses (4 or more) would need to be taken to sustain its therapeutic effects. Two studies were conducted to determine the pharmacokinetics and safety profile of an oral, sustained-release (SR) formulation of fampridine (fampridine-SR, 10-25 mg) administered as a single dose (n = 14) and twice daily for 1 week (n = 16) in patients with chronic, incomplete SCI. Mean plasma concentrations and area under the plasma concentration-time curve were proportional to the dose administered, whereas other pharmacokinetic parameters were independent of dose. Fampridine-SR was absorbed slowly (peak plasma concentration shortly after dosing, 2.6-3.7 hours) and eliminated (plasma half-life, 5.6-7.6 hours), and reached steady state after 4 days of twice-daily administration. Fampridine-SR was well tolerated, with only mild to moderate adverse events reported, and no serious adverse events. The extended plasma half-life of fampridine-SR allows convenient twice-daily dosing. Clinical trials designed to assess neurologic and functional improvement using fampridine-SR in patients with chronic SCI are currently underway.

Pharmacokinetics of an immediate-release oral formulation of Fampridine (4-aminopyridine) in normal subjects and patients with spinal cord injury

Plasma concentration profiles of the K+ channel-blocking compound Fampridine were obtained from (1) control subjects (n = 6) following oral administration of doses of 10, 15, 20, and 25 mg and (2) patients with spinal cord injury (SCI) (n = 11) following a single oral dose of 10 mg of an immediate-release formulation. Plasma concentrations were determined using a reversed-phase ion-pair high-performance liquid chromatography (HPLC) assay with ultraviolet light detection employing liquid extraction. The drug was rapidly absorbed with a tmax approximately 1 hour for both groups; tmax was independent of dose. Cmax and AUC0-infinity were linearly related to dose, and t 1/2 was 3 to 4 hours for both groups. There were no obvious differences in the (10-mg) plasma concentration profiles between control subjects and SCI patients. The drug was well tolerated, with only mild and transient side effects of light-headedness, dysesthesias, and dizziness.

Low 4-aminopyridine concentration-induced contraction is mediated by neuronal noradrenaline in canine saphenous vein

4-Aminopyridine (4-AP), a known inhibitor of the voltage-dependent potassium channels, is able to increase the basal tone of different types of blood vessel preparations. In order to determine the efficiency of 4-AP in veins and to clarify its possible mechanism of action, the aim of the present study was to determine the basal tone and release of radio-labelled tissue noradrenaline (NA) after administration of low 4-AP concentrations. Experiments were performed in canine saphenous vein in the absence and presence of functional endothelium. 4-AP (0.012-5 microM) enhanced the basal tone of venous rings without and with endothelium (maximum tone at 5 microM 4-AP: 2.20 +/- 1.29 and 1.3 +/- 0.57 mN, respectively). NA stores of the venous tissue were loaded by adding 1 mM NA to the tissue for 10 min and then washed out. After loading the NA-stores of venous tissue, 4-AP-induced contractions were significantly increased both in the absence and presence of endothelium (maximum tone at 5 microM 4-AP after loading with NA: 10.51 +/- 3.64 and 10.52 +/- 4.69 mN, respectively). Following NA loading, chemical denervation of the endothelium denuded venous preparations by 0.5 mM 6-hydroxydopamine (6-OHDA) completely abolished the contractions evoked by 4-AP. After incubation of the saphenous preparations with 3H-NA, 5 microM 4-AP significantly increased tritium-efflux from the tissue. These results provide evidence for the efficiency of 4-AP on the basal tone of isolated canine saphenous vein when applied in low concentrations. Furthermore, it is suggested that this action of 4-AP may considerably depend on the release of NA from the perivascular nerve endings.

4-aminopyridine influences heart rate variability in long-standing spinal cord injury

Humans with traumatic spinal myelopathy exhibit intralesional conduction block and autonomic failure as pathophysiologic sequelae of their injury. Analysis of heart rate variability (HRV) provides a means of assessing changes in the function of the autonomic nervous system (ANS) and the cardiac sequelae of injury. Thirteen patients with long-standing spinal cord injury (SCI) and 13 able-bodied controls were studied. Each patient received a single 10-mg dose of an immediate release (IR) formulation of 4-aminopyridine (4-AP). Twenty-four hour heart rate (HR) and HRV data were acquired using a Holter ambulatory electrocardiographic (ECG) monitor. Analysis of acquired data was carried out using a minicomputer programmed to separate ECG R-R intervals into frequency patterns that appear as peaks dispersed along a frequency range of 0.0 to 1.0 Hz. Twenty-four hour baseline, pretreatment low-frequency (LF) HRV power was diminished in all patients with SCI compared with able-bodied-controls and was significantly decreased in tetraplegic patients (P = 0.03). This difference in LF HRV power disappeared during the 24 hours immediately after administration of 4-AP, and mean LF HRV power in tetraplegic patients became indistinguishable from LF HRV power in controls. 4-Aminopyridine appears to influence ANS function and LF HRV in humans with long-standing SCI.

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

Study of the clinical pharmacology of SCI has revealed population-specific patterns of drug metabolism and disposition. PD/PK profiles reflect the changed physiology associated with SCI and correlate well with the neurologic or anatomic level and the magnitude and completeness of the injury. The greatest value of SCI PK/PD profiles lies in their use in developing criteria and strategies for the optimal prescribing of drugs and in therapeutic drug monitoring. Patients with SCI, acute or long-standing, comprise a therapeutically unique and distinct population. Rational, efficacious, and cost-effective approaches to drug development and pharmacotherapy in spinal cord-injured patients can only come about when population-specific PK/PD behavior is incorporated early into the drug development process and used to develop safe, effective therapeutic guidelines.