fluvoxamine and tizanidine

Tizanidine, one of the few oral antispastic therapies approved for use in the USA, has a narrow therapeutic index that can often make optimal patient dosing difficult. We surveyed the published literature for data on potential tizanidine dose relationships to pharmacokinetics, drug safety and effectiveness, as well as to provide practical drug dosing advice.. The number of primary studies that describe tizanidine dose proportionality relationships was somewhat limited, even when including studies that used doses above those currently recommended or data from drug-drug interaction studies that resulted in supra-therapeutic tizanidine concentrations.. There is substantial evidence to show that plasma tizanidine concentrations are linearly related to dose in healthy subjects and patients, although there is a high degree of intersubject variability. The most common adverse events and pharmacodynamic effects are related to plasma concentrations. The clinical implications of the large interpatient variability in plasma tizanidine concentrations and its narrow therapeutic index make it necessary to individualise patient therapy. Practical advice on tizanidine dosing and/or switching between formulations is provided.

Topics: Chemistry, Pharmaceutical; Ciprofloxacin; Clonidine; Dose-Response Relationship, Drug; Drug Interactions; Fluvoxamine; Humans; Multiple Sclerosis; Muscle Relaxants, Central; Treatment Outcome

Our objective was to study the effect of fluvoxamine on the pharmacokinetics and pharmacodynamics of tizanidine, a centrally acting skeletal muscle relaxant.. In a double-blind, randomized, 2-phase crossover study, 10 healthy volunteers took 100 mg fluvoxamine or placebo orally once daily for 4 days. On day 4, each ingested a single 4-mg dose of tizanidine. Plasma concentrations of tizanidine and fluvoxamine and pharmacodynamic variables were measured. A caffeine test was performed on day 3 to examine the role of cytochrome P450 (CYP) 1A2 in tizanidine pharmacokinetics.. On average, fluvoxamine increased the total area under the concentration-time curve [AUC(0- infinity )] of tizanidine 33-fold (range, 14-fold to 103-fold; P =.000002) and the peak plasma concentration 12-fold (range, 5-fold to 32-fold; P =.000001). The mean elimination half-life of tizanidine was prolonged from 1.5 to 4.3 hours (P =.00004) by fluvoxamine. The AUC(0- infinity ) of tizanidine and its increase by fluvoxamine correlated with the caffeine/paraxanthine ratio and its increase, respectively (P <.03). All pharmacodynamic variables revealed a significant difference between the fluvoxamine and placebo phases, eg, in the maximal effects on systolic blood pressure (-35 mm Hg, P =.000009), diastolic blood pressure (-20 mm Hg, P =.00002), heart rate (-4 beats/min, P =.007), Digit Symbol Substitution Test (P =.0003), subjective drug effect (P =.0000001), and drowsiness (P =.0002). In particular, the decrease in systolic blood pressure, to the level of 80 mm Hg or even less, was an alarming finding.. Fluvoxamine seriously affects the pharmacokinetics of tizanidine and increases the intensity and duration of its effects. Inhibition of tizanidine-metabolizing enzyme(s), mainly CYP1A2, by fluvoxamine seems to explain the observed interaction. Because of the potentially hazardous consequences, the concomitant use of tizanidine with fluvoxamine, or other potent inhibitors of CYP1A2, should be avoided.

Topics: Administration, Oral; Adult; Analysis of Variance; Area Under Curve; Clonidine; Cross-Over Studies; Cytochrome P-450 Enzyme System; Dose-Response Relationship, Drug; Double-Blind Method; Drug Administration Schedule; Drug Incompatibility; Drug Interactions; Fluvoxamine; Half-Life; Humans; Male; Muscle Relaxants, Central; Probability; Risk Assessment

Tizanidine is primarily metabolized via cytochrome P450 (CYP) 1A2 and therefore medications that inhibit the enzyme will affect the clearance of tizanidine, leading to increased plasma concentrations of tizanidine and potentially serious adverse events.. Our aim was to study the occurrence of adverse events reported in the FDA Adverse Event Reporting System (FAERS) involving the combination of tizanidine and drugs that inhibit the metabolic activity of CYP1A2.. A disproportionality analysis of FAERS reports from 2004 quarter 1 through 2020 quarter 3 was conducted to calculate the reporting odds ratio (ROR) of reports mentioning tizanidine in a suspect or interacting role or having any role, a CYP1A2 inhibitor, and the following adverse events: hypotension, bradycardia, syncope, shock, cardiorespiratory arrest, and fall or fracture.. A total of 89 reports were identified mentioning tizanidine, at least one CYP1A2 inhibitor, and one of the adverse events of interest. More than half of the reports identified tizanidine as having a suspect or interacting role (n = 59, 66.3%), and the reports more frequently involved women (n = 58, 65.1%). The median age was 56.1 years (standard deviation 17.1). Some of the important safety signals included interactions between tizanidine in a suspect or interacting role and ciprofloxacin (ROR for hypotension 28.1, 95% confidence interval [CI] 19.2-41.2) or fluvoxamine (ROR for hypotension 36.9, 95% CI 13.1-103.4), and also when reported in "any role" with ciprofloxacin (ROR for hypotension 6.3, 95% CI 4.7-8.5), fluvoxamine (ROR for hypotension 11.4, 95% CI 4.5-28.8), and zafirlukast (ROR for falls 16.0, 95% CI 6.1-42.1).. Reports involving tizanidine and a CYP1A2 inhibitor have higher odds of reporting hypotension. This study suggests that concurrent use of tizanidine with CYP1A2 inhibitors may lead to serious health consequences associated with low blood pressure such as falls and fractures.

Topics: Adverse Drug Reaction Reporting Systems; Ciprofloxacin; Clonidine; Cytochrome P-450 CYP1A2; Cytochrome P-450 CYP1A2 Inhibitors; Drug Interactions; Female; Fluvoxamine; Humans; Hypotension; Male; Middle Aged; United States; United States Food and Drug Administration

Topics: Adrenergic alpha-Agonists; Aged; Antidepressive Agents, Second-Generation; Body Temperature; Cerebral Infarction; Clonidine; Drug Interactions; Female; Fluvoxamine; Heart Rate; Humans; Male; Middle Aged; Retrospective Studies