Tizanidine

Absorption

Absorption and Distribution
Following oral administration, tizanidine is essentially completely absorbed. The absolute oral bioavailability of tizanidine is approximately 40% (CV = 24%), due to extensive first-pass hepatic metabolism. Tizanidine is extensively distributed throughout the body with a mean steady state volume of distribution of 2.4 L/kg (CV = 21%) following intravenous administration in healthy adult volunteers. Tizanidine is approximately 30% bound to plasma proteins.
Differences between Tizanidine Capsules and Tizanidine Tablets
Tizanidine capsules and tizanidine tablets are bioequivalent to each other under fasting conditions, but not under fed conditions. A single dose of either two 4 mg tablets or two 4 mg capsules was administered under fed and fasting conditions in an open label, four period, randomized crossover study in 96 human volunteers, of whom 81 were eligible for the statistical analysis. Following oral administration of either the tablet or capsule (in the fasted state), peak plasma concentrations of tizanidine occurred 1 hours after dosing with a half-life of approximately 2 hours. When two 4 mg tablets were administered with food, the mean maximal plasma concentration was increased by approximately 30%, and the median time to peak plasma concentration was increased by 25 minutes, to 1 hour and 25 minutes. In contrast, when two 4 mg capsules were administered with food, the mean maximal plasma concentration was decreased by 20%, the median time to peak plasma concentration was increased 2 to 3 hours.
Consequently, the mean C
maxfor the capsule when administered with food is approximately 66% the C
maxfor the tablet when administered with food.
Food also increased the extent of absorption for both the tablets and capsules. The increase with the tablet (~30%) was significantly greater than with the capsule (~10%). Consequently when each was administered with food, the amount absorbed from the capsule was about 80% of the amount absorbed from the tablet. Administration of the capsule contents sprinkled on applesauce was not bioequivalent to administration of an intact capsule under fasting conditions. Administration of the capsule contents on applesauce resulted in a 15% to 20% increase in C
maxand AUC of tizanidine and a 15 minute decrease in the median lag time and time to peak concentration compared to administration of an intact capsule while fasting.
Figure 1: Mean Tizanidine Concentration vs. Time Profiles For Tizanidine Tablets and Capsules (2 × 4 mg) Under Fasted and Fed Conditions
Metabolism and Excretion
Tizanidine has linear pharmacokinetics over the doses studied in clinical development (1 to 20 mg). Tizanidine has a half-life of approximately 2.5 hours (CV=33%). Approximately 95% of an administered dose is metabolized. The primary cytochrome P450 isoenzyme involved in tizanidine metabolism is CYP1A2. Tizanidine metabolites are not known to be active; their half-lives range from 20 to 40 hours.
Following single and multiple oral dosing of
14C-tizanidine, an average of 60% and 20% of total radioactivity was recovered in the urine and feces, respectively.
Special Populations
Age Effects
No specific pharmacokinetic study was conducted to investigate age effects. Cross study comparison of pharmacokinetic data following single dose administration of 6 mg tizanidine hydrochloride showed that younger subjects cleared the drug four times faster than the elderly subjects. Tizanidine hydrochloride has not been evaluated in children. [see
Use in Specific Populations (
8.4,
8.5)
]
Hepatic Impairment
The influence of hepatic impairment on the pharmacokinetics of tizanidine has not been evaluated. Because tizanidine is extensively metabolized in the liver, hepatic impairment would be expected to have significant effects on pharmacokinetics of tizanidine. Tizanidine hydrochloride is not recommended in this patient population [see Use in Specific Populations
(8.7)]
Renal Impairment
Tizanidine clearance …