Rivastigmine Hydrogen Tartrate 0.5 mg · Hard-gelatin capsules

Although the precise mechanism of action of rivastigmine is unknown, it is thought to exert its therapeutic effect by enhancing cholinergic function. This is accomplished by increasing the concentration of acetylcholine through reversible inhibition of its hydrolysis by cholinesterase. Therefore, the effect of rivastigmine may lessen as the disease process advances and fewer cholinergic neurons remain functionally intact. There is no evidence that rivastigmine alters the course of the underlying dementing process.

Rivastigmine shows linear pharmacokinetics up to 3 mg twice a day but is nonlinear at higher doses. Doubling the dose from 3 mg to 6 mg twice a day results in a 3-fold increase in area under the curve (AUC). The elimination half-life is about 1.5 hours, with most elimination as metabolites via the urine.

Absorption — Rivastigmine is rapidly and completely absorbed. Peak plasma concentrations are reached in approximately 1 hour. Absolute bioavailability after a 3 mg dose is about 36%. Administration of rivastigmine tartrate with food delays absorption (Tmax) by 90 minutes lowers Cmax by approximately 30% and increases AUC by approximately 30%.

Distribution — Rivastigmine is weakly bound to plasma proteins (approximately 40%) over the therapeutic range. It readily crosses the blood-brain barrier, reaching CSF peak concentrations in 1.4 to 2.6 hours. It has an apparent volume of distribution (VD) in the range of 1.8 to 2.7 L/kg.

Metabolism — Rivastigmine is rapidly and extensively metabolized, primarily via cholinesterase-mediated hydrolysis to the decarbamylated metabolite. Based on evidence from in vitro and animal studies, the major cytochrome P450 isozymes are minimally involved in rivastigmine metabolism. Consistent with these observations is the finding that no drug interactions related to cytochrome P450 have been observed in humans.

Elimination — The major pathway of elimination is via the kidneys. Following administration of 14C-rivastigmine to 6 healthy volunteers, total recovery of radioactivity over 120 hours was 97% in urine and 0.4% in feces. No parent drug was detected in urine. The sulfate conjugate of the decarbamylated metabolite is the major component excreted in urine and represents 40% of the dose. Mean oral clearance of rivastigmine is 1.8 ± 0.6 L/min after 6 mg twice a day.

Age — Following a single 2.5 mg oral dose to elderly volunteers (60 years and older, n = 24) and younger volunteers (n = 24), mean oral clearance of rivastigmine was 30% lower in elderly (7 L/min) than in younger subjects (10 L/min).

Gender and Race — Population pharmacokinetic analysis of oral rivastigmine indicated that neither gender (n = 277 males and 348 females) nor race (n = 575 Caucasian, 34 Black, 4 Asian, and 12 Other) affected clearance of the drug.

Body Weight — A relationship between drug exposure at steady-state (rivastigmine and metabolite NAP226-90) and body weight was observed in Alzheimer’s dementia patients. Rivastigmine exposure is higher in subjects with low body weight. Compared to a patient with a body weight of 65 kg, the rivastigmine steady-state concentrations in a patient with a body weight of 35 kg would be approximately doubled, while for a patient with a body weight of 100 kg the concentrations would be approximately halved.

Renal Impairment — Following a single 3 mg dose, mean oral clearance of rivastigmine is 64% lower in moderately impaired renal patients (n = 8, GFR = 10 to 50 mL/min) than in healthy subjects (n = 10, GFR greater than or equal to 60 mL/min); CL/F=1.7 L/min and 4.8 L/min, respectively. In patients with severe renal impairment (n = 8, GFR less than 10 mL/min), mean oral clearance of rivastigmine is 43% higher than in healthy subjects (n = 10, GFR greater than or equal to 60 mL/min); CL/F = 6.9 L/min and 4.8 L/min, respectively. For unexplained reasons, the severely impaired renal patients had a higher clearance of rivastigmine than moderately impaired patients.

Hepatic Impairment — Following a single 3 mg dose, mean oral clearance of rivastigmine was 60% lower in hepatically impaired patients (n = 10, biopsy proven) than in healthy subjects (n = 10). After multiple 6 mg twice a day oral dosing, the mean clearance of rivastigmine was 65% lower in mild (n = 7, Child-Pugh score 5 to 6), and moderate (n = 3, Child-Pugh score 7 to 9) hepatically impaired patients (biopsy proven, liver cirrhosis) than in healthy subjects (n = 10).