The precise mechanism by which XADAGO exerts its therapeutic effects in PD is unknown. XADAGO is an inhibitor of monoamine oxidase B (MAO-B). Inhibition of MAO-B activity, by blocking the catabolism of dopamine, is thought to result in an increase in dopamine levels and a subsequent increase in dopaminergic activity in the brain.
XADAGO inhibits monoamine oxidase B (MAO-B), with more than 1000-fold selectivity over MAO-A. In clinical studies, complete inhibition (>90%) of MAO-B was measured at doses > 20 mg.
Tyramine Challenge Test
In an oral tyramine challenge study, XADAGO produced a distinct but relatively small increase in tyramine sensitivity to increase blood pressure. The results suggest that XADAGO at a dose of 50 mg or 100 mg is relatively selective for inhibiting MAO-B and can be used without dietary tyramine restriction. Relative selectivity of XADAGO for inhibiting MAO-B decreases above the highest recommended daily dosage (100 mg) [see Warnings and Precautions (5.1) and Drug Interactions (7.6)].
The effect of XADAGO on the QTc interval was evaluated in a randomized placebo and positive controlled double-blind, multiple-dose parallel thorough QTc study in 240 healthy subjects. At a dose of 350 mg (3.5 times the maximum recommended dosage), XADAGO did not prolong the QTc interval.
Pharmacokinetics of safinamide is linear over a range of 50 mg to 300 mg (3 times the maximum recommended daily dose). Steady state is reached within 5 to 6 days.
After single and multiple oral dosing under fasting conditions, Tmax of safinamide ranges from 2 to 3 hours. Absolute bioavailability of safinamide is 95% after oral administration, and first pass metabolism is negligible. A slight delay in Tmax was observed in the fed state relative to the fasted condition, but there was no effect on safinamide AUC0−∞ and Cmax [see Dosage and Administration (2.1)].
The volume of distribution (Vss) is approximately 165 L, indicating extensive extravascular distribution. Safinamide is not highly protein bound (unbound fraction is 11 to 12%).
Metabolism and Excretion
In humans, safinamide is almost exclusively eliminated via metabolism (~5% of the drug is eliminated unchanged, mainly in urine), through three main metabolic pathways. One pathway involves hydrolytic oxidation of the amide moiety leading to the primary metabolite ‘safinamide acid’ (NW-1153). Another pathway is oxidative cleavage of the ether bond forming ‘O- debenzylated safinamide’ (NW-1199). Finally, the ‘N-dealkylated acid’ (NW-1689) is formed by oxidative cleavage of the amine bond of either safinamide or the primary safinamide acid metabolite (NW-1153). The ‘N-dealkylated acid’ (NW-1689) undergoes further conjugation with glucuronic acid yielding its acyl glucuronide. NW-1689 is the main circulating metabolite in human plasma, exceeding the exposure of the parent (161% of parent). NW-1689 AG and NW-1153 account for about 18% and 11% of the parent drug exposure, respectively. None of the metabolites has pharmacological activity.
Safinamide is predominantly metabolized by non-microsomal enzymes (cytosolic amidases/MAO-A); CYP3A4 and other CYP iso-enzymes play only a minor role in its overall biotransformation.
The total clearance of safinamide was determined to be 4.6 L/h. Terminal half-life is 20-26 h. The primary route of excretion is through the kidney (76% of safinamide dose recovered in the urine, primarily in the form of inactive metabolites).
Age: Geriatric Population: There are limited clinical data on the use of XADAGO in the elderly (>75 years). These data suggest that the pharmacokinetics of safinamide is not affected by age [see Use in Specific Populations (8.5)].
Race: The pharmacokinetics of safinamide is not influenced by race.
Sex: The pharmacokinetics of safinamide is not influenced by sex.
Hepatic Impairment: The disposition of XADAGO was assessed in subjects with mild and moderate hepatic impairment and compared with subjects with normal hepatic function. A marginal increase in the exposure of safinamide (approximately 30% increase in AUC) was observed in subjects with mild hepatic impairment (Child-Pugh A). In subjects with moderate hepatic impairment (Child-Pugh B), exposure to safinamide was increased by about 80% (CI: 154-215%) [see Dosage and Administration (2.2) and Use in Specific Populations (8.6)]. XADAGO has not been studied in patients with severe hepatic impairment (Child-Pugh C) [see Contraindications (4)].
Renal Impairment: The effect of renal impairment on safinamide pharmacokinetics was investigated in an open-label, parallel-group, single oral dose study in subjects with moderate renal impairment, severe renal impairment, or normal renal function. The pharmacokinetics of safinamide was not affected by impaired renal function.
Drug Interaction Studies
In Vitro Studies: In vitro metabolism studies indicate no meaningful inhibition or induction of Cytochrome P450 (CYP) based enzymes by safinamide and its major metabolites at concentrations that are relevant for dosing. Safinamide or its major metabolites at clinically relevant concentrations are not inhibitors of MAO-A, levodopa decarboxylase or aldehyde dehydrogenase enzymes.
Safinamide is not a substrate of P-gp. Safinamide and its metabolites did not inhibit P-gp or other transporters OCT2, OATP1B1, OATP1B3, BSEP, OAT1/3/4.
In Vivo Studies: Dedicated drug-drug interactions studies conducted with ketoconazole, levodopa (LD), BCRP substrate (rosuvastatin), and CYP1A2 and CYP3A4 substrates (caffeine and midazolam, respectively) did not demonstrate any clinically significant effects on the pharmacokinetic profile of XADAGO, or on the pharmacokinetic profile of co-administered levodopa, rosuvastatin, or CYP1A2 and CYP3A4 substrates.
In carcinogenicity studies in mice and rats, safinamide was administered at oral doses of 0, 50, 100 and 200 mg/kg/day, and 0, 25, 50 and 100 mg/kg/day, respectively, for 2 years. The highest doses tested in both species were approximately 10 times the maximum recommended human dose (MRHD) of 100 mg/day on a body surface area (mg/m2) basis. No evidence of tumorigenic potential was observed in either species.
Safinamide was negative for genotoxicity in in vitro (Ames, mouse lymphoma) and in vivo (mouse micronucleus) assays.
Impairment of Fertility
In a rat fertility study in which males and females were orally administered safinamide (0, 50, 100, 150 mg/kg/day) prior to and during mating and continuing through early pregnancy in females, adverse effects on reproductive function were observed in both males (sperm abnormalities) and females (decreased corpora lutea, increased pre-implantation loss). The no-effect dose for reproductive toxicity (50 mg/kg/day) is approximately 5 times the MRHD on a mg/m2 basis.
Retinal Pathology in Rats
Degeneration and loss of photoreceptor cells were observed in the retina of both albino and pigmented rats at plasma exposures lower than that in humans at the maximum recommended human dose of 100 mg/kg/day. The findings were dose- and time-dependent and progressed from minimal loss to severe outer nuclear cell layer loss after one year of oral dosing with safinamide. In a two year study, total retinal atrophy and scarring and lens opacities (cataracts) were seen at all oral doses tested (0, 25, 50, and 100 mg/kg/day).
In a study in rats dosed orally with safinamide alone or in combination with pramipexole, pramipexole, at a dose (25 mg/kg/day) that did not cause retinal changes, exacerbated the retinal pathology caused by safinamide alone (50 mg/kg/day) in both pigmented and albino rats.
Investigative studies were not able to identify a mechanism underlying the retinal toxicity; the relevance to humans is unknown.
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