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12 CLINICAL PHARMACOLOGY 12.1 Mechanism of Action Solifenacin is a competitive muscarinic receptor antagonist. Muscarinic receptors play an important role in several major cholinergically mediated functions, including contractions of urinary bladder smooth muscle. 12.2 Pharmacodynamics Cardiac Electrophysiology The effect of 10 mg and 30 mg solifenacin succinate (three times the maximum recommended dose) on the QT interval was evaluated at the time of peak plasma concentration of solifenacin in a multi-dose, randomized, double-blind, placebo and positive-controlled (moxifloxacin 400 mg) trial [see Warnings and Precautions ( 5.6 )]. After receiving placebo and moxifloxacin sequentially, subjects were randomized to one of two treatment groups. One group (n=51) completed 3 additional sequential periods of dosing with solifenacin succinate 10, 20, and 30 mg while the second group (n=25) in parallel completed a sequence of placebo and moxifloxacin. Study subjects were female volunteers aged 19 to 79 years. The 30 mg dose of solifenacin succinate (three times the highest recommended dose) was chosen for use in this study because this dose results in a solifenacin exposure that covers those observed upon coadministration of 10 mg solifenacin succinate with strong CYP3A4 inhibitors (e.g., ketoconazole, 400 mg). Due to the sequential dose escalating nature of the study, baseline ECG measurements were separated from the final QT assessment (of the 30 mg dose level) by 33 days. The median difference from baseline in heart rate associated with the 10 and 30 mg doses of solifenacin succinate compared to placebo was -2 and 0 beats/minute, respectively. Because a significant period effect on QTc was observed, the QTc effects were analyzed utilizing the parallel placebo control arm rather than the pre-specified intra-patient analysis. Representative results are shown in Table 2. Table 2: QTc changes in msec (90% CI) from baseline at T max (relative to placebo) 1 Drug/Dose Fridericia method (using mean difference) Solifenacin succinate 10 mg 2 (-3,6) Solifenacin succinate 30 mg 8 (4,13) 1. Results displayed are those derived from the parallel design portion of the study and represent the comparison of Group 1 to time-matched placebo effects in Group 2. Moxifloxacin was included as a positive control in this study and, given the length of the study, its effect on the QT interval was evaluated in 3 different sessions. The placebo-subtracted mean changes (90% CI) in QTcF for moxifloxacin in the three sessions were 11 (7, 14), 12 (8, 17), and 16 (12, 21), respectively. The QT interval prolonging effect of the highest solifenacin succinate dose (three times the maximum therapeutic dose) studied was not as large as that of the positive control moxifloxacin at its recommended dose. However, the confidence intervals overlapped, and this study was not designed to draw direct statistical conclusions between the drugs or the dose levels. 12.3 Pharmacokinetics Absorption After oral administration of solifenacin succinate in healthy volunteers, peak plasma concentrations (C max ) of solifenacin were reached within 3 to 8 hours after administration and, at steady-state, ranged from 32.3 to 62.9 ng/mL for the 5 and 10 mg solifenacin succinate tablets, respectively. The absolute bioavailability of solifenacin is approximately 90%, with plasma concentrations of solifenacin proportional to the dose administered. Effect of Food Solifenacin succinate may be administered without regard to meals. A single 10 mg dose administration of solifenacin succinate with food increased C max and AUC of solifenacin by 4% and 3%, respectively. Distribution Solifenacin is approximately 98% ( in vivo ) bound to human plasma proteins, principally to α 1 -acid glycoprotein. Solifenacin is highly distributed to non-CNS tissues, having a mean steady-state volume of distribution of 600 L. Elimination The elimination half-life (t 1/2 ) of solifenacin following chronic dosing is approximately 45-68 hours. Metabolism Solifenacin is extensively metabolized in the liver. The primary pathway for elimination is by way of CYP3A4; however, alternate metabolic pathways exist. The primary metabolic routes of solifenacin are through N-oxidation of the quinuclidin ring and 4R-hydroxylation of the tetrahydroisoquinoline ring. One pharmacologically active metabolite (4R-hydroxy solifenacin), occurring at low concentrations and unlikely to contribute significantly to clinical activity, and three pharmacologically inactive metabolites (N-glucuronide and the N-oxide and 4R-hydroxy- N-oxide of solifenacin) have been found in human plasma after oral dosing. Excretion Following the administration of 10 mg of 14 C-solifenacin succinate to healthy volunteers, 69% of the radioactivity was recovered in the urine and 23% in the feces over 26 days. Less than 15% (as mean value) of the dose was recovered in the urine as intact solifenacin. The major metabolites identified in urine were N-oxide of solifenacin, 4R-hydroxy solifenacin, and 4R-hydroxy-N-oxide of solifenacin and, in feces, 4R-hydroxy solifenacin. Specific Populations Geriatric Patients Multiple dose studies of solifenacin succinate in geriatric volunteers (65 to 80 years) showed that C max , AUC and t 1/2 values of solifenacin were 20-25% higher compared to the younger adult volunteers (18 to 55 years). [see Use in Specific Populations ( 8.5 ) ] . Patients with Renal Impairment In studies with solifenacin succinate 10 mg, there was a 2.1-fold increase in AUC and a 1.6-fold increase in t 1/2 of solifenacin in patients with severe renal impairment compared to subjects with normal renal function [see Use in Specific Populations ( 8.6 ) ] . Patients with Hepatic Impairment In studies with solifenacin succinate 10 mg, there was a 2-fold increase in the t 1/2 and a 35% increase in AUC of solifenacin in patients with moderate hepatic impairment compared to subjects with normal hepatic function [see Use in Specific Populations ( 8.7 ) ] . Solifenacin succinate has not been studied in patients with severe hepatic impairment. Drug Interaction Studies Strong CYP3A4 Inhibitors In a crossover study, following blockade of CYP3A4 by coadministration of the strong CYP3A4 inhibitor, ketoconazole 400 mg once daily for 21 days, the mean C max and AUC of solifenacin increased by 1.5 and 2.7-fold, respectively [see Dosage and Administration ( 2.4 ) and Drug Interactions ( 7.1 ) ] . CYP3A4 Inducers Because solifenacin is a substrate of CYP3A4, inducers of CYP3A4 may decrease the concentration of solifenacin. Warfarin In a crossover study, subjects received a single oral dose of warfarin 25 mg on the 10 th day of dosing with either solifenacin succinate 10 mg or matching placebo once daily for 16 days. For R -warfarin, when it was coadministered with solifenacin succinate, the mean C max increased by 3% and AUC decreased by 2%. For S -warfarin, when it was coadministered with solifenacin succinate, the mean C max and AUC increased by 5% and 1%, respectively. Oral Contraceptives In a crossover study, subjects received 2 cycles of 21 days of oral contraceptives containing 30 ug ethinyl estradiol and 150 ug levonorgestrel. During the second cycle, subjects received additional solifenacin succinate 10 mg or matching placebo once daily for 10 days starting from the 12 th day of receipt of oral contraceptives. For ethinyl estradiol, when it was administered with solifenacin succinate, the mean C max and AUC increased by 2% and 3%, respectively. For levonorgestrel, when it was administered with solifenacin succinate, the mean C max and AUC decreased by 1%. Digoxin In a crossover study, subjects received digoxin (loading dose of 0.25 mg on day 1, followed by 0.125 mg from days 2 to 8) for 8 days. Consecutively, they received solifenacin succinate 10 mg or matching placebo with digoxin 0.125 mg for an additional 10 days. When digoxin was coadministered with solifenacin succinate, the mean C max and AUC increased by 13% and 4%, respectively. Drugs Metabolized by Cytochrome P450 Enzymes In vitro studies demonstrated that, at therapeutic concentrations, solifenacin does not inhibit CYP1A1/2, 2C9, 2C19, 2D6, or 3A4 derived from human liver microsomes.