Tolvaptan (Page 4 of 7)

10 OVERDOSAGE

Single oral doses up to 480 mg (8 times the maximum recommended daily dose) and multiple doses up to 300 mg once daily for 5 days have been well tolerated in studies in healthy subjects. There is no specific antidote for tolvaptan intoxication. The signs and symptoms of an acute overdose can be anticipated to be those of excessive pharmacologic effect: a rise in serum sodium concentration, polyuria, thirst, and dehydration/hypovolemia.
No mortality was observed in rats or dogs following single oral doses of 2000 mg/kg (maximum feasible dose). A single oral dose of 2000 mg/kg was lethal in mice, and symptoms of toxicity in affected mice included decreased locomotor activity, staggering gait, tremor and hypothermia.
In patients with suspected tolvaptan overdosage, assessment of vital signs, electrolyte concentrations, ECG and fluid status are recommended. Continue replacement of water and electrolytes until aquaresis abates. Dialysis may not be effective in removing tolvaptan because of its high binding affinity for human plasma protein (>98%).

11 DESCRIPTION

Tolvaptan, a selective vasopressin V ₂ -receptor antagonist in tablets for oral use available in 15 mg or 30 mg strength. Tolvaptan is (±) 4’-[(7-Chloro-2,3,4,5-tetrahydro-5-hydroxy-1H-benzazepin-1-yl) carbonyl]-O-tolu-m-toluidide. The empirical formula is C ₂₆ H ₂₅ ClN ₂ O ₃ . Molecular weight is 448.94. The chemical structure is:

Inactive ingredients include corn starch, croscarmellose sodium, lactose monohydrate, magnesium stearate, methyl alcohol, methylene chloride, microcrystalline cellulose and povidone. 30 mg contains FD&C Blue No # 2/Indigo caramine Aluminum Lake as colorant.

12 CLINICAL PHARMACOLOGY

12.1 Mechanism of Action

Tolvaptan is a selective vasopressin V ₂ -receptor antagonist with an affinity for the V ₂ -receptor that is 1.8 times that of native arginine vasopressin (AVP). Tolvaptan affinity for the V ₂ -receptor is 29 times greater than for the V _1a -receptor. When taken orally, 15 to 60 mg doses of tolvaptan antagonize the effect of vasopressin and cause an increase in urine water excretion that results in an increase in free water clearance (aquaresis), a decrease in urine osmolality, and a resulting increase in serum sodium concentrations. Urinary excretion of sodium and potassium and plasma potassium concentrations are not significantly changed. Tolvaptan metabolites have no or weak antagonist activity for human V ₂ -receptors compared with tolvaptan.

12.2 Pharmacodynamics

In healthy subjects receiving a single dose of tolvaptan tablets 60 mg, the onset of the aquaretic and sodium increasing effects occurs within 2 to 4 hours post-dose. A peak effect of about a 6 mEq increase in serum sodium and about 9 mL/min increase in urine excretion rate is observed between 4 and 8 hours post-dose; thus, the pharmacological activity lags behind the plasma concentrations of tolvaptan. About 60% of the peak effect on serum sodium is sustained at 24 hours post-dose, but the urinary excretion rate is no longer elevated by this time. Doses above 60 mg tolvaptan do not increase aquaresis or serum sodium further. The effects of tolvaptan in the recommended dose range of 15 to 60 mg once daily appear to be limited to aquaresis and the resulting increase in sodium concentration.
Plasma concentrations of native AVP may increase (avg. 2 to 9 pg/mL) with tolvaptan administration.
Cardiac Electrophysiology No prolongation of the QT interval was observed with tolvaptan following multiple doses of 300 mg/day for 5 days.

12.3 Pharmacokinetics

In healthy subjects, the pharmacokinetics of tolvaptan after single doses of up to 480 mg and multiple doses up to 300 mg once daily have been examined. In hyponatremia subjects, single and multiple doses up to 60 mg have been studied.
Absorption
In healthy subjects, peak concentrations of tolvaptan are observed between 2 and 4 hours post-dose. Peak concentrations increase less than dose proportionally with doses greater than 240 mg.
The absolute bioavailability of tolvaptan decreases with increasing doses. The absolute bioavailability of tolvaptan following an oral dose of 30 mg is 56% (range 42 to 80%).
Co-administration of 90 mg tolvaptan with a high-fat meal (~1000 calories, of which 50% are from fat) doubles peak concentrations but has no effect on the AUC of tolvaptan; tolvaptan may be administered with or without food.
Distribution
Tolvaptan binds to both albumin and α1-acid glycoprotein and the overall protein binding is >98%; binding is not affected by disease state. The volume of distribution of tolvaptan is about 3 L/kg. The pharmacokinetic properties of tolvaptan are stereospecific, with a steady-state ratio of the S-(-) to the R-(+) enantiomer of about 3. When administered as multiple once-daily 300 mg doses to healthy subjects or to patients with congestive heart failure or ADPKD, tolvaptan’s accumulation factor is <1.2. There is marked inter-subject variation in peak and average exposure to tolvaptan with a percent coefficient of variation ranging between 30 and 60%.
Metabolism and Elimination
Tolvaptan is metabolized almost exclusively by CYP3A. Fourteen metabolites have been identified in plasma, urine and feces; all but one were also metabolized by CYP3A and none are pharmacodynamically active. After oral administration of radiolabeled tolvaptan, tolvaptan was a minor component in plasma, representing 3% of total plasma radioactivity; the oxobutyric acid metabolite was present at 52.5% of total plasma radioactivity with all other metabolites present at lower concentrations than tolvaptan. The oxobutyric acid metabolite shows a plasma half-life of ~180 h. About 40% of radioactivity was recovered in urine (<1% as unchanged tolvaptan) and 59% in feces (19% as unchanged tolvaptan). Following intravenous infusion, tolvaptan half-life is approximately 3 hours. Following single oral doses to healthy subjects, the estimated half-life of tolvaptan increases from 3 hours for a 15 mg dose to approximately 12 hours for 120 mg and higher doses due to more prolonged absorption of tolvaptan at higher doses; apparent clearance is approximately 4 mL/min/kg and does not appear to change with increasing dose.
Specific Populations
Hyponatremia
In patients with hyponatremia of any origin the clearance of tolvaptan is reduced to about 2 mL/min/kg.
Hepatic Impairment
Moderate or severe hepatic impairment or congestive heart failure decrease the clearance and increase the volume of distribution of tolvaptan, but the respective changes are not clinically relevant. Exposure and response to tolvaptan in subjects with creatinine clearance ranging between 79 and 10 mL/min and patients with normal renal function are not different.
Renal Impairment
In a study in patients with creatinine clearances ranging from 10 to 124 mL/min administered a single dose of 60 mg tolvaptan, AUC and C _max of plasma tolvaptan were less than doubled in patients with severe renal impairment (creatinine clearance <30 mL/min) relative to the controls. The peak increase in serum sodium was 5 to 6 mEq/L, regardless of renal function, but the onset and offset of tolvaptan’s effect on serum sodium were slower in patients with severe renal impairment [see Use in Specific Populations ( 8.7)].
Drug Interaction Studies
Impact of Other Drugs on Tolvaptan
Strong CYP3A Inhibitors
Ketoconazole: Tolvaptan’s C _max and AUC were, respectively, 3.5 times and 5.4 times as high following ketoconazole 200 mg given one day prior to and concomitantly with 30 mg tolvaptan [see Contraindications ( 4), Warnings and Precautions ( 5.5) and Drug Interactions ( 7.1)] .
Moderate CYP3A4 inhibitors
Fluconazole: Fluconazole 400 mg given one day prior and 200 mg given concomitantly produced an 80% and 200% increase in tolvaptan C _max and AUC, respectively.
Grapefruit Juice: Co-administration of grapefruit juice and tolvaptan results in an increase in C _max and AUC of 90% and 60% for tolvaptan, respectively [see Drug Interactions ( 7.1)] .
CYP3A4 Inducers
Rifampin: Rifampin 600 mg once daily for 7 days followed by a single 240 mg dose of tolvaptan decreased both tolvaptan C _max and AUC about 85%.
Other Drugs
Co-administration of lovastatin, digoxin, furosemide, and hydrochlorothiazide with tolvaptan has no clinically relevant impact on the exposure to tolvaptan.
Impact of Tolvaptan on Other Drugs
CYP3A Substrates
Tolvaptan is a weak inhibitor of CYP3A. Co-administration of lovastatin and tolvaptan increases the exposure to lovastatin and its active metabolite lovastatin-β hydroxyacid by factors of 1.4 and 1.3, respectively. This is not a clinically relevant change.
P-gp Substrates
Digoxin: Digoxin 0.25 mg was administered once daily for 12 days. Tolvaptan 60 mg, was co-administered once daily on Days 8 to 12. Digoxin C _max and AUC were increased 30% and 20%, respectively.
Transporter Substrates
Tolvaptan is a substrate of P-gp and an inhibitor of P-gp and BCRP. The oxobutyric acid metabolite of tolvaptan is an inhibitor of OATP1B1 and OAT3. Co-administration of tolvaptan with rosuvastatin (BCRP substrate) did not have a clinically significant effect on rosuvastatin exposure. Rosuvastatin C _max and AUC _t increased 54% and 69%, respectively. Administration of rosuvastatin (OATP1B1 substrate) or furosemide (OAT3 substrate) to healthy subjects with elevated oxobutyric acid metabolite plasma concentrations did not meaningfully alter the pharmacokinetics of rosuvastatin or furosemide.
Other Drugs Co-administration of tolvaptan does not appear to alter the pharmacokinetics of warfarin, furosemide, hydrochlorothiazide, or amiodarone (or its active metabolite, desethylamiodarone) to a clinically significant degree.