CLINICAL PHARMACOLOGY
Mechanism Of Action
Tafamidis is a selective stabilizer of TTR. Tafamidis binds to TTR at the thyroxine binding sites, stabilizing the tetramer and slowing dissociation into monomers, the rate-limiting step in the amyloidogenic process.
Pharmacodynamics
A proprietary TTR stabilization assay was utilized as a pharmacodynamic marker and assessed the stability of
the TTR tetramer ex vivo. The TTR stabilization assay quantifies immunoturbidimetric measurement of the
stable TTR tetramer in plasma pre-and post-treatment with 2-day in vitro denaturation with urea. Using this
proprietary assay, a dose-dependent trend for greater TTR tetramer stabilization is observed for VYNDAQEL
80-mg compared to VYNDAQEL 20-mg. However, the clinical relevance of a higher TTR tetramer
stabilization towards cardiovascular outcomes is not known.
VYNDAQEL stabilized both the wild type TTR tetramer and the tetramers of 14 TTR variants tested clinically
after once-daily dosing. Tafamidis also stabilized the TTR tetramer for 25 variants tested ex vivo.
VYNDAQEL and VYNDAMAX may decrease serum concentrations of total thyroxine, without an
accompanying change in thyroid stimulating hormone (TSH). This reduction in total thyroxine values is
probably the result of reduced thyroxine binding to or displacement from transthyretin (TTR) due to the high
binding affinity of tafamidis to the TTR thyroxine receptor. No corresponding clinical findings consistent with
hypothyroidism have been observed.
Biomarkers associated with heart failure (NT-proBNP and Troponin I) favored VYNDAQEL over placebo.
Cardiac Electrophysiology
At approximately 2.2 times the steady state peak plasma concentration (Cmax) at the recommended dose,
tafamidis does not prolong the QTc interval to any clinically relevant extent.
Pharmacokinetics
No clinically significant differences in steady state Cmax and area under the plasma concentration over time
curve (AUC) of tafamidis were observed for VYNDAMAX 61-mg capsule compared to VYNDAQEL
administered as four 20-mg capsules.
Tafamidis exposure increases proportionally over single (up to 480 mg) or multiple (up to 80 mg) (1 to 6 times
the approved recommended dosage) once daily dosing.
The apparent clearance were similar after single and repeated administration of VYNDAQEL 80 mg.
Absorption
Median tafamidis peak concentrations occurred within 4 hours following dosing.
Effect of Food
No clinically significant differences in the pharmacokinetics of tafamidis were observed following administration of a high fat, high calorie meal.
Distribution
The apparent steady state volume of distribution of tafamidis is approximately 18.5 liters. Plasma protein binding of tafamidis is >99% in vitro. Tafamidis primarily binds to TTR.
Elimination
The mean half-life of tafamidis is approximately 49 hours. The apparent oral clearance of tafamidis is
0.263 L/hr. The degree of drug accumulation at steady state after repeated tafamidis daily dosing is approximately 2.5-fold greater than that observed after a single dose.
Metabolism
The metabolism of tafamidis has not been fully characterized. However, glucuronidation has been observed.
Excretion
After a single oral dose of tafamidis meglumine 20 mg, approximately 59% of the dose was recovered in feces
(mostly as the unchanged drug) and approximately 22% of the dose was recovered in urine (mostly as the
glucuronide metabolite).
Specific Populations
No clinically significant differences in the pharmacokinetics of tafamidis were observed based on age,
race/ethnicity (Caucasian and Japanese) or renal impairment.
Patients With Hepatic Impairment
Patients with moderate hepatic impairment (Child-Pugh Score of 7 to 9) had decreased systemic exposure (approximately 40%) and increased clearance (approximately 68%) of tafamidis compared to healthy subjects. As TTR levels are lower in subjects with moderate hepatic impairment than in healthy subjects, the exposure of tafamidis relative to the amount of TTR is sufficient to maintain stabilization of the TTR tetramer in these patients. No clinically significant differences in the pharmacokinetics of tafamidis were observed in patients with mild hepatic impairment (Child Pugh Score of 5 to 6) compared to healthy subjects. The effect of severe hepatic impairment on tafamidis is unknown.
Drug Interaction Studies
Clinical Studies
No clinically significant differences in the pharmacokinetics of midazolam (a CYP3A4 substrate) or on the
formation of its active metabolite (1-hydroxymidazolam) were observed when a single 7.5-mg dose of
midazolam was administered prior to and after a 14-day regimen of VYNDAQEL 20-mg once daily.
In Vitro Studies
Cytochrome P450 Enzymes
Tafamidis induces CYP2B6 and CYP3A4 and does not induce CYP1A2.
Tafamidis does not inhibit CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP3A4/5 or CYP2D6.
UDP glucuronosyltransferase (UGT)
Tafamidis inhibits intestinal activities of UGT1A1 but neither induces nor inhibits other UDP glucuronosyltransferase (UGT) systemically.
Transporter Systems
Tafamidis inhibits breast cancer resistant protein (BCRP). In vitro studies and model predictions show that tafamidis has a low potential to inhibit organic anion transporters OAT1 and OAT3 at clinically relevant concentrations. Tafamidis did not show a potential to inhibit Multi-Drug Resistant Protein (MDR1) (also known as P-glycoprotein; P-gp), organic cation transporter OCT2, multidrug and toxin extrusion transporters MATE1 and MATE2K and, organic anion transporting polypeptide OATP1B1 and OATP1B3.
Clinical Studies
Efficacy was demonstrated in a multicenter, international, randomized, double-blind, placebo-controlled study in 441 patients with wild type or hereditary ATTR-CM (NCT01994889).
Patients were randomized in a 1:2:2 ratio to receive VYNDAQEL 20 mg (n=88), VYNDAQEL 80 mg (administered as four 20-mg VYNDAQEL capsules) (n=176), or matching placebo (n=177) once daily for 30 months, in addition to standard of care (e.g., diuretics). Treatment assignment was stratified by the presence or absence of a variant TTR genotype as well as baseline disease severity (NYHA Class). Transplant patients were excluded from this study. Table 1 describes the patient demographics and baseline characteristics.
Table 1: Patient Demographics and Baseline Characteristics
Characteristic |
Pooled Tafamidis N=264 |
Placebo N=177 |
Age - years |
Mean (standard deviation) |
74.5 (7.2) |
74.1 (6.7) |
Median (minimum, maximum) |
75 (46, 88) |
74 (51, 89) |
Sex - number (%) |
Male |
241 (91.3) |
157 (88.7) |
Female |
23 (8.7) |
20 (11.3) |
TTR Genotype - number (%) |
ATTRm |
63 (23.9) |
43 (24.3) |
ATTRwt |
201 (76.1) |
134 (75.7) |
NYHA Class - number (%) |
NYHA Class I |
24 (9.1) |
13 (7.3) |
NYHA Class II |
162 (61.4) |
101 (57.1) |
NYHA Class III |
78 (29.5) |
63 (35.6) |
Abbreviations: ATTRm = variant transthyretin amyloid, ATTRwt = wild type transthyretin amyloid |
The primary analysis used a hierarchical combination applying the method of Finkelstein-Schoenfeld (F-S) to all-cause mortality and frequency of cardiovascular-related hospitalizations, which was defined as the number of times a subject was hospitalized (i.e., admitted to a hospital) for cardiovascular-related morbidity. The method compared each patient to every other patient within each stratum in a pair-wise manner that proceeded in a hierarchical fashion using all-cause mortality followed by frequency of cardiovascular-related hospitalizations when patients could not be differentiated based on mortality.
This analysis demonstrated a significant reduction (p=0.0006) in all-cause mortality and frequency of cardiovascular-related hospitalizations in the pooled VYNDAQEL 20-mg and 80-mg groups versus placebo (Table 2).
Table 2: Primary Analysis Using Finkelstein-Schoenfeld (F-S) Method of All-Cause Mortality and Frequency of Cardiovascular-Related Hospitalizations
Primary Analysis |
Pooled VYNDAQEL N=264 |
Placebo N=177 |
Number (%) of Subjects Alive* at Month 30 |
186 (70.5) |
101 (57.1) |
Mean Number of Cardiovascular-related Hospitalizations During 30 months (per patient per year) Among Those Alive at Month 30 |
0.297 |
0.455 |
p-value from F-S Method |
0.0006 |
* Heart transplantation and cardiac mechanical assist device implantation are considered indicators of approaching end stage. As such, these subjects are treated in the analysis as equivalent to death. Therefore, such subjects are not included in the count of “Number of Subjects Alive at Month 30” even if such subjects are alive based on 30 month vital status follow-up assessment. |
Analysis of the individual components of the primary analysis (all-cause mortality and cardiovascular-related hospitalization) also demonstrated significant reductions for VYNDAQEL versus placebo.
The hazard ratio from the all-cause mortality Cox-proportional hazard model for pooled VYNDAQEL versus placebo was 0.70 (95% confidence interval [CI] 0.51, 0.96), indicating a 30% relative reduction in the risk of death relative to the placebo group (p=0.026). Approximately 80% of total deaths were cardiovascular-related in both treatment groups. A Kaplan-Meier plot of time to event all-cause mortality is presented in Figure 1.
Figure 1: All-Cause Mortality*
*Heart transplants and cardiac mechanical assist devices treated as death. Hazard ratio from Cox proportional hazards model with treatment, TTR genotype (variant and wild type), and NYHA baseline classification (NYHA Classes I and II combined and NYHA Class III) as factors.
There were significantly fewer cardiovascular-related hospitalizations with VYNDAQEL compared with placebo with a reduction in risk of 32% corresponding to a Relative Risk Ratio of 0.68 (Table 3).
Table 3: Cardiovascular-Related Hospitalization Frequency
|
Pooled VYNDAQEL N=264 |
Placebo N=177 |
Total (%) Number of Subjects with Cardiovascular-related Hospitalizations |
138 (52.3) |
107 (60.5) |
Cardiovascular-related Hospitalizations per Year* |
0.48 |
0.70 |
Pooled VYNDAQEL vs Placebo Treatment Difference (Relative Risk Ratio)* |
0.68 |
p-value* |
<0.0001 |
*This analysis was based on a Poisson regression model with treatment, TTR genotype (variant and wild type), New York Heart Association (NYHA). Baseline classification (NYHA Classes I and II combined and NYHA Class III), treatment-by-TTR genotype interaction, and treatment-by-NYHA baseline classification interaction terms as factors. |
The treatment effects of VYNDAQEL on functional capacity and health status were assessed by the 6-Minute Walk Test (6MWT) and the Kansas City Cardiomyopathy Questionnaire-Overall Summary (KCCQ-OS) score, respectively. A significant treatment effect favoring VYNDAQEL was first observed at Month 6 and remained consistent through Month 30 on both 6MWT distance and KCCQ-OS score (Figure 2 and Table 4).
Figure 2: Change from Baseline to Month 30 in 6MWT Distance and KCCQ-OS Score
Panel A shows change from Baseline to Month 30 in pooled VYNDAQEL patients compared with placebo patients in 6MWT distance.
Panel B shows change from Baseline to Month 30 in pooled VYNDAQEL patients compared with placebo patients in KCCQ-OS score.
The Kansas City Cardiomyopathy Questionnaire-Overall Summary (KCCQ-OS) score is composed of four domains including Total Symptoms (Symptom Frequency and Symptom Burden), Physical Limitation, Quality of Life, and Social Limitation. The Overall Summary score and domain scores range from 0 to 100, with higher scores representing better health status. All four domains favored pooled VYNDAQEL compared to placebo at Month 30, and demonstrated similar treatment effects to the KCCQ-OS score (Figure 2 and Table 4). The distribution for change from Baseline to Month 30 for KCCQ-OS (Figure 3) shows that the proportion of patients with worse KCCQ-OS scores was lower for the pooled VYNDAQEL-treated group compared to placebo, and the proportion with improved scores was higher (Figure 3).
Figure 3: Histogram of Change from Baseline to Month 30 in KCCQ-Overall Summary Score
Table 4: 6MWT Distance and KCCQ-OS Scores
Endpoints |
Baseline Mean (SD) |
Change from Baseline to Month 30, LS Mean (SE) |
Treatment Difference from Placebo LS Mean (95% CI) |
Pooled
VYNDAQEL
N=264 |
Placebo
N=177 |
Pooled
VYNDAQEL |
Placebo |
6MWT
(meters) |
351
(121) |
353
(126) |
-55
(5) |
-131
(10) |
76
(58, 94) |
KCCQ-OS |
67
(21) |
66
(22) |
-7
(1) |
-21
(2) |
14
(9, 18) |
Abbreviations: 6MWT = 6-Minute Walk Test; KCCQ-OS = Kansas City Cardiomyopathy Questionnaire-Overall Summary; SD = standard deviation; LS = least squares; SE = standard error; CI = confidence interval |
Results from the F-S method represented by win ratio for the combined endpoint and its components (all-cause mortality and frequency of CV-related hospitalization) consistently favored VYNDAQEL versus placebo across all subgroups (wild type, variant and NYHA Class I & II, and III), except for CV-related hospitalization frequency in NYHA Class III (Figure 4). Win ratio is the number of pairs of VYNDAQEL-treated patient “wins” divided by number of pairs of placebo patient “wins.” Analyses of 6MWT and KCCQ-OS also favored VYNDAQEL relative to placebo within each subgroup.
Figure 4: Results by Subgroup, Dose, and Components of Primary Analysis
Abbreviations: ATTRm = variant transthyretin amyloid, ATTRwt = wild type transthyretin amyloid, F-S = Finkelstein Schoenfeld, CI = Confidence Interval
*F-S results presented using win ratio (based on all-cause mortality and frequency of cardiovascular hospitalization)
Heart transplants and cardiac mechanical assist devices treated as death.
Results of the primary analysis, 6MWT at Month 30 and KCCQ-OS at Month 30 were statistically significant for both the 80-mg and 20-mg doses of VYNDAQEL vs. placebo, with similar results for both doses.