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Keywords:

  • drug interaction;
  • lasofoxifene;
  • pharmacodynamics;
  • pharmacokinetics;
  • warfarin

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. References

Aim

To investigate the effect of steady-state lasofoxifene on the pharmacokinetics and pharmacodynamics of warfarin.

Methods

Twelve healthy postmenopausal women received warfarin (single 20-mg dose) alone and during lasofoxifene. R- and S-warfarin concentrations, prothrombin time (PT) and international normalized ratio (INR) were determined with each treatment.

Results

Lasofoxifene had no clinically meaningful effect on R- or S-warfarin pharmacokinetics. The S-warfarin area under the plasma concentration–time curve (AUC) was 23% and 67% larger in subjects with *1/*2 and *1/*3 heterozygous mutations, relative to *1/*1, respectively. The mean PT AUC and Cmax ratio (90% confidence interval) was 91.9 (89.6, 94.2) and 84.2 (80.6, 87.8), respectively. INR results were similar.

Conclusions

Lasofoxifene has no clinically meaningful effect on the pharmacokinetics of warfarin. Although the decrease in PT/INR may not be clinically meaningful, more frequent INR monitoring may be considered during lasofoxifene introduction and discontinuation, consistent with warfarin’s label.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. References

Lasofoxifene is a next-generation selective oestrogen receptor modulator (SERM) that is being investigated for prevention and treatment of osteoporosis in clinical studies [1, 2]. Lasofoxifene is metabolized primarily in the liver with less than 2% of the administered dose excreted unchanged in urine [3]. At steady state, the half-life of lasofoxifene in a postmenopausal female population is approximately 6 days.

Warfarin has a narrow therapeutic index and is involved in drug–drug interactions that lead to meaningful clinical consequences. S-warfarin, which is two to five times more potent than the R-enantiomer, is primarily metabolized by CYP2C9, and less by CYP2C19 and CYP3A4, while R-warfarin is metabolized mainly by CYP1A2, with a lesser contribution by CYP3A4 [4]. Coadministration of warfarin with raloxifene, another SERM, resulted in a small increase in systemic warfarin exposure but a small decrease in anticoagulant response, as measured by prothrombin time (PT), neither of which was thought to be clinically significant [5]. The effect may be related to the mechanism of action of raloxifene.

Lasofoxifene was examined in vitro as a potential inhibitor of the metabolism via different cytochrome (CYP) isoforms including diclophenac as the substrate for CYP2C9. Lasofoxifene did not inhibit any of the CYP tested (CYP1A2, CYP2C19, CYP2C9, CYP2D6 and CYP3A4) at relevant concentrations. Although lasofoxifene is highly protein bound (>99%), in vitro studies have shown that lasofoxifene (50 ng ml−1) does not affect the human plasma protein binding of warfarin (2000 and 10 000 ng ml−1) [6]. This amount of lasofoxifene is far above the anticipated concentration in plasma achieved at therapeutic doses. Although there is no in vitro evidence suggesting a pharmacokinetic drug–drug interaction between lasofoxifene and warfarin, the current study was proposed to confirm this and to investigate the effect of steady-state lasofoxifene administration on the pharmacodynamics [as measured by assessment of the international normalized ratio (INR) and PT ratio] of a single dose of warfarin in healthy postmenopausal women.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. References

Subjects and drug administration

Twelve subjects were enrolled in this open-label, nonrandomized, two-period crossover study. Postmenopausal women who were in good health, between the ages of 40 and 65 years, and with CYP2C9 *1/*1 (wild type), *1/*2 or *1/*3 genotypes, were eligible for participation. The local institutional review board approved the protocol and informed consent documents. Written informed consent was received from each subject who participated in the study prior to study enrolment.

Due to the long half-life of lasofoxifene, treatment sequence was not randomized, with warfarin alone administered first. Each subject received four 5-mg warfarin (DuPont Pharmaceuticals, Wilmington, DE, USA) tablets (20-mg warfarin as a single dose) on day 1 (alone) and day 15 (with lasofoxifene). Lasofoxifene was administered on days 8–21 (4-mg lasofoxifene loading dose on day 8 and 0.5 mg on days 9–21).

Pharmacokinetic and pharmacodynamic assessments

Blood samples were collected for determination of lasofoxifene concentrations (7 ml per sample) before lasofoxifene dosing on days 13, 14 and 15 and for determination of warfarin concentrations (5 ml per sample) before warfarin dosing and at 1, 2, 4, 8, 12, 24, 36, 48, 60, 72, 96, 120, 144 and 168 h after dosing on days 1 and 15. Blood samples were centrifuged and plasma was separated and stored frozen at − 20°C until assayed. Lasofoxifene plasma concentrations were determined using a validated liquid chromatography/tandem mass spectrometry (LC/MS/MS) assay with a lower limit of 0.025 mg ml−1 and an upper limit of 6.00 ng ml−1, with accuracy between 102% and 107%. For both S- and R-warfarin, a validated LC/MS/MS assay with a lower limit of 5.0 ng ml−1 and an upper limit of 1000 ng ml−1 was used. The percent coefficient of variation for S-warfarin was within 2.2% and accuracy was between − 3.5% and − 0.5%; for R-warfarin the percent coefficient of variation was within 2.42% and accuracy was between − 3.0% and 2.5%.

For PT/INR determinations, 1.8 ml of venous blood was taken on the same schedule as for the warfarin assays. PT was measured and calibrated against the international standard using a thromboplastin with an International Sensitivity Index (ISI) value of 1.0 to calculate the INR. If a subject’s INR exceeded 2.3 at any time point, vitamin K was to be administered. In that case, the subject’s pharmacodynamic data were to be excluded from the analyses.

Statistical analysis

Pharmacokinetic parameters were determined from plasma concentration time data for R- and S-warfarin using standard noncompartmental methods. Log-transformed maximum plasma concentration (Cmax) and area under the plasma concentration–time curve (AUC) were the primary parameters analysed. Results from analysis of variance (anova) were used to calculate 90% confidence intervals for the ratio (test/reference) least square mean values where the single 20-mg warfarin dose alone was the reference treatment. Consistent with the Food and Drug Administration’s guidance on in vivo drug interaction studies [7], absence of an interaction of lasofoxifene on R- and S-warfarin pharmacokinetics was concluded if the 90% confidence intervals (CIs) for the treatment ratios of log-transformed Cmax and AUC values for both R- and S-warfarin were entirely within the 80–125% range. Pharmacokinetic and statistical analyses were conducted using WinNonlin Pro Version 3.2 (Pharsight Corp, Mountain View, CA, USA).

Pharmacodynamic parameters including PTmax, PT AUC, INRmax, and INR AUC were estimated from PT- and INR-time data using the linear trapezoidal method with no extrapolation (WinNonlin Pro Version 2.1). A similar statistical approach was used for the PT and INR analyses.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. References

Baseline characteristics of the study population included mean age 55.1 ± 3.7 years and mean weight 78.2 ± 11.4 kg; the *1/*1, *1/*2 and *1/*3 CYP 2C9 genotype was identified in eight, two and two subjects, respectively.

Pharmacokinetics and pharmacodynamics

Mean (% coefficient of variation) predose plasma concentrations of lasofoxifene on days 13, 14 and 15 were 3.10 (17.6%), 3.22 (16.4%) and 3.25 (18.5%) ng ml−1, respectively, indicating that steady state had been achieved prior to the administration of single-dose warfarin.

Table 1 shows the pharmacokinetic and pharmacodynamic parameters for both R- and S-warfarin and for PT and INR during warfarin alone and with lasofoxifene. Lasofoxifene had no statistically significant effect on the pharmacokinetics of R- or S-warfarin with the 90% CIs for the ratio of treatment means of both Cmax and AUC0–∞ values within the 80–125% range. The plasma concentration time profiles were almost superimposable for warfarin with or without lasofoxifene (Figure 1A).

Table 1.  Summary of warfarin pharmacokinetic and pharmacodynamic parameters
ParameterLeast squares mean valuesRatio90% confidence interval
Warfarin alone (reference)Warfarin with lasofoxifene (test)
  1. Cmax, Maximum plasma concentration; AUC0–tlqc , area under plasma concentration–time curve from time 0 to the time for the last quantifiable concentration; AUC0– , area under the plasma concentration–time curve from time 0 to infinity; tmax , time to maximum plasma concentration; t1/2, terminal half-life; PTmax , maximum prothrombin time; PT AUC, area under PT–time profile from time 0 postdose to the last time of the study; INRmax , maximum international normalized ratio; INR AUC, area under INR–time profile from time 0 to postdose to the last time of the study.

R-warfarin(N = 12)
Cmax (µg ml−1)   1.53   1.52 99.4 94.5, 105
AUC0–tlqc (µg h−1 ml−1)  69.5  72.1104101, 107
AUC0–∞ (µg h−1 ml−1)  78.0  80.3103 99.9, 106
   Difference 
tmax (h)   1.10   1.27  0.17 
t1/2 (h)   51.2  50.2 −1.0 
S-warfarin (N = 12)
Cmax (µg ml−1)   1.56   1.55 98.9 92.5, 106
AUC0–tlqc (µg h−1 ml−1)  44.5  45.0101 98.3, 106
AUC0–∞ (µg h−1 ml−1)  46.7  47.0101 97.0, 105
   Difference 
tmax (h)   1.10   1.18  0.08 
t1/2 (h)  41.3  38.6 −2.7 
Pharmacodynamics(N = 11)
PTmax (s)  21.22  17.86 84.2 80.6, 87.8
PT AUC (s h−1)2535.82330.0 91.9 89.6, 94.2
INRmax   1.9   1.6 84.3 80.6, 88.1
INR AUC 220 202.4 92.0 89.9, 94.1
image

Figure 1. Mean ± SD R-warfarin and S-warfarin plasma concentration (A) R-warfarin (Alone) (▪), R-warfarin (Combination) (□), S-warfarin (Alone) (●), S-warfarin (Combination (○) and prothrombin time (B) PT (Alone) (●), PT (Combination (○) vs. time profiles following administration of 20 mg warfarin alone and in combination with 0.5 mg lasofoxifene

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Lasofoxifene administration caused a small reduction in the pharmacodynamic effects of warfarin (Figure 1B). The PT and INR time course showed an increase at 72 h postdose following dosing with warfarin only. The reason for this increase is unclear. The 90% CIs for the warfarin PTmax and PT AUC ratios were both within the 80% and 125% range. Mean PT AUC was 8% lower and PTmax was 16% (3.4 s) lower for warfarin plus lasofoxifene than for warfarin alone. The INRmax and INR AUC paralleled these results. Two subjects had increases in INR above 2.3 with one subject receiving vitamin K. Data from this subject were excluded from the pharmacodynamic analysis.

Results showed larger S-warfarin AUC in subjects with heterozygous mutations with mean values of 41.1, 53.9 and 73.0 µg h−1 ml−1 for *1/*1, *1/*2 and *1/*3 genotypes, respectively. There was no difference in R-warfarin AUC between CYP2C9 genotype. Given the small number of subjects and the variability in pharmacodynamic measurements, there was no relationship between pharmacodynamic and pharmacokinetic parameters. Thus, the difference observed in S-warfarin concentrations in subjects with CYP2C9 mutations did not translate into differences in the magnitude of the pharmacodynamic response with or without lasofoxifene.

Adverse events and safety

Overall, adverse events were generally mild or moderate and short in duration, resolving within 1 day. The most commonly reported adverse event that occurred during treatment with warfarin alone was headache (two subjects), while those during treatment with lasofoxifene alone were headache and increased appetite (two subjects each). With the combination of lasofoxifene and warfarin, the most common adverse events were headache and somnolence (three subjects each). No clinically significant changes in clinical laboratory evaluations, physical examinations, vital signs or ECGs were reported with lasofoxifene or warfarin dosing alone or in combination.

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. References

The present study showed that lasofoxifene, under steady-state dosing conditions, has no clinically meaningful effect on the pharmacokinetics of R- or S-warfarin.

Subjects with heterozygous mutations for the CYP2C9 enzyme (*1/*2 and *1/*3) had larger S-warfarin AUCs relative to subjects with the wild-type allele (*1/*1), reflecting the lower oral clearance resulting from the mutation. This is consistent with previous studies that have shown that sensitivity to warfarin is determined by CYP2C9 mutation and not other factors such as plasma vitamin K levels, alcohol consumption, or creatinine and albumin concentrations [8, 9]. As expected, no trends were noted for R-warfarin, which is not metabolized by this isomer.

A small effect of lasofoxifene on warfarin pharmacodynamics was observed, with approximately 8% and 16% (3.4 s) lower mean PT AUC and PTmax, respectively, during coadministration with lasofoxifene. INRmax and INR AUC showed comparable results. The reason for the decrease in PT is unclear but may be related to the oestrogenic effect of SERM. In a study of raloxifene (120 mg day−1), coadministration with warfarin showed an increase in warfarin plasma concentrations but decreases of 8% and 10% in warfarin PT AUC and PTmax, respectively, during coadministration [5]. The mechanism behind these observations is unknown, but a direct effect on circulating coagulation factors mediated by the oestrogenic effect of raloxifene may be involved, as oral oestrogen has been shown to increase plasma concentrations of vitamin K-dependent clotting factors, which would serve to counter the effects of warfarin [10, 11].

The PT effect observed with lasofoxifene was small and may not be clinically significant. However, as this was a single-dose study and the results of chronic warfarin and lasofoxifene coadministration are unknown, it is recommended that PT monitoring be considered when starting or stopping lasofoxifene, consistent with the warfarin label.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. References
  • 1
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    Loebstein R, Yonath H, Peleg D, Almog S, Rotenberg M, Lubetsky A, Roitelman J, Harats D, Halkin H, Ezra D. Interindividual variability in sensitivity to warfarin – nature or nurture? Clin Pharmacol Ther 2001; 70: 15964.
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    Davies T, Fieldhouse G, McNicol GP. The effects of therapy with oestriol succinate and ethinyl estradiol on the haemostatic mechanism in post-menopausal women. Thromb Haemost 1976; 35: 40314.
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    Scarabin PY, Plu-Bureau G, Zitoun D, Bara L, Guize L, Samama MM. Changes in haemostatic variables induced by oral contraceptives containing 50 micrograms or 30 micrograms oestrogen: absence of dose-dependent effect on PAI-1 activity. Thromb Haemost 1995; 74: 92832.