Effects of Ginkgo biloba extract on the pharmacokinetics of bupropion in healthy volunteers

Abstract

AIMS

To assess the effects of Ginkgo biloba extract on the pharmacokinetics of bupropion in healthy volunteers.

METHODS

Fourteen healthy male volunteers (age range 19–25 years) received orally administered bupropion (150 mg) alone and during treatment with G. biloba 240 mg day⁻¹ (two 60-mg capsules taken twice daily) for 14 days. Serial blood samples were obtained over 72 h after each bupropion dose, and used to derive pharmacokinetic parameters of bupropion and its CYP2B6-catalysed metabolite, hydroxybupropion.

RESULTS

Ginkgo biloba extract administration resulted in no significant effects on the AUC_0–∞ of bupropion and hydroxybupropion. Bupropion mean AUC_0–∞ value was 1.4 µg·h ml⁻¹[95% confidence interval (CI) 1.2, 1.6] prior to G. biloba treatment and 1.2 µg·h ml⁻¹ (95% CI 1.1, 1.4) after 14 days of treatment. Hydroxybupropion mean AUC_0–∞ value was 8.2 µg·h ml⁻¹ (95% CI 6.5, 10.4) before G. biloba administration and 8.7 µg·h ml⁻¹ (95% CI 7.1, 10.6) after treatment. The C_max of hydroxybupropion increased from 221.8 ng ml⁻¹ (95% CI 176.6, 278.6) to 272.7 ng ml⁻¹ (95% CI 215.0, 345.8) (P = 0.038) and the t_1/2 of hydroxybupropion fell from 25.0 h (95% CI 22.7, 27.5) to 21.9 h (95% CI 19.9, 24.1) (P = 0.000).

CONCLUSIONS

Ginkgo biloba extract administration for 14 days does not significantly alter the basic pharmacokinetic parameters of bupropion in healthy volunteers. Although G. biloba extract treatment appears to reduce significantly the t_1/2 and increase the C_max of hydroxybupropion, no bupropion dose adjustments appear warranted when the drug is administered orally with G. biloba extract, due to the lack of significant change observed in AUC for either bupropion or hydroxybupropion.

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

• Bupropion, an antidepressant and smoking cessation drug, is metabolized to its active metabolite hydroxybupropion almost exclusively by CYP2B6.

• Ginkgo biloba is among the most commonly used herbal extract in the general population, and is likely to be used by depressed patients receiving bupropion.

• Studies have reported that G. biloba administration to rats markedly increased the CYP content and CYP2B mRNA in the liver, and intake of G. biloba also induced various hepatic CYP enzymes, especially CYP2B-type enzymes.

• There may be drug interactions between G. biloba extract and bupropion (CYP2B6 substrate).

WHAT THIS STUDY ADDS

• Fourteen-day oral administration of G. biloba extract had no statistically significant effect on the pharmacokinetics of bupropion or its active metabolite hydroxybupropion, as measured by AUC, which suggests G. biloba does not significantly affect the metabolism of bupropion following a single oral dose in healthy Chinese men.

Introduction

Bupropion (INN, amfebutamone) is an antidepressant agent that is also effective as an aid to quit cigarette smoking [1]. Cytochrome P450 2B6 (CYP2B6) is the major enzyme responsible for the metabolism of bupropion to its active metabolite hydroxybupropion both in vitro and in vivo[2–4]. Other metabolic pathways include the formation of threohydrobupropion and erythrohydrobupropion by carbonyl reductase, a nonmicrosomal enzyme in the liver and gut [5].

Ginkgo biloba extract (GBE), which is an extract of the leaves of G. biloba tree, is a popular herbal medicine marketed as a dietary supplement and widely used by individuals to improve central nervous system function, for example in cognitive enhancement in dementia [6]. Ginkgo biloba is a mixture, composed mainly of flavone glycosides and terpenoids (ginkgolides and bilobalide) [7, 8]. Ginkgo biloba can be easily purchased by the general population as herbal supplement or health food without prescription in Japan and the USA and as a prescribed drug in some European countries. Given that G. biloba is among the most commonly used herbal extracts in the general population, it is likely to be used by depressed patients receiving bupropion. Thus, studying herb–drug interaction between G. biloba and bupropion is important. Furthermore, previous studies have reported that G. biloba administration to rats markedly increased the CYP content and CYP2B mRNA in the liver [9, 10], and intake of G. biloba also induced various hepatic CYP enzymes, especially CYP2B-type enzymes [11]. As bupropion is a substrate for CYP2B6, it has the potential to interact with co-administered drugs that are inhibitors and inducers of this enzyme. Several drugs are known to interact with bupropion leading to an increase or decrease in bupropion concentrations in humans [12–14]. However, it appears that there are no published studies on the possible effects of G. biloba on the pharmacokinetics of bupropion.

Therefore, the objective of this study was to assess the effect of G. biloba on the pharmacokinetics of bupropion and its major active metabolite hydroxybupropion in healthy volunteers.

Methods

Participants

Fourteen volunteers aged 19–25 years [mean (SD) age 21.3 years (1.4); mean (SD) weight 61.3 kg (8.5)] provided written informed consent approved by the Ethics Committee Board of Central South University (Changsha Hunan, China). All were determined to be healthy by history, physical examination and basic laboratory tests. All subjects were nonsmokers and used no concomitant medications. They also abstained from alcohol, caffeinated beverages, herbal dietary supplements, grapefruit juice and known inducers or inhibitors of CYP throughout the study.

Study design

The study used a two-period, open-label, fixed-sequence design. After an overnight fast, volunteers ingested a single oral dose of 150 mg sustained-release bupropion (Zyban SR; WanTe, Hainan, China) with water (150 ml). Blood samples for pharmacokinetic analysis were taken for 3 days post dose. Following a wash-out period of at least 7 days, volunteers took two 60-mg capsules (120 mg) of G. biloba (Lot No.4684; Now Foods, Boomingdale, IL, USA; http://www.nowfoods.com) twice daily for 14 days. On day 15, a single 150-mg oral dose of bupropion was then administered after overnight fasting. Four hours after bupropion ingestion, they had access to water and received breakfast.

The GBE, containing a minimum of 24% ginkgo flavone glycosides and 6% terpene lactones, was of the same label as the product used in a previous clinical trial [15].

Blood sampling

Venous blood samples (5 ml each) for determination of bupropion and hydroxybupropion concentrations were taken just before and at 0.5, 1, 2, 3, 4, 5, 6, 8, 10,12, 24, 36, 48, 60 and 72 h after administration of bupropion. Samples were collected into tubes containing sodium-heparin anticoagulant and centrifuged at 2000 g for 10 min; plasma was then harvested from the samples and stored at −20°C until assay.

Assay

A 2695 high-performance liquid chromatography system (Waters, Milford, MA, USA), equipped with a vacuum degasser and an auto-sampler, was used in the study. Chromatographic separation was performed on a HyPurity C18, 5-µm analytical column (150 × 2.1 mm i.d.) at 40°C. The mobile phase consisted of acetonitrile and 0.1% formic acid in a ratio of 60 : 40 (v/v). A triple quadrupole tandem mass spectrometer Micromass Quattro Micro instrument (Waters) was equipped with an electrospray source and operated in positive ionization mode. MassLynx 4.1 software package was used for instrument control and data acquisition. The ionspray voltage was set at 3.5 kV and source temperature at 100°C. The collision-activated dissociation was set at 15 V, using argon as the collision gas. The instrument was set up in multiple reactions monitoring, monitoring the transitions for bupropion m\z 240.2 > 184.1, hydroxybupropion m\z 256.6 > 238.1 and propranolol (internal standard) m\z 260.6 > 183.1. To a 1.5-ml polypropylene centrifuge tube, 200 µl of plasma sample was spiked with 400 µl of internal standard (0.243 µg ml⁻¹) solution, and the contents of the tube were vortexed for 10 min followed by centrifugation at 13 000 g for 10 min. The 400-µl supernatant layer was separated and 400 µl 0.1% formic acid was added, vortexed for 20 s, and 10 µl of the sample was injected. The standard curve ranged from 12.5 to 25 600 ng ml⁻¹ for bupropion and from 12.5 to 25 600 ng ml⁻¹ for hydroxybupropion. For all analytes, intraday and interday precision varied between 3.2 and 10.4%, and accuracy ranged from 89.0 to 101.3%.

Data analyses of pharmacokinetics

Pharmacokinetic parameters for bupropion and hydroxybupropion were calculated using noncompartmental method. The maximum concentration (C_max) and the time to C_max (t_max) were obtained directly from the original data. The terminal rate constant (k_e) was obtained by regression analysis of the log-linear portion of the concentration–time curve. The terminal half-life (t_1/2) was calculated as 0.693/k_e. The area under the plasma concentration–time curve (AUC) to the last quantifiable concentration (AUC_0–t) was determined by use of the linear trapezoidal rule. AUC from zero to infinity (AUC_0–∞) was calculated by AUC_0–t + C_t/k_e, where t is the last measurable sampling time and C_t is the last measured plasma concentration. The oral clearance (CL/F) for bupropion was calculated as dose/AUC_0–∞, and further normalized by body weight.

Statistical analysis

The data are expressed as geometric mean and 95% confidence intervals (CI), except for t_max data, which is presented as median and range. Statistical calculations were performed with SPSS for Windows, version 11.5 (SPSS Inc., Chicago, IL, USA), and P-values <0.05 were considered significant. The pharmacokinetic parameters with and without G. biloba treatment were compared by use of a two-tailed paired t-test after logarithmic transformation. The between-treatment t_max data were compared by use of the Wilcoxon signed rank test. Geometric mean ratios with 90% CI were calculated after log transformation of within-subject ratios for pharmacokinetic variables for bupropion and hydroxybupropion. When exposure–response relationships are not well understood or demonstrated, absence of interaction was concluded if these 90% CI values did not exceed the limits of 0.8 to 1.25. The sample size calculation was performed by use of SigmaStat 3.5 (Systat Software, Inc., Chicago, IL, USA) for a two-sided t-test. On the basis of available data with respect to the pharmacokinetics of single-dose bupropion [12], it was calculated that at least eight participants were required to be able to detect a 30% change in the AUC_0–∞ (SD of difference, 30%) of bupropion at a 5% significance level and with 80% statistical power.

Results

Subjects

Bupropion was generally well tolerated by all volunteers; no serious drug-related adverse events occurred during the course of the investigation. All 14 volunteers successfully completed the study. No concomitant drugs were used by any of the volunteers who participated in the study.

Effect of Ginkgo biloba on bupropion and hydroxybupropion pharmacokinetics

The pharmacokinetic parameter estimates for bupropion and hydroxybupropion determined in 14 healthy male subjects are shown in Table 1. Plots of mean (±SE) serum concentrations of bupropion and hydroxybupropion vs. time before and after 14 days of G. biloba administration are shown in Figure 1. GBE administration resulted in no significant effects on the pharmacokinetic parameters of bupropion. Bupropion mean AUC_0–∞ and C_max values were 1.4 µg·h ml⁻¹ (95% CI 1.2, 1.6) and 179.3 ng ml⁻¹ (95% CI 148.5, 216.4), respectively, for control samples and 1.2 µg·h ml⁻¹ (95% CI 1.1, 1.4) and 171.7 ng ml⁻¹ (95% CI 149.2, 197.5), respectively, after G. biloba treatment. In addition, G. biloba administration was associated with significantly increased (P = 0.038) maximum plasma concentration of hydroxybupropion, from 221.8 ng ml⁻¹ (95% CI 176.6, 278.6) to 272.7 ng ml⁻¹ (95% CI 215.0, 345.8).The half-life of hydroxybupropion was significantly decreased (P = 0.000) from 25.0 h (95% CI 22.7, 27.5) to 21.9 h (95% CI 19.9, 24.1), but there was no significant change in AUC_0–∞ of hydroxybupropion.

Table 1.

Pharmacokinetics of bupropion and hydroxybupropion after ingestion of 150 mg bupropion (sustained release), without pretreatment (control) and after 14 days of pretreatment with Ginkgo biloba

Parameter	Control	Ginkgo biloba	Geometric mean ratio and 90% CI	P-value
Bupropion
AUC0–∞ (µg·h ml−1)	1.4 (1.2–1.6)	1.2 (1.1–1.4)	0.90 (0.81, 1.01)	0.138
Cmax (ng ml−1)	179.3 (148.5–216.4)	171.7 (149.2–197.5)	0.96 (0.81, 1.13)	0.643
tmax (h)	2 (1–2)	2 (1–3)		0.317
t1/2 (h)	11.7 (11.1–12.3)	11.7 (11.2–12.2)	1.0 (0.96, 1.04)	0.997
CL/F (l h−1 kg−1)	1.8 (1.5–2.1)	2.0 (1.8–2.2)	1.11 (0.99, 1.24)	0.138
Hydroxybupropion
AUC0–∞ (µg·h ml−1)	8.2 (6.5–10.4)	8.7 (7.1–10.6)	1.05 (0.93, 1.20)	0.475
Cmax (ng ml−1)	221.8 (176.6–278.6)	272.7 (215.0–345.8)	1.23 (1.05, 1.44)	0.038
tmax (h)	4 (3–8)	4 (3–6)		0.089
t1/2 (h)	25.0 (22.7–27.5)	21.9 (19.9–24.1)	0.88 (0.83, 0.92)	0.000

Data are given as geometric mean and 95% CI, except for t_max data, which are presented as median and range. Geometric mean ratio and 90% CI were assessed for the evaluation of equivalence after co-medication with Ginkgo biloba. C_max, maximum plasma concentration; AUC_0–72, area under plasma concentration–time curve from 0 to 72 h; AUC_0–∞, area under plasma concentration–time curve from time 0 to infinity; t_max, time to maximum concentration; t_1/2, elimination half-life; CL/F, total oral clearance; CI, confidence interval.

Figure 1.

Mean plasma concentrations (SE indicated by error bars) of bupropion (a) and hydroxybupropion (b) after a single 150-mg dose of bupropion in 14 healthy male subjects in control phase [without pretreatment (triangles and dashed line)] and after 14-day pretreatment with Ginkgo biloba (squares and solid line)

Discussion

The present study was undertaken to investigate the effects of oral administration of G. biloba on the pharmacokinetics of bupropion in healthy volunteers. Our results show administration of G. biloba (120 mg b.i.d.) for 14 days had no statistically significant effect on the pharmacokinetics of bupropion or its active metabolite hydroxybupropion, as measured by AUC, which suggests G. biloba does not significantly affect the metabolism of bupropion following a single oral dose in healthy Chinese men.

Ginkgo biloba, like most herbal products, contains a large array of active compounds. It contains approximately 30 kinds of flavonoids (e.g. bilobetin, ginkgetin, sciadopitisin, quercetin, isorhamnetin, kaempferol) and their derivatives and terpenoids such as ginkgolide A, ginkgolide B, ginkgolide C and bilobalide [16, 17]. Tremendous variability of the same product may occur between manufacturers and lots [18]. To minimize intraproduct variation in phytochemical content, all of the G. biloba that was utilized in this study originated from the same batch and lot number of the product. The over-the-counter source was chosen for our study based on the premise that one of the advantages of using G. biloba is that it can be easily available to the general population as herbal supplement without prescription in Japan and the USA.

The interpretation of results was based on a homogeneous group of subjects, e.g. young, nonsmoking, male volunteers, thus avoiding confounding factors. Oral contraceptives have been reported [19] to have an inhibitory effect on the activity of CYP2B6. Therefore, female volunteers were excluded from the study. Drug administration was supervised and food intake was standardized on the study days. Blood samples were collected up to 72 h after intake, giving a reliable measure of the pharmacokinetics of bupropion and hydroxybupropion. Unlike competitive enzyme inhibition, enzyme induction by G. biloba is a slow regulatory process because new protein must be synthesized. A 14-day pretreatment was chosen on the basis of previous in vivo studies [20–22] and to prevent unnecessarily long exposure of healthy volunteers to G. biloba.

There are several limitations of the current investigation. First, only one G. biloba product was assessed, and preparations are known to vary in content of various constituents. Second, only one G. biloba dosage was assessed (120 mg twice daily), which does not preclude the possibility that greater doses of G. biloba may affect the pharmacokinetics of bupropion. Third, serum concentrations of erythrohydrobupropion and threohydrobupropion were not measured in this study, so their disposition status remains unknown. However, to the best of our knowledge this is the first study to investigate the interaction between bupropion and G. biloba. The results will add important information to the study of interaction between drugs and herbal products and also help in better use of bupropion in patients with G. biloba.

In conclusion, the data collected in this study indicate no significant effect of G. biloba on the pharmacokinetics of bupropion. Accordingly, on the basis of this study in healthy male volunteers, it appears unlikely that clinically significant drug interactions between G. biloba and bupropion will occur. No bupropion dose adjustments appear warranted when the drug is administered orally with GBE.

Competing interests

None to declare.

This work was supported by research grants from the National Natural Science Foundation of China 30528026, 30300428, 30672497 and 30500623, and by the China Medical Board of New York grant 01-755. We thank Yi Cheng Wang for helping with recruiting the subjects, and Zhi Rong Tan and Hao Chen for technical assistance.