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British Journal of Clinical Pharmacology logoLink to British Journal of Clinical Pharmacology
. 2015 Jan 20;79(2):298–306. doi: 10.1111/bcp.12510

Intake of St John's wort improves the glucose tolerance in healthy subjects who ingest metformin compared with metformin alone

Tore Bjerregaard Stage 1,✉, Rasmus Steen Pedersen 1, Per Damkier 1,2,*, Mette Marie Hougaard Christensen 1, Søren Feddersen 2, John Teilmann Larsen 3, Kurt Højlund 4, Kim Brøsen 1
PMCID: PMC4309635  PMID: 25223504

Abstract

Aims

Our objective was to investigate the steady-state pharmacokinetic and pharmacodynamic interaction between the antidepressive herbal medicine St John's wort and the antidiabetic drug metformin.

Methods

We performed an open cross-over study in 20 healthy male subjects, who received 1 g of metformin twice daily for 1 week with and without 21 days of preceding and concomitant treatment with St John's wort. The pharmacokinetics of metformin was determined, and a 2 h oral glucose tolerance test was performed.

Results

St John's wort decreased the renal clearance of metformin but did not affect any other metformin pharmacokinetic parameter. The addition of St John's wort decreased the area under the glucose concentration–time curve [702 (95% confidence interval, 643–761) vs. 629 min*mmol/L (95% confidence interval, 568–690), P = 0.003], and this effect was caused by a statistically significant increase in the acute insulin response.

Conclusions

St John's wort improves glucose tolerance by enhancing insulin secretion independently of insulin sensitivity in healthy male subjects taking metformin.

Keywords: drug–herb interaction, metformin, pharmacodynamics, pharmacokinetics, St John's wort

What is Already Known about this Subject?

  • The pregnane X receptor agonist rifampicin lowers the plasma concentration of metformin and increases the glucose tolerance in healthy individuals.

  • It has not previously been investigated whether the herbal medicine St John's wort (SJW), which is also an agonist of the pregnane X receptor, exerts the same effect.

What this Study Adds

  • There was no difference in the pharmacokinetics of metformin, except for a small albeit statistically significant reduction in renal clearance after treatment with SJW.

  • We show that SJW increases glucose tolerance in healthy subjects who ingest metformin.

  • The effect was caused by an increase in the acute insulin response.

Introduction

The World Health Organization (WHO) estimates that more than 300 million people worldwide suffer from type 2 diabetes (T2D), and that in 2004, 3.4 million patients died from consequences of T2D. On a global level, WHO projects that the number of deaths caused by diabetes will increase by ∼70% by the year 2030 1.

Metformin is an oral antidiabetic agent used in the pharmacological treatment of T2D and it is first choice due to its positive effect on morbidity and mortality in overweight type 2 diabetics 2. Even though metformin is a well-tolerated drug, the response seems to be insufficient in up to 36% of patients in monotherapy with metformin 3. The exact molecular mechanism of the blood glucose-lowering effect is not fully elucidated, but according to a recent study 4 it seems that metformin supresses the gluconeogenesis through inhibition of the mitochondrial glycerophosphate dehydrogenase. Metformin is not metabolized in the liver or elsewhere but is excreted unchanged in the urine 5. It is a strong base that exists virtually only in its cationic form at physiological pH. This means that metformin has a very low ability to diffuse passively across cellular membranes, and metformin distribution is therefore highly dependent on drug transporters. There are three groups of solute carrier (SLC) transporters, which are involved in the transportation of metformin across cell membranes: the organic cation transporters (OCT, SLC22A1-3); the multidrug and toxin extrusion transporters (MATE, SLC47A1-2); and the plasma membrane monoamine transporters (PMAT, SLC29A4). The hepatic uptake of metformin is mediated primarily by OCT1 and to a lesser extent by OCT3 6–9. It was shown that OCT1-deficient mice had a much lower concentration of metformin in the liver compared with normal mice, indicating the importance of this transporter in the hepatic uptake of metformin 10. OCT2 is involved in the renal excretion of metformin by controlling the basolateral transport of metformin from the blood into the tubular cells in the kidney 11, whereas the H+–drug antiporters multidrug and toxin extrusion 1 and 2 (MATE1 and MATE2) facilitate the extrusion of metformin into the urine at the apical side of the renal tubular cells 12–14.

St John's wort (SJW) is a herbal medicine, available as an over-the-counter medicine in many European countries. It has gained widespread use and popularity in the general population as a herbal medicine with moderate antidepressant effects. St John's wort activates the nuclear receptor pregnane X receptor (PXR), which increases transcription and protein levels of several targets including ABCB1 (also known as P-glycoprotein; P-GP) and cytochrome P450 3A4 (CYP3A4) 15. Furthermore, PXR activation may also increase the transcription of OCT1 in rats 16. Induction of P-GP and CYP3A4 by SJW has been reported to lead to clinically relevant interactions with digoxin, verapamil and cyclosporin, amongst others 17–20. It was recently shown that another PXR agonist, rifampicin, increases the renal clearance of metformin and thus decreases the area under the concentration–time curve (AUC) of metformin and increases the pharmacodynamic response to metformin as well. The authors speculated that the pharmacokinetic and glucose-lowering effect was due to OCT1 upregulation, hence an increase in the uptake of metformin in the liver. This hypothesis was supported by the detection of increased OCT1 mRNA levels in peripheral blood cells after treatment with rifampicin 21.

Depression increases the risk of developing T2D by 37% 22, and T2D seems to double the risk of developing depression 23, and there appears to be a bidirectional association between depression and T2D in women 24. It is plausible that some T2D patients who are treated with metformin also suffer from depression and consequently ingest SJW concomitantly. On this background, we found it of interest to examine whether SJW affects the pharmacokinetic and pharmacodynamic profile of metformin.

Our hypothesis is that intake of SJW increases the transport of metformin into the liver and thus lowers plasma concentration of metformin and increases the response to metformin in healthy individuals.

Methods

Study design

This was an open, cross-over interaction study that consisted of two study periods.

Period 1

The subjects received 1 g metformin twice daily for 1 week to reach steady state (see ‘Study medication and dose’). At 08.00 h on day 7, after a 12 h overnight fast, the subjects ingested 1 g metformin, and the concentrations of metformin were measured at 0 (minimum concentration at steady-state; Css,min), ½, 1, 1½, 2, 2½, 3, 3½, 4, 6, 8, 12 and 24 h after ingestion, and urine was collected for 24 h in two samples (0–12 and 12–24 h). Furthermore, an oral glucose tolerance test (OGTT) was performed simultaneously. The subjects ingested 75 g glucose in an oral solution, and the concentrations of plasma glucose, serum insulin and C-peptide were measured at 0, 30, 60, 90 and 120 min.

Period 2

Following a wash-out period of at least 7 days, the subjects received SJW extract twice daily for 21 days. On days 16–22, metformin was administered. The last SJW capsule was administered the night before the second trial day. The pharmacokinetics of metformin was assessed and an OGTT performed, identical to the procedures in period 1.

Blood collection: OCT1 gene expression

Blood samples for determining OCT1 mRNA concentrations were collected on four separate occasions: (i) at inclusion, when the subjects were screened to determine eligibility; (ii) on day 7 of period 1, after 6 days of treatment with metformin, blood was drawn at 08.00 h before ingestion of metformin; (iii) at the beginning of the trial day in period 2, after 21 days of treatment with SJW and 6 days of treatment with metformin, blood was drawn at 08.00 h before metformin ingestion; (iv) at least 3 weeks after period 2. All samples were stored at −80°C from collection until processing for mRNA.

Study participants

The role of single nucleotide polymorphisms (SNPs) in the interindividual response to metformin is not yet fully elucidated, but several studies suggest that polymorphisms in OCT1 alter the response to metformin treatment 25–27. The current research regarding the SNP rs316019 (808 G>T) in the gene encoding OCT2 is conflicting, but it does appear to affect the renal clearance (CLR) of metformin 28,29. To avoid known and unknown effects of genetic variations, 20 men who were all wild-type for known reduced function SNPs in OCT1 (rs12208357, rs34130495, rs72552763, rs34059508) and OCT2 (rs316019) were included in this study. The inclusion criteria were men aged 18–64 years, normal liver and kidney function, as assessed by bilirubin, alanine amino transferase and plasma creatinine within the normal range and a glycosylated haemoglobin A1c (HbA1c) within the normal range. The exclusion criteria were daily intake of medication requiring prescription, chronic/daily alcohol abuse and/or hypersensitivity to the medication involved in this trial. We did not control for intake of vitamins, minerals and other herbal medicines. The subjects had a median age of 25 years (25th–75th percentile, 24–26.5 years), a median body mass index of 23.6 kg m−2 (25th–75th percentile, 22.4–24.7 kg m−2) and a HbA1c of 32 mmol mol−1 (25th–75th percentile, 31.5–33 mmol mol−1). The subjects were recruited from a pharmacogenetic biobank 30 (EudraCT-number: 2010-020374-41), which stores DNA from 400 individuals.

Study medication and dose

The subjects ingested metformin tablets (Metformin Actavis®, Gentofte, Denmark) to steady state (1 g twice daily) over 1 week. On day 1, 500 mg metformin was ingested twice daily; on day 2, 500 mg was ingested in the morning and 1 g in the evening, followed by 1 g twice daily (08.00 and 20.00 h) for the remaining days. This treatment plan was chosen to try to minimize the adverse effects of metformin. We did not specify whether the subjects would ingest metformin with or without food. One Modigen® (Jemo-Pharm A/S, Stege, Denmark) capsule with 240–294 mg dry extract of SJW (900 μg total hypericin) was ingested twice daily (08.00 and 20.00 h) for 21 days in period 2.

Study procedures

The study was conducted in accordance with the Helsinki Declaration and Good Clinical Practice (GCP) and monitored by the GCP unit, Odense University Hospital, Odense, Denmark. The study protocol was approved by the Danish Health and Medicines Authorities (J. no: 201202075) and the Scientific Ethical Committee (J. no: S-20120174). The trial was registered at http://www.clinicaltrials.gov (identifier NCT01726764).

Analytical method for determination of glucose, insulin and C-peptide in blood

Plasma glucose was determined on anABL800 FLEX® (Radiometer Medical, Brønshøj, Denmark). Insulin and C-peptide were measured on a cobas e 411® (Roche, Hvidovre, Denmark).

Analytical method for determination of metformin in plasma and urine

The concentration of metformin in plasma and urine was determined by high-pressure liquid chromatography; the method has been described in detail elsewhere 31. The within-day coefficient of variation ranged from 3.1 to 7.5% in plasma and from 1.6 to 6.2% in urine. The between-day reproducibility ranged from 2.9 to 5.3% in plasma and from 0.6 to 1.8% in urine. The lower limit of quantification for metformin in plasma and urine was 5 and 40 ng ml−1, respectively.

Relative quantification of OCT1 gene expression

Blood OCT1 mRNA concentrations were measured using relative quantitative real-time PCR (qRT-PCR). Total RNA was extracted from 1.3 ml blood using the NucleoSpin RNA Blood Midi Kit (Macherey-Nagel, Bethlehem, PA, USA), and DNA was removed with the TURBO DNA-free Kit (Ambion, Austin, TX, USA). Equal amounts of RNA (220 ng) were reverse transcribed using Oligo(dT)20 primers and the SuperScript III First-Strand Synthesis System for RT-PCR (Invitrogen, Carlsbad, CA, USA). The qRT-PCR was performed on a StepOne Plus real-time instrument (Applied Biosystems, Foster City, CA, USA) using a human OCT1 TaqMan gene expression primer probe set (Hs00427552_m1) and TaqMan Gene Expression Master Mix (Life Technologies, Grand Island, NY, USA), following the manufacturer's instructions. The following cycling conditions were used: one cycle at 50°C for 2 min, one cycle at 95°C for 10 min, and 40 cycles of 95°C for 15 s and 60°C for 60 s. As recommended by others, we used the qBasePLUS software 32,33 to normalize OCT1 mRNA concentrations to the expression of human GAPDH and ACTB. Human GAPDH and ACTB mRNA concentrations were measured using TaqMan gene expression primer probe sets (Hs99999905_m1 and Hs01060665_g1, respectively) as described for human OCT1.

Statistical analysis and considerations

The size of the study population (n = 20) was calculated to allow a detection of a true difference of >15% in AUC0–24h of metformin with a power of 80% and a significance level of 5% while allowing a 10% dropout rate. Descriptive data are shown as medians with 10th–90th percentiles; others are listed as means with 95% confidence intervals (CIs) unless otherwise specified. Student's paired t-test was used as statistical test to determine significance. Data that did not follow a normal distribution were analysed using the Wilcoxon signed-rank test if logarithmic transformation was insufficient to reach normalization. All P values shown are two tailed. A P value of <0.05 was considered significant. Calculations and statistical analyses were performed using STATA 13 (StataCorp, College Station, TX, USA), except for the analysis of OCT1 mRNA concentrations, which was determined using GraphPad Prism (version 5.0) software (La Jolla, CA, USA).

The pharmacokinetic analysis was performed using the program WinNonlin 6.3 (Pharsight, Mountain View, CA, USA). For AUC determinations of metformin, the linear-up log-down method was used. The AUC0–2h of glucose, insulin and C-peptide was determined using the linear trapezoidal method. The maximal concentration (Cmax) and the time of the maximum concentration of metformin were determined visually. The elimination rate constant (k) was determined as the terminal slope of the log-transformed concentration–time curve, and the half-life elimination time (t1/2½) was determined using the equation t1/2½ = ln(2)/k. The renal clearance of metformin was estimated using the formula: CLR = amount excreted in urine0–24h/AUC0–24h.

The formulas used in the calculation of surrogate markers of insulin resistance, insulin sensitivity and glucose-stimulated insulin secretion, as well as disposition indices, are shown in Table S1.

Results

Twenty healthy male individuals were included, and all completed both study periods. Two subjects were excluded from the analysis of the OGTT because they had not been fasting for 12 h prior to the test. These subjects were instructed to ingest the same type and amount of breakfast in period 2, which means they were eligible to be included in the pharmacokinetic analysis. Additionally, one subject was excluded from both the analysis of the OGTT and the pharmacokinetic analysis due to metformin intake 45 min prior to the OGTT in the first period. Mild gastrointestinal side-effects were reported in all subjects, most probably due to the fast dose titration of metformin. There was a median of 36 days (10th–90th percentiles, 33–45 days) between the two study periods.

Oral glucose tolerance test

There was a statistically significant reduction in the glucose area under the concentration–time curve from 0 to 2 h [AUC0–2h; 702 (95% CI, 643–761) vs. 629 min*mmol/L (95% CI, 568–690), P = 0.003] in period 2 (Figure 1). Figure 2 shows the glucose AUC0–2h for the subjects (n = 17) who were included in the analysis for both periods.

Figure 1.

Figure 1

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The mean plasma glucose concentration (error bars indicate upper and lower SD) in 17 healthy fasting subjects after oral intake of 75 g of glucose on day 7 of daily intake of 1 g of metformin twice daily, without Inline graphic and with Inline graphic 3 weeks of preceding and concomitant treatment with St John's wort, one capsule twice daily. *P < 0.05

Figure 2.

Figure 2

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Glucose area under the plasma concentration–time curve (AUC) during an oral glucose tolerance test in 17 healthy individuals who ingested 1 g metformin twice daily with and without St John's wort

Serum insulin and C-peptide concentrations were statistically significantly higher after SJW intake at 30 min [serum insulin, 391 (95% CI, 304–479) vs. 491 pmol l−1 (95% CI, 373–609), P = 0.03; and C-peptide in plasma, 2015 (95% CI, 1728–2303) vs. 2338 pmol l−1 (95% CI, 1965–2711), P = 0.03], but not at any other time point, nor in AUC0–2h (data not shown).

Estimated values of the homeostasis model of insulin resistance (HOMA-IR) and of surrogate markers for insulin sensitivity (BIGTT-S1, HOMA-IS and Stumvoll) did not change in response to treatment with SJW. Oral glucose-stimulated insulin secretion was evaluated by the acute insulin response (BIGTT-AIR), the insulinogenic index and the corrected insulin response (CIR), and they all increased significantly in response to SJW (P < 0.01; Table 1). The calculation of three different disposition indexes (DI1, DI2 and DI3) based on different estimates of glucose-stimulated insulin secretion and surrogate markers of both hepatic and peripheral insulin sensitivity demonstrated that SJW improved insulin secretion even after adjustment for insulin sensitivity.

Table 1.

Surrogate markers for fasting insulin resistance, insulin sensitivity and glucose-stimulated insulin secretion in 17 subjects who received 1 g metformin twice daily for 1 week without (period 1) and with 21 days of preceding and concomitant treatment with St John's wort (period 2)

Parameter Marker Metformin alone Metformin and St John's wort P Value
Fasting insulin resistance HOMA-IR 1.3 (1.0–1.7) 1.5 (0.9–2.1) 0.86
Insulin sensitivity BIGTT-S10–30–-120 min 10.5 (9.2–11.8) 10.5 (8.9–12.0) 0.98
HOMA-IS 0.9 (0.7–1.1) 1.0 (0.7–1.2) 0.79
Stumvoll 0.11 (0.11–0.12) 0.11 (0.11–0.12) 0.41
Insulin secretion BIGTT-AIR0–30–120 min 2 662 (2246–3078) 3 474 (2675–4273) 0.005
Insulinogenic index 153 (109–196) 230 (134–325) 0.01
Corrected insulin response (CIR) 1 514 (1070–1959) 2 325 (1470–3179) 0.009
Insulin secretion adjusted for insulin sensitivity DI1 1 378 (867–1889) 2 478 (1023–3933) 0.02
DI2 26 644 (23 230–30 058) 33 015 (28 073–37 959) 0.001
DI3 174 (121–228) 273 (163–383) 0.01
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Numbers are listed as means with 95% confidence intervals. P Values are derived from the Wilcoxon signed-rank test. The formulas used to determine these surrogate markers can be found in Table S1. Abbreviations are as follows: BIGTT-AIR, β-cell function, insulin sensitivity and glucose tolerance test for acute insulin response; BIGTT-S1, β-cell function, insulin sensitivity and glucose tolerance test for insulin sensitivity; DI1, DI2 and DI3, disposition index 1, 2 and 3; HOMA-IR, homeostatic model for assessment of insulin resistance; HOMA-IS, homeostatic model for assessment of insulin sensitivity.

Metformin pharmacokinetics

The treatment with SJW caused a small, albeit statistically significant, reduction in CLR of metformin (Table 2). Other pharmacokinetic parameters remained unchanged, as illustrated by the almost identical plasma concentration–time curves of metformin (Figure 3).

Table 2.

Pharmacokinetic parameters of 19 subjects who ingested metformin 1 g twice daily without (period 1) and with 21 days of preceding and concomitant treatment with St John's wort (period 2)

Parameter Metformin alone Metformin and St John's wort Geometric mean ratio P Value
AUCss,0–24h (h μg ml−1) 15.5 (15.1–18.5) 15.9 (13.2–20.1) 0.97 (0.88–1.06) 0.45
Cmax (μg ml−1) 2.3 (1.9–2.7) 2.4 (2.2–3.1) 0.91 (0.82–1.02) 0.10
t1/2 (h) 7.0 (6.0–8.7) 6.8 (6.1–8.4) 0.97 (0.85–1.11) 0.64
CLss/F (l h−1) 76.2 (65.7–79.4) 77.0 (59.5–86.9) 1.04 (0.95–1.13) 0.37
CLR (l h−1) 34.7 (32.6–38.5) 31.0 (27.6–34.2) 1.09 (1.02–1.17) 0.01
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Numbers are listed as medians with 25th–75th percentile range. Geometric means ratios are presented with 95% confidence intervals. Student's paired t-test was performed on log-transformed values. Abbreviations are as follows: AUCss,0–24h, area under the metformin plasma concentration–time curve from 0 to 24 h at steady state; CLR, renal clearance; CLss/F, apparent total clearance at steady state; Cmax, maximal concentration; t1/2, elimination half-life; V/F, apparent volume of distribution.

Figure 3.

Figure 3

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The mean plasma concentration–time curve of metformin after the final dose in 19 healthy subjects who received 1 g metformin to steady-state for 1 week alone Inline graphic and in combination with St John's wort Inline graphic

OCT1 gene expression

In order to further examine if SJW affects the glucose-lowering action of metformin, OCT1 mRNA concentrations were measured in peripheral blood cells as described by Cho et al. 21. For this purpose, blood samples were collected on four occasions: before any treatment; after treatment with metformin for 7 days; after treatment with SJW for 21 days and metformin for 7 days; and at least 3 weeks after the last treatment. There was no statistical significant difference between the four time points in terms of mean OCT1 mRNA concentrations (Figure 4).

Figure 4.

Figure 4

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Scatter plot representing log10 relative organic cation transporter 1 mRNA expression concentrations with mean bar in 17 healthy individuals on four occasions. Expression is normalized to two stably expressed reference genes (ACTB and GAPDH; the genorm average expression stability value, 0.995; coefficient of variation, 0.392)

Discussion

This is the first study to address a possible pharmacokinetic and pharmacodynamic interaction between SJW and metformin. Apart from a minor reduction in the renal clearance, there was no effect of SJW on the steady-state pharmacokinetics of metformin (Figure 3), and we were also unable to demonstrate any effect of SJW on OCT1 mRNA concentrations in peripheral blood cells. In contrast, we found a statistically significant improvement in glucose tolerance after treatment with SJW. Owing to a limitation in our study design, it is unclear whether this is due to a change in the pharmacodynamics of metformin, an increase in the intrahepatic concentration of metformin that is not reflected in the pharmacokinetic parameters or a direct effect of SJW on insulin secretion and glucose metabolism. Furthermore, SJW increased the serum insulin and C-peptide concentrations 30 min after oral glucose intake, and this resulted in reduced plasma glucose concentrations after 60 and 90 min. Accordingly, the glucose AUC0–2h also decreased after treatment with SJW. Using several different indices of insulin sensitivity, reflecting both hepatic and peripheral insulin sensitivity and glucose-stimulated insulin secretion, we demonstrated that a higher insulin secretion explained the improved glucose tolerance after treatment with SJW, even after adjustment for insulin sensitivity. A weakness of our study is that we did not control for the diet of the subjects in the days preceding the OGTT. The crossover design applied in this study means that the subjects are their own controls, and as there was a median of 36 days between the two study periods and we made no interventions on diet, we do not believe that the diet will differ significantly and thus influence the study results.

We observed a statistically significant decrease in CLR of metformin during co-administration with SJW. Cho et al. 21 showed that concomitant ingestion of rifampicin increased the CLR and the effect of metformin in healthy subjects. The effect shown in our study is opposite to the effect reported by Cho et al., and it is biologically counterintuitive. To examine this effect of SJW on the CLR of metformin further, it could be relevant to measure OCT2 mRNA concentrations in kidney cells or, alternatively, in peripheral blood cells; however, OCT2 mRNA cannot be detected in peripheral blood cells 21 and it is unethical to perform kidney biopsies in healthy volunteers. A difference could also have been caused by a difference in kidney function between the two periods. Unfortunately, we did not measure kidney function; however, we do not believe that the renal function in young, healthy men would differ significantly between the study periods. We did not see an effect of SJW on any other metformin pharmacokinetics [elimination half-life (t1/2), Cmax and total apparent clearance (CLss/F)]. We expected that SJW, through PXR activation, would increase cellular concentrations of OCT1 mRNA and protein, which would have caused a decrease in metformin AUC due to increased distribution of metformin into the liver. We did not observe any difference in metformin AUC in our study, which could be explained by the hepatocytes representing only a small compartment in comparison to the plasma. Thus, it is plausible that increased distribution into the hepatocytes would not be reflected in plasma concentrations of metformin. It could also be argued that no difference was observed because OCT1 in hepatocytes was not upregulated, which was supported by our finding that SJW does not increase OCT1 mRNA concentrations in peripheral blood cells. Whether SJW does not affect OCT1 mRNA concentrations or OCT1 mRNA concentrations in blood does reflect hepatic OCT1 mRNA or protein concentrations is unclear. It is unethical to perform liver biopsies in healthy volunteers, and thus we are unable to determine how well OCT1 mRNA in blood correlates with OCT1 mRNA concentrations in the liver.

We determined three surrogate markers that correlate with hepatic (HOMA-IS) and peripheral insulin sensitivity (BIGTT-S1 and Stumvoll index) 34, as well as three indices of oral glucose-stimulated insulin secretion (BIGTT-AIR, insulinogenic index and CIR). From these, we calculated three different disposition indexes (DI1, DI2 and DI3) to evaluate insulin secretion adjusted for insulin sensitivity. St John's wort did not affect insulin sensitivity, but it did increase the glucose-stimulated insulin secretion, even when adjusting for insulin sensitivity (Table 1).

This positive effect on the glucose tolerance in this study does not seem to be explained by an increased hepatic uptake and effect of metformin. Thus, the presumed main mechanism of action of metformin is mediated through regulation of the hepatic gluconeogenesis, and we would not expect to see an effect on the glucose-stimulated insulin secretion, which was shown in this study. This is supported by the finding that hepatic insulin sensitivity (HOMA-IR) is not changed in response to treatment with SJW. Thus, HOMA-IR is known to reflect hepatic insulin sensitivity and basal hepatic glucose production. In the present study, we evaluated the effect of SJW as an add-on to metformin in healthy, normoglycaemic male subjects. This may explain the lack of effect on surrogate markers of hepatic and peripheral insulin sensitivity. Whether the study findings are clinically relevant is difficult to determine, because the subjects are normoglycaemic, but we believe that if the findings can be replicated in patients with T2D this would represent a clinically relevant effect. Studies that examine the effect of SJW on insulin sensitivity and pancreatic β-cell function in subjects with postprandial hyperglycaemia or even manifest diabetes are therefore warranted. If our finding can be confirmed in such conditions, it is possible that SJW may be used in metformin-treated patients with T2D who are affected by depression, instead of the traditional antidepressant which is known to have a negative influence on the glucose homeostasis 35.

In the light of recent findings 36, which indicate that rifampicin, another PXR agonist, impairs the glucose tolerance in healthy individuals, we are not able to rule out the possibility that the effect seen on the oral glucose tolerance shown in this study is caused by SJW alone and does not involve a herb–drug interaction. In vitro studies have shown that the insulin secretion is not affected when isolated β-cells from humans or rats are incubated with SJW for <24 h 37,38. This apparent discrepancy with our results may be due either to the concomitant intake of metformin or to the fact that the human subjects took SJW for 3 weeks. Further studies are warranted. The role of PXR activation in glucose homeostasis is widely discussed, with many conflicting results, as reviewed by Hukkanen et al. 39. However, most in vivo studies indicate that PXR activation leads to reduced glucose tolerance. Interestingly, the effect observed in this study is the opposite of that, that is, SJW improves the glucose tolerance. This supports the notion that the effect seen in this study is caused by a drug–herb interaction. The effect of SJW alone on the glucose tolerance in healthy individuals will be examined in a follow-up study that we are currently performing (clinicaltrials.gov identifier: NCT01921881).

Metformin has been shown to upregulate a small heterodimer partner, which in turns leads to suppression of several nuclear receptors, such as PXR, involved in the hepatic regulation of glucose homeostasis 40. This could further complicate the picture, and could cause the effect observed in this study.

In conclusion, we find that SJW increases the glucose-stimulated insulin response in healthy male subjects receiving metformin, an effect that is not caused by a change in the pharmacokinetics of metformin. The clinical significance of this observation should be the subject of a clinical trial in T2D patients.

Competing Interests

All authors have completed the Unified Competing Interest form at http://www.icmje.org/coi_disclosure.pdf (available on request from the corresponding author) and declare: no support from any organization for the submitted work; no financial relationships with any organizations that might have an interest in the submitted work in the previous 3 years; no other relationships or activities that could appear to have influenced the submitted work.

The authors would like to express gratitude to laboratory technicians Lone Hansen, Charlotte Bøtchiær Olsen, Marie Bonde, Birgitte Damby Sørensen and Susanne Hillbrandt for their contribution to the practical circumstances in this study. Furthermore, MDs Charlotte Gils Andersen and Maija Haastrup have kindly assisted in blood sampling in the study. The Psychiatric Research Council of Southern Denmark and The Danish Council for Independent Research | Medical Science has provided financial support for this study.

Supporting Information

Table S1

Formulas used in this study to determine estimates for fasting insulin resistance, insulin sensitivity and glucose-stimulated insulin secretion

bcp0079-0298-sd1.docx (80.8KB, docx)

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Table S1

Formulas used in this study to determine estimates for fasting insulin resistance, insulin sensitivity and glucose-stimulated insulin secretion

bcp0079-0298-sd1.docx (80.8KB, docx)

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