Research ArticleArticle

In Vivo Bioavailability, Absorption, Excretion, and Pharmacokinetics of [14C]Procyanidin B2 in Male Rats

Stavroula Stoupi, Gary Williamson, Florian Viton, Denis Barron, Laurence J. King, Jonathan E. Brown and Michael N. Clifford
Drug Metabolism and Disposition February 2010, 38 (2) 287-291; DOI: https://doi.org/10.1124/dmd.109.030304

Abstract

Procyanidins are important biologically active compounds, but the pathway and extent of absorption and metabolism are controversial. We conducted a mass balance study to evaluate the total radioactivity excreted in urine and feces after oral administration of [14C]procyanidin B2 to male rats (n = 5). Urine and feces were collected daily from 0 to 96 h. Absolute bioavailability of 14C from [14C]procyanidin B2 was calculated as ∼82% using the values for total urinary 14C. A pharmacokinetic study measured total radioactivity in the blood (n = 9). Blood samples were collected at designated time intervals (0.5–24 h) after administration. Three treatments were used: 1) intravenous, 2) oral higher dose (21 mg/kg b.wt.), and 3) oral lower dose (10.5 mg/kg). Blood concentration of total 14C reached a maximum at ∼6 h after ingestion of [14C]procyanidin B2 (groups II and III), and area under the curve (AUC) was dependent on oral dose. After intravenous or oral administration the terminal half-lives were similar, whereas 8-fold larger values were obtained after oral dosing for total clearance and the apparent volumes of distribution. These pharmacokinetic differences explain the apparently lower 14C bioavailability (8–11%) for [14C]procyanidin calculated from blood [AUC(0–24)] values. After oral administration of [14C]procyanidin B2, 63% was excreted via urine within 4 days. The data suggest that much of the parent compound administered orally is degraded by the gut microflora before absorption and that these microbial metabolites have a different distribution from the compounds circulating after the intravenous dose.

In recent years, polyphenols have been the subject of intense investigation because of their wide distribution in plants and plant foods and their documented association with decreased risk of chronic diseases such as cardiovascular diseases and some forms of cancer (Williamson and Manach, 2005). Proanthocyanidins (condensed tannins), the oligomeric forms of flavan-3-ols, are among the most widespread polyphenols in plants (Ferreira and Slade, 2002) and also in the human diet (Gu et al., 2004). Procyanidins are the commonest type of proanthocyanidin with (+)-catechin and (−)-epicatechin their main constituent units (Ferreira and Slade, 2002; Gu et al., 2004). The chemical structure is complex and consists of several different linkages between the molecules and different arrangements of the two flavanol components. Procyanidin-rich beverages and foods include cocoa, grapes, apples, strawberries, and red wine. The average dietary intake of proanthocyanidins in the United States has been estimated at 58 mg/day (Gu et al., 2004). The most ubiquitous proanthocyanidin dimers present in nature and in human diets are the B-type procyanidins, where the flavan-3-ols are linked via an interflavan bond between the benzylic C-4 carbon of the heterocyclic ring of the upper unit and the C-8 carbon of the flavan-3-ol A-ring of the lower unit (4→8) (Fig. 1). Procyanidins with a 4→6 link have also been reported in nature but occur less frequently (Fig. 2) (Ferreira and Slade, 2002).

Fig. 1.
Fig. 1.

Structure of C(4→8) procyanidins, modified from De Bruyne et al. (1999).

Fig. 2.
Fig. 2.

The structure of C(4→6) procyanidins, modified from De Bruyne et al. (1999).

Previous studies have shown the pharmacokinetics of the monomeric flavan-3-ols (−)-epicatechin and (+)-catechin in vivo in rats (Hackett and Griffiths, 1982; Gott and Griffiths, 1987; Baba et al., 2001b, 2002; Catterall et al., 2003), but very little is known about the bioavailability and pharmacokinetics of procyanidin dimers and/or other related higher molecular weight oligomers, which are consumed at high levels by most of the population. The physiological and biological activities of such abundant compounds will depend on their absorption and metabolism after ingestion.

Early studies in the 1970s reported the presence of unconjugated procyanidin oligomers in urine and in different organs/tissues of rats and mice after oral administration of [14C]flavan-3-ol oligomers (Harmand and Blanquet, 1978). Later studies in animals, using liquid chromatography/electrospray ionization/mass spectrometry analyses, reported the presence, after enzymatic hydrolysis, of unconjugated procyanidin dimers, including B1, B2, B3, B4, and B5, in various tissues including plasma and urine after feeding large doses of proanthocyanidin extracts or proanthocyanidin-rich seeds (Baba et al., 2002; Tsang et al., 2005; Shoji et al., 2006). Likewise, human studies have detected unconjugated procyanidin B1, B2, and B5 in plasma and serum within 30 min of consumption of the test material (Holt et al., 2002; Sano et al., 2003). Unconjugated procyanidin B2 concentration in the systemic circulation reaches a peak at approximately 2 h after administration in both animals (Baba et al., 2002; Shoji et al., 2006) and humans (Holt et al., 2002; Sano et al., 2003). However, the levels of intact procyanidin B2 detected in human plasma (∼10–40 nM) after high doses of proanthocyanidins are lower, sometimes by several orders of magnitude, than the concentrations observed to be effective in various in vitro tests.

Other reports have provided evidence on the in vitro degradation of crude cocoa proanthocyanidins by human colonic bacteria into a complex mixture of smaller molecular weight phenolic acids (Déprez et al., 2000). It is known that these microbial metabolites enter the portal circulation and have been proposed to account, at least in part, for the biological activities of the parent compound. Such microbial metabolites have been detected in the urine of rats in the free form (Gonthier et al., 2003a,b) and/or as mammalian conjugates produced by a combination of O-methylation, glucuronidation, and/or sulfation after absorption (Baba et al., 2001a, 2002; Gonthier et al., 2003b). It has been shown that some flavan-3-ol conjugates (e.g., the 5-β-d-glucuronides) but not all (e.g., 3′-O-methyl) retain their in vitro antioxidant activity (Harada et al., 1999). However, it has been suggested that their transient occurrence and low concentrations in plasma, as well as relatively high redox potential compared with tocopherols and ascorbate, make it unlikely that their antioxidant properties explain any possible benefit to health and that alternative mechanisms should be sought and evaluated (Clifford, 2004). The objective of the current study was to investigate in vivo in male rats the bioavailability, pharmacokinetics, and mass balance of purified [14C]procyanidin B2 (Fig. 3), the major dietary proanthocyanidin.

Fig. 3.
Fig. 3.

Procyanidin B2 radiolabeled at position 2 (∗) of the heterocyclic C-ring of the upper unit.

Materials and Methods

Chemicals.

[14C]Procyanidin B2 was synthesized by Selcia Limited (Ongar, Essex, UK) according to our previously published procedure (Viton et al., 2008). It was received in a liquid form (water solution) in two batches with a specific activity of 49.58 and 78.08 MBq/mmol for the first and second batch, respectively, and used immediately on the same day. Radiochemical purity was confirmed before use by radio-/high-performance liquid chromatography. Solvable was obtained from PerkinElmer Life and Analytical Sciences (Waltham, MA); hydrogen peroxide, scintillation mixture (BioChemika, for radiometry), and EDTA, BioUltra, ≥99.0% (KT) were purchased from Sigma-Aldrich (St. Louis, MO).

Animals.

Wistar albino rats, specific pathogen-free, weighing 200 to 250 g, were purchased from B&K Universal Ltd. (Hull, UK). The animals were kept in controlled conditions within the provisions of Home Office license, UK, and were adapted for 1 week (12-h dark/light cycle), constant temperature of 22 ± 1°C, and 50% humidity with free access to a standard rodent diet (B&K Universal Ltd.) and water. The animals were weighed and divided randomly into three groups (I–III). For the intravenous administration (group I), NaCl was added to [14C]procyanidin B2 to make a solution of 0.9% saline, and a single dose was applied via the caudal vein (∼150 μl/animal) adjusted to provide a dose of 21 mg/kg b.wt. for each animal. For the oral administration, a single dose was applied, approximately 500 μl/animal, adjusted to provide a dose of 21 mg/kg b.wt. (group II) and 10.5 mg/kg b.wt. (group III) for each rat. The weight of the gavage syringe used to deliver the dose was recorded before and after dosing, and the exact amount administered to each rat was calculated by mass difference.

Mass Balance and Blood Pharmacokinetics.

After dosing, rats were housed individually in metabolic cages (Arrowmight Biosciences, Hereford, UK), and urine and feces were collected daily for 4 days at 0, 24, 48, 72, and 96 h after dosing. For the blood pharmacokinetics, approximately 2 mm was removed from the end of the tail with a scalpel blade, and blood samples (0.1 ml) were collected at regular intervals between 0.5 and 24 h (sampling: 0.5, 1, 2, 3, 4, 5, 6, 7, 9, 12, 15, 18, 20, 22, and 24 h) into lithium-heparinized centrifuge microtubes.

Mass Balance Analysis.

Duplicate aliquots of urine (250 μl) were mixed with 5 ml of scintillation mixture, and total 14C was counted. Fecal homogenates (10% w/v in water) were prepared, and duplicated aliquots (250 μl) were placed into scintillation vials. Each sample was digested using Solvable (160 μl; PerkinElmer Life and Analytical Sciences). Samples were placed in a shaking water bath (50°C for 1 h). They were allowed to cool to room temperature, and then 30 μl of EDTA (0.1 M) and 160 μl of H2O2 (30%, v/v) were added. The vials were incubated further (50°C for 1 h) and then allowed to cool to room temperature. Scintillation mixture was added (5 ml) to each sample, and radioactivity content was determined by liquid scintillation counting (Wallac 1400 DSA, 2.50; PerkinElmer Wallac, Gaithersburg, MD). Total radioactivity content was quantified by liquid scintillation counting, and each analysis was carried out in triplicate. Total 14C in urine and feces was calculated as a percentage of the dose administered. Animal data were analyzed individually and are presented as mean ± S.D. for five animals.

Analysis of Blood Pharmacokinetics.

Duplicate aliquots (40 μl) of blood were placed into scintillation vials. Each sample was digested and decolorized as described for the mass balance. Measurements for each sample were taken in triplicate. The concentration of radioactivity was calculated as micrograms of [14C]procyanidin B2 equivalents per gram of blood, and the pharmacokinetic parameters were calculated using the PK solutions software package (version 2.0; Summit Research Services, Montrose, CO) and fitted to a noncompartmental model. Animal data were analyzed individually and are presented as mean ± S.D. for nine animals.

Results

Mass Balance.

Excretion of radioactivity from 0 to 96 h was higher in urine than in feces (Table 1). During the first 24 h after administration, ∼69% of the dose was excreted in urine for the intravenous group (I), whereas ∼58% was recovered for both oral groups (II and III). Total urinary excretion at the end of the collection period was calculated as ∼76% after intravenous dosing (group I) and 62 to 63% after oral dosing (groups II and III), respectively. Fecal excretion after intravenous administration was approximately 25% within the first 24 h and increased to ∼28% at the end of the collection period. After oral administration, almost the entire amount excreted in the feces (∼41%) at the end of the collection period was recorded within 24 h (∼39–40%) (Table 1) for both groups II and III.

View this table:
TABLE 1

Excretion of total radioactivity following single intravenous and oral administrations of [14C]procyanidin B2 in male rats

[14C]Procyanidin B2 was administered to rats as either a single intravenous dose (group I received 21 mg/kg) or oral dose (group II received 21 mg/kg; group III received 10.5 mg/kg). Data represent total 14C radioactivity, i.e., the parent compound and/or metabolites. Results are mean ± S.D. for nine animals.

After intravenous administration of [14C]procyanidin B2, approximately 93% of the total dose was excreted within the first 24 h. After oral administration, total radioactivity excreted in urine and feces from groups II and III was calculated as ∼98 and ∼99%, respectively (≤24 h after administration). Low levels of total radioactivity were excreted even 96 h after administration for all three groups of animals (I, II, and III). The sum of total radioactivity collected from urine and feces in all three groups of animals suggests full recovery (103.2 ± 7.2%) within experimental error. Results are the mean ± S.D. for five animals. Bioavailability (F) obtained from the ratios of the total amount of 14C excreted in urine after oral dosing (U∞oral) to the total amount of the 14C excreted in the urine after intravenous dosing (U∞iv) was calculated as 82 to 83% (Table 2).

View this table:
TABLE 2

Summary of blood pharmacokinetics

[14C]Procyanidin B2 was administered to rats as a single intravenous dose (group I received 21 mg/kg) or oral dose (group II received 21 mg/kg; group III received 10.5 mg/kg). Data were analyzed using the PK Solutions 2.0 pharmacokinetic software and fitted to a noncompartmental model. Results are mean ± S.D. for nine animals.

Blood Pharmacokinetics.

Total 14C radioactivity in blood was plotted against time after intravenous and oral administration (Figs. 4 and 5, respectively). After intravenous administration (group I), a prominent secondary peak shows a transient increase in total blood 14C radioactivity at approximately 7 to 8 h after administration (Fig. 4). The blood profiles of total 14C radioactivity for both oral doses (groups II and III, respectively) are compared in Fig. 5 and show no evidence of a secondary peak. Results are the mean ± S.D. for three animals per time point.

Fig. 4.
Fig. 4.

Profiles of total radioactivity in blood sampled from male rats. The concentration of radioactivity is expressed as microgram equivalents of procyanidin B2 per milliliter after intravenous administration of [14C]procyanidin B2 for group I (21 mg/kg). Results are mean ± S.D. for nine animals.

Fig. 5.
Fig. 5.

Profiles of total 14C radioactivity in blood sampled from male rats in group II (21 mg/kg, square symbols) and group III (10.5 mg/kg, round symbols). The concentration of radioactivity is expressed as microgram equivalents of procyanidin B2 per milliliter after oral dosing. Results are mean ± S.D. for nine animals.

The pharmacokinetic parameters calculated from the noncompartmental analysis subsequent to intravenous or oral doses (groups I, II, and III, respectively) are shown in Table 2. After intravenous administration (group I), the terminal half-life was 6.67 ± 0.95 h, and the rate constant for elimination (kel) was 0.12 ± 0.02/h. Total clearance [Clp(iv)] and the apparent volume of distribution (Vd) were calculated as 0.56 ± 0.1 ml/min and 1.28 ± 0.21 l/kg, respectively.

After oral administration, the maximum blood concentration (Cmax) for group II was twice that of group III, and the Tmax values were similar for both groups. Similar values were also calculated for total clearance [Clp(oral)] and apparent Vd for both groups II and III, assuming bioavailability of 83%.

The area under the curve [AUC(0–24)] value for group I was 149 ± 21 μg · h/min, but those for group II and group III were only 11 and 8% of this value, respectively. Consequently, use of these AUC(0–24) values gave much lower values for bioavailability than those obtained from the urinary data.

Discussion

The present study, in which pure radiolabeled [14C]procyanidin B2 was administered to male rats, defines its bioavailability, absorption, excretion, and pharmacokinetics. Note that in this report, data represent cumulative total radioactivity, i.e., the parent compound and/or its metabolites.

A summary of the mass balance study is illustrated in Table 1. After intravenous administration, ∼76% of the dose was excreted via urine by the end of the collection period (≤96 h) (Table 1), reflecting extensive renal clearance. However, ∼28% of the dose was excreted in the feces, indicating biliary excretion along with a prominent secondary peak (Fig. 4), suggesting enterohepatic recycling.

Studies with rats fed unlabeled procyanidins have reported the excretion of unchanged procyanidins in urine to be very low (<0.5%) (Baba et al., 2002; Tsang et al., 2005; Shoji et al., 2006), and although methylated forms have been observed (Shoji et al., 2006), there are no reports of glucuronidated or sulfated metabolites. This is in contrast to studies on the flavanol monomers [(+)-catechin and (−)-epicatechin], in which substantial amounts of glucuronide and sulfate conjugates have been observed in urine within 24 h, accounting for 27 and 36% of the dose, respectively (Tsang et al., 2005). Baba et al. (2001b) also reported that conjugated forms accounted for the majority (approximately 70–90%) of the flavanol monomers excreted in urine but did not express this as a percentage of the dose. Baba et al. (2002) reported the excretion of trace amounts of epicatechin after feeding rats with unlabeled procyanidin B2, and again a substantial portion of this epicatechin was present as methylated and glucuronidated derivatives, but the exact percentage was not clearly expressed. There is also evidence for the absorption of phenolic acids produced from ingested procyanidins by the gut microflora and their excretion in urine. Gonthier et al. (2003b) reported that 14 phenolic acids together accounted on a molar basis for 6.5% of the ingested procyanidin B3.

In this study, the first to use pure [14C]procyanidin B2, ∼58% of the ingested procyanidin and/or its 14C mammalian and microbial metabolites were excreted in urine (≤24 h) after administration of higher or lower oral doses (groups II and III, respectively) (Table 1). Although excretion in urine in the current study was substantially higher than previously reported, none of the earlier studies was able to investigate all the forms excreted.

Maximum concentrations (Cmax) of conjugated metabolites in the systemic circulation after administration of grape seed extract containing oligomeric and polymeric procyanidins (Tsang et al., 2005; Shoji et al., 2006) and after 3-palmitoyl-(+)-catechin administration (Hackett and Griffiths, 1982) in rats have been reported to be between 3 and 8 h, in the range of the Tmax of 14C observed after oral dosing of [14C]procyanidin in the present study. Such conjugates, because of their large molecular weight and relative hydrophilicity, would be expected to undergo biliary excretion in the rat and enterohepatic recirculation; however, such recycling is not necessarily complete (Benet and Massoud, 1984) as indicated in this study, where after administration of both higher or lower doses (groups II and III, respectively), ∼40% of the total radioactivity of [14C]procyanidin B2 was excreted in the feces (≤96 h after administration) (Table 1). However, the 14C detected in the feces could include unmetabolized procyanidin B2, its microbial breakdown products, procyanidin B2 metabolites excreted in bile, and procyanidin B2 metabolites effluxed from the enterocytes.

In the present study, approximately 60% of the oral dose was recovered in the urine (≤96 h after administration) (Table 1). Similar behavior with flavonoids in rats has been observed by others reporting total 14C excretion in urine to reach up to 60 or 67% after a single administration of [14C](+)-catechin (40 mg/kg) (Gott and Griffiths, 1987) or genistein (4 mg/kg), respectively (Coldham and Sauer, 2000).

Almost the entire dose was excreted within the first 24 h (Table 1), and such observations are in fair agreement with the terminal half-lives (t1/2) for groups II and III, respectively (Table 2). Likewise, previous in vivo studies with rats have reported the total excretion of flavan-3-ols and/or their metabolites to occur within the first 24 h after administration (Hackett and Griffiths, 1982; Gott and Griffiths, 1987; Catterall et al., 2003; Tsang et al., 2005). Extensive enterohepatic cycling of related flavonoids and their conjugated metabolites has been reported elsewhere in male rats (Gott and Griffiths, 1987; Coldham and Sauer, 2000; Silberberg et al., 2006) and has been reported to be proportional to the dose administered reaching a net transfer of biliary secreted conjugates of up to 50% of the dose (Silberberg et al., 2006).

The Clp are not the same after intravenous and oral dosing as shown in Table 2. This suggests that the radioactivity counted after oral dosing was present predominantly in a form(s) other than the parent compound administered. Because the terminal half-lives were similar after intravenous and oral dosing, it is proposed that predominantly the difference in the clearance values is a result of the 8-fold larger Vd after oral dosing compared with the intravenous dosing (Table 2). This emphasizes that the measurement of 14C is monitoring different chemical entities after oral dosing compared with intravenous dosing. In addition, the 8-fold larger Vd calculated from the total 14C after oral dosing (higher or lower), compared with the intravenous dosing, indicates that some of the compounds formed after the oral dosing are comparatively more hydrophobic than the parent compound. This observation is consistent with previous findings (Stoupi et al., 2009) that the gut flora metabolites produced in an in vitro model of procyanidin B2 catabolism have greater retention times (14.06–22.71 min) compared with the substrate (13.53 min) during reversed-phase high-performance liquid chromatography.

Bioavailability (F) from the U∞ values (Table 1) was calculated as ∼82% (Table 2). The discrepancy between the F values obtained from the AUCs and U∞ (Table 2) is attributable to the difference in the clearance values between intravenous and oral dosing (Table 2), reflecting the difference in the compounds being monitored. In the present study, a significant level of radioactivity was detected in blood even at 0.5 h after oral administration, suggesting that some of the [14C]procyanidin B2 was absorbed in the stomach or upper gastrointestinal tract, probably in the intact form (Fig. 5). However, the maximal concentrations (Cmax) for total 14C in blood were not attained until 5 to 6 h after oral administration, suggesting that much of the radioactivity was absorbed from the distal part of the small intestine and/or the colon. Taken collectively, these data strongly suggest that much of the radioactivity detected in the blood after oral administration is derived from low molecular weight phenolic acids and other compounds produced from procyanidin B2 by microorganisms in the gut before absorption, as has been previously shown for procyanidin B3 by rat cecal microflora (Gonthier et al., 2003b), crude proanthocyanidins by human colonic microflora (Déprez et al., 2000), and procyanidin B2 by human colonic microflora (Stoupi et al., 2009) in vitro.

Similar observations have been made previously but have been explained by suggesting that the major bioavailable form of procyanidin dimers (B2 and B5) was (−)-epicatechin formed by depolymerization based on studies using an in vitro small intestinal model (Spencer et al., 2001). Subsequent studies in humans showed that this depolymerization did not occur in vivo (Rios et al., 2002). In this regard, it is interesting to note that after a single oral dose of (−)-[3-3H]epicatechin (4.5 mg/kg), rats excreted 2.9 and 90.3% of the total radioactivity in urine and feces, respectively, and the bioavailability was 7.8% calculated from the U∞ values (Catterall et al., 2003). This behavior is in marked contrast to that observed with [14C]procyanidin B2 and also suggests that in the present study little if any of the procyanidin was metabolized to an intermediate monomer, consistent with previous reports (Stoupi et al., 2009). In the present study, Cmax increased in proportion with the oral dose, the value for group II being twice that of group III (Table 2), thus providing no evidence for saturation at least at these two doses and suggesting linear first-order elimination at such doses.

To conclude, [14C]procyanidin B2 is bioavailable in male rats (∼82%) as calculated from the 14C U∞ values, and most of the radioactivity is absorbed from the distal part of the small intestine and/or the colon (Tmax ≈ 6 h). These data suggest that a greater fraction of the parent compound is transformed by the gut microflora and that it may be these metabolites that are responsible for any biological activities or health benefits that might be associated with proanthocyanidin consumption.

Acknowledgments.

We thank Selcia Ltd. for the synthesis of the labeled procyanidin B2, Kim Morton and Graham Moorey for assisting with the animal handling in this study, Penny Giorgio as radiation protection adviser, Natalya Roxborough as radiation protection officer, and Dr. Laurent Fay for facilitating funding.

Footnotes

  • This work was funded by the European Union 6th framework project FLAVO.

  • Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.

    doi:10.1124/dmd.109.030304

  • Clp
    total clearance
    Vd
    apparent volumes of distribution
    AUC
    area under the curve.

    • Received September 19, 2009.
    • Accepted November 5, 2009.

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In Vivo Bioavailability, Absorption, Excretion, and Pharmacokinetics of [14C]Procyanidin B2 in Male Rats

Stavroula Stoupi, Gary Williamson, Florian Viton, Denis Barron, Laurence J. King, Jonathan E. Brown and Michael N. Clifford
Drug Metabolism and Disposition February 1, 2010, 38 (2) 287-291; DOI: https://doi.org/10.1124/dmd.109.030304
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