Diaheles Research aml Clinit'al Practice. 4 (1988) 21—29
`Elsevier
`
`DRC 00181
`
`223
`
`Metformin in the digestive tract
`
`N. Vidonl, S. Chaussadel, M. Noelz, C. Franchisseurl, B. Huchet1 and J. J. Bernierl
`
`1INSERM U290. H(3pi1al Sainl—Lazare, 75010 Paris. France and 2S.E.R.P.A.. 92/52 Suresnes Cedm‘, France
`
`(Received 1 April 1987. revision received 9 September 1987. accepted 14 September 1987)
`
`
`
`Key H'UI'l/A‘.‘ Metformin; Digestive tract
`
`
`Summary
`
`After ingestion of metformin. a drug of the biguanide class. there are gastrointestinal side effects in the form
`of nausea and vomiting. and about 30% of the drug is recovered in feces. The purpose of this work was to
`explain these two phenomena. Two sets of experiments were carried out.
`Study 1 evaluated the gastroduodenal (GD) absorption in six healthy volunteers by means of an intubation
`method, employing a twin—lumen tube introduced into the intestine and another into the stomach. Metformin
`1 g was introduced into the stomach with a homogenized meal containing a non-absorbable marker,
`l4C-
`PEG 4000; another marker. PEG 4000, was perfused continuously into the duodenum at the ampulla of
`Vater. Samples of GD contents were collected every 15 min during 4 h. Metformin was poorly absorbed
`from the stomach, about 10% over a 4-h period. It did not modify the gastric emptying ofa meal but induced
`a duodeno-gastric reflux in five out of six subjects. About 20% of the amount of drug emptied from the
`stomach were absorbed from the duodenum. The delivery process was the rate—limiting factor for metformin
`absorption from the duodenum. The AUC/24 h increased as the absorption rate from the duodenum in-
`creased.
`
`Study 2 investigated in six healthy volunteers, using another intestinal perfusion technique. the jejunal and
`ileal absorption of metformin. Metformin 400 mg in saline solution was perfused, over a 2-h period. below
`an inflated balloon. directly into either the jejunum or the ileum. The mean amount of drug absorbed along
`a 25-cm segment was low. and similar from the jejunum and ileum: 10.8% and 8.8% respectively. When the
`drug was perfused into the jejunum, the AUC values were about 2.5 times higher than the values when the
`drug was perfused into the ileum. These results suggest that the whole intestine is necessary for a sufficient
`absorption of the drug.
`
`Address for correspondence: Miss N. Vidon. INSERM U290.
`Hopital Saint-Lazare. 107 bis rue du faubourg Saint-Denis.
`75010 Paris. France.
`
`ml
`'
`1
`any Year}
`Metformm has been ”com-mendedfor
`m the treatment 0f maturIIY'00set diabetes~ [1 13
`now the most commonly prescribed oral hypogly-
`
`Introduction
`
`0168-8227/88/30350 «‘1 1988 Elsevier Science Publishers B.V. (Biomedical Division)
`
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`

`

`224
`
`cemic drug of the biguanide class. In spite of its
`widespread use, little is known about the pharma-
`cokinetics of this drug. Previous investigations [1—
`4] have shown that after ingestion of metformin,
`the cumulative excretion in urine over 48 h is about
`
`50% of the ingested drug amount while the recov-
`ery in feces is about 30%. Moreover, it has been
`suggested that metformin could induce inhibition
`of gastric emptying in the rat [5] and the dog [6]
`and that this inhibition could be responsible for the
`gastrointestinal side effects of biguanides (nausea,
`vomiting). The purpose of this work was to study
`in healthy volunteers (1) the effect of metformin on
`gastric emptying ofa meal, and (2) the gastrointes-
`tinal absorption of this drug by a direct method.
`
`Materials and methods
`
`Subjects
`
`Studies were performed on 12 healthy volunteers
`(all men), aged 25.0 :1: 1.2 years (mean :1: SEM).
`No subject had a history of bOWel disease or dia-
`betes. All subjects gave written informed consent,
`the protocol having been approved by the Ethical
`Subcommittee of the Saint-Lazare Hospital. Their
`weight was 71.9 :1: 1.3 kg and their height was 178.9
`:1: 1.4 cm. The subjects were instructed to take no
`
`drug during the 8 days preceding the study and
`none other than metformin during the 3 days of
`intubation.
`
`General procedure
`
`The intubation techniques facilitate investigation of
`the gastrointestinal absorption of drugs in man
`[7—9]. A complete description of the absorption of
`metformin from stomach to colon is given in this
`paper. The effect of metformin on postprandial gas-
`tric function is also reported.
`Two sets of experiments were performed. In the
`first study (A), metformin was ingested with an ho-
`mogenized meal; the gastric emptying of metformin
`and its gastroduodenal absorption were studied by
`applying the technique described by Bernier et al.
`[10], as improved by Malagelada et al. [11] and Vi-
`don et a1. [12]. This involved, after an intubation of
`
`the stomach and duodenum. measuring gastric
`
`functions by dilution of two aqueous non—absorb-
`able markers, one present in the meal,
`the other
`simultaneously perfused into the duodenum.
`The second study (B) involved perfusing metfor-
`min, dissolved in a saline solution, directly into the
`upper jejunum or the upper ileum; the drug ab-
`sorption was measured from the 25-cm segment
`below each perfusion point.
`
`Solutions
`
`I. Unlabelled polyethylene glycol (PEG
`Study A.
`4000, 10 g/l) in normal saline solution was perfused
`into the duodenum as a duodenal recovery marker
`at a flow rate of 2 ml/min.
`II. The homogenized meal consisted of 90 g ten—
`derloin steak, 70 g white bread, 14.5 g olive oil, 90
`g pear sherbet and 190 ml water containing 30 uCi
`of
`l4C—labelled polyethylene glycol 4000 (”C-
`PEG). The total caloric value was 490 kcal, made
`
`up of approximately 50% carbohydrates, 30% fat
`and 20% proteins. After homogenizing,
`the vol—
`ume, osmolality and pH of the meal were 400 ml,
`490 mOsm/kg and 5.6, respectively.
`
`Study 8. The perfused solution contained: NaCl
`130 mM, KC] 5 mM, mannitol 30 mM, l4C-PEG
`
`15 uCi/l and metformin 700 mg/l.
`
`Experimental design
`Study A. Experiments were carried out over a 3-
`day period in six subjects aged 23.7 :1: 0.2 years.
`On the first day, subjects were intubated with a
`double—lumen tube. The meal was given on two
`consecutive days (days 2 and 3), with or without 1
`g of metformin in a random order.
`
`After an overnight fast, the tube was positioned
`under fluoroscopic control with the perfusion site
`of unlabelled PEG 4000 at the ampulla of Vater,
`the aspiration site being located 20 cm distally, near
`the ligament of Treitz. A gastric tube was then posi-
`tioned with its tip in the antrum. The volunteers
`adopted a sitting position for the duration of the
`study.
`
`The meal was introduced into the stomach by the
`gastric tube. Duodenal perfusion of PEG 4000 was
`started immediately before the ingestion of the
`
`

`

`meal. The meal was introduced over a period of 164
`:l: 15 s. Gastric and duodenal contents were sam-
`
`pled every 15 min for 4 h, duodenal contents being
`aspirated at a rate of 1 ml/min. At the end of the
`experiment,
`the gastric contents were completely
`aspirated. The stomach was then rinsed with 250 ml
`of normal saline solution to recover all the meal
`marker.
`
`Blood samples were taken at 0, 15, 30, 60, 90,
`120, 150, 180, 210, 240, 360, 480 min and 24 h when
`
`the meal contained drug.
`
`Study B. Experiments were carried out over a 3-
`day period in six subjects aged 25.9 :I: 2.1 years.
`On the first day, subjects were intubated with a
`four-lumen tube which also incorporated an occlu-
`sive balloon [13]. The technique has been described
`elsewhere [8].
`The segment length was 25 cm. The solution was
`perfused at a rate of 5 ml/min over 2 h. After an
`
`equilibration period to obtain a hydrodynamic
`steady state, five successive 15-min samples were
`collected.
`
`The jejunal and ileal absorption was studied on
`days 2 and 3, respectively.
`Blood samples were taken at 0, 15, 30, 45, 60, 90,
`120, 150, 180, 210, 240, 360 and 480 min. For the
`
`jejunal study only, a blood sample was also taken
`at 24 h.
`
`Analytical method
`Study A.
`In each gastric and intestinal sample, the
`cold PEG concentration was measured by the tur-
`bidimetric method of Hyden [l4];
`l“'C-PEG was
`measured in a scintillation counter; metformin con—
`
`centrations were measured by a high-performance
`liquid chromatography (HPLC) method.
`
`In each intestinal sample, cold PEG and
`Study B.
`metformin concentrations were measured as in
`
`study A. In both studies, metformin plasma con-
`centrations were measured by HPLC.
`
`225
`
`gastric emptying of the meal and drug and the gas-
`tric secretion were measured using previously re—
`ported procedures [1 l]. The duodenal absorption of
`the drug was calculated using measured gastric and
`duodenal concentrations of markers and metfor-
`
`min. Gastric absorption of the drug was estimated
`from the difference between the amount given with
`the meal and that leaving the stomach (amount
`passing the pylorus + amount removed with gas-
`tric
`samples).
`Areas
`under
`the
`plasma
`concentration—time curves (AUC) were calculated
`
`by the trapezoidal rule. The peak plasma concen-
`tration of metformin (Cmax) and times to reach the
`peak level (Tmax) were determined.
`
`Study B. The absorption of metformin along the
`studied segment per unit of time was calculated ac-
`cording to the usual formulae for perfusions [15].
`Absorption was defined as the difference between
`the amount entering and that recovered at the sam—
`pling site.
`Data are expressed as means :t SEM. Wilcox-
`on’s t-test for paired or non-sequential and corre-
`lation analysis were used for statistical compari—
`sons.
`
`Results
`
`Study A
`Gastric absorption of metformin. Total l4C-PEG
`recovery was measured by adding 1“C—PEG con—
`tained in gastric samples, the final aspiration, the
`gastric lavage and the amount passing the ligament
`of Treitz. Recovery was nearly complete, being 95.1
`i 2.7 and 95.7 :t 1.4% with and without metfor-
`
`min, respectively. The amount of drug lost from the
`stomach by gastric emptying and sampling over the
`study was 81.9% of the amount ingested. The met-
`formin to 14C-F‘EG ratio in each gastric sample
`over 4 h postprandially also reflects gastric absorp—
`tion of the drug; it was 86.7 :I: 1.0%. These results
`indicate that about 13% of metformin were ab-
`
`Calculations and statistical analysis
`
`sorbed from the stomach over a 4-h period.
`
`Study A. The postprandial volume of gastric con-
`tents, its fraction emptied into the duodenum, the
`
`Gastric emptying. Gastric emptying of the meal
`
`

`

`226
`
`TABLE I
`
`INTRAGASTRIC VOLUME OF MEAL EXPRESSED AS PERCENT OF INGESTED MEAL VOLUME IN RELATION TO
`TIME
`
`
`Intragastric volume of meal
`
`
`
`Time after meal ingestion
`l h
`2 h
`3 h
`4 h
`
`With metformin
`Without metformin
`
`1.6 i 0.9
`12.9 i 3.3
`36.9 i 3.4
`[.6
`66.5 :1:
`
`
`
`37.4 i 3.0 13.9 i 4.867.2 :I: 2.5 5.5 a: 5.0
`
`varied little between subjects, the intragastric vol-
`ume of meal in relation to time was not statistically
`different with or without metformin (Table 1). At
`the end of the 4 h, almost all of the initial meal had
`left the stomach.
`
`Gastric secretion. Total gastric volume is made up
`of the volume of the meal present in the stomach
`plus the volume of salivary and gastric secretions.
`These salivary and gastric secretions varied con-
`siderably between subjects but were not signifi-
`cantly modified by metformin. Their values were
`1040 :t 230 and 977 :1: 154 ml with and without
`
`metformin, respectively.
`
`In five subjects out of six,
`Duodenogastric reflux.
`there was a duodenogastric reflux when the meal
`contained metformin.
`Indeed. gastric juice was
`
`Gastric emptying
`o! Mellormin
`mg 15'
`
`100
`
`50
`
`0
`
`0 15 30
`
`60
`
`90
`
`120
`
`150
`
`180
`
`210
`
`240 Mlnules
`
`Fig.
`
`l. Gastric emptying of drug in relation to time over a 4-h
`period after ingestion with a meal (mean i SEM).
`
`stained yellow by bile acids as early as 20 min after
`ingestion of the meal in four subjects and continued
`so over more than 2 h. Cold PEG was detected in
`
`three gastric samples from five subjects. When met-
`formin was not ingested. gastric juice was never
`stained by bile acids. In one subject, cold PEG was
`detected during 3 h and 45 min after ingesting the
`meal with and without drug respectively; no side
`effects were associated with duodenogastric reflux.
`
`Gastric emptying and duodenal absorption of met-
`formin.
`Gastric emptying of metformin did not
`vary between subjects. The amount of drug emptied
`within a period of4 h was 78.4 :t 2.2% of the total
`amount
`that could have been emptied.
`i.e..
`the
`amount ingested minus the amounts sampled over
`4 h.
`
`The amount of drug emptied every 15 min varied
`considerably during the Study (Fig. 1). The duo—
`denal absorption of this drug (Y) is closely related
`to the gastric emptying rate (X) (Fig 2). The rela—
`
`nun/20 m. =
`(r = 0.73): Ymg/ls
`linear
`tion is
`——
`1.1;
`the slope of 0.22 for the
`0.22Xmg,15 min
`regression line indicates that only 22% of the drug
`emptied from the stomach was absorbed from the
`duodenum. Individual absorptive capacities for this
`segment ranged from 11 to 31% of that delivered
`at the pylorus over 4 h.
`
`Plasma metformin concentrations. The highest
`maximum plasma concentrations of metformin
`
`(Table 2) were recorded for subjects 5 and 6 who
`exhibited the highest mean duodenal absorption of
`
`

`

`Metformin
`duodenal
`absorption
`
`mg/15 min. /20 cm
`100
`
`50
`
`0
`
`50
`
`100
`
`Metformin Gastric delivery
`mg/15 min.
`
`Fig. 2. Duodenal absorption of metformin. Relationship be-
`tween gastric delivery and duodenal absorption rate. ‘y‘ is the
`duodenal absorption rate. ‘x’ the delivery rate from the stomach
`(mean of six subjects).
`
`drug over 4 h. 27 and 31% respectively. Low Cm“
`and low AUC values were associated with low rates
`
`of absorption. Interindividual differences in Cmax
`and bioavailability (Fig. 3) were related to the
`amount of metformin absorbed in the duodenum.
`
`227
`
`in order that sufficient metformin can be detected
`
`in the plasma to give an AUC value.
`
`Study 8
`The difference between absorption rates of metfor-
`min measured in a 25-cm segment of the jejunum
`and ileum was not statistically significant. The ab-
`sorption rates were 10.8 :b 2.6 and 8.8 :1: 1.1% of
`the amount of drug perfused into the jejunum and
`ileum, respectively.
`The peak plasma concentration of metformin
`(Fig. 4) and the AUC/24 h (Table 3) when the drug
`was perfused into the jejunum were significantly (P
`< 0.05) higher than the values when the drug was
`
`perfused into the ileum. The respective AUC values
`were 280 :1: 24 mg - min - 1'1 and 123 :h 14 mg -
`min - 1‘1.
`
`In subject 3, on day 3, the perfusion point was in
`the transverse colon. Metformin was not detected
`
`in plasma over 8 h except at 150 min; at that time
`the drug plasma concentration was only 0.15 mg/l
`which is
`the minimum detectable level by this
`method of analysis.
`
`Discussion
`
`A minimum of 50 mg of metformin must be ab-
`sorbed in the duodenum over 4 h. i.e.. 3 mg/15 min
`
`The first study showed that metformin did not
`modify the rate of gastric emptying in healthy
`
`200 mg Duodenal absorption
`01 metformin
`over on.
`
`O
`
`O O
`
`100
`
`TABLE 2
`
`INDIVIDUAL VALUES OF Cum. Tmnx AND AUC/24 h
`AFTER INGESTION OF METFORMIN WITH A MEAL
`(STUDY A)
`
`
`AUC
`mg.min.l"
`
`100
`
`Subject
`Cm“
`Tm“
`AUC/24 h
`(mg/l)
`(h)
`(mg-min-l‘ ')
`
`
`50
`
`1
`2
`3
`4
`5
`6
`
`1.17
`0.96
`0.62
`1.18
`1.70
`1.71
`
`3
`3
`4
`4
`3.5
`6
`
`478
`253
`422
`656
`1062
`1 141
`
`O
`
`0
`
`669
`1.22
`Mean
`
`
`0.17SEM 147
`
`Fig. 3. Duodenal absorption of metformin. Relationship be-
`tween AUC/24 h and duodenal absorption rates over 4 h after
`ingestion with a meal. r = 0.989.
`
`

`

`228
`
`metformine plasma
`concentration Ing/mL]
`o Jegunal
`of
`0 Heel perfusion
`
`0.2 lmtnulesl
`
`0.6
`0.5
`0.4
`0.3
`
`0
`
`so
`
`IOU ISD 200 250 300 350 400 «so 500
`
`the stomach where it was about only 10% over 4
`h.
`
`In the second study, the amount of drug perfused
`into the jejunum was similar to that reaching the
`jejunum when 1 g of metformin was ingested with
`the homogenized meal in study A. The amounts of
`drug absorbed appeared to be directly related to the
`amounts delivered to the intestine by the stomach.
`Extrapolation of these results to the total small
`bowel length (about 180 cm between the ligament
`of Treitz and the cecum) suggests that about 80%
`of the l g of ingested drug with the meal would be
`absorbed in the gastrointestinal tract, the remaining
`20% would reach the colon and would not be ab-
`sorbed.
`
`The same method of extrapolation when drug in
`saline solution was perfused into the jejunum or
`ileum shows that 50% and 30%,
`respectively,
`would be absorbed, suggesting that the length of
`the small intestine would be a limiting factor of
`metformin absorption; that explains the difference
`between the observed AUC/24 h during the jejunal
`and ileal perfusion. These results are in agreement
`with those of Pentikainen et al. [3] who obtained a
`bioavailability of 50—60% when the drug was given
`with 200 ml water after an overnight fast.
`
`Fig. 4. Metformin plasma concentrations in relation to time
`when drug is perfused into jejunum (O) or ileum (0).
`
`human volunteers. This absence of effect is in con-
`
`trast with an animal study [5,6] which showed that
`
`inhibition of gastric
`biguanides could induce
`emptying. However, metformin did induce duode-
`nogastric reflux in five subjects. The mechanism of
`this reflux is not known but could be the conse-
`
`quence of the action of biguanides on pyloric activ-
`ity. The drug was absorbed from the stomach, duo-
`denum, jejunum and ileum but probably not from
`
`the colon; its absorption rate throughout the gas-
`trointestinal tract was relatively weak, specially in
`
`TABLE 3
`
`INDIVIDUAL VALUES OF Cum. T"m AND AUC/24 h AFTER PERFUSION OF METFORMIN (STUDY B)
`
`Cm“ and AUC values were calculated for a dose of 400 mg metformin presented to the absorption site (the amount of metformin
`perfused minus the amounts sampled during 2 h).
`
`Subject
`
`Perfusion into
`
`
`Jejunum
`Ileum
`
`
`C"m
`Tm“
`AUC/24 h
`Cm,
`Tm“
`AUC/24 h
`(mg/l)
`(min)
`(mg.min.l"‘)
`(mg/l)
`(min)
`(mgtmin-l")
`
`
`1
`2
`3
`4
`5
`6
`
`0.52
`0.57
`1.04
`0.53
`0.59
`0.62
`
`180
`120
`150
`180
`180
`150
`
`236
`256
`397
`263
`266
`264
`
`0.29
`0.30
`
`0.33
`0.23
`0.42
`
`180
`150
`
`150
`210
`180
`
`133
`104
`
`89
`122
`169
`
`123
`174
`0.31
`280
`160
`0.65
`Mean
`
`
`
`
`
`
`0.08 10 24 0.03 llSEM 14
`
`

`

`These results could also explain the difference in
`bioavailability observed when subjects ingested an
`aqueous solution of metformin. two tablets of Glu-
`cophage retard or a formulation which provided a
`slower release of the drug [2,16]. The bioavailability
`will be higher when the drug is ingested with (at the
`beginning of) a meal than under fasting conditions.
`The exact mechanism by which biguanides exert
`their hypoglycemic activity in diabetic patients has
`been of interest for many years and several mech-
`anisms of action have been proposed. It has been
`suggested that biguanides could induce an inhibi-
`tion of glucose absorption [17,18]. It has also been
`shown that biguanides improve glucose tolerance
`after ingestion of glucose whereas they do not after
`intravenous administration of glucose [19]. The bi-
`guanides could inhibit glucose absorption by in—
`hibiting Na+-glucose transport across brush-bor—
`der enterocytes. The degree of inhibition of glucose
`transport could be linked to the concentration of bi-
`guanides in the small intestine. Thus the weak ab-
`sorption rate of metformin from the duodenum. je-
`junum and ileum would maintain a relatively high
`concentration of the drug along the small intestine
`which could be involved in the inhibition of glucose
`absorption. If this is indeed the case. their improve-
`ments in absorption rate could lead to reduced hy-
`poglycemic activity.
`
`Acknowledgement
`
`The authors are indebted to Dr. G. L. Nicholson
`
`for reviewing the English text.
`
`References
`
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`3 Pentikainen. P.J.. Neuvonen. P.J. and Penttila. A. (1979)
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