Diabetes Research and Clinical Practice, 4 (1988) 223-229
`Elsevier
`
`DRC 00181
`
`223
`
`Metformin in the digestive tract
`
`N. Vidon!, S. Chaussade!, M. Noel?, C. Franchisseur!, B. Huchet! and J. J. Bernier!
`
`INSERMU.290, Hopital Saint-Lazare, 75010 Paris, France and ?S.E.R.P.A., 92152 Suresnes Cedex, France
`
`(Received 1 April 1987, revision received 9 September 1987, accepted 14 September 1987)
`
`
`
`Key words: Metformin; Digestive tract
`
`
`Summary
`
`After ingestion of metformin, a drug of the biguanideclass, 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. Twosets of experiments were carried out.
`Study | evaluated the gastroduodenal (GD) absorption in six healthy volunteers by meansof an intubation
`method, employing a twin-lumen tube introduced into the intestine and anotherinto the stomach. Metformin
`1 g was introduced into the stomach with a homogenized meal containing a non-absorbable marker,
`'*C-
`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 of a 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. Thedelivery process was therate-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 anotherintestinal perfusion technique, the jejunal and
`ileal absorption of metformin. Metformin 400 mgin 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 waslow, 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.
`
`Addressfor correspondence: Miss N. Vidon. INSERM U.290.
`Hopital Saint-Lazare, 107 bis rue du faubourg Saint-Denis,
`75010 Paris, France.
`
`me
`1
`£
`any yeas
`Metformin has been recommicneled iior
`in the treatment of micRRLEY=OTiSe diabetes. It
`is
`now the most commonly prescribed oral hypogly-
`
`Introduction
`
`0168-8227/88/$03.50 «*: 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 [l-
`4] have shownthat 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 of a 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 + 1.2 years (mean + 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 + 1.3 kg and their height was 178.9
`+ 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. Theeffect of metformin on postprandial gas-
`tric function is also reported.
`Twosets 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 Malageladaetal. [11] and Vi-
`donet al. [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
`1. Unlabelled polyethylene glycol (PEG
`Study A.
`4000, 10 g/l) in normalsaline 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 pCi
`of
`'*C-labelled polyethylene glycol 4000 ('4C-
`PEG). Thetotal 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 B. The perfused solution contained: NaCl
`130 mM, KCl 5 mM, mannitol 30 mM, !4C-PEG
`15 pCi/l and metformin 700 mg/I.
`
`Experimental design
`Study A. Experiments were carried out over a 3-
`day period in six subjects aged 23.7 + 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 wasintroduced 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 overa period of 164
`+ 15s. 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 wasthen 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 + 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 and3, 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 wasalso taken
`at 24h.
`
`Analytical method
`Study A.
`In each gastric and intestinal sample, the
`cold PEG concentration was measured by the tur-
`bidimetric method of Hyden [14];
`'*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.
`
`Calculations and statistical analysis
`Study A. The postprandial volumeofgastric con-
`tents, its fraction emptied into the duodenum, the
`
`225
`
`gastric emptying of the meal and drug andthegas-
`tric secretion were measured using previously re-
`ported procedures [11]. The duodenal absorption of
`the drug wascalculated using measured gastric and
`duodenal concentrations of markers and metfor-
`min. Gastric absorption of the drug was estimated
`from the difference between the amountgiven 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 (Cyax) and times to reach the
`peak level (Tinax) 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-
`plingsite.
`Data are expressed as means + 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 '*C-PEG
`recovery was measured by adding '*C-PEG con-
`tained in gastric samples, the final aspiration, the
`gastric lavage and the amountpassing the ligament
`of Treitz. Recovery was nearly complete, being 95.1
`+ 2.7 and 95.7 + 1.4% with and without metfor-
`min, respectively. The amountof drug lost from the
`stomachby gastric emptying and sampling over the
`study was 81.9% of the amount ingested. The met-
`formin to !*C-PEG ratio in each gastric sample
`over 4 h postprandially also reflects gastric absorp-
`tion of the drug; it was 86.7 + 1.0%. These results
`indicate that about 13% of metformin were ab-
`sorbed from the stomach overa 4-h period.
`
`Gastric emptying. Gastric emptying of the meal
`
`

`

`226
`
`TABLE |
`
`INTRAGASTRIC VOLUME OF MEAL EXPRESSED AS PERCENT OF INGESTED MEAL VOLUME IN RELATION TO
`TIME
`
`
`
`
` Intragastric volume of meal
`
`
`Time after meal ingestion
`lh
`2h
`3h
`4h
`
`With metformin
`Without metformin
`
`16+ 0.9
`12.9 + 3.3
`36.9 + 3.4
`66.5 + 1.6
`
`
`
`37.4 + 3.0 13.9 + 4.867.2 + 2.5 5.5 + 5.0
`
`varied little between subjects, the intragastric vol-
`ume of meal in relation to time was notstatistically
`different with or without metformin (Table 1). At
`the end of the 4h, 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 + 230 and 977 + 154 ml with and without
`metformin, respectively.
`
`In five subjects out ofsix,
`Duodenogastric reflux.
`there was a duodenogastric reflux when the meal
`contained metformin.
`Indeed, gastric juice was
`
`Gastric emptying
`of Metformin
`mg 18°
`
`100
`
`so
`
`0
`
`0 15 30
`
`60
`
`90
`
`120
`
`6150
`
`180
`
`210
`
`240 Minutes
`
`Fig.
`
`|. Gastric emptying of drug in relation to time over a 4-h
`period after ingestion with a meal (mean + 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 of 4h was 78.4 + 2.2%ofthetotal
`amount
`that could have been emptied, 1.e.,
`the
`amount ingested minus the amounts sampled over
`4h.
`The amount of drug emptied every 15 min varied
`considerably during the study (Fig. 1). The duo-
`denal absorption ofthis drug (Y) is closely related
`to the gastric emptying rate (X) (Fig. 2). The rela-
`tion is
`linear
`(r = 0.73): Yingsis min/20
`em =
`0.22Xme/is min — I.1;
`the slope of 0.22 for the
`regression line indicates that only 22% of the drug
`emptied from the stomach was absorbed from the
`duodenum. Individual absorptive capacities forthis
`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
`
`0
`
`mg/15 min. /20 cm
`
`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
`duodenalabsorption rate, *x’ the delivery rate from the stomach
`(mean of six subjects).
`
`drug over 4 h, 27 and 31%respectively. Low Cmax
`and low AUC valueswereassociated with low rates
`of absorption. Interindividual differences in Cyax
`and bioavailability (Fig. 3) were related to the
`amount of metformin absorbed in the duodenum.
`A minimum of 50 mg of metformin must be ab-
`sorbed in the duodenum over4 h, i.e., 3 mg/15 min
`
`227
`
`in order that sufficient metformin can be detected
`in the plasmato give an AUC value.
`
`Study B
`The difference between absorption rates of metfor-
`min measured in a 25-cm segmentof the jejunum
`and ileum wasnotstatistically significant. The ab-
`sorption rates were 10.8 + 2.6 and 8.8 + 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 weresignificantly (P
`< 0.05) higher than the values when the drug was
`perfused into the ileum. The respective AUC values
`were 280 + 24 mg-min-1~'and 123 + 14mg-
`min - 17}.
`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
`
`The first study showed that metformin did not
`modify the rate of gastric emptying in healthy
`
`200 mg Duodenal absorption
`of metformin
`over 4h
`
`e
`
`s °
`
`100
`
`TABLE2
`
`INDIVIDUAL VALUES OF Coax. Tmax AND AUC/24 h
`AFTER INGESTION OF METFORMIN WITH A MEAL
`(STUDYA)
`
`
`AUC
`mg.min.I7 4
`
`100
`
`Subject
`Cmax
`Tmax
`AUC/24 h
`(mg/l)
`(h)
`(mg-min.I7')
`
`
`50
`
`l
`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
`1141
`
`oO
`
`0
`
`Fig. 3. Duodenal absorption of metformin. Relationship be-
`669
`1.22
`Mean
`tween AUC/24 h and duodenal absorption rates over 4hafter
`
`0.17SEM 147
`
`ingestion with a meal. r = 0.989.
`
`

`

`228
`
`Metformine plasma
`concentration ing/mt)
`© Jerunal
`or
`@ Ileal perfusron
`
`a2 {minutes}
`
`0
`
`SQ
`
`100
`
`150 200 250 300 380 400 450 500
`
`the stomach where it was about only 10% over 4
`h.
`
`In the second study, the amountof drug perfused
`into the jejunum was similar to that reaching the
`jejunum when | g of metformin was ingested with
`the homogenized meal in study A. The amounts of
`drug absorbed appearedto bedirectly 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 | g of ingested drug with the meal would be
`absorbedin the gastrointestinal tract, the remaining
`20% would reach the colon and would not be ab-
`sorbed.
`The same method of extrapolation when drugin
`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 (<) or ileum (@).
`
`human volunteers. This absence of effect is in con-
`trast with an animal study [5,6] which showedthat
`biguanides could induce
`inhibition of gastric
`emptying. However, metformin did induce duode-
`nogastric reflux in five subjects. The mechanism of
`this reflux is not known but could be the conse-
`quenceof 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
`
`TABLE3
`
`INDIVIDUAL VALUESOF Coax» Tmax AND AUC/24 h AFTER PERFUSION OF METFORMIN (STUDY B)
`
`Cmax 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
`Tleum
`
`
`Cra
`Tmax
`AUC/24 h
`Cymax
`Tmax
`AUC/24 h
`(mg/l)
`(min)
`(mg-min.17')
`(mg/l)
`(min)
`(mg-min.1~')
`
`
`l
`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 11SEM 14
`
`

`

`These results could also explain the difference in
`bioavailability observed when subjects ingested an
`aqueoussolution of metformin, two tablets of Glu-
`cophage retard or a formulation which provided a
`slowerrelease of the drug [2,16]. The bioavailability
`will be higher when the drugis ingested with (at the
`beginning of) a meal than underfasting 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 notafter
`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 concentrationofbi-
`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
`
`to
`
`Sirtori, C. R., Franceschini, G., Galli-Kienle, M. et al. (1978)
`Disposition of metformin (NN-dimethylbiguanide) in man.
`Clin. Pharmacol. Ther. 24, 683-693.
`Noel, M. (1980) Kinetic study of normal and sustained re-
`lease dosage forms of metformin in normal subjects. J. Int.
`Biomed. Data |, 9-20.
`Pentikainen, P.J., Neuvonen, P.J. and Penttila, A. (1979)
`Pharmacokinetics of metformin after intravenous and oral
`administration to man. Eur. J. Clin. Pharmacol. 16, 195-202.
`4 Tucker, G. T., Casey, C., Phillips, P.J., Connor, H., Ward,
`J.D. and Woods, H.F. (1981) Metformin kinetics in healthy
`
`tae
`
`229
`
`wa
`
`a
`
`~
`
`oO
`
`subjects and in patients with diabetes mellitus. Br. J. Clin.
`Pharmacol. 12, 235-246.
`Ghareb, A., Botros, M., Saba, J.A. et al. (1969) Mechanism
`of action of biguanides on glucose metabolism. Ains Shams
`Med. J. 20, 313-318.
`:
`Forster, H., Hager, E. and Mehnert, H. (1965) Der Einfluss
`von Butylbiguanid im Tierversuch auf die Resorption von
`Glukose and Fruktose. Arzneimittelforschung (Drug Res.)
`15, 1340-1344.
`
`Jobin, G., Cortot, A., Godbillon, J. et al. (1985) Investiga-
`tion of drug absorption from the gastrointestinal tract of
`man. I. Metoprolol in the stomach, duodenum andjejunum.
`Br. J. Clin. Pharmacol. 19, 97S-105S.
`Vidon, N., Evard, D., Godbillon, J. et al. (1985) Investiga-
`tion of drug absorption from the gastrointestinal tract of
`man. II. Metoprolol in the jejunumand ileum. Br. J. Clin.
`Pharmacol. 19, 1078-1128.
`Godbillon, J., Evard, D., Vidon, N. et al. (1985) Investiga-
`tion of drug absorption from the gastrointestinal tract of
`man. II]. Metoprolol in the colon. Br. J. Clin. Pharmacol.
`19, 1135-1188.
`Bernier, J.J. and Lebert, A. (1971) Vitesse de l’évacuation de
`lestomac et du duodénum au cours de l’hyperglycémie pro-
`voquée per os. Biol. Gastroentérol. 4, 351-352.
`Malagelada, J.R., Longstreth, G.F., Summerskill, W.H.J.
`and Go, V.L.W. (1976) Measurementof gastric functions
`during digestion of ordinary solid meals in man. Gastroen-
`terology 70, 203-210.
`Vidon, N., Muschart, J.M., Cosnes, J., Ruskoné, A. and Ber-
`nier, J.J. (1979) Etude critique de l‘estimation de la vidange
`gastrique par la méthode de perfusion duodénale d’une sub-
`stance non absorbable a faible débit. Gastroenterol. Clin.
`Biol. 3, 549-552.
`.
`Phillips, S. F. and Summerskill, W. H. J. (1966) Occlusion
`of the jejunum for intestinal perfusion in man. MayoClin.
`Proc. 41, 224-231.
`Hyden, S (1955) A turbidimetric method for the determina-
`tion of higher polyethylene glycols in biological materials.
`Ann. R. Agric. Coll. Sweden 22, 139-145.
`Modigliani, R., Rambaud, J.C. and Bernier, J.J. (1973) The
`method of intraluminal perfusion of the human small intes-
`tine. I. Principle and technique. Digestion 9, 176-192.
`Pentikainen, P.J. (1986) Bioavailability of metformin. Com-
`parison of solution, rapidly dissolving tablet, and three sus-
`tained release products. Int. J. Clin. Pharm. Ther. Tox. 24,
`213-220.
`
`Czyzyk, A., Tawecki, J., Sadowski, J., Ponikowska, I. and
`Szczepanik, Z. (1968) Effect of biguanides on intestinal ab-
`sorption of glucose. Diabetes 17, 492-499.
`Caspary, W.F. (1977) Biguanides and intestinal absorptive
`function. Acta Hepato-Gastroenterol. 24, 473-480.
`Hollobaugh, S.L., Rao, B. and Kruger, F.A. (1970) Studies
`on the site and mechanism of action of phenphormin. I. Evi-
`dencefor significant ‘non-peripheral’ effects of phenphormin
`on glucose metabolism in normal subjects. Diabetes 19,
`45-49.
`
`