Department of Pharmaceutics, School of Pharmacy (D.S., M.F., A.H.)
and David R. Bloom Center for Pharmacy (M.F., A.H.), The Hebrew
University of Jerusalem, Jerusalem, Israel; and Diabetes Unit (I.R.),
Hadassah University Hospital, Jerusalem, Israel
Metformin, a commonly used antidiabetic drug, exerts its
glucose-lowering effect due to metabolic activities at several sites of
action (biophases), including liver, intestine, muscle cells, and
adipocytes. The relative contribution of the individual biophases to
the overall glucose-lowering effect is not known. Thus, the aims of
this investigation were to study the influence of mode of drug
administration on the kinetics of glucose-lowering action of metformin
in diabetic rats and identify the contribution of different sites of
action to the overall response. Streptozotocin diabetic rats received
metformin in crossover fashion via intraduodenal, intravenous, and
intraportal routes as bolus dose or infusion regimens designed to yield
similar pharmacokinetic profiles. Metformin plasma concentrations and
blood glucose levels were measured following each mode of
administration. Despite the similarity in the concentration-time profiles obtained for different routes of metformin administration, intraduodenal administration produced larger response than intraportal metformin infusion, and lowest response was observed following intravenous administration. This finding indicates that a significant "first-pass" pharmacodynamic effect, which occurs in the
presystemic sites of action (liver and the gastrointestinal wall),
contributes to the overall glucose-lowering response of metformin. We
applied a combined pharmacokinetic-pharmacodynamic modeling approach to study the nature of the first-pass pharmacodynamic effect. The observed data were successfully described by a novel integrated indirect response pharmacokinetic-pharmacodynamic model that
revealed a correlation between the temporal metformin concentrations
that transit the portal vein and through the gut wall rather than with drug concentrations that accumulated in the liver and the intestinal wall.
 |
Introduction |
Metformin, a biguanide glucose-lowering agent, is
commonly used for management of type 2 diabetes. Despite the wide
clinical use of metformin, the mechanism of its action is not fully
understood. The glucose-lowering effect of metformin is apparently
composed of a combination of several distinct activities in various
organs and tissues (Hermann and Melander, 1992
; Cusi and DeFronzo,
1998), including: 1) decreased hepatic glucose output due to decreased hepatic gluconeogenesis and increased glycogenesis and lipogenesis (Christiansen and Hellerstein, 1998); 2) reduced rate of intestinal glucose absorption (Wilcock and Bailey, 1991); and 3) increased glucose
uptake by muscle cells and adipocytes (Bailey et al., 1996). The
multifactorial mechanism of action of metformin and the complex nature
of glucose homeostasis in vivo obscures the dose-response relationship
of the metabolic effects in individual organs and tissues
(Wiernsperger, 1996). As a result, the relative significance of the
above-mentioned sites of metformin action (biophases) to produce
metabolic effects remains unknown and is still a matter of continuous
debate (Bailey et al., 1994; Abbasi et al., 1998; Cusi and DeFronzo,
1998).
In preliminary investigations, we have found that mode of metformin
administration affects the kinetics and extent of its pharmacological
action. A bolus peroral administration of the drug produced stronger
and longer glucose-lowering response than i.v. administration of an
equivalent dose (Stepensky et al., 1998). At the time, the mechanism of
this finding was not elucidated. In general, it could be attributed to
pharmacokinetic
(PK1) factors
(i.e., changes in the drug concentration versus time profiles at
biophases), pharmacodynamic (PD) factors (e.g., nonlinearity of the
concentration-effect relationship), or both PK and PD mechanisms (Castaneda-Hernandez et al., 1994; Hoffman, 1998; Hoffman and Stepensky, 1999).
Previously, we reported similar effects of mode of administration on
magnitude of response for the lipid-lowering drugs niacin and
bezafibrate. Slow input of these drugs to the gut produces a
significantly augmented hypolipidemic response compared with administration of the drug by equivalent rate and extent directly to a
peripheral vein (Lomnicky et al., 1998; Hoffman et al., 1999). These
outcomes result from the targeting of the drug to presystemic biophases
by continuous administration of the drug to the gastrointestinal (GI)
tract and were termed "first-pass pharmacodynamic effect" (Hoffman
et al., 1999; Stepensky et al., 2001). Similarly, it has been reported
that for an active derivative of simvastatin, continuous mode of drug
administration to the GI tract produced stronger cholesterol-reducing
effect than oral bolus administration of this compound to dogs
(McClelland et al., 1991).
In the present investigation, we studied the relationship between
metformin disposition and glucose-lowering effects. The specific aims
were to investigate the influence of the mode of metformin
administration on the kinetics of its glucose-lowering action; to
distinguish between the pharmacokinetic and pharmacodynamic mechanisms
that affect the kinetics of action; and to identify the contribution of
the different biophases (i.e., systemic versus GI and liver) to the
overall glucose-lowering effect.
For these purposes, we developed a novel PK-PD model, which was used to
study the contribution of each of three sites of metformin action to
the glucose-lowering effect. To our knowledge, this study applied the
indirect response PK-PD model for the first time in a case where the
measured response is influenced simultaneously by three different sites
of action. This modeling approach was necessary to resolve the
contribution of individual sites of action to the overall
glucose-lowering effect of metformin.
| |
Experimental Procedures |
Materials.
Metformin hydrochloride was kindly provided by Teva Pharmaceutical
Industries, Ltd. (Netanya, Israel). Phenformin hydrochloride and
streptozotocin were purchased from Sigma-Aldrich (Rehovot, Israel). All
other reagents used in this study were of analytical or HPLC grade.
Animals.
Male Sabra rats (200-250 g; Animal Breeding Unit, The Hebrew
University of Jerusalem, Israel) were used in this study. This investigation adhered to the principles of laboratory animal care (National Institutes of Health publication 85-23, revised 1985). The
animals were housed under standard conditions with a 12-h light/dark
cycle with free access to water and food (regular rat chow) with the
exception of food deprivation during the period of blood sampling
throughout the PK-PD experiments.
An experimentally induced model of type 2 diabetes was produced
by streptozotocin injection (50 mg/kg, i.p.). Degree of diabetes was
assessed 5 days later by measurements of blood glucose levels using a
Glucometer Elite blood glucose meter (Bayer, Brussels, Belgium). Rats
with blood glucose below 140 mg/dl following an overnight fast and
above 300 mg/dl at fed conditions were selected for the experiment. The
baseline time course of blood glucose concentrations was checked on
several occasions to ensure that the metabolic status of the rats
remained stable throughout the whole experimental period.
Surgery.
To enable drug administration, cannulas (PE-50 intramedic polyethylene
tubing; BD Biosciences, San Jose, CA) were implanted in the duodenum
and blood vessels (jugular and portal vein) of the rats. Portal vein
cannulation was performed according to the method described by Strubbe
et al. (1999) with slight modifications. The surgery was performed
under anesthesia (9% ketamine and 1% xylazine solution, i.p.; 1.0 ml/kg) at least 5 days prior to initiation of the experiments. The
cannulas were exteriorized at the dorsal part of the neck, which made
it possible to carry out the investigation in nonanesthetized and
unrestrained rats.
Experimental Protocols.
The streptozotocin diabetic rats (n = 6) received
metformin in a crossover experimental design via the following modes:
1) intraduodenal bolus (450 mg/kg); 2) a constant rate intraduodenal infusion (4 h, total dose 450 mg/kg); 3) a constant rate intravenous infusion (4 h, total dose 200 mg/kg); 4) a variable rate intravenous infusion (total dose 200 mg/kg); 5) a variable rate intraportal infusion (total dose 200 mg/kg); and 6) vehicle bolus administration (double-distilled water and saline via intraduodenal and intravenous routes, respectively).
The washout period between the drug administrations was at least 6 days. For parenteral modes of drug administration, metformin was
dissolved in saline, and for intraduodenal administration, metformin
was dissolved in double-distilled water. Metformin doses were selected
on the basis of preliminary experiments to produce similar systemic
exposure (measured as area under the concentration-time curve)
following gastrointestinal and parenteral modes of drug administration.
Variable rate infusions were designed to mimic the plasma drug
concentrations versus time profile attained following the intraduodenal
infusion mode of administration. For this purpose, the infusion rate
was gradually elevated and then reduced at 1-h steps (infusion rate of
5, 11, 18, 40, 51, 37.5, 27.5, and 10 mg/kg/h for a total dose of 200 mg/kg for both variable rate intravenous and intraportal infusions).
Administration of constant and variable rate infusions was performed by
means of a microprocessor-controlled syringe pump (Pump 22; Harvard
Apparatus, Holliston, MA).
For each mode of metformin administration, blood samples (120-µl)
were collected from the tail artery at 0, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, and 10 h (and additional samples at 0.25 and 4.25 h for a
constant rate intravenous infusion). Blood glucose levels were
immediately measured by a Glucometer Elite blood glucose meter. Plasma
samples were obtained from the rest of the blood (by centrifugation at
3500 rpm for 10 min) and stored at 
20°C
pending analysis.
Analytical Procedures.
Plasma metformin concentrations were determined by an HPLC method
applying a Kontron HPLC system (Kontron, Zurich, Switzerland) and
LiChrospher 100 RP-18 column (Merck, Darmstadt, Germany). The detection
was at 234 nm, and phenformin was applied as the internal standard. The
mobile phase consisted of 0.01M
Na2HPO4 solution (pH = 6.5), methanol, and acetonitrile (20:3:6, v/v). The quantitation limit
was 100 ng/ml. Intraassay and interassay coefficients of variation were
5 and 9%, respectively.
PK Model.
A multicompartment PK model was used to describe the pharmacokinetics
of metformin (see Fig. 1). Transfer of
metformin between different compartments was assumed to occur according
to a first-order kinetic process along the arrows with corresponding
rate constants:
|
(1)
|
|
(2)
|
|
(3)
|
|
(4)
|
where X1,
X2,
X3, and
X4 are metformin amounts in GI lumen,
GI wall, liver, and systemic compartments, respectively. The rate
constants are: kg0, drug elimination
with feces; kgg, drug transfer from
the GI lumen to GI wall compartment;
kgl, drug transfer from the GI wall to
liver compartment; kls and
ksl, drug transfer from the liver to
systemic compartment and in the opposite direction, ksg, drug transfer from the systemic
to GI wall compartment; and ks0, drug
elimination with urine. Metformin is administered into GI lumen, liver,
or systemic compartments following intraduodenal, intraportal, and
intravenous modes of administration, respectively (see Fig. 1).
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Fig. 1.
The pharmacokinetic model of metformin
disposition in rat.
The total metformin content in the gastrointestinal tract is
composed of two fractions: free drug in the lumen (GI lumen) and
the drug within the intestinal wall (GI wall). Additional compartments
are attributed to liver and systemic circulation (including blood,
different organs, and tissues). Transfer of metformin between different
compartments is assumed to occur according to the first-order kinetic
processes along the arrows with corresponding rate constants
(kgg, kgl, etc.).
Elimination occurs from the GI lumen or systemic compartments (with
kg0 and ks0 rate
constants, respectively).
|
|
The structure of the PK model was based on available data of metformin
pharmacokinetics and the physiology of intestinal and portal blood
circulation. Metformin is not metabolized in the body, oral
bioavailability reaches 50 to 60%, and the rest of the dose is
excreted in the feces (Tucker et al., 1981; Wiernsperger, 1996). Due to
the biguanide chemical properties of metformin, the drug is
characterized by a unique distribution behavior; following intraduodenal (or oral) administration, part of the metformin is
reversibly adsorbed to the luminal surface of the intestinal wall
(Hermann and Melander, 1992). Metformin tends also to accumulate in the
GI wall following both oral and intravenous administration (Sirtori et
al., 1978; Wilcock and Bailey, 1994). This accumulation is not
attributed to the GI lumen since no fecal excretion of metformin was
observed following i.v. administration (Pentikainen et al., 1979;
Tucker et al., 1981). Thus, the "GI wall" compartment introduced in
this model receives metformin via arterial blood supply to the
intestine (with rate constant ksg)
and, in addition, comprises the drug amounts that are absorbed from the
GI lumen. According to the unidirectional blood flow through the portal system, the PK model allows metformin passage through both the GI wall
and liver before reaching the systemic circulation following the
intraduodenal route and only through the liver for intraportal administration.
PK-PD Model.
The overall glucose-lowering effect of metformin was attributed to
inhibition of glucose production or stimulation of glucose utilization
at the individual biophases according to the sigmoidal Emax model:
|
(5)
|
|
(6)
|
|
(7)
|
where Iliver is inhibition of
glucose production in the liver (L) and
SGI and
SS are stimulation of glucose
utilization in the GI tract (GI) and by muscle and fat tissues (S),
respectively. To reduce the number of estimated parameters, the
equations were written in terms of drug amounts and not concentrations,
thereby casting off the need for estimating the volumes of the PK
compartments and biophases. Therefore, drug effect at certain biophase
is a function of metformin amount (AL,
AGI, and
AS), maximum effect (Imax L,
Emax GI, and
Emax S), metformin amount at the
biophase that produces 50% of maximal effect
(IA50 L, EA50 GI, and
EA50 S), and the shape factor
(nL,
nGI, and
nS).
A combination of indirect response models I and IV (Dayneka et al.,
1993) was applied to describe the time course of metformin glucose-lowering effect, according to the following equation
|
(8)
|
where the response parameter R is the
metformin-related change in glucose blood concentration;
kin is the zero-order rate of glucose
input into the body; and kout is the
first-order rate of glucose utilization.
Linked PK-PD Model.
Two approaches were used to model the kinetics of metformin effect(s).
Model A assumed that the amounts of metformin accumulated in
"liver", GI wall, and "systemic" compartments (see Fig. 1) were
responsible for the glucose-lowering effects.
Model B was based on the presumption that only the metformin amount
reaching the GI tract and liver at a given time point (i.e., drug flux
through intestinal microvessels and the portal vein, respectively),
rather than overall accumulated amount, is related to the observed
glucose-lowering effect. The drug flux through intestinal microvessels
was calculated as amounts of metformin transferred from "GI lumen"
to GI wall compartment: Qgg = X1 · kgg (see Fig. 1). Due to the
first-pass effect, metformin concentrations in the portal vein are
derived from metformin concentrations in the systemic circulation,
infused drug (for intraportal mode of administration), and drug
absorption in the GI tract (for intraduodenal bolus and infusion modes
of administration). Therefore, metformin flux through the portal vein
(Qgl) was calculated as the sum of systemically derived amounts, drug amounts infused in the portal vein
(in the case of intraportal infusion), and transfer of metformin from
GI wall to liver compartment that was derived from the absorption from
GI lumen (for intraduodenal bolus and infusion). Systemically derived
drug amounts in the portal vein were calculated as metformin plasma
concentrations multiplied by portal blood flow (5.92 ± 0.97 ml/min/100 g body weight; Sakaeda et al., 1998).
For the purpose of PK-PD modeling according to model B, estimated
values of drug flux through intestinal microvessels and the portal vein
(Qgg and
Qgl) were introduced instead of the
amounts at the GI wall and liver (AGI
and AL, respectively) in eqs. 6 and 5.
PK-PD linking for systemic compartment (i.e., adipocytes and muscle
cells) was not different in models A and B.
Data Analysis.
To overcome the circadian variations in blood glucose levels and reveal
the metformin-driven glucose-lowering effect, the baseline time course
of blood glucose levels (following vehicle administration) was
subtracted from the observed blood glucose levels following the drug
administration. Calculation of area under concentration versus time
curve (AUC) and area under effect versus time curve (AUEC) values was
performed using the WinNonlin program (version 1.1; Pharsight
Corporation, Mountain View, CA) by means of the noncompartmental
analysis method.
Analysis of mean pharmacokinetic and pharmacodynamic data following
various modes of administration was performed with ADAPT II
Pharmacokinetic/Pharmacodynamic Systems Analysis Software (Biomedical Simulations Resource, Los Angeles, CA) applying the mean likelihood objective function (D'Argenio and Shumitzky, 1997). The variance was
described by the linear model,
|
(9)
|
where a and b are the variance parameters.
The PK fits were done simultaneously for seven data sets: five sets of
mean plasma concentration versus time data for various modes of
administration and two estimated data sets representing GI and liver
amount versus time data for i.v. bolus administration. The estimated
amounts of metformin versus time profiles in the GI tissues and liver
were calculated using intestine/plasma and liver/plasma ratios (10 and
5.0, respectively) based on metformin distribution data in
streptozotocin diabetic mice (Wilcock and Bailey, 1994). In the second
stage, the PK-PD model was fitted to the mean glucose-lowering effect
versus time data (simultaneous fit of five modes of administration)
using the estimated PK parameters as fixed values.
Statistical Analysis.
The Kruskal-Wallis analysis of variance with subsequent Newman-Keuls
multiple comparisons test were applied for analysis of AUC and AUEC
values following different modes of metformin administration. A
p value of less than 0.05 was termed significant.
| |
Results |
The PK and PD Data.
The mean concentration versus time profiles following different modes
of metformin administration are presented in Fig.
2. It can be seen that a constant rate
i.v. infusion produced a gradual increase in metformin plasma
concentrations and reached a steady state within 2 to 3 h. The PK
results for constant rate intravenous infusion served for calculation
of clearance and volume of distribution, which were 2.02 l/kg/h and
1.31 l/kg, respectively. These values are comparable (up to 3- to
4-fold difference when normalized by weight) with the results obtained
in human studies (Pentikainen et al., 1979, 1986; Tucker et al., 1981;
Scheen, 1996).
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Fig. 2.
Plasma metformin concentrations following
administration of 450 mg/kg metformin as intraduodenal bolus or
intraduodenal infusion or 200 mg/kg metformin as constant rate i.v.
infusion, variable rate i.v. infusion, or variable rate intraportal
infusion.
Data points are the mean observed pharmacokinetic data, and solid lines
are the best fits according to the PK model.
|
|
Following intraduodenal bolus administration, the peak plasma metformin
concentrations were obtained 3 h after drug administration and
then declined gradually. The concentration-time profile following a
constant rate intraduodenal infusion was characterized by slow kinetics
of drug absorption to the systemic circulation; the peak plasma
concentration was attained 1 h after the termination of the infusion.
The plasma concentration-time profiles of metformin following variable
rate i.v. and intraportal infusions were designed to mimic the PK
profile obtained following constant rate intraduodenal infusion (see
Fig. 2). No significant differences in AUC values were found for the
different modes of metformin administration that were studied [see
Table 1; the results of intraduodenal bolus and constant rate intraduodenal infusion were published previously (Stepensky et al., 2001)].
At the beginning of the experiment, mean plasma glucose levels were
approximately 400 mg/100 ml, and following vehicle administration, they
declined gradually, reaching approximately 320 mg/100 ml at the end of
the data collection period (10 h; graph not shown). The time course of
glucose-lowering effects following different modes of metformin
administration is shown in Fig. 3. It can
be seen that both the kinetics and magnitude of the glucose-lowering effect were highly dependent on the mode of metformin administration. Intraduodenal bolus and infusion produced the highest extent of glucose-lowering effects as evidenced by the augmented AUEC values (see
Table 1). Intraportal infusions produced intermediate effects, and the
lowest extent of pharmacological effect was observed for constant and
variable i.v. infusion modes of metformin administration.
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Fig. 3.
Glucose-lowering effects following
administration of 450 mg/kg metformin as intraduodenal bolus or
intraduodenal infusion or 200 mg/kg metformin as constant rate i.v.
infusion, variable rate i.v. infusion, or variable rate intraportal
infusion.
To reveal the metformin-driven glucose-lowering effect, the baseline
time course of blood glucose levels (following vehicle administration)
was subtracted from the observed blood glucose levels following the
drug administration. Data points are the mean observed glucose-lowering
effects, and the solid lines are the best fits according to model B.
|
|
PK-PD Modeling.
The PK model adequately captured the concentration-time data following
all the modes of metformin administration (Fig. 2). The values of
estimated PK parameters are presented in Table
2. The values of the Akaike and Schwartz
criteria for the results of PK modeling were 209 and 261, respectively.
Estimated exposures of biophases to metformin according to model A are
shown in Fig. 4, and those according to
model B are presented in Fig. 5. PK-PD
analysis according to model A could not describe the observed time
course of the drug effects (see Discussion). PK-PD analysis
according to model B provided adequate description of the time course
of the drug effects (see Fig. 3). The estimated values of the PD
parameters are presented in Table 2. To reduce degrees of freedom, the
values of nL,
nGI, and
nS were set to 2, 5, and 5, respectively, based on the preceding PK-PD fitting process. The values
of the Akaike and Schwartz criteria for the results of PK-PD modeling
were 515 and 549, respectively.
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Fig. 4.
Model A predicted exposure of biophases to
metformin following administration of 450 mg/kg metformin as
intraduodenal bolus or intraduodenal infusion or 200 mg/kg metformin as
constant rate i.v. infusion, variable rate i.v. infusion, or variable
rate intraportal infusion.
Note overlapping curves of the variable rate intraportal and i.v.
infusions for the GI and systemic biophases.
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Fig. 5.
Model B predicted exposure of biophases to
metformin following administration of 450 mg/kg metformin as
intraduodenal bolus or intraduodenal infusion or 200 mg/kg metformin as
constant rate i.v. infusion, variable rate i.v. infusion, or variable
rate intraportal infusion.
Note overlapping curves of the variable rate intraportal and i.v.
infusions for the systemic biophase.
|
|
| |
Discussion |
Preliminary work by Stepensky et al. (1998), as well as previous
reports of Marchetti et al. (1987), have revealed a lack of direct
correlation between the magnitude of glucose-lowering effect and blood
metformin concentrations. To clarify the underlying pharmacokinetic
and/or pharmacodynamic mechanism(s), we investigated the
concentration-effect relationship following metformin infusion by
different routes. The major difference between infusion of metformin to
the duodenum and the peripheral vein is the targeting of higher drug
concentrations to presystemic biophases over a prolonged period of
time. This difference may account for the larger magnitude of effect
observed for intraduodenal infusion despite the similarity in AUC
values (see Table 1). However, this conclusion is questionable because
the pattern of the concentration-time profile of the drug in the
systemic circulation differed considerably between the intraduodenal
and i.v. infusions.
To further investigate this phenomenon, we applied a unique
experimental strategy of variable rate i.v. infusion of the drug. The
infusion rates were selected to yield the same metformin plasma concentration-time profile as that following infusion of the drug to
the duodenum (see Fig. 2). This approach bypassed the PK constraints that evolve due to the slow intestinal absorption rate of metformin (i.e., flip-flop pharmacokinetics) and enabled a clear appreciation of
the impact of the administration route on the magnitude of effect. The
only difference between the two modes/routes of infusion was that the
first pass of metformin from the GI tract, via the portal vein and the
liver, into the systemic circulation was of notable significance to the
glucose-lowering effect. This difference in magnitude of response is
therefore termed a first-pass PD effect (Lomnicky et al., 1998; Hoffman
et al., 1999).
To distinguish between the contribution of metformin effects on the GI
wall during the first pass and its effects on the liver, the drug was
infused directly to the portal vein of unrestrained rats using an input
function that produced the same concentration-time profile as the
intraduodenal (and i.v.) infusion. The results reconfirmed that
administration of metformin via the portal-hepatic pathway produces a
stronger glucose-lowering response than direct administration to the
systemic circulation. The results also clarify the contribution of the
first pass of the GI wall to the overall enhanced activity, as the
magnitude of effect following direct intraportal infusion yielded a
somewhat weaker glucose-lowering response than intraduodenal administration.
Modeling.
To better understand the mode of administration dependence, we employed
a PK-PD modeling approach that enabled an estimation of drug
concentration-time data at the biophases, which otherwise are not
accessible for sampling. This mathematically oriented approach
estimates the major kinetic rate constants and the contribution of
individual biophases to the overall drug effect. Due to the complex
pharmacokinetic behavior of metformin and the contribution of three
different sites of action to the overall glucose-lowering effect,
complex models had to be produced. The number of estimated parameters
reached 8 for the pharmacokinetic model and 11 for the pharmacodynamic
model. To reduce the number of the estimated parameters, we fixed the
values of the shape factors in the final run. This fixation was based
on extensive modeling efforts applying different initial estimates of
the shape factors that revealed that the value of
nL tends to be 2 and values of
nGI and
nS tend to be 5. We performed a
simultaneous fit of all the experimental results (first PK and then PD)
that were collected in a crossover design from the same experimental
animals, thus considerably enhancing the power of the parameter
estimation and thereby the validity of the derived conclusions.
PK Model.
The PK model described the distribution of metformin versus time at
different sites, including the proposed biophases (see Fig. 1). The
simultaneous fit procedure yielded estimated PK parameters that
adequately described the observed concentration-time data of the five
investigated modes of metformin administration (Fig. 2). The fitting
produced similar values of the rate constants of absorption and
elimination from the GI lumen (kg0 and
kgg, respectively; see Table 2),
indicating an overall GI bioavailability of approximately 50%, which
is in accordance with the 40-60% bioavailability reported in most
studies (Wiernsperger, 1996). The rate constant representing metformin
elimination from the systemic circulation (ks0) was higher than the absorption
rate constant, confirming flip-flop PK behavior of metformin (Scheen,
1996; Cusi and DeFronzo, 1998).
PK-PD Model.
Since metformin affects blood glucose levels indirectly by altering the
rate of glucose production and utilization, the indirect response PK-PD
modeling approach (Dayneka et al., 1993) was applied to describe the
glucose-lowering effect of metformin. In this investigation, a combined
indirect PD model was used for the first time to describe the effect of
the drug on the kinetics of the measured response by affecting both the
input and output parameters (models I and IV, respectively; Dayneka et
al., 1993). For each biophase, metformin concentration versus time
profiles estimated from the PK model were linked to the elevation or
reduction of blood glucose (eqs. 5-8).
Model A was based on the assumption that concentrations of metformin
accumulated in the GI wall, liver, and systemic compartments specified
in the PK model were responsible for the glucose-lowering effects.
However, exposure of each of these compartments to metformin, according
to model A, was similar for all the modes of drug administration (see
Fig. 4). For example, intraportal and i.v. infusion exposure to
metformin according to model A was similar for liver biophase and
identical for the "GI" and systemic biophases, despite the major
differences in the observed magnitude of glucose-lowering effect (AUEC)
(see Fig. 3 and Table 1). Hence, model A is not the proper model, and
the drug concentrations at these PK compartments do not represent the
concentration "seen" by the receptors associated with metformin action.
Model B was based on the working hypothesis that there is a correlation
between the temporal metformin concentrations in the portal vein and
within the intestinal wall (for liver and GI wall compartments,
respectively) and the observed glucose-lowering effect. It differs from
model A, which focused on metformin concentrations that accumulated in
the above-mentioned compartments. For the liver biophase, this
presumption is based on the known heterogeneity in liver anatomical
structure, termed "hepatic zonation" (Gebhardt, 1992; Jungermann
and Thurman, 1992), and the unique organization of the liver blood
supply from both portal vein and hepatic artery. Focusing on the
variable rate i.v. and intraportal infusions, model B predicts higher
exposure of liver biophase following intraportal infusion (Fig. 5).
Appropriate description of the pharmacodynamic data by model B (Fig. 3)
indicates that the magnitude of the glucose-lowering effect of
metformin in liver is related to the drug concentrations in the portal
vein. This finding is in accord with previous knowledge on metformin
pharmacologic activity that is mediated through cell membrane events
(Klip and Leiter, 1990; Wiernsperger, 1996). An additional possibility
is the rapid equilibrium between the metformin concentrations at the
portal vein and an intracellular site of action. Thus, the liver/portal
biophase could be the subpopulation of hepatocytes whose cellular
membrane is exposed to portal blood.
In intraduodenal bolus administration, adsorption of metformin to the
intestinal wall and low rate of absorption (i.e., flip-flop PK) lead to
prolonged elevation of portal drug concentrations compared with i.v.
administration. Even more prolonged elevation of portal metformin
concentrations was achieved following intraduodenal infusion.
Therefore, intraduodenal bolus and infusion produced higher portal-vein
metformin concentrations and thereby enhanced exposure of the
liver/portal biophase to metformin compared with i.v. infusions (see
Fig. 5). This conclusion is confirmed by the findings that the portal
vein concentrations were consistently higher (by approximately 50%)
than drug concentrations in the systemic circulation (inferior vena
cava) following oral metformin administration to streptozotocin
diabetic mice (Wilcock and Bailey, 1994).
GI administration of metformin, in addition to elevated portal
exposure, leads also to higher exposure of the GI biophase to the drug.
This is apparent from the comparison between the magnitude of
glucose-lowering effect following GI and intraportal modes of metformin
administration. model B assumes that the main activity of metformin at
the GI wall is contributed from drug that reaches the GI biophase from
the GI lumen following intestinal absorption, whereas the impact of the
arterial blood input is negligible. The good fit between the
response-time profile predicted by model B and the experimental data
substantiates the underlying assumption. It means that drug molecules
that permeate the intestinal wall account for the first-pass
phenomenon, whereas the relatively large amounts of the drug that
accumulate there have only a negligible contribution to that effect.
It should be noted that the streptozotocin-induced model of diabetes as
applied in this investigation is not a perfect mimic of type 2 diabetes
with regard to insulin resistance in peripheral tissues (Weiss et al.,
1995). Thus, the contribution of presystemic biophases to the overall
glucose-lowering effect may be accentuated in this model. It is,
therefore, suggested that the route of administration dependence of
metformin in insulin-resistant type 2 diabetes be examined to validate
the outcomes and refine the extrapolation of the conclusions of the
current investigation to understanding the PK-PD of the drug in
clinical situations.
We thank Dr. Joshua Backon for valuable suggestions. This work is part
of D. Stepensky's Ph.D. dissertation.
Abbreviations used are:
PK, pharmacokinetic;
PD, pharmacodynamic;
GI, gastrointestinal;
AUC, area under concentration
versus time curve;
AUEC, area under effect versus time curve;
HPLC, high-performance liquid chromatography.
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Abstract
Introduction
