Introduction

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Drugs administered orally must first be absorbed, either passively or via facilitated transport, across the intestinal luminal membrane to reach the systemic circulation. Much is known about the various intestinal transport proteins that participate in the uptake of drugs (Tsuji and Tamai, 1996; Lin et al., 1999). Additionally, the intestine possesses metabolic enzymes, notably the conjugating enzymes, UDP-glucuronosyltransferases, glutathione S-transferases (Dubey and Singh, 1988; Ilett et al., 1990; Koster et al., 1995), and cytochrome P-450 3A (Watkins et al., 1987; Peters and Kremers, 1989; Kolars et al., 1992; Lampen et al., 1995; Paine et al., 1996, 1997). In some instances, metabolism by the intestine was noted only during absorption and not on subsequent circulation through the intestinal tissue. That intestinal metabolism is "route dependent", being greater with oral than with i.v. dosing, was observed for acetaminophen (Pang et al., 1986), enalapril (Pang et al., 1985), and morphine (Doherty and Pang, 2000), and for the conversion of the prodrug ()6-aminocarbovir to ()carbovir (Wen et al., 1999) in the perfused rat small intestine preparation. The observation was repeated for the oxidation of midazolam in man (Paine et al., 1996, 1997). Furthermore, a 170-kDa protein, the P-glycoprotein (Pgp),² has been identified to be responsible for drug efflux into the intestinal lumen (Thiebault et al., 1987; Hunter et al., 1990; Hsing et al., 1992; Saitoh and Aungst, 1995; Smit et al., 1998). Intestinal metabolism and exsorption effectively reduce the bioavailability of orally administered agents (Gibaldi et al., 1971; Leu and Huang, 1995; Doherty and Pang, 1997; Lown et al., 1997; Arimori and Nakano, 1998; Wacher et al., 1998; Hall et al., 1999; Lin et al., 1999).

Despite the large body of information on intestinal exsorption and metabolism, only a few models exist to correlate these physiological processes with the overall drug absorption or bioavailability (Barr and Riegelman, 1970; Crouthamel et al., 1975; Stigsby and Krag, 1983; Nakashima et al., 1984; Choi et al., 1995; Yu and Amidon, 1998; Ito et al., 1999). Although the models would account for multiple-site/regional absorption, metabolism, secretion, or even diffusion within the tissue, few would forecast route-dependent intestinal metabolism. An exception is the model proposed by Klippert and Noordhoek (1985) that suggests shunting of intestinal blood for prediction of route-dependent metabolism.

In this communication, a physiologically based Segregated-Flow Model (SFM) was developed to explain route-dependent intestinal metabolism; the model encompassed differential blood perfusions to distinct tissue layers of the intestine. The properties of the model were investigated upon engendering intestinal blood flow, the intestinal metabolic, secretory, and intrinsic clearances, tissue-partitioning characteristics (diffusion-limited versus flow-limited distribution) of substrate, and presence of eliminatory pathways in parallel organs to predict the intestinal clearance and systemic availability. The segregated flows could be rationalized because distinct blood flow patterns have been noted for various tissue layers of the intestinethe mucosa, submucosa, and musculariswith each contributing to one of three functions of the small intestine, absorption, secretion, and motility (Granger et al., 1980), and the serosa that lies inferior to the muscularis. The large surface area for absorption is attributed to the villi and microvilli of the mucosa, and metabolizing enzymes are located within enterocytes at the villus tip (Kolars et al., 1992; Lown et al., 1997). It has been noted that the majority of "resting" intestinal blood flow, some 60 to 70% of the intestinal flow, is distributed to the mucosa-submucosa because of greater metabolic demand (Schurgers and de Blaey, 1984), with approximately 18% (MacFerran and Mailman, 1977), 5 to 7% (Mailman, 1978; Granger et al., 1980), or 10 to 30% (Svanvik, 1973; Micflikier et al., 1976) of the intestinal blood flow perfusing the enterocyte layer of the villus tips where the majority of the absorptive, metabolic, and Pgp activities reside. Because flow perfusing the site of elimination can influence the disposal of drugs and because there are differing blood flow distributions to various tissue layers of the small intestine, it becomes important to view intestinal drug metabolism beyond what is ordinarily considered in traditional, compartmental, or physiological models, in which the absorptive layer is assumed to receive 100% of the total intestinal blood flow.

Two physiological models for the intestine were examined: the Traditional Model (TM) (Fig. 1A) and the SFM (Fig. 1B). Removal by other parallel eliminating organs exists, and the effective clearance is described by CL_others. Common features of the models include the interconnection of the blood compartment (central or reservoir compartment in this instance) to the intestinal tissue via the circulation. Only first order transport and removal processes are considered, and for the sake of simplicity, the drug is assumed to be completely unbound.

View larger version (23K): [in this window] [in a new window]   Fig. 1.   Schematic presentations of TM (A) and SFM (B) for intestinal absorption, metabolism, and secretion of substrates given orally or i.v.; the conditions considered mimic the recirculating perfused small intestine preparation with additional (parallel) clearances occurring within the central or reservoir compartment. Refer to text for details.

Traditional Model. The intestine is subdivided into the vascular (intestinal blood), cellular (tissue), and luminal subcompartments (Fig. 1A). The tissue is supplied with blood from the superior mesenteric artery with the flow rate, Q_I; venous blood returns through the portal vein to the reservoir. The exchange of substrate between the cellular and vascular compartments is described by the intrinsic transport clearance terms CL_d1 and CL_d2 that characterize, respectively, transport from intestinal blood into intestinal tissue and vice versa. The rate constant for absorption of the substrate across the luminal membrane is denoted by k_a, whereas luminal removal of the drug, either by metabolism, fecal excretion, and/or gastrointestinal transit, is represented by rate constant k_g. Once in the intestinal tissue, the drug undergoes biotransformation, and is transported out to blood or effluxed into lumenprocesses that are described by intrinsic clearance terms CL_m, CL_d2, and CL_sec, respectively (Doherty and Pang, 2000).

Segregated-Flow Model. This model is an expansion of the physiological model normally developed for the intestine, but it further recognizes the subtle demarcation of tissue layers and distributions in blood supply. The notion of flow-bypass of tissular regions of the intestine was also recognized by Klippert and Noordhoek (1985). Drug in the serosal blood compartment equilibrates with tissue with the transfer clearances CL_d3 and CL_d4, whereas drug in the mucosal-blood/enterocyte-blood compartment equilibrates with tissue with the transfer clearances CL_d1 and CL_d2. The absorptive, metabolic, and efflux activities within the villus tips of the enterocyte compartment are denoted by the rate constant, k_a, and the intrinsic clearances, CL_m and CL_sec, respectively (see Fig. 1B).

Experimental Procedures

Mass-balanced equations were written for the TM and the SFM. For emphasis of intestinal metabolism, secretion, and absorption, the system described was similar to that for the recirculating system of the perfused intestine preparation (Doherty and Pang, 2000).

Traditional Model. For the rate of change of drug in the reservoir (compartment "R"):

(1)

For the rate of change of drug in the intestinal blood (compartment "int,b"):

(2)

For the rate of change of drug and formation of metabolite {mi} in the intestinal tissue (compartment "int"):

(3)

(3A)

For the rate of change of drug in the intestinal lumen (compartment "lumen"):

(4)

Segregated-Flow Model. For the rate of change of drug in the reservoir (compartment "R"):

(5)

For the rate of change of drug and rate of formation of metabolite {mi} in enterocyte layer of mucosa (compartment "en"):

(6)

(6A)

For the rate of change of drug in the mucosal blood to enterocyte compartment (compartment "en,b"):

(7)

For the rate of change of drug in the serosal blood (compartment "s,b"):

(8)

For the rate of change of drug in the compartment comprising of the serosa and other intestinal structures (compartment "s"):

(9)

For the rate of change of drug in the intestinal lumen (compartment "lumen"):

(10)

It is noteworthy that if Q_en equals Q_I, the SFM simplifies to the TM.

Simulation. Values of the intestinal clearance and the systemic and intestinal availabilities were either simulated with the equations (eqs. 1 to 10, with the program, Scientist, Micromath, Salt Lake City, UT) or calculated using the solutions obtained for both the TM and the SFM. Various values for the volume, flow, and transport and intrinsic clearances (Table 1) were placed into rows and columns of the worksheet in Excel (Version 5.0 for Macintosh, Microsoft, Seattle, WA) and substituted into the solved equations (see Table 2) for estimation of the various parameters. The overall intestinal flow rate was set as 8 ml/min. Because literature values for the blood flow to the absorptive enterocyte layer of the mucosa vary greatly, ranging from 5 to 30% (Svanvik, 1973; MacFerran and Mailman, 1977; Mailman, 1978; Granger et al., 1980), the average flow to this compartment was assigned 10% of intestinal flow for the sake of simplicity, and the remaining compartmentthe serosa and other intestinal structuresreceived the other 90% of flow; the volumes were partitioned in the same fashion. Furthermore, simulation was performed with transport clearances between blood and tissue compartments being identical for the TM (CL_d = CL_d1 = CL_d2) and for SFM (CL_d = CL_d1 = CL_d2 = CL_d3 = CL_d4). The value of CL_d was set either as 0.5 or 50 ml/min, because these represented conditions of drugs of low (diffusion-limited distribution) and high (flow-limited distribution) permeability, respectively. The intestinal metabolic intrinsic clearance (CL_m, ranging from 0.1 to 50 ml/min), the exsorption or secretory intrinsic clearance (CL_sec, ranging from 0 to 50 ml/min), and values of the absorption rate constant (k_a, from 0.01 to 10 min¹) were varied under a nonchanging k_g (0.5 min¹) to study the influence of these factors on the area under the curve, clearance, and bioavailability estimates.

1

Fitting of Morphine Data to the TM and SFM. The utility of the SFM versus the TM was appraised with the recent data of Doherty and Pang (2000) in which morphine (M), a substrate which is absorbed, glucuronidated, and secreted, was given both systemically and intraduodenally to the recirculating, vascularly perfused rat small intestine preparation. The models (Fig. 1) were extended to describe not only the disposition of M but also for the formation of the metabolite, morphine-3-glucuronide (M3G), by the rat intestine preparation; in this instance, CL_others was set to zero (Fig. 2). For TM, influx/efflux of M into the intestinal tissue from the blood is characterized by the transport clearance parameter, CL1 and CL2, respectively (Fig. 2A). Once M enters the intestinal tissue, it undergoes biotransformation to M3G with the intestinal metabolic clearance, CL11, or is exsorbed across the luminal (denoted by the secretory intrinsic clearance CL3). The absorption intrinsic clearance of M from the intestinal lumen is denoted by CL4, and the luminal degradation clearance, CL12. M3G, once formed in the intestinal tissue, can either efflux out to the perfusate blood (CL10) or be excreted into the lumen (CL7), where there exists deconjugation of the glucuronide metabolite (with CL5) and glucuronidation of M (with CL6). The influx and efflux clearances for M3G across the basolateral membrane are denoted by CL9 and CL10, respectively. The data had been fitted to mass balance relationships developed previously (see Appendix of Doherty and Pang, 2000) to describe events occurring during the traverse of M and M3G across the intestine. The intrinsic clearances for drug and metabolite absorption and luminal degradation, CL4, CL8, and CL12, respectively, become the corresponding rate constants upon division by the volume of the lumen, V_lumen.

View larger version (24K): [in this window] [in a new window]   Fig. 2.   Models for the TM and SFM in describing the metabolism of M to M3G in the recirculating, perfused rat liver preparation. The TM was described previously by Doherty and Pang (2000).

	Results

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Analytical Solutions. Mathematical solutions for the AUC values of i.v. and p.o. administrations, obtained from inversion of the square matrices, were used to calculate the total and intestinal clearances, and systemic and intestinal availabilities for both the TM and SFM, when membrane transport clearances were distinct (CL_d1  CL_d2, and CL_d1  CL_d2  CL_d3  CL_d4) (Table 2); these solutions readily provided simplified versions when the transport clearances were equal (CL_d1 = CL_d2, and CL_d1 = CL_d2 = CL_d3 = CL_d4). The solutions differed only in the flow rate terms: Q_I for the TM and Q_en for SFM. The presence of other clearance (CL_others > 0) did not influence expressions for the intestinal clearance and systemic bioavailability, solved for the first time when absorption, luminal degradation, and intestinal secretion and metabolism are all present. The solutions were complex relations encompassing the termsblood flow rate to the intestinal tissue/enterocyte layer, transport clearance, intestinal metabolic intrinsic clearance, exsorption intrinsic clearance, and the luminal degradation (k_g) and absorption (k_a) rate constants, and CL_others. The AUC values were simplified when CL_others was zero: AUC_R,p.o. were the same for the TM and SFM although the AUC_R,i.v. differed due to the flow terms: Q_I for the TM and Q_en for SFM, as did CL_I, F_sys, and F_I. Interestingly, the transport clearances of drug across the serosal membrane (CL_d3 and CL_d4) and the serosal flow rate (Q_s) were absent in the solutions of the SFM. This is due to the role of the serosa serving only as a noneliminating, drug-distribution compartment (Fig. 1B). Because of exsorption of drug and readsorption, the absorption rate constant, k_a, and the luminal degradation rate constant, k_g, were present in the solutions of CL_t, CL_I, F_sys, and F_I. In the absence of secretion by Pgp, the constants k_a and k_g are absent in the equations for CL_t, CL_I, and F_I, except for AUC_R,p.o. and F_sys, which are influenced by F_abs (Table 2).

Simulations.

Effects of intestinal metabolism and secretion on CL_I, F_sys, and F_I at constant F_abs (0.667, with k_a and k_g equal to 1 and 0.5 min¹, respectively) The intestinal clearance (CL_I), systemic availability (F_sys), and intestinal availability (F_I) were found not to be influenced by the presence of other eliminatory pathways (CL_others > 0). CL_I was affected directly by both the intestinal secretory and metabolic intrinsic clearances (Fig. 3). The magnitude of the intestinal clearance for any combination of CL_sec (from 0 to 50 ml/min) and CL_m (from 0.1 to 50 ml/min) was greater for the TM (Fig. 3, A and B, top) than for the SFM (Fig. 3, C and D, bottom). As expected, CL_I increased with increasing CL_sec and CL_m, and the increases were more obvious for a highly permeable (flow-limited) substrate (transport intrinsic clearance = 50 ml/min, Fig. 3, B and D). These changes were more gradual for the TM (Fig. 3B), but were more abrupt for the SFM (Fig. 3D). By contrast, F_I was modulated by CL_sec and CL_m in an inverse manner (Fig. 4), and the changes were more gradual for drugs with high permeability (cf. Fig. 4, B and D to Fig. 4, A and C) and with the TM. For drugs will low permeability, values of F_I decreased dramatically to almost a constant value upon increasing the CL_m and CL_sec from 0 to 10 ml/min; further increases in CL_m and CL_sec were, however, ineffective in decreasing the value of F_sys, which was already close to zero (Fig. 4, A and C). The trends for F_sys were identical to those for F_I inasmuch as F_abs was constant due to the nonchanging k_a and k_g (data not shown; values were lower because of the fraction, F_abs).

View larger version (41K): [in this window] [in a new window]   Fig. 3.   Simulated effects of CLsec and CLm on CLI for the TM (A and B) and the SFM (C and D), based on parameters shown in Table 1 (ka and kg = 1 and 0.5 min1, respectively). Membrane transport clearance (CLd) was fixed at 0.5 ml/min and at 50 ml/min for illustration of drugs of the low and high permeability, respectively.

View larger version (44K): [in this window] [in a new window]   Fig. 4.   Simulated effects of CLsec and CLm on FI for the TM (A and B) and the SFM (C and D), based on parameters shown in Table 1 (ka and kg = 1 and 0.5 min1, respectively). CLd was fixed at 0.5 and 50 ml/min for illustration of drugs of the low and high permeability, respectively.

View larger version (28K): [in this window] [in a new window]   Fig. 5.   Comparison of the ratios of CLI, Fabs, and FI simulated for the SFM and the traditional model when the CLsec and CLm were altered. The absorption and luminal degradation constants, ka and kg, were kept constant at 1 and 0.5 min1, respectively.

Effects of CL_sec, k_a, and k_g on Fsys when CL_m = 0. In absence of metabolism, secretion and absorption represented the processes effecting the cycling of drug between lumen and intestine. However, the overall bioavailability depended not only on the values of CL_sec and k_a, but also on k_g, the "luminal degradation" constant associated with gastrointestinal transit time or loss. When k_g was set to zero, CL_I became zero regardless of the value of CL_sec because of drug reabsorption and total lack of loss in the system (CL_m and k_g = 0). High secretion tended to be offset with rapid absorption (high k_a) when minimal loss existed in the lumen (k_g = 0.01 min¹), and the systemic availability tended to remain close to unity (data not shown). At increasing values of k_g (0.5 min¹), however, F_sys became attenuated (Fig. 6), and the trend persisted with even higher k_g (10 min¹) (data not shown).

View larger version (38K): [in this window] [in a new window]   Fig. 6.   Effects of ka and CLsec on Fsys according to the TM (A and B) and the SFM (C and D), based on parameters shown in Table 1. CLd was fixed at 0.5 and 50 ml/min for illustration of drugs of the low and high permeability, respectively. CLm was fixed at 0 ml/min, and kg was set as 0.01 min1; similar trends with lower values for Fsys were observed at kg = 10 min1.

Effects of CL_m and k_a on Fsys when CL_sec = 0 and k_g = 0.5 min¹. In the absence of secretion (CL_sec = 0), increasing the values of k_a failed to alter AUC_R,i.v. or CL_I (see Table 2) but increased values of F_sys, the single parameter changing with k_a. The greatest changes existed for drugs with low CL_d; whereas changes were more gradual for the high-permeability drugs (Fig. 7). Similar trends were observed at CL_sec = 5 ml/min, albeit the values for F_sys were attenuated (data not shown). F_sys bore an inverse relation to CL_m. It was noted that values of F_sys for the SFM were consistently smaller than those for the TM, and the ratios of the values were always less than one.

View larger version (51K): [in this window] [in a new window]   Fig. 7.   Effect of ka and CLm on Fsys according to the TM (A and B) and the SFM (C and D), based on parameters shown in Table 1. CLd was fixed at 0.5 and 50 ml/min for illustration of drugs of the low and high permeability, respectively. CLsec was fixed at 0 ml/min, and kg was set as 0.5 min1.

Effects of CL_others, CL_m, and CL_d on metabolism with constant k_a (0.05 min¹). The simulation with Scientist according to the differential equations revealed different extents in intestinal metabolism between i.v. and p.o. doses for the SFM and TM when values of CL_others, CL_m, and CL_d were varied in the absence of secretion and luminal loss (CL_sec and k_g = 0). When CL_others = 0, intestinal metabolism accounted for 100% of the administered i.v. and p.o. doses regardless of the value of CL_d for drug because metabolism was the only route of removal (data not shown). With degradation or loss occurring within the lumen (k_g > 0), however, the percentage of dose metabolized by intestine could become greater for the i.v. over the p.o. dose due to incomplete absorption (F_abs < 1).

View larger version (14K): [in this window] [in a new window]   Fig. 8.   Effect of CLd and CLothers on intestinal metabolism when intestinal secretion and luminal loss are nonexistent (CLsec and kg = 0) according to the TM (A) and the SFM (B).

Application of SFM: Fitting of Morphine Data. The optimized parameters obtained from simultaneous fitting of the systemic and oral data of M and M3G to the TM and SFM are summarized in Table 3. Parameter estimation for M was more reliable because the S.D. values of the estimates were less than the values of the estimates. Expectedly, those for M3G were much less reliable due to the very high S.D. values of the estimates. This situation was not unique because the metabolite was not given, and there were too many fitted parameters. Nonetheless, least-square fitting was best with a weighting scheme of unity, and the resultant fits generally yielded good correlation with the data (Table 3, Fig. 9). The quality of the fits was, however, better for the SFM. Although an adequate fit of the TM was observed for intraduodenal data (Fig. 9B), a systematic trend existed for the fit to the i.v. data of M; M3G formation, though not detected in the system, was over-predicted (Fig. 9A). The SFM furnished, in comparison, superior fits, as shown by the higher value for the MSC (Model Selection Criterion), the slightly improved correlation coefficient, the lower RSS or residual sum of square of residuals (Table 3), and increased randomness in the residual plots (Fig. 10). An improved fit was observed with the i.v. data since the serosal compartment effectively provided a distribution space for M (Fig. 9A). The fitted value for the fraction of the intestinal flow perfusing the enterocyte layer (f_Q) was very low, representing only 2.4% of the total intestinal flow, and was different from zero or unity. If f_Q were unity, the SFM would simplify to the TM.

View larger version (22K): [in this window] [in a new window]   Fig. 9.   Fitting of the SFM (- - - -) to data on the metabolism of M to M3G. M was given i.v. (A) and intraduodenally (B) to the recirculating perfused rat liver preparation (data of Doherty and Pang, 2000). The SFM was more superior in describing the data compared to TM () described by Doherty and Pang (2000). Note that M3G was not observed after the i.v. dosing of M although a trace amount of M3G was predicted to be formed according to the SFM, and 3-fold that was predicted with the TM (A).

View larger version (15K): [in this window] [in a new window]   Fig. 10.

	Discussion

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The overall systemic availability of an orally administered substrate depends on the outcome between intestinal absorption and elimination by first-pass organs such as the intestine, liver, and lungs. Indeed, the importance of the intestine as an ingress organ in regulating the net absorption of drugs into the portal circulation is well recognized (Rowland 1972; Doherty and Pang, 1997). However, unlike the attention given to the examination of physiological variables influencing liver drug clearance (for review, see Pang et al., 1998), removal processes such as metabolism and secretion (or exsorption) and the physiological variables such as intestinal flow and gastrointestinal transit time on intestinal clearance and availability have not been fully investigated.

Until now, modeling and computer fitting of drug absorption have been based on a simplistic view of the intestine, where the tissue is considered as a homogeneous compartment separated from the lumen compartment by an apical membrane and from the organ blood by a basolateral membrane. Although these compartmental models have been applied to describe the intestinal absorption of various agents, the models lack consideration of one or more of the processes that are critical in determining reliably the overall clearance of the intestine. More specifically, the model assumed by Barr and Riegelman (1970) allowed for efflux and intracellular metabolism of orally administered drugs but did not include the transfer constant from the blood compartment to the tissue. Crouthamel et al. (1975), on the other hand, included the reversible transfer of drugs between the tissue and blood compartments, but both intestinal secretion and metabolism were ignored in modeling of the pharmacokinetics of sulfaethidole. Transport processes, such as the exchange from blood to tissue or the efflux from tissue to lumen, and intestinal metabolic activities were absent in the kinetic models proposed by Choi et al. (1995) and Nakashima et al. (1984). Recently, Ito et al. (1999) introduced a theoretical pharmacokinetic model to relate the influence of intestinal CYP3A4 metabolism, Pgp efflux, and intracellular diffusion on drug absorption. Not unlike both of our TM and SFM, Ito's model was able to predict the inverse relationship between bioavailability and metabolism and/or efflux. However, the transport clearance term that describes the partitioning of drug from the circulation to the epithelial cells was absent, precluding the intestinal accumulation or exsorption of i.v. administered drugs, and transfer processes between the gut lumen and epithelial cells were omitted in their definition of absorption clearance. The extended compartmental absorption and transit model developed by Yu and Amidon (1998) had simultaneously considered passive absorption, saturable absorption, degradation, and transit kinetics in the small intestine. But processes such as luminal and intracellular metabolism and exsorption were excluded. The present model is developed to comprehensively illustrate the interaction between the effective flow to the intestine, the absorption rate constant, intestinal enzymatic and secretory activities, and the influence of other clearances on systemic bioavailability. The SFM, based on the view that the absorptive site of the intestine receives only a portion of the overall organ blood flow, is in theory not dissimilar to the bypass phenomenon proposed by Klippert and Noordhoek (1985), with the exception that the flow rate to the intestinal tissue is conserved and drug distributes into the nonabsorptive and noneliminatory layer of the serosa and submucosa.

A close scrutiny of the SFM and TM reveals notable differences because of the different effective perfusion of the absorptive/metabolic/secretory layer. Theoretical solutions for both the TM and SFM differ only in the flow terms (Q_I versus Q_en) (see Table 2). Elimination within other parallel (non first pass) organs fails to affect the intestinal clearance, as expected of the additivity of organ clearances among parallel elimination pathways, and does not impact on bioavailability. The present communication also uncovers that, for both the SFM and TM, CL_I and F_I are directly/inversely related to the intestinal metabolic and exsorption intrinsic clearances (CL_m and CL_sec) and blood flow to the absorptive layer (Figs. 3 and 4); the parameters are additionally affected by k_a and k_g when there is drug exsorption (Table 2). Values for the SFM are, however, consistently lower than those for the TM (Fig. 5).

The frequent question addressed on whether the role of Pgp on secretion is overemphasized (Lin et al., 1999) can now be answered. The exsorption of substrate from the intestinal tissue to the lumen (CL_sec > 0) exerts a direct influence on F_sys; the larger the exsorption clearance, the less the systemic availability. Drug secretion by Pgp, viewed best in absence of metabolism and loss from lumen, reveals that secretion may be obliterated when drug absorption is rapid (Fig. 6). However, the concurrent absence of secretion and metabolism (CL_sec = 0; CL_m = 0) will result in a dramatic increase in the systemic (or intestinal) availability.

The difference in flow between the models also affects the extents of intestinal metabolism. The condition was best shown when CL_sec and k_g = 0; a greater difference in the extent of intestinal metabolism is found between the p.o. and i.v. doses with the SFM (see Fig. 8). According to the SFM, the lowered flow rate perfusing the enterocyte layer renders lower values of intestinal clearance, because there is reduced drug delivery to intestinal enzymes or secretory sites. However, during oral absorption, the entire orally administered dose must traverse the enterocyte layer before the substrate enters the circulation. The consequence of the partial flow to the enterocyte compartment leads to sluggish dispersal of drug into the circulation and a longer transit time within the intestinal tissue. The differential exposure with the site of administration results in different extents of metabolism by intestinal enzymes and exsorption, and contributes to the observation of route-dependent metabolism (Klippert and Noordhoek, 1985; Pang et al., 1985, 1986; Wen et al., 1999). Intestinal metabolism may then be viewed effectively as a single preabsorptive event, occurring predominantly during the absorption of the substrate across the luminal membrane and is substantially lower upon recirculation of the drug. It has been noted that flow can also be a limiting factor of intestinal absorption because it affects the net substrate flux from the lumen into the circulation and vice versa (Crouthamel et al., 1975; Winne, 1978; Schurgers and de Blaey, 1984). However, the flow rate to the enterocyte layer is now recognized as critical to intestinal clearance and bioavailability. Although the nature of the change remains largely untested, the magnitude of this flow is expected to be of paramount importance to the initial absorptive flux and drug extraction as well as on subsequent recirculation of the substrate.

Finally, the confirmatory evidence that the SFM is the better explanation of intestinal metabolism is substantiated by the fit to the experimental data of M. Statistically, the fits of the SFM to data on route-dependent glucuronidation of M in the vascularly perfused intestine preparation (data of Doherty and Pang, 2000) are improved over those afforded by the TM (Table 3, Fig. 9). In particular, the fit of the SFM to the i.v. data of M was superior because the distribution phase was better described by the SFM due to the presence of the serosal compartment acting as the storage/distribution compartment (Fig. 9A). The tissue partitioning ratio (value of 8) for M for the SFM was more reasonable than the much higher value of 22 predicted for the TM (CL2/CL1 or CL_d2/CL_d1), when levels of total radioactivity in the tissue were low (5 to 6% dose). Although there were notable levels of M3G accumulated in the reservoir after the intraduodenal dose, M3G was not detected after i.v. administration. The total level of M3G predicted by the SFM was lower for the SFM (6.6% for TM and 2% for the SFM).

Currently, the intestine is regarded as a single compartment. The SFM is physiologically sound and affords a plausible explanation of route-dependent metabolism. Due to the many examples of route-dependent metabolism of the intestine, it is anticipated that the proposed intestinal SFM may be important in future endeavors to accurately relate in vitro parameters with in vivo physiological events on absorption and bioavailability. Moreover, this model may be readily expanded to describe the physiological segmental divisions of the intestineduodenum, jejunum, and ileumand transport and metabolic or secretory heterogeneity within these segments (Dubey and Singh, 1988; Fei et al., 1994; Saitoh and Aungst, 1995; Aldini et al., 1996; Paine et al., 1997). With the development of these kinds of models, predictions on the first pass removal/metabolism and drug-drug interactions within the intestinal tissue would then be made accurately.