Mechanism and site dependency of intestinal mucosal transport and metabolism of thymidine analogues

In situ kinetic modelling of intestinal efflux in rats: functional characterization of segmental differences and correlation within vitro results

The objective was to devise and apply a novel modelling approach to combine segmental in situ rat perfusion data and in vitro cell culture data, in order to elucidate the contribution of efflux in drug absorption kinetics. The fluoroquinolone CNV97100 was used as a model P-gp substrate. In situ intestinal perfusion was performed in rat duodenum, jejunum, ileum and colon to measure the influence of P-gp expression on efflux. Inhibition studies of CNV97100 were performed in the presence of verapamil, quinidine, cyclosporin A and p-aminohippuric acid. Absorption/efflux parameters were modelled simultaneously, using data from both in situ studies as well as in vitro studies. The maximal efflux velocity was modelled as a baseline value, corrected for each segment based on the expression level. CNV97100 passive diffusional permeability (P(diff)) and its affinity for the efflux carrier (K(m)) were assumed to be the same in all segments. The results indicate the new approach to combine in situ data and in vitro data succeed in yielding a unified, quantitative model for absorption/efflux. The model incorporated a quantitative relationship between P-gp expression level and the efflux functionality, both across in situ and in vitro systems, as well across different intestinal segments in the in situ studies. Permeability values decreased from duodenum to ileum in accordance with the increasing P-gp expression levels in rat intestine. The developed model reflects a strong correlation between in vitro and in situ results, including intrinsic differences in surface area. The successful application of a model approach to combine absorption data from two different experimental systems holds promise for future efforts to predict absorption results from one system to a second system.

The effects of sinomenine on intestinal absorption of paeoniflorin by the everted rat gut sac model

Paeoniflorin and sinomenine, derived from the root of Paeonia lactiflora Pall. (family Ranunculaceae) and the stem of Sinomenium acutum Rehder & Wilson (family Menispermaceae), respectively, have been, and are currently, widely used for treatment of rheumatic and arthritic diseases in China and Japan. Our previous studies demonstrated that sinomenine could significantly improve the bioavailability of paeoniflorin in rats, but the underlying mechanisms remain unknown. The present study aims to investigate the intestinal kinetic absorptive characteristics of paeoniflorin as well as the absorptive behavior influenced by co-administration of sinomenine using an in vitro everted rat gut sac model. The results showed a good linear correlation between the paeoniflorin absorption in sac contents and the incubation time from 0 to 90 min. However, the concentration dependence showed that a non-linear correlation exists between the paeoniflorin absorption and its concentrations from 10 to 160 microM, and the absorption was saturated at about 80 microM of the drug. Sinomenine at 16 and 136 microM concentrations could significantly enhance the absorption of paeoniflorin (20 microM) by 1.5- and 2.5-fold, respectively. Moreover, two well-known P-glycoprotein inhibitors, verapamil and quinidine, could significantly elevate the absorption of paeoniflorin by 2.1- and 1.5-fold, respectively. Furthermore, sinomenine in a pattern, which influenced paeoniflorin's absorption, manifested as similar to that of P-glycoprotein inhibitors. In conclusion, sinomenine significantly enhance the intestinal absorption of paeoniflorin, subsequently improve the bioavailability of paeoniflorin. The mechanism underlying the improvement of paeoniflorin's bioavailability was proposed that sinomenine could decrease the efflux transport of paeoniflorin by P-glycoprotein.

Segmental intestinal transporters and metabolic enzymes on intestinal drug absorption

Recently, a physiologically-based, segregated flow model that incorporates separate intestinal tissue and flow to both a nonabsorptive and an absorptive outermost layer (enterocytes) was shown to better describe the observations on route-dependent morphine glucuronidation in the rat small intestine than a traditional physiologically-based model. These theoretical models were expanded, as the segmental segregated flow model and the segmental traditional model, to view the intestine as three segments of equal lengths receiving equal flows to accommodate heterogeneities in segmental transporter and metabolic functions. The influence of heterogeneity in absorptive, exsorptive, and metabolic functions on drug clearance, bioavailability (F), and metabolite formation after intravenous and oral dosing was examined for the intestine when the tissue was the only organ of removal. Simulations were performed for first-order conditions, when drug partitioned readily (flow-limited distribution) or less readily (membrane-limited distribution) into intestinal tissue, and for different gastrointestinal transit times. The intestinal clearance was found to be inversely related to the rate constant for absorption of a drug that was subjected to secretion and was positively correlated with the metabolic and secretory intrinsic clearances. F was positively correlated with the absorption rate constant but was inversely related to the metabolic and secretory intrinsic clearances. The gastrointestinal transit time decreased metabolite formation, increased clearance, and decreased F. The simulations further showed that a descending metabolic intrinsic clearance yielded a lower F and an ascending segmental distribution of metabolic intrinsic clearance yielded a higher F.

Transport, metabolism and elimination mechanisms of anti-HIV agents

Currently available anti-HIV drugs can be classified into three categories: nucleoside analogue reverse transcriptase inhibitors, non-nucleoside reverse transcriptase inhibitors, and protease inhibitors. Knowledge of these anti-HIV drugs in various physiological or pharmacokinetic compartments is essential for design and development of drug delivery systems for the treatment of HIV infection. The input and output of anti-HIV drugs in the biological systems are described by their transport and metabolism/elimination in this review. Transport mechanisms of anti-HIV agents across various biological barriers, i.e., gastrointestinal wall, skin, mucosa, blood cerebrospinal barrier, blood-brain barrier, placenta, and cellular membranes, are discussed. Their fates during and after systemic absorption and their metabolism-related drug interactions are reviewed. Many anti-HIV drugs presently marketed in the US bear some significant drawbacks such as relatively short half-life, low bioavailability, poor penetration into the central nervous system, and undesirable side effects. Efforts have been made to design drug delivery systems for the anti-HIV agents to: (1) reduce the dosing frequency; (2) increase the bioavailability and decrease the degradation/metabolism in the gastrointestinal tract; (3) improve the CNS penetration and inhibit the CNS efflux; and (4) deliver them to target cells selectively with minimal side effects. We hope to stimulate further interests in the area of controlled delivery of anti-HIV agents by providing current status of transport and metabolism/elimination of these agents.

Gastrointestinal absorption of drugs: methods and studies

Physico-chemical descriptors of drug molecules are often not adequate in predicting their oral bioavailability. In vitro methods can be useful in evaluating some of the different factors contributing to bioavailability. While physical parameters such as drug solubility may effect oral bioavailability, in most cases, the major determining factors are likely to be metabolism, and absorption at the intestinal level. Metabolism may be preabsorptive, as with peptides, or during absorption, particularly as a result of the activity of the intracellular enzyme CYP3A4. Absorption may be transcellular (membrane diffusion, carrier-mediated, endocytosis) or paracellular, while p-glycoprotein activity in the apical cell membrane may limit bioavailability by expelling drugs from the mucosal cells. Knowledge of the absorption mechanism is important in determining formulation strategies. The different in vitro techniques used to study absorption have advantages and disadvantages. Ussing chambers can be useful to measure bidirectional transport, but most studies use simple salt media, and full tissue viability is doubtful. Caco-2 cell monolayers are human cells, but the system is static, and gives very low rates of transport, and exagerated enhancement of the paracellular route compared with small intestine. The rat everted gut sac incubated in tissue culture medium maintains tissue viability and gives reliable data, although it is a closed system. In situ perfusion gives no information on events at the cellular level, and absorption may be reduced by anaesthesia and surgical manipulation. In vivo perfusion in man, with multichannel tubes, gives valuable data, but is not practical for screening. Pharmacokinetic modelling can also give useful data such as the existence of different absorption sites. Permeability values from the literature show that for small hydrophilic molecules, which pass by the paracellular route, the improved everted sac gives values close to those for humans, while values with Caco-2 cells are orders of magnitude lower.

Influence of the microporous substratum and hydrodynamics on resistances to drug transport in cell culture systems: calculation of intrinsic transport parameters

Although cell culture models are increasingly used to study drug transport and metabolism, the influence of the substratum on the transport properties of the cell monolayer has not been studied in great detail. Furthermore, the use of effective (or apparent) permeabilities (Peff) assumes that the contribution of the microporous filter substratum and the aqueous boundary layer (ABL) to transport are negligible or are at least constant for a series of drugs. In the present study, the permeabilities of the substratum, ABL, and monolayer were obtained for a series of compounds at variable flow rates in side-by-side diffusion chambers. Comparisons of transport properties were made between cell monolayers grown on substrata made of polycarbonate (PC) and polyester (PE). All paracellular markers demonstrated a reduction in permeability and a corresponding increase in transepithelial electrical resistance (TEER) through PE-grown monolayers. The permeabilities of two carrier-mediated compounds, phenylalanine and proline, were 55% higher and 48% lower through PE-grown monolayers than through the PC-grown monolayers, respectively. The resistance to progesterone transport attributed to the PE and PC filters was large (71% and 27% of total resistance, respectively) at a flow rate of 20 mL/min, indicating that the monolayer was not the rate-limiting transport barrier. Therefore, for highly permeable compounds, reporting Peff has limited value since it is an indicator of the transport properties of the substratum rather than of the monolayer. These results demonstrate that substratum properties (e.g., membrane composition, pore size, etc.) significantly affect the barrier properties of the Caco-2 cell monolayer. The most probable mechanism is by the modulation of the functional expression of nutrient and ion transporters resulting in variable transcellular and paracellular transport properties. These results further demonstrate the importance of calculating intrinsic membrane transport parameters if the monolayer is not maintained as the rate-determining barrier in the transport experiment. Using higher flow rates and higher porosity substrata supports may help maintain the monolayer as the rate-limiting transport barrier.

Absorption of D-glucose in the rat studied using in situ intestinal perfusion: a permeability-index approach

PURPOSE

A permeability-index approach was developed and used to study the transport of D-glucose in the jejunum and ileum of rats.

METHODS

The effective permeability coefficient (Pe) of [3H]D-glucose and [14C]antipyrine (an internal standard) in jejunum and ileum of four rats was determined using an in situ rat intestinal perfusion technique. The permeability ratio of the test compound (D-glucose) to the internal standard was defined as the permeability-index (P(i)), which was mathematically independent of the length and surface area of the intestinal segment perfused. Using this approach, the transport of [3H]D-glucose in jejunum and ileum of eight animals was investigated at concentrations ranging from 1 to 300 mM. The tissue/perfusate distribution of [3H]D-glucose and [14C]antipyrine at steady state was also determined.

RESULTS

The variability (%CV) in P(i) of D-glucose was only approximately 5%, compared with 23-36% in Pe values of D-glucose or antipyrine alone. The permeability and tissue distribution of [14C]antipyrine were unaffected by the presence of D-glucose. In contrast, the permeability and tissue distribution of [3H]D-glucose were concentration-dependent in both jejunum and ileum. The transport of D-glucose was studied assuming that the transport was mediated by a carrier (with maximum flux, Vmax and dissociation constant, Km) as well as by non-saturable transport (Pd). The maximum transport capacity for D-glucose in jejunum (0.522 mumole/min/cm2) was twice that in ileum (0.199 mumole/min/cm2), but the affinity (1/Km) was less than half of that in ileum (1/(48.2 mumole/mL) vs. 1/(21.4 mumole/mL)), rendering a similar active transport efficiency (Vmax/Km) in these two regions. The non-saturable permeability (Pd) in jejunum (44.6 x 10(-4) cm/min) was approximately twice that in ileum (20.4 x 10(-4) cm/min).

CONCLUSIONS

The permeability-index approach yielded parameters with reduced variability by eliminating potential imprecisions in length and surface area measurements of the intestinal segments perfused. D-glucose was transported via carrier-mediated systems in both jejunum and ileum, with different transport capacity and affinity in these two regions.

Molecular cloning and functional expression of cDNAs encoding a human Na+-nucleoside cotransporter (hCNT1)

We report identification of a new human nucleoside transporter protein by molecular cloning and functional expression of its cDNA. Previously, we used expression selection in Xenopus oocytes to isolate a cDNA from rat jejunal epithelium encoding the pyrimidine-selective Na+-dependent nucleoside transporter rCNT1 (Q.-Q. Huang, S. Y. M. Yao, M. W. L. Ritzel, A. R. P. Paterson, C. E. Cass, and J. D. Young. J. Biol. Chem. 269: 17757-17760, 1994). cDNAs for a human homologue of rCNT1, designated hCNT1, have been isolated from human kidney by hybridization cloning and reverse transcriptase polymerase chain reaction amplification strategies. hCNT1 was 83% identical to rCNT1 in amino acid sequence and exhibited the transport characteristics of an Na+-dependent nucleoside transporter with selectivity for pyrimidine nucleosides and adenosine when expressed in Xenopus oocytes. Deoxyadenosine, which undergoes net renal secretion, and guanosine were poor permeants. hCNT1 did, however, transport 3'-azido-3'-deoxythymidine. This is the first demonstration that members of the CNT family exist in human cells and provides evidence of their involvement in the renal transport of physiological nucleosides and nucleoside drugs. The hCNT1 gene was mapped to chromosome 15q25-26.

Oral absorption of anti-acquired immune deficiency syndrome nucleoside analogues. 2. Carrier-mediated intestinal transport of stavudine in rat and rabbit preparations

The intestinal transport and metabolism of stavudine (d4T), a nucleoside analogue of thymidine used in the treatment of AIDS, was studied using single-pass intestinal perfusion (SPIP), intestinal brush-border membrane vesicles (BBMV), and mucosal homogenates in rats and rabbits. In the SPIP, d4T demonstrated concentration-dependent mean wall permeability (P+/-w) at perfusate concentrations ranging from 0.001 to 25 mM. In coperfusion studies using 0.1 mM thymidine, 1 mM formycin B, or 5 microM NBTI as putative inhibitors of d4T transport, the P+/-w of 5 microM d4T was reduced to 48%, 62%, and 70% of the control value, respectively, suggesting the involvement of multiple nucleoside carriers in the intestinal uptake of d4T. d4T uptake in rat BBMV was significantly greater in the presence of a sodium ion gradient compared with a sodium-free (choline) gradient. The permeability of d4T, in the presence of a sodium gradient, was concentration-dependent and inhibited by 10 mM thymidine but not significantly reduced by 10 mM formycin B. In the presence of 10 microM NBTI, the permeability of d4T was not inhibited; however; the binding of d4T to rat and rabbit BBMV was significantly reduced. Formycin B did not significantly reduce the d4T uptake in rat or rabbit BBMV suggesting that d4T does not interact with the purine-selective N1 nucleoside carrier. However, because formycin B inhibited d4T uptake in the SPIP and since d4T inhibited formycin B uptake in rat but not rabbit BBMV, it appears to interact with the N3 carrier which has been demonstrated in rat but not rabbit intestine. Also, an interaction with the sodium-independent facilitative transporter at the basolateral membrane cannot be ruled out. The low hybrid K(m) and high passive permeability of d4T likely account for the lack of saturable absorption behavior observed in humans, whereas the brush-border and intracellular stability of d4T preserve the high bioavailability observed after oral dosing.

Intestinal absorption barriers and transport mechanisms, including secretory transport, for a cyclic peptide, fibrinogen antagonist

PURPOSE

The intestinal absorption of DMP 728, a cyclic peptide fibrinogen antagonist, was examined with the goals of identifying the cause(s) of its low oral bioavailability and understanding the mechanisms of its intestinal transport.

METHODS

In vitro partitioning, metabolism, and permeation through rat intestinal segments were evaluated.

RESULTS

DMP 728 had low lipophilicity and low intestinal permeation rates relative to model compounds. In addition, DMP 728 in vitro intestinal permeation in the secretory direction greatly exceeded transport in the absorptive direction. The secretory transport was saturable, glucose-dependent, and was inhibited by verapamil and by a monoclonal antibody to P-glycoprotein. DMP 728 likewise inhibited the secondary transport of verapamil. Mucosal-to-serosal permeation rates increased in going from the proximal to distal intestinal sites, but were lower than serosal-to-mucosal permeation rates for each site.

CONCLUSIONS

Net secretory transport and low lipophilicity are the major barriers to absorption of DMP 728.

Determination intestinal metabolism and permeability for several compounds in rats. Implications on regional bioavailability in humans

PURPOSE

To investigate the regional differences in small intestinal (SI) metabolism and permeability for several compounds and to ascertain the potential significance of these differences on the reported reductions in regional bioavailability in humans.

METHODS

The regional SI metabolism and permeability of captopril, didanosine (ddI), mannitol, ofloxacin and zidovudine (ZDV) were investigated in rats using a Single Pass Perfusion (SPIP) procedure or intestinal homogenates.

RESULTS

ddI was metabolized to a greater extent in the upper SI whereas captopril was metabolized to a greater extent in the lower SI. Relatively low homogenate concentrations resulted in significant degradation of captopril in the upper and lower SI. All other compounds were stable and changes in the buffer system or the initial concentration did not affect the results. The SI permeabilities of all compounds, with the exception of mannitol, decreased linearly with respect to SI location and the slopes of the corresponding normalized regression lines were not significantly different.

CONCLUSIONS

It has been reported that captopril and ddI demonstrate regional intestinal bioavailability in several species including humans. The current results suggest that the reported reduction in the lower SI bioavailability of captopril may be a result of a reduction in permeability and an increase in intestinal metabolism whereas for ddI, the reduction in the lower SI bioavailability appears to be attributable to a reduction in intestinal permeability. Other factors such as luminal metabolism may also significantly effect regional differences in the intestinal bioavailability of ddI or captopril. Based on these results, a strong possibility exists that ofloxacin and ZDV may also demonstrate regional differences in intestinal bioavailability.

Acyclovir permeation enhancement across intestinal and nasal mucosae by bile salt-acylcarnitine mixed micelles

The purpose of this study was to investigate the absorption enhancement of acyclovir, an antiviral agent, by means of bile salt-acylcarnitine mixed micelles. The specificity, site dependence, palmitoyl-DL-carnitine chloride (PCC) concentration dependence, and effects of absorption promoters on acyclovir absorption via the nasal cavity (N) and four different intestinal segments of the rat, i.e., duodenum (D), upper jejunum (UJ), combined lower jejunum and ileum (LJ), and colon (C) were evaluated. The present study employed the rat in situ nasal and intestinal perfusion techniques and utilized sodium glycocholate (NaGC), three acylcarnitines, and their mixed micelles as potential nasal and intestinal absorption promoters. Acylcarnitines used were DL-octanoylcarnitine chloride (OCC), palmitoyl-DL-carnitine chloride (PCC), and DL-stearoylcarnitine chloride (SCC). All acylcarnitines and NaGC by themselves produced negligible enhancement of acyclovir absorption in the rat intestine, while OCC and SCC were totally ineffective in the nasal cavity. However, the mixed micellar solutions of NaGC with PCC or SCC could significantly increase the mucosal membrane permeability of acyclovir in the colon and nasal cavity. On the other hand, NaGC-OCC mixed micelles slightly increased the absorption of acyclovir by both routes. When a mixed micellar solution of NaGC with PCC was used, the rank order of apparent acyclovir permeability (Papp; cm/sec), corrected for surface area of absorption, was N (10.54 +/- 0.62 x 10(-5)) > D (6.82 +/- 0.30 x 10(-5)) > LJ (2.90 +/- 0.08 x 10(-5)) > C (2.54 +/- 0.14 x 10(-5)) > UJ (2.30 +/- 0.22 x 10(-5)).(ABSTRACT TRUNCATED AT 250 WORDS)