Intestinal secretion of drugs. The role of P-glycoprotein and related drug efflux systems in limiting oral drug absorption

Improved intestinal absorption of sulpiride in rats with synchronized oral delivery systems

The goals of this study were to examine whether formulations, capable of releasing sulpiride (SP) in synchrony with the p-Glycoprotein (P-gp) inhibitors, verapamil (Ver) or quinidine (Qn) can increase SP relative bioavailability and to suggest a rationale approach for oral administration of SP. Jejunum of anesthetized rats was perfused with 200 or 400 microg/ml of SP either alone or together with 98 microg/ml of Ver. It was observed that while an increasing SP concentration did not cause an increase in SP blood levels, the addition of Ver or Qn to the perfusion solution caused a profound increase in SP absorption. Erodible matrix tablets, exhibiting a range of erosion rates, were prepared by manipulating the ratios of hydroxypropylmethylcellulose (HPMC) in the matrices. The tablets were designed to release the low water soluble SP and the highly water soluble Qn concomitantly over 1, 2 or 4 h. In all cases, the synchronous release increased SP bioavailability after intra-intestinal administration. The increase varied from 2.6- to 3.9-fold for the fast and the slow release formulations, respectively (compared with a control administration of a powdered mixture of SP and Qn). It is speculated that the poor oral bioavailability of SP was caused by brush border P-gp efflux. Synchronous release delivery systems of SP containing also the P-gp inhibitor Qn were able to increase SP bioavailability after intestinal administration in the rat. It is concluded that oral bioavailability of poorly absorbed drugs, in which absorption is restricted by gut wall secretory transport, may be improved by formulating them with functional adjuvants in synchronous release drug carriers.

The role of P-glycoprotein in determining the oral absorption and clearance of the NK2 antagonist, UK-224,671

UK-224,671 has been shown to exhibit low oral bioavailability in vivo due to poor absorption from the GI tract. The purpose of this study was to investigate the underlying reason for this observation. In Caco-2 cell flux experiments, the absorptive (A to B) flux of UK-224,671 was low, consistent with poor in vivo absorption. However, flux in the B to A direction was significantly greater, suggesting that UK-224,671 can permeate the membrane of the gut wall cell. Such a Caco-2 cell flux is indicative of transporter mediated efflux, possibly by P-glycoprotein. In P-glycoprotein knockout mice, the oral bioavailability of UK-224,671 was 22%, representing a significant increase over the P-glycoprotein expressing wild type mice (<2%). However, in the knockout mice absorption was still incomplete, suggesting that both P-glycoprotein mediated efflux and poor membrane permeation combine to limit the oral absorption of UK-224,671 in wild type mice. Lack of P-glycoprotein expression had no effect on the clearance of UK-224,671 in mice, which suggests that uptake from the blood into the excretory cell is mediated by a transporter other than P-glycoprotein. Bile duct cannulated rat experiments show that approximately 20% of the clearance of UK-224,671 occurs by direct secretion across the gut wall into the faeces. This clearance pathway requires UK-224,671 to cross both the basolateral and apical membranes of the gut wall cell. P-glycoprotein is likely to be involved in the passage of the compound across the apical membrane as has been observed for other P-glycoprotein substrates.

HMG-CoA reductase inhibitors: assessing differences in drug interactions and safety profiles

OBJECTIVE

To review the cytochrome P450 system and associated metabolic differences between the HMG-CoA reductase inhibitors.

DATA SOURCES

A MEDLINE search (1993-99) was conducted for English-language articles using key search terms including adverse drug reactions, cytochrome P450, drug metabolism, drug interactions, hydroxymethylglutaryl-CoA (HMG-CoA) reductase inhibitors, myopathy, and rhabdomyolysis.

STUDY SELECTION

Review articles, clinical trials, and case reports concerning HMG-CoA reductase inhibitor metabolism, drug interactions, and adverse drug reactions were evaluated.

DATA EXTRACTION

By the author. No software or assistants were used to extract information from the chosen studies.

DATA SYNTHESIS

The cytochrome P450 enzymes, which can be divided into families, subfamilies, and isoenzymes, act as a major catalyst for drug oxidation in the liver. CYP3A4 is a major enzyme, accounting for about 60% of drug metabolic capacity in the liver and 70% of such function in the intestine. Lovastatin, simvastatin, and atorvastatin are substrates of CYP3A4, whereas fluvastatin is metabolized by CYP2C9. Pravastatin is not extensively metabolized by either of these isoenzymes; rather, it is transported into hepatocytes by a sodium-independent, carrier-mediated uptake system that normally transports bile acids. Compared with other statins, pravastatin thus has a reduced potential for drug interactions with other substrates, inhibitors, or inducers of the CYP3A4 and CYP2C9 systems.

CONCLUSION

Pharmacists must understand the functions of these enzymes to identify potential drug interactions, especially in high-risk patient populations, and to make appropriate therapeutic recommendations that prevent or minimize adverse clinical outcomes.

Effect of gastrointestinal inflammation and age on the pharmacokinetics of oral microemulsion cyclosporin A in the first month after bone marrow transplantation

Cyclosporin A (CsA) absorption is highly variable in BMT patients. Neoral, a new microemulsion formulation of CsA, permits increased absorption with less variable pharmacokinetic parameters in non-BMT patients. We evaluated the pharmacokinetics of CsA after BMT in patients received microemulsion CsA. Two oral doses of 3mg/kg were given 48 h apart between 14 and 28 days after allogeneic BMT in 20 adults, and one dose in seven children, while subjects were receiving a continuous i.v. infusion of CsA. Whole blood samples were taken throughout the dosing interval to calculate the incremental CsA exposure using maximum concentration (Cmax), time to Cmax (tmax), concentration at 12 h after the dose (C12), the area under the concentration-time curve (AUC), and to establish inter- and intra-patient pharmacokinetic variability. Drug exposure was substantially lower in children than adults, with an AUC of 861+/-805 vs 2629+/-1487 micromg x h/l (P = 0.001), respectively, and absorption was delayed and diminished in both groups by comparison with solid organ recipients. Intra-patient variability in adults for AUC was high at 0.59+/-0.34, while inter-patient variability, measured as the coefficient of variation (c.v.), was 0.55 for the first and 0.54 for the second dose. In adults, gastrointestinal (GI) inflammation due to either mucositis or GVHD resulted in a higher AUC of 3077+/-1551 microg x h/l compared to 1795+/-973 microg x h/l (P = 0.02), and a similar trend was observed in children. AUC seemed little affected by the CsA formulation (liquid or capsule), or co-administration with liquids or food. Trough (12 h) CsA levels correlated poorly with incremental AUC. Sparse sample modeling of the AUC using two-point predictors taken at 2.5 and 5 h after dosing accurately approximated AUC in adults (r2 = 0.94), while 1.5 and 5 h was superior in children (r2 = 0.98). These data suggest that 12 h postdose trough measurements of CsA may not be the most appropriate way to evaluate CsA blood concentrations in order to establish therapeutic efficacy in BMT patients. Based on this study, the dose of microemulsion CsA should be adjusted based on recipient age, and the presence of GI inflammation secondary to mucositis or GVHD. These data would suggest that sparse sampling at time points earlier than the trough more accurately reflects the AUC and may correlate more closely with therapeutic efficacy early post-BMT.

Lipids, lipophilic drugs, and oral drug delivery-some emerging concepts

Lipid-based dose forms, which encompass a wide variety of compositional and functional characteristics, can be advantageously utilized for the formulation of lipophilic drugs. There has been a traditional reluctance to develop lipid-based dose forms due to potential problems of chemical and physical instability, and a paucity of knowledge regarding formulation design algorithms and technology transfer issues. However, there is a current resurgence of interest in lipid-based dose forms due to potential commercial and pharmaceutical benefits, and the industry trend towards the discovery/development of increasingly hydrophobic (and potent) new chemical entities. This mini-review describes some emerging formulation and biopharmaceutic strategies that hold promise for better understanding how to design and evaluate lipid-based dose forms.

Active efflux kinetics of etoposide from rabbit small intestine and colon

The aim of the present study was to investigate the directional transport kinetics of etoposide in rabbit intestinal tissues using side-by-side diffusion chambers. Etoposide is a routinely used mixed-mechanism 'efflux' inhibitor; however, its absorptive and secretory transport kinetics in rabbit intestinal tissues, a commonly used animal model, have not yet been reported. Kinetic studies revealed that the apical (AP) to basolateral (BL) (i.e. absorptive) transport of etoposide was not apparently mediated by specialized transporters, whereas secretion (i.e. BL to AP transport) by intestinal tissues was concentration dependent and saturable. Half-saturation constant values (K(m), mean+/-standard deviation (S.D.)) ranged from 53.6+/-35.8 microM to 168.7+/-127.3 microM, consistent with previous results from our group in intestinal tissues from other species and Caco-2 cell monolayers. Secretory permeability was greatest in the ileum, whereas values in the upper small intestine and colon were approximately equal, and represented only 50% of the value in the ileum. The ileal secretory transport of etoposide was temperature dependent, with the activation energy (E(a)) >4 kCal/mole at 5 microM, suggesting the involvement of the active, energy dependent mechanism. Etoposide inhibition by verapamil and saquinavir, known inhibitors of intestinal secretion, was characterized as competitive with K(i)'s equal to 193.0+/-164.4 microM and 72.6+/-53.5 microM, respectively. The current results demonstrate that the absorptive transport of etoposide in rabbit tissue was not mediated by specialized carriers, and that secretory transport was regionally dependent, mediated by a transporter or transporters, the K(m)'s were in the micromolar range, and involved the energy dependent mechanism(s). The relatively low k(m) of etoposide compared with its aqueous solubility (0.25-0.34 mM, pH 5-6.5, 25 degrees C) makes it the excellent mixed-mechanism competitive inhibitor for determining the secretory transport properties of putative drug substrates. Understanding the in vitro secretory transport kinetics of etoposide provides a mechanistic basis for ongoing studies exploring the functional role of 'efflux' in vivo.

Influence of dietary components on the gastrointestinal metabolism and transport of drugs

There is widespread recognition that the ingestion of a meal is associated with a number of physiologic changes (gastric pH, gastric emptying, hepatic blood flow, etc.) that can significantly alter the rate and extent of drug absorption. It is also well recognized that the components of food can alter drug absorption through alterations in drug solubility. The nutritional status of a patient can also contribute to variability in the pharmacokinetics of certain drugs. The more recent finding that grapefruit juice can increase the bioavailability of certain drugs, by reducing presystemic intestinal metabolism, has led to renewed interest in the area of 'food-drug interactions.' Particular interest has focused on the effects of the grapefruit flavonoid, naringin, and the furanocoumarin, 6',7'-dihydroxybergamottin, on the activity of intestinal CYP3A4. The possibility that grapefruit juice might affect drug absorption via an interaction with intestinal P-glycoprotein (P-gp) is also being explored. The growing use of herbal extracts and phytopharmaceuticals raises a new challenge-will the use of these products cause changes in the pharmacokinetics of 'conventional' drugs? As a case in point, consider the phytoestrogenic isoflavones, which are being promoted for a number of health benefits. Isoflavones such as genistein and daidzein can inhibit oxidative and conjugative metabolism in vitro and interact with transporters such as P-gp and the canalicular multispecific organic anion transporter. Given that P-gp and canalicular multispecific organic anion transporter are involved in the intestinal absorption and biliary excretion of a wide range of drugs and metabolites, it is reasonable to suspect that isoflavones may alter drug disposition in humans. However, this possibility has not been explored.

Caco-2 versus Caco-2/HT29-MTX co-cultured cell lines: permeabilities via diffusion, inside- and outside-directed carrier-mediated transport

PURPOSE

The objective of this study was a systematic characterization and evaluation of cell culture models based on mixtures of Caco-2/HT29-MTX co-cultures for their use in screening for drug absorption and intestinal permeability in comparison to the properties of the respective mono-cultures.

METHODS

Co-cultures of Caco-2 cells (absorptive-type) and HT29-MTX cells (goblet-type) were set up. Three different co-cultures (initial seeding ratios Caco-2/HT29-MTX: 90/10, 70/30, and 50/50) were grown on permeable filter supports, and monolayers were used for permeability studies with model compounds for paracellular absorption (atenolol, furosemide, H334/75, mannitol, terbutaline), transcellular absorption (antipyrine, ketoprofen, metoprolol, piroxicam), carrier-mediated absorption (D-glucose, Gly-Pro, and L-phenylalanine) as well as substrates for carrier-mediated secretion via P-glycoprotein (cimetidine and talinolol). Electrophysiological and microscopic controls were performed to characterize the cell cultures.

RESULTS

For compounds undergoing passive intestinal absorption permeabilities were generally higher in co-cultures than in Caco-2 monolayers, yielding highest values in pure HT29-MTX monolayers. This difference was most obvious for compounds transported via the paracellular pathway, where HT29-MTX cells may be up to 30 times more permeable than Caco-2 cells, whereas for lipophilic and highly permeable compounds, the difference in permeability values was less obvious. For drugs undergoing intestinal secretion mediated by P-glycoprotein, co-cultivation of Caco-2 cells with HT29-MTX cells led to increased apical to basolateral permeability which was decreased in the opposite direction, consistent with the fact that HT29-MTX cells do not express P-glycoprotein. When a carrier-mediated absorption mechanism is involved, the permeabilities observed were lower than the values reported for human small intestine and co-cultivation of HT29-MTX cells with Caco-2 cells resulted in even lower values as compared to the plain Caco-2 cultures.

CONCLUSIONS

Co-cultures of HT29-MTX and Caco-2 cells offer the opportunity of modifying the permeability barrier of the cell monolayers both with respect to paracellular resistance and secretory transport via P-gp. Thus, in special cases, they allow more flexibility in adapting the in vitro system to the in vivo situation as compared to the monocultures. Another advantage is the obvious robustness of the method with respect to the reproducibility of the results. A problem remaining, however, is the quantitative expression of carriers involved in intestinal uptake of many nutrients and drugs.

Optimizing oral absorption of peptides using prodrug strategies

Prodrug strategies applied to peptides have tended to focus on modification of a single functional group (e.g., N-terminal end). Recently, our laboratory introduced the concept of making cyclic prodrugs of peptides as a way to modify their physicochemical properties sufficiently to allow them to permeate biological barriers (i.e., intestinal mucosa). This cyclization strategy required the development of new 'chemical linkers,' including an acyloxyalkoxy linker, a phenylpropionic acid linker, and a coumarinic acid linker. All three chemical linkers were designed to be susceptible to esterase metabolism (slow step), leading to a cascade of chemical reactions (fast steps) that result in release of the peptide. These cyclic prodrug strategies have been applied to opioid peptides in an attempt to stabilize them to metabolism and/or improve their intestinal mucosal permeation. Specifically, we prepared acyloxyalkoxy-, phenylpropionic acid- and coumarinic acid-based cyclic prodrugs of [Leu(5)]-enkephalin (H-Tyr-Gly-Gly-Phe-Leu-OH) and its metabolically stable analog DADLE (H-Tyr-D-Ala-Gly-Phe-D-Leu-OH) and determined their metabolic and biopharmaceutical properties. The cyclic prodrugs of these opioid peptides were shown to have: (i) favorable physicochemical properties (e.g., increased lipophilicity) for membrane permeation; (ii) unique solution structures (e.g., beta-turns) that reduce their hydrogen bonding potential; and (iii) metabolic stability to exo- and endopeptidases. The cell membrane permeation characteristics of [Leu(5)]-enkephalin, DADLE and the cyclic peptide prodrugs were evaluated using Caco-2 cell monolayers, a cell culture model of the intestinal mucosa. The phenylpropionic acid- and coumarinic acid-based cyclic prodrugs of [Leu(5)]-enkephalin and DADLE were shown to have significantly better cell permeation characteristics than the parent opioid peptides. Furthermore, these cyclic prodrugs were shown to be transcellular permeants (in contrast to the opioid peptides, which are paracellular permeants), and were not substrates for polarized efflux systems. Surprisingly, the acyloxyalkoxy-based prodrugs of [Leu(5)]-enkephalin and DADLE were shown to exhibit very low permeation through Caco-2 cell monolayers, which could be attributed to their substrate activity for efflux systems.

Oral absorption of the HIV protease inhibitors: a current update

Although the human immunodeficiency virus (HIV) protease inhibitors are highly effective, they are characterized by low and/or variable bioavailability with limited penetration into the central nervous system (CNS). Their clinical use is limited by patient compliance and by drug-drug interactions. The effect of drug solubility on their oral absorption has been investigated but further evaluation of this relationship is required. First pass metabolism appears to be significant for the HIV protease inhibitors and they are extensively metabolized by cytochrome P450 (CYP) 3A4. Recent studies suggest that these drugs are substrates for the P-glycoprotein efflux pump, which can limit their intestinal absorption and their transport across the blood-brain barrier. Drugs inducing or inhibiting CYP3A4 and/or P-glycoprotein may influence the bioavailability of the HIV protease inhibitors. The low bioavailability, variable absorption and drug-drug interactions of the HIV protease inhibitors may be related to the variability of cytochrome P450 and P-glycoprotein expression and to possible CYP3A4/P-glycoprotein interactions. To improve oral HIV protease inhibitor therapy, it is essential to mechanistically characterize the cell specific, tissue specific and regional intestinal dependencies of drug transport, secretory transport, metabolism and P-glycoprotein/CPY3A4 interactions. This report reviews the physicochemical characteristics and pharmacokinetics of the HIV protease inhibitors while considering the relationships between their hepatic and intestinal metabolism, low bioavailability, variable absorption and drug-drug interactions.

Role of P-glycoprotein-mediated secretion in absorptive drug permeability: An approach using passive membrane permeability and affinity to P-glycoprotein

It has been shown in vivo and in vitro that P-glycoprotein (P-gp) may be able to influence the permeability of its substrates across biological membranes. However, the quantitative contribution of the secretion process mediated by P-gp on the overall permeability of membranes has not been determined yet. In particular, observations need to be clarified in which substrates showing high affinity to P-glycoprotein, e.g., verapamil, apparently do not seem to be greatly influenced by P-gp in their permeability and consequently also with respect to their extent of GI-absorption after oral administration, whereas weaker substrates of P-gp, e.g., talinolol, have clearly shown P-gp-related absorption phenomena such as nonlinear intestinal permeability and bioavailability. Experiments with Caco-2 cell monolayers and mathematical simulations based on a mechanistic permeation model should aid in clarifying the underlying mechanism for these observations and quantifying the influence of passive membrane permeability and affinity to P-gp to the overall transmembrane drug flux. In addition, the concentration range of drug at which P-glycoprotein-mediated transport across the biological membrane is relevant should be examined. The permeability of various drugs in Caco-2 monolayers was determined experimentally and modeled using a combination of passive absorptive membrane permeability and a Michaelis-Menten-type transport process in the secretory direction. The passive permeabilities were experimentally obtained for the apical and basolateral membrane by efflux experiments using Caco-2 monolayers in the presence of a P-gp inhibitor. The Michaelis-Menten parameters were determined by a newly developed radioligand-binding assay for the quantification of drug affinity to P-gp. The model was able to accurately simulate the permeability of P-glycoprotein substrates, with differing passive membrane permeabilities and P-glycoprotein affinities. Using the outlined approach, permeability vs donor-concentration profiles were calculated, and the relative contribution of passive and active transport processes to the overall membrane permeability was evaluated. A model is presented to quantitatively describe and predict direction-dependent drug fluxes in Caco-2 monolayers by knowing the affinity of a compound to the exsorptive transporter P-gp and its passive membrane permeability. It was shown that a combination of high P-gp affinity with good passive membrane permeability, e.g., in the case of verapamil, will readily compensate for the P-gp-mediated reduction of intestinal permeability, resulting in a narrow range in which the permeability depends on the apical drug concentration. On the other hand, the permeability of compounds with low passive membrane permeability (e. g., talinolol) might be affected over a wide concentration range despite low affinity to P-gp.

Synthesis and binding properties of photoactivable biotin-conjugated verapamil derivatives for the study of P-170 glycoprotein

The design and synthesis of two photoactivable biotin-labeled analogues of verapamil (6 and 7) is reported. Preliminary evaluation of the biological profile of 6 (EDP 137) and 7 (EDP 141) shows that they have comparable affinities to that of verapamil for P-170, the protein responsible for multidrug resistance (MDR). Since both appear to bind irreversibly to the protein and the presence of biotin in their structure makes them easily detectable by avidin, they promise to be of great help in studying the protein and its mechanism of action.

The roles of P-glycoprotein and intracellular metabolism in the intestinal absorption of methadone: in vitro studies using the rat everted intestinal sac

Methadone is used as a treatment for opiate detoxification in methadone maintenance programs. Intra- and inter-patient variations in methadone bioavailability have been observed after oral methadone treatment and this makes it difficult to predict a dosing regimen. Intestinal absorption and metabolism could explain these variations. The in vitro gut sac model was used to study the intestinal absorption of methadone, and it confirmed that methadone is a substrate for P-glycoprotein. The transport of methadone was increased in presence of P-gp inhibitors verapamil and quinidine. The appearance of a major metabolite of methadone, 2-ethylidene-1, 5-dimethyl-3, 3-diphenyl pyrrolidine (EDDP) in the gut sac contents also demonstrated the existence of intestinal metabolism of methadone.

The metabolism of antihistamines and drug interactions: the role of cytochrome P450 enzymes

The non-sedating antihistamines show a diversity of fates in the body and the parent drugs and metabolites may differ in their biological properties. Clinically significant interactions with inhibitors of cytochrome P450 have been reported primarily for terfenadine, which has the potential for cardiac toxicity, and is metabolized to fexofenadine, an antihistamine without cardiac effects. Astemizole shares many of these characteristics and important safety-related interactions are likely. Loratadine undergoes extensive metabolism so that pharmacokinetic interactions could occur, but they would be of little clinical importance because of the lack of cardiac activity of the parent drug and its metabolites. Ebastine also undergoes pharmacokinetic interactions, the significance of which is dependent on clarification of the extent of any relevant cardiotoxicity of both ebastine and its metabolite. Interactions would not be clinically important for cetirizine and fexofenadine which do not show cardiac effects and are eliminated with little metabolism.

Design, synthesis, and in vitro activity of catamphiphilic reverters of multidrug resistance: discovery of a selective, highly efficacious chemosensitizer with potency in the nanomolar range

On the basis of the results obtained in previous research, three series of compounds (A-C), derived from verapamil, were designed and synthesized to obtain drugs able to revert multidrug resistance (MDR), an acquired resistance that frequently impairs cancer chemotherapy. The ability of the obtained compounds to revert MDR was evaluated on anthracycline-resistant erythroleukemia K 562 cells, measuring the uptake of THP-adriamycin (pirarubicin) by continuous spectrofluorometric monitoring of the decrease of the fluorescence signal of the anthracycline at 590 nm (lambdaex = 480 nm), after incubation with cells. Cardiovascular activity, which is responsible for unwanted side effects, was also evaluated. The results obtained show that many of the compounds studied are potent reverters of MDR and are endowed with reduced cardiovascular activity. One of the compounds (7, MM36) presents a pharmacological profile (unprecedented nanomolar potency, high reversal of MDR, low cardiovascular activity) that makes it a promising drug candidate to treat MDR and a useful tool for studying P-glycoprotein.

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.

The effect of conformation of the acyloxyalkoxy-based cyclic prodrugs of opioid peptides on their membrane permeability

In an earlier study using Caco-2 cells, an in vitro cell culture model of the intestinal mucosa, we have shown that the acyloxyalkoxy-based cyclic prodrugs 3 and 4 of the opioid peptides [Leu5]-enkephalin(1, H-Tyr-GLY-Gly-Phe-Leu-OH) and DADLE(2, H-Tyr-D-Ala-Gly-Phe-D-Leu-OH), respectively, were substrates for apically polarized efflux systems and therefore less able to permeate the cell monolayers than were the opioid peptides themselves. In an attempt to explain how structure may influence the recognition of these cyclic prodrugs as substrates by the apically polarized efflux systems, we have determined the possible solution conformations of 3 and 4 using spectroscopic techniques (2D-NMR, CD) and molecular dynamics simulations. Spectroscopic as well as computational studies indicate that cyclic prodrug 4 exhibits a major and a minor conformer in a ratio of 3:2 where both conformers exhibit gamma and beta-turn structures. Spectroscopic, as well as molecular dynamics, studies indicate that the difference between the two conformers involves a cis/trans inversion occurring at the amide bond between the promoiety and Tyr1. The major conformer has a trans amide bond between the promoiety and Tyr1, whereas the minor conformer has a cis amide bond. The spectroscopic data indicate that cyclic prodrug 3 has a structure similar to that of the major conformer in cyclic prodrug 4. It has recently been reported that a particular arrangement of polar groups and spatial separation distances is required for substrate recognition by P-glycoprotein. When the conformation of the acyloxyalkoxy linker was investigated in the major and minor conformers of cyclic prodrug 4, with respect to distances between the polar functional groups, this ideal fixed spatial orientation was observed. Interestingly this same spatial orientation of polar functional groups was not observed for other cyclic prodrugs prepared by our laboratory using different chemical linkers (coumarinic acid and phenylpropionic acid) but the same opioid peptides that had previously been shown not to be substrates for the apically polarized efflux systems. Therefore, we hypothesize that the structure and/or the flexibility of the acyloxyalkoxy linker itself allows cyclic prodrugs 3 and 4 to adopt conformations that permit ideal arrangement of polar groups in the linker and their fixed spatial orientation. This possibly induces the substrate activity of cyclic prodrugs 3 and 4 for the apically polarized efflux systems.

Metabolism and drug interactions of 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors in transplant patients: are the statins mechanistically similar?

3-Hydroxy-3-methylglutaryl coenzyme A reductase (EC 1.1.1.88) inhibitors are the most effective drugs to lower cholesterol in transplant patients. However, immunosuppressants and several other drugs used after organ transplantation are cytochrome P4503A (CYP3A, EC 1.14.14.1) substrates. Pharmacokinetic interaction with some of the 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors, specifically lovastatin and simvastatin, leads to an increased incidence of muscle skeletal toxicity in transplant patients. It is our objective to review the role of drug metabolism and drug interactions of lovastatin, simvastatin, pravastatin, fluvastatin, atorvastatin, and cerivastatin. In the treatment of transplant patients, from a drug interaction perspective, pravastatin, which is not significantly metabolized by CYP enzymes, and fluvastatin, presumably a CYP2C9 substrate, compare favorably with the other statins for which the major metabolic pathways are catalyzed by CYP3A.