Mechanism by which orally administered

For class 4 BDDCS drugs, predicting the effect of a high-fat meal on drug absorption is difficult, as a combination of interactions of both class 2 and 3 compounds is possible. Conversely, when fatty diet effects appear, they are mostly exhibited by an increase in the drug bioavailability, resulting from the combination of enhanced solubilization of a drug in the GI, as well as the inhibition of efflux transporters. Intestinal metabolism is normally triggered by digestive enzymes secreted by the pancreas, such as lipases; amylase; and peptidases, including chymotrypsin and trypsin, as well as those that are originated from the intestinal flora of the colon found mainly within the lower part of the GI tract.

In addition, the first-pass metabolism, which includes intracellular and brush-border metabolism, occurs on the enterocyte surface by enzymes present within the membrane of the brush border.

Brush-border metabolism occurs mainly in the small intestine. Isomaltase, alkaline phosphatase, sucrose, and other peptidases contribute to the brush border metabolism Barthe et al. First-pass metabolism might limit oral absorption. Intracellular metabolism occurs in the enterocytes and mainly involves phase-I metabolizing enzymes, including cytochrome P enzymes such as CYP3A4; several phase-II conjugating enzymes associated with reactions such as sulfation and glucuronidation; and other enzymes such as esterases Gibson and Skett, Although the intestinal epithelium is a site for pre-absorptive metabolism, it can act as a major site for the delivery of ester-type pro-drugs such as aspirin Thummel et al.

In addition to the intestinal epithelium, hepatic first-pass metabolism represents the major metabolic barrier. Membrane transporters can be categorized into two types: uptake and efflux transporters; they facilitate the transport of drugs and endogenous compounds out or into the cells. Thus, membrane transporters are important determinants for oral drug absorption, disposition, and bioavailability Shugarts and Benet, The main uptake transporters that enable xenobiotic transport of drugs into the cells belong to the solute carrier SLC superfamily, whereas the efflux transporters belong to the ABC superfamily Giacomini et al.

Most of these membrane transporters utilize ATP to pump substrates against a concentration gradient. In the small intestine and largely in the colon, P-gp is mainly located in the brush border surface of enterocytes where it acts as a defense barrier against exogenous compounds. Furthermore, CYP3A4 is co-localized with P-gp in mature enterocytes and has overlapping substrate specificity Watkins, Thus, most of substrate drugs might be metabolized by pumping them out of the enterocytes into the lumen via P-gp before they can be reabsorbed again into the cells, thereby prolonging their exposure to CYP3A4 Watkins, This mechanism limits the bioavailability of many drugs Gibson and Skett, Moreover, it can lead to drug-drug interactions, especially when drugs are made to inhibit P-gp or CYP3A4 Thummel, The main factors that affect drug absorption after oral administration are summarized in Table 3.

Development of oral formulations for drugs with poor aqueous solubility requires the understanding of barriers. Drug solubility is a key element of the low oral bioavailability of hydrophobic drugs Boyd et al. Other factors related to low bioavailability of hydrophobic drugs are food effect, gastric irritation, slow onset of action, lack of dose proportionality, and high intra- and inter-subject variability Singh and Kim, Therefore, many approaches are utilized to improve the aqueous solubility of drugs Table 4.

Formulation considerations such as surfactant selection, particle size reduction, and salt selection need to be carefully screened to develop formulations of poorly soluble drugs.

Traditionally, a combination of surfactants has been utilized for improving the oral absorption of drugs Wong et al. Surfactants contain a hydrophilic head and hydrophobic tail, in which both the hydrophilic and lipophilic groups help the drug molecules in localizing at the interface, thereby diminishing the interfacial tension.

Surfactants improve the bioavailability of drugs via several mechanisms, which include enhancing the solubility and permeability of drugs by momentarily opening tight intracellular junctions.

However, the use of surfactants at higher concentrations can become a safety concern and requires careful consideration Lawrence, In these techniques, the particle size of pharmaceuticals is reduced considerably, which in turn increases their surface area and subsequently the dissolution rate.

A brief summary of the formulation approaches for various BCS class drugs is shown in Table 5. Salt formation is the common conventional method for enhancing the oral absorption of weakly acidic and basic drugs Serajuddin, In general, salts of weakly acidic and basic drugs have higher solubility than their corresponding pure forms. Among the salt forms approved by the Food and Drug Administration FDA , hydrochloride and methanesulfonate mesylate are the most common ions for basic drugs, whereas sodium and calcium are the most common ions used for acidic drugs Lam et al.

The pH solubility profile can be used to increase the aqueous solubility of a drug by adjusting the pH. Furthermore, the capability of a salt to alter the overall medium pH is especially important because the micro-environmental conditions in the diffusion layer have been shown to represent a critical role in enhancing the dissolution rate of drug molecules Yang et al. A basic drug with a higher pKa, maximum intrinsic solubility, and lower salt solubility has been shown to favor salt formation under increased pH; in contrast, for an acidic compound, lower pKa and increased intrinsic solubility yield a lower pH, thereby increasing the possibility for salt formation.

Nevertheless, an error and trial process is required to identify and select the most suitable salt form for drugs. A prodrug is a chemical derivative of a main drug; it needs to undergo enzymatic biotransformation in the body to convert to an active drug.

The prodrug approach is a common chemical modification to improve drug properties, including aqueous solubility, lipophilicity, stability, mucosal membrane permeability, and therapeutic index. The most common prodrug types include ester, amide, carbonate, carbamate, azo, glucuronidic, and glycosidic bonds. In addition, polar moieties such as polyethylene glycol PEG are commonly included in drug molecules Greenwald et al.

The improvement of the oral absorption of a PEGylated prodrug is partially attributed to the bypass of P-gp efflux pump and cytochromes P metabolism Choi and Jo, ; Hussain et al. In addition, the inhibitory activity of efflux pump by several conjugates such as PEG-based detergents have been reported Veronese and Pasut, Among these detergents, polysorbate Tweens and tocopheryl poly ethylene glycol succinate 1, TPEGS are the most commonly used in oral drug delivery.

The prodrugs should be inert, nontoxic, and metabolizable. The prodrug design can improve the oral bioavailability of drugs by enhancing their water solubility and gastrointestinal permeability and overcoming first-pass metabolism. Prodrugs can improve the carrier-mediated absorption of charged or polar drugs with negligible passive absorption Shah et al. Further, they can target specific bioactivation mechanisms or colon bacterial microflora to achieve site-specific drug delivery Schacht et al.

Lipophilic esters are the most commonly used for oral prodrugs; they can enhance drug absorption by improving membrane permeability and absorption via the lymphatic route Charman and Porter, Some representative examples of oral prodrugs are listed in Table 6.

Solid dispersion indicates the dispersion of one or more drugs in an inert excipient or matrix, in the solid form. It is usually prepared using the melting fusion , solvent evaporation, co-precipitation, melting—extrusion, or spray drying method Serajuddin, Solid dispersions are generally formulated using a hydrophilic polymer and a poor water-soluble drug. In solid dispersions, the physical state of the active pharmaceutical ingredient is notably transformed from the crystalline to amorphous state Serajuddin, Solid dispersions of drugs in an amorphous state are rarely eutectic and thus remain metastable and thermodynamically active, leading to their supersaturation in the GI fluid.

This leads to a greater concentration gradient and thus increased dynamic force for drug transport across the cellular membrane. Moreover, the dissolution rate and bioavailability of solid dispersions of poorly water-soluble drugs are considerably higher because of the increased surface area and wettability owing to the reduced particle size.

The melting method is commonly used for developing scalable quantities of pharmaceutical formulations, but it is not applicable to thermolabile compounds Serajuddin, Common pharmaceutical excipients suitable for solid dispersions include cellulosic compounds such as hydroxypropyl cellulose HPC or hydroxypropyl-methylcellulose, PEG, polyvinylpyrrolidone, polyvinyl alcohol, and crospovidone Serajuddin, ; Newman, The bioavailability of orally administered cyclosporine A CsA , a BCS class II drug, was improved by preparing its solid dispersion formulation using the wet-milling method and HPC hydrophilic polymers.

The amorphous solid dispersion of CsA showed significant increase in the C max and AUC to about 5-fold, leading to enhanced therapeutic efficacy in inflammatory disease treatment and organ transplantation Onoue et al.

Inclusion complex formation with drug molecules is another approach to improve their aqueous solubility; it allows to control the release rates of lipophilic drugs; mask the taste of bitter drugs; and maximize the tolerance of oral drug formulations by minimizing the irritation of the drugs after oral administration Loftsson and Brewster, Moreover, it has the added advantage of improving the stability of drugs, especially esters, by shielding chemically labile substances from potentially harsh environmental conditions and reducing their enzymatic degradation, hydrolysis, or oxidation.

Cyclodextrins are chains of cyclic oligomers enclosing 6, 7, and 8 d -glucopyranose structures named alpha, beta, and gamma-cyclodextrins, respectively. Thus, they increase the aqueous solubility of drugs that are slightly soluble or water-insoluble to boost their oral bioavailability.

At present, more than 85 different oral drug formulations based on complexation are available in the market Choudhury et al. Co-crystals can be described as crystalline solids consisting of two or more molecular and ionic compounds held together by non-covalent forces Blagden et al. They might be considered as the crystalline counterpart of solid dispersions. The formation and sustenance of co-crystals in a supersaturated solution can enhance drug absorption and oral bioavailability Kwei et al.

Orally consumed co-crystals act similar to a single unit and partition into the intestinal membrane as a hydrophobic unit. This approach involves the co-administration of an additional concentration of a counter ion. The ionic compounds dissociate when diluted after administration in the GI medium. In principle, this strategy is simple and eliminates the need for chemical modification or prodrug design. Ion pairs need to have desired characteristics such as high lipophilicity, sufficient aqueous solubility, biocompatibility, and physiological stability.

The most commonly used counter ions are phthalic acid, succinic acid, and benzoic acid. However, these counter ions used for ion pairing can compete with endogenous compounds such as sialic acid, bile acids, and phosphoglycerides Varshosaz et al. In addition, most counter ions are not safe, which could cause membrane irritation and toxicity, particularly at higher doses.

Ion pairing by using naphthoic acid as a counter ion has been used to deliver highly polar antiviral drugs Miller et al. A study showed that itraconazole cocrystals with succinate, maleate, and tartrate behaved in a identical manner to the amorphous form, however its solubility improved from fourth to twentieth fold in comparison to that of the crystalline form of the drug Remenar et al.

Various absorption enhancers are known to increase drug permeability in the intestine, especially for BCS class III drugs. Compounds such as surfactants, cholesterol, glycerides, salicylates, bile salts,, and chelating agents are used as absorption enhancers Aungst, Most absorption enhancers increase the transport of hydrophilic drugs by altering their paracellular permeability LeCluyse and Sutton, However, some absorption enhancers might cause mucosal damage and systemic toxicity.

Ethylene diamine tetraacetic acid EDTA is commonly used as a paracellular permeation enhancer to deplete calcium and magnesium in the tight junctions Lemmer and Hamman, However, strategies involving the modulation of the permeability of tight junctions have safety concerns. Once tight junctions are opened, not only drugs but also other toxic molecules can be transported across the intestinal membrane. Conversely, transcellular promoters can increase the oral absorption of drugs primarily by fluidizing, solubilizing, or reorganizing the intracellular phospholipids, causing the disruption of membrane integrity.

Examples of such enhancers include tartaric acid, sodium salicylate, sodium caprate, and sodium caprylate. The co-administration of metabolism and efflux pump inhibitors has been shown to overcome multidrug resistance and increase the oral bioavailability of drugs that are substrates of efflux transporters such as P-glycoprotein, MRP2, and BCRP Pang, Recently, many excipients have been shown to modulate the function of efflux transporters, such as polyethoxylated castor oil Cremophor EL , polysorbates Tweens , poloxamers pluronic P85 , tocopherol polyethylene glycol 1, succinate TPGS , and polyethylene glycol PEG Murakami and Takano, Although the exact mechanism of P-gp inhibition is not yet known, polymeric excipients might act via one or more mechanisms that include competing with the substrate-binding site on the efflux transporter, altering the membrane lipid fluidity, protecting the drug while by-passing the efflux transporter, inhibiting the efflux pump ATPase activity, or directly acting on the P-gp protein on the mucosal surface Takano et al.

These studies indicate that the improved oral bioavailability of drugs administered with TPEGS might, in part, be attributed to the inhibition of P-gp pump Dintaman and Silverman, Mucoadhesive polymers such as dextran, chitosan, polycarbophil, and polyacrylic acid have been shown to affect the activity of intestinal protease enzymes, mainly trypsin, chymotrypsin, and carboxypeptidases, which might be useful for the oral delivery of metabolically labile drugs as well as increase their residence time.

The co-administration of grapefruit juice and ketoconazole was shown to reduce the pre-systemic metabolism by CYP3A4 present in the intestinal enterocytes Dresser et al. Similarly, the administration of erythromycin was found to increase the oral bioavailability of cyclosporine by selectively inhibiting hepatic CYP3A4 metabolism. Even though this approach seemed to be effective, the co-administration of metabolism inhibitors and dietary components such as grapefruit juice might not likely be approved by the FDA and other regulatory agencies for routine clinical practice.

Initially, drugs were not particularly developed to inhibit P-gp activity; basically, they had other therapeutical properties, as well as a low affinity for transporters. For instance, the first-generation P-gp inhibitors such as verapamil Nobili et al. However, most of these drugs were less specific and had more adverse effects. Second generation P-gp inhibitors such as biricodar VX are potent and selective, but have adverse pharmacokinetic interactions Leonard et al.

Several third generation P-gp inhibitors such as zosuquidar LY and tariquidar XR have been developed and ongoing studies being explored in clinical studies Planting et al. Lipid-based formulations have been used for the oral administration of drugs that are poorly soluble in water, such as BCS classes II and IV drugs.

Depending on their composition, size, and chemical characteristics, lipid-based systems can be further classified into lipid solutions, lipid suspensions, emulsions, multiple emulsions, micro- and nanoemulsions, self-emulsifying and self-micro-emulsifying systems, solid lipid nanoparticles, solid lipid dispersions, niosomes, and liposomes.

Lipid-based formulations are an attractive approach for oral application owing to their inherent biocompatibility, particle size versatility, scaling-up ability, and cost-effectiveness Hauss, A representative list of lipid-based formulations marketed for oral administration is shown in Table 8.

Most of these formulations can be administered as liquid-filled hard capsules or tablets as well as oral liquids in the form of solutions or suspensions.

Moreover, these dosage forms can be utilized for sustained- or immediate-release formulations. A lipid-based carrier is effective for the oral delivery of small hydrophobic molecules via several mechanisms. One of the main mechanisms is to enhance the dissolution rate and solubility in the GI tract.

The digestion of lipids is started in the stomach by gastric lipases. Shear forces in the digestive tract and stomach emptying assist in the emulsification of the drug before emptying into the duodenum.

Secretion of pancreatic enzyme lipase together with its co-factor co-lipase facilitates the breakdown of intake glycerides to diglycerides, monoglycerides, and fatty acids. The existence of fatty diets in the intestine also stimulates the gallbladder biliary secretions of bile salt, cholesterol, and phospholipids.

Due to the presence of bile salt, the yields of lipid digestion are consequently assembled into a colloidal structure, including vesicles, mixed micelles, and micellar carrier.

These carriers increase the solubilization of the drugs in the intestine. Furthermore, the nature and composition of the formulation such as lipids, surfactants, co-solvents, and complexation agents as well as bile salts and phospholipids contribute to the enhanced absorption Hauss, ; Savla et al. Thus, lipid-based formulations maintain a higher drug concentration gradient for facilitating the diffusion of drugs across the unstirred aqueous layer and then to the mucosal membrane.

Interestingly, the co-administration of a fatty food has also been shown to provide similar advantages to improve drug dissolution and bioavailability McClements and Xiao, This also explains why most lipid-based formulations have reduced food effects compared to conventional oral formulations Fatouros et al.

Poorly water-soluble drugs administered using lipid-based systems can be protected against enzymatic degradation. Interestingly, lipid-based carriers such as liposomes have been shown to be absorbed in an intact state by pinocytosis across the epithelial membrane and occasionally through the lymphatic system Lee, Lymphatic transport of lipophilic drugs e.

In addition, pancreatic lipase digestion results in the breakdown of triglycerides into monoacylglyceride and free fatty acid molecules that can interact with fatty acid transporters present on the apical membrane and mediate drug absorption.

While the common forms of lipids such as cholesterol, phospholipid, and tocopherol are absorbed via the chylomicron pathway, the majority of lipids and lipid digestion products are absorbed via fatty acid transporters Porter et al. Recently, more evidence suggests that lipid-based formulations inhibit the efflux transporter P-gp Sachs-Barrable et al. The sustained-release formulations prepared as a semi-solid lipid matrix filled in hard gelatin capsules seem to prolong the absorption of drugs even under a fed state, which could be attributed to the delay in the gastric emptying effect.

Although lipid-based delivery systems have the potential as drug carriers, obtaining the overall consistency, including in physicochemical properties, drug encapsulation, drug-release kinetics, and particle size, is difficult, especially with liposomes and solid lipid nanoparticles Phan et al. Furthermore, the ability of lipids to incorporate drugs differs in regard to their crystallization and polymorphism properties, which usually results in undesirable interactions and inconsistency Parve et al.

In addition, the availability and range of lipid-based excipients is limited Dahan and Hoffman, Drug solubility can be enhanced and drug precipitation after exposure to the GI environment can be avoided by incorporating poorly soluble compounds in surface-active agents, known as copolymers. Micellar systems occur in dynamic equilibrium in three systems in a surfactant solution: monomeric surfactant, micellar aggregates, and surfactants adsorbed as a film at the interface.

Micellar carriers are formed when the concentration of surfactant is above the critical micellar concentration CMC Ribeiro et al. Amphiphilic copolymers comprising of hydrophobic and hydrophilic blocks freely associated into micelles when dissolved in an aqueous environment. The hydrophobic domains of the copolymers form the core and the hydrophilic tails form the external shell of the micelles.

The lipophilic core serves as a container for loading lipophilic drugs, whereas the corona stabilizes the interface between the hydrophobic drug and aqueous medium. Micellar carriers can be utilized to increase the solubility of lipophilic drugs by incorporating them in the micellar core Gaucher et al. Poloxamers surfactant series is one of the most commonly used block copolymers in drug delivery.

The CMC of these copolymers range from 10 —5 to 10 —8 M, whereas that of standard low molecular weight surfactants range from 10 —3 to 10 —4 M Kwon and Okano, As micelles have the CMC of 10 —6 M, their ability to withstand dilution than is greater than that of surfactants with low molecular weight.

The study showed that the micellar composition was important in determining the desired drug release profile Sant et al. Moreover, these micellar systems can be chemically modified through the conjugation of antibodies on their side chains for improved target specificity. It is worth noting that antibody-conjugated micelles may undergo rapid clearance from blood circulation due to their accumulation in the liver, mainly in the absence of adequate target antigens Musacchio and Torchilin, The exponential development of nanotechnology has allowed the development of new oral drug delivery systems.

Numerous natural or synthetic based polymers have been utilized to prepare oral drug delivery systems. Some natural polymers commonly used include dextran, chitosan, gelatin, and alginate, and the synthetic based polymers used as oral drug delivery carriers include polylactide-coglycolide PLGA , polylactide PLA , polycaprolactone PCL , polyglycolide, polycyanoacrylate, and polyaziridine Ritika et al.

The nanotechnology approach involves the formulation of drugs by using particles that are in the nanometer size range of 10—1, nm. Nanocarriers can be prepared using many methods that can be divided into two groups. Top-down techniques: They are based on the reduction of the particle size of relatively large polymers into small particles; they involve processes that apply high shear, ultrasonication, cavitations, homogenization, microfluidization, spray drying, or milling Pinto Reis et al.

Bottom-up techniques: They are based on the growth of particles formed from individual particles; they are mainly known as phase separation methods. Examples include spray-freezing liquid, coordinated crystallization during freeze-drying, and pharmaceutical technologies that are based on supercritical fluid de Waard et al. Reducing the particle sizes to the nanometer scale results in larger effective surface area, eventually enhancing the dissolution rate and solubility of drugs Mei et al.

Examples of nanocarriers used for the oral drug administration are shown in Table 9. Polymeric nanocarriers can be used to deliver insoluble drugs, target the drugs to specific regions of the GI tract, minimize food effect on drug absorption, facilitate transcytosis of drugs across the mucosal membrane, and permit receptor-mediated intracellular drug delivery Mei et al.

Thus, polymeric nanocarrier-based drug delivery can enhance the specificity, tolerability, and efficacy of therapeutic agents.

Examples of patented nano-formulations employed for the oral delivery of drugs are shown in Table Nanoparticle uptake by enterocytes or M cells and stability in the GI tract depend mainly on the particle size, surface characteristics, molecular weight, and chemical composition Kulkarni and Feng, For oral drug delivery, particle size plays an important role because it influences particle adhesion and interaction with the mucosal membrane and the drug-release kinetics Kulkarni and Feng, ; Alqahtani, Several studies have indicated that the uptake of particles with size diameters of nm is higher than that of larger particles in the rat GI mucosa Desai et al.

Biodegradable polymers used in drug delivery have many advantages over non-biodegradable polymers; the former are safe and undergo complete degradation after drug release. For example, PLGA nanoparticles degrade into lactic acid and glycolic acid via hydrolysis, polycyanoacrylate nanoparticles degrade into cyanoacetate and formaldehyde, and protein nanoparticles are enzymatically degraded to basic amino acids and peptides Ratnaparkhi and Gupta Jyoti, Polymers for oral delivery of drugs often have an upper limit on the concentration of polymers that is nontoxic Islam et al.

For sustained release applications of potent drugs, the precise concentration of drug desired should be calculated and the formulation optimized accordingly by tuning the polymeric constituents or formulation process.

Bioerosion or swelling of polymers results in the diffusion of drugs from the nanoparticles in tunable release profile.

Polymers can be modified to exhibit the preferred release profile through cross-linking or chemical conjugation of the encapsulated drug Alqahtani et al. Polymers can also be combined with hydrogels or scaffolds to further fine-tune the desired release profile Liu, Biopolymers such as protein-based nanoparticles have been used owing to their desirable features, including generally recognized as safe GRAS status and biodegradability and correspond to amino acids. Some protein-based polymers that have been investigated for oral delivery include whey proteins, casein, gelatin, soy proteins, zein, and wheat proteins Shukla and Cheryan, ; Dong et al.

They can be used as drug delivery systems and in the food industry Hurtado-Lopez and Murdan, ; Liu et al. Furthermore, the amphiphilic characteristic of proteins allows them to interact equally well with both the drug and intestinal mucosa Islam et al. Therefore, nanoparticles obtained from natural proteins have an enhanced tendency for biological interaction and facilitate surface modification for the attachment of numerous drugs and target specific areas by using ligands.

Protein-based nanoparticles have been synthesized using both water-soluble and non-soluble proteins Perumal and Podaralla, Human serum albumin and bovine albumin are examples of water-soluble carriers, whereas zein and gliadin are lipophilic carriers.

However, the use of protein polymers for oral drug delivery has been less studied, especially with regard to the understanding of enzymatic stability, drug-release kinetics, and absorption mechanism. The challenges relate to nano-formulations yield, polydispersity, sonochemistry and throughput, are still the biggest obstacle against the widespread application of these systems, this prevents them from being scaled up to commercial levels and from entering the mass production for pharmaceutical industry Pathak and Thassu, TABLE List of patented formulations related to nanoparticles for oral drug delivery.

Schematic representation of the drug absorption barriers and mechanisms of nanoparticle transport across the intestinal epithelium, which include transcellular transport, receptor-mediated transport, and M-cell-mediated transport. The upper age limit used to distinguish the pediatric population differs between countries; usually, adolescents are considered up to the age of 18 or 21 years. Different age groups have different physiological and pharmacokinetic considerations, as well as ability to handle formulations.

Although efforts have been made to develop and design new pediatric formulations, the development of an acceptable pediatric formulation remains a challenging task. Furthermore, the development of pediatric formulations is hindered by the absence of market share or economic stimulus, since the majority of the patients is comprised of adults; dilemma in developing formulations that are adequately taste-masked; methodology and ethical requirements for clinical trials in children; and high costs associated with development research, manufacturing, and storage Strickley, Many physiological and maturational changes occur in growing children.

These age-related changes impact the absorption, disposition, and metabolism of drugs. For example, the pH of the GI tract is different between adults and children Lam et al. Similarly, differences also exist in gastric residence time, gastric emptying time, intestinal transit time, P-gp expression in the GI tract, and bacterial population composition Strolin Benedetti and Baltes, In addition, differences exist in the total body water content, enzymatic activity, and blood flow as well as in fat content.

From the formulation prospective, many pharmaceutically active ingredients have poor water solubility, stability, or an unpleasant taste, thereby rendering pediatric drug development a challenging task. Children cannot swallow large tablets and capsules and are unable to accept formulations that taste bitter or bad.

In addition to children, many geriatric and ill adults, such as patients in surgeries or recovering from comas, also unable to swallow tablets and capsules, thereby expanding the need for pediatric formulation. Clinically desirable pediatric dosage forms involve a solid dosage form or an orally dissolvable formulation that is tasteless; formulated with safe and minimal additives; and in a appropriate dosage form that is stable even after exposure to heat and humidity.

In the interests of transparency, eLife publishes the most substantive revision requests and the accompanying author responses. Tu and colleagues present an interesting body of work describing what they feel is a novel method of recycling of glucuronidated metabolites, specifically those of phenolic compounds. Your article has been reviewed by 2 peer reviewers, and the evaluation has been overseen by a Reviewing Editor and Olga Boudker as the Senior Editor.

The following individual involved in review of your submission has agreed to reveal their identity: Noelle S Williams Reviewer 2. The reviewers have discussed the reviews with one another and the Reviewing Editor has drafted this decision to help you prepare a revised submission. First, because many researchers have temporarily lost access to the labs, we will give authors as much time as they need to submit revised manuscripts. We are also offering, if you choose, to post the manuscript to bioRxiv if it is not already there along with this decision letter and a formal designation that the manuscript is "in revision at eLife ".

Please let us know if you would like to pursue this option. If your work is more suitable for medRxiv, you will need to post the preprint yourself, as the mechanisms for us to do so are still in development. Tu and colleagues present a body of work describing a novel method of recycling of glucuronidated metabolites, specifically those of phenolic compounds.

The authors argue that this has relevance for drug-drug interactions involving the OATP transporters and as a means to shuttle glucuronidated metabolites to the colon since the metabolism of phenolics by intestinal cells to glucuronidated forms as well as subsequent recycling means there should be large amounts of these metabolites in the colon. It would also be helpful if there were more data in between and uM. While the difference is dramatic, it is hard to know the relative roles of metabolism of the aglycone to the glucuronide versus uptake and transport of the aglycone form.

It might be helpful to evaluate levels of the aglycone and glucuronide inside the liver after this experiment. This seems somewhat arbitrary and if they truly wanted to assess the role of saturable protein binding, should do a protein binding experiment. It seems it would have been better to directly evaluate uptake of the aglycone form as previously suggested and just eliminate this evaluation. This doesn't make sense.

One should evaluate the same compound that is glucuronidated at multiple sites to evaluate the role of glucuronidation location — and correlate this with cell based studies with the transporters. It was also unclear why the 7 location was so relevant. The authors should elaborate. Does the latter have something to do with Table S4?

If so, that table should be referenced here. This seems something included to satisfy a previous reviewer without any reference to the relevance of the finding to the authors' overall conclusion.

The manuscript has been improved but there are some remaining issues that need to be addressed, as outlined below:. Only the recycling ratio of liver was determined. We believe It is important to delineate HER from EHR since the formation and disposition of phase II metabolites could be greatly different in disease condition related to different organs that could lead to differences in drug efficacy and toxicity both in systemic circulation and intestinal lumen. We believe that the differences in phase II enzymes, efflux and uptake transporters in different metabolic organs such as liver and intestine based on gender and disease conditions plays a significant role in determining the impact of EHR vs HER on the drug disposition.

We have modified the introduction as suggested by the reviewers. The contents are now made more directed to general introduction. We have added the clinical relevance for understanding the difference between EHR and HER recycling processes with references. We also got the feedback from colleagues not in the same area of research on the introduction and incorporate their comments in the introduction.

Instead we plotted the excretion rates against steady-state blood concentration. The saturation in the excretion rate as a function of blood concentration is expected if the blood concentration is correlated with the biliary excretion of wogonoside. This means that a higher blood concentration will result in a higher rate of excretion until the transporter is saturated. However, the current data are enough to show that the transporter is involved because process is saturable. The following is the revised changes along with page and paragraph information.

Also, there was no significant reduction in the uptake of Wogonin in the presence of OATP inhibitors, suggesting that the hepatic uptake of aglycone is primarily by passive diffusion and not active uptake.

Lastly, in Caco-2 cells studies, flavonoid aglycones always displayed high permeability []. As stated in the answer to the question 3, there is no evidence to suggest that uptake of flavonoid aglycones is slow, and therefore become the rate-limiting step in hepatic glucuronide formation. Based on the uptake experiments performed in our lab, we believe that uptake of aglycones into the hepatocytes is equal to or faster than the corresponding glucuronides.

This is supported by the ratio of liver concentration of wogonin and wogonoside Table S1 when aglycone is infused in portal vein. The 7-position is highly relevant because major dietary important flavonoids have a hydroxyl group at 7 position, and several UGTs are shown to have region-selective glucuronidation at this position [4].

For flavonoid compounds such as genistein [5], chrysin [6], apigenin [7], wogonin [8], and baicalein [9], at least one of their main glucuronides are conjugated at the 7position. We agreed with the reviewer that it would be better to study this problem with a flavonoids that have OH groups and then make synthetic glucuronide for each of these OH positions, but this was not scope of the current study as it requires huge amount of efforts to purify mono-glucuronides from a mixture of other mono-glucuronides.

Multiple monoglucuronides are expected if we use biosynthesis method, and chemical synthesis of these mono-glucuronides is beyond the scope of this work. However, we have turned down the importance of this 7-position in our discussion.

Since Oatps were reported to be male-predominant in rat liver [10], we hypothesized that sex differences could cause the changes of OATP expression levels and required investigating. In addition, E2G, a precursor for female sex hormone and a classical OATP substrate, has a drastically different blood concentrations in male and female.

Therefore, we tested some of our model compounds to verify whether the sex difference matters. Whereas experiment at higher concentration requires a much large quantities of glucuronides, which are generally not commercially available and hard to obtain via biosynthesis.

Another reason we did not use higher concentration because it is possible that these glucuronides would have competitive inhibition when infused together. Appendix 2 supplement Table S6. Luteolin was previously reported to be metabolized into Lut-di-G, which is consistent with our observation [11].

We agreed that we did not articulate that clearly. The latter will save our time to develop an analysis method to measure the glucuronide. This has been updated in the section 3. We modified section 3. Of all the results from the Fisher exact test Appendix 4 supplement Table S1 , 22 combinations were eliminated based on this criterion. Fang, Y. Nutrients, Tian, X.

Int J Pharm, Ahmed, I. ACS Omega, Singh, R. J Agric Food Chem, Yuan, B. AAPS J, Mohos, V. Drug Metab Dispos, Griffiths, L. Smith, Metabolism of apigenin and related compounds in the rat. Metabolite formation in vivo and by the intestinal microflora in vitro. Biochem J, Chen, X. Zhang, L. Hou, W. Toxicol Appl Pharmacol, Wang, L. The statement is based on results shown in two papers. Our data in this paper also confirmed that ezetimibe was extensively metabolized in rat small intestine and underwent HER almost exclusively.

Teeter, J. Meyerhoff, Environmental fate and chemistry of raloxifene hydrochloride. Environ Toxicol Chem, Sun, D. Cancer Prev Res Phila , Kemp, D. Fan, and J. Stevens, Characterization of raloxifene glucuronidation in vitro: contribution of intestinal metabolism to presystemic clearance.

Kokawa, Y. Eur J Pharm Sci, Strassburg, C. Manns, and R. We conclude that orally administered leminoprazole, via direct effect toward the gastric mucosa, stimulates mucus synthesis in rats probably through nitric oxide mediation.

The stimulatory effect of leminoprazole on mucus synthesis is independent of its antisecretory effect. Abstract We examined the mechanism by which leminoprazole an acid pump inhibitor stimulates mucus synthesis in rats. Within the bone marrow, the macrophages degraded the large beta-1,3-glucans into smaller soluble beta-1,3-glucan fragments that were taken up by the CR3 of marginated granulocytes.

These granulocytes with CR3-bound beta-1,3-glucan-fluorescein were shown to kill iC3b-opsonized tumor cells following their recruitment to a site of complement activation resembling a tumor coated with mAb. Abstract Antitumor mAb bind to tumors and activate complement, coating tumors with iC3b.