enhanced oral bioavailability of doxorubicin in a dendrimer drug delivery system

Enhanced Oral Bioavailability of Doxorubicin in aDendrimer Drug Delivery System

WEILUN KE, YANSONG ZHAO, RONGQIN HUANG, CHEN JIANG, YUANYING PEI

Department of Pharmaceutics, School of Pharmacy, Fudan University, P.O. Box 130,Shanghai 200032, People’s Republic of China

Received 2 March 2007; revised 27 June 2007; accepted 5 July 2007

Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jps.21155

Corresponde7053; Fax: þ86-E-mail: jiangche

Journal of Pharm

� 2007 Wiley-Liss

2208 JOURN

ABSTRACT: PAMAM dendrimers can permeate across intestinal epithelial barrierssuggesting their potential as oral drug carriers. In the present study, we havedeveloped a drug–PAMAM complex for oral administration. The loading of a modeldrug, doxorubicin into PAMAM, the cellular uptake and Pharmacokinetics of thedoxorubicin–PAMAM complex were studied. As the results, the cellular uptake ofdoxorubicin in Caco-2 cells treated with the doxorubicin–PAMAM complex wasincreased significantly with an increase in concentration and time, as compared to thattreated with free doxorubicin. And the transport efficiency of the doxorubicin–PAMAMcomplex from the mucosal side to the serosal side was 4–7 times higher than that of freedoxorubicin in different segments of small intestines of rat. The doxorubicin–PAMAMcomplex led to the bioavailability that was more than 200-fold higher than that of freedoxorubicin after oral administration. These results indicate that PAMAM dendrimeris a promising novel carrier to enhance the oral bioavailability of drug, especially for theP-glycoprotein (P-gp) substrates. � 2007 Wiley-Liss, Inc. and the American Pharmacists

Association J Pharm Sci 97:2208–2216, 2008

Keywords: PAMAM; doxorubicin; P-gp
; oral bioavailability
INTRODUCTION

The oral route is by far the most convenient onefor drug administration. However, a variety ofdrugs are difficult to cross the intestinal epithe-lium into the blood circulation, resulting in poororal bioavailability. Some of them are not only thepoorly soluble drugs, but also are substrates of theefflux transporters and cytochrome P 450 (CYP).

Particulate drug delivery systems such aspolymeric microspheres,1 nanoparticles,2 lipo-somes,3 and the self-microemulsifying drug deliv-ery system (SMEDDS)4 offer great promise toincrease drug absorption at intestine. Particulate

nce to: Chen Jiang (Telephone: þ86-21-5423-21-5423-7186;[email protected])

aceutical Sciences, Vol. 97, 2208–2216 (2008)

, Inc. and the American Pharmacists Association

AL OF PHARMACEUTICAL SCIENCES, VOL. 97, NO. 6, JUN

systems are well known to be able to deliver drugswith higher efficiency and fewer adverse sideeffects.5,6 And some encapsulated formulationsmay be further engineered to deliver substancessuch as P-glycoprotein (P-gp) inhibitors or othertherapeutic molecules together with cytotoxicdrugs to achieve stronger anticancer activityin drug-resistant cancer cells.7–9 Mechanismsestablished so far including inhibition of P-gp bythe polymers make up the drug carriers,10,11 andincrease cellular drug uptake by endocytosis of thedrug carriers.12 At present, there are few studiesdevoted to the investigation of avoiding P-gpefflux of drugs with particulate drug deliverysystems at intestine.

Dendrimers have a number of applications inseveral pharmaceutical fields such as enhancingthe solubility of poorly soluble drug, enhancingthe delivery of DNA, and as carriers for thedevelopment of drug delivery system. Dendrimers

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ENHANCED ORAL BIOAVAILABILITY OF DOXORUBICIN 2209

have been shown to act as potential carrier/delivery systems that cross cell barriers atsufficient rates by both paracellular and transcel-lular pathways.13

Recently, some studies showing that PAMAMdendrimers can permeate across intestinal epithe-lial barriers suggest the potential as oral drugcarriers of dendrimers. However, limited data areavailable for the characteristics and mechanismsof PAMAM dendrimers, on their permeabilityacross the intestinal epithelial cell monolayersin vitro. The objective of this study is to evaluatethe potential of a PAMAM–drug complex toenhance oral bioavailability of P-gp substrate.

Doxorubicin, which is a P-gp and CYP substratewith poor bioavailability after the oral administra-tion14 was selected as a model drug. Caco-2 cell linewas chosen for this study because this cellline is known to express P-gp and CYP.14 In thepresent study, we have developed doxorubicin–PAMAM complex for oral administration. Byincorporating doxorubicin with PAMAM dendri-mers we sought to enhance cellular delivery, as wellas bypassing P-gp-mediated secretory efflux anddegradation by CYP, to increase its adsorptive drugtransport.

MATERIAL AND METHODS

Materials

PAMAM G3 (molecular weight is 6909, molecularradius is 3.6 nm, and surface group is –NH2),N-tris(hydroxymethyl)methyl-2-aminoethanesul-fonic acid (TES) and 3H-mannitol were pur-chased from Sigma (Sigma–Aldrich, Steinhein,Germany). Doxorubicin (molecular weight is 580)was obtained from Haizheng Medical DrugCo. (Zhejiang, China). Cyclosporin A (CsA) wasobtained from Guangdong Medical Drug Co.(Guangzhou, China).

Methods

Cell Culture

Caco-2 cells were grown in Dulbecco’s modifiedEagle’s medium containing 10% fetal calf serum,1% nonessential amino acids, 2 mM L-glutamine,100 U/mL penicillin G and 100 mg/mL strep-tomycin at 378C in a humidified atmosphere of5% CO2/95% air. All cells in this study were

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between passage 55 and 60. P-gp was detected inCaco-2 cells (data not shown).

Incorporation of Doxorubicin–PAMAMComplex and Release of DoxorubicinFrom the Doxorubicin–PAMAM Complex

The doxorubicin–PAMAM complex was preparedfollowing the method previously described15 withminor modifications. Doxorubicin (1.72 mmol) and0.492 mmol PAMAM (about 3.5:1 molar ratioof doxorubicin–PAMAM) were dissolved in 4 mL10 mM pH 7.5 TES and the solution was stirredovernight. The PAMAM dendrimer incorporatingdoxorubicin was extracted overnight using 3 mLof chloroform. Five hundred microliter of thesolution in the chloroform layer was removed,added with 500 mL of 0.1 M hydrochloric acid andcentrifuged (3000 rpm, 10 min). The water layerwhich contains doxorubicin hydrochloride wasseparated for detecting. The absorbance of dox-orubicin was measured on a microplate fluoro-meter (Perkin-Elmer) at Ex/Em 537/584 nm.

The release of doxorubicin from doxorubicin–PAMAM complex (1 mL of the preceding solution)was studied in 10 mM pH 7.5 TES at 378C usingdialysis bags (molecular weight cut off 1000).The doxorubicin released at various times, up to48 h was measured on a microplate fluorometer atEx/Em 537/584 nm.

MTT Cytotoxicity Assay of PAMAM in Caco-2 Cells

For cytotoxicity assay, Caco-2 cells were seeded ata density of 4.5� 103 cells/well in 96-well plates,and incubated for 96 h. PAMAM was dissolved inphosphate buffer solution (PBS, pH 7.4) at a seriesof concentrations. Caco-2 cells were incubated for4 h in different solutions with a series concentra-tion of PAMAM. After 4-h incubation, the mediawere discarded and 100 mL of the MTT solution(0.5 mg/mL in PBS) was added to each well of96-well plates. The incubation was performed at378C for 2 h after the media were discarded. Then100mL of DMSO solution was added into each wellto solubilize formazan crystals. The culture plateswere placed on an orbital shaker at 378C for10 min. The absorbance was measured at the testwavelength of 570 nm and the reference wave-length of 630 nm by a microplate reader (Bio-Rad, Tokyo, Japan). Cells without the addition ofPAMAM solution served as positive controls.Assays with each PAMAM concentration wererepeated four times.

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Uptake Experiment With Caco-2

To establish cellular drug uptake profiles, cellswere plated onto 24-well plates at a density ofapproximately 40000 and cultured for 14 days.In the time-dependent experiment, 400 mLof doxorubicin solution or doxorubicin–PAMAMcomplex suspension or doxorubicin solution mixedwith CsA or doxorubicin–PAMAM complexmixed with CsA was added into each well toinitiate cellular drug uptake. All treatments wereadjusted to 17.2 mM doxorubicin with Hank’sSolution. Blank Hank’s Solution was used as thenegative control. At predetermined time intervals(5–90 min), supernatant was removed, and cellswere washed with ice-cold PBS (pH 7.6) and lysedwith PBS containing 1% Triton-X. Doxorubicinconcentrations in the cell lysates were measuredwith a microplate fluorometer at an excitationwavelength lex¼ 478 nm and an emission wave-length lem¼ 594 nm.

In the concentration-dependent experiment, aseries of concentrations (from 5 to 100 mM) ofdoxorubicin solution or doxorubicin–PAMAMcomplex was added into each well to initiatecellular drug uptake. The cells were incubated for30 min and the solution was treated as the time-dependent experiment.

To adjust for background fluorescence fromthe cellular components, doxorubicin standardiza-tion curves were also prepared using cell lysates.Cellular doxorubicin uptake is expressed asnanomoles per milligram of protein. Proteinconcentrations of the cell lysates were determinedby the Bradford colorimetric assay.16

In vitro of Permeability of Doxorubicin,Doxorubicin–PAMAM, and Doxorubicin MixedWith CsA Through Intestinal Mucosa of Rat (A–B)

Male Sprague-Dawley (SD) rats weighingbetween 230 and 270 g were deprived of food for1 day, and provided with only double distilledwater until the rats were anesthetized with etherbefore the experiment. The duodenum, jejunum,and ileum of the rat intestines (approximately10 cm each) were taken, and the segmentswere immediately immersed in oxygenated Krebssolution, stripped off longitudinal muscle andmyenteric plexus, and two to four adjacent piecesfrom each segment were mounted in Ussingchambers (World Precision Instruments, NarcoScientific, Mississauga, Ontario, Canada). Thechamber exposed 1 cm2 of tissue surface area to4 mL of circulating oxygenated Krebs buffer at

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378C. The buffer contained (in mM) 115 NaCl, 1.25CaCl2, 1.2 MgCl2, 2.0 KH2PO4, and 25 NaHCO3

(pH 7.35). The buffer in mucosal compartmentalso contained doxorubicin–PAMAM (added with0.5 mCi/mL 3H-mannitol as maker) or doxorubicin(108 mM).

During a 90 min incubation period a sample of200 mL was taken from the acceptor chamber atvarious times (15–90 min), and the volume wasreplaced by incubation medium preequilibrated at378C. The samples were measured on a microplatefluorometer at Ex/Em 537/584 nm.

The transport rate (dQ/dt) was calculated byplotting the amount of polymer penetratingthe basolateral side versus time (15, 30, 45, 60,75, and 90 min) and then determining the slopeof this relationship. The apparent permeabilitycoefficient (Papp, cm s�1) was calculated fromthe following equation: Papp¼ (dQ/dt)/(C0�A),where dQ/dt is the permeability (mg/s), C0 is theinitial polymer concentration on the apical side ofcell monolayers (mg/mL), and A is the surface areaof the membrane filter in cm2.

Pharmacokinetics of theDoxorubicin–PAMAM Complex

The rat was intragastric administrated with 5 mLof 1 mg/mL (1.72 mM) doxorubicin. Collect about0.5 mL of the blood of the rat at various times(1–24 h). Then the blood was centrifuged(8000 rpm, 20 min), and 50 mL of the plasmawas mixed with 200 mL 6% hydrochlorine-ethanoland stored overnight. The mixture was centri-fuged (8000 rpm, 20 min) and the supernatant wasanalyzed using a microplate fluorometer at Ex/Em537/584 nm.

Statistics

Data are expressed as the mean�SD. The signi-ficance of differences between groups and controlwas evaluated by using Student’s t-test orANOVA, followed by Scheffe’s test. Differencesbetween means were considered to be significantwhen the p-value was less than 0.05.

RESULTS

Incorporation and Release of Doxorubicin Fromthe Doxorubicin–PAMAM Complex

The formation of doxorubicin–PAMAM complexhas been achieved using PAMAM G3 and molar

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ratio of doxorubicin to PAMAM 2:1. The resultsdictate that almost 95% of doxorubicin wasincorporated into the dendrimer. The release ofdoxorubicin from doxorubicin–PAMAM complexappears to be slower and lower (74.5% during24 h) as compared to that from the doxorubicinsolution (more than 95% during 3 h) as shown inFigure 1.

Figure 2. Time-dependent uptake of doxorubicinby Caco-2 cells and effect of Cyclosporin A. Eachpoint represents the mean�SEM of four experiments.Significance of differences was determined by usingANOVA followed by Bonferroni’s t-test: �p< 0.05;��p< 0.01; ��p< 0.001 versus free doxorubicin.

Cellular Uptake of the Doxorubicin–PAMAMComplex by Caco-2 Cells

Figure 2 represents the time course of the cellularuptake of doxorubicin by Caco-2 cells with thedoxorubicin solution or the doxorubicin–PAMAMcomplex. Doxorubicin is a substrate for P-gp, andCsA is known to be an inhibitor of P-gp andthus prevents the efflux of P-gp substrates fromthe cells. The higher accumulation of doxorubicinin Caco-2 cells was observed in the presence of10 mg/mL of CsA, as compared to that treated withfree doxorubicin (Fig. 2), evidently, due to theinhibition of P-gp efflux by CsA. And using thedoxorubicin–PAMAM complex, the absorptiveamount in Caco-2 cells was significantly increasedover all time points as compared to the other twogroups without PAMAM. Meanwhile, the additionof CsA did not cause significant increase of uptakeof doxorubicin–PAMAM complex.

The cellular uptake of doxorubicin in Caco-2cells was scarcely altered with an increase in theconcentration with the doxorubicin solution as itis shown in Figure 3. In contrast, the cellular

Figure 1. The release of doxorubicin from doxorubi-cin–PAMAM complex in TES at 378C during a 24 hperiod. Each point represents the mean of three inde-pendent experiments.

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uptake of doxorubicin of the doxorubicin–PAMAMcomplex was increased significantly with an in-crease in the concentration over the range from5 to 100 mM.

Absorptive Transport Characteristics of theDoxorubicin–PAMAM Complex in Intestine of Rats

The time-dependent mucosal-to-serosal transportof doxorubicin (108 mM) was observed in differentsegments of small intestines of rat. As shown inFigure 4, the amount of doxorubicin in the serosalfluid increased slightly with time in all segments

Figure 3. Concentration-dependent uptake of doxor-ubicin by Caco-2 cells. Each point represents themean�SEM of four experiments. Significance ofdifferences was determined by using ANOVA fol-lowed by Bonferroni’s t-test: ��p< 0.001 versus freedoxorubicin.


Figure 4. Doxorubicin permeability across freshlyexcised small intestinal mucosa of rat. Indicated valuesare means of at least four experiments�SEM. Signi-ficance of differences was determined by using ANOVAfollowed by Bonferroni’s t-test: �p< 0.05; ��p< 0.01;��p< 0.001 versus free doxorubicin.

Figure 5. Plasma profiles of doxorubicin after asingle oral administration of different doxorubicinformulations in rats. Indicated values are means ofat least four experiments�SEM. Significance of dif-ferences was determined by using ANOVA followedby Bonferroni’s t-test: ��p< 0.01; ��p< 0.001 versus freedoxorubicin.

2212 KE ET AL.

treated with the doxorubicin solution. However,the significantly higher transport of doxorubicinwas achieved by using a doxorubicin–PAMAMcomplex, and the amount of doxorubicin in theserosal fluid increased linearly from 15 up to90 min. The transport of doxorubicin in the ileumtreated with the doxorubicin–PAMAM complexwas slightly higher than that in the duodenumand the jejunum. The transport of doxorubicin inthe ileum treated with the doxorubicin–PAMAM


complex was significantly higher than thattreated with CsA. At 90 min, the transportefficiency of the doxorubicin–PAMAM complexfrom the mucosal side to the serosal side was 4–7 times higher than that of the doxorubicinsolution. And the doxorubicin concentration inthe serosal side of all segments treated withthe doxorubicin solution was almost the same. Itis shown that the slight increase in permeabilityof 3H-mannitol across small intestinal mucosa ofall segments upon incubation with doxorubicin–PAMAM complex (Fig. 4).

Pharmacokinetics of theDoxorubicin–PAMAM Complex

The oral concentration–time curve after a singledose of different doxorubicin formulations in ratsis shown in Figure 5. Pharmacokinetic para-meters were calculated with a noncompartmentalmethod using the standard methods again.The time to peak (Tmax) and the peak concentra-tion (Cmax) were measured directly from theconcentration–time curve and AUC by the lineartrapezoidal rule (Tab. 1). All calculations wereperformed with the software package WinNonlinProgram (Version 5.2, Pharsight Corporation,Mountain View, CA, USA). At all time points,the doxorubicin plasma concentrations of ratstreated with the doxorubicin–PAMAM complexwere significantly higher than those treated withdoxorubicin solution. The Cmax values of doxor-ubicin in the doxorubicin–PAMAM complex were

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Table 1. Pharmacokinetic Parameters ofDoxorubicin After Oral Administration of DifferentFormulations (n¼ 4)

Parameters Doxorubicin Doxorubicin–PAMAM

Tmax (h) 0.5 0.5Cmax (mM) 0.20� 0.05 7.63� 0.11AUC (mg/mL)�h 0.78� 0.22 246.88� 156.12

Pharmacokinetic parameters were calculated with a non-compartmental method using the standard methods again. Thetime to peak (Tmax) and the peak concentration (Cmax) weremeasured directly from the concentration-time curve and AUCby the linear trapezoidal rule.


higher than that obtained with the solution. TheAUC values of doxorubicn after oral administra-tion of the doxorubicin–PAMAM complex weremore than 300-fold higher than those obtainedwith the doxorubicin solution. From these results,we can conclude that doxorubicin absorption wasincreased significantly by employing the doxor-ubicin–PAMAM formulation compared with freedoxorubicin solution.

Cytotoxicity Assay

The data presented in Figure 6, clearly show thelack of cytotoxicity of PAMAM (i.e., of complex-forming materials) against Caco-2 cells even at100 mM.

Figure 6. Cytotoxicity of PAMAM towards Caco-2cells as analyzed by MTT assay. Cytotoxicity wasexpressed as survival rate versus the control grouptreated with PBS. Results are means�SEM of tripli-cate experiments.

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DISCUSSION

In the present study, we have demonstrated thatdoxorubicin entrapped in PAMAM dendrimerefficiently can be used successfully to deliverdrugs across small intestine epithelial cell. Thedoxorubicin–PAMAM complex led to the bioavail-ability that was <700-fold higher than that of freedoxorubicin.

Since the PAMAM dendrimer has a basic andhydrophobic interior, the PAMAM dendrimerscould encapsulate and retain doxorubicin with ahydrophobic or acidic character by their PAMAMdendrimer moieties in aqueous environments.Doxorubicin is a hydrophobic drug. The resultsshow that doxorubicin has high entrapmentefficiency in PAMAM G3. As is shown inFigure 1, the release of doxorubicin from thecomplex, which may be free or weakly interactedone, was about 60% of the total amount atmaximum, and the rest 40% may be reallyincorporated and be hardly released. And it hasbeen reported that the dendrimers of highergenerations have a higher ability to carry drugscompared with the lower generation dendrimers.Using PAMAM G4 as a drug carrier, a doxorubicinto PAMAM molar ratio of 3:1 is sufficient inorder to achieve an almost 97% incorporationof doxorubicin into the dendrimers.15 However,cytotoxicity of PAMAM dendrimers increases withthe generation for cationic dendrimers,17 and thecytotoxicity of PAMAM G3 is reported to be low.18

In our study, it is showed that PAMAM at highconcentration (>100 mM) caused a significantdecrease in cell viability. This result indicatedthat the cytotoxicity of PAMAM is minimal at theconcentration used in this study.

Doxorubicin is a substrate for P-gp. On theother hand, CsA is known to be an inhibitor of P-gp and thus prevents the efflux of P-gp substratesfrom the cells. We have found that significantlyhigher accumulation of doxorubicin in Caco-2cells was observed in cells treated with CsA ascompared to untreated cells (Fig. 2), evidently,due to the inhibition to P-gp efflux and CYPmetabolism by CsA. However, when the samecells were incubated with doxorubicin encapsu-lated into PAMAM dendrimer, the presence orabsence (Fig. 2) of CsA did not influence doxor-ubicin accumulation in the cells. The mostevident explanation for this phenomenon isthat doxorubicin–PAMAM complex is capable ofbypassing the drug efflux by P-gp and CYPmetabolism.


2214 KE ET AL.

To study the transport characteristics ofdoxorubicin–PAMAM complex further, the adsorp-tive transport of doxorubicin was investigatedin isolated intestinal mucosa of rat. Similar toresults of Caco-2 cells, the transport rate ofdoxorubicin–PAMAM complex was significantly(p< 0.01) greater than free doxorubicin, and nosignificant difference in different small intestinalsegments. P-gp expression is rich in the lowersmall intestine.19 As expected, the transport offree doxorubicin was increased in the presence ofthe P-gp and CYP inhibitor CsA (10 mM) in ileum,which is the lower small intestine. The resultswere consistent with doxorubicin being a sub-strate of the intestinal efflux transporter P-gp,which is functionally active in these cells.

PAMAM dendrimer was not as functioning as aP-gp inhibitor. Add the extent of enhancement ofdoxorubicin transport was significantly (p< 0.01)greater than the presence of the P-gp inhibitorCsA. The fact that the transport of substrates ofP-gp was not enhanced in the presence of PAMAMdendrimer would suggest that the route ofdrug transport is endocytosis-mediated transcel-lular route.20 In addition, it has been reportedthat transport of PAMAM across Caco-2 cellmonolayers involves in part paracellular trans-port.13,21,22 Our results also show an increasein permeability of 3H-mannitol, a known para-cellular permeability marker, across smallintestinal mucosa of all segments upon incubationwith doxorubicin–PAMAM complex (Fig. 4). Theresults that the Papp of doxorubicin–PAMAMcomplex was significantly (p< 0.01) greater thanthat of 3H-mannitol would suggest that the routeof drug transport is primarily transcellular. Ourresults suggested that when entraped to PAMAMdoxorubicin is able to cross intestine epitheliumvia a transcellular route and a paracellulartransport of PAMAM. And the endocytosismechanism of PAMAM should be clarified in thefuture study. In addition, deriving profit fromshelter of PAMAM, doxorubicin could bypass theP-gp efflux and CYP metabolism.

To confirm in vivo that doxorubicin–PAMAMcomplex can cross the intestinal barrier andbe absorbed, pharmacokinetic studies wereperformed in rats. The oral dose was at a lowconcentration (about 0.5 mM PAMAM) whichwould not cause noticeable cytotoxicity at Caco2cells. And this concentration might be lower onthe membrane of epithelial cell after diluted ingastrointestinal tract and with the large surfacearea of intestine which is made up of villus and


microvillus. For a single oral dose, there wassignificant improvement in the relative bioavail-ability of the drug in PAMAM complex to that offree doxorubicin (Fig. 5). And it was clearlyshowed that the effect of PAMAM-complex onthe increased systemic exposure of orallyadministered doxorubicin was the result of anincreased uptake which enhanced the oral bioa-vailability of doxorubicin. From the results thatdoxorubicin–PAMAM complex can significantlyenhance the transport of doxorubicin from themucosa side to the serosal side and the bioavail-ability of doxorubicin, it is suggested thatdoxorubicin is released mainly in the serosal sideor in the plasma after transepithelial transport,but not in cells.

It has been reported that coadministration ofthe P-gp and CYP3A4 inhibitor also improvedgreatly the oral bioavailability of the substrate ofP-gp. Most of the reports indicated that forimproving some anticancer drug oral bioavail-ability, an efficient P-gp/CYP3A4 comodulationwas necessary.23–25 However, due to using theP-gp and CYP3A4 inhibitor, the normal physio-logical function of the intestine epithelial cellsshould be disturbed, and the pharmacokineticsthe drug of coadministration could not predictexactly. In our study, deriving profit from shelterof PAMAM, doxorubicin could be free fromrecognization of P-gp and bypass the P-gp effluxand CYP metabolism, but not affect P-gp and CYPfunctions.

Although it has been reported that thedendrimer-anticancer drug delivery system couldenhance anticancer activity by increasing cellularuptake and cellular retention affords,15,26 thetherapeutic potential of this delivery systemshould be investigated in the future study onthe tumor cells and the tumor-bearing animalmodel.

CONCLUSION

The doxorubicin–PAMAM complex demonstratedtime-dependent and concentration-dependentuptake by Caco-2 cells. Moreover, this internaliza-tion, opposite to the uptake of the free form in thepresence and absence of the P-gp inhibitor, wasnot influenced by the P-gp efflux, which assumesP-gp-independent accumulation into cells. Andafter the oral administration, a higher bioavail-ability was obtained by doxorubicin–PAMAMcomplex. Since oral delivery of doxorubicin is

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limited because of the multidrug P-gp effluxpump, which is abundantly present in thegastrointestinal tract, drug–PAMAM complexmay represent a potential oral delivery systemfor the P-gp subtracts.

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enhanced oral bioavailability of doxorubicin in a dendrimer drug delivery system

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