Nanomedicine: Nanotechnology, B

Pharmacology

A review of in vitro–in vivo investigations on dendrimers:

the novel nanoscopic drug carriers

Umesh Gupta, B Pharm, Hrushikesh Bharat Agashe, M Pharm,

Abhay Asthana, M Pharm, Narendra K. Jain, M Pharm, PhD4

Pharmaceutics Research Laboratory, Department of Pharmaceutical Sciences, Dr. Hari Singh Gour University, Sagar 470 003, India

Received 24 December 2005; accepted 7 April 2006

www.nanomedjournal.com

Abstract Dendrimers have emerged as one of the most interesting themes for researchers as a result of their

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doi:10.1016/j.nano.20

No financial confli

4 Corresponding

Gour Nagar, Sagar, M

E-mail address: u

unique architecture and macromolecular characteristics. Several groups are involved in exploring

their potential as versatile carriers in drug delivery. The use of dendrimers in drug delivery has been

reviewed extensively. The increasing relevance of the potential of dendrimers in drug delivery

emphasizes the need to explore the routes by which they can be administered. The present review

focuses on dendrimer-mediated drug delivery based on various routes of administration, a topic that

has received little attention in the available literature. With this focus in mind, we present a

comprehensive exploration of the recent advances in the investigational aspects of these nanoscopic

polymeric devices. Also included are some in vitro studies that present data suggestive of their

possible application in different routes of administration.

D 2006 Published by Elsevier Inc.

Key words: Dendrimers; Nanoscopic drug carriers; Drug delivery; Routes of administration

In all controlled-release devices used currently, with the

exception of mechanical pumps, polymers control the rate of

release [1]. In these controlled- and sustained-release devices

a drug candidate is usually attached to or entrapped within a

polymer. Many such polymeric drug carrier systems have

been extensively reviewed [2-4]. The efficacy of dendrimers

as ideal drug carrier systems is also being studied worldwide

by the scientific community; indeed it is one of the most

rapidly expanding research areas. Dendrimers are three-

dimensional, highly branched monodispersed macromole-

cules, which are obtained by an iterative sequence of reaction

steps producing a precise, unique branching structure [5].

Unique structures of dendrimers include highly branched and

well-defined globular structures with controlled surface

functionality, adding to their potential as new scaffolds for

drug delivery [6].

nt matter D 2006 Published by Elsevier Inc.

06.04.002

ct of interest was reported by the authors of this paper.

author. Department of Pharmaceutical Sciences,

adhya Pradesh 470003, India.

meshsemail@yahoo.co.in (N.K. Jain).

Dendritic cores can act in a bhost Q capacity for bguestsQ(that is, drug molecules); in this way they have been

reported to release drug in a controlled manner [7,8].

Dendrimers have been reported to host both hydrophilic and

hydrophobic drugs, thus demonstrating their versatility. The

nanoscopic particle size of dendrimers (ranging from 1 to

100 nm) makes them less susceptible to uptake by the

reticuloendothelial system. Because of their nanoscopic size

dendrimers have already been reported to transfect cells

[9-11]. Dendrimers have recently been used successfully in

gene delivery [12-18], as magnetic resonance imaging

agents [19-21], as solubilizing agents [22-28], and in other

nonpharmaceutical fields such as desalination [29].

The role of dendrimers as drug carrier nanosystems has

added new dimensions to the concept of controlled drug

delivery. A plethora of literature already available specifi-

cally focuses on drug delivery applications and future

prospects of dendrimers [30-36]. Most of these studies

describe physically entrapped or chemically conjugated

drug delivery. This article gives an overview of various

routes of administration of dendrimers. There are many in

vitro studies available demonstrating the usefulness of

iology, and Medicine 2 (2006) 66–73

Table 1

Drugs studied using different dendrimers and routes of administration

Serial No. Routes of administration Dendrimer Drug

1 IV PEGylated PAMAM dendrimer 5-Fluorouracil [37]Galactose-coated PPI dendrimer Primaquine phosphate [39]Polyester dendrimer Doxorubicin [40]

IM PEGylated peptide dendrimer Artemether [38]2 Transdermal PAMAM dendrimers Tamsulosin [46]

PAMAM dendrimers Indomethacin [47]3 Ophthalmic PAMAM dendrimers Tropicamide [48]

Pilocarpine [48]4 Oral PAMAM dendrimers 5-Fluorouracil [51]

U. Gupta et al. / Nanomedicine: Nanotechnology, Biology, and Medicine 2 (2006) 66–73 67

dendrimers as a drug delivery tool but very few in vivo

studies (Table 1). The available in vivo studies also do not

reveal any commonly adopted pattern of route of adminis-

tration; on the contrary, many variations have been

observed. We have compiled the results of different in vivo

studies and categorized them according to their routes of

administration. A few in vitro studies suggest the potential

of dendrimers upon their in vivo administration. These

studies have highlighted certain features of dendrimeric

carriers such as receptor-mediated uptake, drug release in

environments simulating that of some organs, or avoidance

of efflux mechanisms—all of which indirectly suggest an in

vivo applicability of dendrimers. Such studies are here

considered separately, emphasizing the feature they display

that is applicable to in vivo administration.

In vivo studies

Intravenous administration

The inherent toxicity associated with polycationic poly-

mers such as dendrimers limits their clinical usefulness.

Among the many options explored to overcome this lim-

itation, attachment of polyethylene glycol (PEG) to the

dendrimer (PEGylation) is prominent.

Bhadra et al [37] reported a PEGylated dendritic

architecture for delivery of the anticancer agent 5-fluorouracil

(5-FU). The researchers conjugated PEG monomethyl ether

(M-PEG-5000) to the periphery of fourth-generation (4G)

polyamidoamine (PAMAM) dendrimers. This imparted two

positive features to the nanometric drug carrier. First, the drug

loading capacity of the dendrimer was enhanced signif-

icantly. This was believed to be an effect of steric hindrance

by M-PEG, increasing the congestion at the dendritic

periphery, and of the additional functional groups made

available by M-PEG for electronic interaction with the

drug. Second, the hemolytic toxicity of PEGylated den-

drimers was minimized significantly compared with non-

PEGylated systems.

Further, the PEGylated systems were able to release the

drug in a sustained fashion for as long as 6 days in vitro.

Intravenous administration of plain drug, non-PEGylated,

and PEGylated systems determined increased mean resi-

dence time (MRT) for both non-PEGylated and PEGylated

systems (approximately 6 and 13 times, respectively).

The same group of researchers [38] reported PEGylated

peptide dendrimers for delivery of artemether. In this study

the authors synthesized a peptide dendrimer on a PEG core

with l-lysine as a repeating unit. Artemether was found to

form a complex with the dendritic interior as a result of

hydrogen bonding and hydrophobic interactions. It was

found that 4G and 5G peptide dendrimers incorporated

approximately 10 and 18 molecules of artemether, respec-

tively. The authors went on to conjugate chondroitin sulfate

A (CSA) to this system, thus further increasing drug loading

(25 to 40 drug molecules) depending on the generation and

the degree of conjugation. They also found that CSA

conjugation reduced hemolytic toxicity and macrophage

toxicity. CSA-conjugated systems proved to be effective in

removing ring and trophozoidal forms of Plasmodium

falciparum in culture in vitro. Upon intramuscular admin-

istration both conjugated and unconjugated systems were

observed to be effective as sustained-release tools.

Glycodendrimers have also been considered for drug

delivery applications. The ability of glycodendrimers to

target primaquine phosphate to the liver was explored by

Bhadra et al [39]. A 5.0G polypropylene imine (PPI)

dendrimer (Figure 1) was conjugated with galactose to

produce galactose-coated dendrimers. This carbohydrate-

coated system was anticipated to accumulate in a higher

concentration in liver along with the drug primaquine

phosphate as estimated 2 hours after administration in liver.

It was found that in the case of carbohydrate-coated

dendrimers the hepatic accumulation of primaquine phos-

phate was as high as 50.7% compared with 25.7% in the case

of the plain PPI dendrimer formulation of the drug. Almost

30% of drug was found in hepatic parenchyma when

primaquine phosphate was delivered in galactose-coated

formulations, as opposed to approximately 20% in the case of

the plain dendritic system.

Another group of researchers [40] synthesized polyester

dendritic architectures using 2,2-bishydroxymethyl pro-

pionic acid as the monomer unit [41]. Authors synthesized

three polyester dendritic molecules having molecular

weights of 3790 Da (I), 11,500 Da (II), and 23,500 Da(III)

having 2,2-bishydroxymethyl propionic acid monomer. To

the III molecule composed of three-arm poly(ethylene)

oxide–dendrimer hybrid; the potent anticancer drug doxo-

rubicin (DOX) was covalently attached via an acid-labile

hydrazone linkage. In the biologic evaluation all the

Fig 2. PAMAM dendrimer (3.0G).

Fig 1. Typical structure of a 5.0G polypropylene imine dendrimer.

U. Gupta et al. / Nanomedicine: Nanotechnology, Biology, and Medicine 2 (2006) 66–7368

compounds were highly water soluble and nontoxic. Model

compound III was found to have highest circulatory half-life

(72 minutes) as compared with the lower molecular weight

systems I and II. Therefore, a DOX)–compound III

conjugate was prepared for drug delivery studies. The

polymer-DOX conjugate showed no significant accumula-

tion in any vital organ including the liver, heart, and lungs;

this is a significant improvement over the administration of

free drug, which partitions into organs such as liver and

heart. A 100% release was achieved after 10 minutes,

3 hours, 26.5 hours, and 10 days at pH 2.5, 4.5, 5.5, and 6.5,

respectively. Thus hydrazone linkage provides a suitable

system for pH-dependent release (i.e., compatible with

conditions found in tumors). DOX conjugate was also found

to be less toxic than the free drug.

Recently Asthana et al [25] solubilized flurbiprofen (FB),

a nonsteroidal anti-inflammatory drug, in 3.0G PAMAM

dendrimers (Figure 2) and studied the possibilities of this

drug-loaded dendrimer for controlled and site-specific drug

delivery. FB was solubilized at different pH levels (pH 2.0,

7.0, and 10.0) and at different concentrations (0.1%, 0.2%,

0.3%, 0.4% w/v in water) of 3.0G PAMAM dendrimers.

Drug loading with DF2 (0.2% w/v solution of dendrimer +

drug) and DF4 (0.4% w/v solution of dendrimer + drug)

formulations was found to be 960 Ag/mL and 1177 Ag/mL,

respectively. Solubility enhancement of drug was found to

depend on concentration. In vitro–release studies of drug

from DF2 and DF4 formulations were carried out in

phosphate-buffered saline (PBS) (0.1 M, pH 7.4) alone,

0.1 M pH 7.4 PBS containing 0.1% albumin, 0.1 M pH 6.2

PBS and deionized water. Initially rapid drug release was

observed (up to 40% in 3 hours); the rate slowed later,

indicating controlled release. The authors concluded that the

later slowing of release might be due to the binding of drug

to terminal primary amine groups as well as hydrophobic

encapsulation of drug in the dendritic interior. The DF2 drug

formulation on intravenous administration showed better

anti-inflammatory action than the free drug FB. Anti-

inflammatory action was maximum (75%) at the fourth

hour that continued up to 8 hours and even after a 25% anti-

inflammatory effect was observed with DF2 formulations.

However, in the case of plain drug (FB), the maximum

effect (75%) was displayed at hour 3 but decreased below

50% after 4 hours. Pharmacokinetic studies of DF2 and

FB reveal a significant difference in terminal half-life,

distribution volume at steady state, and MRT values. MRT

value and terminal half-life of DF2, respectively, were

almost two and threefold greater than FB. Biodistribution

studies of drugs in organs such as liver, spleen, paw,

stomach, kidney and lung indicated more localization of

drug in an inflamed paw.

Neerman et al [42] synthesized a 3G melamine den-

drimer according to a reported procedure [43] and proposed

this as a potential drug carrier. This observation was based

on acute (48 hours) and subchronic (6 weeks) in vivo

toxicity studies of these dendrimers in mice after intraper-

itoneal injection [44]. These data suggested that the

melamine dendrimers could serve as potential drug delivery

vehicle. In addition, they offer two significant advantages

over other dendrimers, including PAMAM. The interior of

these dendrimers is significantly more hydrophobic, pro-

viding opportunity to include hydrophobic guests. Future

studies could further support the candidacy of this dendritic

polymer in drug delivery.

Transdermal studies

In the first report of transdermal drug delivery of

dendrimers Wang et al [45] found that dendrimers can be

used to enhance penetration. In this study tamsulosin, a

U. Gupta et al. / Nanomedicine: Nanotechnology, Biology, and Medicine 2 (2006) 66–73 69

selective a1A-adrenoreceptor antagonist, was administered

transdermally in a polyhydroxy alkanoate (PHA)–based

system with 3G PAMAM dendrimers.

PHA, which is composed of 3-hydroxyhexanoic acid

(8%) and 3-hydroxyoctanoic acid (92%), was mixed with

tamsulosin and G3 PAMAM dendrimer and thereafter cast

into a transdermal patch. In vitro studies were performed in

shed snakeskin of Python reticulatus. The amount of

tamsulosin permeated per day from plain PHA matrix

through snakeskin was 15.7 Ag/cm2, whereas for dendrimer

containing PHA matrix it was found to be 24 Ag/cm2. The

clinical dose of tamsulosin of 200 Ag/ day can be achieved

easily by preparing a 10-cm2 patch using this transdermal

drug delivery system. The potential of dendrimers as

transdermal drug permeation enhancers through a PHA

matrix was also explained in their further studies. The

mechanism was explored using x-ray analysis, and it was

observed that crystallization of drug was promoted in a PHA

matrix in the presence of dendrimers. These crystals were

proposed to have a highly ordered orientation, which caused

limited diffusion direction of tamsulosin thereby promoting

its permeation [46].

Chauhan et al [47] also used dendrimers to deliver drug

transdermally. These types of dendrimers included 4G

(PAMAM) amine-terminated dendrimers, hydroxyl-termi-

nated 4G PAMAM dendrimers, and 4.5G PAMAM den-

drimers with carboxylate groups on the surface. All

dendrimers were responsible for solubility enhancement of

indomethacin, a hydrophobic model drug used in the study.

The increase in the flux of the drug indomethacin was

proportional to the increase in the concentration of the

dendrimer. It was observed that the ability of the dendrimers

to enhance the permeation of indomethacin into rat skin is

related to their ability to solubilize the drug in their

structure. The authors proposed that dendrimers present

the drug on the skin surface in solubilized form, and the

drug’s higher affinity for lipophilic stratum corneum causes

it to be partitioned into this layer.

Ophthalmic delivery

In a uniquely designed study to investigate the potential

of dendrimers as an ophthalmic drug delivery system

Vandamme and Brobeck [48] used 1.5G, 3.5G (peripheral

-COOH), and 4G PAMAM dendrimers with peripheral

amine groups, and 2G and 4G (with hydroxyl terminal

groups) PAMAM dendrimers. Several investigations were

successfully conducted, including residence time of these

systems in the eye, irritation caused, and pharmacologic

performance of drugs entrapped within them. Pilocarpine

nitrate (PiNO3) or tropicamide were the drugs considered

for this study (1% w/v). The authors found that the mean

ocular residence time for 1.5 G, 2 G (OH), and 4 G (OH)

were comparable to carbopol (approximately 4 to 5 hours)

when 25 AL and 0.2% (w/v) of aqueous solution was

instilled in eyes of male New Zealand albino rabbits. All the

dendrimers in concentrations up to 2.0% (w/v) or lower

were found to be weakly irritant, because dendrimers neither

caused any ocular irritation and nor induced a watering

reflex. Results of a miotic activity test on albino rabbits

indicated that these PAMAM dendrimer solutions improved

the bioavailability of PiNO3 compared with control and also

prolonged the miotic effect, indicating increased precorneal

residence time. Except 4G, in the case of mydriatic activity

tests of tropicamide, all the dendrimer solutions enhanced

pharmacological activity compared with controls.

Oral delivery

Very few reports are available on the ability of dendrimers

to cross gastrointestinal (GI) membranes. Wiwattanapatapee

et al [49] investigated transport of cationic PAMAM (3G

and 4G) and anionic PAMAM dendrimers (2.5G, 3.5G, and

5.5G) across the intestine of adult rats. They used the everted

rat intestinal sac method as an in vitro model. Investigations

with 125I-labeled dendrimers suggested that transport across

the intestinal membrane was charge dependent. For cationic

dendrimers, in general, it is reported that tissue uptake was

higher than serosal transport. However, the tissue uptake of

5.5G dendrimers was considerably higher than that of

the remaining anionic dendrimers. The serosal transfer rate

of all the anionic dendrimers was similar. 2.5G and 3.5G

dendrimers were assumed to be transported through third-

phase endocytosis. In contrast, 5.5G and cationic dendrimers

were observed to be taken up by specific or nonspecific

adsorptive endocytosis.

Florence et al [50] studied oral uptake of lysine dendrimers

with 16 surface alkyl (C12) chains. A 4G dendrimer with the

diameter of 2.5 nm was studied for its absorption through the

oral route in female Sprague-Dawley rats. It was found that at

6 hours the amount of dendrimers accumulated in the

stomach, small intestine, and large intestine taken together

was 20% of the administered dose. However, after 24 hours

this concentration fell to 1%. Hence the authors concluded

that 20% of dendrimers present at 6 hours was attributable to

an absorbed, adsorbed, or otherwise associated population of

dendrimers, which was either excreted (i.e., unabsorbed

fraction) or cleared (i.e. absorbed fraction) with time. To

ensure the amount of dendrimers taken up the authors carried

out organ distribution studies. They found that in organs such

as spleen, liver, and kidney, and in blood, the presence of

dendrimers was maximum at 6 hours after administration;

however, it was found to be negligible at or beyond 24 hours.

They further evaluated the comparative role played by Peyers

patches and enterosides of the small and large intestine in

dendrimer uptake and observed that in small intestine

dendrimer was preferentially taken up by Peyers patches,

whereas in large intestine enterosides played a more

important role.

In another study Tripathi et al [51] synthesized dendrimer

grafts (DGs) for delivery of 5-FU. 4G PAMAM dendrimers

were modified on their surface by attaching a fatty acid,

palmitoyl chloride, to give DGs. An average of about

0.396 Ag/mL of 5-FU was loaded in the DGs. After this

U. Gupta et al. / Nanomedicine: Nanotechnology, Biology, and Medicine 2 (2006) 66–7370

phospholipids were coated on these DGs. Drug loading in

the phospholipids-coated DG was found to be 52.9% F1.3% w/w. In vitro release studies revealed that there was no

release of 5-FU during thefirst 48 hours, and only 4.3% was

released in 7 days, showing their sustained-release charac-

teristics. In vivo studies in albino rats resulted in an oral

bioavailability of about 64.8%, which is about 1.7-fold more

than free drug bioavailability (57.4%).

Studies of in vivo potential of dendrimers

Some in vitro studies are clearly suggestive of the potential

of dendrimers for oral delivery. Oral delivery of any drug is a

challenge in its own context. Variations in theGI environment

from site to site, the presence of enzymes in GI fluids, and

peristaltic movement complicate the design of a system for

oral delivery. In addition, the presence of an efflux pump such

as p-glycoprotein–mediated efflux, which is active in

intestine, makes the task even more difficult. D’Emanuele

et al [52] synthesized prodrug by binding propranolol with

3G and lauroyl-3G PAMAM dendrimer and determined the

effect of these conjugates on transport of propranolol across

adenocarcinoma cell line CaCo-2. As a p-glycoprotein efflux

transporter substrate, propranolol is a poorly bioavailable

drug. Conjugates of 3G PAMAM dendrimers having two,

four, and six molecules of propranolol were synthesized; in

addition, propranolol-lauroyl dendrimer conjugates having

two and six lauroyl chains were also synthesized. In transport

studies of propranolol, propranolol-dendrimer conjugate and

propranolol-lauroyl-dendrimer across CaCo-2 cells it was

found that propranolol was able to bypass the p-glycoprotein

efflux state transporter when conjugated. The apical (A) to

basolateral (B) apparent permeability coefficient (Papp) of

propranolol was increased in the case of propranolol-

dendrimer conjugates, and this was further enhanced using

propranolol-lauroyl-dendrimer conjugates. Researchers

designed formulations as GxLyPz where x is the dendrimer

generation, y is the number of lauroyl chains, and z represents

number of propranolol molecules attached. AYB Papp of G3

L2 P2 was found to be approximately 3.5 times that of G3P2

and approximately 3.5 times that of G2P2. Themechanism by

which dendrimers enhance AYB transport (analogous to

absorption) was found to be endocytosis-mediated trans-

epithelial transport. The authors concluded that these

dendrimers were useful in enhancing bioavailability.

The variations in pH and enzymatic activity in the GI

tract can, however, also be used to advantage, and several

drug delivery systems making use of enzymes as a trigger

for drug release have been reported [53-55]. The usefulness

of dendrimers in pH-triggered oral delivery of sulfasalazine

was suggested by Wiwattanapatapee et al [56]. Two types of

dendrimer conjugates for colonic delivery of 5-amino

salicylic acid were synthesized and their release character-

istics studied. PAMAM dendrimers of 3G were conjugated

with two types of spacers [ p-aminobenzoic acid (PABA)

and p-aminohippuric acid (PAH)) via amide linkage. Both

spacers are azo linkers, and the drug 5-amino salicylic acid

(5-ASA) was bound to both conjugates via azo linkage.

Conjugates using PAH as the spacer carry three times more

5-ASA than conjugates using PABA as the spacer. Both

dendrimer conjugates (PAMAM–PABA–SA and PAMAM–

PAH–SA) were incubated in homogenates of small intesti-

nal and cecal content of albino rats in vitro for 12 hours. It

was found that in cecal content about 28% and 38% of a

dose of 5-ASA was released from PAMAM-PABA-SA and

PAMAM-PAH-SA, respectively, which increased up to the

24th hour. In contrast, in the case of small intestine

homogenate the release of 5-ASA was about 4.5% and

7.2% for PAMAM-PABA-SA and PAMAM-PAH-SA,

respectively. In the case of small intestine homogenate a

significant amount of PABA-SA (3.8%) and PAH-SA

(12.5%) were also reported to be released. Release of

5-ASA in cecal content was due to activity of azo reductase,

which led to breakdown of the azo bond between the spacer

and drug molecule. About 45% and 57% of 5-ASA was

released from PAMAM-PABA-SA and PAMAM-PAH-SA

conjugates, respectively, in 24 hours. It was found that this

release was much slower (80% in 6 hours) than that of

5-ASA from sulfasalazine; it was assumed that this was due

to the highly branched structure limiting the enzyme

cleavability of the azo bond. This proves that dendrimers

certainly could be explored for pH-based drug targeting. A

thorough in vivo investigation, however, is warranted to

provide further evidence for this claim.

Availability of several functional groups on the dendrimer

periphery permits the attachment of targeting moieties.

Targeting of anticancer drugs could prove extremely bene-

ficial, because they have severe toxicity profiles. The use of

dendrimers for tumor targeting is one of the most interesting

fields. Apart from a few reports [40], no in vivo reports

describing biodistribution of dendrimer-conjugated antican-

cer agents are available; however, the use of targeting

moieties such as folic acid has demonstrated encouraging

results. Quintana et al [57] designed dendrimer-based

therapeutic conjugates with methotrexate (MTX) for tumor

cell targeting. They conjugated folate residues to 5G

PAMAM dendrimers along with fluorescein isothiocyanate

(FITC; Fluorochrome) for targeting as well as detection of

such conjugates in tumor cells. MTX was conjugated to 5G-

FITC-FA (folic acid, FA) conjugates via amide and ester

linkages. Both types of conjugates were internalized in the

KB cell line of human epidermoid carcinoma, which over-

expresses folate receptor. In vitro internalization of these

conjugates was found to be as efficient as conjugates of

dendrimers to FAwithout drug. Plain MTX was fourfold less

effective in killing tumor cells than drug conjugates through

ester linkages. If these results are reproduced on intravenous

administration of dendrimers, then that might prove a pivotal

breakthrough in cancer therapy.

PEG has been used for several years for designing long-

circulating drug carriers. Some PEGylated dendrimers are

also intended for this purpose. In addition to some in vivo

U. Gupta et al. / Nanomedicine: Nanotechnology, Biology, and Medicine 2 (2006) 66–73 71

studies on PEGylated dendrimers [26,37,38,58,59], some in

vitro studies are also available. Kojima et al [60]

synthesized PAMAM dendrimers having PEG grafts and

studied their ability to encapsulate two anticancer drugs:

adriamycin (ADR) and MTX. M-PEG with average

molecular weights of 550 and 2000 Da were attached to

end groups of 3G and 4G PAMAM dendrimers. ADR was

found to be encapsulated in the M-PEG-attached 3G and

4G PAMAM dendrimers. The study revealed the contribu-

tion of both dendrimer and PEG chains. The maximum

numbers of ADR molecules per dendrimer for M-PEG

(550)-3G, M-PEG (2000)-3G, M-PEG (550) 4G, and

M-PEG (2000)-4G dendrimers were approximately 1.2,

1.3, 1.6, and 6.5, respectively. It was clearly demonstrated

that encapsulation increases with generation as well as

molecular weight of PEG attached. Another drug studied

was MTX, which is acidic in nature. The maximum

numbers of MTX molecules associated with the M-PEG

(550)-3G, M-PEG (2000)-3G, M-PEG (550)-4G, and

M-PEG (2000)-4G dendrimers are approximately 10, 13,

20, and 260 mol/mol of dendrimers, respectively. It was

inferred that the increased encapsulation of MTX by

dendrimer as compared with ADR was due to acid-base

interaction between MTX and amino groups of dendrimer.

In vitro–release studies revealed that ADR was readily

released from modified dendrimers as free drug, whereas

the release of MTX from MTX-loaded M-PEG (2000)-4G

PAMAM dendrimer was slower than the free drug in an

aqueous solution of low ionic strength.

In yet another study by Ooya et al [26] ethylene glycol–

based DGs and star-shaped polymers were used to solubilize

and control the release of paclitaxel. Paclitaxel solubility in

water is about 0.3 Ag/mL. The authors synthesized 3G, 4G,

and 5G polyglycerol dendrimers, as well as graft and star-

shaped PEG methacrylate (PEGMA) polymers and investi-

gated their role in solubilization. Paclitaxel solubility was

increased using all these polymers. At a low percentage 3G,

4G, and 5G dendrimers increased solubility up to 270-,

370-, and 430-fold, respectively, compared with solubility

of paclitaxel in water. However, at 80% weight concentra-

tion paclitaxel solubility was increased to as high as 1.8 to

2.3 mg/mL (8000-fold). In vitro studies revealed that all

drug was released from the dendrimer solution at around

96 hours, whereas in the case of poly (PEGMA) and star-

shaped poly (PEGMA) the release rate was much slower

than with paclitaxel-loaded dendrimers. Furthermore, obser-

vations suggest that paclitaxel was not entrapped in the

dendritic structure; instead its retention was apparently due

to the effect of the high density of ethylene glycol units

of dendrimer.

Yang et al [58] conjugated penicillin V to PEGylated

PAMAM dendrimers of 2.5G and 3G via ester and amide

linkages. Penicillin V, a carboxylic acid–containing drug,

was conjugated to 2.5G (PAMAM dendrimers with 32 car-

boxylic groups on surface) via ester linkage and to 3.0G

PAMAM dendrimers (with 32 primary amino groups

on surface) via amide linkage. When these penicillin V–

conjugated PEG-PAMAM (3.0G) dendrimers were tested

against the Staphylococcus aureus strain of bacteria to assess

its antimicrobial activity, the drug was bioavailable after

cleavage of its ester bonding to dendrimers. Sideratou et al

[59] investigated the solubilization and release properties of

PEGylated diaminobutane (DAB)–PPI dendrimers using

pyrene, betamethasone valerate (BV), and bethamethasone

dipropionate (BD). Two types of PEGylated dendrimers—

weakly PEGylated (DAB64-4PEG) and densely PEGylated

(DAB64-8PEG) dendrimers—along with DAB64 den-

drimers were used for the study. Pyrenes as well as BD

and BV were successfully solubilized within interiors of

dendrimer but only partially in PEG coat. Densely PEGy-

lated derivatives solubilized higher concentrations of pyrene.

For DAB64-8PEG the loading was 13 wt% and 7 wt% for

BVand BD, respectively, whereas for DAB64-4PEG it was 6

wt% and 4 wt%, respectively. In conclusion, the enhanced

solubilization of these drugs in PEGylated dendrimers

secures their application as promising controlled-release

drug carriers, which can be protected by the poly(ethylene

oxide) coating on the dendritic surface.

Conclusions

Dendrimer-based drug delivery is currently one of the

most widely explored scientific areas. Many studies describ-

ing the usefulness of dendrimers in sustained and targeted

drug delivery are available. Most of these studies are in vitro

in nature, and only a few routes of administration are

attempted for drug delivery. Some very interesting conclu-

sions can be drawn from these studies. It has been shown that

dendrimers can cross GI membranes and can also be useful

for local pH- and enzyme-dependent delivery in the GI tract.

Dendrimer-mediated administration of several drugs resulted

in improvement in their pharmacokinetics profile. Den-

drimers were responsible for improvement in transdermal

flux and prolongation of corneal retention time. All these

results prove the versatility of dendrimers, and some very

important in vitro studies with in vivo potential further

endorse this versatility. More detailed studies on the routes

already investigated and studies on other routes for den-

drimer-mediated drug delivery are required, yet the existing

data emphasize the potential of dendrimers as drug carriers

via various routes. However, the toxicologic status of

candidate dendrimers must be established conclusively

before drawing any final conclusions in this regard.

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