Dendrimer: A 21st century nanobiopolymer

Access of drug to the target site is hindered by various physical and biological constraints in the living systems. To surmount these barriers nanosystems having size range in nanometers are continuously being explored by scientists. These nanomedicines could have tremendous potential to revolutionize the current treatment strategies as supported by various reports. These nanomedicines work by selectively delivering the therapeutic moiety in the vicinity of diseased site in the living systems. This selective delivery reduces the associated toxicity of drug by minimizing the exposure of drug to the normal cells with increased accumulation into the diseased site.

The continuous demand for an optimum formulation to conquer the troubles of conventional drug delivery system compelled the emergence of novel drug delivery systems. Polymers are one of the key components of majority of novel drug delivery systems like microparticles, nanoparticles, polymeric micelles, drug-polymer conjugates, and a new macromolecular architecture “Dendrimer”. Dendrimers are the result of continuous advancements in the field of polymer science and synthesized first times by two different groups of scientists, simultaneously during the period of 1970-1990; by Buhleier and coworkers, and Tomalia and coworkers.

The word dendrimer is derived from a Greek term dendron that means “tree” because the architecture of dendrimers is composed of a number of branching units similar to a tree. Dendrimers are the macromolecular architectures designed synthetically and characterized by the presence of large number of branching points with three dimensional globular shapes. “Arborols”, “Cascade molecules” and “Dendritic molecules” are the other terms used synonymously for dendrimers in literature. Monodispersity is the unique property of dendrimers, which is attributed to their well defined size and molecular weight. Multivalency is another characteristic feature of dendrimer, which is due to the presence of large number of surface groups. The presence of these tailor-made surface groups renders dendrimers as a host or template for drug delivery and other biomedical applications like diagnostic agent.

Dendrimers are constituted of three distinct architectural components, as following:
Core: Center of dendrimer, which may consist of an atom or a molecule having at least two identical chemical functional groups.

Branches: These are the repeat units, which emanate from the focal point, core. The geometrical repetition of branch points results in a series of radially concentric layers known as “generations”.

Terminal functional groups: These are located at the surface of dendritic architecture which is critical factor in determining the properties of dendritic polymers (Figure 1).

Architectural comparison between dendrimers and polymers
In contrast to linear polymers, dendrimers are characterized by precisely controlled architecture with tailor made surface groups, which could be tuned finely. In the family of polymeric architecture dendrimers comprise the fourth class as dendritic architecture after linear, cross linked and branched polymers. Further, dendritic macromolecular architecture may be divided into subclasses; hyperbranched polymer, dendrigraft polymers, dendrons, and dendrimers. In contrast with polymers, dendrimers are synthesized by controlled chemical reactions leading to synthesis of monodisperse, macromolecular and globular dendritic architecture.

Step-by-step growth method is used for synthesis of dendrimers and the branching elements are represented as generation number. Divergent and convergent growth methods are the two main methods for the synthesis of dendrimers. Divergent synthesis of dendrimers starts from core which is grown away radially and leads to extension to the periphery. Convergent synthesis starts from the surface proceeding towards interior before the attachment of pre-synthesized dendrons to the focal point, core.

Different types of dendrimers
A variety of dendritic molecules have been explored thoroughly for drug delivery including poly(propyleneimine) (PPI), polyamidoamine (PAMAM), poly-l-lysine (PLL), triazine, melamine, polyethylene glycol (PEG), carbohydrate, citric acid, poly(glycerol-co-succinic acid), poly(glycerol), and phosphorous dendrimers (Figure 2). A number of dendrimers with different core molecules and surface functional groups are being synthesized and have been classified into the following classes:

• Plain dendrimers
• PPI dendrimer
• PAMAM dendrimer
• Poly(amidoamine-organosilicon) or PAMAMOS dendrimers
• Polyester dendrimer
• Polyether dendrimer
• Polyether imine dendrimers
• Poly-l-lysine dendrimer
• Phosphate dendrimer
• Melamine dendrimer
• Citric acid dendrimer
• Triazine dendrimer

Dendrimers based on macromolecules

• Peptide dendrimers
• Carbohydrate dendrimers
• PEG dendrimers

Surface engineered dendrimer

• Acetylated dendrimer
• Amino acids/peptides conjugated dendrimer
• Antibody conjugated dendrimer
• Carbohydrate conjugated dendrimer
• Folate conjugated dendrimer
• PEGylated dendrimer
• SiRNA conjugated dendrimer
• Tuftsin conjugated dendrimer

Specialized dendrimers

• Amphiphilic dendrimer
• Chiral dendrimer
• Domino dendrimer
• Hybrid dendrimer
• Liquid crystalline dendrimer
• Micellar dendrimer
• Multilingual dendrimer
• Multiple antigen peptide dendrimer
• Tecto dendrimer

Dendrimers as nanobiopolymers: pharmaceutical applications
Most of the currently used drugs suffer with inadequate biodistribution and pharmacokinetic properties, which result into poor therapeutic responses and numerous side effects to healthy organs. These problems associated with conventional formulations of therapeutic agents could possibly be resolved using a nanometric carrier system, which can modulate body drug distribution leading to improved therapeutic efficacy. Initially linear polymers were explored for drug delivery and biomedical applications. Highly branched polymers having properties different from linear polymers were the subsequent action to scientists due to presence of large number of branches. In the current decade, dendrimers have emerged as the most innovative architecture that are being explored for drug delivery. In addition to unique architectural characteristics like monodispersity, precisely controlled size and shape, the biological properties of dendrimers including their ability to deliver the loaded content intracellularly, improved pharmacokinetic profile, and longer retention time with appropriate tissue distribution profile; render the status of ideal nanopolymers for dendrimers for the delivery of therapeutic agents like anticancer drugs, antimalarial drugs, antitubercular drugs, anti-HIV drugs, antifungal agents etc. The unique properties of dendrimers with associated advantages are summarized in Table 1 and their possible biomedical applications are presented graphically in

Properties of dendrimers
The superior potential of dendrimers in the drug delivery over other nanocarriers is due to their unique properties ascribed to its specific architecture synthesized by step-by-step controlled reactions. These salient properties of dendrimers include monodispersity, nanometric size, globular shape, biocompatibility, surface charge and toxicity, multivalency, interactions with biological membrane and pharmacokinetic properties; which have been discussed in the following section of this article.

Nanometric size and shape
The nanometric size and well defined shape of dendrimers allows them to cross the biological membrane with reduced risk of premature clearance from the body. The unique dendritic architecture of dendrimers is the result of high level of control during their synthesis. Size of dendrimers ranges from several to tens of nanometers in diameter and increases with generation number. The nanometric size of dendrimers matches the size to a number of biological structures, for example, 5.0G PAMAM dendrimers is approximately the same size and shape as hemoglobin (Hb) (5.5 nm diameter).

Monodispersity
Applications of linear polymers in drug delivery suffer with the problem of polydispersity. Dendrimers are the class of dendritic polymers that can be constructed with a well-defined molecular structure which, unlike polydisperse linear polymers makes them monodisperse. Monodispersity of dendrimers, as confirmed by various analytical methods including mass spectroscopy, size exclusion chromatography (SEC), gel electrophoresis and transmission electron microscopy (TEM), offers scientists the possibility to work with a tool for well-defined and reproducible scalable size.

Periphery charge, polyvalency and toxicity
As discussed earlier dendrimers comprise of three structural units- core, branching units and terminal end groups. Surface functional groups of dendrimers may possess positive, negative or neutral charges, which are critical in determining the chemical and biological properties as well as in the exploration of dendrimers as drug delivery vehicles. Due to this surface charge cationic dendrimers like PAMAM, PPI and PLL can form complexes with negatively charged DNA. Further, the positive charges of dendrimers facilitate their interaction with negatively charged biological membranes leading to applicability of dendrimers for intracellular drug delivery. Despite of such advantages the polyvalency of dendrimers also poses challenges like toxicities including hemolytic toxicity, cytotoxicity etc. Fortunately anionic and neutrally charged dendrimers reveal less toxic or even nontoxic manifestations. Surface engineering of dendrimers as well as designing of biodegradable/biocompatible dendrimers may assist in alleviating the toxicity of dendrimers (Figure 4). Thus polyvalency is a vital factor that has important implication on the properties of dendrimers as well as in the field of dendrimers-mediated drug delivery.

Biocompatibility
Despite of their inherent toxicity, the cationic dendrimers have been considered as ‘smart’ delivery vehicle due to their ability to cross biological barriers, propensity as intracellular drug delivery vehicle, long retention time and targeting propensity. Generally cationic dendrimers with surface amine functionality like PAMAM and PPI dendrimers display concentration-dependent toxicity and hemolysis whereas neutral or anionic groups terminated dendrimers have shown comparatively less toxicity and hemolysis (Figure 4).

Advantageously, masking of cationic end groups or conversion of end groups of dendrimers to neutral or anionic groups have resulted in dendrimers with decreased toxicity or even non-toxic dendrimers as investigated in in vitro and in vivo studies. Polyester, polyether dendrimers are the examples of neutral dendrimers and surface engineered dendrimers including PEGylated and glycosylated dendrimers etc.

Conclusion & future prognosis
Nanobiopolymer based delivery systems offer enhanced therapeutic benefit because of their targeting ability, propensity to bypass drug resistance, intracellular drug delivery, high drug payload and multivalency to carry multiple drug molecules along with targeting moiety or imaging agents. Superior therapeutic efficacy has been observed with emerging new class of drug delivery systems based on nanobiopolymeres. In this scenario dendrimeric nanobiopolymer has been successful in drawing the attention of scientists due to its fascinating drug delivery propensity along with ability to function as important diagnostic agents.  Biomedical applications of the most advanced nanobiopolymer, dendrimers, have witnessed tremendous advances in the last two decades.

Dendrimers as delivery devices have offered controlled drug disposition in the biological environment with maximum therapeutic index. They could emerge as promising delivery devices with fascinating diagnostic applications in the modern medicine systems. Dendrimers with high loading efficiency have provided an excellent platform for the attachment of drug, gene, targeting moieties and homing devices,

imaging agents etc. In the recent scenario dendrimers are being explored as a means for tissue regeneration. The success of dendrimers in the field of nanomedicine lies in the unique characteristics of dendrimers i.e. they could be tuned finely to be explored in different vistas of modern drug delivery scenario including drug targeting.