Polyphosphazenes: Synthesis, structures, properties, applications

Biodegradable polyphosphazenes for drug delivery applications

Biodegradable polymers such as poly(alpha-hydroxy acids), poly(anhydrides), poly(ortho esters), poly(amino acids) and polyphosphazenes have raised considerable interest as short-term medical implants due to their transient nature. Among these, polyphosphazenes are a relatively new class of polymers, quite distinct from all the biodegradable polymers synthesized so far, due to their synthetic flexibility and versatile adaptability for applications. These are high molecular weight, essentially linear polymers with an inorganic backbone of alternating phosphorous and nitrogen atoms bearing two side groups attached to each phosphorous atom. Controlled tuning of physico-chemical properties, including biodegradability, can be achieved in this class of polymers via macromolecular substitutions. Biodegradable polyphosphazenes, due to their hydrolytic instability, nontoxic degradation products, ease of fabrication and matrix permeability, are an excellent platform for controlled drug delivery applications. This review discusses the mode of degradation and drug delivery applications of biodegradable polyphosphazenes.

Pyrazolylcyclotriphosphazene containing pendant polymers: synthesis, characterization, and phosphate ester hydrolysis using a Cu(II)-metalated cross-linked polymeric catalyst

A multi-pyrazolyl cyclotriphosphazene containing polymerizable group N(3)P(3)(3,5-Me(2)Pz)(5)(O-C(6)H(4)-p-C(6)H(4)-p-CH=CH(2)) (2) has been prepared from the corresponding chloro derivative N(3)P(3)Cl(5)(O-C(6)H(4)-p-C(6)H(4)-p-CH=CH(2)) (1). The X-ray structures of 1 and 2 have been determined. Compound 2 undergoes ready metalation with CuCl(2) to afford N(3)P(3)(3,5-Me(2)Pz)(5)(O-C(6)H(4)-p-C(6)H(4)-p-CH=CH(2)).CuCl(2) (3). Model compound N(3)P(3)(3,5-Me(2)Pz)(5)(O-C(6)H(4)-p-CHO).CuCl(2) (6) has been prepared and characterized by spectroscopy and X-ray crystallography. In this compound, the coordination around copper is distorted trigonal bipyramidal, and the cyclotriphosphazene coordinates in a non-gem N(3) mode. Compound 2 has been copolymerized with divinylbenzene to afford cross-linked multisite coordinating polymer CPPL which is readily metalated with CuCl(2) to afford copper-containing polymer CPPL-Cu. The coordination environment around copper in CPPL-Cu has been evaluated by obtaining its EPR, optical, and IR spectra and comparing them with those of model compounds 3 and 6. The utility of CPPL-Cu as a heterogeneous catalyst has been demonstrated in the phosphate ester hydrolysis involving three model phosphate esters: p-nitrophenyl phosphate (pNPP), bis(p-nitrophenyl) phosphate (bNPP), and 2-(hydroxypropyl)-p-nitrophenyl phosphate (hNPP). In all of these reactions, a significant rate enhancement of ester hydrolysis is observed. Detailed kinetic analyses to evaluate Michaelis-Menten parameters have also been carried out along with experiments to elucidate the effect of pH, solvent, and temperature on the rate of hydrolysis. Recycling experiments on the hydrolysis of pNPP with CPPL-Cu shows that it can be recycled several times over without affecting the rates.

Use of polyphosphazenes for skeletal tissue regeneration

The hydrolytically unstable polyphosphazenes, poly [(imidazolyl) (methylphenoxy) phosphazenes] and poly [ethyl glycinato) (methylphenoxy) phosphazenes], were studied as potential polymeric supports for cells in tissue regeneration. For bone repair, their specific function would be to support osteoblast growth, forming a bone-polymer matrix. MC3T3-E1 cells (an osteogenic cell line) were seeded onto polymer matrices and cell adhesion and growth as well as polymer degradation were examined. Both imidazolyl- and ethyl glycinato-substituted polyphosphazenes supported the growth of MC3T3-E1 cells. An increase in the content of the imidazolyl side group resulted in a reduction in cell attachment and growth on the polymer surface and an increase in the rate of degradation of the polymer. In contrast, substitution with the ethyl glycinato group favored increased cell adhesion and growth and also an increase in the rate of degradation of the polymers. Thus, the polyphosphazenes represent a system whereby cell growth and degradation can be modulated by varying the nature of the hydrolytically unstable side chain. This in vitro evaluation suggests that the polyphosphazenes may be suitable candidate biomaterials for the construction of a cell-polymer matrix for tissue regeneration.