Delivery of Osteogenic Regulatory Growth Factors - Engineered Bone: Tissue Engineering Intelligence Unit - page 118

surface. 118 It seems that degradation and resorption reduce the size of microspheres larger than
10 µ m to a point where they become susceptible to phagocytosis by macrophages and foreign
body giant cells. 119,120
The design of PHA carriers for bone regeneration must integrate a number of features
required for cells to attach, differentiate and express the set of appropriate gene products lead-
ing to tissue regeneration.
As with ceramics, the porosity of the device appears to be an essential parameter. Porosity is
defined by the pore size, range, void volume and pore distribution. These features contribute to
the macromolecular internal space available for bone ingrowth. Poly (lactic-coglycolic acid)
particles with pore sizes of 150 to 300 µ m diameter which corresponds to the porosity of
human haversian bone, were manufactured to allow for cell ingression into pores. 111 One study
has shown that disk implant with a high pore size (300-350 µ m) demonstrated the best
osteoconductive ability in a standard intraosseous calvarial wound. 121
Moreover, the carrier shape will have an impact on the rate of polymer degradation and
influence the pharmacokinetics for drug delivery. Block devices do not allow easy host cellular
infiltration and apparently elicit significant foreign body reactions. Microparticulate PLG co-
polymer were therefore preferred because they present numerous advantages. First, they pro-
vide a delivery system with a high exchange surface, which potentially enhance the growth
factor availability. Second, they resorb rapidly with minimal foreign body reactions and can be
incorporated into mouldable implant. However, to provide a surgically convenient composite
suitable for implantation into irregularly shaped skeletal defects, a third component is neces-
sary to yield a semi-solid paste. Different biopolymer solutions have been used: blood clot,
calcium cross-linked alginate, cellulose and its derivatives.
The rhBMP-2 release from porous microspheres made of 50:50 PLG (50PL/50PG) was
evaluated in vitro and indicated a triphasic process. 122,123 These porous matrices retained BMP
in both « free » and « bound » forms. The initial phase represented loss of free protein from the
porous particles over 3-4 days (growth factor present on the surface and within the pores of the
PLG particles). A lag period followed during which bound rh-BMP-2 is retained until 14-21
days. During the third phase that began after 3 weeks, mass loss of the bioerodible particles
began and consequently resulted in renewed release of the bound protein. The combination of
PLG with carboxymethylcellulose (CMC) or methylcellulose (CM) polymer delayed the BMP
release.
Particulate PLG combined with various « thickening agents » have been tested with BMP to
accelerate osseous regeneration in various critical size defect models: calvaria, 124-126 femur, ra-
dius, 111,125,127 maxillary cleft, 128 and spinal fusion. 11,129 The addition of thickening agents to
PLA particulates probably influenced the effectiveness of the osseous regeneration and it re-
mained difficult to evaluate the exact role played by each component, especially when the
agent was an autologous blood clot. Blood clot contained cells and platelets containing numer-
ous growth factors such as TGFb1 and PDGF. All these in vivo experiments with particulate
delivery systems gave promising results without major adverse tissue responses. However, in
some cases, the authors observed a dislocation of the carrier coming from soft tissue move-
ments and blood ooze from the recipient bed. 111 In order to improve the plasticity of the
material (i.e., the ability to mould to the bone defect), Saito et al synthesised polylactic
acid-polyethylene glycol block copolymers (PL-PEG). They prepared diverse copolymers with
various sizes of PLA and PEG and tested them as BMP-carrier on a rat ectopic model. 115,130-132
It appeared that the ratio of PL size to PEG size and the total molecular size are essential factors
for a good carrier and influence degradation rate, swelling ratio, and implant hardness. To
improve the resorption of the polymer and thereby optimise the delivery system of BMPs, the
same authors synthesised a new copolymer based on a random insertion of p-dioxanone (DX)
in the PL segment. This PL-DX-PEG polymer exhibited good degradation characteristics, and
when mixed with rhBMP-2, demonstrated ectopic bone formation and healing of large bone
defects. 133