The use of angiogenic growth factors such as VEGF is an appealing
strategy to improve the vascularization of a construct. Although a single
bolus of VEGF improved revascularization in animal models [77], this
approach might have some limitations due to protein instability
in vivo and possible undesirable side effects. Indeed, careful dosing of
VEGF is required, as overexpression of this angiogenic growth factor
leads to aberrant bone formation, bone marrow fibrosis, abnormal
blood vessels and extramedullary hematopoiesis [78]. Current
approaches are therefore more focusing on localized and sustained
delivery of growth factors, permitting prolonged exposure of
regenerating tissues to lower doses [79,80]. To this end, numerous natural
[81], synthetic [82] and composite [83] materials have been proposed as
delivery matrices. Alternatively, advances in gene therapy also facilitate
the controlled release of angiogenic growth factors, either by injection
of (viral) vectors or by using genetically modified implanted cells. Indeed,
direct injection of a recombinant adeno-associated virus overexpressing
VEGF induced mandibular condylar growth in mice [84]. Furthermore,
genetically induced expression of VEGF in MSC has been shown to
improve bone formation in critical sized defects in mice, either
when cells were used alone or in combination with different BMPs
and angiopoietin-1 [85,86].
To conclude, stimulation of an angiogenic response aids bone regeneration
in different preclinical models. However, the timing and dosing
of angiogenic growth factors have to be very tightly regulated, as severe
adverse effects might occur upon overstimulation.