Ischemia/reperfusion (I/R) injury is a considerable insult to skeletal muscle, often resulting in prolonged functional deficits. The purpose of the current study was to evaluate the controlled release of the pro-regenerative growth factor, insulin-like growth factor-I (IGF-I), from a biodegradable polyethylene glycol (PEG)ylated fibrin gel matrix and the subsequent recovery of skeletal muscle from I/R. To accomplish this, the hind limbs of male Sprague–Dawley rats were subjected to 2-h tourniquet-induced I/R then treated with saline, bolus IGF-I (bIGF), PEGylated fibrin gel (PEG-Fib), or IGF-I conjugated PEGy-lated fibrin gel (PEG-Fib-IGF). Functional and histological evaluations were performed following 14 days of reperfusion, and muscles from 4-day reperfusion animals were analyzed by Western blotting and histological assessments. There was no difference in functional recovery between saline, bIGF, or PEG-Fib groups. However, PEG-Fib-IGF treatment resulted in significant improvement of muscle function and structure, as observed histologically. Activation of the PI3K/Akt pathway was significantly elevated in PEG-Fib-IGF muscles, compared to PEG-Fib treatment, at 4 days of reperfusion, suggesting involvement of the pathway PI3K/Akt as a mediator of the improved function. Surprisingly, myoblast activity was not evident as a result of PEG-Fib-IGF treatment. Taken together, these data give evidence for a protective role for the delivered IGF. These results indicate that PEG-Fib-IGF is a viable therapeutic technique in the treatment of skeletal muscle I/R injury.

Therapeutic delivery of regeneration-promoting biological factors directly to the site of injury has demonstrated its efficacy in various injury models. Several reports describe improved tissue regeneration following local injection of tissue specific growth factors, cytokines and chemokines. Evidence exists that combined cytokine/growth factor treatment is superior for optimizing tissue repair by targeting different aspects of the regeneration response. The purpose of this study was to evaluate the therapeutic potential of the controlled delivery of stromal cell-derived factor-1alpha (SDF-1α) alone or in combination with insulin-like growth factor-I (SDF-1α/IGF-I) for the treatment of tourniquet-induced ischemia/reperfusion injury (TK-I/R) of skeletal muscle. We hypothesized that SDF-1α will promote sustained stem cell recruitment to the site of muscle injury, while IGF-I will induce progenitor cell differentiation to effectively restore muscle contractile function after TK-I/R injury while concurrently reducing apoptosis. Utilizing a novel poly-ethylene glycol PEGylated fibrin gel matrix (PEG-Fib), we incorporated SDF-1α alone (PEG-Fib/SDF-1α) or in combination with IGF-I (PEG-Fib/SDF-1α/IGF-I) for controlled release at the site of acute muscle injury. Despite enhanced cell recruitment and revascularization of the regenerating muscle after SDF-1α treatment, functional analysis showed no benefit from PEG-Fib/SDF-1α therapy, while dual delivery of PEG-Fib/SDF-1α/IGF-I resulted in IGF-I-mediated improvement of maximal force recovery and SDF-1α-driven in vivo neovasculogenesis. Histological data supported functional data, as well as highlighted the important differences in the regeneration process among treatment groups. This study provides evidence that while revascularization may be necessary for maximizing muscle force recovery, without modulation of other effects of inflammation it is insufficient.