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One-Stage Positron Emission Tomography and Magnetic Resonance Imaging to Assess Mesenchymal Stem Cell Survival in a Canine Model of Intervertebral Disc Degeneration.

Intervertebral disc degeneration (IVDD) is a major health problem. Although mesenchymal stem cells (MSCs) have been used to promote IVD regeneration, the actual survival time of implanted MSCs in IVDs has never been studied noninvasively and continuously in vivo. To investigate survival of implanted MSCs in vivo, this study used a canine model of degenerated IVD and MSCs transfected with a mutant herpes simplex type-1 virus thymidine kinase and labeled with magnetic iron oxide nanoparticles (MION). One-stage positron emission tomography (PET) and magnetic resonance (MR) imaging were carried out 3 days and 2 weeks, 3 weeks, and 4 weeks after implantation of MSCs into IVDs with surgically induced degeneration. Pfirrmann disc degeneration grade determined from the MR images indicated that the repair progress of degenerated IVD stopped 3 weeks after MSC implantation. Meanwhile, MION signal strength, signal contrast ratio (%), and low signal area (mm2) did not change significantly from that seen 3 days after cell implantation until 4 weeks [751.43 (4 weeks) ±52.67 (3 days) vs. 225.34 ± 35.62; 47.37 ± 5.01 vs. 85.37 ± 10.54; 1.78 ± 0.31 vs. 5.29 ± 1.35; P < 0.01, respectively]. Accumulation of the PET reporter probe, 9-(4-[18F]-fluoro-3-hydroxymethylbutyl)-guanine, was dramatically decreased at 3 weeks after MSC implantation. These results demonstrated that MSCs could survive no more than 3 weeks after implantation into IVDs with surgically induced degeneration, suggesting that MSCs could contribute to IVD repair for the first 3 weeks after implantation. The results also indicate that PET imaging could be used reliably to quantify the survival of implanted MSCs, whereas MION with MR imaging would likely be unsuitable for long-term tracking of MSCs in IVDs.