Targeted delivery of osteogenic drugs for bone tissue engineering
Murphy, Matthew Brian
Mikos, Antonios G.
Doctor of Philosophy
To create a more efficient and effective method of osteogenic drug delivery in vivo, drugs were modified with high calcium affinity moieties including pamidronate, poly(aspartic acid), and poly(glutamic acid). To test the initial hypothesis that modified drugs can demonstrate the same bone binding capabilities of pamidronate, poly(aspartic acid), and poly(glutamic acid), these motifs were conjugated to model peptides and exhibited high affinity to hydroxyapatite (HA). An in vitro controlled release experiment was conducted for native and modified TP508. Native and modified TP508 drugs were loaded in PLGA-PEG microparticles. Porous PPF scaffolds were injected with these drug-loaded particles, and in some instances with HA microparticles (20-50 or 50-100 mum). Less mineral surface resulted in less binding of drugs after release from the PLGA-PEG carriers and therefore a greater release than with the large HA particles. A final study was performed in the presence of 383 ng/mL collagenase, which cleaved the TP508 from the bone-binding domains at the point of the degradable peptide linker sequence. The dose effect of TP508 was established by delivering 0, 25, 50, and 100 mug TP508 loaded into PPF scaffolds and implanted in a sized rat cranial defect. After 4 weeks, microCT analysis of the skulls revealed a statistically significant increase in bone formation for the 50 mug dose compared to controls and the 25 mug dose. Based on these findings, an equivalent of 50 mug TP508 or modified drugs were delivered from PLGA-PEG microspheres in the presence of 20-50 mum HA microparticles in the PPF scaffolds' pore network, which revealed no significant differences between drug groups. These results were promising in that this strategy of drug modification had no apparent negative effect on the bioactivity of TP508. Another finding of this work was that the incorporation of HA into PPF composites resulted in significantly greater bone formation, even after subtraction of the initial amount of HA. The addition of this osteoconductive material stimulated an increase in new bone over 4 weeks for both the control and 50 mug TP508 groups.