Targeted Drug Delivery to Bone: Pharmacokinetic and Pharmacological Properties of Acidic Oligopeptide-Tagged Drugs

Site-specific drug delivery to bone is considered to be achievable by utilizing acidic amino acid homopeptides. We found that fluorescence-labeled acidic amino acid (L-Asp or L-Glu) homopeptides containing six or more residues bound strongly to hydroxyapatite, which is a major component of bone, and were selectively delivered to and retained in bone after systemic administration. We explored the applicability of this result for drug delivery by conjugation of estradiol and levofloxacin with an L-Asp hexapeptide. We also similarly tagged an enzyme, tissue-nonspecific alkaline phosphatase, to see whether this would improve the efficacy of enzyme replacement therapy. The L-Asp hexapeptide-tagged drugs, including the enzyme, were selectively delivered to bone in comparison with the untagged drugs. It was expected that the ester linkage to the hexapeptide would be susceptible to hydrolysis in situ, releasing the drug or enzyme from the acidic oligopeptide. An in vivo experiment confirmed the efficacy of L-Asp hexapeptide-tagged estradiol and levofloxacin, although there was some loss of bioactivity of estradiol and levofloxacin in vitro, suggesting that the acidic hexapeptide was partly removed by hydrolysis in the body after delivery to bone. The adverse effect of estradiol on the uterus was greatly reduced by conjugation to the hexapeptide. These results support the usefulness of acidic oligopeptides as bone-targeting carriers for therapeutic agents. We present some pharmacokinetic and pharmacological properties of the L-Asp hexapeptide-tagged drugs and enzyme.