| Abstract
| - The membrane permeation in vivo of therapeutic proteins may be enhanced by conjugation of theprotein to cationic import peptides, such as the tat protein of the human immune deficiency virus.The organ uptake, expressed as a percent of injected dose (ID) per gram of tissue, is a function of bothmembrane permeability and the area under the plasma concentration curve (AUC), which is a functionof the plasma pharmacokinetics. The purpose of the present studies was to examine the effect of thetat peptide on the plasma AUC of a model exogenous protein, streptavidin, and to examine the extentto which changes in the plasma AUC influence organ uptake (%ID/g) of the protein. The cationicportion of the tat protein is comprised of a lysine/arginine-rich sequence, designated tat48-58. A biotinanalogue of this cationic peptide, tat-biotin, was radioiodinated and injected intravenously into ratswith or without conjugation to streptavidin. The unconjugated tat-biotin peptide was nearlyinstantaneously cleared from plasma by all tissues with a very high systemic clearance of 29 ± 4mL/min/kg and a high systemic volume of distribution of 4160 m± 450 mL/kg. The plasma clearanceof the tat-biotin/streptavidin conjugate, 1.37 ± 0.01 mL/min/kg, was reduced relative to the clearanceof unconjugated tat peptide, but was higher than the plasma clearance of the unconjugated streptavidin,0.058 ± 0.005 mL/min/kg. Conjugation of cationic import peptides such as tat48-58 to higher molecularweight proteins results in a marked increase in the rate of removal of the protein from the circulation,which is reflected in the reduced plasma AUC. In summary, tat conjugation of a protein has opposingeffects on membrane permeation and the plasma AUC. Therefore, the organ %ID/g is not increasedin proportion to the increase in membrane permeation caused by tat conjugation of proteins.
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