Drug Targeting Organ-Specific Strategies

5.3 Renal Delivery Using Macromolecular Carriers: The Low Molecular Weight Protein Approach 139
Figure 5.10. Accumulation of a radiolabelled LMWP in the lysosomes of the proximal tubular cell.
Electron microscope autoradiography of renal proximal tubular cells from a rat injected i.v. with [125I]tyramine-cellobiose-labelled
cytochrome-c, 4 h prior to fixation through the abdominal aorta. An
intense lysosomal accumulation of the protein is observed in three dark electron-dense lysosomes . A
few grains are seen over the apical endocytic apparatus. Part of the luminal brush border is found in the
upper right hand corner. Magnification, x 25 000. Unpublished data from E. I. Christensen, Arhus,
Denmark, and M. Haas, Groningen, Netherlands.
5.3.3 Renal Catabolism of LMWP-conjugates
5.3.3.1 Renal Catabolism of Native LMWPs
Morphological (Figure 5.10) and biochemical studies have established that after endocytosis
by the proximal tubular cell, LMWPs migrate via endosomes to the proteolytically active
lysosomes [78,79]. Within the lysosomes the LMWPs are degraded into small peptides and
single amino acids.Whereas the renal uptake rate of various LMWPs appeared to be similar,
LMWPs are catabolized with distinct individual differences in their catabolic rate as indicated
from the difference in the rate of decline of radioactivity in the kidney (Figure 5.11). The
rate of catabolism seemed unrelated to the size or charge of the protein alone [80,81]. Probably
multiple structural factors play a role in this process. A crucial factor may be the different
endosomal migration times of LMWPs from the tubular lumen to the lysosomes. Whereas
cytochrome-c accumulated in the lysosomes within 3 min, lysozyme seemed to migrate for
20 min before the commencement of degradation [72, 82].Also the intrinsic activity of the reabsorbed
protein may play a role. For instance, the long renal half-life of aprotinin, an inhibitor
of proteolytic enzymes, may be explained by an inhibition of its own degradation, as
suggested by Bianchi [71]. These studies suggest that the LMWP method of renal drug targeting
results in cell-selective delivery followed by controlled drug release which can be manipulated
at various stages of the renal deposition process. The lysosomes are stacked with a
variety of proteolytic enzymes in an acidic environment. Programmed drug release from a
drug–carrier conjugate may therefore be achieved using peptide, ester or acid-labile bonds