Drug Targeting Organ-Specific Strategies

5.3 Renal Delivery Using Macromolecular Carriers: The Low Molecular Weight Protein Approach 141
en–lysine was gradually released from the conjugate. This catabolite was subsequently eliminated
from the kidney and after a single injection, drug levels in the renal tissue gradually decreased
with a t 1/2 of 160 min (Figure 5.9a).
No detectable amounts of naproxen or its lysine conjugates were found in the plasma after
administration of the conjugate and it can be inferred that excretion into the urine is the
crucial process which determines the elimination rate t 1/2. The lack of diffusion into the
bloodstream is a favourable property in relation to unwanted extra-renal effects.
5.3.3.3 Renal Catabolism of Captopril–Lysozyme
After renal uptake, captopril was rapidly released from the conjugate as indicated by the
rapid decrease in renal captopril levels with time (Figure 5.9b). The difference in renal t 1/2 of
naproxen and captopril after delivery with lysozyme is likely to be due to an unequal rate of
release from the lysozyme conjugates. Whereas naproxen–lysozyme requires a peptidase for
cleavage, captopril is released from the conjugate enzymatically by β-lyase and/or non-enzymaticaly
by thiol-disulfide exchange with endogenous thiols. To reduce the rate of captopril–lysozyme
breakdown, two different cross-linking reagents, SPDP and SMPT, were tested.
Although an SMPT link between two proteins is in principle less susceptible to disulfide
reduction [83], no difference in degradation rate was found between the SPDP and the
SMPT captopril–lysozyme conjugates (Kok et al., unpublished data).
5.3.4 Effects of Targeted Drugs Using an LMWP as Carrier
5.3.4.1 Renal Effects of Naproxen–Lysozyme
Having obtained promising kinetic profiles, the potential renal effects of naproxen–lysozyme
in the rat were investigated [84]. Naproxen, as an inhibitor of cyclooxygenase, blocks
prostaglandin synthesis. Among other effects, naproxen reduced furosemide-stimulated urinary
excretion of prostaglandin E 2 (PGE 2) as well as the natriuretic and diuretic effects of
furosemide. Studies with the conjugate showed that naproxen–lysozyme treatment clearly
prevents furosemide-induced excretion of PGE 2. This occurred with a dose of naproxen that
was not effective in the unconjugated form. Surprisingly, this effect occurred in the absence
of a change in natriuretic and diuretic response to furosemide. In this respect the pharmacological
effect differed from treatment with a high dose of free naproxen. An explanation for
these differences remains to be found. One possibility is that there is a difference in the intra-renal
kinetics of the NSAID compared with the parent drug. Free naproxen is extensively
reabsorbed in the distal tubule of the kidney via which route it may effectively inhibit
prostaglandin synthesis in the medullary interstitial cells. On the other hand, naproxen–lysine
is more hydrophilic and may be unable to reach the sites of prostaglandin synthesis involved
in the furosemide-induced excretion of sodium and water. These data shows that renal
drug targeting preparations can also be used as a tool to unravel the mechanisms of renal
therapeutic effects.