Drug Targeting Organ-Specific Strategies

142 5 Delivery of Drugs and Antisense Oligonunucleotides to the Proximal Tubular Cell
5.3.4.2 Renal and Systemic Effects of Captopril–Lysozyme
With regard to the pharmacological effects of the captopril–lysozyme conjugate, the following
observations were made (Kok et al., unpublished data). The extent of ACE-inhibition in
the plasma and kidney tissue was measured after i.v. administration of captopril–lysozyme
and an equimolar dose of free captopril. It was shown that conjugation to lysozyme caused a
similar though more sustained inhibition of renal ACE-activity by captopril.The inhibition of
plasma ACE-activity was clearly reduced but not entirely prevented by conjugation of captopril
to lysozyme. Possibly, the S-S linked drug conjugate is partly degraded in the circulation.
It is also possible that after degradation of the conjugate in the kidney, captopril was
transported back into the bloodstream. The rapid intracellular release may provide a sufficient
driving force for transport across the basolateral membranes.
Captopril–lysozyme did not significantly affect systemic blood pressure whereas an
equimolar dose of captopril alone decreased blood pressure significantly. Whereas free captopril
(5 mg kg –1 ) completely prevented an angiotensin-I-induced blood pressure increase, an
equimolar amount of captopril–lysozyme did not. However, in line with the direct ACE activity
measurements in renal tissue and plasma, in captopril–lysozyme-treated rats the angiotensin-I-induced
blood pressure increase was lower than in untreated rats, suggesting that
systemic activity was not fully prevented.
Neither free nor conjugated captopril affected glomerular filtration. Renal plasma flow increased
to the same degree after treatment with free or conjugated captopril (1 mg kg –1 ).Although
the complete dose–effect relationship was not studied, we can conclude that conjugation
of captopril to lysozyme did not prevent the drug from acting on the renal plasma
flow. Whether this effect is determined by intra-renal or systemic ACE-inhibition remains to
be investigated.
At present, the synthesis of lysozyme conjugates with ACE-inhibitors other than captopril
is under investigation. Some of these ACE-inhibitors may be advantageous for renal delivery.
The amount of conjugate required for therapy can be reduced when using an ACE-inhibitor
with a higher affinity for ACE (e.g. lisinopril). Furthermore, the stability of the conjugate in
plasma may be increased by using an ACE-inhibitor which is conjugated to lysozyme via a
linkage that is highly stable in plasma (e.g. lisinopril can in principle be coupled via an acidsensitive
spacer).
5.3.5 Renal Disease and LMWP Processing
Proteinuria is one of the most prominent abnormalities found in renal disease and is one of
the factors held responsible for the progressive loss of renal function. As a consequence of
the glomerular leakage of proteins, the proximal tubular cells are exposed to increasing
amounts of protein. This pathological condition can be anticipated to influence the deposition
and metabolism of protein-linked drugs. It is likely, in such a situation, that drug–LMWP
conjugates will have to compete with the overload of protein for tubular uptake as well as for
catabolism. The effect of proteinuria on the renal processing of LMWPs has been examined
in a number of studies [85–92]. Collectively, these studies clearly indicate that the effect of
proteinuria on renal uptake and degradation of LMWPs depends on the severity and dura-