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This paper reviews the design of a drug targeting strategy for renal specific delivery and endorenal release of drugs using low-molecular-weight proteins (LMWPs). In general, LMWPs are known to be filtered and subsequently reabsorbed by the proximal tubular cells of the kidneys. Within these cells LMWPs are catabolized in lysosomes due to prevalence of proteolytic enzymes and pH of 4-5. As such, these LMWPs might serve as drug carriers that release drugs site-specifically in the kidneys. The ability of the kidney to release the parent drug from drug-LMWP conjugates and drug-spacer derivatives by enzymatic or chemical hydrolysis of the connecting bond has been tested by incubation experiments in renal cortex homogenates and lysosomal lysates. Drug-LMWP conjugates of naproxen, sulfamethoxazole and dopamine have also been studied in vivo. The pharmacokinetics of these conjugates have been examined in freely moving rats and compared to those obtained after injection of equivalent doses of unreacted mixtures of drug and protein. In addition, the disposition of the drug-conjugated LMWP has been studied by radioactive-imaging with a gamma-camera. These studies demonstrated that the kidneys are the main site of uptake of drug-conjugated LMWPs and that the drugs are renally uncoupled and finally excreted in the urine. gamma-Camera experiments revealed a predominant renal uptake of radio-iodinated drug-conjugated LMWPs. Coadministration of an excess of the native LMWP resulted in a pronounced decrease of urinary (parent) drug excretion, suggesting competition for proximal tubular uptake between the LMWP and the drug-conjugated LMWP. Endorenal parent drug release from the LMWP was demonstrated for drugs with carboxyl groups and amino groups. The NSAID, naproxen, was shown to be released from the LMWP conjugate in vivo, provided that coupling via its carboxyl group was performed via an ester bond using a L-lactic acid spacer. If naproxen was directly coupled to the protein via an amide linkage, renal proteolysis of the LMWP-conjugated drug gave rise to the formation of a naproxen-lysine derivative. Interestingly, this product has been demonstrated to be at least as active in inhibiting prostaglandin synthesis as naproxen itself. Sulfamethoxazole was shown to be endorenally released from a drug-LMWP conjugate, if its primary amino group was coupled to the LMWP via an acid-sensitive spacer. Apart from the altered kinetics of the drug itself, the selective renal distribution was further demonstrated by the absence of its hepatically formed metabolite N-4-acetylsulfamethoxazole in urine. This metabolite was abundantly present in the control experiments after injection of the drug in its uncoupled form. Parent drug regeneration in the kidney was further demonstrated for a dopamine-LMWP conjugate, linked through its amino group via a similar acid-sensitive spacer. Within a relatively low dose range of lysozyme, a dose-dependent endorenal dopamine release could be demonstrated. In principle, drugs released within the proximal tubular cell may leave this cell at the basolateral membrane (blood site) or at the brush-border membrane (urine site). Renal delivery of LMWP-conjugated naproxen resulted in detectable plasma levels of the parent drug. In contrast, in the case of LMWP-conjugated sulfamethoxazole and dopamine, no (increase in) plasma levels of the free drugs were measured.

Drug targeting to the kidney with low molecular weight proteins