Specific drug delivery to the kidney.

The mesangial cells of the glomerulus, the proximal tubular cells and the interstitial fibroblasts are the first choice targets for renal drug delivery since they play a pivotal role in many disease processes in the kidney. In the present review, only targeting to the proximal tubular cell is addressed because only this has been studied extensively. Two approaches of drug delivery to the proximal tubular cell have been studied up to now, the prodrug/softdrug and low-molecular-weight protein (LMPWP) approach. Most research on tubular specific drug delivery has focused on the development of amino-acid prodrugs that, after delivery, require activation by more or less kidney-selective enzymes. Large differences in renal selectivity are found. For some prodrugs, a rapid removal of the released drug from the kidney explained the low renal selectivity whereas for others, cleavage in non-target tissue and insufficient transport across the cell to the enzyme site seemed mainly responsible. The LMWP approach is based on drug attachment to a protein (<30 kD) that is freely filtered through the glomerulus and after accumulation is selectively catabolized in the lysosomes of the proximal tubular cell. Using LMWPs as drug carriers, a higher renal selectivity can be attained and a broader range of drugs can be attached while the rate of drug release can also be manipulated. The studies with captopril-lysozyme and naproxen-lysozyme clearly showed that targeting resulted in a higher renal selectivity and that drugs delivered into and regenerated in the proximal tubular cell exert renal selective pharmacological activity. Further testing will provide more definite data on the added value of this delivery technology.