Regional haemodynamic effects of dopamine and its prodrugs L-dopa and gludopa in the rat and in the glycerol-treated rat as a model for acute renal failure.

1. In this study the renal selectivity of dopamine and its prodrugs L-dopa and gludopa, with respect to their effects on regional blood flow, vascular resistance and central haemodynamics was investigated in normal rats and in rats with glycerol-induced acute renal failure (ARF). 2. In normal, anaesthetized rats, dopamine as well as its prodrugs caused a dose-dependent reduction of vascular resistance in the kidney (RR), mesentery (MR) and hindquarters (HQR) (dose range: dopamine: 0.1-5 mumol kg-1 h-1; L-dopa and gludopa: 1-200 mumol kg-1 h-1). Blood pressure and heart rate were affected at the highest dose only. 3. Administration of glycerol induced a preferential renal vasoconstriction; renal blood flow (-60%) and vascular resistance (+190%) were significantly more affected than MR (+40%) and HQR (+60%). This was only ameliorated by a low rate (10 mumol kg-1 h-1) infusion of gludopa: the glycerol-induced reduction of renal flow and increase in RR were significantly attenuated. A high dose of gludopa (100 mumol kg-1 h-1) or any dose of L-dopa or dopamine did not induce this beneficial effect. The glycerol-induced increase in MR and HQR was not attenuated by any of the treatments used. 4. The results indicate that gludopa is not renally selective at a pharmacodynamic level in normal, anaesthetized rats. Contrary to this, a low dose of gludopa does cause a renal selective vasodilatation and reduction of RR in rats with glycerol-induced ARF. This difference could be explained by a difference in renal vascular tone between normal rats and glycerol-induced ARF rats. A high dose ofgludopa does not cause these renal-selective effects: renal resistance and renal flow are at the same level as following glycerol and saline. This is probably due to the systemic effects of the released dopamine.

Renal selective N-acetyl-L-gamma-glutamyl prodrugs: studies on the selectivity of some model prodrugs.

1. In this study, a number of structurally different N-acetyl-L-gamma-glutamyl prodrugs were investigated with respect to selective uptake by the kidney in male Wistar rats. 2. All prodrugs were tested in vitro in rat kidney slices and kidney homogenate to study their uptake and conversion. It was found that the prodrugs of para-nitroaniline (agPNA), aminophenyl acetic acid (agAFA), sulphamethoxazole (agSM), sulphadimethoxine (agSDM), propranolol (agPP) and metoprolol (agMP) were accumulated by a probenecid-sensitive carrier. The prodrug of 4'-aminoantipyrine (agAAP) was not accumulated by a probenecid- or buthionine sulphoximine-sensitive carrier. Unlike all other prodrugs, agAAP and agMP were not, or only a very limited extent converted to the parent compound in vitro. 3. agPNA, agAFA and agPP were also investigated in vivo. The tissue distribution of the prodrugs and the parent drugs was established, as was their urinary excretion and pharmacokinetic behaviour. agPNA and agAFA showed selective uptake by the kidney, in contrast to agPP which accumulated in the liver. The distribution of the parent compounds following prodrug administration was as follows: agPNA was found in kidney and plasma: agAFA in kidney only; agPP in liver only. 4. The factors which determine the selectivity of N-acetyl-L-gamma-glutamyl prodrugs are discussed. The main factors are: the transport into the kidney, the conversion rate, the residence time of the prodrug in the kidney and the presence or absence of competition for uptake and conversation by other tissues, e.g. the liver. It is concluded that this prodrug approach offers the possibility of delivering drugs selectively to the kidney, but also that it is not universally applicable.

Renal selective N-acetyl-L-gamma-glutamyl prodrugs. III. N-acetyl-L-gamma-glutamyl-4'-aminowarfarin is not targeted to the kidney but is selectively excreted into the bile.

The pharmacokinetics of N-acetyl-L-gamma-glutamyl-4'-aminowarfarin (AGAW) was studied in the rat. The aim of this prodrug was to cause a renal-specific inhibition of the vitamin K cycle as a result of renal-specific release of the active drug 4'-aminowarfarin (AW). In vitro, it was found that kidney and liver homogenates and cytosol were able to convert the prodrug. In vivo, plasma concentrations of AW rose only slowly after a dose of 10 mg/kg AGAW i.v. to give a maximum concentration of about 3 micrograms AW/ml at t = 14 to 24 h. The tissue distribution of AGAW and AW was measured after 10 mg/kg AGAW i.v. It was found that AGAW did not accumulate in the kidney (9.7 micrograms/g in the kidney; 83 micrograms/ml in plasma at t = 60 min). AW concentrations were very low (0.1 microgram/ml or mg at t = 60 min). These results suggest that AGAW is not transported via a carrier into the kidney. The uptake of AGAW in vitro by rat kidney slices was investigated. It was found that AGAW did not accumulate in the slices. Neither did AGAW influence the accumulation of N-acetyl-gamma-glutamyl sulfamethoxazole in kidney slices. A second explanation for the lack of selectivity of AGAW in vivo could be its high (approximately 90%) plasma protein binding. Instead of being targeted to the kidney, however, AGAW was found to be excreted via a carrier-mediated mechanism into the bile: 50% of the dose was recovered unchanged in the bile within 3 hr.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal selective N-acetyl-gamma-glutamyl prodrugs. II. Carrier-mediated transport and intracellular conversion as determinants in the renal selectivity of N-acetyl-gamma-glutamyl sulfamethoxazole.

The mechanism of activation of the prodrug N-acetyl-L-gamma-glutamyl sulfamethoxazole (AGSM) and of gamma-glutamyl sulfamethoxazole (GSM) as a model for the mechanism of the renal selectivity of N-acetyl-gamma-glutamyl prodrugs was investigated. The hypothesis was tested that this selectivity is due largely to a carrier-mediated transport followed by an intracellular conversion of the prodrug to the active drug, in contrast to another mechanism. The transport of AGSM and GSM was studied with the use of kidney slices. AGSM accumulated in the slices. At 75 microM substrate concentration, the slice to medium ratio was 2.5 +/- 0.2. This accumulation was inhibited by the anion transport inhibitor probenecid (82% inhibition at 1.0 mM) and by the gamma-glutamyl transport inhibitor buthionine sulfoximine (60% at 1.0 mM). Acivicin, L-(alpha S,5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazole acetic acid did not inhibit AGSM accumulation at 0.1 mM, a concentration sufficient to inhibit the enzyme gamma-glutamyl transpeptidase; at 1.0 mM, however, AGSM accumulation was inhibited by 44%. These results suggest that the accumulation of AGSM is caused by an active transport process. GSM did not accumulate in the slices, but was completely converted to sulfamethoxazole (SM) after a 90-min incubation. Accumulation of AGSM was also seen in vivo: at 20 min after AGSM administration (10 mg.kg-1) the plasma, kidney and liver concentrations were 73 +/- 6, 110 +/- 7 and 37 +/- 5 micrograms.g-1, respectively. This accumulation could be inhibited by buthionine sulfoximine but not by acivicin.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal selective N-acetyl-gamma-glutamyl prodrugs: a study on the mechanism of activation of the renal vasodilator prodrug CGP 22979.

1. In this study the processes underlying the renal selectivity of the vasodilator prodrug CGP 22979 (N-acetyl-L-glutamic acid-N-[N2-(5-n-butyl-2-pyridyl) hydrazide]) were studied in rats. 2. The active drug CGP 18137 (2-hydrazino-5-n-butyl pyridine) selectively accumulated in the renal tissue following administration of the prodrug. 3. The kidney concentrations of active drug following prodrug administration were significantly lower than control values when either buthionine sulphoximine, glutathione or probenecid was coadministered (29 +/- 11; 33 +/- 14 and 61 +/- 20% of control values, respectively). Inhibition of gamma-glutamyl transpeptidase by AT-125 did not cause a significant decrease of renal CGP 18137 levels. 4. In order to correlate tissue drug concentrations with pharmacological effect, the renal haemodynamic responses to CGP 22979 were measured and the effect of buthionine sulphoximine, glutathione and AT-125 on these responses evaluated. All three of the compounds attenuated the renal response to the prodrug: an approximately 50% lesser decrease in renal resistance was found. The compounds had no effect on the haemodynamic actions of CGP 18137 itself. 5. In vitro, it was found that kidney cytosol was able to convert the prodrug, whereas microsomes were not, unless acylase was added. 6. The results indicate that, upon prodrug administration, gamma-glutamyl transpeptidase is not involved in the renal accumulation of CGP 18137 but is partly responsible for the renal haemodynamic responses to CGP 22979. Active transport of the prodrug into the tubular cells appears to be the major reason for the renal selectivity. A model is proposed for the renal action of CGP 22979, in which the important parts are the uptake of the prodrug via a transport system followed by an intracellular conversion to the active drug.