Takashi Dan

The Activity of AA-193, A New Uricosuric Agent, in Animals

A new uricosuric agent, 5-chloro-7, 8-dihydro-3-phenylfuro[2, 3-g]-1, 2-benzisoxazole-7-carboxylic acid (AA–193), was compared with other uricosurics in the rat, mouse and cebus monkey.

Hypouricemic and uricosuric actions of AA-193 in a hyperuricemic rat model

In normal rats, consecutive administrations of AA-193 for 7 days maintained the dose-dependent uricosuric activity without significant changes of the plasma urate level. In clearance studies, AA-193 produced an increase in the fractional excretion of urate (FEua) namely an inhibition of the net urate reabsorption in the nephron, which was probably dependent on the plasma concentration of the agent. During in vitro studies, 1 mmol/L AA-193 had no effect on liver uricase activity and 0.2 mmol/L AA-193 did not inhibit xanthine dehydrogenase activity. Therefore, it is unlikely that AA-193 at physiologic doses has a significant effect on either the production or degradation of urate. To assess the hypouricemic effect of AA-193 derived from its uricosuric effect, we used uricase-inhibited rats produced by oxonate feeding. In the hyperuricemic rat model, consecutive administrations of AA-193 for 7 days increased urate excretion and decreased the plasma urate level. We conclude that AA-193 has a hypouricemic effect caused by increases in urate excretion in hyperuricemic rats.

Uricosurics inhibit urate transporter in rat renal brush border membrane vesicles

It has been proposed that a urate-anion exchanger system in brush border membrane vesicles (BBMV), which mediates hydroxyl ion (OH-) gradient-dependent urate uptake, is the most likely route for the mediation of urate transport in the first step of urate reabsorption in the proximal tubules. Luminal drugs which inhibit urate reabsorption would inhibit the transport of urate into the cell by blocking the urate-anion exchanger. To confirm this hypothesis, we investigated the inhibitory effects of well-known uricosuric drugs on the OH-/urate exchange in BBMV. The rank order of potency was benzbromarone greater than tienilic acid greater than sulfinpyrazone greater than probenecid, which is consistent with clinical doses in man. AA-193 (5-chloro-7,8-dihydro-3-phenylfuro[2,3-g]-1,2-benzisoxazole-7-carb oxylic acid), an excellent candidate for a uricosuric, exerted the most potent inhibition on urate uptake (Ki = 0.12 microM). In contrast with that by stilbene disulfonates, the inhibition by AA-193 or benzbromarone was reversible.

A selective uricosuric action of AA-193 in rats

SummaryWe evaluated the uricosuric action of AA-193 by comparing it with the effect on PAH secretion in rats, using in vivo and in vitro techniques. The i.v. administration of AA-193 elevated the fractional excretion of urate (FEurate) significantly in a dose-dependent manner at doses from 0.1 to 10 mg/kg. Only at the highest dose of 10 mg/kg did AA-193 cause a momentary decrease in FEPAH. On the other hand, tienilic acid and probenecid reduced FEPAH at uricosuric effective doses. To compare the inhibitory effects of uricosurics on urate reabsorption and PAH secretion more directly, we investigated the effects of uricosurics on the OH− gradient-dependent urate uptake in brush border membrane vesicles and the net PAH accumulation in cortical slices. The relation between the affinity of uricosuric drug for urate and PAH transporters corresponds well with the difference between the effect on FEurate and that on FEPAH. The relative affinity of AA-193 for the urate uptake is 83-fold greater than that for the PAH accumulation.These results support the assumption that, in contrast with the other uricosurics, AA-193 has a much higher affinity for urate reabsorption system than that for the common pathway of weak organic acids in rats.

Substrate specificity of urate transporter in rat renal brush border membranes

To further demonstrate the substrate specificity of urate-anion exchanger in rat renal brush border membrane vesicles, the hydroxyl ion gradient-dependent [2-14C] urate uptake was studied by a rapid filtration technique. The [2-14C] urate uptake was more sensitive to unlabeled urate than to unlabeled xanthine and hypoxanthine. In addition, urate derivatives which are methylated at the positions 3 and 9 hardly inhibited the urate uptake. Because of the substrate specificity, the urate-anion exchanger in brush border membranes appears to selectively use urate as the endogenous substrate.

25 THE ACTIVITY OF AA-193, A NEW URICOSURIC AGENT, IN ANIMALS

A new uricosuric agent, 5-chloro-7,8-dihydro-3-phenylfuro[2,3-g]-1,2-benzisoxazole-7-carboxylic acid (AA-193) was compared with other uricosurics in the rat, mouse and cebus monkey, because the effects of those drugs were known to be highly species-dependent.Probenecid and tienilic acid which in the human kidney block the reabsorption of filtered and secreted urate, increased the urate excretion in rats. But benzbromarone, an inhibitor of the latter reabsorption in man, did not have the uricosuric activity. Thus, the post-filtered reabsorption of urate is probably dominant in rats. We found that in rats AA-193 is the most potent uricosuric yet reported. In mice, probenecid not only had so-called paradoxical actions but stimulated the urinary urate wasting after administration of pyrazinamide. Aspirin also responded paradoxically. These data suggested that the renal transport system of urate in the mouse is similar to that in man. AA-193 as well as benzbromarone enhanced the urate excretion dose-dependently in normal condition, but the effects were different in pyrazinamide-suppression tests in mice. In cebus monkeys, the uricosuric and hypouricemic effects of AA-193 were more patent than those of probenecid and similar to those of tienilic acid, but less than those of benzbromarone. Benzbromarone had a considerable role in post-secreted reabsorption in the mokey.Conclusion: AA-193 is a new class of uricosuric agent that controls the renal reabsorption of filtrated urate particularly.