Drug targeting to the liver with bile acids: THE “trojan horse” resurrected?

Bile acids are selectively taken up from portal blood into the liver by specific transport systems in the hepatocyte plasma membrane. Therefore, studies were performed to evaluate the potential of bile acids as shuttles to deliver drugs specifically to the liver. The alkylating cytostatic drug chlorambucii and the fluorescent prolyl‐4‐hydroxylase inhibitor 4‐nitrobenzo‐2‐oxa‐1,3‐diazol‐β‐Ala‐Phe‐5‐oxaproline‐Gly were covalently linked via an amide bond to 7α, 12α,‐dihydroxy‐3β‐(ω‐aminoalkoxy)‐5‐β‐cholan‐24‐oic acid. The chlorambucil‐bile acid conjugates S 2521, S 2539, S 2567, and S 2576 inhibited Na + ‐dependent [ 3 H]taurocholate uptake in a concentration‐dependent manner both into isolated rat hepatocytes and rabbit ileal brush border membrane vesicles, whereas the parent drug chlorambucil showed no significant inhibitory effect. The chlorambucil‐bile acid conjugates were able to prevent photoaffinity labeling of bile acid binding proteins in rat hepatocytes by the photolabile [ 3 H]7,7‐azo derivative of taurocholic acid indicating their bile acid character. The chlorambucil‐bile acid conjugate S 2577 was able to alkylate proteins demonstrating the drug character conserved in the hybridmolecules. Liver perfusion experiments revealed a secretion profile of the chlorambucil‐bile acid conjugate S 2576 into bile very similar to taurocholate compared to chlorambucil which is predominantly excreted by the kidney. 4‐Nitrobenzo‐2‐oxa‐1,3‐diazol‐β‐Ala‐Phe‐5‐oxaproline‐Gly‐t‐butylester (S 4404), a fluorescent peptide inhibitor of prolyl‐4‐hydroxylase, was not transported in intact form from portal blood into bile in contrast to its bile acid conjugate S 3744; about 25% of the peptide‐bile acid conjugate S 3744 was secreted in intact form into bile within 40 min compared with less than 4% of the parent oxaprolylpeptide S 4404. In conclusion, these studies reveal that modified bile acid molecules can be used as “Trojan horses” to deliver a drug molecule specifically into the liver and the biliary system. This offers important pharmacological options for the development of liver‐specific drugs. This study was designed to determine whether the conjugation product of L‐T 3 with cholic acid would result in a liver‐targeted compound (CGH 509A) with hypocholesterolemic (HC) activity significantly dissociable from cardiovascular (CV) and thyroxine‐suppressing (TS) effects normally observed with thyroid hormone. Evaluation of HC activity in lipemic rats showed that CGH 509A was 6 times less potent than L‐T 3 with ED 25 values estimated at 150 and 25 nmol/kg, respectively. CV function measured as changes in atrial rate, atrial tension and heart weight was determined in euthyroid rats. CGH 509A was at least 64 times less cardio‐stimulant than L‐T 3 , with minimum effective doses estimated at 2350 and 37 nmol/kg, respectively. TS activity was assessed in euthyroid rats as the potency of any compound to reduce plasma T 4 levels. CGH 509A was 50 times less potent than L‐T 3 with ED 50 values estimated at 900 and 18 nmol/kg, respectively. From these results, it is clear that, while L‐T 3 was equally potent on HC, CV and TS activities, the HC potency of CGH 509A was at least 15 and 6 times greater than its CV and TS potencies, respectively.