Requisite structural characteristics for benzodiazepine inhibition of triiodothyronine uptake into a human liver cell line.

Previously, we reported potent inhibition of triiodo-L-thyronine (T3) cellular uptake into a human liver cell line (HepG2) by central and peripheral receptor specific benzodiazepine (BZ) compounds and our working hypothesis of BZ's as direct competitors for the iodothyronine transporter, displacing T3 but not acting as a substrate for transport. In this report, we list other reported uptake inhibitors and compare them to 23 benzodiazepine receptor ligands, in their potency to inhibit cellular uptake of T3. The most potent inhibitors are restricted to the benzodiazepine class. From the BZ structure-activity relationship (SAR) for inhibition, we see that the nonfused phenyl ring may be essential for activity and the strongest relationship is seen with substitution at R2' where Cl greater than F greater than H. Substitution at R4' and hydroxyl substitution at R3 enhances potency as will alkyl groups at R1 or on the imidazole group in the 1,2-annelated series. With R7 substitution, Cl is preferred over NO2 but not necessarily H when R4' = Cl; this may reflect a slightly different orientation of the molecule with large aliphatic R1 groups and/or R4' substitution. The carbonyl at R2 in the 1,4 benzodiazepine series, enhances their potency. The resultant structure-activity relationship highlights the importance of the halogen-substituted nonfused phenyl ring and seems unique relative to other described benzodiazepine sites and/or effects.